5.6 The Gestalt Principles of Perception

Learning objectives.

By the end of this section, you will be able to:

  • Explain the figure-ground relationship
  • Define Gestalt principles of grouping
  • Describe how perceptual set is influenced by an individual’s characteristics and mental state

   In the early part of the 20th century, Max Wertheimer published a paper demonstrating that individuals perceived motion in rapidly flickering static images—an insight that came to him as he used a child’s toy tachistoscope. Wertheimer, and his assistants Wolfgang Köhler and Kurt Koffka, who later became his partners, believed that perception involved more than simply combining sensory stimuli. This belief led to a new movement within the field of psychology known as Gestalt psychology. The word gestalt literally means form or pattern, but its use reflects the idea that the whole is different from the sum of its parts. In other words, the brain creates a perception that is more than simply the sum of available sensory inputs, and it does so in predictable ways. Gestalt psychologists translated these predictable ways into principles by which we organize sensory information. As a result, Gestalt psychology has been extremely influential in the area of sensation and perception (Rock & Palmer, 1990).

Gestalt perspectives in psychology represent investigations into ambiguous stimuli to determine where and how these ambiguities are being resolved by the brain. They are also aimed at understanding sensory and perception as processing information as groups or wholes instead of constructed wholes from many small parts. This perspective has been supported by modern cognitive science through fMRI research demonstrating that some parts of the brain, specifically the lateral occipital lobe, and the fusiform gyrus, are involved in the processing of whole objects, as opposed to the primary occipital areas that process individual elements of stimuli (Kubilius, Wagemans & Op de Beeck, 2011).

One Gestalt principle is the figure-ground relationship. According to this principle, we tend to segment our visual world into figure and ground. Figure is the object or person that is the focus of the visual field, while the ground is the background. As the figure below shows, our perception can vary tremendously, depending on what is perceived as figure and what is perceived as ground. Presumably, our ability to interpret sensory information depends on what we label as figure and what we label as ground in any particular case, although this assumption has been called into question (Peterson & Gibson, 1994; Vecera & O’Reilly, 1998).

An illustration shows two identical black face-like shapes that face towards one another, and one white vase-like shape that occupies all of the space in between them. Depending on which part of the illustration is focused on, either the black shapes or the white shape may appear to be the object of the illustration, leaving the other(s) perceived as negative space.

The concept of figure-ground relationship explains why this image can be perceived either as a vase or as a pair of faces.

   Another Gestalt principle for organizing sensory stimuli into meaningful perception is proximity . This principle asserts that things that are close to one another tend to be grouped together, as the figure below illustrates.

The Gestalt principle of proximity suggests that you see (a) one block of dots on the left side and (b) three columns on the right side.

   How we read something provides another illustration of the proximity concept. For example, we read this sentence like this, notl iket hiso rt hat. We group the letters of a given word together because there are no spaces between the letters, and we perceive words because there are spaces between each word. Here are some more examples: Cany oum akes enseo ft hiss entence? What doth es e wor dsmea n?

We might also use the principle of similarity to group things in our visual fields. According to this principle, things that are alike tend to be grouped together (figure below). For example, when watching a football game, we tend to group individuals based on the colors of their uniforms. When watching an offensive drive, we can get a sense of the two teams simply by grouping along this dimension.

When looking at this array of dots, we likely perceive alternating rows of colors. We are grouping these dots according to the principle of similarity.

   Two additional Gestalt principles are the law of continuity (or good continuation) and closure. The law of continuity suggests that we are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines (figure below). The principle of closure states that we organize our perceptions into complete objects rather than as a series of parts (figure below).

Good continuation would suggest that we are more likely to perceive this as two overlapping lines, rather than four lines meeting in the center.

Closure suggests that we will perceive a complete circle and rectangle rather than a series of segments..

   According to Gestalt theorists, pattern perception, or our ability to discriminate among different figures and shapes, occurs by following the principles described above. You probably feel fairly certain that your perception accurately matches the real world, but this is not always the case. Our perceptions are based on perceptual hypotheses: educated guesses that we make while interpreting sensory information. These hypotheses are informed by a number of factors, including our personalities, experiences, and expectations. We use these hypotheses to generate our perceptual set. For instance, research has demonstrated that those who are given verbal priming produce a biased interpretation of complex ambiguous figures (Goolkasian & Woodbury, 2010).

Template Approach

Ulrich Neisser (1967), author of one of the first cognitive psychology textbook suggested pattern recognition would be simplified, although abilities would still exist, if all the patterns we experienced were identical. According to this theory, it would be easier for us to recognize something if it matched exactly with what we had perceived before. Obviously the real environment is infinitely dynamic producing countless combinations of orientation, size. So how is it that we can still read a letter g whether it is capitalized, non-capitalized or in someone else hand writing? Neisser suggested that categorization of information is performed by way of the brain creating mental  templates , stored models of all possible categorizable patterns (Radvansky & Ashcraft, 2014). When a computer reads your debt card information it is comparing the information you enter to a template of what the number should look like (has a specific amount of numbers, no letters or symbols…). The template view perception is able to easily explain how we recognize pieces of our environment, but it is not able to explain why we are still able to recognize things when it is not viewed from the same angle, distance, or in the same context.

In order to address the shortfalls of the template model of perception, the  feature detection approach to visual perception suggests we recognize specific features of what we are looking at, for example the straight lines in an H versus the curved line of a letter C. Rather than matching an entire template-like pattern for the capital letter H, we identify the elemental features that are present in the H. Several people have suggested theories of feature-based pattern recognition, one of which was described by Selfridge (1959) and is known as the  pandemonium model suggesting that information being perceived is processed through various stages by what Selfridge described as mental demons, who shout out loud as they attempt to identify patterns in the stimuli. These pattern demons are at the lowest level of perception so after they are able to identify patterns, computational demons further analyze features to match to templates such as straight or curved lines. Finally at the highest level of discrimination, cognitive demons which allow stimuli to be categorized in terms of context and other higher order classifications, and the decisions demon decides among all the demons shouting about what the stimuli is which while be selected for interpretation.

what perceptual hypothesis

Selfridge’s pandemonium model showing the various levels of demons which make estimations and pass the information on to the next level before the decision demon makes the best estimation to what the stimuli is. Adapted from Lindsay and Norman (1972).

Although Selfridges ideas regarding layers of shouting demons that make up our ability to discriminate features of our environment, the model actually incorporates several ideas that are important for pattern recognition. First, at its foundation, this model is a feature detection model that incorporates higher levels of processing as the information is processed in time. Second, the Selfridge model of many different shouting demons incorporates ideas of parallel processing suggesting many different forms of stimuli can be analyzed and processed to some extent at the same time. Third and finally, the model suggests that perception in a very real sense is a series of problem solving procedures where we are able to take bits of information and piece it all together to create something we are able to recognize and classify as something meaningful.

In addition to sounding initially improbable by being based on a series of shouting fictional demons,  one of the main critiques of Selfridge’s demon model of feature detection is that it is primarily a  bottom-up , or  data-driven processing system. This means the feature detection and processing for discrimination all comes from what we get out of the environment. Modern progress in cognitive science has argued against strictly bottom-up processing models suggesting that context plays an extremely important role in determining what you are perceiving and discriminating between stimuli. To build off previous models, cognitive scientist suggested an additional  top-down , or  conceptually-driven account in which context and higher level knowledge such as context something tends to occur in or a persons expectations influence lower-level processes.

Finally the most modern theories that attempt to describe how information is processed for our perception and discrimination are known as  connectionist   models. Connectionist models incorporate an enormous amount of mathematical computations which work in parallel and across series of interrelated web like structures using top-down and bottom-up processes to narrow down what the most probably solution for the discrimination would be. Each unit in a connectionist layer is massively connected in a giant web with many or al the units in the next layer of discrimination. Within these models, even if there is not many features present in the stimulus, the number of computations in a single run for discrimination become incredibly large because of all the connections that exist between each unit and layer.

The Depths of Perception: Bias, Prejudice, and Cultural Factors

   In this chapter, you have learned that perception is a complex process. Built from sensations, but influenced by our own experiences, biases, prejudices, and cultures , perceptions can be very different from person to person. Research suggests that implicit racial prejudice and stereotypes affect perception. For instance, several studies have demonstrated that non-Black participants identify weapons faster and are more likely to identify non-weapons as weapons when the image of the weapon is paired with the image of a Black person (Payne, 2001; Payne, Shimizu, & Jacoby, 2005). Furthermore, White individuals’ decisions to shoot an armed target in a video game is made more quickly when the target is Black (Correll, Park, Judd, & Wittenbrink, 2002; Correll, Urland, & Ito, 2006). This research is important, considering the number of very high-profile cases in the last few decades in which young Blacks were killed by people who claimed to believe that the unarmed individuals were armed and/or represented some threat to their personal safety.

Gestalt theorists have been incredibly influential in the areas of sensation and perception. Gestalt principles such as figure-ground relationship, grouping by proximity or similarity, the law of good continuation, and closure are all used to help explain how we organize sensory information. Our perceptions are not infallible, and they can be influenced by bias, prejudice, and other factors.


Openstax Psychology text by Kathryn Dumper, William Jenkins, Arlene Lacombe, Marilyn Lovett and Marion Perlmutter licensed under CC BY v4.0. https://openstax.org/details/books/psychology

Review Questions:

1. According to the principle of ________, objects that occur close to one another tend to be grouped together.

a. similarity

b. good continuation

c. proximity

2. Our tendency to perceive things as complete objects rather than as a series of parts is known as the principle of ________.

d. similarity

3. According to the law of ________, we are more likely to perceive smoothly flowing lines rather than choppy or jagged lines.

4. The main point of focus in a visual display is known as the ________.

b. perceptual set

Critical Thinking Question:

1. The central tenet of Gestalt psychology is that the whole is different from the sum of its parts. What does this mean in the context of perception?

2. Take a look at the following figure. How might you influence whether people see a duck or a rabbit?

A drawing appears to be a duck when viewed horizontally and a rabbit when viewed vertically.

Personal Application Question:

1. Have you ever listened to a song on the radio and sung along only to find out later that you have been singing the wrong lyrics? Once you found the correct lyrics, did your perception of the song change?

figure-ground relationship

Gestalt psychology

  • good continuation

pattern perception

perceptual hypothesis

principle of closure

Key Takeaways

1. This means that perception cannot be understood completely simply by combining the parts. Rather, the relationship that exists among those parts (which would be established according to the principles described in this chapter) is important in organizing and interpreting sensory information into a perceptual set.

2. Playing on their expectations could be used to influence what they were most likely to see. For instance, telling a story about Peter Rabbit and then presenting this image would bias perception along rabbit lines.

closure:  organizing our perceptions into complete objects rather than as a series of parts

figure-ground relationship:  segmenting our visual world into figure and ground

Gestalt psychology:  field of psychology based on the idea that the whole is different from the sum of its parts

good continuation:  (also, continuity) we are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines

pattern perception:  ability to discriminate among different figures and shapes

perceptual hypothesis:  educated guess used to interpret sensory information

principle of closure:  organize perceptions into complete objects rather than as a series of parts

proximity:  things that are close to one another tend to be grouped together

similarity:  things that are alike tend to be grouped together

Review Questions

According to the principle of ________, objects that occur close to one another tend to be grouped together.

Our tendency to perceive things as complete objects rather than as a series of parts is known as the principle of ________.

According to the law of ________, we are more likely to perceive smoothly flowing lines rather than choppy or jagged lines.

The main point of focus in a visual display is known as the ________.

  • perceptual set

Critical Thinking Question

The central tenet of Gestalt psychology is that the whole is different from the sum of its parts. What does this mean in the context of perception?

Take a look at the following figure. How might you influence whether people see a duck or a rabbit?

Answer: Playing on their expectations could be used to influence what they were most likely to see. For instance, telling a story about Peter Rabbit and then presenting this image would bias perception along rabbit lines.

Personal Application Question

Have you ever listened to a song on the radio and sung along only to find out later that you have been singing the wrong lyrics? Once you found the correct lyrics, did your perception of the song change?

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Module 6: Sensation and Perception

What you’ll learn to do: define perception and give examples of gestalt principles and multimodal perception.

Poster depicting a drawing of an eye with circles of cyan, magenta, and yellow.

Seeing something is not the same thing as making sense of what you see. Why is it that our senses are so easily fooled? In this section, you will come to see how our perceptions are not infallible, and they can be influenced by bias, prejudice, and other factors. Psychologists are interested in how these false perceptions influence our thoughts and behavior.

Learning Objectives

  • Give examples of gestalt principles, including the figure-ground relationship, proximity, similarity, continuity, and closure
  • Define the basic terminology and basic principles of multimodal perception
  • Give examples of multimodal and crossmodal behavioral effects

Gestalt Principles of Perception

In the early part of the 20th century, Max Wertheimer published a paper demonstrating that individuals perceived motion in rapidly flickering static images—an insight that came to him as he used a child’s toy tachistoscope. Wertheimer, and his assistants Wolfgang Köhler and Kurt Koffka, who later became his partners, believed that perception involved more than simply combining sensory stimuli. This belief led to a new movement within the field of psychology known as Gestalt psychology . The word gestalt literally means form or pattern, but its use reflects the idea that the whole is different from the sum of its parts. In other words, the brain creates a perception that is more than simply the sum of available sensory inputs, and it does so in predictable ways. Gestalt psychologists translated these predictable ways into principles by which we organize sensory information. As a result, Gestalt psychology has been extremely influential in the area of sensation and perception (Rock & Palmer, 1990).

One Gestalt principle is the figure-ground relationship . According to this principle, we tend to segment our visual world into figure and ground. Figure is the object or person that is the focus of the visual field, while the ground is the background. As Figure 1 shows, our perception can vary tremendously, depending on what is perceived as figure and what is perceived as ground. Presumably, our ability to interpret sensory information depends on what we label as figure and what we label as ground in any particular case, although this assumption has been called into question (Peterson & Gibson, 1994; Vecera & O’Reilly, 1998).

An illustration shows two identical black face-like shapes that face towards one another, and one white vase-like shape that occupies all of the space in between them. Depending on which part of the illustration is focused on, either the black shapes or the white shape may appear to be the object of the illustration, leaving the other(s) perceived as negative space.

Figure 1. The concept of figure-ground relationship explains why this image can be perceived either as a vase or as a pair of faces.

Another Gestalt principle for organizing sensory stimuli into meaningful perception is proximity . This principle asserts that things that are close to one another tend to be grouped together, as Figure 2 illustrates.

Illustration A shows thirty-six dots in six evenly-spaced rows and columns. Illustration B shows thirty-six dots in six evenly-spaced rows but with the columns separated into three sets of two columns.

Figure 2. The Gestalt principle of proximity suggests that you see (a) one block of dots on the left side and (b) three columns on the right side.

How we read something provides another illustration of the proximity concept. For example, we read this sentence like this, notl iket hiso rt hat. We group the letters of a given word together because there are no spaces between the letters, and we perceive words because there are spaces between each word. Here are some more examples: Cany oum akes enseo ft hiss entence? What doth es e wor dsmea n?

We might also use the principle of similarity to group things in our visual fields. According to this principle, things that are alike tend to be grouped together (Figure 3). For example, when watching a football game, we tend to group individuals based on the colors of their uniforms. When watching an offensive drive, we can get a sense of the two teams simply by grouping along this dimension.

An illustration shows six rows of six dots each. The rows of dots alternate between blue and white colored dots.

Figure 3. When looking at this array of dots, we likely perceive alternating rows of colors. We are grouping these dots according to the principle of similarity.

Two additional Gestalt principles are the law of continuity (or good continuation) and closure . The law of continuity suggests that we are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines (Figure 4). The principle of closure states that we organize our perceptions into complete objects rather than as a series of parts (Figure 5).

An illustration shows two lines of diagonal dots that cross in the middle in the general shape of an “X.”

Figure 4. Good continuation would suggest that we are more likely to perceive this as two overlapping lines, rather than four lines meeting in the center.

An illustration shows fragmented lines that would form a circle if they were connected. Another illustration shows fragmented lines that would form a square if they were connected.

Figure 5. Closure suggests that we will perceive a complete circle and rectangle rather than a series of segments.

Link to Learning

Watch this podcast  showing real world illustrations of Gestalt principles.

According to Gestalt theorists, pattern perception , or our ability to discriminate among different figures and shapes, occurs by following the principles described above. You probably feel fairly certain that your perception accurately matches the real world, but this is not always the case. Our perceptions are based on perceptual hypotheses : educated guesses that we make while interpreting sensory information. These hypotheses are informed by a number of factors, including our personalities, experiences, and expectations. We use these hypotheses to generate our perceptual set. For instance, research has demonstrated that those who are given verbal priming produce a biased interpretation of complex ambiguous figures (Goolkasian & Woodbury, 2010).

Dig Deeper: The Depths of Perception: Bias, Prejudice, and Cultural Factors

In this module, you have learned that perception is a complex process. Built from sensations, but influenced by our own experiences, biases, prejudices, and cultures, perceptions can be very different from person to person. Research suggests that implicit racial prejudice and stereotypes affect perception. For instance, several studies have demonstrated that non-Black participants identify weapons faster and are more likely to identify non-weapons as weapons when the image of the weapon is paired with the image of a Black person (Payne, 2001; Payne, Shimizu, & Jacoby, 2005). Furthermore, White individuals’ decisions to shoot an armed target in a video game is made more quickly when the target is Black (Correll, Park, Judd, & Wittenbrink, 2002; Correll, Urland, & Ito, 2006). This research is important, considering the number of very high-profile cases in the last few decades in which young Blacks were killed by people who claimed to believe that the unarmed individuals were armed and/or represented some threat to their personal safety.

Think It Over

Have you ever listened to a song on the radio and sung along only to find out later that you have been singing the wrong lyrics? Once you found the correct lyrics, did your perception of the song change?

Colorful image of fears, lines, and circles that, when seen together, look like a woman's face.

Figure 6. The way we receive the information from the world is called sensation while our interpretation of that information is called perception. [Image: Laurens van Lieshou]

Multi-Modal Perception

Although it has been traditional to study the various senses independently, most of the time, perception operates in the context of information supplied by multiple sensory modalities at the same time. For example, imagine if you witnessed a car collision. You could describe the stimulus generated by this event by considering each of the senses independently; that is, as a set of  unimodal stimuli. Your eyes would be stimulated with patterns of light energy bouncing off the cars involved. Your ears would be stimulated with patterns of acoustic energy emanating from the collision. Your nose might even be stimulated by the smell of burning rubber or gasoline. However, all of this information would be relevant to the same thing: your perception of the car collision. Indeed, unless someone was to explicitly ask you to describe your perception in unimodal terms, you would most likely experience the event as a unified bundle of sensations from multiple senses. In other words, your perception would be multimodal . The question is whether the various sources of information involved in this multimodal stimulus are processed separately by the perceptual system or not.

For the last few decades, perceptual research has pointed to the importance of multimodal perception : the effects on the perception of events and objects in the world that are observed when there is information from more than one sensory modality. Most of this research indicates that, at some point in perceptual processing, information from the various sensory modalities is integrated . In other words, the information is combined and treated as a unitary representation of the world.

Behavioral Effects of Multimodal Perception

Although neuroscientists tend to study very simple interactions between neurons, the fact that they’ve found so many crossmodal areas of the cortex seems to hint that the way we experience the world is fundamentally multimodal. Our intuitions about perception are consistent with this; it does not seem as though our perception of events is constrained to the perception of each sensory modality independently. Rather, we perceive a unified world, regardless of the sensory modality through which we perceive it.

It will probably require many more years of research before neuroscientists uncover all the details of the neural machinery involved in this unified experience. In the meantime, experimental psychologists have contributed to our understanding of multimodal perception through investigations of the behavioral effects associated with it. These effects fall into two broad classes. The first class— multimodal phenomena —concerns the binding of inputs from multiple sensory modalities and the effects of this binding on perception. The second class— crossmodal phenomena —concerns the influence of one sensory modality on the perception of another (Spence, Senkowski, & Roder, 2009).

Multimodal Phenomena

Audiovisual speech.

Multimodal phenomena concern stimuli that generate simultaneous (or nearly simultaneous) information in more than one sensory modality. As discussed above, speech is a classic example of this kind of stimulus. When an individual speaks, she generates sound waves that carry meaningful information. If the perceiver is also looking at the speaker, then that perceiver also has access to  visual patterns that carry meaningful information. Of course, as anyone who has ever tried to lipread knows, there are limits on how informative visual speech information is. Even so, the visual speech pattern alone is sufficient for very robust speech perception. Most people assume that deaf individuals are much better at lipreading than individuals with normal hearing. It may come as a surprise to learn, however, that some individuals with normal hearing are also remarkably good at lipreading (sometimes called “speechreading”). In fact, there is a wide range of speechreading ability in both normal hearing and deaf populations (Andersson, Lyxell, Rönnberg, & Spens, 2001). However, the reasons for this wide range of performance are not well understood (Auer & Bernstein, 2007; Bernstein, 2006; Bernstein, Auer, & Tucker, 2001; Mohammed et al., 2005).

How does visual information about speech interact with auditory information about speech? One of the earliest investigations of this question examined the accuracy of recognizing spoken words presented in a noisy context, much like in the example above about talking at a crowded party. To study this phenomenon experimentally, some irrelevant noise (“white noise”—which sounds like a radio tuned between stations) was presented to participants. Embedded in the white noise were spoken words, and the participants’ task was to identify the words. There were two conditions: one in which only the auditory component of the words was presented (the “auditory-alone” condition), and one in both the auditory and visual components were presented (the “audiovisual” condition). The noise levels were also varied, so that on some trials, the noise was very loud relative to the loudness of the words, and on other trials, the noise was very soft relative to the words. Sumby and Pollack (1954) found that the accuracy of identifying the spoken words was much higher for the audiovisual condition than it was in the auditory-alone condition. In addition, the pattern of results was consistent with the Principle of Inverse Effectiveness: The advantage gained by audiovisual presentation was highest when the auditory-alone condition performance was lowest (i.e., when the noise was loudest). At these noise levels, the audiovisual advantage was considerable: It was estimated that allowing the participant to see the speaker was equivalent to turning the volume of the noise down by over half. Clearly, the audiovisual advantage can have dramatic effects on behavior.

Another phenomenon using audiovisual speech is a very famous illusion called the “McGurk effect” (named after one of its discoverers). In the classic formulation of the illusion, a movie is recorded of a speaker saying the syllables “gaga.” Another movie is made of the same speaker saying the syllables “baba.” Then, the auditory portion of the “baba” movie is dubbed onto the visual portion of the “gaga” movie. This combined stimulus is presented to participants, who are asked to report what the speaker in the movie said. McGurk and MacDonald (1976) reported that 98 percent of their participants reported hearing the syllable “dada”—which was in neither the visual nor the auditory components of the stimulus. These results indicate that when visual and auditory information about speech is integrated, it can have profound effects on perception.

Tactile/Visual Interactions in Body Ownership

Not all multisensory integration phenomena concern speech, however. One particularly compelling multisensory illusion involves the integration of tactile and visual information in the perception of body ownership. In the “rubber hand illusion” (Botvinick & Cohen, 1998), an observer is situated so that one of his hands is not visible. A fake rubber hand is placed near the obscured hand, but in a visible location. The experimenter then uses a light paintbrush to simultaneously stroke the obscured hand and the rubber hand in the same locations. For example, if the middle finger of the obscured hand is being brushed, then the middle finger of the rubber hand will also be brushed. This sets up a correspondence between the tactile sensations (coming from the obscured hand) and the visual sensations (of the rubber hand). After a short time (around 10 minutes), participants report feeling as though the rubber hand “belongs” to them; that is, that the rubber hand is a part of their body. This feeling can be so strong that surprising the participant by hitting the rubber hand with a hammer often leads to a reflexive withdrawing of the obscured hand—even though it is in no danger at all. It appears, then, that our awareness of our own bodies may be the result of multisensory integration.

Crossmodal Phenomena

Crossmodal phenomena are distinguished from multimodal phenomena in that they concern the influence one sensory modality has on the perception of another.

Visual Influence on Auditory Localization

Man posing with his ventriloquist doll.

Figure 7. Ventriloquists are able to trick us into believing that what we see and what we hear are the same where, in truth, they are not. [Image: Indiapuppet]

A famous (and commonly experienced) crossmodal illusion is referred to as “the ventriloquism effect.” When a ventriloquist appears to make a puppet speak, she fools the listener into thinking that the location of the origin of the speech sounds is at the puppet’s mouth. In other words, instead of localizing the auditory signal (coming from the mouth of a ventriloquist) to the correct place, our perceptual system localizes it incorrectly (to the mouth of the puppet).

Why might this happen? Consider the information available to the observer about the location of the two components of the stimulus: the sounds from the ventriloquist’s mouth and the visual movement of the puppet’s mouth. Whereas it is very obvious where the visual stimulus is coming from (because you can see it), it is much more difficult to pinpoint the location of the sounds. In other words, the very precise visual location of mouth movement apparently overrides the less well-specified location of the auditory information. More generally, it has been found that the location of a wide variety of auditory stimuli can be affected by the simultaneous presentation of a visual stimulus (Vroomen & De Gelder, 2004). In addition, the ventriloquism effect has been demonstrated for objects in motion: The motion of a visual object can influence the perceived direction of motion of a moving sound source (Soto-Faraco, Kingstone, & Spence, 2003).

Auditory Influence on Visual Perception

A related illusion demonstrates the opposite effect: where sounds have an effect on visual perception. In the double-flash illusion, a participant is asked to stare at a central point on a computer monitor. On the extreme edge of the participant’s vision, a white circle is briefly flashed one time. There is also a simultaneous auditory event: either one beep or two beeps in rapid succession. Remarkably, participants report seeing two visual flashes when the flash is accompanied by two beeps; the same stimulus is seen as a single flash in the context of a single beep or no beep (Shams, Kamitani, & Shimojo, 2000). In other words, the number of heard beeps influences the number of seen flashes!

Participate in the double-flash experiment here .

Take a look at the bouncing balls illusion  here .

Another illusion involves the perception of collisions between two circles (called “balls”) moving toward each other and continuing through each other. Such stimuli can be perceived as either two balls moving through each other or as a collision between the two balls that then bounce off each other in opposite directions. Sekuler, Sekuler, and Lau (1997) showed that the presentation of an auditory stimulus at the time of contact between the two balls strongly influenced the perception of a collision event. In this case, the perceived sound influences the interpretation of the ambiguous visual stimulus.

Crossmodal Speech

Several crossmodal phenomena have also been discovered for speech stimuli. These crossmodal speech effects usually show altered perceptual processing of unimodal stimuli (e.g., acoustic patterns) by virtue of prior experience with the alternate unimodal stimulus (e.g., optical patterns). For example, Rosenblum, Miller, and Sanchez (2007) conducted an experiment examining the ability to become familiar with a person’s voice. Their first interesting finding was unimodal: Much like what happens when someone repeatedly hears a person speak, perceivers can become familiar with the “visual voice” of a speaker. That is, they can become familiar with the person’s speaking style simply by seeing that person speak. Even more astounding was their crossmodal finding: Familiarity with this visual information also led to increased recognition of the speaker’s auditory speech, to which participants had never had exposure.

Similarly, it has been shown that when perceivers see a speaking face, they can identify the (auditory-alone) voice of that speaker, and vice versa (Kamachi, Hill, Lander, & Vatikiotis-Bateson, 2003; Lachs & Pisoni, 2004a, 2004b, 2004c; Rosenblum, Smith, Nichols, Lee, & Hale, 2006). In other words, the visual form of a speaker engaged in the act of speaking appears to contain information about what that speaker should sound like. Perhaps more surprisingly, the auditory form of speech seems to contain information about what the speaker should look like.

In the late 17th century, a scientist named William Molyneux asked the famous philosopher John Locke a question relevant to modern studies of multisensory processing. The question was this: Imagine a person who has been blind since birth, and who is able, by virtue of the sense of touch, to identify three dimensional shapes such as spheres or pyramids. Now imagine that this person suddenly receives the ability to see. Would the person, without using the sense of touch, be able to identify those same shapes visually? Can modern research in multimodal perception help answer this question? Why or why not? How do the studies about crossmodal phenomena inform us about the answer to this question?

closure:  organizing our perceptions into complete objects rather than as a series of parts crossmodal phenomena : effects that concern the influence of the perception of one sensory modality on the perception of another double flash illusion : the false perception of two visual flashes when a single flash is accompanied by two auditory beeps figure-ground relationship:  segmenting our visual world into figure and ground Gestalt psychology:  field of psychology based on the idea that the whole is different from the sum of its parts< good continuation:  (also, continuity) we are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines integrated : the process by which the perceptual system combines information arising from more than one modality McGurk effect : an effect in which conflicting visual and auditory components of a speech stimulus result in an illusory percept multimodal : of or pertaining to multiple sensory modalities multimodal perception : the effects that concurrent stimulation in more than one sensory modality has on the perception of events and objects in the world multimodal phenomena : effects that concern the binding of inputs from multiple sensory modalities pattern perception:  ability to discriminate among different figures and shapes perceptual hypothesis:  educated guess used to interpret sensory information principle of closure:  organize perceptions into complete objects rather than as a series of parts proximity:  things that are close to one another tend to be grouped together rubber hand illusion : the false perception of a fake hand as belonging to a perceiver, due to multimodal sensory information sensory modalities : a type of sense; for example, vision or audition similarity:  things that are alike tend to be grouped together unimodal : of or pertaining to a single sensory modality

  • Modification, adaptation, and original content. Provided by : Lumen Learning. License : CC BY: Attribution
  • Modification of content on multi-modal perception. Provided by : Lumen Learning. License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike
  • Perception Poster Show Concept. Authored by : Jon Ashcroft. Located at : https://www.flickr.com/photos/theilluminated/5704563713 . License : CC BY: Attribution
  • Gestalt Principles of Psychology. Authored by : OpenStax College. Located at : http://cnx.org/contents/[email protected]:cOcxAR_r@5/Gestalt-Principles-of-Percepti#Figure_05_06_Continuity . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/content/col11629/latest/.
  • Multi-Modal Perception. Authored by : Lorin Lachs. Provided by : California State University, Fresno. Located at : http://nobaproject.com/modules/multi-modal-perception . Project : The Noba Project. License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike
  • Perceiving is Believing - Crash Course Psychology #7. Provided by : CrashCourse. Located at : https://www.youtube.com/watch?v=n46umYA_4dM . License : Other . License Terms : Standard YouTube License
  • The McGurk Effect. Provided by : BBC. Located at : https://youtu.be/G-lN8vWm3m0?t=32s . License : Other . License Terms : Standard YouTube License
  • The Rubber Hand Illusion. Provided by : BBC. Located at : https://youtu.be/sxwn1w7MJvk . License : Other . License Terms : Standard YouTube License

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Perception: A Very Short Introduction

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2 (page 7) p. 7 Perceptual theories—direct, indirect, and computational

  • Published: October 2017
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‘Perceptual theories—direct, indirect, and computational’ considers three different conceptions of what it means to perceive and the processes involved in each theory. The origins of indirect or constructivist theory can be traced back to Hermann von Helmholtz in the 19th century, who emphasized the importance of experience in shaping our perceptual abilities. It was assumed that the primary purpose of perception was to create subjective experiences. The American psychologist James Gibson first suggested a direct theory—that the primary role of perceptual processes was to guide action. Since the 1960s, there have been many attempts to model the perceptual processes using computer algorithms, with David Marr at MIT being the most influential figure.

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Action-based Theories of Perception

Action is a means of acquiring perceptual information about the environment. Turning around, for example, alters your spatial relations to surrounding objects and, hence, which of their properties you visually perceive. Moving your hand over an object’s surface enables you to feel its shape, temperature, and texture. Sniffing and walking around a room enables you to track down the source of an unpleasant smell. Active or passive movements of the body can also generate useful sources of perceptual information (Gibson 1966, 1979). The pattern of optic flow in the retinal image produced by forward locomotion, for example, contains information about the direction in which you are heading, while motion parallax is a “cue” used by the visual system to estimate the relative distances of objects in your field of view. In these uncontroversial ways and others, perception is instrumentally dependent on action. According to an explanatory framework that Susan Hurley (1998) dubs the “Input-Output Picture”, the dependence of perception on action is purely instrumental:

Movement can alter sensory inputs and so result in different perceptions… changes in output are merely a means to changes in input, on which perception depends directly. (1998: 342)

The different action-based theories of perception, reviewed in this entry, challenge the Input-Output Picture. They maintain that perception can also depend in a noninstrumental or constitutive way on action (or, more generally, on capacities for object-directed motor control). This position has taken many different forms in the history of philosophy and psychology (for overviews, see Mandik 2005; Chemero 2011; Creem‐Regehr & Kunz 2010; Nanay 2013; Drayson 2017; Springle & Machamer 2016; Grush & Springle 2019; and Ferretti 2021). Most action-based theories of perception in the last 300 years, however, have looked to action in order to explain how vision, in particular, acquires either all or some of its spatial representational content . Accordingly, these are the theories on which we shall focus here.

This entry is historically structured. We begin in Section 1 by discussing George Berkeley’s Towards a New Theory of Vision (1709), the historical locus classicus of action-based theories of perception, and one of the most influential texts on vision ever written. Berkeley argues that the basic or “proper” deliverance of vision is not an arrangement of voluminous objects in three-dimensional space, but rather a two-dimensional manifold of light and color. We then turn to a discussion of Lotze, Helmholtz, and the local sign doctrine. The “local signs” were felt cues for the mind to know what sort of spatial content to imbue visual experience with. For Lotze, these cues were “inflowing” kinaesthetic feelings that result from actually moving the eyes, while, for Helmholtz, they were “outflowing” motor commands sent to move the eyes.

In Section 2, we discuss sensorimotor contingency theories, which became prominent in the latter half of the 20 th century. These views maintain that an ability to predict the sensory consequences of self-initiated actions is necessary for perception. Among the motivations for this family of theories is the problem of visual direction constancy —why do objects appear to be stationary even though the locations on the retina to which they reflect light change with every eye movement?—as well as experiments on adaptation to optical rearrangement devices (ORDs) and sensory substitution.

