Topic 2Perception

Outline of course

• Perception Theories of perceptual organisation • Gregory’s top down/indirect theory of perception • Gibson’s bottom up/direct theory of perception

• Development of perception • The development of perceptual abilities, including depth/distance, visual

constancies. • Perceptual development, including infant and cross-cultural research

• Face recognition and visual agnosias • Bruce and Young’s theory of face recognition, including case studies and

explanations of prosopagnosia

Perception Theories of perceptual organisation• Perception Theories of perceptual organisation • Gregory’s top down/indirect theory of perception • Gibson’s bottom up/direct theory of perception

• Intro 13.21

Gregory’s top down/indirect theory of perception

• Gregory (1970, 1972, 1973) is a present-day constructivist theorist who argues that perception is an active and constructive process in which top-down processing is an essential element. He believes that the data (information) we derive from the images that impinge on the retinas are insufficient for us to build up an accurate picture of the world. He refers to the data obtained as “fragmentary scraps”. Gregory believes that various cognitive processes are involved in perception. Schemas based on expectations are amongst the most important of these.

Hollow Face

• Constructivists like Gregory suggest that our perception of the world is strongly affected by our existing schemas. • Schemas are like categories into which we incorporate the information from

our experiences. One example of a schema in the world of visual perception is that of faces. Our “face” schema tells us that however different one face is from another, they have certain common features, such as a nose that protrudes, two eyes, a mouth, and so on. One of Gregory’s favourite demonstrations of the role of schemas based on expectation is the “hollow face illusion” in which people view a hollowed mask of a face. Although there is sufficient information to see the mask as hollow, we still perceive a normal face. Our schema of a face overrides the evidence of our own eyes and we accept an incorrect hypothesis.

• According to Gregory, perception is a series of hypothesis-testing exercises. As we encounter certain life experiences, we “call up” the most appropriate, the most relevant, or the most recent schemas and use these to provide the context in which to interpret our present experience. Most of the time these hypotheses are accurate—they fit our expectations. Sometimes, however, the schema does not fit the occasion and perception is made difficult or inaccurate.

Gregory’s Explanation of Visual Illusions• Gregory argued that we treat two-dimensional illusions as if they

were three-dimensional, even though we know they are only two-dimensional. Gregory suggests the misapplied size–constancy theory explains several illusions.

• Gregory suggests that when we look at the Müller–Lyer illusion and the “fins” out of context, there are no cues to help us perceive their length so we draw on existing hypotheses and expectations. These tell us that the figure with the fins diverging represents an inside corner and the one with the fins converging represents an outside corner. Since it is part of our expectation (according to Gregory) that outside corners are usually further away than inside ones, when the vertical line is the same length, the “outside” corner one is perceived as longer since it is perceived as further away.

In a sense, we perceive a two-dimensional figure as if it were a three-dimensional one. This may well be true since if these fins are presented as luminous figures in a dark room, they do appear three-dimensional.

http://www.ted.com/talks/al_seckel_says_our_brains_are_mis_wired?language=en

Summary

• A lot of information reaches the eye, but much is lost by the time it reaches the brain (Gregory estimates about 90% is lost).

• Therefore, the brain has to guess what a person sees based on past experiences. We actively construct our perception of reality.

• Richard Gregory proposed that perception involves a lot of hypothesis testing to make sense of the information presented to the sense organs.

• Our perceptions of the world are hypotheses based on past experiences and stored information.

• Sensory receptors receive information from the environment, which is then combined with previously stored information about the world which we have built up as a result of experience.

• The formation of incorrect hypotheses will lead to errors of perception (e.g. visual illusions like the Necker cube).

RESEARCH EVIDENCE FOR GREGORY’S TOP-DOWN/INDIRECT THEORY

• Evidence that expectations influence our everyday perception comes from situations in which expectations produce errors in perception. For example, proofreading often involves numerous mistakes. Daneman and Stainton (1993) found that errors were more common when readers were proofreading an essay handwritten by them rather than an essay written by someone else. Their extreme familiarity with their own writing meant that they focused less on possible errors in their own essay.

RESEARCH EVIDENCE FOR

• Bruner and Goodman (1947) asked rich and poor children to estimate the size of coins. The poor children overestimated the size of every coin more than did the rich children, which may reflect the greater value placed on money by the poor children.