Section 3 examines two other important 20 th century theories. According to what we shall call the motor component theory , efference copies generated in the oculomotor system and/or proprioceptive feedback from eye-movements are used together with incoming sensory inputs to determine the spatial attributes of perceived objects. Efferent readiness theories , by contrast, look to the particular ways in which perceptual states prepare the observer to move and act in relation to the environment. The modest readiness theory , as we shall call it, claims that the way an object’s spatial attributes are represented in visual experience can be modulated by one or another form of covert action planning. The bold readiness theory argues for the stronger claim that perception just is covert readiness for action.

In Section 4, we move to the disposition theory , most influentially articulated by Gareth Evans (1982, 1985), but more recently defended by Rick Grush (2000, 2007). Evans’ theory is, at its core, very similar to the bold efferent readiness theory. There are some notable differences, though. Evans’ account is more finely articulated in some philosophical respects. It also does not posit a reduction of perception to behavioral dispositions, but rather posits that certain complicated relations between perceptual input and behavioral provide spatial content. Grush proposes a very specific theory that is like Evans’ in that it does not posit a reduction, but unlike Evans’ view, does not put behavioral dispositions and sensory input on an undifferentiated footing.

1.1 Movement and Touch in the New Theory Of Vision

1.2 objections to berkeley’s theory, 1.3 lotze, helmholtz, and the local sign doctrine, 2.1.1 objections to the efference copy theory, 2.1.2 alternatives to the efference copy theory, 2.2.1 held’s experiments on prism adaptation, 2.2.2 challenges to the reafference theory, 2.3.1 evidence for the enactive approach, 2.3.2 challenges to the enactive approach, 3.1 the motor component theory (embodied visual perception), 3.2.1 the modest readiness theory, 3.2.2 the bold readiness theory, 4. skill/disposition theories, other internet resources, related entries, 1. early action-based theories.

Two doctrines dominate philosophical and psychological discussions of the relationship between action and space perception from the 18 th to the early 20 th century. The first is that the immediate objects of sight are two-dimensional manifolds of light and color, lacking perceptible extension in depth. The second is that vision must be “educated” by the sense of touch—understood as including both kinaesthesis and proprioceptive position sense—if the former is to acquire its apparent outward, three-dimensional spatial significance. The relevant learning process is associationist: normal vision results when tangible ideas of distance (derived from experiences of unimpeded movement) and solid shape (derived from experiences of contact and differential resistance) are elicited by the visible ideas of light and color with which they have been habitually associated. The widespread acceptance of both doctrines owes much to the influence of George Berkeley’s New Theory of Vision (1709).

The Berkeleyan approach looks to action in order to explain how depth is “added” to a phenomenally two-dimensional visual field. The spatial ordering of the visual field itself, however, is taken to be immediately given in experience (Hatfield & Epstein 1979; Falkenstein 1994; but see Grush 2007). Starting in the 19 th century, a number of theorists, including Johann Steinbuch (1770–1818), Hermann Lotze (1817–1881), Hermann von Helmholtz (1821–1894), Wilhelm Wundt (1832–1920), and Ernst Mach (1838–1916), argued that all abilities for visual spatial localization, including representation of up/down and left/right direction within the two-dimensional visual field, depend on motor factors, in particular, gaze-directing movements of the eye (Hatfield 1990: chaps. 4–5). This idea is the basis of the “local sign” doctrine, which we examine in Section 2.3 .

There are three principal respects in which motor action is central to Berkeley’s project in the New Theory of Vision (1709). First, Berkeley argues that visual experiences convey information about three-dimensional space only to the extent that they enable perceivers to anticipate the tactile consequences of actions directed at surrounding objects. In §45 of the New Theory, Berkeley writes:

…I say, neither distance, nor things placed at a distance are themselves, or their ideas, truly perceived by sight…. whoever will look narrowly into his own thoughts, and examine what he means by saying, he sees this or that thing at a distance, will agree with me, that what he sees only suggests to his understanding, that after having passed a certain distance, to be measured by the motion of his body, which is perceivable by touch, he shall come to perceive such and such tangible ideas which have been usually connected with such and such visible ideas.

And later in the Treatise Concerning the Principles of Human Knowledge (1734: §44):

…in strict truth the ideas of sight, when we apprehend by them distance and things placed at a distance, do not suggest or mark out to us things actually existing at a distance, but only admonish us what ideas of touch will be imprinted in our minds at such and such distances of time, and in consequence of such or such actions. …[V]isible ideas are the language whereby the governing spirit … informs us what tangible ideas he is about to imprint upon us, in case we excite this or that motion in our own bodies.

The view Berkeley defends in these passages has recognizable antecedents in Locke’s Essay Concerning Human Understanding (1690: Book II, Chap. 9, §§8–10). There Locke maintained that the immediate objects of sight are “flat” or lack outward depth; that sight must be coordinated with touch in order to mediate judgments concerning the disposition of objects in three-dimensional space; and that visible ideas “excite” in the mind movement-based ideas of distance through an associative process akin to that whereby words suggest their meanings: the process is “performed so constantly, and so quick, that we take that for the perception of our sensation, which is an idea formed by our judgment.”

A long line of philosophers—including Condillac (1754), Reid (1785), Smith (1811), Mill (1842, 1843), Bain (1855, 1868), and Dewey (1891)—accepted this view of the relation between sight and touch.

The second respect in which action plays a prominent role in the New Theory is teleological. Sight not only derives its three-dimensional spatial significance from bodily movement, its purpose is to help us engage in such movement adaptively :

…the proper objects of vision constitute an universal language of the Author of nature, whereby we are instructed how to regulate our actions, in order to attain those things, that are necessary to the preservation and well-being of our bodies, as also to avoid whatever may be hurtful and destructive of them. It is by their information that we are principally guided in all the transactions and concerns of life. (1709: §147)

Although Berkeley does not explain how vision instructs us in regulating our actions, the answer is reasonably clear from the preceding account of depth perception: seeing an object or scene can elicit tangible ideas that directly motivate self-preserving action. The tactual ideas associated with a rapidly looming ball in the visual field, for example, can directly motivate the subject to shift position defensively or to catch it before being struck. (For recent versions and critical assessments of the view that perception of the environment’s spatial layout is systematically sensitive to the subject’s abilities and goals for action, see Proffitt 2006, 2008; Bennett 2011; Firestone 2013; and Siegel 2014).

The third respect in which action is central to the New Theory is psychological. Tangible ideas of distance are elicited not only by (1) visual or “pictorial” depth cues such as object’s degree of blurriness (objects appear increasingly “confused” as they approach the observer), but also by kinaesthetic, muscular sensations resulting from (2) changes in the vergence angle of the eyes (1709: §16) and (3) accommodation of the lens (1709: §27). Like many contemporary theories of spatial vision, the Berkeleyan account thus acknowledges an important role for oculomotor factors in our perception of distance.

Critics of Berkeley’s theory in the 18 th and 19 th centuries (for reviews, see Bain 1868; Smith 2000; Atherton 2005) principally targeted three claims:

Most philosophers and perceptual psychologists now concur with Armstrong’s (1960) assessment that the “single point” argument for claim (a)—“distance being a line directed end-wise to the eye, it projects only one point in the fund of the eye, which point remains invariably the same, whether the distance be longer or shorter” (Berkeley 1709: §2)—conflates spatial properties of the retinal image with those of the objects of sight (also see Condillac 1746/2001: 102; Abbott 1864: chap. 1). In contrast with claim (a), we should note, both contemporary “ecological” and information-processing approaches in vision science assume that the spatial representational contents of visual experience are robustly three-dimensional: vision is no less a distance sense than touch.

Three sorts of objections targeted on claim (b) were prominent. First, it is not evident to introspection that visual experiences reliably elicit tactile and kinaesthetic images as Berkeley suggests. As Bain succinctly formulates this objection:

In perceiving distance, we are not conscious of tactual feelings or locomotive reminiscences; what we see is a visible quality, and nothing more. (1868: 194)

Second, sight is often the refractory party when conflicts with touch arise. Consider the experience of seeing a three-dimensional scene in a painting: “I know, without any doubt”, writes Condillac,

that it is painted on a flat surface; I have touched it, and yet this knowledge, repeated experience, and all the judgments I can make do not prevent me from seeing convex figures. Why does this appearance persist? (1746/2001: I, §6, 3)

Last, vision in many animals does not need tutoring by touch before it is able to guide spatially directed movement and action. Cases in which non-human neonates respond adaptively to the distal sources of visual stimulation

imply that external objects are seen to be so…. They prove, at least, the possibility that the opening of the eye may be at once followed by the perception of external objects as such, or, in other words, by the perception or sensation of outness. (Bailey 1842: 30; for replies, see Smith 1811: 385–390)

Here it would be in principle possible for a proponent of Berkeley’s position to maintain that, at least for such animals, the connection between visual ideas and ideas of touch is innate and not learned (see Stewart 1829: 241–243; Mill 1842: 106–110). While this would abandon Berkeley’s empiricism and associationism, it would maintain the claim that vision provides depth information only because its ideas are connected to tangible ideas.

Regarding claim (c), many critics denied that the supposed “habitual connexion” between vision and touch actually obtains. Suppose that the novice perceiver sees a remote tree at time 1 and walks in its direction until she makes contact with it at time 2 . The problem is that the perceiver’s initial visual experience of the tree at time 1 is not temporally contiguous with the locomotion-based experience of the tree’s distance completed at time 2 . Indeed, at time 2 the former experience no longer exists. “The association required”, Abbott thus writes,

cannot take place, for the simple reason that the ideas to be associated cannot co-exist. We cannot at one and the same moment be looking at an object five, ten, fifty yards off, and be achieving our last step towards it. (1864: 24)

Finally, findings from perceptual psychology have more recently been leveled against the view that vision is educated by touch. Numerous studies of how subjects respond to lens-, mirror-, and prism-induced distortions of visual experience (Gibson 1933; Harris 1965, 1980; Hay et al. 1965; Rock & Harris 1967) indicate that not only is sight resistant to correction from touch, it will often dominate or “capture” the latter when intermodal conflicts arise. This point will be discussed in greater depth in Section 3 below.

Like Berkeley, Hermann Lotze (1817–1881) and Hermann von Helmholtz (1821–1894) affirm the role played by active movement and touch in the genesis of three-dimensional visuospatial awareness:

…there can be no possible sense in speaking of any other truth of our perceptions other than practical truth. Our perceptions of things cannot be anything other than symbols, naturally given signs for things, which we have learned to use in order to control our motions and actions. When we have learned to read those signs in the proper manner, we are in a condition to use them to orient our actions such that they achieve their intended effect; that is to say, that new sensations arise in an expected manner (Helmholtz 2005 [1924]: 19, our emphasis).

Lotze and Helmholtz go further than Berkeley in maintaining that bodily movement also plays a role in the construction of the two-dimensional visual field, taken for granted by most previous accounts of vision (but for exceptions, see Hatfield 1990: ch. 4).

The problem of two-dimensional spatial localization, as Lotze and Helmholtz understand it, is the problem of assigning a unique, eye-relative (or “oculocentric”) direction to every point in the visual field. Lotze’s commitment to mind-body dualism precluded looking to any physical or anatomical spatial ordering in the visual system for a solution to this problem (Lotze 1887 [1879]: §§276–77). Rather, Lotze maintains that every discrete visual impression is attended by a “special extra sensation” whose phenomenal character varies as a function of its origin on the retina. Collectively, these extra sensations or “local signs” constitute a “system of graduated, qualitative tokens” (1887 [1879]: §283) that bridge the gap between the spatial structure of the nonconscious retinal image and the spatial structure represented in conscious visual awareness.

What sort of sensation, however, is suited to play the individuating role attributed to a local sign? Lotze appeals to kinaesthetic sensations that accompany gaze-directing movements of the eyes (1887 [1879]: §§284–86). If P is the location on the retina stimulated by a distal point d and F is the fovea, then PF is the arc that must be traversed in order to align the direction of gaze with d . As the eye moves through arc PF , its changing position gives rise to a corresponding series of kinaesthetic sensations p 0 , p 1 , p 2 , … p n , and it is this consciously experienced series, unique to P , that constitutes P’s local sign. By contrast, if Q were rather the location on the retina stimulated by d , then the eye’s foveating movement through arc QF would elicit a different series of kinaesthetic sensations k 0 , k 1 , k 2 , … k n unique to Q .

Importantly, Lotze allows that retinal stimulation need not trigger an overt movement of the eye. Rather, even in the absence of the corresponding saccade, stimulating point P will elicit kinaesthetic sensation p 0 , and this sensation will, in turn, recall from memory the rest of the series with which it is associated p 1 , … p n .

Accordingly, though there is no movement of the eye, there arises the recollection of something, greater or smaller, that must be accomplished if the stimuli at P and Q , which arouse only a weak sensation, are to arouse sensations of the highest degree of strength and clearness. (1887 [1879]: §285)

In this way, Lotze accounts for our ability to perceive multiple locations in the visual field at the same time.

Helmholtz 2005 [1924] fully accepts the need for local signs in two-dimensional spatial localization, but makes an important modification to Lotze’s theory. In particular, he maintains that local signs are not feelings that originate in the adjustment of the ocular musculature, i.e., a form of afferent, sensory “inflow” from the eyes, but rather feelings of innervation ( Innervationsgefühlen ) produced by the effort of the will ( Willensanstrengung ) to move the eyes, i.e., a form of efferent, motor “outflow”. In general, to each perceptible location in the visual field there is an associated readiness or impulse of the will ( Willensimpuls ) to move eyes in the manner required in order to fixate it. As Ernst Mach later formulates Helmholtz’s view: “The will to perform movements of the eyes, or the innervation to the act, is itself the space sensation” (Mach 1897 [1886]: 59).

Helmholtz favored a motor outflow version of the local sign doctrine for two main reasons. First, he was skeptical that afferent registrations of eye position are precise enough to play the role assigned to them by Lotze’s theory (2005 [1924]: 47–49). Recent research has shown that proprioceptive inflow from ocular muscular stretch receptors does in fact play a quantifiable role in estimating direction of gaze, but efferent outflow is normally the more heavily weighted source of information (Bridgeman 2010; see Section 2.1.1 below).

Second, attempting a saccade when the eyes are paralyzed or otherwise immobilized results in an apparent shift of the visual scene in the same direction (Helmholtz 2005 [1924]: 205–06; Mach 1897 [1886]: 59–60). This finding would make sense if efferent signals to the eye are used to determine the direction of gaze: the visual system “infers” that perceived objects are moving because they would have to be in order for retinal stimulation to remain constant despite the change in eye direction predicted on the basis of motor outflow.

Although Helmholtz was primarily concerned to show that “our judgments as to the direction of the visual axis are simply the result of the effort of will involved in trying to alter the adjustment of the eyes” (2005 [1924]: 205–06), the evidence he adduces also implies that efferent signals play a critical role in our perception of stability in the world across saccadic eye movements. In the next section, we trace the influence of this idea on theories in the 20 th century.

2. Sensorimotor Contingency Theories

Action-based accounts of perception proliferate diversely in 20 th century. In this section, we focus on the reafference theory of Richard Held and the more recent enactive approach of J. Kevin O’Regan, Alva Noë, and others. Central to both accounts is the view that perception and perceptually guided action depend on abilities to anticipate the sensory effects of bodily movements. To be a perceiver it is necessary to have knowledge of what O’Regan and Noë call the laws of sensorimotor contingency —“the structure of the rules governing the sensory changes produced by various motor actions” (O’Regan & Noë 2001: 941).

We start with two sources of motivation for theories that make knowledge of sensorimotor contingencies necessary and/or sufficient for spatially contentful perceptual experience. The first is the idea that the visual system exploits efference copy , i.e., a copy of the outflowing saccade command signal, in order to distinguish changes in visual stimulation caused by movement of the eye from those caused by object movement. The second is a long line of experiments, first performed by Stratton and Helmholtz in the 19 th century, on how subjects adapt to lens-, mirror-, and prism-induced modifications of visual experience. We follow up with objections to these theories and alternatives.

2.1 Efference and Visual Direction Constancy

The problem of visual direction constancy (VDC) is the problem of how we perceive a stable world despite variations in visual stimulation caused by saccadic eye movements. When we execute a saccade, the image of the world projected on the retina rapidly displaces in the direction of rotation, yet the directions of perceived objects appear constant. Such perceptual stability is crucial for ordinary visuomotor interaction with surrounding the environment. As Bruce Bridgeman writes,

Perceiving a stable visual world establishes the platform on which all other visual function rests, making possible judgments about the positions and motions of the self and of other objects. (2010: 94)

The problem of VDC divides into two questions (MacKay 1973): First, which sources of information are used to determine whether the observer-relative position of an object has changed between fixations? Second, how are relevant sources of information used by the visual system to achieve this function?

The historically most influential answer to the first question is that the visual system has access to a copy of the efferent or “outflowing” saccade command signal. These signals carry information specifying the direction and magnitude of eye movements that can be used to compensate for or “cancel out” corresponding displacements of the retinal image.

In the 19 th century, Bell (1823), Purkyně (1825), and Hering (1861 [1990]), Helmholtz (2005 [1924]), and Mach (1897 [1886]) deployed the efference copy theory to illuminate a variety of experimental findings, e.g., the tendency in subjects with partially paralyzed eye muscles to perceive movement of the visual scene when attempting to execute a saccade (for a review, see Bridgeman 2010.) The theory’s most influential formulation, however, came from Erich von Holst and Horst Mittelstädt in the early 1950s. According to what they dubbed the “reafference principle” (von Holst & Mittelstädt 1950; von Holst 1954), the visual system exploits a copy of motor directives to the eye in order to distinguish between exafferent visual stimulation, caused by changes in the world, and reafferent visual stimulation, caused by changes in the direction of gaze:

Let us imagine an active CNS sending out orders, or “commands” … to the effectors and receiving signals from its sensory organs. Signals that predictably come when nothing occurs in the environment are necessarily a result of its own activity, i.e., are reafferences . All signals that come when no commands are given are exafferences and signify changes in the environment or in the state of the organism caused by external forces. … The difference between that which is to be expected as the result of a command and the totality of what is reported by the sensory organs is the proportion of exafference…. It is only this difference to which there are compensatory reflexes; only this difference determines, for example during a moving glance at movable objects, the actually perceived direction of visual objects. This, then, is the solution that we propose, which we have termed the “reafference principle”: distinction of reafference and exafference by a comparison of the total afference with the system’s state—the “command”. (Mittelstädt 1971; translated by Bridgeman et al. 1994: 251).

It is only when the displacement of the retinal image differs from the displacement predicted on the basis of the efference copy, i.e., when the latter fails to “cancel out” the former, that subjects experience a change of some sort in the perceived scene (see Figure 1 ). The relevant upshot is that VDC has an essential motoric component: the apparent stability of an object’s eye-relative position in the world depends on the perceiver’s ability to integrate incoming retinal signals with extraretinal information concerning the magnitude and direction of impending eye movements.

[Three parts to the image, the first, labeled 'a.', has at the top an apple and at the bottom an eyeball looking straight up with an arrow going from the bottom of the eyeball to the apple; the arrow is labeled EC=0 and the eyeball, A=0. The second, labeled 'b.', is like the first except the eyeball is looking slightly clockwise of straight up and the arrow follows the line of sight; a second arrow goes from the apple to the bottom of the eyeball; the eyeball is labeled A=-10; the first arrow, EC=+10 (there two equations line up horizontally with A=0 and EC=0 respectively from the first part). The third part is not labeled and consists of a circle divided into quarters with a '+' in the top quarter and a '-' in the bottom quarter. The equation 'EC=+10' in part b has an arrow going from it to the '+' quadrant of the circle. The equation 'A=-10' from part b has an arrow going from it to the '-' quadrant of the circle. From the right side of the circle is an arrow that points to an equation 'EA=EC+A=0'; the arrow is labeled 'Comparator'. ]

Figure 1: (a) When the eye is stationary, both efference copy (EC) and afference produced by displacement of the retinal image (A) are absent. (b) Turning the eye 10° to the right results in a corresponding shift of the retinal image. Since the magnitude of the eye movement specified by EC and the magnitude of retinal image displacement cancel out, no movement in the world or “exafference” (EA) is registered.

The foregoing solution to the problem of VDC faces challenges on multiple, empirical fronts. First , there is evidence that proprioceptive signals from the extraocular muscles make a non-trivial contribution to estimates of eye position, although the gain of efference copy is approximately 2.4 times greater (Bridgeman & Stark 1991). Second , in the autokinetic effect , a fixed luminous dot appears to wander when the field of view is dark and thus completely unstructured. This finding is inconsistent with theories according to which retinotopic location and efference copy are the sole determinants of eye-relative direction. Third , the hypothesized compensation process, if psychologically real, would be highly inaccurate since subjects fail to notice displacements of the visual world up to 30% of total saccade magnitude (Bridgeman et al. 1975), and the locations of flashed stimuli are systematically misperceived when presented near the time of a saccade (Deubel 2004). Last , when image displacements concurrent with a saccade are large, but just below threshold for detection, visually attended objects appear to “jump” or “jiggle” against a stable background (Brune and Lücking 1969; Bridgeman 1981). Efference copy theories, however, as Bridgeman observes,

do not allow the possibility that parts of the image can move relative to one another—the visual world is conceived as a monolithic object. The observation would seem to eliminate all efference copy and related theories in a single stroke. (2010: 102)

The reference object theory of Deubel and Bridgeman denies that efference copy is used to “cancel out” displacements of the retinal image caused by saccadic eye-movements (Deubel et al. 2002; Deubel 2004; Bridgeman 2010). According to this theory, visual attention shifts to the saccade target and a small number of other objects in its vicinity (perhaps four or fewer) before eye movement is initiated. Although little visual scene information is preserved from one fixation to the next, the features of these objects as well as precise information about their presaccadic, eye-relative locations is preserved. After the eye has landed, the visual system searches for the target or one of its neighbors within a limited spatial region around the landing site. If the postsaccadic localization of this “landmark” object succeeds, the world appears to be stable. If this object is not found, however, displacement is perceived. On this approach, efference copy does not directly support VDC. Rather, the role of efference copy is to maintain an estimate of the direction of gaze, which can be integrated with incoming retinal stimulation to determine the static, observer-relative locations of perceived objects. For a recent, philosophically oriented discussion, see Wu 2014.

A related alternative to the von Holst-Mittelstädt model is the spatial remapping theory of Duhamel and Colby (Duhamel et al. 1992; Colby et al. 1995). The role of saccade efference copy on this theory is to initiate an updating of the eye-relative locations of a small number of attended or otherwise salient objects. When post-saccadic object locations are sufficiently congruent with the updated map, stability is perceived. Single-cell and fMRI studies show that neurons at various stages in the visual-processing hierarchy exploit a copy of the saccade command signal in order to shift their receptive field locations in the direction of an impending eye movement microseconds before its initiation (Merriam & Colby 2005; Merriam et al. 2007). Efference copy indicating an impending saccade 20° to the right, in effect, tells relevant neurons:

If you are now firing in response to an item x in your receptive field, then stop firing at x . If there is currently an item y in the region of oculocentric visual space that would be coincident with your receptive field after a saccade 20° to the right, then start firing at y .

Such putative updating responses are strongest in parietal cortex and at higher levels in visual processing (V3A and hV4) and weakest at lower levels (V1 and V2).

2.2 The Reafference Theory

In 1961, Richard Held proposed that the reafference principle could be used to construct a general “neural model” of perception and perceptually guided action (for recent applications, see Jékely et al. 2021). Held’s reafference theory goes beyond the account of von Holst and Mittelstädt in three main ways. First , information about movement parameters specified by efference copy is not simply summated with reafferent stimulation. Rather, subjects are assumed to acquire knowledge of the specific sensory consequences of different bodily movements. This knowledge is contained in a hypothesized “correlational storage” area and used to determine whether or not the reafferent stimulations that result from a given type of action match those that resulted in the past (Held 1961: 30). Second , the reafference theory is not limited to eye movements, but extends to “any motor system that can be a source of reafferent visual stimulation”. Third , knowledge of the way reafferent stimulation depends on self-produced movement is used for purposes of sensorimotor control: planning and controlling object-directed actions in the present depends on access to information concerning the visual consequences of performing such actions in the past.

The reafference theory was also significantly motivated by studies of how subjects adapt to devices that alter the relationship between the distal visual world and sensory input by rotating, reversing, or laterally displacing the retinal image (for helpful guides to the literature on this topic, see Rock 1966; Howard & Templeton 1966; Epstein 1967; and Welch 1978). We will refer to these as optical rearrangement devices (or ORDs for short). In what follows, we review experimental work on ORDs starting with the American psychologist George Stratton in the late 19th century.

Stratton conducted two dramatic experiments using a lens system that effected an 180º rotation of the retinal image in his right eye (his left eye was kept covered). The first experiment involved wearing the device for 21.5 hours over the course of three days (1896); the second experiment involved wearing the device for 81.5 hours over the course of 8 days (1897a,b). In both cases, Stratton kept a detailed diary of how his visual, imaginative, and proprioceptive experiences underwent modification as a consequence of inverted vision. In 1899, he performed a lesser-known but equally dramatic three-day experiment, using a pair of mirrors that presented his eyes with a view of his own body from a position in space directly above his head ( Figure 2 ).

[a line drawing of a man standing and looking up at about a 45 degree angle. Above him is a horizontal line labeled at the left end 'A' and right end 'B'. A second line goes from the 'B' end at about a -60 degree angle to point approximately horizontal to the man's neck that point is labeled 'D'. To the right of D is the dotted line drawing of a horizontal man, head closest to D; feet labeled 'E'. Fromt the first man's eyes is a short line, labeled 'C', going up at about a 45 degree angle approximately in the direction of the point labeled 'B'.]

Figure 2: The apparatus designed by Stratton (1899). Stratton saw a view of his own body from the perspective of mirror AB, worn above his head.

In both experiments, Stratton reported a brief period of initial visual confusion and breakdown in visuomotor skill:

Almost all movements performed under the direct guidance of sight were laborious and embarrassed. Inappropriate movements were constantly made; for instance, in order to move my hand from a place in the visual field to some other place which I had selected, the muscular contraction which would have accomplished this if the normal visual arrangement had existed, now carried my hand to an entirely different place. (1897a: 344)

Further bewilderment was caused by a “swinging” of the visual field with head movements as well as jarring discord between where things were respectively seen and imagined to be:

Objects lying at the moment outside the visual field (things at the side of the observer, for example) were at first mentally represented as they would have appeared in normal vision…. The actual present perception remained in this way entirely isolated and out of harmony with the larger whole made up by [imaginative] representation. (1896: 615)

After a seemingly short period of adjustment, Stratton reported a gradual re-establishment of harmony between the deliverances of sight and touch. By the end of his experiments on inverted vision, it was not only possible for Stratton to perform many visuomotor actions fluently and without error, the visual world often appeared to him to be “right side up” (1897a: 358) and “in normal position” (1896: 616). Just what this might mean will be discussed below in Section 2.2 .

Another influential, if less dramatic, experiment was performed by Helmholtz (2005 [1924]: §29), who practiced reaching to targets while wearing prisms that displaced the retinal image 16–18° to the left. The initial tendency was to reach too far in the direction of lateral displacement. After a number of trials, however, reaching gradually regained its former level of accuracy. Helmholtz made two additional discoveries. First, there was an intermanual transfer effect : visuomotor adaptation to prisms extended to his non-exposed hand. Second, immediately after removing the prisms from his eyes, errors were made in the opposite direction, i.e., when reaching for a target, Helmholtz now moved his hand too far to the right. This negative after-effect is now standardly used as a measure of adaptation to lateral displacement.

Stratton and Helmholtz’s findings catalyzed a research tradition on ORD adaptation that experienced its heyday in the 1960s and 1970s. Two questions dominated studies conducted during this period. First, what are the necessary and sufficient conditions for adaptation to occur? In particular, which sources of information do subjects use when adapting to the various perceptual and sensorimotor discrepancies caused by ORDs? Second, just what happens when subjects adapt to perceptual rearrangement? What is the “end product” of the relevant form of perceptual learning?

Held’s answer to the first question is that subjects must receive visual feedback from active movement, i.e., reafferent visual stimulation , in order for significant and stable adaptation to occur (Held & Hein 1958; Held 1961; Held & Bossom 1961). Evidence for this conclusion came from experiments in which participants wore laterally displacing prisms during both active and passive movement conditions. In the active movement condition, the subject moved her visible hand back and forth along a fixed arc in synchrony with a metronome. In the passive movement condition, the subject’s hand was passively moved at the same rate by the experimenters. Although the overall pattern of visual stimulation was identical in both conditions, adaptation was reported only when subjects engaged in self-movement. Reafferent stimulation, Held and Bossom concluded on the basis of this and other studies,

is the source of ordered contact with the environment which is responsible for both the stability, under typical conditions, and the adaptability, to certain atypical conditions, of visual-spatial performance. (1961: 37)

Held’s answer to the second question is couched in terms of the reafference theory: subjects adapt to ORDs only when they have relearned the sensory consequences of their bodily movements. In the case of adaptation to lateral displacement, they must relearn the way retinal stimulations vary as a function of reaching for targets at different body-relative locations. This relearning is assumed to involve an updating of the mappings from motor output to reafferent sensory feedback in the hypothesized “correlational storage” module mentioned above.

The reafference theory faces a number of objections. First , the theory is an extension of von Holst and Mittelstädt’s reafference principle, according to which efference copy is used to cancel out shifts of the retinal image caused by saccadic eye movements. The latter was specifically intended to explain why we do not experience object displacement in the world whenever we change the direction of gaze. There is nothing, at first blush, however, that is analogous to the putative need for “cancellation” or “discounting” of the retinal image in the case of prism adaptation. As Welch puts it, “There is no visual position constancy here, so why should a model originally devised to explain this constancy be appropriate?” (1978: 16).

Second , the reafference theory fails to explain just how stored efference-reafference correlations are supposed to explain visuomotor control. How does having the ability to anticipate the retinal stimulations that would caused by a certain type of hand movement enable one actually to perform the movement in question? Without elaboration, all that Held’s theory seems to explain is why subjects are surprised when reafferences generated by their movements are non-standard (Rock 1966: 117).

Third , adaptation to ORDs, contrary to the theory, is not restricted to situations in which subjects receive reafferent visual feedback, but may also take place when subjects receive feedback generated by passive effector or whole-body movement (Singer & Day 1966; Templeton et al. 1966; Fishkin 1969). Adaptation is even possible in the complete absence of motor action (Howard et al. 1965; Kravitz & Wallach 1966).

In general, the extent to which adaptation occurs depends not on the availability of reafferent stimulation, but rather on the presence of either of two related kinds of information concerning “the presence and nature of the optical rearrangement” (Welch 1978: 24). Following Welch, we shall refer to this view as the “information hypothesis”.

One source of information present in a displaced visual array concerns the veridical directions of objects from the observer (Rock 1966: chaps. 2–4). Normally, when engaging in forward locomotion, the perceived radial direction of an object straight ahead of the observer’s body remains constant while the perceived radial directions of objects to either side undergo constant change. This pattern also obtains when the observer wears prisms that displace the retinal image to side. Hence, “an object seen through prisms which retains the same radial direction as we approach must be seen to be moving in toward the sagittal plane” (Rock 1966: 105). On Rock’s view, at least some forms of adaptation to ORDs can be explained by our ability to detect and exploit such invariant sources of spatial informational in locomotion-generated patterns of optic flow.

Another related source of information for adaptation derives from the conflict between seen and proprioceptively experienced limb position (Wallach 1968; Ularik & Canon 1971). When this discrepancy is made conspicuous, proponents of the information hypothesis have found that passively moved (Melamed et al. 1973), involuntarily moved (Mather & Lackner 1975), and even immobile subjects (Kravitz & Wallach 1966) exhibit significant adaption. Although self-produced bodily movement is not necessary for adaptation to occur, it provides subjects with especially salient information about the discrepancy between sight and touch (Moulden 1971): subjects are able proprioceptively to determine the location of a moving limb much more accurately than a stationary or passively moved limb. It is the enhancement of the visual-proprioceptive conflict rather than reafferent visual stimulation, on this interpretation, that explains why active movement yields more adaptation than passive movement in Held’s experiments.

A final objection to the reafference theory concerns the end product of adaptation to ORDs. According to the theory, adaptation occurs when subjects learn new rules of sensorimotor dependence that govern how actions affect sensory inputs. There is a significant body of evidence, however, that much, if not all, adaptation rather occurs at the proprioceptive level. Stratton, summarizing the results of his experiment on mirror-based optical rearrangement, wrote:

…the principle stated in an earlier paper— that in the end we would feel a thing to be wherever we constantly saw it —can be justified in a wider sense than I then intended it to be taken…. We may now, I think, safely include differences of distance as well, and assert that the spatial coincidence of touch and sight does not require that an object in a given tactual position should appear visually in any particular direction or at any particular distance. In whatever place the tactual impression’s visual counterpart regularly appeared, this would eventually seem the only appropriate place for it to appear in. If we were always to see our bodies a hundred yards away, we would probably also feel them there. (1899: 498, our emphasis)

On this interpretation, the plasticity revealed by ORDs is primarily proprioceptive and kinaesthetic, rather than visual. Stratton’s world came to look “right side up” (1897b: 469) after adaptation to the inverted retinal image because things were felt where they were visually perceived to be—not because, his “entire visual field flipped over” (Kuhn 2012 [1962]: 112). This is clear from the absence of a visual negative aftereffect when Stratton finally removed his inverting lenses at the end of his eight-day experiment:

The visual arrangement was immediately recognized as the old one of pre-experimental days; yet the reversal of everything from the order to which I had grown accustomed during the past week, gave the scene a surprising, bewildering air which lasted for several hours. It was hardly the feeling, though, that things were upside down. (1897b: 470)

Moreover, Stratton reported changes in kinaesthesis during the course of the experiment consistent with the alleged proprioceptive shift:

when one was most at home in the unusual experience the head seemed to be moving in the very opposite direction from that which the motor sensations themselves would suggest . (1907: 156)

On this view, the end product of adaptation to an ORD is a recalibration of proprioceptive position sense at one or more points of articulation in the body (see the entry on bodily awareness ). As you practice reaching for a target while wearing laterally displacing prisms, for example, the muscle spindles, joint receptors, and Golgi tendon organs in your shoulder and arm continue to generate the same patterns of action potentials as before, but the proprioceptive and kinaesthetic meaning assigned to them by their “consumers” in the brain undergoes change: whereas before they signified that your arm was moving along one path through the seven-dimensional space of possible arm configurations (the human arm has seven degrees of freedom: three at the wrist, one at the elbow, and three at the shoulder), they gradually come to signify that it is moving along a different path in that kinematic space, namely, the one consistent with the prismatically distorted visual feedback you are receiving. Similar recalibrations are possible with respect to sources of information for head and eye position. After adapting to laterally displacing prisms, signals from receptors in your neck that previously signified the alignment of your head and torso, for example, may come to signify that your head is turned slightly to the side. For discussion, see Harris 1965, 1980; Welch 1978: chap. 3; and Briscoe 2016.

2.3 The Enactive Approach

The enactive approach defended by J. Kevin O’Regan and Alva Noë (O’Regan & Noë 2001; Noë 2004, 2005, 2010; O’Regan 2011) is best viewed as an extension of the reafference theory. According to the enactive approach, spatially contentful, world-presenting perceptual experience depends on implicit knowledge of the way sensory stimulations vary as a function of bodily movement. “Over the course of life”, O’Regan and Noë write,

a person will have encountered myriad visual attributes and visual stimuli, and each of these will have particular sets of sensorimotor contingencies associated with it. Each such set will have been recorded and will be latent, potentially available for recall: the brain thus has mastery of all these sensorimotor sets. (2001: 945)

To see an object o as having the location and shape properties it has it is necessary (1) to receive sensory stimulations from o and (2) to use those stimulations in order to retrieve the set of sensorimotor contingencies associated with o on the basis of past encounters. In this sense, seeing is a “two-step” process (Noë 2004: 164). It is important to emphasize, however, that the enactive approach distances itself from the idea that vision is functionally dedicated, in whole or in part, to the guidance of spatially directed actions: “Our claim”, Noë writes,

is that seeing depends on an appreciation of the sensory effects of movement (not, as it were, on the practical significance of sensation)…. Actionism is not committed to the general claim that seeing is a matter of knowing how to act in respect of or in relation to the things we see. (Noë 2010: 249)

The enactive approach also has strong affinities with the sense-data tradition. According to Noë, an object’s visually apparent shape is the shape of the 2D patch that would occlude the object on a plane perpendicular to the line of sight, i.e., the shape of the patch projected by the object on the frontal plane in accordance with the laws of linear perspective. Noë calls this the object’s “perspectival shape” (P-shape) (for other accounts of the perspectival nature of perception, see Hill 2022, chap. 3; Green & Schellenberg 2018; Lande 2018; and Green 2022). An object’s visually apparent size, in turn, is the size of the patch projected by the object on the frontal plane. Noë calls this the object’s “perspectival size” (P-size). Appearances are “perceptually basic” (Noë 2004: 81) because in order to see an object’s actual spatial properties it is necessary both to see its 2D P-properties and to understand how they would vary (undergo transformation) with changes in one’s point of view. This conception of what it is to perceive objects as voluminous space-occupiers is closely to akin to views defended by Russell (1918), Broad (1923), and Price (1950). It also worth mentioning that the enactive approach has strong affinities to views in the phenomenological tradition that are beyond the scope of this entry (but for discussion, see Thompson 2005; Hickerson 2007; and the entry on phenomenology ).

The enactive approach rests its case on three main sources of empirical support. The first derives from experiments with optical rearrangement devices (ORDs), discussed in Section 2.2 above. Hurley and Noë (2003) maintain that adaptation to ORDs only occurs when subjects relearn the systematic patterns of interdependence between active movement and reafferent visual stimulation. Moreover, contrary to the proprioceptive change theory of Stratton, Harris, and Rock, Hurley and Noë argue that the end product of adaptation to inversion and reversal of the retinal image is genuinely visual in nature: during the final stage of adaptation, visual experience “rights itself”.

In Section 2.2 above, we reviewed empirical evidence against the view that active movement and corresponding reafferent stimulation are necessary for adaptation to ORDs. Accordingly, we will focus here on Hurley and Noë’s objections to the proprioceptive-change theory. According to the latter, “what is actually modified [by the adaptation process] is the interpretation of nonvisual information about positions of body parts” (Harris 1980: 113). Once intermodal harmony is restored, the subject will again be able to perform visuomotor actions without error or difficulty, and she will again feel at home in the visually perceived world.

Hurley and Noë do not contest the numerous sources of empirical and introspective evidence that Stratton, Harris, and Rock adduce for the proprioceptive-change theory. Rather they reject the theory on the basis of what they take to be an untoward epistemic implication concerning adaptation to left-right reversal:

while rightward things really look and feel leftward to you, they come to seem to look and feel rightward. So the true qualities of your experience are no longer self-evident to you. (2003: 155)

The proprioceptive-change theory, however, does not imply such radical introspective error. According to proponents of the theory, experience normalizes after adaptation to reversal not because things that really look leftward “seem to look rightward” (what this might mean is enigmatic at best), but rather because the subjects eventually become familiar with the way things look when reversed—much as ordinary subjects can learn to read mirror-reversed writing fluently (Harris 1965: 435–36). Things seem “normal” after adaptation, in other words, because subjects are again able to cope with the visually perceived world in a fluent and unreflective manner.

A second line of evidence for the enactive approach comes from well-known experiments on tactile-visual sensory substitution (TVSS) devices that transform outputs from a low-resolution video camera into a matrix of vibrotactile stimulation on the skin of one’s back (Bach-y-Rita 1972, 2004) or electrotactile stimulation on the surface of one’s tongue (Sampaio et al. 2001).

At first, blind subjects equipped with a TVSS device experience its outputs as purely tactile. After a short time, however, many subjects cease to notice the tactile stimulations themselves and instead report having quasi-visual experiences of the objects arrayed in space in front of them. Indeed, with a significant amount of supervised training, blind subjects can learn to discriminate spatial properties such as shape, size, and location and even to perform simple “eye”-hand coordination tasks such as catching or batting a ball. A main finding of relevance in early experiments was that subjects learn to “see” by means of TVSS only when they have active control over movement of the video camera. Subjects who receive visual input passively—and therefore lack any knowledge of how (or whether) the camera is moving—experience only meaningless, tactile stimulation.

Hurley and Noë argue that passively stimulated subjects do not learn to “see” by means of sensory substitution because they are unable to learn the laws of sensorimotor contingency that govern the prosthetic modality:

active movement is required in order for the subject to acquire practical knowledge of the change from sensorimotor contingencies characteristic of touch to those characteristic of vision and the ability to exploit this change skillfully. (Hurley & Noë 2003: 145)

An alternative explanation, however, is that subjects who do not control camera movement—and who are not otherwise attuned to how the camera is moving—are simply unable to extract any information about the structure of the distal scene from the incoming pattern of sensory stimulations. In consequence they do not engage in “distal attribution” (Epstein et al. 1986; Loomis 1992; Siegel & Warren 2010): they do not perceive through the changing pattern of proximal stimulation to a spatially external scene in the environment. For development of this alternative explanation in the context of Bayesian perceptual psychology, see Briscoe 2019.

A final source of evidence for the enactive approach comes from studies of visuomotor development in the absence of normal, reafferent visual stimulation. Held & Hein 1963 performed an experiment in which pairs of kittens were harnessed to a carousel in a small, cylindrical chamber. One of the kittens was able to engage in free circumambulation while wearing a harness. The other kitten was suspended in the air in a metal gondola whose motions were driven by the first harnessed kitten. When the first kitten walked, both kittens moved and received identical visual stimulation. However, only the first kitten received reafferent visual feedback as the result of self-movement. Held and Hein reported that only mobile kittens developed normal depth perception—as evidenced by their unwillingness to step over the edge of a visual cliff, blinking reactions to looming objects, and visually guided paw placing responses. Noë (2004) argues that this experiment supports the enactive approach: in order to develop normal visual depth perception it is necessary to learn how motor outputs lead to changes to visual inputs.

There are two main reasons to be skeptical of this assessment. First , there is evidence that passive transport in the gondola may have disrupted the development of the kittens’ innate visual paw placing responses (Ganz 1975: 206). Second , the fact that passive kittens were prepared to walk over the edge of a visual cliff does not show that their visual experience of depth was abnormal. Rather, as Jesse Prinz (2006) argues, it may only indicate that they “did not have enough experience walking on edges to anticipate the bodily affordances of the visual world”.

The enactive approach confronts objections on multiple fronts. We focus on just three of them here (but for discussion and potential replies, see Block 2005; Prinz 2006; Briscoe 2008; Clark 2009: chap. 8; Block 2012; and Hutto & Myin 2017). First , the approach is essentially an elaboration of Held’s reafference theory and, as such, faces many of the same empirical obstacles. Evidence, for example, that active movement per se is not necessary for perceptual adaptation to optical rearrangement ( Section 2.2 ) is at variance with predictions made by the reafference theory and the enactive approach alike.

A second line of criticism targets the alleged perceptual priority of P-properties. According to the enactive approach, P-properties are “perceptually basic” (Noë 2004: 81) because in order to see an object’s intrinsic, 3D spatial properties it is necessary to see its 2D P-properties and to understand how they would undergo transformation with variation in one’s point of view. When we view a tilted coin, critics argue, however, we do not see something that looks—in either an epistemic or non-epistemic sense of “looks”—like an upright ellipse. Rather, we see what looks like a disk that is partly nearer and partly farther away from us. In general, the apparent shapes of the objects we perceive are not 2D but have extension in depth (Austin 1962; Gibson 1979; Smith 2000; Schwitzgebel 2006; Briscoe 2008; Hopp 2013).

Support for this objection comes from work in mainstream vision science. In particular, there is abundant empirical evidence that an object’s 3D shape is specified by sources of spatial information in the light reflected or emitted from the object’s surfaces to the perceiver’s eyes as well as by oculomotor factors (for reviews, see Cutting & Vishton 1995; Palmer 1999; and Bruce et al. 2003). Examples include binocular disparity, vergence, accommodation, motion parallax, texture gradients, occlusion, height in the visual field, relative angular size, reflections, and shading. That such shape-diagnostic information having once been processed by the visual system is not lost in conscious visual experience of the object is shown by standard psychophysical methods in which experimenters manipulate the availability of different spatial depth cues and gauge the perceptual effects. Objects, for example, look somewhat flattened under uniform illumination conditions that eliminate shadows and highlights, and egocentric distances are underestimated for objects positioned beyond the operative range of binocular disparity, accommodation, and vergence. Results of such experimentation show that observers can literally see the difference made by the presence or absence of a certain cue in the light available to the eyes (Smith 2000; Briscoe 2008; for debate on whether 2D perspectival properties should be introduced into scientific explanations of shape perception, see Bennett 2012; McLaughlin 2016; Lande 2018; Morales & Firestone 2020; Linton 2021; Burge & Burge 2022; Cheng et al. 2022; and Morales & Firestone 2023).

According to the influential dual systems model (DSM) of visual processing (Milner & Goodale 1995/2006; Goodale & Milner 2004), visual consciousess and visuomotor control are supported by functionally and anatomically distinct visual subsystems (these are the ventral and dorsal information processing streams, respectively). In particular, proponents of the DSM maintain that the contents of visual experience are not used by motor programming areas in the primate brain:

The visual information used by the dorsal stream for programming and on-line control, according to the model, is not perceptual in nature …[I]t cannot be accessed consciously, even in principle. In other words, although we may be conscious of the actions we perform, the visual information used to program and control those actions can never be experienced. (Milner & Goodale 2008: 775–776)

A final criticism of the enactive approach is that it is empirically falsified by evidence for the DSM (see the commentaries on O’Regan & Noë 2001; Clark 2009: chap. 8; Briscoe 2014; and the essays collected in Gangopadhyay et al. 2010): the bond it posits between what we see and what we do is much too tight to comport with what neuroscience has to tells us about their functional relations.

The enactivist can make two points in reply to this objection. First, experimental findings indicate that there are a number of contexts in which information present in conscious vision is utilized for purposes of motor programming (see Briscoe 2009 and Briscoe & Schwenkler 2015). Action and perception are not as sharply dissociated as proponents of DSM sometimes claim.

Second, the enactive approach, as emphasized above, rejects the idea that the function of vision is to guide actions. It

does not claim that visual awareness depends on visuomotor skill, if by “visuomotor skill” one means the ability to make use of vision to reach out and manipulate or grasp. Our claim is that seeing depends on an appreciation of the sensory effects of movement (not, as it were, on the practical significance of sensation). (Noë 2010: 249)

Since the enactive approach is not committed to the idea that seeing depends on knowing how to act in relation to what we see, it is not threatened by empirical evidence for a functional dissociation between visual awareness and visually guided action.

3. Motor Component and Efferent Readiness Theories

At this point, it should be clear that the claim that perception is active or action-based is far from unambiguous. Perceiving may implicate action in the sense that it is taken constitutively to involve associations with touch (Berkeley 1709), kinaesthetic feedback from changes in eye position (Lotze 1887 [1879]), consciously experienced “effort of the will” (Helmholtz 2005 [1924]), or knowledge of the way reafferent sensory stimulation varies as a function of movement (Held 1961; O’Regan & Noë 2001; Hurley & Noë 2003).

In this section, we shall examine two additional conceptions of the role of action in perception. According to the motor component theory , as we shall call it, efference copies generated in the oculomotor system and/or proprioceptive feedback from eye-movements are used in tandem with incoming sensory inputs to determine the spatial attributes of perceived objects (Helmholtz 2005 [1924]; Mack 1979; Shebilske 1984, 1987; Ebenholtz 2002; Briscoe 2021). Efferent readiness theories , by contrast, appeal to the particular ways in which perceptual states prepare the observer to move and act in relation to the environment. The modest readiness theory , as we shall call it, claims that the way an object’s spatial attributes are represented in visual experience is sometimes modulated by one or another form of covert action planning (Festinger et al. 1967; Coren 1986; Vishton et al. 2007). The bold readiness theory argues for the stronger, constitutive claim that, as J.G. Taylor puts its, “perception and multiple simultaneous readiness for action are one and the same thing” (1968: 432).

As pointed out in Section 2.3.2 , there are numerous, independently variable sources of information about the spatial layout of the environment in the light sampled by the eye. In many cases, however, processing of stimulus information requires or is optimized by recruiting sources of auxiliary information from outside the visual system. These may be directly integrated with incoming visual information or used to change the weighting assigned to one or another source of optical stimulus information (Shams & Kim 2010; Ernst 2012).

An importantly different recruitment strategy involves combining visual input with non-perceptual information originating in the body’s motor control systems, in particular, efference copy, and/or proprioceptive feedback from active movement (kinaesthesis). The motor component theory, as we shall call it, is premised on evidence for such motor-modal processing.

The motor component theory can be made more concrete by examining three situations in which the spatial contents of visual experience are modulated by information concerning recently initiated or impending bodily movements:

  • Apparent direction: The retinal image produced by an object is ambiguous with respect to the object’s direction in the absence of extraretinal information concerning the orientation of the eye relative to the head. While there is evidence that proprioceptive inflow from muscle spindles in the extraocular muscles is used to encode eye position, as Sherrington 1918 proposed, outflowing efference copy is generally regarded as the more heavily weighted source of information (Bridgeman & Stark 1991). The problem of visual direction constancy was discussed in detail in Section 2.1 above.
  • Apparent distance and size: When an object is at close range, its distance in depth from the perceiver can be determined on the basis of three variables: (1) the distance between the perceiver’s eyes, (2) the vergence angle formed by the line of sight from each eye to the object, and (3) the direction of gaze. Information about (1) is updated in the course of development as the perceiver’s body grows. Information about (2) and (3), which vary from one moment to the next, is obtained from efference copy of the motor command to fixate the object as well as proprioceptive feedback from the extraocular muscles. Since an object’s apparent size is a function of its perceived distance from the perceiver and the angle it subtends on the retina (Emmert 1881), information about (2) and (3) can thus modulate visual size perception (Mon-Williams et al. 1997).
  • Apparent motion: In the most familiar case of motion perception, the subject visually tracks a moving target, e.g., a bird in flight, against a stable background using smooth pursuit eye movements. As Bridgeman et al. 1994 note, smooth pursuit “reverses the movement conditions on the retina: the tracked object sweeps across the retina very little, while the background undergoes a brisk motion” (p. 255). Nonetheless, it is the target that appears to be in motion while the environment appears to be stationary. There is evidence that the visual system is able to compensate for pursuit induced retinal motion by means of efference-based information about the changing direction of gaze (for a review, see Furman & Gur 2012). This has been used to explain the travelling moon illusion (Post & Leibowitz 1985: 637). Neuropsychological findings indicate that failure to integrate efference-based information about eye movement leads to a breakdown in perceived background stability during smooth pursuit (Haarmeier et al. 1997; Nakamura & Colby 2002; Fischer et al. 2012).

The motor component theory is a version of the view that perception is embodied in the sense of Prinz 2009 (see the entry on embodied cognition ). Prinz explains that

embodied mental capacities, are ones that depend on mental representations or processes that relate to the body…. Such representations and processes come in two forms: there are representations and processes that represent or respond to body, such as a perception of bodily movement, and there are representations and processes that affect the body, such as motor commands (2009: 420).

The three examples presented above provide empirical support for the thesis that visual perception is embodied in this sense. For additional examples, see Ebenholtz 2002: chap. 4, and for further discussion of various senses of embodiment, see Alsmith & de Vignemount 2012, de Vignemont & Alsmith 2017, Bermúdez 2018, Stoneham 2018, and Berendzen 2023.

3.2 The Efferent Readiness Theory

Patients with frontal lobe damage sometimes exhibit pathological “utilization behaviour” (Lhermitte 1983) in which the sight of an object automatically elicits behaviors typically associated with it, such as automatically pouring water into a glass and drinking it whenever a bottle of water and a glass are present (Frith et al. 2000: 1782). That normal subjects often do not automatically perform actions afforded by a perceived object, however, does not mean that they do not plan, or imaginatively rehearse, or otherwise represent them. (On the contrary, recent neuroscientific findings suggest that merely perceiving an object often covertly prepares the motor system to engage with it in a certain manner. For overviews, see Jeannerod 2006 and Rizzolatti 2008.)

Efferent readiness theories are based on the idea that covert preparation for action is “an integral part of the perceptual process” and not “merely a consequence of the perceptual process that has preceded it” (Coren 1986: 394). According to the modest readiness theory , as we shall call it, covert motor preparation can sometimes influence the way an object’s spatial attributes are represented in perceptual experience. The bold readiness theory , by contrast, argues for the stronger, constitutive claim that to perceive an object’s spatial properties just is to be prepared or ready to act in relation to the object in certain ways (Sperry 1952; Taylor 1962, 1965, 1968).

A number of empirical findings motivate the modest readiness theory. Festinger et al. 1967 tested the view that visual contour perception is

determined by the particular sets of preprogrammed efferent instructions that are activated by the visual input into a state of readiness for immediate use. (p. 34)

Contact lenses that produce curved retinal input were placed on the right eye of three observers, who were instructed to scan a horizontally oriented line with their left eye covered for 40 minutes. The experimenters reported that there was an average of 44% adaptation when the line was physically straight but retinally curved, and an average of 18% adaptation when the line was physically curved but retinally straight (see Miller & Festinger 1977, however, for conflicting results).

An elegantly designed set of experiments by Coren 1986 examined the role of efferent readiness in the visual perception of direction and extent. Coren’s experiments support the hypothesis that the spatial parameter controlling the length of a saccade is not the angular direction of the target relative to the line of sight, but rather the direction of the center of gravity (COG) of all the stimuli in its vicinity (Coren & Hoenig 1972; Findlay 1982). Importantly,

the bias arises from the computation of the saccade that would be made and, hence, is held in readiness, rather than the saccade actually emitted. (Coren 1986: 399)

The COG bias is illustrated in Figure 3 . In the first row (top) , there are no extraneous stimuli near the saccade target. Hence, the saccade from the point of fixation to the target is unbiased. In the second row , by contrast, the location of an extraneous stimulus (×) results in a saccade from the point of fixation that undershoots its target, while in the third row the saccade overshoots its target. In the fourth row , changing the location of the extraneous stimulus eliminates the COG bias: because the extraneous stimulus is near the point of fixation rather than the saccade target, the saccade is accurate.

[Two columns of 4 dots each, each pair of dots on the same horizontal. The left column has the words 'Fixation Point' at the top and a blue arrow pointing from those words to the top of the column. The right column has the words 'Saccade Target' and a similar blue arrow. The top (or first) pair of dots has an arrow curving from the left dot to the right dot. The second pair as a 'x' to the left of the right dot and an arrow curving from the left dot to a point between the 'x' and the right dot. The third pair has a 'x' to the right of the right dot and an arrow curving from the left dot to a point between the right dot and the 'x'. The fourth pair as a 'x' to the left of the left dot and an arrow curving from the left dot to the right dot.]

Figure 3: The effect of starting eye position on saccade programming (after Coren 1986: 405)

The COG bias is evolutionarily adaptive: eye movements will bring both the saccade target as well as nearby objects into high acuity vision, thereby maximizing the amount of information obtained with each saccade. Motor preparation or “efferent readiness” to execute an undershooting or overshooting saccade, Coren found, however, can also give rise to a corresponding illusion of extent (1986: 404–406). Observers, e.g., will perceptually underestimate the length of the distance between the point of fixation and the saccade target when there is an extraneous stimulus on the near side of the target (as in the second row of Figure 3 ) and will perceptually overestimate the length of the distance when there is an extraneous stimulus on the far side of the target (as in the third row of Figure 3 ).

According to Coren, the well-known Müller-Lyer illusion can be explained within this framework. The outwardly turned wings in Müller-Lyer display shift the COG outward from each vertex, while the inwardly turned wings in this figure shift the COG inward. This influences both saccade length from vertex to vertex as well as the apparent length of the central line segments. The influence of COG on efferent readiness to execute eye movements, Coren argues (1986: 400–403), also explains why the line segments in the Müller-Lyer display can be replaced with small dots while leaving the illusion intact as well as the effects of varying wing length and wing angle on the magnitude of the illusion.

The modest readiness theory holds that the way an object’s spatial attributes are represented in visual experience is sometimes modulated by one or another form of covert action planning. The bold readiness theory argues for a stronger, constitutive claim: to perceive an object’s spatial properties just is to be prepared or ready to act in relation to the object in certain ways. We begin by examining J.G. Taylor’s “behavioral theory” of perception (Taylor 1962, 1965, 1968).

Taylor’s behavioral theory of perception identifies the conscious experience of seeing an object’s spatial properties with the passive activation of a specific set of learned or “preprogrammed” motor routines:

[P]erception is a state of multiple simultaneous readiness for actions directed to the objects in the environment that are acting on the receptor organs at any one moment. The actions in question have been acquired by the individual in the course of his life and have been determined by the reinforcing contingencies in the environment in which he grew up. What determines the content of perception is not the properties of the sensory transducers that are operated on by stimulus energies from the environment, but the properties of the behaviour conditioned to those stimulus energies …. (1965: 1, our emphasis)

According to Taylor’s theory, sensory stimulation gives rises to spatially contentful visual experience as a consequence of associative, reinforcement learning: we perceive an object as having the spatial attribute G when the types of proximal sensory stimulation caused by the object have been conditioned to the performance of actions sensitive to G (1962: 42). The conscious experience of seeing an object’s distance, e.g., is constituted by the subject’s learned readiness to perform specific whole body and limb movements that were reinforced when the subject previously received stimulation from objects at the same remove. In general, differences in the spatial content of a visual experience are identified with differences in the subject’s state of “multiple simultaneous readiness” to interact with the objects represented in the experience.

The main problem with Taylor’s theory is one that besets behaviorist theories of perception in general: it assumes that for any visible spatial property G , there will be some distinctive set of behavioral responses that are constitutive of perceiving the object as having G . The problem with this assumption, as Mohan Matthen (1988) puts it,

there is no such thing as the proper response, or even a range of functionally appropriate responses, to what perception tells us. (p. 20, see also Hurley 2001: 17)

The last approach we shall discuss has roots in, and similarities to, many of the proposals covered above, but is most closely aligned with the bold readiness theory. We will follow Grush (2007) in calling this approach the disposition theory (see Grush 2007: 394, for discussion of the name). The primary proponent of this position is Gareth Evans, whose work on spatial representation focused on understanding how we manage to perceive objects as occupying locations in egocentric space. Related views have been defended by Peacocke 1992, chap. 3; Simmons 2003; Schellenberg 2007, 2010; Briscoe 2009, 2014; Ward et al. 2011; Alsmith 2012; Mandrigin 2021; de Vignemont 2021; and Nave et al. 2022.

The starting point of Evans’ theory is that the subject’s perceptual systems have isolated a channel of sensory input, an “information link”, through which she receives information about the object. The information link by itself does not allow the subject to know the location of this object. Rather, it is when the information link is able to induce in the subject appropriate kinds of behavioral dispositions that it becomes imbued with spatial import:

The subject hears the sound as coming from such-and-such a position, but how is the position to be specified? Presumably in egocentric terms (he hears the sound as up, or down, to the right or to the left, in front or behind). These terms specify the position of the sound in relation to the observer’s own body; and they derive their meaning in part from their complicated connections with the subject’s actions . (Evans 1982: 155)

This is not a version of a motor theory (e.g., Poincaré 1907: 71). The behavioral responses in question are not to be understood as raw patterns of motor activations, or even muscular sensations. Such a reduction would face challenges anyway, since for any location in egocentric space, there are an infinite number of kinematic configurations (movements) that would, for example, effect a grasp to that location; and for any kinematic configuration, there are an infinite number of dynamic profiles (temporal patterns of muscular force) that would yield that configuration. The behavioral responses in question are overt environmental behavior :

It may well be that the input-output connections can be finitely stated only if the output is described in explicitly spatial terms (e.g., ‘extending the arm’, ‘walking forward two feet’, etc.). If this is so, it would rule out the reduction of the egocentric spatial vocabulary to a muscular vocabulary. But such a reduction is certainly not needed for the point being urged here, which is that the spatial information embodied in auditory perception is specifiable only in a vocabulary whose terms derive their meaning partly from being linked with bodily actions. Even given an irreducibility, it would remain the case that possession of such information is directly manifestable in behaviour issuing from no calculation; it is just that there would be indefinitely many ways in which the manifestation can occur. (Evans 1982: 156)

Also, on this proposal, all modalities are in the same boat. As such the disposition theory is more ambitious than most of the theories already discussed, which are limited to vision. Not only is there no reduction of perceptual spatial content to a “muscular vocabulary”, there is also no reduction of the spatial content of some perceptual modalities to that of one or more others—as there was for Berkeley, who sought to reduce the spatial content of vision to that of touch, and whose program forced a distinction between two spaces, visual space and tangible space:

The spatial content of auditory and tactual-kinaesthetic perceptions must be specified in the same terms—egocentric terms. … It is a consequence of this that perceptions from both systems will be used to build up a unitary picture of the world. There is only one egocentric space, because there is only one behavioural space. (Evans 1982: 160)

Relatedly, for Evans it is not even the case that spatial perceptual content, for all modalities, is being reduced to behavioral dispositions. Rather, perceptual inputs and behavioral outputs jointly and holistically yield a single behavioral space:

Egocentric spatial terms are the terms in which the content of our spatial experiences would be formulated, and those in which our immediate behavioural plans would be expressed. This duality is no coincidence: an egocentric space can exist only for an animal in which a complex network of connections exists between perceptual input and behavioural output. A perceptual input—even if, in some loose sense, it encapsulates spatial information (because it belongs to a range of inputs which vary systematically with some spatial facts)—cannot have a spatial significance for an organism except in so far as it has a place in such a complex network of input-output connections. (Evans 1982: 154) Egocentric spatial terms and spatial descriptions of bodily movement would, on this view, form a structure familiar to philosophers under the title “holistic”. (Evans 1982: 156, fn. 26)

This last point and the associated quotes address a common misconception of the disposition theory. It would be easy to read the theory as providing a proposal of the following sort: A creature gets sensory information from a stimulus, and the problem is to determine where that stimulus is located in egocentric space; the solution is that features of that sensory episode induce dispositions to behavior targeting some egocentric location . While this sort of thing is indeed a problem, it is relatively superficial. Any creature facing this problem must already have the capacity to grasp egocentric spatial location contents, and the problem is which of these ready-at-hand contents it should assign to the stimulus. But the disposition theory is addressing a deeper question: in virtue of what does this creature have a capacity to grasp egocentric spatial contents to begin with? The answer is that the creature must have a rich set of interconnections between sensory inputs (and their attendant information links) and dispositions for behavioral outputs.

Rick Grush (2000, 2007) has adopted Evans’ theory, and attempted to clarify and expand upon it, particularly in three areas: first, the distinction between the disposition theory and other approaches; second, the neural implementation of the disposition theory; and finally the specific kinds of dispositions that are relevant for the issue of spatial experience.

The theory depends on behavioral dispositions. Grush (2007) argues that there are two distinctions that need to be made: first, the organism might possess i) knowledge of what the consequences (bodily, environmental, or sensory) of a given action will be; or ii) knowledge of which motor commands will bring about a given desired end state (of the body, environment, or sensory channels) (Grush 2007: 408). I might be able to recognize that a series of moves someone shows me will force my grandmaster opponent into checkmate (knowledge of the first sort, the consequences of a given set of actions), and yet not have been anywhere near the skill level to have come up with that series of moves on my own (knowledge of the second sort, what actions will achieve a desired effect). Sensorimotor contingency theorists appeal to knowledge of the first sort—though Noë (2004: 90) flirts with appealing to knowledge of the second sort to explain the perceptual grasp of P-shapes; to the extent he does, he is embracing a disposition theoretic account of P-shapes. Disposition theorists, and bold readiness theorists ( Section 3.2.2 ) appeal to knowledge of the second sort. These are the dispositions of the disposition theory: given some goal, the organism is disposed to execute certain actions.

This leads to the second distinction, between type-specifying and detail-specifying dispositions. Grush (2007: 393) maintains that only the latter are directly relevant for spatial perception. A type-specifying disposition is a disposition to execute some type of behavior with respect to an object or place. For example, an organism might be disposed to grasp, bite, flee, or foveate some object. This sort of disposition is not relevant to the spatial content of the experience on the disposition theory. Rather, what are relevant are detail-specifying dispositions: the specifics of how I am disposed to act to execute any of these behavior types. When reaching to grab the cup to take a drink (type), do I move my hand like so (straight ahead, say), or like such (off to the right)? When I want to foveate or orient towards (behavior type) the ant crawling up the wall, do a I move my head and eyes like this , or like that ?

This latter distinction allows the disposition theory to answer one of the main objections to the bold readiness theory (described at the end of section 3.2.2 ) that there is no single special disposition connected to perceiving any given object. That is true of type-specifying dispositions, but not of detail-specifying dispositions. Given the ant’s location there is indeed a very limited range of detail specifying dispositions that will allow me to foveate it (though this might require constraints on possible actions, such as minimum jerk or other such constraints).

Grush (2007; 2009) has proposed a detailed implementation of the disposition theory in terms of neural information processing. The proposal involves more mathematics than is appropriate here, and so a quick qualitative description will have to suffice (for more detail, see Grush 2007; 2009). The basic idea is that relevant cortical areas learn sets of basis functions which, to put it very roughly, encode equivalence classes of combinations of sensory and postural signals (for discussion, see Pouget et al. 2002). For example, many combinations of eye orientation and location of stimulation on the retina correspond to a visual stimulus that is directly in front of the head. Sorting such bodily postural information (not just eye orientation, but any postural information that affects sensation, which is most) and sensory condition pairs into useful equivalence classes is the first half of the job.

What this does is encode incoming information in a way that renders it ready to be of use in guiding behavior, since the equivalence classes are precisely those for which a given kind of motor program is appropriate. The next part corresponds to how this information, so represented, can be used to produce the details of such a motor program. For every type of action in a creature’s behavioral repertoire (grasp, approach, avoid, foveate, bite, etc.) its motor areas have a set of linear coefficients, easily implemented as a set of neural connection strengths, and when these are applied to a set of basis function values, a detailed behavior is specified. For example, when a creature senses an object O 1 , a set of basis function values B 1 for that stimulus is produced. If the creature decides to execute overt action A 1 , then the B 1 basis function values are multiplied by the coefficient corresponding to A 1 . The result is an instance of behavior type A 1 executed with respect to object O 1 . If the creature had decide instead to execute action A 2 , with respect to O 1 , the B 1 basis function values would have been multiplied by the A 2 set of coefficients, and the result would be a motor behavior executing A 2 on object O 1 .

Accordingly, the disposition theory has a very different account of what is happening with sensory substitution devices than Susan Hurley and Alva Noë (see Section 2.3.2 above). On the disposition theory, what allows the user of such a device to have spatial experience is not the ability to anticipate how the sensory input will change upon execution of movement as the sensorimotor contingency theory would have it. Rather, it is that the subject’s brain has learned to take these sensory inputs together with postural signals to produce sets of basis functions that poise the subject to act with respect to the object that is causing the sensory signals (see Grush 2007: 406).

One objection to disposition theories is what Hurley has called The Myth of the Giving :

To suppose that … the content of intentions can be taken as unproblematically primitive in explaining how the content of experience is possible, is to succumb to the myth of the giving. (Hurley 1998: 241)

The idea behind this objection is that one is simply shifting the debt from one credit card to another when one takes as problematic the spatial content of perception, and then appeals to motor behavior as the supplier of this content. For then, of course, the question will be: Whence the spatial content of motor behavior?