RESEARCH EVIDENCE FOR

• Ashley, Harper, and Runyon (1951,) introduced an ingenious modification to the experimental design used by Bruner and Goodman (1947). They hypnotised adult participants into believing they were rich or poor. The size estimates of coins were consistently larger when the participants were in the “poor” state.

RESEARCH EVIDENCE AGAINST GREGORY’S TOP-DOWN/INDIRECT THEORY

• Bruner and Goodman (1947) may not support expectation because there is a simpler explanation. The rich children would have had more familiarity with coins, and this may have made them more accurate in their size estimates.

RESEARCH EVIDENCE AGAINST

• DeLucia and Hochberg (1991) reported findings that are very hard (or impossible) to explain using Gregory’s theory. They presented observers with three-dimensional 2-foot-high fins placed in a line on the floor in an arrangement like that shown in the Müller–Lyer. The usual illusion effect was obtained even though it was obvious that all the fins were the same distance from the observers. This finding can’t be explained by Gregory’s misapplied size–constancy theory.

EVALUATION OF GREGORY’S TOP-DOWN/INDIRECT THEORY• Strong evidence that top-down processing influences perception.

Gregory and other indirect theorists have performed the valuable service of showing that top-down processes shouldn’t be ignored and there is strong evidence for this.• Considers motivational and emotional factors. Gregory’s theory is wide-

ranging in that it assumes that motivational and emotional factors can influence perception, and so it is more holistic than other theories.• Explains inaccuracies in perception. Misapplied size–constancy does

explain inaccuracies in perception. The artificiality of such evidence is a weakness however; proofreading errors show that expectations do play a role in real life.

EVALUATION

• It does not apply to many other variations of the illusion. The most obvious problem with the theory is that there are numerous variations on the Müller–Lyer that produce just as strong an illusion, if not stronger, and yet cannot be explained in terms of misapplying the rules of size constancy. The illusion still works if the lines are horizontal, if they are absent, or if the fins are replaced by squares, circles, or other shapes. None of these figures could be said to represent the corner of a building.• Artificiality of research may limit bottom-up processing and exaggerate top-

down-processing. The research involves the very rapid presentation of visual stimuli. Brief presentation reduces the impact of bottom-up processes, thus allowing more scope for top-down processes to operate. This is very artificial because in real life we have longer than a fraction of a second to look at an object.

EVALUATION

• The information received is not as fragmented and inadequate as Gregory states. Gregory points to the inadequacy and ambiguity of visual information. For example, that an object in the distance can produce an identical image on the retina as a small object in the foreground, so we cannot judge the size of the retinal image alone. Hence, how do we perceive depth if not on the basis of additional cognitive processes? However, as Gibson pointed out, we have a wealth of other information in the optic array available in addition to relative size: the texture gradient of the surroundings, the height in the horizontal field, and the relative size of other objects, to name but a few. The optic array we have at our disposal is far more complex than Gregory proposed.

Test

• Essay hand in Friday

Gibson’s bottom up/direct theory of perception

• The main theme of Gibson’s theory (1950, 1966, 1979) is that the sensations received by the visual system are so highly organised and rich in information that we do not need to interpret them to make them meaningful; we can perceive them directly. This means that there are no intermediate stages between light reaching the retina and an animal’s response to it. According to Gibson, all animals use information from the visual system to move around their environment without needing to make use of internal hypotheses or thought processes—perception is automatic. Because of the emphasis on environmental stimuli, Gibson’s theory is also known as the theory of ecological optics.

The Optic Array

The most important source of information is the optic array, which contains all the visual information available at the retina. Gibson argued that as we move around the environment, we effortlessly detect information from the whole of the optic array. Since this optic array provides a great deal of unambiguous information, little or no information processing is required. This information comes in many forms, including optic flow patterns, movement, and texture gradients.

The Optic Flow

Gibson was particularly interested in the problems pilots experience when landing. Imagine you are a pilot coming into land. The point you focus on, called the “pole”, is static, while the remainder of your field of vision is moving. The further from the pole an object or scene is, the quicker it appears to move. All of this information is known as the optic flow pattern, and it provides unmistakable information on direction, speed, and altitude.