The disposition theory, however, does not posit any such unilateral reduction (though Taylor’s bold readiness theory arguably does, see Section 3.2.2 above). As discussed above, Evans explicitly claims that the behavioral space is holistically determined by both behavior and perception. And on Grush’s account spatial content is implemented in the construction of basis function values, and these values coordinate transitions from perceptual input to behavioral output. As such, they are highly analogous to inferences whose conditions of application are given in sensory-plus-postural terms and whose consequences of application manifest in behavioral terms. The import of the states that represent these basis function values is no more narrowly motor than the meaning of a conditional can be identified with its consequent (or its antecedent, for that matter) in isolation.

Another very common objection, one that is often leveled at many forms of motor theory, has to do with the fact that even paralyzed people, with very few possibilities for action, seem capable in many cases of normal spatial perception. Such objections would, at a minimum, place significant pressure on any views that explain perceptual content by appeal to actual behavior. It is also easy to see how even hypothetical behavior would be called into question in such cases, since in many such cases behavior is not physically possible. Grush’s theory (2007), right or wrong, has something specific to say about this objection. Since spatial content is taken to be manifested in the production of basis function values in the cortex, the prediction is that any impairments manifesting farther down the chain, the brain stem or spinal cord, for example, need have no direct effect on spatial content. So long as the relevant brain areas have the wherewithal to produce sets of basis function values suitable for constructing a motor sequence (if multiplied by the action-type-specific coefficients), then the occasioning perceptual episode will have spatial content.

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Hypothesis and theory article, understanding human perception by human-made illusions.

what perceptual hypothesis

  • 1 Department of General Psychology and Methodology, University of Bamberg, Bamberg, Germany
  • 2 Bamberg Graduate School of Affective and Cognitive Sciences (BaGrACS), Bamberg, Germany

It may be fun to perceive illusions, but the understanding of how they work is even more stimulating and sustainable: They can tell us where the limits and capacity of our perceptual apparatus are found—they can specify how the constraints of perception are set. Furthermore, they let us analyze the cognitive sub-processes underlying our perception. Illusions in a scientific context are not mainly created to reveal the failures of our perception or the dysfunctions of our apparatus, but instead point to the specific power of human perception. The main task of human perception is to amplify and strengthen sensory inputs to be able to perceive, orientate and act very quickly, specifically and efficiently. The present paper strengthens this line of argument, strongly put forth by perceptual pioneer Richard L. Gregory (e.g., Gregory, 2009 ), by discussing specific visual illusions and how they can help us to understand the magic of perception.

About the Veridicality of Perception

The relationship between reality and object.

Sensory perception is often the most striking proof of something factual—when we perceive something, we interpret it and take it as “objective”, “real”. Most obviously, you can experience this with eyewitness testimonies: If an eyewitness has “seen it with the naked eye”, judges, jury members and attendees take the reports of these percepts not only as strong evidence, but usually as fact—despite the active and biasing processes on basis of perception and memory. Indeed, it seems that there is no better, no more “proof” of something being factual knowledge than having perceived it. The assumed link between perception and physical reality is particularly strong for the visual sense—in fact, we scrutinize it only when sight conditions have been unfortunate, when people have bad vision or when we know that the eyewitness was under stress or was lacking in cognitive faculties. When people need even more proof of reality than via the naked eye, they intuitively try to touch the to-be-analyzed entity (if at all possible) in order to investigate it haptically. Feeling something by touch seems to be the ultimate perceptual experience in order for humans to speak of physical proof ( Carbon and Jakesch, 2013 ).

We can analyze the quality of our perceptual experiences by standard methodological criteria. By doing so we can regularly find out that our perception is indeed mostly very reliable and also objective ( Gregory and Gombrich, 1973 )—but only if we employ standard definitions of “objective” as being consensual among different beholders. Still, even by meeting these methodological criteria, we cannot give something in evidence about physical reality. It seems that knowledge about the physical properties of objects cannot be gained by perception, so perception is neither “veridical” nor “valid” in the strict sense of the words—the properties of the “thing in itself” remain indeterminate in any empirical sense ( Kant, 1787/1998 ). We “reliably” and “objectively” might perceive the sun going up in the morning and down in the evening; the physical relations are definitely different, as we have known at least since Nicolaus Copernicus’s proposed heliocentricism—it might also be common sense that the Earth is a spheroid for most people, still the majority of people have neither perceived the Earth as spherical nor represented it like that; one reason for this is that in everyday life contexts the illusion of a plane works perfectly well to guide us in the planning and execution of our actions ( Carbon, 2010b ).

Limitations of the Possibility of Objective Perception

The limitations of perception are even more far reaching: our perception is not only limited when we do not have access to the thing in itself, it is very practically limited to the quality of processing and the general specifications of our perceptual system. For instance, our acoustic sense can only register and process a very narrow band of frequencies ranging from about 16 Hz–20 kHz as a young adult—this band gets narrower and narrower with increasing age. Typically, infrasonic and ultrasonic bands are just not perceivable despite being essential for other species such as elephants and bats, respectively. The perception of the environment and, consequently, the perception and representation of the world as such, is different for these species—what would be the favorite music of an elephant, which preference would a bat indicate if “honestly asked”? What does infrasonic acoustics sound and feel like? Note: infrasonic frequencies can also be perceived by humans; not acoustically in a strict sense but via vibrations—still, the resulting experiences are very different (cf. Nagel, 1974 ). To make such information accessible we need transformation techniques; for instance, a Geiger-Müller tube for making ionizing radiation perceivable as we have not developed any sensory system for detecting and feeling this band of extremely high frequency electromagnetic radiation.

But even if we have access to given information from the environmental world, it would be an illusion to think of “objective perception” of it—differences in perception across different individuals seem to be obvious: this is one reason for different persons having different tastes, but it is even more extreme: even within a lifetime of one person, the perceptual qualities and quantities which we can process change. Elderly people, for instance, often have yellowish corneas yielding biased color perception reducing the ability to detect and differentiate bluish color spectra. So even objectivity of perceptions in the sense of consensual experience is hardly achievable, even within one species, even within one individual—just think of fashion phenomena ( Carbon, 2011a ), of changes in taste ( Martindale, 1990 ) or the so-called cycle of preferences ( Carbon, 2010a )! Clearly, so-called objective perception is impossible, it is an illusion.

Illusory Construction of the World

The problem with the idea of veridical perception of the world is further intensified when taking additional perceptual phenomena, which demonstrate highly constructive qualities of our perceptual system, into account. A very prominent example of this kind is the perceptual effect which arises when any visual information which we want to process falls on the area of the retina where the so-called blind spot is located (see Figure 1 ).


Figure 1. Demonstration of the blind spot, the area on the retina where visual information cannot be processed due to a lack of photoreceptors . The demonstration works as follows: Fixate at a distance of approx. 40 cm the X on the left side with your right eye while having closed your left eye—now move your head slightly in a horizontal way from left to right and backwards till the black disc on the right side seems to vanish.

Interestingly, visual information that is mapped on the blind spot is not just dropped—this would be the easiest solution for the visual apparatus. It is also not rigidly interpolated, for instance, by just doubling neighbor information, but intelligently complemented by analysing the meaning and Gestalt of the context. If we, for example, are exposed to a couple of lines, the perceptual system would complement the physically non-existing information of the blind spot by a best guess heuristic how the lines are interconnected in each case, mostly yielding a very close approximation to “reality” as it uses most probable solutions. Finally, we experience clear visual information, seemingly in the same quality as the one which mirrors physical perception—in the end, the “physical perception” and the “constructed perception”, are of the same quality, also because the “physical perception” is neither a depiction of physical reality, but is also constructed by top-down processes based on best guess heuristic as a kind of hypothesis testing or problem solving ( Gregory, 1970 ).

Beside this prominent example which has become common knowledge up to now, a series of further phenomena exist where we can speak of full perceptual constructions of the world outside without any direct link to the physical realities. A very intriguing example of this kind will be described in more detail in the following: When we make fast eye movements (so-called saccades) our perceptual system is suppressed, with the result that we are functionally blind during such saccades. Actually, we do not perceive these blind moments of life although they are highly frequent and relatively long as such—actually, Rayner et al. estimated that typical fixations last about 200–250 ms and saccades last about 20–40 ms ( Rayner et al., 2001 ), so about 10% of our time when we are awake is susceptible to such suppression effects. In accordance with other filling-in phenomena, missing data is filled up with the most plausible information: Such a process needs hypotheses about what is going on in the current situation and how the situation will evolve ( Gregory, 1970 , 1990 ). If the hypotheses are misleading because the underlying mental model of the situation and its further genesis is incorrect, we face an essential problem: what we then perceive (or fail to perceive) is incompatible with the current situation, and so will mislead our upcoming action. In most extreme cases, this could lead to fatal decisions: for instance: if the model does not construct a specific interfering object in our movement axis, we might miss information essential to changing our current trajectory resulting in a collision course. In such a constellation, we would be totally startled by the crash, as we would not have perceived the target object at all—this is not about missing an object but about entirely overlooking it due to a non-existing trace of perception.

Despite the knowledge about these characteristics of the visual system, we might doubt such processes as the mechanisms are working to so great an extent in most everyday life situations that it provides the perfect illusion of continuous, correct and super-detailed visual input. We can, however, illustrate this mechanism very easily by just observing our eye movements in a mirror: when executing fast eye movements, we cannot observe them by directly inspecting our face in the mirror—we can only perceive our fixations and the slow movements of the eyes. If we, however, film the same scene with a video camera, the whole procedure looks totally different: Now we clearly also see the fast movements; so we can directly experience the specific operation of the visual system in this respect by comparing the same scene captured by two differently working visual systems: our own, very cognitively operating, visual system and the rigidly filming video system which just catches the scene frame by frame without further processing, interpreting and tuning it. 1 We call this moment of temporary functional blindness phenomenon “saccade blindness” or “saccade suppression”, which again illustrates the illusionary aspects of human perception “saccadic suppression”, Bridgeman et al., 1975 ; “tactile suppression”, Ziat et al., 2010 ). We can utilize this phenomena for testing interesting hypotheses on the mental representation of the visual environment: if we change details of a visual display during such functional blind phases of saccadic movements, people usually do not become aware of such changes, even if very important details, e.g., the expression of the mouth, are changed ( Bohrn et al., 2010 ).

Illusions by Top-Down-Processes

Gregory proposed that perception shows the quality of hypothesis testing and that illusions make us clear how these hypotheses are formulated and on which data they are based ( Gregory, 1970 ). One of the key assumptions for hypothesis testing is that perception is a constructive process depending on top-down processing. Such top-down processes can be guided through knowledge gained over the years, but perception can also be guided by pre-formed capabilities of binding and interpreting specific forms as certain Gestalts. The strong reliance of perception on top-down processing is the essential key for assuring reliable perceptual abilities in a world full of ambiguity and incompleteness. If we read a text from an old facsimile where some of the letters have vanished or bleached out over the years, where coffee stains have covered partial information and where decay processes have turned the originally white paper into a yellowish crumbly substance, we might be very successful in reading the fragments of the text, because our perceptual system interpolates and (re-)constructs (see Figure 2 ). If we know or understand the general meaning of the target text, we will even read over some passages that do not exist at all: we fill the gaps through our knowledge—we change the meaning towards what we expect.


Figure 2. Demonstration of top-down processing when reading the statement “The Grand Illussion” under highly challenging conditions (at least challenging for automatic character recognition) .

A famous example which is often cited and shown in this realm is the so-called man-rat-illusion where an ambiguous sketch drawing is presented whose content is not clearly decipherable, but switches from showing a man to showing a rat—another popular example of this kind is the bistable picture where the interpretation flips from an old woman to a young woman an v.v. (see Figure 3 )—most people interpret this example as a fascinating illusion demonstrating humans’ capability of switching from one meaning to another, but the example also demonstrates an even more intriguing process: what we will perceive at first glance is mainly guided through the specific activation of our semantic network. If we have been exposed to a picture of a man before, or if we think of a man or have heard the word “man”, the chance is strongly increased that our perceptual system interprets the ambiguous pattern towards a depiction of a man—if the prior experiences were more associated with a rat, a mouse or another animal of such a kind, we will, in contrast, tend to interpret the ambiguous pattern more as a rat.


Figure 3. The young-old-woman illusion (also known as the My Wife and My Mother-In-Law illusion) already popular in Germany in the 19th century when having been frequently depicted on postcards . Boring (1930) was the first who presented this illusion in a scientific context (image on the right) calling it a “new” illusion (concretely, “a new ambiguous figure”) although it was very probably taken from an already displayed image of the 19th century within an A and P Condensed Milk advertisement ( Lingelbach, 2014 ).

So, we can literally say that we perceive what we know—if we have no prior knowledge of certain things we can even overlook important details in a pattern because we have no strong association with something meaningful. The intimate processing between sensory inputs and our semantic networks enables us to recognize familiar objects within a few milliseconds, even if they show the complexity of human faces ( Locher et al., 1993 ; Willis and Todorov, 2006 ; Carbon, 2011b ).

Top-down processes are powerful in schematizing and easing-up perceptual processes in the sense of compressing the “big data” of the sensory inputs towards tiny data packages with pre-categorized labels on such schematized “icons” ( Carbon, 2008 ). Top-down processes, however, are also susceptible to characteristic fallacies or illusions due to their guided, model-based nature: When we have only a brief time slot for a snapshot of a complex scene, the scene is (if we have associations with the general meaning of the inspected scene at all) so simplified that specific details get lost in favor of the processing and interpretation of the general meaning of the whole scene.

Biederman (1981) impressively demonstrated this by exposing participants to a sketch drawing of a typical street scene where typical objects are placed in a prototypical setting, with the exception that a visible hydrant in the foreground was not positioned on the pavement besides a car but unusually directly on the car. When people were exposed to such a scene for only 150 ms, followed by a scrambled backward mask, they “re-arranged” the setting by top-down processes based on their knowledge of hydrants and their typical positions on pavements. In this specific case, people have indeed been deceived, because they report a scene which was in accordance with their knowledge but not with the assessment of the presented scene—but for everyday actions this seems unproblematic. Although you might indeed lose the link to the fine-detailed structure of a specific entity when strongly relying on top-down processes, such an endeavor works quite brilliantly in most cases as it is a best guess estimation or approximation—it works particularly well when we are running out of resources, e.g., when we are in a specific mode of being pressed for time and/or you are engaged in a series of other cognitive processes. Actually, such a mode is the standard mode in everyday life. However, even if we had the time and no other processes needed to be executed, we would not be able to adequately process the big data of the sensory input.

The whole idea of this top-down processing with schematized perception stems from F. C. Bartlett’s pioneering series of experiments in a variety of domains ( Bartlett, 1932 ). Bartlett already showed that we do not read the full information from a visual display or a narrative, but that we rely on schemata reflecting the essence of things, stories, and situations being strongly shaped by prior knowledge and its specific activation (see for a critical reflection of Bartlett’s method Carbon and Albrecht, 2012 ).

Perception as a Grand Illusion

Reconstructing human psychological reality.

There is clearly an enormous gap between the big data provided by the external world and our strictly limited capacity to process them. The gap widens even further when taking into account that we not only have to process the data but ultimately have to make clear sense of the core of the given situation. The goal is to make one (and only one) decision based on the unambiguous interpretation of this situation in order to execute an appropriate action. This very teleological way of processing needs inhibitory capabilities for competing interpretations to strictly favor one single interpretation which enables fast action without quarrelling about alternatives. In order to realize such a clear interpretation of a situation, we need a mental model of the external world which is very clear and without ambiguities and indeterminacies. Ideally, such a model is a kind of caricature of physical reality: If there is an object to be quickly detected, the figure-ground contrast, e.g., should be intensified. If we need to identify the borders of an object under unfavorable viewing conditions, it is helpful to enhance the transitions from one border to another, for instance. If we want to easily diagnose the ripeness of a fruit desired for eating, it is most helpful when color saturation is amplified for familiar kinds of fruits. Our perceptual system has exactly such capabilities of intensifying, enhancing and amplifying—the result is the generation of schematic, prototypical, sketch-like perceptions and representations. Any metaphor for perception as a kind of tool which makes photos is fully misleading because perception is much more than blueprinting: it is a cognitive process aiming at reconstructing any scene at its core.

All these “intelligent perceptual processes” can most easily be demonstrated by perceptual illusions: For instance, when we look at the inner horizontal bar of Figure 4 , we observe a continuous shift from light to dark gray and from left to right, although there is no physical change in the gray value—in fact only one gray value is used for creating this region. The illusion is induced by the distribution of the peripheral gray values which indeed show a continuous shift of gray levels, although in a reverse direction. The phenomenon of simultaneous contrast helps us to make the contrast clearer; helping us to identify figure-ground relations more easily, more quickly and more securely.


Figure 4. Demonstration of the simultaneous contrast, an optical illusion already described as phenomenon 200 years ago by Johan Wolfgang von Goethe and provided in high quality and with an intense effect by McCourt (1982) : the inner horizontal bar is physically filled with the same gray value all over, nevertheless, the periphery with its continuous change of gray from darker to lighter values from left to right induce the perception of a reverse continuous change of gray values . The first one who showed the effect in a staircase of grades of gray was probably Ewald Hering (see Hering, 1907 ; pp. I. Teil, XII. Kap. Tafel II), who also proposed the theory of opponent color processing.

A similar principle of intensifying given physical relations by the perceptual system is now known as the Chevreul-Mach bands (see Figure 5 ), independently introduced by chemist Michel Eugène Chevreul (see Chevreul, 1839 ) and by physicist and philosopher Ernst Waldfried Josef Wenzel Mach ( Mach, 1865 ). Via the process of lateral inhibition, luminance changes from one bar to another are exaggerated, specifically at the edges of the bars. This helps to differentiate between the different areas and to trigger edge-detection of the bars.


Figure 5. Chevreul-Mach bands. Demonstration of contrast exaggeration by lateral inhibition: although every bar is filled with one solid level of gray, we perceive narrow bands at the edges with increased contrast which does not reflect the physical reality of solid gray bars.

Constructing Human Psychological Reality

This reconstructive capability is impressive and helps us to get rid of ambiguous or indeterminate percepts. However, the power of perception is even more intriguing when we look at a related phenomenon. When we analyze perceptual illusions where entities or relations are not only enhanced in their recognizability but even entirely constructed without a physical correspondence, then we can quite rightly speak of the “active construction” of human psychological reality. A very prominent example is the Kanizsa triangle (Figure 6 ) where we clearly perceive illusory contours and related Gestalts—actually, none of them exists at all in a physical sense. The illusion is so strong that we have the feeling of being able to grasp even the whole configuration.


Figure 6. Demonstration of illusory contours which create the clear perception of Gestalts . The so-called Kanizsa triangle named after Gaetano Kanizsa (see Kanizsa, 1955 ), a very famous example of the long tradition of such figures displayed over centuries in architecture, fashion and ornamentation. We not only perceive two triangles, but even interpret the whole configuration as one with clear depth, with the solid white “triangle” in the foreground of another “triangle” which stands bottom up.

To detect and recognize such Gestalts is very important for us. Fortunately, we are not only equipped with a cognitive mechanism helping us to perceive such Gestalts, but we also feel rewarded when having recognized them as Gestalts despite indeterminate patterns ( Muth et al., 2013 ): in the moment of the insight for a Gestalt the now determinate pattern gains liking (the so-called “Aesthetic-Aha-effect”, Muth and Carbon, 2013 ). The detection and recognition process adds affective value to the pattern which leads to the activation of even more cognitive energy to deal with it as it now means something to us.


Perceptual illusions can be seen, interpreted and used in two very different aspects: on the one hand, and this is the common property assigned to illusions, they are used to entertain people. They are a part of our everyday culture, they can kill time. On the other hand, they are often the starting point for creating insights. And insights, especially if they are based on personal experiences through elaborative processes actively, are perfect pre-conditions to increase understanding and to improve and optimize mental models ( Carbon, 2010b ). We can even combine both aspects to create an attractive learning context: by drawing people’s attention via arousing and playful illusions, we generate attraction towards the phenomena underlying the illusions. If people get really interested, they will also invest sufficient time and cognitive energy to be able to solve an illusion or to get an idea of how the illusion works. If they arrive at a higher state of insight, they will benefit from understanding what kind of perceptual mechanism is underlying the phenomenon.

We can of course interpret perceptual illusions as malfunctions indicating the typical limits of our perceptual or cognitive system—this is probably the standard perspective on the whole area of illusions. In this view, our systems are fallible, slow, malfunctioning, and imperfect. We can, however, also interpret illusory perceptions as a sign of our incredible, highly complex and efficient capabilities of transforming sensory inputs into understanding and interpreting the current situation in a very fast way in order to generate adequate and goal-leading actions in good time (see Gregory, 2009 )—this view is not yet the standard one to be found in beginners’ text books and typical descriptions or non-scientific papers on illusions. By taking into account how perfectly we act in most everyday situations, we can experience the high “intelligence” of the perceptual system quite easily and intuitively. We might not own the most perfect system when we aim to reproduce the very details of a scene, but we can assess the core meaning of a complex scene.

Typical perceptual processes work so brilliantly that we can mostly act appropriately, and, very important for a biological system, we can act in response to the sensory inputs very fast—this has to be challenged by any technical, man-made system, and will always be the most important benchmark for artificial perceptual systems. Following the research and engineering program of bionics ( Xie, 2012 ),where systems and processes of nature are transferred to technical products, we might be well-advised to orient our developments in the field of perception to the characteristic processing of biological perceptual systems, and their typical behavior when perceptual illusions are encountered.

Conflict of Interest Statement

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


This paper was strongly inspired by Richard L. Gregory’s talks, texts and theories which I particularly enjoyed during the first years of my research career. The outcome of these “perceptions” changed my “perception on reality” and so on “reality” as such. I would also like to thank two anonymous reviewers who put much effort in assisting me to improve a previous version of this paper. Last but not least I want to express my gratitude to Baingio Pinna, University of Sassari, who edited the whole Research Topic together with Adam Reeves, Northeastern University, USA.

  • ^ There is an interesting update in technology for demonstrating this effect putting forward by one of the reviewers. If you use the 2nd camera of your smartphone (the one for shooting “selfies”) or your notebook camera and you look at your depicted eyes very closely, then the delay of building up the film sequence is seemingly a bit longer than the saccadic suppression yielding the interesting effect of perceiving your own eye movements directly. Note: I have tried it out and it worked, by the way best when using older models which might take longer for building up the images. You will perceive your eye movements particular clearly when executing relatively large saccades, e.g., from the left periphery to the right and back.

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Keywords: optical illusion, delusion, deception, reality, perception, representation, validity, truth

Citation: Carbon C-C (2014) Understanding human perception by human-made illusions. Front. Hum. Neurosci. 8 :566. doi: 10.3389/fnhum.2014.00566

Received: 01 June 2014; Accepted: 11 July 2014; Published online: 31 July 2014.

Reviewed by:

Copyright © 2014 Carbon. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Claus-Christian Carbon, Department of General Psychology and Methodology, University of Bamberg, Markusplatz 3, D-96047 Bamberg, Germany e-mail: [email protected]

This article is part of the Research Topic

The Future of Perceptual Illusions: From Phenomenology to Neuroscience

Perceptual Set In Psychology: Definition & Examples

Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

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Perceptual set in psychology refers to a mental predisposition or readiness to perceive stimuli in a particular way based on previous experiences, expectations, beliefs, and context. It influences how we interpret and make sense of sensory information, shaping our perception and understanding of the world.

Perceptual set theory stresses the idea of perception as an active process involving selection, inference, and interpretation (known as top-down processing ).

The concept of perceptual set is important to the active process of perception.  Allport (1955) defined perceptual set as:

“A perceptual bias or predisposition or readiness to perceive particular features of a stimulus.”

Perceptual set is a tendency to perceive or notice some aspects of the available sensory data and ignore others.  According to Vernon, 1955 perceptual set works in two ways:

  • The perceiver has certain expectations and focuses attention on particular aspects of the sensory data: This he calls a Selector”.
  • The perceiver knows how to classify, understand and name selected data and what inferences to draw from it. This she calls an “Interpreter”.

It has been found that a number of variables, or factors, influence perceptual set, and set in turn influences perception. The factors include:

• Expectations • Emotion • Motivation • Culture

Expectation and Perceptual Set

(a) Bruner & Minturn (1955) illustrated how expectation could influence set by showing participants an ambiguous figure “13” set in the context of letters or numbers e.g.

percpetual set Bruner Minturn

The physical stimulus “13” is the same in each case but is perceived differently because of the influence of the context in which it appears. We EXPECT to see a letter in the context of other letters of the alphabet, whereas we EXPECT to see numbers in the context of other numbers.

(b) We may fail to notice printing/writing errors for the same reason. For example:

1. “The Cat Sat on the Map and Licked its Whiskers”.

percpetual set

(a) and (b) are examples of interaction between expectation and past experience.

(c) A study by Bugelski and Alampay (1961) using the “rat-man” ambiguous figure also demonstrated the importance of expectation in inducing set. Participants were shown either a series of animal pictures or neutral pictures prior to exposure to the ambiguous picture. They found participants were significantly more likely to perceive the ambiguous picture as a rat if they had had prior exposure to animal pictures.

percpetual set expectation

Motivation / Emotion and Perceptual Set

Allport (1955) has distinguished 6 types of motivational-emotional influence on perception:

(i) bodily needs (e.g. physiological needs) (ii) reward and punishment (iii) emotional connotation (iv) individual values (v) personality (vi) the value of objects.

(a) Sandford (1936) deprived participants of food for varying lengths of time, up to 4 hours, and then showed them ambiguous pictures. Participants were more likely to interpret the pictures as something to do with food if they had been deprived of food for a longer period of time.

Similarly Gilchrist & Nesberg (1952), found participants who had gone without food for the longest periods were more likely to rate pictures of food as brighter. This effect did not occur with non-food pictures.

(b) A more recent study into the effect of emotion on perception was carried out by Kunst- Wilson & Zajonc (1980). Participants were repeatedly presented with geometric figures, but at levels of exposure too brief to permit recognition.

Then, on each of a series of test trials, participants were presented a pair of geometric forms, one of which had previously been presented and one of which was brand new.  For each pair, participants had to answer two questions: (a) Which of the 2 had previously been presented? ( A recognition test); and (b) Which of the two was most attractive? (A feeling test).

The hypothesis for this study was based on a well-known finding that the more we are exposed to a stimulus, the more familiar we become with it and the more we like it.  Results showed no discrimination on the recognition test – they were completely unable to tell old forms from new ones, but participants could discriminate on the feeling test, as they consistently favored old forms over new ones. Thus information that is unavailable for conscious recognition seems to be available to an unconscious system that is linked to affect and emotion.

Culture and Perceptual Set

percpetual set culture

Elephant drawing split-view and top-view perspective. The split elephant drawing was generally preferred by African children and adults .

(a) Deregowski (1972) investigated whether pictures are seen and understood in the same way in different cultures. His findings suggest that perceiving perspective in drawings is in fact a specific cultural skill, which is learned rather than automatic. He found people from several cultures prefer drawings which don”t show perspective, but instead are split so as to show both sides of an object at the same time.

In one study he found a fairly consistent preference among African children and adults for split-type drawings over perspective-drawings. Split type drawings show all the important features of an object which could not normally be seen at once from that perspective. Perspective drawings give just one view of an object. Deregowski argued that this split-style representation is universal and is found in European children before they are taught differently.

(b) Hudson (1960) noted difficulties among South African Bantu workers in interpreting depth cues in pictures. Such cues are important because they convey information about the spatial relationships among the objects in pictures. A person using depth cues will extract a different meaning from a picture than a person not using such cues.

Hudson tested pictorial depth perception by showing participants a picture like the one below. A correct interpretation is that the hunter is trying to spear the antelope, which is nearer to him than the elephant. An incorrect interpretation is that the elephant is nearer and about to be speared. The picture contains two depth cues: overlapping objects and known size of objects. Questions were asked in the participants native language such as:

What do you see? Which is nearer, the antelope or the elephant? What is the man doing?

The results indicted that both children and adults found it difficult to perceive depth in the pictures.

percpetual set culture

The cross-cultural studies seem to indicate that history and culture play an important part in how we perceive our environment. Perceptual set is concerned with the active nature of perceptual processes and clearly there may be a difference cross-culturally in the kinds of factors that affect perceptual set and the nature of the effect.

Allport, F. H. (1955). Theories of perception and the concept of structure . New York: Wiley.

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Bugelski, B. R., & Alampay, D. A., (1961). The role of frequency in developing perceptual sets. Canadian Journal of Psychology , 15, 205-211.

Deregowski, J. B., Muldrow, E. S. & Muldrow, W. F. (1972). Pictorial recognition in a remote Ethiopian population. Perception , 1, 417-425.

Gilchrist, J. C.; Nesberg, Lloyd S. (1952). Need and perceptual change in need-related objects. Journal of Experimental Psychology , Vol 44(6).

Hudson, W. (1960). Pictorial depth perception in sub-cultural groups in Africa. Journal of Social Psychology , 52, 183-208.

Kunst- Wilson, W. R., & Zajonc, R. B. (1980). Affective discrimination of stimuli that cannot be recognised. Science , Vol 207, 557-558.

Necker, L. (1832). LXI. Observations on some remarkable optical phenomena seen in Switzerland; and on an optical phenomenon which occurs on viewing a figure of a crystal or geometrical solid . The London and Edinburgh Philosophical Magazine and Journal of Science, 1 (5), 329-337.

Sanford, R. N. (1936). The effect of abstinence from food upon imaginal processes: a preliminary experiment. Journal of Psychology: Interdisciplinary and Applied , 2, 129-136.

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Why people should be skeptical when evaluating the accuracy of their perceptual set?

People should be skeptical when evaluating the accuracy of their perceptual set because it can lead to biased and subjective interpretations of reality. It can limit our ability to consider alternative perspectives or recognize new information that challenges our beliefs. Awareness of our perceptual sets and actively questioning them allows for more open-mindedness, critical thinking, and a more accurate understanding of the world.

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Key Theories On The Psychology Of Perception

Perception is defined as “ the process or result of becoming aware of objects, relationships, and events by means of the senses, which includes such activities as recognizing, observing, and discriminating.” It allows us to notice and then interpret stimuli around us so we can understand and respond accordingly. While perception may seem simple, it’s actually a complex and highly individualized process with many psychological components and implications. Below, we’ll cover the basics of perception psychology along with a few of the leading theories on this topic.

How we perceive the world around us

Let’s start with a brief overview of the basic mechanisms of perception—that is, the ways in which we’re able to perceive the world around us. Scientists now recognize seven senses that humans can use to gather information about our surroundings:

  • Visual perception: sight perceived through the eyes
  • Auditory perception: sounds perceived through the ears
  • Gustatory perception: awareness of flavor and taste on the tongue 
  • Olfactory perception: smelling via the nose
  • Tactile perception: awareness of sensation on the skin
  • Vestibular sense: perception of balance and motion
  • Proprioception: perception of the body’s position in space

A brief introduction to perception psychology

Perception psychology is a division of cognitive psychology that studies how humans receive and understand the information delivered through the senses. As mentioned above, perception is a network of bodily systems and sense organs that receive information and then process it. As we interact with the physical world, our brains interpret this information to make sense of what we experience. 

Our brains also automatically attempt to group perceptions to help us understand and interpret our world. There are six main principles the human mind uses to organize what it perceives:

grouping things that look like each other. Items with the same shape, size, and/or color make up parts of perceived patterns that appear to belong together.

grouping things according to how physically close they are to each other. The closer together they are, the more likely the brain will identify them as a group—even if they don’t have any connection to each other.

the tendency to perceive individual elements as a whole rather than a series of parts


perceiving all elements of an image before recognizing the parts of it. For example, you may sense a car before recognizing the color, make, or who is inside. 

seeing a partial image and filling in the gaps of what is believed should be there. This ability allows one to overlook a partial understanding and perceive the situation in its entirety despite missing information. 

a tendency to simplify complex stimuli into a simple pattern. An example is looking at a complex building and being aware of the front door while not registering the structure’s many other features.

Main perception psychology theories today

Psychologists and researchers continue to explore the nuances of this complex field. As of today, here’s a brief overview of some of the key perception psychology theories out there. Note that none of these completely explains the process in every instance; this field of study is ongoing.

Perception psychology according to Bruner

Jerome S. Bruner was an American psychologist who theorized that people go through various processes before they form opinions about what they have observed. According to Bruner, people use different informational cues to ultimately define their perceptions. This information-seeking continues until the individual comes across a familiar part and the mind categorizes it. If signals are distorted or do not fit a person’s initial perceptions, the images are forgotten or ignored while a picture forms on the most familiar perceptions. 

Perception psychology according to Gibson

James J. Gibson is another American psychologist who studied perception psychology. Gibson is known for his philosophy of the direct theory of visual perception in particular, also called the “bottom-up” theory. He believed we can explain visual perception solely in terms of the environment, beginning with a sensory stimulus. In each stage of the perceptual process, the eyes send signals to the brain to continue analyzing until it can conclude what the person is seeing.

Gibson theorized that the starting point of visual perception begins with the pattern of light that reaches our eyes. These signals then form the basis of our understanding of perceptions because they convey unambiguous information about the spatial layout we perceive. He further defined perception according to what he called affordances. He identified six affordances of perception, including:

  • Optical array: the patterns of light that travel from the environment to the eyes
  • Relative brightness: the perception that brighter, more evident objects are closer than darker, out-of-focus objects
  • Texture gradient: The grain of texture becomes less defined as an object recedes, indicating that the object may be further in the distance.
  • Relative size: Objects that are farther away will appear smaller.
  • Superimposition: When one image partially blocks another, the viewer sees the first image as being closer to them. Superimposition is similar to inattentional blindness , in which the eye cannot see an object because another object fully engages it.
  • Height in the visual field: Objects that are further away from the viewer typically appear higher in the visual field.