Perception and Action

In the normal course of our daily lives, we may not be flying aeroplanes, but we are moving around our environment. Gibson consistently argued that there are very close links between perception and action. We do not perceive our world from a static viewpoint but by moving around and interacting in it, and this supports the idea that perception is entirely dependent on action.

When an observer moves, the entire optic array is transformed. It is this changing scenery that provides crucial information in the perception of the world. Gibson was critical of other theorists in the field of perception for concentrating on how we perceive a static object (or even glimmer of light) when we stare at it. He pointed out that this is simply not how we perceive in ordinary everyday life.

Invariants

As an animal moves around in its natural environment, it identifies those aspects of it that remain the same—that are invariant. These invariants provide essential information about the layout of the environment. This makes the layout of objects in space unambiguous, so they can be picked up directly by the perceiver. For a pilot, the “pole” is an invariant.

Texture Gradients

Another invariant is texture gradient in the natural world. When we look at almost any scene, the texture of things changes as they get further away (think of a sandy beach, or brickwork); in essence, the texture appears finer in the distance than in the foreground. Since texture gradient is a characteristic of almost any surface in the natural world around us, this is yet another cue to perception. Again, it is an aspect of the environment that is visible to the observer, not one that needs to be constructed in the brain.

Resonance

Gibson believed that we are able to make use of the rich source of environmental information because our nervous system has been specially attuned by evolution to receive it. He drew an analogy to a radio in the same way that this resonates with the information contained in the electromagnetic radiation so we are equipped to make direct use of information from the environment without the need for constant judgements and hypothesis testing, as implied by constructivist theories. Like a radio, we operate as a single unit and if one part goes wrong, the system does not work. In a similar way, Gibson argued, the nervous system works as a single unit when perceiving.

Affordances

According to Gibson, we can only understand an animal’s perceptual system in the context within which it has evolved, particularly in terms of the optic arrays to which it has been exposed. Within this context, we attach meaning to the type of visual information we receive. Constructivist theorists believe that these meanings are attached by means of “top-down” processes, as a result of our memory and expectations based on past experience. However, Gibson disagreed. He argued that all the potential uses of an object, called affordances, are perceived directly. We would, for example, “afford” a ladder the properties of being able to assist ascent or descent, by considering the gaps between successive rungs in relation to the length of our legs. The importance of action is again of great importance here. They emphasise that our perception of affordances (our perception of the possibilities for action offered by the environment) can only be accurately assessed if we move around. The notion of affordances is therefore important in this theory because it emphasises that all the information needed to make sense of the environment is directly present in the visual input and that perception depends on action.

horizon ratio relation

How much an object is above the horizon to how much appears below remains constant (invariant) as the object gets closer or further away from the observer.

Visual Cliff

• Link• Handout

RESEARCH EVIDENCE FOR GIBSON’S DIRECT PERCEPTION THEORY Warren and Hannon (1988) produced films consisting of patterns of moving dots simulating the optic flow that would be produced if someone were moving in a given direction. As predicted by Gibson, observers used the optic-flow information to make accurate judgements of the direction in which they were heading.

RESEARCH EVIDENCE FOR

• According to Milner and Goodale (1995, 1998) we have two visual perceptual systems, which often interact with each other and so aren’t entirely separate. The vision-for-perception system is used to decide that the animal in front of us is a cat or a buffalo or to admire a beautiful painting. The vision-for-action system is used for visually guided action. It resembles what Gibson had in mind when he argued that perception and action are very closely linked and so supports Gibson’s theory.

RESEARCH EVIDENCE FOR

• Haart et al. (1999) found the Müller–Lyer illusion is only obtained when the vision-for-perception system is used. There was now no illusion effect at all when the vision-for-action system was used. This finding has also been obtained with other visual illusions. This shows that the vision-for-action system generally operates in the way proposed by Gibson.

RESEARCH EVIDENCE AGAINST GIBSON’S DIRECT PERCEPTION THEORY • Van den Berg and Brenner (1994) provide evidence that optic flow

doesn’t sufficiently account for depth perception. We only need one eye to use optic-flow information. However, judgements about the direction in which observers were moving were more accurate when they used two eyes rather than only one. This happened because of what is known as binocular disparity—the slight difference in images on the retina of each eye allows observers to obtain additional information about the relative depths of objects.