Perception psychology according to Gregory

Richard Langton Gregory was a British psychologist and Emeritus Professor of Neuropsychology at the University of Bristol. Gregory was also the author of the constructivist theory of perception, or the "top-down" theory—which takes the opposite approach of Gibson’s “bottom-up” theory. It assumes that our cognitive processes—including memory and perception—result from our continuously generating hypotheses about the world from the top down. In other words, we recognize patterns by understanding the context in which we perceive them. 

Consider handwriting as an example. The handwriting of many individuals can be difficult for others to read; however, if we can pick out a few words here or there, it helps us understand the text’s context, and that helps us figure out the words we could not read. In other words, Gregory's theory assumes we have previous knowledge of what we are perceiving in addition to the stimulus itself. Because stimuli can often be ambiguous, correctly perceiving it requires a higher level of cognition because we must draw from stored knowledge or past experiences to help us understand our perceptions. He believed perception is based on our accumulated knowledge, and that we actively construct perceptions whether they’re correct or not—though an incorrect hypothesis can lead to errors in perception.

Exploring perception with a therapist

The way we perceive objects, individuals, events, and our environment can have a significant impact on our mood, emotions, and behaviors. In some cases, our perceptions can be distorted, which can lead to distressing feelings or even symptoms of a mental health condition like depression or anxiety. Talk therapy— cognitive behavioral therapy (CBT) in particular—is one way to learn how to recognize any cognitive distortions you may be experiencing and shift your thoughts in a more realistic, balanced, and healthy direction. 

Regularly attending in-person therapy sessions is not possible for everyone. Some may not have adequate provider options in their area, while others may have trouble commuting to and from in-office sessions. In cases like these, online therapy can represent a viable alternative. A platform like BetterHelp can match you with a licensed therapist who you can meet with via video, phone, and/or in-app messaging, all from the comfort of home. Research suggests that virtual therapy is “no less efficacious” than the in-person variety in many cases, so you can generally feel confident in selecting whichever format may work best for you.

What are examples of perception in psychology?

Some examples of types of perception include taste perception, such as being able to identify various flavors in what you’re eating, or visual perception, such as being able to identify and distinguish between a rock, a tree, and a flower. 

What is the simple definition of perception?

The simple, specific meaning of perception is how we use our five senses—plus our senses of balance and our perception of our own body position—to experience the world around us. Perception involves actions like seeing, touching, tasting, and smelling in order to take in our surroundings and then using automatic neural processing to make sense of them.

What are the 4 stages of perception?

The perception process involves four basic stages. First, the individual is exposed to a stimulus through their environment and becomes aware of it through one or more of their perception skills, or senses. Second, their brain registers the stimulus based on the information gathered through the sense(s). Next, the information is organized based on a person’s existing knowledge and beliefs. Finally, the person interprets the stimulus based on their own knowledge and beliefs, such as a good or bad smell, a dangerous or non-dangerous animal, a pleasant or grating sound, etc. 

What is perceptual psychology simple?

Perceptual psychology is made up of various theories from studies over the years about why and how we take in information from the environments around us and perceive things in a certain way. There are many elements that go into why a person may perceive something the way they do, such as existing knowledge, beliefs, culture, and even mental health. Perceptual psychologists study these unconscious processes that contribute to a person’s perception. 

What are 4 examples of perception?

Perception refers to how we see and make sense of the world around us. Four examples include seeing a sunset, smelling a fragrant flower, hearing music playing, and touching a soft blanket.

What is an example of perception in human behavior?

Perception is how our sensory organs detect or perceive stimuli in our surroundings. An example of perception as it relates to human behavior is two people seeing a dog and reacting differently based on their past experiences, knowledge, and beliefs. One might react in fear because they were chased by a dog as a child or in disgust because they think all dogs smell bad. The other might react in excitement and go to interact with the dog because they have a beloved dog of their own at home.

What is an example of perception effect?

There are several different perception effects the human mind uses to categorize, organize, and make sense of the world, and of which we are typically not consciously aware. For example, we’re likely to unconsciously group things that resemble each other—such as objects of the same shape, size, or color—because our brain tells us that they belong together.

What are the 3 factors that influence perception?

There are many different factors that can affect perception, so much so that the field of perception psychology, a type of social psychology, is devoted to examining and understanding them. A few examples of factors that could influence the way an individual perceives something include past experiences, prior knowledge, and cultural values.

What is an example of perception and personality?

The way we perceive words and sounds, sights, smells, tastes, and other forms of stimuli is influenced by our personality. For example, the words one person perceives through auditory signals and then interprets to find offensive may be welcomed by another due to a natural tendency toward humor, optimism/pessimism, etc. 

What is perception and why is it important?

Overall, perception is our ability to identify stimuli in the world around us and interpret it according to our own values, personality, culture, and other factors. It’s important because it’s the means through which we sense and then interpret the world around us. 

  • Exploring Extinction Psychology Medically reviewed by Arianna Williams , LPC, CCTP
  • Is Transpersonal Psychology Right For Me? Medically reviewed by April Justice , LICSW
  • Psychologists
  • Relationships and Relations

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6.1 The Process of Perception

Learning objectives.

By the end of this section, you will be able to:

  • Discuss how salience influences the selection of perceptual information.
  • Explain the ways in which we organize perceptual information.

Perception is the process of selecting, organizing, and interpreting sensory information. This cognitive and psychological process begins with receiving stimuli through our primary senses (vision, hearing, touch, taste, and smell). This information is then passed along to corresponding areas of the brain and organized into our existing structures and patterns, and then interpreted based on previous experiences (Figure 6.1). How we perceive the people and objects around us directly affects our communication. We respond differently to an object or person that we perceive favorably than we do to something or someone we find unfavorable. But how do we filter through the mass amounts of incoming information, organize it, and make meaning from what makes it through our perceptual filters and into our social realities?

Circular graphic showing the three aspects of the process of perception; selection, organization, and interpretation

Selecting Information

We take in information through all five of our senses, but our perceptual field (the world around us) includes so many stimuli that it is impossible for our brains to process and make sense of it all. So, as information comes in through our senses, various factors influence what actually continues on through the perception process (Fiske & Taylor, 1991). Selecting is the first part of the perception process, in which we focus our attention on certain incoming sensory information (Figure 6.2). Think about how, out of many other possible stimuli to pay attention to, you may hear a familiar voice in the hallway, see a pair of shoes you want to buy from across the mall, or smell something cooking for dinner when you get home from work. We quickly cut through and push to the background all kinds of sights, smells, sounds, and other stimuli, but how do we decide what to select and what to leave out?

A group of coworkers talking at a crowded conference.

We tend to pay attention to information that is salient. Salience is the degree to which something attracts our attention in a particular context. The thing attracting our attention can be abstract, like a concept, or concrete, like an object. For example, a person’s identity as a Native American may become salient when they are protesting at the Columbus Day parade in Denver, Colorado. Or a bright flashlight shining in your face while camping at night is sure to be salient. The degree of salience depends on three features (Fiske & Tayor, 1991). We tend to find things salient when they are visually or aurally stimulating, they meet our needs or interests, or when they do or don’t meet our expectations.

Visual and Aural Stimulation

It is probably not surprising to learn that visually and/or aurally stimulating things become salient in our perceptual field and get our attention. Creatures ranging from fish to hummingbirds are attracted to things like silver spinners on fishing poles or red and yellow bird feeders. Having our senses stimulated isn’t always a positive thing though. Think about the couple that won’t stop talking during the movie or the upstairs neighbor whose subwoofer shakes your ceiling at night. In short, stimuli can be attention-getting in a productive or distracting way. However, we can use this knowledge to our benefit by minimizing distractions when we have something important to say. It’s probably better to have a serious conversation with a significant other in a quiet place rather than a crowded food court.

Needs and Interests

We tend to pay attention to information that we perceive to meet our needs or interests in some way. This type of selective attention can help us meet instrumental needs and get things done. When you need to speak with a financial aid officer about your scholarships and loans, you sit in the waiting room and listen for your name to be called. Paying close attention to whose name is called means you can be ready to start your meeting and hopefully get your business handled. When we don’t think certain messages meet our needs, stimuli that would normally get our attention may be completely lost. Imagine you are in the grocery store and you hear someone say your name. You turn around, only to hear that person say, “Finally! I said your name three times. I thought you forgot who I was!” A few seconds before, when you were focused on figuring out which kind of orange juice to get, you were attending to the various pulp options to the point that you tuned other stimuli out, even something as familiar as the sound of someone calling your name. We select and attend to information that meets our needs.

We also find information salient that interests us. Of course, many times, stimuli that meet our needs are also interesting, but it’s worth discussing these two items separately because sometimes we find things interesting that don’t necessarily meet our needs (Figure 6.3). I’m sure we’ve all gotten sucked into a television show, video game, or random project and paid attention to that at the expense of something that actually meets our needs like cleaning or spending time with a significant other. Paying attention to things that interest us but don’t meet specific needs seems like the basic formula for procrastination that we are all familiar with.

Teenager holding a controller, playing a video game.

In many cases we know what interests us and we automatically gravitate toward stimuli that match up with that. For example, as you filter through radio stations, you likely already have an idea of what kind of music interests you and will stop on a station playing something in that genre while skipping right past stations playing something you aren’t interested in. Because of this tendency, we often have to end up being forced into or accidentally experiencing something new in order to create or discover new interests. For example, you may not realize you are interested in Asian history until you are required to take such a course and have an engaging professor who sparks that interest in you. Or you may accidentally stumble on a new area of interest when you take a class you wouldn’t otherwise because it fits into your schedule. As communicators, you can take advantage of this perceptual tendency by adapting your topic and content to the interests of your audience.


The relationship between salience and expectations is a little more complex. Basically, we can find expected things salient and find things that are unexpected salient. While this may sound confusing, a couple examples should illustrate this point. If you are expecting a package to be delivered, you might pick up on the slightest noise of a truck engine or someone’s footsteps approaching your front door. Since we expect something to happen, we may be extra tuned in to clues that it is coming. In terms of the unexpected, if you have a shy and soft-spoken friend who you overhear raising the volume and pitch of their voice while talking to another friend, you may pick up on that and assume that something out of the ordinary is going on. For something unexpected to become salient, it has to reach a certain threshold of difference. If you walked into your regular class and there were one or two more students there than normal, you may not even notice. If you walked into your class and there was someone dressed up as a wizard, you would probably notice. So, if we expect to experience something out of the routine, like a package delivery, we will find stimuli related to that expectation salient. If we experience something that we weren’t expecting and that is significantly different from our routine experiences, then we will likely find it salient.

There is a middle area where slight deviations from routine experiences may go unnoticed because we aren’t expecting them. To go back to the earlier example, if you aren’t expecting a package, and you regularly hear vehicle engines and sidewalk foot traffic outside your house, those pretty routine sounds wouldn’t be as likely to catch your attention, even if it were slightly more or less traffic than expected. This is because our expectations are often based on previous experience and patterns we have observed and internalized, which allows our brains to go on “autopilot” sometimes and fill in things that are missing or overlook extra things. Look at the following sentence and read it aloud:

Percpetoin is bsaed on pateetrns, maening we otfen raech a cocnlsuion witouht cosnidreing ecah indviidaul elmenet.

This example illustrates a test of our expectation and an annoyance to every college student. We have all had the experience of getting a paper back with typos and spelling errors circled. This can be frustrating, especially if we actually took the time to proofread. When we first learned to read and write, we learned letter by letter. A teacher or parent would show us a card with A-P-P-L-E written on it, and we would sound it out. Over time, we learned the patterns of letters and sounds and could see combinations of letters and pronounce the word quickly. Since we know what to expect when we see a certain pattern of letters, and know what comes next in a sentence since we wrote the paper, we don’t take the time to look at each letter as we proofread. This can lead us to overlook common typos and spelling errors, even if we proofread something multiple times. Now that we know how we select stimuli, let’s turn our attention to how we organize the information we receive.

Organizing Information

Organizing is the second part of the perception process, in which we sort and categorize information that we perceive based on innate and learned cognitive patterns. Three ways we sort things into patterns are by using proximity, similarity, and difference (Coren, 1980).

In terms of proximity, we tend to think that things that are close together go together (Figure 6.4). For example, have you ever been waiting to be helped in a business and the clerk assumes that you and the person standing near you are together? The moment usually ends when you and the other person in line look at each other, then back at the clerk, and one of you explains that you are not together. Even though you may have never met that other person in your life, the clerk used a basic perceptual organizing cue to group you together because you were standing in proximity to one another.

Chart of coffee beans grouped by different varieties.

We also group things together based on similarity. We tend to think similar-looking or similar-acting things belong together. For example, a group of friends that spend time together are all males, around the same age, of the same race, and have short hair. People might assume that they are brothers. Despite the fact that many of their features are different, the salient features are organized based on similarity and they are assumed to be related (Figure 6.5).

Group of friends taking selfie in a field.

We also organize information that we take in based on difference. In this case, we assume that the item that looks or acts different from the rest doesn’t belong with the group (Figure 6.6). For example, if you ordered ten burgers and nine of them are wrapped in paper and the last is in a cardboard container, you may assume that the burger in the container is different in some way. Perceptual errors involving people and assumptions of difference can be especially awkward, if not offensive. Have you ever attended an event, only to be mistaken as an employee working at the event, rather than a guest at the event?

Jelly beans sorted into different containers based on flavor.

These strategies for organizing information are so common that they are built into how we teach our children basic skills and how we function in our daily lives. I’m sure we all had to look at pictures in grade school and determine which things went together and which thing didn’t belong. If you think of the literal act of organizing something, like your desk at home or work, we follow these same strategies. If you have a bunch of papers and mail on the top of your desk, you will likely sort papers into separate piles for separate classes or put bills in a separate place than personal mail. You may have one drawer for pens, pencils, and other supplies and another drawer for files. In this case you are grouping items based on similarities and differences. You may also group things based on proximity, for example, by putting financial items like your checkbook, a calculator, and your pay stubs in one area so you can update your budget efficiently. In summary, we simplify information and look for patterns to help us more efficiently communicate and get through life.

Simplification and categorizing based on patterns aren’t necessarily a bad thing. In fact, without this capability we would likely not have the ability to speak, read, or engage in other complex cognitive/behavioral functions. Our brain innately categorizes and files information and experiences away for later retrieval, and different parts of the brain are responsible for different sensory experiences. In short, it is natural for things to group together in some ways. There are differences among people, and looking for patterns helps us in many practical ways. However, the judgments we place on various patterns and categories are not natural; they are learned and culturally and contextually relative. Our perceptual patterns do become unproductive and even unethical when the judgments we associate with certain patterns are based on stereotypical or prejudicial thinking.

We also organize interactions and interpersonal experiences based on our firsthand experiences. Misunderstandings and conflict may result when two people experience the same encounter differently. Punctuation refers to the structuring of information into a timeline to determine the cause (stimulus) and effect (response) of our communication interactions (Sillars, 1980). Applying this concept to interpersonal conflict can help us see how the process of perception extends beyond the individual to the interpersonal level. This concept also helps illustrate how organization and interpretation can happen together and how interpretation can influence how we organize information and vice versa.

Where does a conflict begin and end? The answer to this question depends on how the people involved in the conflict punctuate, or structure, their conflict experience. Punctuation differences can often escalate conflict, which can lead to a variety of relationship problems (Watzlawick, Bavelas, & Jackson, 1967). For example, Linda and Joe are on a project team at work and have a deadline approaching. Linda has been working on the project over the weekend in anticipation of her meeting with Joe first thing Monday morning. She has had some questions along the way and has e-mailed Joe for clarification and input, but he hasn’t responded. On Monday morning, Linda walks into the meeting room, sees Joe, and says, “I’ve been working on this project all weekend and needed your help. I e-mailed you three times! What were you doing?” Joe responds, “I had no idea you e-mailed me. I was gone all weekend on a camping trip.” In this instance, the conflict started for Linda two days ago and has just started for Joe. So, for the two of them to most effectively manage this conflict, they need to communicate so that their punctuation, or where the conflict started for each one, is clear and matches up. In this example, Linda made an impression about Joe’s level of commitment to the project based on an interpretation she made after selecting and organizing incoming information. Being aware of punctuation is an important part of perception checking, which we will discuss later. Let’s now take a closer look at how interpretation plays into the perception process.

Interpreting Information

Although selecting and organizing incoming stimuli happens very quickly, and sometimes without much conscious thought, interpretation can be a much more deliberate and conscious step in the perception process. Interpretation is the third part of the perception process, in which we assign meaning to an experience using a mental structure known as schema. A  schema  is a cognitive tool for organizing related concepts or information. Schemata are like databases of stored, related information that we use to interpret new experiences. Overtime we incorporate more and more small units of information together to develop more complex understandings of new information.

We have an overall schema about education and how to interpret experiences with teachers and classmates (Figure 6.7). This schema started developing before we even went to preschool based on things that parents, peers, and the media told us about school. For example, you learned that certain symbols and objects like an apple, a ruler, a calculator, and a notebook are associated with being a student or teacher. You learned new concepts like grades and recess, and you engaged in new practices like doing homework, studying, and taking tests. You also formed new relationships with classmates, teachers, and administrators. As you progressed through your education, your schema adapted to the changing environment. How smooth or troubling schema reevaluation and revision is varies from situation to situation and person to person. For example, some students adapt their schema relatively easily as they move from elementary, to middle, to high school, and on to college and are faced with new expectations for behavior and academic engagement. Other students don’t adapt as easily, and holding onto their old schema creates problems as they try to interpret new information through old, incompatible schema.

An empty college classroom with individual desks.

It’s also important to be aware of schemata because our interpretations affect our behavior. For example, if you are doing a group project for class and you perceive a group member to be shy based on your schema of how shy people communicate, you may avoid giving them presentation responsibilities in your group project because you do not think shy people make good public speakers.

As we have seen, schemata are used to interpret others’ behavior and form impressions about who they are as a person. To help this process along, we often solicit information from people to help us place them into a preexisting schema. In the United States and many other Western cultures, people’s identities are often closely tied to what they do for a living. When we introduce others, or ourselves, occupation is usually one of the first things we mention. Think about how your communication with someone might differ if he or she were introduced to you as an artist versus a doctor. We make similar interpretations based on where people are from, their age, their race, and other social and cultural factors.

In summary, we have schemata about individuals, groups, places, and things, and these schemata filter our perceptions before, during, and after interactions. As schemata are retrieved from memory, they are executed, like computer programs or apps on your smartphone, to help us interpret the world around us. Just like computer programs and apps must be regularly updated to improve their functioning, we update and adapt our schemata as we have new experiences.

  • Perception is the process of selecting, organizing, and interpreting information. This process affects our communication because we respond to stimuli differently, whether they are objects or persons, based on how we perceive them.
  • Given the massive amounts of stimuli taken in by our senses, we only select a portion of the incoming information to organize and interpret. We select information based on salience. We tend to find salient things that are visually or aurally stimulating and things that meet our needs and interests. Expectations also influence what information we select.
  • We organize information that we select into patterns based on proximity, similarity, and difference.
  • We interpret information using schemata, which allow us to assign meaning to information based on accumulated knowledge and previous experience.

Discussion Questions

  • Take a moment to look around wherever you are right now. Take in the perceptual field around you. What is salient for you in this moment and why? Explain the degree of salience using the three reasons for salience discussed in this section.
  • As we organize information (sensory information, objects, and people) we simplify and categorize information into patterns. Identify some cases in which this aspect of the perception process is beneficial. Identify some cases in which it could be harmful or negative.
  • Think about some of the schemata you have that help you make sense of the world around you. For each of the following contexts—academic, professional, personal, and civic—identify a schema that you commonly rely on or think you will rely on. For each schema you identified note a few ways that it has already been challenged or may be challenged in the future.

Remix/Revisions featured in this section

  • Small editing revisions to tailor the content to the Psychology of Human Relations course.
  • Added and changed some images as well as changed formatting for photos to provide links to locations of images and CC licenses.
  • Added doi links to references to comply with APA 7 th edition formatting reference manual.


CC Licensed Content, Original Modification, adaptation, and original content.  Provided by : Stevy Scarbrough. License : CC-BY-NC-SA

CC Licensed Content Shared Previously Communication in the Real World. Authored by: University of Minnesota. Located at:   https://open.lib.umn.edu/communication/chapter/2-1-perception-process/ License: CC-BY-NC-SA 4.0

Coren, S. (1980). Principles of perceptual organization and spatial distortion: The Gestalt illusions.  Journal of Experimental Psychology: Human Perception and Performance,  6(3) 404–12. https://doi.org/10.1037/0096-1523.6.3.404

Fiske, S. T., & Taylor, S. E. (1991).  Social Cognition,  2nd ed. New York, NY: McGraw Hill.

Payne, B. K. (2001). Prejudice and perception: The role of automatic and controlled processes in misperceiving a weapon. Journal of Personality and Social Psychology,  81(2) 181–92. https://doi.org/10.1037/0022-3514.81.2.181

Rozelle, R. M. & Baxter, J. C. (1975). Impression formation and danger recognition in experienced police officers. Journal of Social Psychology, 96 (1), 53-63. https://doi.org/10.1080/00224545.1975.9923262

Sillars, A. L. (1980). Attributions and communication in roommate conflicts. roommate Conflicts.  Communication Monographs,  47(3), 180–200. https://doi.org/10.1080/03637758009376031

Watzlawick, P., Bavelas, J. B., & Jackson, D. D. (1967).  Pragmatics of human communication: A study of interactional patterns, pathologies, and paradoxes. New York, NY: W. W. Norton.

Psychology of Human Relations Copyright © by Stevy Scarbrough is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Action-Specific Influences on Perception and Post-Perceptual Processes: Present Controversies and Future Directions

John w. philbeck.

1 Department of Psychology, The George Washington University, Washington, DC

2 School of Psychology, University of Wollongong, Wollongong, NSW

Jessica K. Witt

3 Department of Psychology, Colorado State University, Fort Collins, CO

The action-specific perception account holds that people perceive the environment in terms of their ability to act in it. In this view, for example, decreased ability to climb a hill due to fatigue makes the hill visually appear to be steeper. Though influential, this account has not been universally accepted, and in fact a heated controversy has emerged. The opposing view holds that action capability has little or no influence on perception. Heretofore, the debate has been quite polarized, with efforts largely being focused on supporting one view and dismantling the other. We argue here that polarized debate can impede scientific progress and that the search for similarities between two sides of a debate can sharpen the theoretical focus of both sides and illuminate important avenues for future research. In this paper, we present a synthetic review of this debate, drawing from the literatures of both approaches, to clarify both the surprising similarities and the core differences between them. We critically evaluate existing evidence, discuss possible mechanisms of action-specific effects, and make recommendations for future research. A primary focus of future work will involve not only the development of methods that guard against action-specific post-perceptual effects, but also development of concrete, well-constrained underlying mechanisms. The criteria for what constitutes acceptable control of post-perceptual effects and what constitutes an appropriately specific mechanism vary between approaches, and bridging this gap is a central challenge for future research.

According to the action-specific account of perception, people perceive spatial properties of the environment in terms of their ability to act in it ( Proffitt, 2006 ; Witt, 2011a ). For example, people wearing a heavy backpack judge objects as being farther away than people who are not wearing a backpack ( Proffitt et al., 2003 ). According to this account, an object’s geometrical properties (for example, its size, shape, or distance) can be perceived differently depending on the perceiver’s ability to perform an intended action, even when the visual information about the object is exactly the same. Presumably, objects appear farther away because the heavy backpack increases the energetic demands involved in walking to them. Evidence for the action-specific account of perception comes from a variety of experimental paradigms. For example, the ability to reach to targets influences estimated distance to the targets ( Kirsch, Herbort, Butz, & Kunde, 2012 ; Kirsch & Kunde, 2013a ; Osiurak, Moragado, & Palluel-Germain, 2012 ; Witt, Proffitt, & Epstein, 2005 ). The energetic costs associated with ascending a hill influence the estimated slant of the hill ( Bhalla & Proffitt, 1999 ). The size of the body, and its associated effects on a person’s ability to act, influences estimates of object size ( Linkenauger, Leyrer, Buelthoff, & Mohler, 2013 ; van der Hoort, Guterstam, & Ehrsson, 2011 ). And performance, or probability of successfully performing an action, influence estimates of the target’s size and speed ( Lee, Lee, Carello, & Turvey, 2012 ; Witt & Proffitt, 2005 ; Witt & Sugovic, 2010 , 2012 ). These linkages between ability and perceptual judgments have been interpreted as evidence that a person’s ability to act influences the perception of geometrical properties of the environment.

The empirical findings and theoretical interpretations of the action-specific approach have given rise to a heated controversy. Much of the debate has centered on the extent to which past evidence of action-specific effects should be conceived as stemming from differences in how the spatial properties of things appear (that is, differences in the underlying perceptual representation) versus differences in output processing (that is, differences in how people select a behavioral response to communicate about their perception; Durgin et al., 2009 ; Firestone, 2013 ; Hutchison & Loomis, 2006 ; Woods, Philbeck & Danoff, 2009 ; Shaffer & Flint, 2011 ). When people don a heavy backpack, for example, do their distance judgments increase because the objects now visually appear to be farther away, or instead because people guess that the experimenters expect them to increase their judgments, and they comply by inflating their responses? This is a fundamental concern that cuts across many domains: if we ask people to describe some aspect of their experience (e.g., their sensory experiences, their cognitions, or their social perceptions), to what degree does their reply actually reflect the experience we want to know about? What factors influence how people communicate about their experiences? What methods may be used to mitigate or control output-related processes to obtain more “pure” measures of the underlying representation of interest? Why does it matter to distinguish between underlying representations and output-related processes, and when might it not matter? These issues are not new to psychological research, but the recent action-specific perception debate has brought them under an intense level of scrutiny. This scrutiny stands to yield new perspectives on perennial issues that are common to so many domains in psychology.

The distinction between the underlying perception and post-perceptual output processes often goes unappreciated. For some, if a person happens to give an accurate verbal judgment of an object’s distance, this is a straightforward indication that perception itself is accurate, and no other psychological processes need be considered. From this perspective, debating the distinction between perceptual vs. output processing has little meaning. For perception researchers, however, discriminating between perceptual and output processing is vitally important, and thus it is critical at the outset to motivate the importance of this distinction. First and foremost, the ability to predict and modify future behavior rests crucially on the accuracy of one’s model of the psychological processes underlying the behavior. Distinguishing perceptual from output-level processes is important for diagnosing and treating neurological disorders, for example. Treatment would proceed quite differently if a patient’s deficit in estimating distances stemmed from difficulty in manipulating numbers (to take one possible output-related process) rather than impaired distance perception. Similarly, in a functional neuroimaging scan, the function of an activated brain region would be interpreted quite differently if a task primarily changes how the participant complies with implicit social demands (to take another output process) rather than changing the visual appearance of an object.

Interventions that seek to improve safety or performance by enhancing visual perception (e.g., in driving, aviation, military, or sports settings) might also be impacted, if the intervention only influencing how people respond in the specific social context of a laboratory experiment, rather than influencing real-world perception when there are no experimenters present. There is agreement among researchers from both sides of the debate that discriminating between perception and output processing is crucial, but they differ strongly in terms of how they feel about the impact of output processing on perceptual judgments: Researchers who have adopted the action-specific perception perspective tend to believe that output processing plays little or no role in explaining past evidence of action-specific effects, while other researchers tend to believe that output processing accounts for most or all of this past evidence. At a more fundamental level, the debate hinges on researchers’ willingness to consider any possible influence of the observer’s transient state of action capability on the perception of geometrical properties of the environment, such as object sizes, object distances, and so forth. We will label the alternative perspectives as “action-specific perception” and “action-resistant perception”, to highlight the degree to which each perspective considers perception to be influenced by action capability.

The literature is currently flooded by one-sided viewpoints championing one perspective over the other (e.g. Durgin et al., 2009 ; Firestone, 2013 ; Proffitt, 2006 , 2009 , 2013 ; Proffitt & Linkenauger, 2013 ; Witt, 2011a , b ; Witt & Sugovic, 2012 , 2013a , b ). The thesis of this paper is that a definitive resolution of this debate must include a nuanced evaluation of the possibility that both positions have explanatory power. Indeed, these positions need not be considered mutually exclusive; the relative contribution of action-specific perception and post-perceptual influences might be dynamic and vary dramatically from situation to situation. This being the case, a satisfying account of the action-specific perception issue must characterize the factors that determine the relative contribution of perceptual effects to output-related effects across different situations, as well as the mechanisms underlying these factors.

One drawback of the current literature is that, although there is an extensive corpus of work investigating post-perceptual influences on behavioral responses, many of these studies were conducted long before the notion of action-specific perception was formulated. Thus, there are no consolidated reviews of this wide-ranging literature that are framed in the context of action-specific perception. Readers new to the debate (and even some intimately familiar with it) may find it difficult to appreciate why post-perceptual processing plays such a prominent role in the contentious scientific conversations surrounding these issues. Because the two perspectives differ so strongly in terms of the role they ascribe to post-perceptual processing, there is a pressing need for a synthetic review of this literature. One goal of this paper is to provide a summary of the work underlying post-perceptual processing that captures why these processes are so salient and compelling for some visual space perception researchers as explanations of action-specific effects. We will also propose an extension of the action- resistant perception view that for the first time explicitly characterizes how this approach might account for bona fide action-specific influences on perception (as opposed to characterizing how it might account for action-specific influences on behavioral responses strictly through output processing).

To lay the groundwork for this endeavor, we will first describe a framework that underlies both the action-resistant perception approach and the action-specific perception approach. We then review various types of output processes that might impact judgments of geometrical properties of the environment. From there, we review methods that might be used to discriminate experimentally between genuine perceptual effects and output processes. We then compare and contrast three classes of mechanisms that could generate genuine action-specific influences on perception. Next, we make recommendations for particularly fruitful and theoretically meaningful research directions for future study. This section identifies a set of “best practice” methodologies that show promise for bilateral acceptance. We conclude with thoughts about the relative strengths of the two approaches and a summary of the insights gained through our emphasis on the similarities between them.

A “Modal Model”

For our starting point, we begin with a characterization of the basic model of perceptual processing that both underlies many space perception approaches (for concrete examples, see Foley [1978 ; 1991 ], and Gogel, 1990 , among a host of others; see Wagner, 2006 for a review) and was the original starting point for the action-specific approach. Many existing theories are designed to account for somewhat different phenomena and thus differ in their specific features. For our purposes, the differences between these theories are relatively unimportant, and as such we will emphasize the global aspects they share in common. The resulting “modal model” is shown in Figure 1 . Although the model presented here is framed in terms of visually perceived distance, a similar organization could apply to other perceptual dimensions (e.g., geographical slant or object size) or other perceptual modalities (e.g., audition). While it is admittedly incomplete, the model is intended to characterize the mutually agreed-upon conceptual distinctions that are crucial for discussing the similarities and differences between approaches and their related controversies.

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A modal model of perception. This model captures many of the structural features that the action-resistant perception and action-specific perception approaches share in common. The model shows the relation between visual cues, non-visual factors, perception, and post-perceptual processing. Dashed lines indicate feedback connections. See text for details.

The major components of the model are as follows. All current theories of visual space perception, including the action-specific perception approach, acknowledge the crucial role of visual information for constructing visual perceptual representations. In our backpack example given earlier, there is no controversy surrounding the idea that visual cues are important for specifying the perceived distance of the target, regardless of whether or not the observer is wearing a backpack. In the case of visually-perceived distance, this role is supported by a vast literature ( Cutting & Vishton, 1995 ; Sedgwick, 1986 ; DaSilva, 1985 ). Various forms of visual information specifying an object’s distance are available to the brain (e.g., absolute disparity and angular declination). Our perceptual experience of distance is based (at least in part) on these sources of information, or “cues”. One way to describe the linkage between the cues and perceptual experience is by characterizing the relative effectiveness of each cue in determining perceived distance. Often, the cues appear to be combined according to a Bayesian weighted averaging rule, with the effectiveness weights being determined by the cue reliability ( Ernst & Banks, 2002 ; Knill, 2007 ; see also Massaro & Friedman, 1990 ). The most reliable cues get weighted most heavily in determining how an object is perceived ( Kersten & Yuille, 2003 ; Landy, Maloney & Johnston, 1995 ). In principle, attention could play a role in determining the effectiveness weights of specific cues ( Gogel & Tietz, 1977 ; Gogel & Sharkey, 1989 ). If a particular cue is unreliable or is simply not present, the observer’s perception is largely determined by a combination of the remaining cues. Variations of this framework can account for many aspects of perceived space and layout, including situations involving disruptions or gaps in the ground surface ( Sinai, Ooi & He, 1998 ), well-lit versus darkened viewing contexts ( Ooi & He, 2007 ) and visible contact with the ground versus no visible ground contact ( Bian, Braunstein & Andersen, 2005 ).

In much of the research underlying the modal model, perceived distance is taken to be an internal conscious experience that can be explicitly reported. As an internal experience, perceived distance can only be measured via some kind of behavior. A variety of behaviors might be used, and each behavior is assumed to be subject to a transformation that maps the internal experience of perceived distance onto a behavioral output. This transformation could introduce bias in the behavioral response with respect to the perceived distance. We will refer to this transformation as post-perceptual or output-level processing. In terms of our earlier backpack example, it is possible that donning a backpack has no effect on the perceived target distance, but instead changes some aspect of processing that occurs “downstream” from perception—how people choose a number to describe the distance they perceive, for instance. This possibility has been a central point of contention in the debate, and we will discuss these issues in more detail shortly.

Finally, there is some provision in the modal model for the overt behavioral indications of distance to feed back to earlier stages of processing. Both perspectives agree that information stored from past perceptual experience can serve as a non-visual factor that shapes future perception (e.g., memories that underlie perceptual learning; Epstein, 1967 ; Kellman & Massey, 2013 ). Similarly, for action-based responses, moving the body can influence the optic flow field, thereby providing additional visual cues for perceiving and acting ( Gibson, 1979 ). From the action-specific perspective, recurrent connections between action-related signals generated during or in anticipation of a behavioral response and non-visual inputs to perception are particularly crucial. The question remains as to the extent of these recurrent connections.

Figure 1 shows only one box for behavioral response, but in fact this model should generalize well to a variety of response types. That is, many explanations for differences that might arise between response types can be framed in terms of existing components of the model. For example, an observer might verbally estimate a target’s distance as 5 m, but walk 6 m if attempting to walk to the target without vision (using the so-called blindwalking response). This could occur because both response types respond to the same value of perceived distance, but differ in the kind of post-perceptual processing that they tend to engage ( Philbeck & Loomis, 1997 ): observers might tend to stop short when walking if they are concerned about stepping on the target, whereas they may tend to be more influenced by their assumed skill at using numbers if giving a verbal report. Alternatively, systematic differences in distance judgments between response types might happen because there are differences in the perception that controls the response (via differences in visual cues, cue weightings, or non-visual factors).

Given the central importance afforded to action capability in the action-specific perception approach, it may seem surprising that verbal reports and visual matching tasks, rather than action-based measures, have heretofore been the primary response modes used when investigating action-specific perceptual effects (e.g. Bhalla & Proffitt, 1999 ; Proffitt et al., 2003 ; Witt et al., 2004 ). A primary reason for this is that manipulating action capability can alter the action response itself in ways that have nothing to do with the underlying perceptual representation. For example, if action capability were manipulated by asking participants to throw a pencil versus an anvil at a target, the pencil could be thrown farther, but it would be a mistake to attribute this result purely to differences in perceived target distance. In addition, some action-based responses, particularly those that involve on-line visual control of rapid, precise movements, are thought to be guided by visual information processed in a dorsal cortical pathway that is anatomically and functionally distinct from a ventral pathway that presumably subserves conscious visual perception ( Milner & Goodale, 1995 ). The action-specific perception approach seeks to explain conscious visual perception, however, and accordingly has focused on responses that are thought to be controlled by the ventral visual stream. For these reasons, much of the action-specific perception work has used verbal reports and visual matching tasks—response modes that are nearly universal in perception work and are often thought to be sensitive to conscious perception ( Da Silva, 1985 ). Nevertheless, developing new action-based measures is an important ongoing effort that stands to significantly broaden the scope of research questions that can be addressed in this domain ( Witt & Sugovic, 2013b ).

Output-Level Factors as Alternative Explanations

As we foreshadowed earlier, discriminating between action-specific influences on perception versus output-level processes is a central issue for both the action-specific and action-resistant approaches. Both acknowledge that output-level effects govern the calibration of overt behavioral responses with respect to the underlying perceived distance. A key difference between approaches, however, concerns their assumptions about the relative involvement of perception-level processes versus output-level processes as an explanation of action-specific effects. The action-specific account places special emphasis on the role of the observer’s task and abilities to perform the intended action for visual perception. Some have suggested that this role is so fundamental that perception itself may be implemented in the brain by engaging the motor control pathways that are likely to be required to act on a target ( Witt & Proffitt, 2008 ; Witt, South, & Sugovic, 2014 ; Witt, Sugovic, & Taylor, 2012 ). Accordingly, in the action-specific perception account, a person’s ability to act directly influences perception itself, and output-level processing is thought to play a negligible role in explaining action-specific response patterns. The action-resistant perception approach, meanwhile, tends to place more emphasis on the possible impact of output-level factors on responses as the driving factor underlying action-specific effects.

For some, the vigor and tenacity with which adherents of the action-resistant perception approach pursue output-level explanations of action-specific effects can be mystifying. After all, evidence of action-specific effects comes from studies that use commonly-accepted methods for studying perception such as magnitude estimation (e.g., verbal reports and blind walking) and psychophysics. These have been interpreted as valid measures of perception in many studies. Given the use of these common methods, why is there such resistance to the idea that action-specific effects reflect genuine differences in perception? Going further, if one assumes that researchers from the action-resistant perception perspective accept these methods uncritically as measures of perception in other research contexts, but then do not accept them in action-specific perception contexts, this apparent double-standard could be seen as an unfounded bias on the part of the critics.

For researchers from the action-resistant perception perspective, however, acceptance of a behavioral response as an indicator of perception is not so uncritical as it might seem. In this view, the possible influence of post-perceptual biases is not something that can be tested and definitively ruled out for all possible contexts. This approach assumes that any behavioral indication of perception is potentially subject to post-perceptual biases—even common measures such as verbal reports and blind walking. It also assumes that the influence of output biases is sensitive to contextual factors: output biases could play a negligible role in one experimental context, but play a larger role in other contexts. Thus, the possibility of bias by output factors must be considered in each experimental context. These assumptions are supported by decades of research that have been devoted to characterizing the kinds of output-level factors that exist and delineating the circumstances under which they are manifest (e.g., Asch, 1955 ; Baird, 1963 ; Carlson, 1977 ; Da Silva, 1985 ; Durgin et al., 2009 ; Epstein, 1963 ; Gilinsky, 1955 ; Gogel, 1974 ; Gogel & Da Silva, 1987 ; Hastorf, 1950 ; Mershon, Kennedy & Falacara, 1977 ; Poulton, 1979 ; Rogers & Gogel, 1975 ; Stevens, 1957 ; Witt & Sugovic, 2013a ; Woods et al., 2009 ). Here, we consider some of the most prominent forms of output-level factors that can account for apparent action-specific perceptual effects. The variety and extent of this evidence provides some motivation for why output factors are at the forefront of action-resistant perception researchers’ minds when evaluating data from action-specific perception experiments.

Demand Characteristics

Certain aspects of the experimental design or experimenter behavior could unintentionally communicate the hypothesis of the experiment ( Orne, 1959 ; Weber & Cook, 1972 ). Such cues to the experimental hypothesis are called demand characteristics. Behavioral experiments involving humans are typically conducted within a social setting (involving experimenters interacting with participants); if participants believe they know the experimental hypotheses, they may feel compelled to produce responses that support those hypotheses, perhaps in order to fulfill an implied social contract ( Durgin et al., 2009 ; Orne, 1962 ). In studies investigating action-specific perceptual effects, a particular concern is that when participants arrive at the experimental setting, they may hold preexisting beliefs about the linkage between action capabilities and their experiences when interacting with the world. If this is true, aspects of the experimental methodology might encourage participants to respond in a way that conforms to this belief, without there being any difference in the underlying perceptual variable under study. For example, based on their own experience, participants may believe that carrying something heavy should make one’s destination “seem” farther away. If someone holding this belief is asked to estimate object distances while carrying a heavy box, he or she may interpret the box as a cue that the responses should be inflated to conform with the hypothesized linkage between heavy objects and distances ( Durgin et al., 2009 ; Proffitt, 2006 ; Woods et al., 2009 ).

Importantly, researchers operating within the action-specific perception perspective have been mindful of possible demand characteristics in their experiments and have taken steps to reduce task demand (for example, by using cover stories and distractor tasks) in even the earliest work in this domain ( Bhalla & Proffitt, 1999 ; Proffitt et al., 1995 ), although not in every case (e.g. Proffitt et al., 2003 ; Witt et al., 2004 ). Even so, uncontrolled cues to the experimental hypotheses can be very difficult to completely eliminate, and thus some task demand might exist despite researchers’ best intentions. Indeed, experimenters’ motivation to search for such influences could be subtly influenced by whether or not these influences stand to support or disconfirm their hypotheses. In the context of the action-specific perception controversy, these thoughts highlight the need for researchers on both sides of the debate to scrutinize their own motivations so as to strike an appropriate balance in considering the possible role of demand characteristics in individual experiments.

Manipulations of action capability by backpack encumberment

One of the earliest paradigms used for studying action-specific perceptual effects is a valuable example of the issues involved in evaluating experimental demand. Bhalla and Proffitt (1999) noted that when participants donned a heavy backpack, their judgments of hill slope were systematically larger compared to judgments made by unencumbered participants. Having attempted to reduce the role of task demands by using a cover story, the authors interpreted this result as stemming from perceptual differences due to the observers’ capability to act on the hill. In this view, because climbing the hill while wearing the backpack would be more effortful, observers visually perceived the hill to be steeper than when they were unencumbered. This paradigm was designed to capture the personal experiences of the researchers, in which they perceived hills to be steeper when hiking with a heavy backpack. This real-world situation strikes the current authors as one in which bona fide action-specific perceptual effects are especially likely to be manifested, if they are manifested anywhere.

Several aspects of the experimental paradigm, however, make it depart from its real-world analog in ways that reasonably would be expected to increase its sensitivity to demand characteristics and reduce its sensitivity to action-specific perceptual effects. For example, unlike in real world situations, observers know they are participating in an experiment, and the experimenter asks them to put on the heavy backpack as part of the experiment. Frank Durgin et al. have argued that in fact demand characteristics are likely to play the primary explanatory role in this paradigm. In one study ( Durgin et al., 2009 ), one group of participants was asked to wear a backpack, but a cover story was given that the reason was to carry special equipment. The intention of this cover story was to make participants think that wearing the backpack had no bearing on the experimental hypotheses. The researchers predicted that when participants judged the slope of a ramp while wearing a backpack, those who were not given the cover story would be more susceptible to task demands and therefore estimate the ramp to be steeper than those who were given the cover story. The results confirmed this prediction.

This study has been criticized on several grounds, namely its use of a short ramp, the potential impact of demand characteristics in the cover story encouraging observers to ignore possible perceptual effects, and differences between the two groups in the felt weight of the backpack ( Proffitt, 2009 ). Subsequent work involving a real hill has replicated the lack of effect for encumberment when a cover story is provided to disguise the purpose of the backpack ( Shaffer, McManama, Swank & Durgin, 2013 ). An additional complication is that cover story manipulations might themselves influence the perceiver’s intention to act—a factor that has been shown to be crucial for eliciting action-specific effects ( Witt et al., 2004 , 2005 , 2010 ). For example, if a cover story makes the backpack seem so incidental that perceivers’ intention to act discounts the backpack completely, then neither approach would predict an effect. Indeed, those in the cover story condition of the first experiment ( Durgin et al., 2009 ) reported that the backpack felt significantly lighter than did those in the no-cover story condition, suggesting that participants given the cover story may have been discounting the backpack. Perceived backpack weight was not assessed in a second, similar experiment ( Shaffer et al., 2013 ).

Even if a suitable cover story could be constructed, there are reasons to question whether this paradigm is sufficiently sensitive to bona fide action-specific effects on perception. One issue is that participants are typically asked to stand on a flat field in front of a hill and have little exposure to walking around with the backpack. This gives participants virtually no time to gain experience with the relative energetic costs associated with climbing the hill with versus without the backpack. Participants are also typically not given an explicit goal of walking up the hill. The presence of action-specific effects on perception are thought to be strongly linked with one’s intention to act ( Witt et al., 2004 , 2005 , 2010 ), and thus if participants do not intend to walk up the hill, one would not expect action-specific perceptual mechanisms to be engaged. Altering the paradigm to correct for these shortcomings runs the risk of increasing the possible involvement of task demands.

In sum, although some contention continues to surround this paradigm, there is clear cause for concern with regards to its susceptibility to demand characteristics. This does not conclusively show that experimental demand (rather than differences in perception) was responsible for past evidence of backpack effects; similarly, this susceptibility to demand in experimental settings does not necessarily rule out bona fide action-specific perceptual differences in the real-world hiking scenario that was the original inspiration of this research domain. Indeed, it is our opinion that if action-specific perceptual effects occur anywhere, they are especially likely to occur under this kind of scenario, in which there are large differences in energetic costs (i.e., due to wearing a heavy backpack and fatigue). It does illustrate, however, that it is particularly difficult in this paradigm to disentangle perceptual effects from post-perceptual influences. To date, the specific paradigms that have been used to assess these effects are not adequate for understanding how people perceive slopes when encumbered.

Compliance with task demand

The impact of demand characteristics in an experiment depends on factors that determine whether or not participants notice the demand, as well as on factors that determine their level of compliance ( Asch, 1955 ; Durgin et al., 2012 ; Shaffer et al., 2013 ; Witt & Sugovic, 2013a ). These factors likely vary from person to person as well as between specific experimental contexts. Although there is a rich literature on the social psychology of compliance more generally (e.g., Cialdini & Goldstein, 2004 ), the issue has only recently been taken up in the context of action-specific manipulations. In principle, any intentional behavior might be subject to demand effects. Durgin et al. have reported evidence of compliance with experimental demand in verbal, visual matching, and action-based judgments (e.g., matching one’s hand orientation to a given geographical slant; Durgin, Klein et al., 2012 ; Durgin, Hajnal et al., 2010 ; Durgin, 2013 ; Li & Durgin, 2009 ).

Presumably, when demand characteristics are driving the results under manipulations of action capability, participants (1) make an inference about the pattern of responses they “should” produce, and (2) modify their responses to match these expectations. With respect to the first assumption, it is important to consider why participants wearing a backpack, for example, would infer that a hill “should” seem steeper. Where would this inference come from other than past experience with hills looking or seeming steeper under loads? Indeed, many action-specific experiments are motivated by real-life experiences in athletes such as baseball players who claim the ball looked as big as a grapefruit after a successful hit, or tennis players who claim that the game seems to move in slow motion when they are “in the zone”.

An alternative view is that an inference that hills “should” seem steeper under loads could come from non-perceptual kinds of past experience. For example, if one is estimating the distance to one’s car when carrying heavy grocery bags, the car’s distance could be encoded and remembered in an abstract, propositional format (“the car is farther than I can comfortably carry these bags”), rather than as a genuine perception that the car visually appeared farther when carrying the bags. One could argue that the affordances provided by the distant car really do change when one is under a load; this being the case, inferences that the car “should” seem farther away under loads could be based on memories of how the affordances of distant objects change under loads. Of course, the action-specific perception account would argue that the bags could indeed make the car visually appear farther way. Nevertheless, the point here is that memory of abstract (though perhaps highly salient) associations could constitute an important source of demand characteristics; the inferences underlying these effects--i.e., inferences about the linkage between effort and geographical slant, egocentric distance, size, and so forth--are certainly based on past experience interacting with the world, but the experience people remember may not necessarily be a perceptual one. Similarly, anecdotal reports of past experiences in the context of athletics may not be founded on perception (as we have defined it here), even though they are expressed in perceptual terms. By contrast, proponents of the action-specific perception approach find such anecdotal reports quite compelling and feel that the simpler interpretation is the one that takes these reports at face value.

With respect to the second assumption mentioned above (that participants modify their responses to match their expectations about the experimental hypotheses), it is important to note that not all participants will modify their responses according to expectations (e.g., Durgin et al. 2009 ). In the classic Asch conformity experiments, for which there was heightened pressure to select an obviously incorrect answer, only 30% of participants adjusted their response accordingly ( Asch, 1955 ). This tendency for only some participants to be compliant has important implications, because the action-resistant and action-specific accounts make unique predictions for how compliant and non-compliant individuals should respond under manipulations of action capability. According to the action-specific account that these manipulations result in genuine perceptual differences, the effects should be apparent not only in individuals who are generally willing to comply, but also in individuals who tend to resist complying. By contrast, if demand characteristics effects are the sole mechanism underlying apparent action-specific effects, these effects should only be apparent in compliant individuals.

In research leveraging this logic, Witt and Sugovic (2013b) examined the effect of blocking ability (defined in terms of the size of a visible paddle in a Pong-like computer game) on speed judgments. Instructions were designed to bias participants’ responses toward reporting faster (or slower) ball speeds. For example, some participants were instructed to be sure to correctly classify all fast balls as fast and to make no errors in classifying any fast balls as slow, whereas other participants were instructed to be sure to correctly classify all slow balls as slow. Participants then judged the speed of balls while attempting to hit them with paddles of various sizes. Afterwards, the participants were divided into “compliant” and “non-compliant” groups according to whether or not the overall pattern of their speed judgments was shifted in the direction suggested by the instructions. In particular, those who were instructed to be sure to classify all fast balls as fast were considered to be compliant if their mean classifications were faster than the group mean, and non-compliant if their classifications were slower (and vice versa for the other group). The results showed that almost all participants reported faster ball speeds when using a smaller paddle, regardless of their measured level of compliance.

Importantly, this pattern held true even for the “non-compliant” group, who demonstrated an unwillingness to comply with the task demands given in the instructions. If they were unwilling to comply with the demand in the instructions, it is unclear why they would feel any more compelled to comply with a task demand relating to a specific linkage between paddle size and ball speed. It is possible that these participants were navigating within a complex hierarchy of compliance rules, such that they felt compelled to comply with some task demands but were unwilling to comply with others. However, a much simpler and more plausible explanation is that the observed linkage between paddle size and reported ball speed was rooted in genuine perceptual effects and had nothing to do with task demands. This presents strong evidence that experimental demand is unlikely to account for this particular action-specific effect. This technique of determining individual compliancy and examining the relationship between compliancy and action-specific effects is one way to examine, and potentially rule out, explanations that focus on demand characteristics.

Evaluation of past paradigms

Some paradigms are more susceptible to task demands than others. Paradigms in which the key experimental manipulation is obvious (such as putting on a heavy backpack or throwing a heavy ball) are particularly problematic. Ultimately it may be impossible to modify such paradigms to rule out an influence of task demands, and as such, we question whether these paradigms can adequately assess any potential perceptual effect. The paradigms that seem to mitigate task demands best are ones for which experimental manipulations are undetectable by participants. One potential example is the use of sugary drinks versus drinks with artificial sweetener. In one such study, participants were not able to differentiate the drinks based on taste, but the group that drank the sugary drink (which presumably increased their available energy by way of increasing their caloric intake) estimated hills to be less steep than did the group that received no calories ( Schnall et al., 2010 ). However, it is not always possible to conceal the manipulations fully and this can have complex effects on the data. For example, Durgin et al. (2012) have reported that even an artificial sweetener manipulation can be detectable for a sizeable subset of participants, and that participants who had fasted for 3 hours and therefore had lowered blood sugar were more susceptible to experimental demand (i.e., judged a hill as steeper) than participants who had not fasted. Shaffer et al. (2013) argue that participants with higher blood sugar may be more capable of resisting experimental demand. One can imagine improving this paradigm via a combination of food science and psychophysics to develop a truly undetectable caloric intake manipulation, but if blood sugar level is indeed linked with resistance to demand, this would call into question this paradigm as a means of mitigating task demands. As such, this link deserves further exploration. For example, one might predict that if it takes energy to resist demand characteristics, a link between blood sugar and compliance should be observed in other experimental paradigms that have been used to study compliance (for example, the foot-in-the-door procedure or the low-ball procedure; Burger, 1999 ; Cialdini, Cacioppo, Bassett & Miller, 1978 ), rather than just action-based settings. If such a link fails to generalize, this paradigm could establish a gold standard for mitigating task demands.

Another approach for concealing the importance of capability to act from participants is to leverage individual differences in their pre-existing baseline level of action capability and analyze their data according to their capability. In this way, participants are not exposed to an obvious action-based manipulation such as donning a heavy backpack; they simply make perceptual judgments. For example, in one study, participants judged the steepness of a staircase, and then were allowed to choose a snack or beverage as compensation ( Taylor-Covill & Eves, 2014 ). The researchers assessed the participants’ action capability indirectly by examining the sugar content of their chosen snack or beverage. The idea was that those who chose a more sugary option were more likely to have depleted energetic resources compared with those who chose less sugary options, a notion supported by several lines of evidence (e.g., Mehta et al., 2012 ; Page et al., 2011 ; Piech, Pastorino & Zald, 2010 ; Wang, Novemsky, Dhar & Baumeister, 2010 ). It is unlikely in this paradigm that participants would be aware of any task demand encouraging them to change their hill slope judgments according to their energetic resources or to alter their food choice according to their slope judgments.

Paradigms that leverage individual differences are only effective at concealing the hypothesized importance of action capability if participants are not aware that they are being recruited for their level of action capability. Participants recruited at, for example, a pain clinic, or an assisted living facility ( Witt et al., 2009 ; Sugovic & Witt, 2013 ; respectively) might be likely to infer that their age or chronic pain status is relevant to the hypotheses of the experiment in some way. Another pitfall of the individual differences approach can arise if the participants’ action capability status is inferred on the basis of a variable that covaries with a variety of other factors. For example, several studies have examined the effects of age ( Bhalla & Proffitt, 1999 ; Bian & Andersen, 2013 ; Sugovic & Witt, 2013 ) on perceptual judgments, and it is possible that age-related differences in these judgments stems from declines in action capability. However, aging is associated with a host of perceptual and cognitive changes that could impact responses without action capability being involved at all. The success of the individual differences approach for assessing perceptual effects, then, rests crucially on recruiting in a way that does not telegraph the importance of action capability to participants, and on ensuring that participants are well-matched apart from their action capability status.

Response Attitudes

Importantly, not all response biases are the result of participants consciously misreporting their perception when responding so as to comply with an inferred experimental hypothesis. They may make naïve assumptions about what aspect of the experimental context is the focus of the study, and then adopt a “response attitude” ( Epstein, 1963 ; Gilinsky, 1955 ; Rock, 1986 ) that gives that assumed aspect more weight as they generate their responses. As an example, consider the following. Researchers typically have precise definitions in mind for the concepts they study, but naïve experimental participants do not necessarily conceive of these concepts so precisely. The instruction to “tell me how far away that mountain is” may seem unambiguous, but from the perspective of a naïve participant, there are several possible interpretations ( Carlson, 1977 ; Mack, 1978 ; Woods et al., 2009 ). It may refer to the physical distance of the mountain recalled from memory (e.g., “The map I saw this morning said the mountain is 50 km away”); it might be a distance estimate developed through explicit inferential reasoning when viewing other aspects of the scene (e.g., “I can see 10 streetlights between here and the mountain, and those are at least 2 km apart, so the mountain is at least 20 km away”). It might be the visually-perceived distance (which is sometimes much shorter than physical distances (e.g., the mountain may appear only 10 km away on a clear day). It could even encompass influences from more abstract factors (e.g., being in a hurry may make the mountain “seem” farther away in some sense). Conceivably, each of these connotations could result in a numerically different distance judgment, without there being any difference in the underlying perceived distance. Importantly, some response attitudes could alter judgments by way of focusing the observer’s attention on output-level effects; other response attitudes could have legitimate perceptual effects--for example, by way of eliciting changes in eye movements or attention to specific spatial features of the environment (see below).

One approach to controlling for differences in response attitude is to inform participants of several possible attitudes, and then instruct them to ignore all but one of these when responding. This approach has been used extensively in studies of perceived size and distance, albeit primarily in reduced-cue environments (for reviews, see Carlson, 1977 ; Da Silva, 1985 ). Woods et al. (2009) used this kind of instruction manipulation in a well-lit outdoor environment. All participants were informed about three possible factors that might influence distance judgments: (1) the visual appearance of an object’s distance, (2) the physical distance of the object, and (3) non-visual factors. Participants were instructed to respond on the basis of one of these factors (specified in advance by the experimenter) when judging the distance of a target object. Participants threw either a heavy ball or a lighter ball several times without vision toward a target (presumably engaging action-specific perception mechanisms; Witt et al., 2004 ), and then verbally estimated the target distance. According to the action-specific perception account, throwing the heavy ball should require more effort than throwing the lighter ball, and accordingly the target object should appear farther away. The apparent and physical distance instruction groups showed no differences in distance judgments across the ball weight manipulation. Only the group that was instructed to take “all non-visual factors into account” when responding showed the predicted difference. If the apparent and/or physical (objective) instructions resulted in judgments that were indicative of the apparent target distance, the results suggest that the ball weight manipulation did not influence perception and that the “non-visual factors” effects may have been due to post-perceptual processes. That is, participants in this condition may have taken some salient aspect of the experimental methodology (e.g., the heavy ball), formed a hypothesis about the effect of ball weight on distance judgments, and then produced responses consistent with that hypothesis. More generally, in the absence of explicit instructions specifying which attitude to adopt, subtle aspects of experimenter behavior could influence the response attitude participants assume they should use. Use of specific instructions provides one way to constrain the possible response attitudes that participants may adopt and could help distinguish between perceptual and post-perceptual processing.

This is currently an under-explored area within action-specific paradigms. Nearly all paradigms have asked participants to report on physical or objective properties of the environment (e.g. “What size is the object?”), but, with the exception of Woods et al. (2009) , few have gone on to clarify more precisely what response attitude participants should adopt. Thus, in principle, many existing studies from the action-specific perception approach are potentially subject to biases due to response attitudes. One mitigating factor is that if participants are unable to intuit what the hypotheses are, there should not be any systematic differences associated with an action-specific manipulation, even if participants did adopt a response attitude that took non-visual factors into account. For example, if experimenters have been successful in completely concealing an action-specific manipulation (e.g., manipulation of blood sugar; Schnall et al., 2010 ; Taylor-Covill & Eves, 2014 ), one would not expect participants to adopt a response attitude that takes that factor into account. Thus, many of the same issues discussed in the Demand Characteristics section above are relevant for evaluating the effect of response attitudes on past paradigms.

One other mitigating factor is that one might expect some action-based measures to be less sensitive to biases stemming from poor control of response attitudes (though perhaps not completely immune). In particular, if a participant is asked to blindwalk to an object or catch a moving object (e.g., Stefanucci & Proffitt, 2009 ; Witt et al., 2010 ; Witt & Sugovic, 2013a ), the task arguably is framed in a way that emphasizes responding according to the physical location rather than non-visual factors.

To some extent, manipulating action capability within subjects can provide some measure of control for output effects relating to response attitudes. The validity of this approach hinges on the assumption that individuals will tend to adopt the same response attitude across all levels of the action capability manipulation. In principle, this assumption could be verified directly in post-experiment questionnaires, although no experiment investigating action-specific effects has yet done this. A potential drawback of this approach, however, is that by exposing each participant to all levels of action capability, the design may increase a paradigm’s susceptibility to demand characteristics. Thus, the potential benefit of within-subjects designs for mitigating response attitude effects must be weighed against their potential drawbacks in terms of demand characteristics.

Misattribution Effects

Closely related to response attitude effects are misattribution effects. Here, despite instructions to report an object’s size, distance, or speed, participants could be reporting on some other aspect of the stimulus entirely. For instance, when asked about the size of a softball, players might instead report on how easy they think it might be to hit ( Lee, Lee, Carello, & Turvey, 2012 ). According to Gibson (1979) , the primary objects of perception are affordances. In this view, it stands to reason that even when asked about size or speed, a participant might (implicitly) report about affordances instead. Misattribution effects are most likely to be of concern in experiments for which an action is frequently performed or at least strongly implied, such as reaching for or grasping objects, trying to catch moving objects, and jumping over gaps.

Experimenter Effects

Even if the hypotheses of an experiment are not communicated to participants, systematic differences in how an experimenter treats the participants during testing could bias the results in a way that happens to coincide with the experimental predictions, thus resulting in apparent action-specific perceptual effects when there are none. For example, if throwing a heavy ball at a target makes one group perform less accurately than a group throwing a light ball, an experimenter may unconsciously provide more encouragement to the heavy ball group or emphasize different aspects of the methodology ( Woods et al., 2009 ). This could result in systematic biases in the group responses, without there being any inferences or assumptions about the experimental hypotheses on the part of the participants. Thus, great care is required to ensure that all individuals and groups are treated identically. Woods et al. (2009) have argued that interpersonal consistency is best maintained when experimenters adopt a neutral affect. In this view, neutral affect guards against unintentional inconsistencies of the kind noted above, stemming from a more positive, supportive affect. Deviations from neutral affect may also be more difficult to characterize when describing the study’s methodology.

Like task demands, experimenter effects can be difficult for a researcher to detect in his or her own experiments. Even if the experimenter treats all participants the same, the participants might be influenced systematically by some personal characteristic that varies from experimenter to experimenter. Within a study, this kind of effect can be minimized by having the same experimenter test all participants; if there are multiple experimenters, no experimenter should test a disproportionate number of participants in each group. These issues are difficult to control across different laboratories, however, as characteristics of the experimenters are rarely reported.

Some paradigms control for experimenter effects more effectively than others. One technique for controlling for experimenter effects is to use double-blind methods, in which the experimenters who interact with participants are themselves unaware of the experimental hypotheses. The Woods, Philbeck and Danoff (2009) study, described in the Response Attitudes section above, is one of the few that has used this technique in the context of action-specific manipulations. Under double-blind conditions, when participants received instructions that emphasized either apparent distance or physical distance, there was no replication of the ball weight effect reported in Witt et al., (2004) . It is not clear that experimenter effects were responsible for past evidence of ball weight effects, however, because in another study, Woods et al. attempted to replicate the methods of Witt et al. (2004) as closely as possible (including using experimenters who knew about the hypotheses and interacted with participants from both the heavy and light ball groups), but again there were no ball weight effects. Further research is required to determine the reason for the differing results between Woods et al. (2009) and Witt et al. (2004) . Unreported differences in experimenter behavior may be responsible. Nevertheless, the Woods et al. study represents a particularly well-controlled attempt to study action-specific effects. Keeping experimenters blind to the accuracy of participant’s responses (both verbal and throwing responses, in this paradigm) would further guard against the possibility of experimenter effects across treatment groups.

Even if experimenters are not blind to the experimental hypotheses, this may not be a concern to the extent that (1) experimenter interactions with participants are minimized (e.g., by computerized testing) or (2) experimenter interactions with participants are highly stereotyped or follow a written script. Thus, paradigms in which there are extensive unscripted interactions with experimenters who know the hypotheses are potentially more open to experimenter effects. Many studies investigating action-specific effects fall into this category. Examples include the backpack and ball throwing studies ( Bhalla & Proffitt, 1999 ; Witt et al. 2004 ), studies involving chronic pain or older adults ( Bian & Anderson, 2013 ; Sugovic & Witt, 2013 ; Witt et al., 2009 ) and studies involving swimmers and parkour athletes ( Witt et al., 2011 ; Taylor et al., 2011 ; respectively). By contrast, other studies from the action-specific perception perspective have used methods that would be expected to reduce the likelihood of experimenter effects. For example, in several experiments involving athletes, the experimenters did not know the athletes’ performance levels, thus mitigating potential experimenter effects (e.g. softball: Witt & Proffitt, 2005 ; golf: Witt et al., 2008 ). For other experiments, the interaction between experimenter and participant were minimal and restricted mainly to initial instructions. These include tennis and Pong studies ( Witt & Sugovic, 2010 , 2012 ), and reaching studies ( Kirsch & Kunde, 2013a ; Morgado et al., 2013 ; Witt et al., 2005 ; Witt, 2011b ).

Determining the possible role of experimenter effects in specific studies is not straightforward. If a published study does not mention attempting to minimize experimenter effects, this does not necessarily mean that no efforts were taken in this regard. For many researchers, interacting with participants in a stereotyped way with neutral affect is considered to be a fundamental principle of good methodological practice, and even when these practices are followed during testing, they may not be explicitly mentioned in published work. Nevertheless, given that effective techniques for minimizing experimenter effects are available (e.g., double-blind designs), these techniques should be used to the extent possible in future work in this domain. In addition, any efforts to control experimenter – participant interactions should be explicitly reported.

Memory Effects

There are many examples of action-specific effects for which the target is visible during the response (e.g. Bhalla & Proffitt, 1999 ; Proffitt et al., 2003 ; Linkenauger et al., 2009 , 2010 ; Linkenauger, Witt & Proffitt, 2011 ; Witt & Dorsch, 2009 ; Witt, Linkenauger, Bakdash, & Proffitt, 2008 ; van der Hoort et al., 2011 , 2014 ; Witt et al., 2004 , 2005 ; Witt & Sugovic, 2012 , 2013b ). However, when the target is not visible, processes related to memory are a possible factor in explaining any effect. Presumably, the vision-based perceptual experience of an object’s distance fades away almost instantaneously after vision of the object is occluded, but nevertheless, representations of the object’s location can endure after vision is occluded (e.g., the “spatial image”; Loomis & Philbeck, 2008 ). Behavioral indications of geometrical object properties are often collected after vision is occluded. In the case of blindfolded walking, for instance, observers view the target object and then attempt to indicate its location by walking to it while blindfolded (e.g., among many others: Philbeck & Loomis, 1997 ; Rieser, Ashmead, Talor, & Youngquist, 1990 ; Thomson, 1980 ; Witt et al., 2004 ; Wu, Ooi, & He, 2004 ). Because vision is not available during the walk, the walk must be based on a memory representation rather than directly on visual perception. Such responses could still be informative about perception if the remembered target location was the same as the perceived location ( Loomis & Philbeck, 2008 ). In the transformation from perceptual to memorial representations, however, biases could emerge, such that the remembered location is no longer coincident with the perceived location. Conceivably, an observer’s memory for an object’s location might be influenced by expectations or inferences about where the object “should” be, and these influences may well depend on action capabilities ( Cooper, Sterling, Bacon & Bridgeman, 2012 ). If behavioral responses are based upon the remembered location rather than the perceived location (because, for example, the judgment is made once the object is no longer in view), action-specific effects on memory could be mistaken for bona fide action-specific effects on perception .

As a concrete example, consider the case in which an observer reports on the distances to two equidistant locations, A and B. He might remember that Location A was more difficult to walk to than Location B because he carried an anvil to A and was unencumbered in his walk to B. If memory of the additional effort to walk influences memory of the walked distance, he thus might remember A as being farther away from an initial vantage point than B, even though he did not visually perceive it to be farther way. Put differently, the affordances provided by Locations A and B were different because of differences in the observer’s action capability, and memory of the location might be influenced by the difference in affordances rather than by differences in visually perceived distance.