RESEARCH EVIDENCE AGAINST

• Rushton et al. (1998) provide evidence that perceived visual direction is more important than optic-flow information. In their study they produced a conflict between perceived visual direction (the direction in which observers seemed to be heading) and optic-flow information. The conflict was achieved by the observers wearing prisms that provided misleading information about visual direction but accurate information about optic flow. If optic-flow information were all-important, the prisms should have had no effect on observers’ direction of walking—they should have walked straight towards the target. In fact, however, observers walked along a curved path as would be expected if they were focusing on perceived visual direction. This finding is bad news for Gibson’s theory because it suggests that optic-flow information is less important than she assumed.

• However, Gibson’s approach is threatened by the existence of a large number of visual illusions in which perception is very inaccurate. In the Müller–Lyer illusion, participants are asked to compare the lengths of two vertical Most people think one of the vertical lines is longer than the other, when in fact they are the same length. In the Ebbinghaus illusion, the central circle surrounded by smaller circles looks larger than a central circle of the same size surrounded by larger circles. In the Ponzo illusion), rectangle A looks larger than rectangle B although they are actually the same length. These illusions would not occur if perception was solely bottom-up as Gibson suggests.

Ebbinghaus illusion

RESEARCH EVIDENCE AGAINST

• Menzel’s (1978) research shows Gibson’s key omission: he ignored cognition. Chimpanzees were carried around a field, and were shown the locations of 20 pieces of food. When each chimpanzee was then released, it moved around the field picking up the food efficiently. There could be no relevant information in the light reaching the chimpanzees (because they were now moving independently rather than being carried), and so they must have made use of stored information in long-term memory to guide their search, which Gibson did not account for.

EVALUATION OF GIBSON’S DIRECT PERCEPTION THEORY • It draws attention to the rich source of information available to the perceiver.

Gibson pointed to the amazingly rich source of stimulation that is available to a moving perceiver and to the fact that the light reflected from objects must be organised in some way. Previously, theorists had concentrated only on the image on the retina, not the way in which this changed as the observer moved. Gibson pointed out that we have far more information than that at our disposal.

• It accounts well for everyday perceptions. Gibson’s theory accounts well for the considerable accuracy of our everyday perception. To those who argue that many visual illusions show that our perception is not accurate, his reply was that such data are obtained because people are presented with ambiguous, brief, or disjointed stimuli in very artificial conditions that have little relationship to perception in everyday conditions. He had a point, as research shows no visual illusions with the perception-for-action system.

EVALUATION

• It takes account of evolution. Considering that all animals have evolved in response to the demands of their particular environment, it is highly probable that they do possess some in-built systems for receiving external information that is important to their survival.• An underestimation of the difficulties of perceiving the world. Even though

Gibson has shown that various perceptions (such as the perception of depth) could be accomplished by external cues alone, that does not mean that they are accomplished in this way. Gibson underestimated the difficulty involved in detecting and using the aspects of the environment in order to perceive the world correctly. The processes involved in identifying invariants in the environment, in discovering affordances, and in producing resonance, are all much more complex than was assumed by Gibson. None of these achievements of visual perception happens as rapidly and automatically as Gibson claimed.

EVALUATION

• Gibson exaggerated the role of optic-flow information. This is not the only factor that we use to move successfully around the environment. Such information is sometimes used, but it is much less important than assumed by Gibson, as there are other factors, including visual direction and binocular disparity.

• The need for cognitive processes in perception of objects. Few cells or neural networks have been found that pick up the very complex features of the world. It may be that we have the innate capability to directly perceive something very basic and essential for survival, such as depth, but we may not have cells that respond to a particular object, such as a goose. Some cognitive processes must need to operate when we identify such a creature, as supported by Menzel’s (1978) research on chimpanzees, as discussed previously.

• Ignored the additional meaning attached to objects. Gibson provides a valuable account of seeing (especially when related to action), but he did not consider the additional meaning that is attached to what is seen.

Question

• Outline two theories of perceptual organisation (8)• Evaluate one of the theories of perceptual organisation outlined in

your answer (14)