In the context of judging the size of a hole after throwing a marble successfully or unsuccessfully into the hole, Cooper et al. (2012) found that size judgments did not depend on participants’ throwing success when the hole remained visible (perception condition), but did depend on throwing success when the hole was occluded (memory condition). This suggests that action capability has its effect on memory, rather than perception, in this paradigm. The generalizability of such memory effects to explaining other past action-specific effects remains unclear, however. Memory has not been differentiated from perception in a number of studies of action-specific effects, including those involving softball ( Witt & Proffitt, 2005 ), swimming ( Witt et al., 2011 ), and tennis ( Witt & Sugovic, 2010 ). It is therefore possible that the findings in these studies are attributable to action-specific effects on memory rather than perception, but as yet there is no empirical basis for evaluating this possibility. Furthermore, no studies that have found an effect in perception (i.e. while the target is still visible) have addressed whether the effects get bigger in memory. Thus, the relative roles of perceptual and memory processes in these paradigms remains unknown.

Other Output Effects

All of the foregoing factors may be said to influence the extent to which a behavioral judgment accurately reflects the underlying percept. Other output-related effects have been described, however. Blindfolded walking judgments of egocentric distance, for example, have been found to be subject to response compression biases—a form of stimulus range biases (regression to the mean; e.g., Li, Sun et al., 2013 ). Estimates of geographical slant obtained by manipulating a palm board device have been shown to be subject to anchoring effects, in which judgments initiated from a vertical palm board position differ systematically from those initiated from a horizontal position ( Shaffer et al., 2014 ). Visual matching tasks are also subject to anchoring effects, such that larger values of a match are often obtained when the adjustable comparison stimuli are initially positioned farther apart than when they are initially close together (e.g. Witt et al., 2004 ).

Furthermore, many experiments have collected spatial judgments using some variation of the magnitude estimation technique, and this method is known to be subject to a variety of other kinds of biases that presumably occur at the output level (e.g., stimulus range biases and stimulus spacing biases; Poulton, 1979 ). Much work has been aimed at validating magnitude estimation methods and characterizing the kinds of factors that influence them (e.g., Stevens, 1957 ). The resounding message of this work is that “avoiding all the biases requires exceedingly rigorous investigations” ( Poulton, 1979 ). To complicate matters, these biases may be applied unconsciously by observers when responding. Even action-based responses are not immune to unconscious biases. Involuntary and unconscious motor activity can occur as a result of suggestion or expectations. This effect, known as ideomotor action, has been implicated via double-blinded studies as the driving force behind phenomena such as facilitated communication and the motion of Ouija board pointers and dowsing rods (e.g., Burgess et al., 1998 ; for a review, see Spitz, 1997 ).

Although output-related effects have been acknowledged in the action-specific perception literature, they have not been particularly concerning to proponents of the action-specific approach because it has often been assumed that the effect of many output biases would be consistent across all action-specific conditions. To the extent that this is true, differences between conditions could not be attributable to these kinds of biases (e.g. Proffitt et al., 2003 ; Witt et al., 2004 ). Thus, the key consideration is not so much whether a particular response is subject to output-related biases (because one could validly argue that this is true for all behavioral responses), but instead whether the response is subject to biases that operate differently depending on one’s action capability. This remains an active focus of investigation.

Discriminating between perceptual effects and output-level effects

This extensive literature tends to weigh heavily in the minds of researchers from the action-resistant perception perspective when interpreting any response as a possible indication of perception, even if that response has been interpreted as sensitive to perception in past work (e.g., verbal reports; Da Silva, 1985 ). From the action-resistant perception perspective, output processes take on a devious character akin to Arthur Conan Doyle’s Professor Moriarty—capable of outwitting even a brilliant and determined adversary such as Sherlock Holmes. At least some of the resistance to the notion of action-specific effects stems from the impression that researchers from the action-specific perception approach have not done enough to thwart Moriarty—i.e., have not done due diligence in considering output-level explanations of the observed effects. At worst, failing to aggressively pursue alternative explanations that have such extensive empirical support runs the risk of coming across as confirmation bias (a tendency to only seek out evidence that supports the theory). As a reaction to this line of criticism, proponents of the action-specific perception account have become increasingly mindful of post-perceptual factors when interpreting their results and have begun to apply a variety of techniques to determine the potential role of output-related processes in action-specific effects. We have reviewed some techniques for controlling or mitigating the effects of specific output-level factors, above. Here, we review other strategies that have been employed and discuss their relative strengths and weaknesses.


As we have discussed, a recurring criticism of the action-specific perception approach is that when action-specific effects have been found in past work, they are likely due to post-perceptual processes such as demand characteristics rather than because perception itself has been influenced by the participants’ action capabilities. One argument against this criticism makes reference to the large number of studies in which action-specific perceptual influences have been reported: in this view, if one’s action capability indeed plays a strong role in determining what one perceives, evidence of this role should be observable in a wide variety of situations.

Much research supports this generalizability prediction. Action-specific effects have been reported in athletes, community members, psychology students, adolescents, and special populations such as older adults and those with chronic pain (e.g. Bhalla & Proffitt, 1999 ; Bian & Andersen, 2013 ; Cañal-Bruland & van der Kamp, 2009 ; Sugovic & Witt, 2011 , 2013 ; Taylor, Witt, & Sugovic, 2011 ; Witt et al., 2009 ; Witt, Schuck, & Taylor, 2011 ). Effects have also been reported in a variety of dimensions including estimates of size, distance, slant, height, shape, speed, and weight (e.g. Cañal-Bruland & van der Kamp, 2009 ; Doerrfeld, Sebanz, & Shiffrar, 2012 ; Gray, 2013 ; Linkenauger et al., 2013 ; Proffitt, Bhalla, Gossweiler, & Midgett, 1995 ; Proffitt, Stefanucci, Banton, & Epstein, 2003 ; Stefanucci & Proffitt, 2009 ; Witt, 2011b ; Witt & Sugovic, 2010 , 2012 ). Effects have been reported with a variety of manipulations, including ones in which the manipulation is obvious to participants, such as donning a backpack or wielding a tool (e.g. Osiurak et al., 2012 ; Witt et al., 2005 ), as well as with less obvious ones such as drinking juice with sugar versus artificial sweetener or the use of individual differences rather than direct experimental manipulations (e.g. Bhalla & Proffitt, 1999 ; Schnall, Zadra, & Proffitt, 2010 ; Taylor-Covill & Eves, 2014 ; but see also Shaffer et al., 2013 ).

Generalizability is an important prediction that should be satisfied if one’s action capability indeed plays a strong role in determining what one perceives. By the same token, this kind of evidence is not fully diagnostic with regard to the overall viability of the action-specific perception approach. If output-level biases happen to be very common and occur in a wide variety of situations, the existence of a large number of studies showing action-specific effects might just as likely be due to output-level effects as genuine perceptual influences. Thus, determining the overall likelihood of output-level biases across a wide variety of situations remains an important topic for future research.

Converging Operations

One technique for bolstering evidence that an experimental manipulation influences perception itself rather than post-perceptual processes involves comparing the pattern of responses across multiple response modes. This “converging operations” strategy has been used by researchers from both the action-resistant and action-specific perception perspectives. Importantly, many of these studies from the action-resistant perception perspective that have used this technique compared responses under situations in which the stimulus environments and visual cues are the same across response types, as is often the case in action-specific perception experiments ( Foley, 1977 ; Philbeck & Loomis, 1997 ; Philbeck, Woods, Kontra, & Zdenkova, 2010 ; Gogel et al., 1985 ). Evidence for action-specific perceptual effects has come from experiments using verbal reports, visual matching tasks, and action-based tasks. For example, increased anticipated effort for walking has been linked both with increased verbal estimates of target distances and with increased walked distances when participants attempt to walk to previously-viewed targets without vision ( Witt, Proffitt, & Epstein, 2004 , 2010 ; but see also Corlett, Byblow & Taylor, 1990 ). Another example comes from a task requiring participants to use a computerized fish-catching simulation with large or small nets ( Witt & Sugovic, 2013b ); virtual fish that are easier to catch as a result of using a larger net lead to verbal reports that the fish is moving slower and also to delayed net release times, which serves as an action-based measure of apparent speed. A third example comes from throwing or pushing a beanbag to land at the target’s location ( Linkenauger, Bülthoff, & Mohler, 2015 ). In these cases, multiple indicators of distance and speed reveal converging evidence.

Particularly strong converging evidence that an effect is perceptual can come from indirect measures. Here, the assumption is that if participants are asked to report on some other dimension than the one of interest, they will be less likely to infer what the experimental hypotheses are and consciously adjust their responses to produce the predicted pattern. For example, given the relationship between retinal size and distance, apparent size can be an indicator of distance ( Epstein, Park & Casey, 1961 ; Sedgwick, 1986 )—in such an experiment, participants would judge the size of an object, rather than its distance, and the size judgments would then be converted into distances using a formula such as Emmert’s Law (in which distance equals the object size divided by the tangent of the visual angle that the object subtends). Indirect measures of perceived distance (specifically, the observer’s perceived height above the ground surface) using judgments of apparent size have revealed effects of fear on perceived heights, such that perceived height, as measured indirectly via judgments of object sizes, were larger when participants were fearful of heights ( Stefanucci & Proffitt, 2009 ). In addition, other factors such as apparent shape and apparent parallelism have revealed similar patterns of effects as verbal estimates and visual matching tasks and thus can be considered indicators of apparent distance ( Witt, 2011b ).

Confirmation of the action-specific perception predictions across a wide variety of contexts and measurement techniques has been taken as the most compelling evidence available that action-specific response patterns can be perceptual in origin. Going further, the similarity of response patterns across so many contexts and measurement techniques has been taken to suggest that a common process underlies this pattern across contexts (e.g. Witt, 2011b ; Witt et al., 2010 ; following Foley, 1977 ; Philbeck & Loomis, 1997 ): that is, most or all past evidence of action-specific response patterns is due to bona fide action-specific differences in perception. Thus, from the action-specific perception perspective, this research suggests that post-perceptual processing plays a relatively small or even negligible role in explaining action-specific responses in most contexts.

For researchers operating within the action-resistant perception framework, generalizability and converging operations can be convincing methodologies for addressing the possibility of output-level biases, but even these methods can be subject to systematic output biases and should be interpreted with caution. In the case of indirect measures, one cannot automatically assume that participants draw no inferences about the experimental predictions simply because they are asked to report on size rather than height or distance—ultimately, this assumption might be valid, but it must be approached with the same level of scrutiny as other methods. In short, a nuanced consideration of possible output-level influences in each experimental context is warranted before interpreting action-specific effects as stemming from perception in that context.

Overall Evaluation of Past Paradigms

Earlier, we reviewed existing paradigms in terms of their ability to resist various forms of post-perceptual influence. Here we take a more comprehensive perspective to evaluate the overall likelihood that existing results reflect genuine action-specific influences on perception. In one sense, very few past studies provide effective controls for a broad variety of output-related factors simultaneously, and by this criterion, some researchers might feel that virtually no existing studies provide truly compelling evidence. Nevertheless, some studies provide more compelling evidence than others. Perhaps there is the most cause for doubt in studies that have shown sensitivity to specific kinds of output-related factors. For example, the backpack encumberment and ball throwing paradigms show sensitivity to demand characteristics under at least some circumstances ( Durgin et al., 2009 ; Woods et al., 2009 ); the link between throwing success and size judgments has been shown to be sensitive to action-specific effects on memory ( Cooper et al., 2012 ). The extent to which past evidence of action-specific effects using these paradigms is indeed due to output effects is unclear, but suspicion that output effects were responsible in the past is certainly heightened by evidence that the paradigm is sensitive to such effects.

There is broad agreement that studies are more compelling to the extent that they provide effective controls for specific kinds of output effects. Double-blind methodology can provide a means of controlling for experimenter effects ( Taylor-Colvill & Eves, 2014 ; Woods et al., 2009 ). Paradigms involving undetectable action-specific manipulations would be a gold standard for minimizing demand characteristics. One candidate is the use of drinks containing sugar versus artificial sweetener as a manipulation of action capability ( Schnall et al., 2010 ). There has been some contention surrounding this paradigm ( Shaffer et al., 2013 ), but additional development may be able to make it more broadly compelling. Another paradigm that shows promise for minimizing demand characteristics is to collect perceptual judgments before assessing the participant’s action capability ( Taylor-Covill & Eves, 2014 ). In this way, participants do not know that their action capability is relevant to the study at the time they are making their judgments, and experimenters do not know participants’ action capability during the judgments because it has not been measured yet. Thus, this paradigm can be effective for controlling both demand and experimenter effects.

Paradigms using individual differences, such as drink or food preference, can provide a useful way to conceal from participants the hypothesized importance of their action capability and thereby minimize demand characteristics. However, as we have argued, this approach is only effective if the recruitment process does not suggest to participants that their action capability (or some factor related to it) is relevant to the hypotheses. For example, several studies involving athletes collected perceptual judgments before assessing performance, but participants were aware that recruitment took place at softball fields and golf courses ( Witt & Proffitt, 2005 ; Witt et al., 2008 ). Thus, this may have made the hypothesized importance of their athletic ability salient. As another example, recruiting younger and older adults at an assisted living facility ( Sugovic & Witt, 2013 ) likely made age a salient factor, and recruiting patients and employees at a chronic pain clinic ( Witt et al., 2009 ) likely made pain a salient factor. In contrast, recruiting adults at a public shopping center and measuring their weight and BMI after collecting all perceptual measures is an effective way to conceal an interest in body size ( Sugovic & Witt, 2011 ).

A potential drawback is that attempts to make action capability manipulations undetectable by participants may also minimize the likelihood of finding genuine effects of action on perception. This could happen, for example, if the manipulation is made undetectable by using treatment conditions that contain only very small differences in energetic demands. For instance, if a backpack is worn for only a brief period of time and participants have no reason to anticipate ascending the hill, the conditions are not optimized to find an effect in perception, if one should exist. The presence of the backpack is clearly detectable, but under these conditions, the difference in energetic demands between the backpack and no-backpack conditions is likely quite small. This reduces the sensitivity of the experiment for detecting genuine action-specific perceptual effects and increases the possibility that output-related factors may underlie systematic differences in the results. For this reason, paradigms that involve very small differences in action capability tend to be less compelling as evidence of action-specific perceptual effects.

Another example of potentially insufficient variation in action capability is that of throwing a heavy ball ( Witt et al., 2004 ). Throwing a 2lb (.91kg) ball to targets ranging from 4–10m does not require substantially more effort than throwing a .7lb (.32kg) ball. This could explain why this manipulation sometimes produces significant effects on estimated distance ( Witt et al., 2004 ) and sometimes does not ( Woods et al., 2009 ). A significant challenge for future research is to balance the need to minimize output-related factors, such as task demands, against the need to maximize the amount of variation in action capability; small variations in action capability are not likely to affect perception appreciably.

Yet another paradigm that we question is dart-throwing, at least when conducted with novice dart throwers. Although significant effects of action capability have been reported using this paradigm ( Wesp et al., 2004 ; Cañal-Bruland et al., 2010 ), novice dart-throwers have poor control over the precision and accuracy of their performance. This would be expected to dramatically reduce the sensitivity of this paradigm to action-specific perceptual effects. Poor sensitivity also increases the likelihood that small variations in methodology across laboratories could lead to inconsistent results. More anecdotally, recent work by one of the current authors (Witt, unpublished data) asked participants to maneuver an airplane icon in a computer display through an aperture or to drop a cargo load from the airplane into an aperture on the ground and then estimate the size of the aperture. Action capability was manipulated by changing factors such as the speed and height of the airplane. None of the action capability manipulations influenced size judgments in this 2D setting. Given that action-specific effects have been found with respect to estimated size of virtual 3D objects (e.g. Linkenauger et al., 2013 ) and estimated speed of 2D objects (e.g. Witt & Sugovic, 2010 , 2012 ), the reason why these 2D size estimation paradigms failed to elicit effects remains unclear. Although resolving this issue may yield important information about the conditions under which action-specific effects are and are not manifested, the paradigm itself does not appear to provide reliable evidence of these effects.

An example of a paradigm that has made substantial progress towards controlling multiple output-related factors is the so-called Pong task, discussed earlier, in which participants make judgments of ball speed on a computer monitor after they attempt to block the ball with different-sized paddles ( Witt & Sugovic, 2010 , 2012 ). Work using this paradigm has leveraged individual differences in compliance to assess the possible effect of experimental demand ( Witt & Sugovic, 2013b ). This work suggests that demand plays a relatively small role in judgments of ball speed in this paradigm. This paradigm is also effective for minimizing experimenter effects, in that participants perform the task without interacting with the experimenter, apart from the initial instruction phase. Furthermore, paddle size is typically manipulated within-subjects randomly on a trial-by-trial basis; on the plausible assumption that participants would be unlikely to rapidly switch between different response attitudes under these conditions, this paradigm thus also provides a measure of control over response attitudes. The manipulation of paddle size is certainly obvious to participants, so additional strategies to minimize other output-related factors are still warranted. To mitigate this concern, Witt and Sugovic (2013a) used an indirect, action-based measure ( Witt & Sugovic, 2013a ). Rather than continuously controlling the paddle, participants pressed a trigger to shoot the paddle up, and the challenge was to time the release of the paddle just right in order to catch the target (a “fish” in this case). The center of each paddle was positioned similarly for all 3 paddle sizes so that to maximize catching performance, the different-sized paddles should be released at the same time-point. Participants, however, tended to wait longer to release the large paddle than the small paddle. The experimenters took differences in the time to release the various-sized paddles as an indirect measure of differences in perceived ball speed; in this view, participants waited longer to release the big paddle compared to the small paddle because they saw the fish as moving slower and therefore had to wait longer. Although no paradigm is unassailable, Pong-like tasks such as these arguably come closest to providing evidence of action-related perceptual effects under conditions that control for multiple output-related processes.

Strategies for minimizing the influence of output-related processes in judgments of distance and other spatial aspects of the environment are summarized in Table 1 . Although ultimately it may be impossible to definitively rule out all output-related processing as an explanation for action-specific effects (or any purported perceptual effect), steps may be taken to maximize the discriminability of perceptual and post-perceptual effects and make a convincing case that action-specific effects can be genuinely perceptual. We will outline some of these steps in the “Recommendations for Future Research” section.

Recommended strategies for evaluating or minimizing the role of post-perceptual processes.

Mechanisms for Genuine Influences on Perception

We have focused thus far on post-perceptual effects that could be mistaken as differences in the underlying perceptual representation, as well as methods for minimizing the influence of such factors in experimentation. To date, these issues have taken center stage of the controversy surrounding the action-specific approach. We now turn to a more neglected area: the mechanisms that could drive bona fide differences in perception. Given that the debate that has dominated the literature has been about the potential role of output-related factors, the consequence is that much less attention has been paid to similarities and differences between the structural features of the action-resistant perception approach and action-specific perception approach. There has been relatively little emphasis on this issue from the action-resistant perception perspective, but nevertheless some possible mechanisms have been discussed, and to some degree accepted, by researchers from this perspective. We will next discuss three possible mechanisms by which one’s potential to act could influence perception.

(1) Modification of Visual Information or Visual Processing

One way that action capability could affect visual perception is by influencing the type or quality of the visual information that gets picked up by the observer. Importantly, this mechanism can be accommodated within the modal model. Even though the distal cue configuration may remain constant, an observer’s goals and abilities may affect where she looks and what she pays attention to ( Castelhano, Mack & Henderson, 2009 ; Henderson, 2003 ; Rothkopf, Ballard & Hayhoe, 2007 ); variations in eye movements can accordingly impact task performance (e.g., putting performance in golf; Vickers, 1992 ). These eye movements and/or shifts in attention might then impact perception. Here, we review evidence suggesting that shifts in eye movements and attention can influence the appearance of geometrical properties of the environment.

Depth and egocentric distance

Perceived depth and distance can be influenced by eye position and eye movements, presumably by incorporating extra-retinal oculomotor signals ( Collewijn & Erkelens, 1990 ; Foley & Richards, 1972 ; Gogel & Tietz, 1977 ; Wist & Summons, 1976 ). If one fixates one object but pays attention to another object at a different distance, this can influence the perceived distance of the fixated object, albeit minimally ( Gogel & Tietz, 1977 ). In outdoor environments, observers localize objects less accurately in egocentric distance when they move their direction of gaze from a far distance inwards toward the object than if they scan from their feet outwards ( Wu, Ooi & He, 2004 ). A similar dependence on scanning direction does not occur in indoor room-sized environments, perhaps because the ground surface plays a less important role when other planar surfaces (walls and a ceiling) are present ( Gajewski, Wallin & Philbeck, 2014a ). Wu, He and Ooi (2008) have shown that distance judgments are less accurate if observers’ initial attention (and presumably, their direction of gaze) is biased toward locations that lie beyond the peripersonal ground surface than if their initial attention is biased toward locations within this nearby region. This suggests that task-related factors that interfere with processing of visual information from the nearby ground surface can impact distance judgments. Other work has shown that completely occluding the nearby ground plane in outdoor environments has little effect on distance judgments relative to unrestricted viewing conditions ( Gajewski et al., 2014b ). The explanation for these apparently conflicting reports remains unresolved. The role of eye movements and gaze direction on perceived distance may play a more pronounced role in reduced-cue settings, contexts in which only a very brief glimpse of a novel environment is available, or in more cluttered environments ( Gajewski et al., 2014a , b ).

Size and speed

In the context of relatively impoverished 2D displays, attended objects have been found to appear larger than unattended objects ( Anton-Erxleben, Henrich, & Treue, 2007 ), and fixated objects appear larger than those in the periphery ( Newsome, 1972 ). Epstein and Broota (1986) , furthermore, found that when attention was diverted by asking participants to make numerosity judgments of spots on a stimulus card rather than size judgments of the card, subsequent size judgments were biased toward the card’s projective size. Similar issues have been studied in the domains of obstacle avoidance and steering (e.g., Franchak & Adolph, 2010 ; Land & Hayhoe, 2001 ; Matthis & Fajen, 2014 ; Patla & Vickers, 2003 ), albeit not in connection with testing for effects of action capability. In an aviation context, novice pilots’ judgments of airport runway size in a flight simulator were correlated with their runway fixation time, as well as with several measures of landing performance ( Gray, Navia, & Allsop, 2014 ). This correlational analysis cannot determine whether these differences in fixation were directly responsible for the differences in judged runway size, however. As such, the role of attention and/or eye position in determining perceived size is uncertain.

Cañal-Bruland, Zhu, van der Kamp, and Masters (2011) examined the role of attention by blocking the view of a golf hole, so as to reduce attention directed to the hole. Putting performance did not influence judged hole size when the hole was occluded, but did influence judged hole size when the hole was visible. The researchers also diverted attention away from the hole by requiring participants to putt the ball between two markers before reaching the hole. The intention was to force attention to the markers rather than the hole. Again, this manipulation eliminated the relationship between putting performance and perceived hole size. These results suggest that attention to the hole was key for eliciting a linkage between action capability and judged size.

Other research, however, does not support the idea that attention is a critical factor. In one set of experiments, a participant’s body was rendered in a virtual environment to be twice as big or half its size. Participants estimated the size of virtual objects placed nearby. The objects were judged to be bigger when placed next to the larger rendered body ( van der Hoort et al., 2011 ). In a follow-up experiment ( van der Hoort & Ehrsson, 2014 ), the researchers constrained participants to look at a stationary fixation point, thereby controlling any differences in eye movements. The effect of rendered body size on perceived object size was just as big when eye movements were constrained as when they were not, suggesting that eye movements and/or attention do not play a role in this particular action-specific effect.

Witt and Sugovic (submitted) found that eye movements do not play a role in explaining the effect of blocking ability on apparent ball speed. In this paradigm, blocking ability is manipulated by varying the size of a paddle in a 2D computerized display and participants make judgments of ball speed after attempting to hit the ball with the paddle ( Witt & Sugovic, 2010 , 2012 , 2013a , b ). Witt, Sugovic and Woodman controlled for eye movements by constraining participants to fixate the ball via a secondary task, and also restricted their analyses to trials in which eye tracking showed that participants indeed fixated the ball. The previously-reported influence of paddle size on apparent ball speed persisted even when eye movements were held constant across the paddle size manipulation.

Differences in eye movements/attention due to action capability

For purposes of evaluating this mechanism as a possible explanation of action-specific effects, a more fundamental question is whether there are indeed natural shifts in eye movements or attention associated with differences in action capability. Few studies have investigated this issue. At least in the case of egocentric distance estimation, Gajewski et al. (2014a) found no differences between observers’ natural patterns of eye movements in a distance judgment task as a function of whether a verbal or a blindwalking distance judgment was required. Furthermore, there was no apparent linkage between the natural pattern of eye movements and the accuracy of distance judgments; observers who adopted the strategy of steadily fixating the target performed equally well as those who looked around the room before responding. Additional research is required to determine the generalizability of these findings to other manipulations of intentions to act. Nevertheless, these null results suggest that systematic differences in eye movements and/or attention, if present, may be too small to play a robust role in mediating any bona fide action-related perceptual component in paradigms involving egocentric distance judgments (e.g., Proffitt et al., 2006 ; Witt et al., 2004 ).

Under some circumstances, task manipulations clearly do influence the deployment of eye movements and attention, and differences in eye movements and/or attention clearly can influence perceptual aspects of the environment such as distance and size. However, no research to date provides a satisfactory demonstration of a direct connection between action, attention, and perceived geometrical properties of the environment. Much work remains to be done to evaluate this mechanism, but at present it appears unlikely to provide a robust, across-the-board explanation of action-based effects. It may play a role in specific situations, however, and as such should be evaluated whenever possible.

(2) Modification of Non-visual Processes

Researchers from both the action-specific and action-resistant perception perspectives tend to conceive of perception as a phenomenal experience that excludes influences from conscious cognitive processing, such as explicit reasoning or mental arithmetic. Similarly, there is undeniably some degree of cognitive impenetrability ( Fodor, 1983 ; Pylyshyn, 1999 ) in layout perception; for example, if a mountain visually appears to be 10 km away, knowing that it is physically 30 km away does not alter the perception that it appears much closer. Both perspectives also agree that the output-level processes discussed above should not be considered to influence perception. Nevertheless, both approaches allow for some kinds of “top-down” influences on perception.

From the action-resistant perception approach, there is an established literature on perceptual learning, in which past experience is thought to influence on-line perception ( Fine & Jacobs, 2002 ). There is evidence that different visual cues to distance require different amounts of time to be extracted during the initial stages of an eye fixation ( Gajewski et al., 2010 ). Given this, and the fact that the region of high acuity is limited to the central 1 deg, some degree of persistence or integration of information is required to maintain perceptual stability of spatial layout. Assumptions or expectations that an object is resting on the ground can influence the object’s perceived location ( Wu, He & Ooi, 2014 ). Similarly, other research suggests that information gained during long glimpses of an environment (e.g., 5 sec) influences subsequent distance judgments when targets are glimpsed very briefly (e.g., 100 ms; Gajewski et al., 2010 ; Gajewski et al., 2014 ). Finally, the specific-distance tendency ( Gogel, 1990 ; Gogel & Tietz, 1973 ; Owens & Leibowitz, 1976 ) and the intrinsic ground plane ( Ooi, Wu & He, 2006 ) are thought to be internal biases that influence perceived distance to the extent that visual information about an object’s distance is unreliable (e.g., beyond the effective range of distance cues in well-lit natural environments), or unavailable (e.g., in darkened environments). Because these biases do not arise directly from visual stimulation, they might be considered yet another type of non-visual influence on visual perception. Taken together, there are several precedents for the idea that at least some kinds of non-visual factors can influence perception, although the full scope of these influences remains poorly understood.

The action-specific perception approach claims that non-visual action-related factors play a crucial role in perception. However, these processes are thought to be very different than the specific distance tendency and intrinsic ground plane. For example, in this view, action influences perception even when reliable visual cues are available. Also, the information about action is dynamic and detected at the time of perception, rather than being based on a stored representation or internal bias ( Witt & Proffitt, 2008 ). Importantly, this approach holds that the mechanisms responsible for these non-visual influences on perception are limited to unconscious motor-related processes, thus preserving the idea that perception is cognitively impenetrable with respect to conscious knowledge.

Contrasting with the possibility that behavioral potential modifies perception by altering non-visual internal biases or assumptions, the influence of unconscious motor-related processes on visual perception may well be a kind of multimodal interaction ( Witt & Riley, 2014 ). Information from the motor system concerning the perceiver’s ability to act could serve as a non-visual input that gets integrated with the visual cues to determine perceived layout, distance, or size. Much research supports the notion that visual information can influence audition and vice-versa (for a review, see Shams, Kamitani & Shimojo, 2004 ) even though these effects are typically not included in models of visual perception like the modal model. Often, multimodal interactions are well-described by Bayesian cue combination, in the sense that the influence of each modality is weighted by its relative reliability ( Ernst & Banks, 2002 ; see also Yang & Purves, 2003 ). Little is known about this in the context of motor-related influences on perception. Indeed, the reliability of this kind of motor information remains unknown. Nevertheless, the notion of multimodal integration provides a conceptual framework, and potentially a toolbox of methodological techniques, for studying action-specific motor-related influences on visual perception ( Witt & Riley, 2014 ). Intention to act might be operationalized using electromyelographic or electroencephalographic data in future work (e.g., van Elk, van Schie, Neggers & Bekkering, 2010 ).

(3) Scaling of Visual Information

More recently, Proffitt and Linkenauger (2013) have proposed a “perceptual ruler” hypothesis to explain action-specific effects ( Proffitt & Linkenauger, 2013 ). Optical information specifying size, distance, and other spatial properties of objects takes the form of visual angles. For example, binocular disparity is a difference in the visual angle separating the images of two objects between the two eyes. Likewise, texture gradients are informative about the slant of a surface because the angular size of the visible texture elements changes in a systematic way depending on the surface slant. Consequently, in order to perceive dimensions such as distance, size, and slant, optical information needs to be scaled from angles to these dimensions. Because optical information about spatial properties comes in the form of angles, the scaling mechanism must be a non-visual factor. Proffitt and Linkenauger (2013) argue that this scaling mechanism is, and must be, derived from the body.

The perceptual ruler hypothesis is supported by a variety of empirical findings that demonstrate effects of body size on judgments of spatial properties. For instance, manipulations of simulated body size in virtual environments have been shown to influence judgments of the size of other environmental objects ( van der Hoort, Guterstam, & Ehrsson, 2011 ; van der Hoort & Ehrsson, 2014 ). Furthermore, hand size influences the reported size of graspable objects ( Linkenauger et al., 2013 ; Linkenauger, Mohler, & Proffitt, 2011 ; Linkenauger, Ramenzoni, & Proffitt, 2010 ; Linkenauger, Witt, & Proffitt, 2011 ). Effective arm length, which has been manipulated by giving participants tools with which to reach, influences reported distance to objects presented beyond arm’s reach ( Bloesch, Davoli, Roth, Brockmole, & Abrams, 2012 ; Davoli, Brockmole, & Witt, 2012 ; Osiurak et al., 2012 ; Witt, 2011b ; Witt & Proffitt, 2008 ; Witt et al., 2005 ). According to this perceptual ruler account, it is not just the physical size of the body that provides a scaling mechanism but also the physiological potential and behavioral repertoire of the body. This inclusion allows for the mechanism to explain all action-specific effects to date.

From the action-resistant perception approach, there is general agreement that the information provided by the known egocentric and exocentric distance cues comes in the form of visual angles or visual directions, and that these angular values must be scaled by some aspect of the body in order to be informative ( Cutting & Vishton, 1995 ; Sedgwick, 1986 ). For each cue, a trigonometric equation can be defined that relates three values: a specific aspect of the observer’s body, an optical (angular) value, and the target’s distance. These equations explicitly show how the body-based units establish a scale that allows visual angles to be transformed into perceived distance. For instance, the distance of a target on the ground is given by the observer’s eye height divided by the tangent of the object’s angular declination below eye level ( Cutting & Vishton, 1995 ; Gajewski, Philbeck, Wirtz & Chichka, 2014 ; Mon-Williams & Bingham, 2008 ; Ooi, Wu & He, 2001 ; Sedgwick, 1986 ). In a similar way, eye height, along with shoulder width, can be used to scale the visual angles necessary to perceive the width of apertures (Warren & Whang, 1987). The existence of this kind of formal relationship constitutes an “informationally grounded” scaling mechanism, as described at length by Firestone (2013) . In cases for which the body- or action-based scaling is informationally grounded, the claim that the body provides a perceptual ruler is generally acceptable to vision scientists from both the action-resistant and action-specific perception approaches. However, more controversially, some body- or action-based scaling factors have been proposed that are not obviously rooted in explicit, informationally-grounded equations.

One example involves using hand size to perceive the size of a small, graspable object. Of course, the size of an unfamiliar object can be determined by its distance from the observer and the visual angle that the object subtends: size is equal to the tangent of the visual angle times distance. Distance can be determined via one or more of the known visual egocentric distance cues, which, as we have argued, are already scaled by body-based factors. The issue here, however, is whether hand size provides a scale for perceiving the object size, over and above the known egocentric distance cues. A relationship between hand size and object size can be informationally grounded, but only if certain criteria are met. These include that the object is already in the hand or next to the hand and that the observer is familiar with the size of her own hand. In this case, the ratio of the visual angles subtended by the hand and object is available. The object’s linear size can be determined with respect to this ratio, because the object size will be this same proportion of the known linear hand size. Several studies demonstrate effects of hand size on perceived object size when these criteria are met ( Linkenauger et al., 2010 ; Linkenauger et al., 2013 ). However, if any of the specified criteria are not met, the assumptions necessary to scale object size from hand size using these equations are no longer valid. Nevertheless, Linkenauger, Witt and Proffitt (2011) found that hand size influences judged object size even when the hand is not visible and the object and hand are at different distances—conditions that fail to satisfy the assumptions for informational grounding.

Proffitt (2013) has argued that body scaling might be mediated through calibration-like processes, involving learned relationships between visual angles and the outcomes of actions. Proffitt likens this kind of scaling to that of visually-guided actions. When performing a visually-guided action such as catching a fly ball or braking a vehicle to avoid a collision, representations of ball position or distance to obstacle are not necessary to perform the actions successfully. In these cases, the affordance is specified by optical variables, and once an observer has learned the relationship between the optical variable and the action, the observer can successfully perform the action without a representation of size or distance. Proffitt (2013) notes that body scaling in situations like using hand size to perceive the size of a small, graspable object could operate in the same way: observers learn the relationship between hand size, physical object size, and the object’s visual angle, and later are able to perceive object size on the basis of hand size and the object’s visual angle alone. A significant outstanding challenge is for proponents of the perceptual ruler account to specify and demonstrate how aspects of the body can provide a scaling mechanism when these parts of the body are not visible. Finding an informational basis for performance outcomes (such as proportion of softballs hit or golf putts made successfully) seems even more challenging.

For Firestone (2013) , the lack of an informational basis for linking one’s body or action capability and the environment is a serious weakness that undermines the logical foundation of the entire action-specific perception approach. In contrast, from the action-specific perception perspective, research investigating these mechanisms is still in its infancy, and development of well-constrained explanations is the ultimate goal even if such explanations are not available at present. Nevertheless, according to this perspective, a particular body part or action capability can still play a functional role in scaling optical information even if there is no formal informational basis for linking the body or action capability with optical information and properties of the environment. To this extent, there is disagreement between the two approaches concerning the level of specificity and constraint in the hypothesized underlying mechanisms that are required to constitute acceptable evidence for action-specific perceptual effects.

Summary and Discussion

To date, relatively little theoretical or empirical work has been devoted to distinguishing this kind of action-specific perceptual influence from those that might operate by changing the proximal information available to the visual system through eye movements or attention. As mentioned above, the action-specific perception approach proposes that action-specific non-visual factors can influence perception even when there are no differences in either the distal or proximal visual information about distance ( Proffitt, 2006 ; Witt, 2011a ). Indeed, one goal of this paper is to prompt more research on the mechanisms that could underlie action-specific effects on perception.

There is nothing about these mechanisms, as described here, that would make them mutually-exclusive. Thus, all three could play a role in relative proportions that vary depending on the context. Similarly, these proportions might be different depending on what kind of geometrical property is under consideration (e.g., geographical slant versus egocentric distance versus object size): task-dependent eye movements might play more of a role for judgments of slant while body-based scaling of visual information may play more of a role for distance judgments, for instance. Although the tendency in the literature has been to put forward one or another of these mechanisms as capable of explaining all action-specific effects, few authors have taken the stronger stance that one mechanism is the one and only correct explanation for such effects, to the exclusion of the others. Ruling out other possible mechanisms that are not mutually exclusive is a sizeable challenge, and thus supporting the stronger assertions requires especially compelling theoretical and empirical arguments. At any rate, determining the relative weighting of these three mechanisms, and characterizing them in more detail, are important topics for future research. Techniques are available for recording eye movements and controlling or manipulating the locus of attention, so determining the role of action-specific differences in visual cues (via eye movements or attention) will be relatively straightforward, assuming that appropriate methods are in place to minimize output-related processes. Although some work of this kind is in progress, much remains unknown and this is a particularly fruitful direction for future research. Distinguishing between non-visual influences of action-related information and the perceptual ruler account may be more challenging.

Moving Forward: Recommendations for Future Research

Methodological concerns.

The above discussion shows not only the remarkable complexity and subtlety of post-perceptual processing, but also the great difficulty of controlling for, or otherwise mitigating against, the influence of these factors when studying action-specific effects. While it may be impossible to completely rule out post-perceptual processes as explanations for action-specific effects, there are several methodological features that can help make a more compelling case that action capability manipulations have influenced perception itself (see also Table 1 ).

Participants in all groups should be treated as identically as possible, to avoid possible experimenter effects. Interpersonal consistency can be enhanced by several means. Experimenters should use neutral affect when interacting with participants, and instructions should either be presented in written form or be highly consistent across participants. Experimenters should not be seen to exert effort themselves, as this could serve to emphasize that action capability is somehow relevant to the experimental hypotheses. Experimenters should also avoid giving feedback or encouragement to participants, because this interaction could have the effect of treating the treatment groups differently. Ideally, the experimenters should be blind to the experimental hypotheses, to further guard against experimenter effects. When feasible, the instructions should specify what response attitude participants should adopt when responding (e.g., report on apparent versus objective distance, ignore abstract connotations of distance). This can guard against variability stemming from individual differences in the assumed response attitude.

Naturally, care should also be taken to conceal the experimental hypotheses from participants. This is especially crucial for building a compelling case for action-specific perceptual effects. Past work has attempted to do this through the use of cover stories (as in Bhalla & Proffitt, 1999 ; Durgin et al., 2009 ), non-obvious manipulations (as in Schnall et al., 2010 ), and between-subjects instead of within-subjects manipulations ( Witt et al., 2004 ; Woods, Philbeck, & Danoff, 2009 ). Indirect methods (e.g. Stefanucci & Proffitt, 2009 ; Witt, 2011b ) may also be of use—because participants are asked to report on a different aspect of the scene than the one of interest, they may be less likely to infer the correct experimental hypothesis. Another approach is to group participants according to naturally-occurring differences in action capability rather than experimentally manipulating it (e.g. Bhalla & Proffitt, 1999 ; Sugovic & Witt, 2011 , 2013 ).

Of course, use of these methods for concealing experimental hypotheses does not guarantee that they will be successful, and indeed the extent to which the hypotheses have been concealed in a given experimental paradigm can itself become a source of controversy ( Durgin et al., 2009 ; Proffitt, 2013 ). Ideally, any attempt to conceal the hypotheses should be accompanied by some measure of its success. Surveys can be useful in assessing the extent to which participants may have been aware of the experimental hypotheses and/or were influenced by them. There is a rich empirical literature on how to construct and administer unbiased surveys (e.g. Schwarz, 1999 ), and although so far there has been little development of how best to construct surveys specifically in visual space perception contexts, the established principles of survey construction should be consulted as this development moves forward. In this regard, it is important to bear in mind that even if participants are able to deduce an experimental hypothesis when responding to a survey, this does not necessarily mean that they were aware of the hypothesis at the time of making perceptual judgments nor that this awareness influenced their perceptual judgments.

Underlying Mechanisms

In addition to these methodological recommendations, our review has also identified several more substantive issues that suggest important directions for future research. Chief among these is a pressing need to characterize the mechanisms underlying action-specific effects. We have highlighted the importance of controlling or manipulating eye movements and attention for identifying action-specific effects stemming from these factors. Developing methodologies for distinguishing between the other possible mechanisms outlined above remains an important challenge for future research.

Perceptual Stability and Phenomenology

A concern frequently raised about action-specific effects on perception is that they challenge the idea that perception is stable. There are two aspects of this concern. One is phenomenological, pertaining to the extent to which changes in perception ought to be noticeable. Another pertains to how actions might be calibrated to perceptions that change with the observers’ transient state of action capability and intention to act. These issues have important implications for future research. We will address these two aspects in turn.

First, if perception changes when one’s potential to act changes, it is reasonable to expect that the perceiver should notice such changes ( Firestone, 2013 ). In this view, these transient changes in perception would constitute a breakdown in perceptual stability that arguably runs contrary to many people’s subjective experience. Upon donning a backpack, hills do not suddenly appear steeper. Upon grasping a tool, objects do not suddenly appear closer. Firestone (2013) argues that this lack of phenomenological experience of a change is evidence against the idea of action-specific perceptual effects. The central issue at stake concerning the phenomenology of action-specific perception is how much and what kind of change in perception is required for the change to become subjectively noticeable. Perception may tend to resist rapid fluctuations during eye fixations, for example, but be more likely to change rapidly (and unnoticed) across saccades or shifts of attention to match changes in action capability. There is a large literature addressing such issues in the domain of attention—particularly with reference to change blindness (e.g., Rensink, O’Regan & Clark, 1997 ; Simons, Franconeri & Reimer, 2000 ) and to contrast appearance (e.g., Anton-Erxleben, Abrams & Carrasco, 2010 ; Schneider & Komlos, 2008 ). The full relevance of this literature for perception of spatial features such as size, distance and slant remains poorly understood, however.

Witt (in press) has shown that people are not aware of a change in their perception of size in a dynamic version of the Ebbinghaus illusion. Observers viewed a center circle surrounded by large inducer circles, which switched to become small inducer circles (or vice versa). When probed afterwards about whether they noticed any change in the center circle size at the time of the switch, a large majority of participants responded that they did not. It is possible, even likely, that observers experienced the illusion both before and after the switch, but that there was some hysteresis of perception or continuity of experience during the switch itself such that the perceived size remained unchanged for a short time. That is, observers were not aware of a change in center circle size because the visual system tends to resist or filter out rapid changes in perception. Similarly, one might not be aware of a sudden change in perceived distance when putting on a heavy backpack because the visual system filters out this kind of sudden perceptual change and tends to maintain perceptual stability for some time. Important questions that remain open concern how rapidly perception can change (e.g., in the face of changes in action capability), and to what extent perception exhibits persistence when visual cues or action capability change. Answering these questions could add critical insights into, or constraints upon, the conditions under which action-specific changes in perception might be manifest. Thus, these issues constitute an important potential focus for future research.

Second, perceptual stability may seem to be a crucial precondition for effective control of actions. Perceptual coordinates must be transformed to motor coordinates prior to the execution of an action, and in principle this calibration transformation can correct for biases within perception and support accurate control of actions. If a stationary object appears to be at different distances depending on one’s transient intention to act, however, this might seem to present an insurmountable challenge for these calibration processes and should lead to widespread errors in the control of actions. A transformation function that is effective when a target is physically 10 m away but appears to be at 9 m would not be effective when that same target at 10 m appears to be at 11m. In this view, the fact that our actions continue to be relatively accurate could be taken as evidence that action-specific effects are not perceptual, because otherwise transformation functions would lead to significantly more errors.

However, actions could continue to be well-calibrated to the environment despite action-specific differences in perception if the calibrating transformations are themselves action-specific. For instance, consider an experiment examining heavy versus light ball throwing on estimated distance ( Witt et al., 2004 ). There might be one transformation function for converting perceived distance to throwing coordinates for the light ball and another for the heavy ball. So even if the target looks to be at one distance when intending to throw the light ball and another distance when intending to throw the heavy ball, the transformation functions for each ball could be calibrated to the perceptual bias associated with each ball. Although it may seem implausible to have separate calibration functions for an object depending on the effort associated with throwing to it, the complexity could be reduced if there is a common calibration function for throwing actions that is scaled by one or two effort-related parameters. When the intention to act is established, the effort parameter is set and the appropriate transformation from perceptual coordinates into motor coordinates is applied. This idea has not been tested empirically, and important issues would need to be worked out to characterize this kind of scheme adequately (for example, to what extent is the effort parameter under conscious control). Nevertheless, under this kind of scheme, even if there were action-specific failures of perceptual stability, these failures would not be problematic for maintaining reasonably accurate control of actions. A variety of techniques for investigating motor calibration have been developed (e.g., Durgin et al., 2005 ; Kurtzer, Dizio & Lackner, 2005 ; Pan, Coats & Bingham, 2014 , among a host of others), and a promising area of future research is to use such techniques to flesh out calibration functions in the domain of human egocentric distance perception.

Replicability and Reproducibility

Earlier we discussed apparent failures to replicate certain aspects of previous action-specific perceptual studies (e.g., Durgin et al., 2009 ; Durgin et al., 2012 ; Hutchison & Loomis, 2006 ; Shaffer et al., 2013 ; Woods et al., 2009 ). Some of these apparent failures to replicate have themselves been criticized on methodological grounds (e.g., Proffitt, 2009 , 2013 ; Proffitt, Stefanucci, Banton & Epstein, 2006 ), and there have been many replications of action-specific findings (Bloesch et al., 2011; Cañal-Bruland et al., 2010 ; 2012 ; Cañal-Bruland & Van der Kamp, 2009 ; Cañal-Bruland et al., 2011 ; Davoli et al., 2012 ; Doerrfeld et al., 2012 ; Eves et al., 2014 ; Gray, 2013 ; Gray et al., 2014 ; Kirsch et al., 2012 ; Kirsch & Kunde, 2013a , b ; Kuylen et al., 2014 ; Lee et al., 2012 ; Morgado et al., 2013 ; Osiurak et al., 2012 ; Taylor-Covill & Eves, 2013 ; 2014 ; Thomas et al., 2014 ; van der Hoort & Ehrsson, 2014 ; van der Hoort et al., 2011 ; Wesp et al., 2004 ; White et al., 2013 ). Controversies aside, these dialogs in the literature have illuminated important methodological issues that are important to bear in mind in future research—many of which are summarized in this review. Thus, attempts to replicate past work, not only within laboratories, but across laboratories with a history of past collaboration and, most importantly, across completely unrelated laboratories, is a vital means of characterizing the boundary conditions for action-specific influences and output-related effects. Demonstrating reproducibility across laboratories is crucial for any experimental finding, as this can serve to bolster confidence that systematic effects in the data are not due to specific aspects of a particular experimental context (for example, particular experimenters, laboratory spaces, stimulus materials, procedures, and so on). Such efforts will also be important for establishing norms for future computations of the size of effects to be detected and the sample sizes needed to detect effects of a given size. As methodologies have become more refined for studying action-specific effects, a potentially fruitful avenue of future research would be to attempt to replicate past work in a way that contrasts the original methodology with some of the more refined methods. This would provide an empirical foundation for evaluating the role of the factor that the methodology is designed to control. For example, if using neutral affect when interacting with participants is intended to minimize experimenter effects, experimentally manipulating experimenter affect in a previously-test action-based paradigm (e.g., ball throwing; Witt et al. 2004 ) would provide an empirical test of this idea.

Conclusions: Moving Towards Reconciliation

The action-specific perception and action-resistant perception approaches assume a very similar underlying model, which we have called the modal model. Both approaches assume that visual information plays a primary role in determining perceptual experience, and both approaches agree that output-related processing is important to control or otherwise factor out when interpreting behavioral responses as indicators of perception. Both also agree that there are some mechanisms that could elicit bona fide influences of action capability on perception, for example, by way of task-dependent differences in eye movements or attention or by the influence of certain non-visual factors. Importantly, this similarity in the assumed model provides a common language for conceptualizing the possible mechanisms underlying action-specific effects on behavioral indications of perception. Research from within the action-resistant perception perspective describes mechanisms that could potentially give rise to action-specific perceptual effects (e.g., eye movements or attention) and that generate non-visual influences on perception (e.g., perceptual biases and perceptual learning).

There are also important differences between the two approaches, however, specifically in terms of the emphasis they place on specific parts of the model. The assumed role of output-level factors is a primary difference between these approaches. It is important to emphasize that although the action-specific perception approach acknowledges that output-level factors can and do influence behavioral indications of perception, this approach proposes that these factors play virtually no role in explaining results of most, if not all, of the experiments targeting action-specific influences on perception. In contrast, the action-resistant perception account assumes that output-related factors account for the large majority of action-related effects.

Another major difference is that the action-specific perception perspective allows for direct influences on perception that do not stem from attention, eye movements, or differential selection or weighting of visual cues. Two alternative mechanisms have been proposed. For one, action-relevant information is integrated with visual information in a similar manner as the multimodal integration of auditory and visual information ( Witt & Riley, 2014 ). For another, action-relevant information provides a mechanism for scaling visual information from angles into units such as distance and size ( Proffitt & Linkenauger, 2013 ). One of the current limitations of the action-specific approach is the relatively sparse amount of empirical data that addresses the underlying mechanism.

Insights Gained by Rejecting All-or-Nothing Conceptualizations of Action-Specific Perception

One insight underlies a recurring theme in this review: namely, that simply debating whether or not action-specific perceptual effects exist is an overly-coarse level of analysis. If one rejects all forms of non-visual influences on perception, one must also reject a subset of research from the action-resistant perception perspective. Alternatively, if one embraces this subset of the action-resistant perception literature, one must acknowledge that legitimate action-specific perceptual effects might occur under at least some circumstances. This perspective highlights the importance of characterizing the boundary conditions under which action-specific perceptual effects do or do not occur (while ruling out output processing as much as possible) and identifying the mechanisms underlying bona fide perceptual effects.

A second insight comes from considering why some examples of non-visual influences on perception (such as those coming from the action-specific perception approach) have met with more intense criticism than other examples rooted more firmly in the action-resistant perception approach (e.g., the Specific Distance Tendency and Intrinsic Ground Plane; Gogel, 1990 ; Ooi, Wu & He, 2006 ). One notable difference is that work on non-visual factors has often involved reduced-cue viewing contexts, in which glowing targets are seen in otherwise dark environments. This permits greater control over potentially relevant visual information and accordingly enhances the opportunity to unmask influences of non-visual factors. Reduced-cue contexts thus generally afford a more well-constrained analysis of the factors that play a role in behavioral judgments than do more naturalistic viewing conditions. By contrast, for many action-specific perception researchers, more ecological viewing conditions are favored because the interest is in how perception operates in the context of action rather than isolated from action. This is not to say that reduced-cue settings are necessary for acceptance of evidence of non-visual influences on perception, but just that reduced-cue settings may have contributed to their acceptance. Other potential factors in the acceptance of some non-visual influences on perception by the action-resistant perception approach are that much of the work has been cross-validated using a variety of direct and indirect behavioral measures, that it has generally adopted a cautious and incremental approach, and that it has generally treated post-perceptual influences as complex and insidious.

We acknowledge that there is no “magic formula” that will guarantee bilateral acceptance in a contentious domain. Ultimately, bilateral acceptance of action-specific perceptual effects hinges in part upon the degree to which this research program can (1) adequately guard against post-perceptual effects, and (2) specify concrete and well-constrained underlying mechanisms. In addition, given that definitively ruling out output-level explanations of action-specific effects may be impossible, caution should also be expressed when interpreting results as favoring action-specific influences on perception. At a minimum, there should be some acknowledgement of the complexity of the issue, with some discussion of the relative likelihood of various forms of output-level biases in the studied context. Research founded in the action-specific perception approach has been increasingly mindful of many of these issues. To some extent, the criteria for what constitutes acceptable control of post-perceptual effects and what constitutes an appropriately specific mechanism vary between approaches, and bridging this gap is a central challenge for future research.


This work was sponsored in part by NIH R01-EY021771 awarded to JWP and NSF BSC-1348916 and NSF BSC-1314162 awarded to JKW.

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  • 22 April 2024

Your perception of time is skewed by what you see

  • Lilly Tozer

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A row of six clocks showing different times at night

Previous research has shown that our perception of time is linked to our senses. Credit: Karol Serewis/SOPA Images/LightRocket/Getty

How the brain processes visual information — and its perception of time — is heavily influenced by what we’re looking at, a study has found.

In the experiment, participants perceived the amount of time they had spent looking at an image differently depending on how large, cluttered or memorable the contents of the picture were. They were also more likely to remember images that they thought they had viewed for longer.

The findings, published on 22 April in Nature Human Behaviour 1 , could offer fresh insights into how people experience and keep track of time.

“For over 50 years, we’ve known that objectively longer-presented things on a screen are better remembered,” says study co-author Martin Wiener, a cognitive neuroscientist at George Mason University in Fairfax, Virginia. “This is showing for the first time, a subjectively experienced longer interval is also better remembered.”

Sense of time

Research has shown that humans’ perception of time is intrinsically linked to our senses. “Because we do not have a sensory organ dedicated to encoding time, all sensory organs are in fact conveying temporal information” says Virginie van Wassenhove, a cognitive neuroscientist at the University of Paris–Saclay in Essonne, France.

Previous studies found that basic features of an image, such as its colours and contrast, can alter people’s perceptions of time spent viewing the image. In the latest study, researchers set out to investigate whether higher-level semantic features, such as memorability, can have the same effect.

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The researchers first created a set of 252 images, which varied according to the size of the scene and how cluttered it was, then developed tests to determine whether those characteristics affected the sense of time in 52 participants. For example, an image of a well-stocked cupboard would be defined as being smaller but more cluttered than one featuring an empty warehouse. Participants were shown each image for less than a second, and asked to rate the time they were shown a specific image as ‘long’ or ‘short’.

When viewing larger or less-cluttered scenes, participants were more likely to experience time dilation; thinking that they had viewed the picture for longer than they actually did. The opposite effect — time constriction — occurred when viewing smaller-scale, more cluttered images.

The researchers suggest two possible explanations for these distortions. One posits that visual clutter is perceived as harder to navigate and move through, whereas the other says that clutter impairs our ability to recognize objects, making it harder to mentally encode the visual information. These difficulties could both lead to time constriction.

Memorable sights

To investigate whether more-memorable images could have an effect on time perception, the researchers showed 48 participants a set of 196 images rated according to their memorability by a neural network. Participants not only experienced time dilation when looking at more-memorable images, but were also more likely to remember those images the next day.

Sleep loss impairs memory of smells, worm research shows

The images were then applied to a neural-network model of the human visual system, one that could process information over time, unlike other networks that take in data only once. The model processed more-memorable images faster than less-memorable ones. A similar process in the human brain could be responsible for the time-dilation effect when looking at a memorable image, says Wiener. “It suggests that we use time to gather information about the world around us, and when we see something that’s more important, we dilate our sense of time to get more information.” This adds to converging evidence that suggests a link between memorability and increased brain processing, says van Wassenhove.

Questions remain about exactly how people perceive time and how this interacts with memory. “We’re still missing pieces of the puzzle,” says Wiener. The next step would be to validate the findings with a larger sample of participants, and to refine the model of the visual system, he adds. Van Wassenhove suggest that future studies could use neuroimaging to study brain activity during perception tests. Eventually, Wiener hopes to test whether the brain could be stimulated artificially to influence the way it processes time and memory.

doi: https://doi.org/10.1038/d41586-024-01169-3

Ma, A.C., Cameron, A.D., Wiener, M. Nature Hum. Behav. https://doi.org/10.1038/s41562-024-01863-2 (2024).

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Why The Love Hypothesis Could Kickstart More Romance Film Adaptations

Quick links, what is the love hypothesis about, the love hypothesis could pave the way for other unconventional adaptations, are authors like ali hazelwood and emily henry changing the perception of romance.

  • Ali Hazelwood's The Love Hypothesis originally began as a Rey and Kylo Ren fanfiction focusing on the characters in a STEM setting, and the novel has achieved great success.
  • The upcoming film adaptation of The Love Hypothesis could pave the way for more unconventional book adaptations.
  • Authors like Ali Hazelwood and Emily Henry and works like Bridgerton and Red, White and Royal Blue are changing the perception of romance novels.

Ali Hazelwood's romance novel The Love Hypothesis took BookTok by storm in 2021, and part of its whimsical appeal was that it began as Star Wars fan fiction. Originally published in 2018 on Archive of Our Own as a work called "Head Over Feet," it detailed a modern interpretation of the relationship between Rey and Kylo Ren set against the backdrop of Stanford's graduate program. Though all the references to Star Wars were cut in the final draft of the novel, the similarities in characters are still there, and it's been a major part of why the novel was so successful.

Successful enough, in fact, that in October 2022, it was announced that Bisous Pictures, which specializes in romantic films, acquired the rights to the novel. The film adaptation is currently in pre-production. Depending on how successful it is, The Love Hypothesis has the potential to pave the way for more romance adaptations -- especially those that originated in equally unconventional locations.

Updated on April 15th, 2024 by Fawzia Khan: While The Love Hypothesis movie is still very much in preproduction and there are few updates about it, fans stay on tenterhooks for the STEM romance to come to life on screen. Ali Hazelwood's book is truly an outlier -- a fanfiction work that was turned into an independent novel, set in STEM, a setting so unusual that it had not been explored before. However, Hazelwood's storytelling turns even the science lab into a romantic playground, giving impetus to all sorts of romance subgenres that might not have been greenlit earlier. This feature has been updated with further information about romance novel adaptations and their future.

10 High Fantasy Romance Movies That Combine Love with Adventure

Star wars rebels foreshadowed rey and kylo ren's force bond.

The Love Hypothesis begins explosively: Stanford graduate student Olive plants a kiss on Dr. Adam Carlsen, a known grump who has gained a reputation for tanking the research dreams of many students. The kiss culminates into something bigger, and Olive decides to enter a fake relationship with Adam Carlsen in order to convince her best friend, Anh that she is over her ex-boyfriend Jeremy, whom Anh has feelings for. While Olive wants Anh to pursue happiness, Adam's motivations for the fake relationship lie in his research funding, which has been frozen by the university as they predict that he will leave their lab and move to another. Being in a relationship would give him a sense of permanence at Stanford, and Olive would regain a sense of dignity once Anh would pursue her romance without guilt. Neither Olive nor Adam is too enthused about this arrangement -- after all, Adam Carlsen is the bane of most graduate students' existence, and he's known throughout the program for his ruthlessness and, at times, rudeness. The Love Hypothesis has all the makings of a romance book headed to the big screen.

  • The Love Hypothesis was published on September 14, 2021.
  • It has a 4.15/5 rating on Goodreads, with nearly 150,000 reviews.

However, as their relationship progresses, they each have to come to terms with their feelings, which are beginning to transcend far beyond what their initial arrangement entailed. Their romance may be a farce, but Hazelwood uses well-known tropes in a fresh manner to make audiences feel butterflies as the two protagonists interact. The realities of life in academia, and Olive's own tragic backstory bring a lot of depth into the plot. Their growing love for each other is marred many times, especially when Adam's old friend, Dr. Tom Benton, decides to harrass Olive and derail her research, which makes her distance herself from him. The Love Hypothesis is a beautiful love story that will translate flawlessly to the big screen , especially because of how unique it is.

10 Best Magical Romance Movies With The Most Whimsical Plots

The Love Hypothesis is the most recent in a long line of movies adapted from fan works. In fact, Ali Hazelwood's trajectory from fanfic writer to New York Times bestselling author happened when Thao Le from the Sandra Dijkstra Literary Agency saw her works online and reached out to her to submit. In the past, authors like E.L. James have tried to downplay that the origins of their novels (the Fifty Shades trilogy, in this case) lay in fiction that was associated with existing properties. Seeing how the success of The Love Hypothesis was intrinsically tied up with the appeal of the Star Wars couple, it is becoming clear that the landscape of publication and cinematic adaptation has had a total overhaul. Increasingly, publishing houses are turning to well-known fan fiction authors to revise their works and send them out into the world. Often, these books sell well in part because of their obvious association with a better-known intellectual property, which then makes them prime candidates to be turned into films.

Similarly, the City of Bones series (including the associated movies and Shadowhunters television show ) has its roots in the Harry Potter series. If fan fiction evolves into a prevalent source for movies, the possibilities for future adaptations are endless. Films and TV shows based on graphic novels, such as Nimona and Heartstopper, have recently gained traction . Video game movies, no matter how controversial they tend to be, are slowly making their way into the cultural eye with big titles such as Uncharted or Five Nights at Freddy's . Though romantic movies in the past have been primarily based on published novels or entirely original, the increasing number of fan fiction and graphic novel adaptations could lead to a broader future for the romantic genre. If The Love Hypothesis is successful (which it likely will be), rom-coms may see a major renaissance, and perhaps the source material for them will be diversified as well.

10 Best TV Series With Amazing Romances

In short, yes. Both romance novels and fanfiction were long considered guilty pleasure genres, consumed by those who liked reading adult or "spicy" content. Fortunately, the success of The Love Hypothesis and other such books has brought this genre into the mainstream, as larger and larger studios are queuing up to adapt romance novels into movies. As "women-centric" movies become box office hits, romance novels have further opportunities for getting that coveted adaptation. In addition to Ali Hazelwood, Emily Henry has become a studio favorite, with every one of her romance novels getting the go-ahead for big-screen adaptations.

Additionally, romance is no longer just about heterosexual couples. Red, White, and Royal Blue proved that LGBTQ+ love stories are very much the next step for the romance genre; a much-needed update to keep it current and with the times. Bridgerton's roaring success also brought forth an important aspect: romance fans want to hear diverse stories of different cultures too to reaffirm their belief that love is love, no matter where one is from. A growing acceptance of the romance genre, and the recognition of it as a true art form worthy of investment and adaptation has brought fresh growth to it, as well as to cinema.

Why The Love Hypothesis Could Kickstart More Romance Film Adaptations


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    The hypothesis that internal representations are hypotheses that play a key role in perception and action formed the basis of the cognitive revolution (e.g., Gregory, 1980; Neisser, 1967) and within social psychology, implicit attitudes, stereotyping and prejudice are predicated on the idea that information inside the head shapes experience of ...

  6. Perceptual theories—direct, indirect, and computational

    The idea of direct perception can also be thought to be more consistent with what we know about the perceptual systems of animals lower down on the phylogenetic scale. Their behaviour appears to be controlled more directly by the available sensory information and there appears to be no need to invoke ideas of inference or hypothesis testing ...

  7. Perceptions as hypotheses

    To understand perception, the signal codes and the stored knowledge or assumptions used for deriving perceptual hypotheses must be discovered. Systematic perceptual errors are important clues for appreciating signal channel limitations, and for discovering hypothesis-generating procedures.

  8. Perceptions as Hypotheses

    Abstract. Philosophers concerned with perception traditionally consider phenomena of perception which may readily be verified by individual observation and a minimum of apparatus. Experimental psychologists and physiologists, on the other hand, tend to use elaborate experimental apparatus and sophisticated techniques, so that individual ...

  9. Beliefs and desires in the predictive brain

    The first is perceptual inference, where incoming sensory signals are used to adjust hypotheses at higher levels, such that the hypotheses more closely match the outside world.

  10. Optimality and heuristics in perceptual neuroscience

    A crowning achievement of optimality theory in perception is the application of principles of statistical decision-making, in the form of signal detection theory, to psychophysical measurement 3,4 ...

  11. The Common Kind Theory and The Concept of Perceptual Experience

    In this paper, I advance a new hypothesis about what the ordinary concept of perceptual experience might be. To a first approximation, my hypothesis is that it is the concept of something that seems to present mind-independent objects. Along the way, I reveal two important errors in Michael Martin's argument for the very different view that the ordinary concept of perceptual experience is ...

  12. Embodiment and the Perceptual Hypothesis

    The Perceptual Hypothesis opposes Inferentialism, which is the view that our knowledge of others' mental features is always inferential. The claim that some mental features are embodied is the claim that some mental features are realised by states or processes that extend beyond the brain. The view I discuss here is that the Perceptual ...

  13. Frontiers

    If perception corresponds to hypothesis testing ( Gregory, 1980 ); then visual searches might be construed as experiments that generate sensory data. In this work, we explore the idea that saccadic eye movements are optimal experiments, in which data are gathered to test hypotheses or beliefs about how those data are caused.

  14. PDF Paradigms of Perception

    Perception: Conscious sensory experience. Electrical signals that represent something (eg. seeing a tiger) are somehow transformed into your experience of seeing a "tiger". Recognition: Our ability to place an object in a category, such as "tiger," that gives its meaning. Some questions about Perception.

  15. Action-based Theories of Perception

    Action-based Theories of Perception. First published Wed Jul 8, 2015; substantive revision Tue Sep 19, 2023. Action is a means of acquiring perceptual information about the environment. Turning around, for example, alters your spatial relations to surrounding objects and, hence, which of their properties you visually perceive.

  16. Understanding human perception by human-made illusions

    Gregory proposed that perception shows the quality of hypothesis testing and that illusions make us clear how these hypotheses are formulated and on which data they are based (Gregory, 1970). One of the key assumptions for hypothesis testing is that perception is a constructive process depending on top-down processing. Such top-down processes ...

  17. Attention and Conscious Perception in the Hypothesis Testing Brain

    The core idea is that conscious perception correlates with activity, spanning multiple levels of the cortical hierarchy, which best suppresses precise prediction error: what gets selected for conscious perception is the hypothesis or model that, given the widest context, is currently most closely guided by the current (precise) prediction errors 5.

  18. Perceptual Set In Psychology: Definition & Examples

    The hypothesis for this study was based on a well-known finding that the more we are exposed to a stimulus, the more familiar we become with it and the more we like it. ... Perception, 1, 417-425. Gilchrist, J. C.; Nesberg, Lloyd S. (1952). Need and perceptual change in need-related objects. Journal of Experimental Psychology, Vol 44(6).

  19. Key Theories On The Psychology Of Perception

    Visual perception: sight perceived through the eyes. Auditory perception: sounds perceived through the ears. Gustatory perception: awareness of flavor and taste on the tongue. Olfactory perception: smelling via the nose. Tactile perception: awareness of sensation on the skin. Vestibular sense: perception of balance and motion.

  20. 6.1 The Process of Perception

    Perception is the process of selecting, organizing, and interpreting information. This process affects our communication because we respond to stimuli differently, whether they are objects or persons, based on how we perceive them. Given the massive amounts of stimuli taken in by our senses, we only select a portion of the incoming information ...

  21. Action-Specific Influences on Perception and Post-Perceptual Processes

    More recently, Proffitt and Linkenauger (2013) have proposed a "perceptual ruler" hypothesis to explain action-specific effects (Proffitt & Linkenauger, 2013). Optical information specifying size, distance, and other spatial properties of objects takes the form of visual angles. For example, binocular disparity is a difference in the visual ...

  22. PDF Perception as hypothesis testing

    perception involves testing hypotheses, and that once a hypothesis is established it may prevent or delay the acceptance of an alternative hypothesis. This interpreta­ tion thus attributes the negative effect of prior exper-This research was supported by a grant to the senior author

  23. Embodiment and the Perceptual Hypothesis

    The Perceptual Hypothesis opposes Inferentialism, which is the view that our knowledge of others' mental features is always inferential. The claim that some mental features are embodied is the claim that some mental features are realised by states or processes that extend beyond the brain. The view I discuss here is that the Perceptual ...

  24. Your perception of time is skewed by what you see

    Your perception of time is skewed by what you see. Features of a scene such as size and clutter can affect the brain's sense of how much time has passed while observing it. Previous research has ...

  25. Why The Love Hypothesis Could Kickstart More Romance Film Adaptations

    Ali Hazelwood's romance novel The Love Hypothesis took BookTok by storm in 2021, and part of its whimsical appeal was that it began as Star Wars fan fiction. Originally published in 2018 on ...