THEORIES OF PERCEPTION
In order to receive information from the environment we are equipped with sense organs eg eye, ear, nose. Each sense organ is part of a sensory system which receives sensory inputs and transmits sensory information to the brain. A particular problem for psychologists is to explain the process by which the physical energy received by sense organs forms the basis of perceptual experience. Sensory inputs are somehow converted into perceptions of desks and computers, flowers and buildings, cars and planes; into sights, sounds, smells, taste and touch experiences.
A major theoretical issue on which psychologists are divided is the extent to which perception relies directly on the information present in the stimulus. Some argue that perceptual processes are not direct, but depend on the perceiver's expectations and previous knowledge as well as the information available in the stimulus itself. This controversy is discussed with respect to Gibson (1966) who has proposed a direct theory of perception which is a 'bottom-up' theory, and Gregory (1970) who has proposed a constructivist (indirect) theory of perception which is a 'top-down' theory.
EXPLAINING PERCEPTION - A TOP-DOWN APPROACH
Helmholtz (1821-1894) is considered one of the founders of perceptual research. He argued that between sensations and our conscious perception of the real world there must be intermediate processes. Such processes would be, for example, 'inferential thinking' - which allows us to go beyond the evidence of the senses (these inferences are at an unconscious level). Thus Helmholtz was an early Constructivist who believed perception is more than direct registration of sensations, but that other events intervene between stimulation and experience.
An early illustration that supports the idea of perceptions as modifiable constructions rather than the direct responses to pattern of stimulation is the 'Ames Room'. This room is of an irregular shape with a receding rear wall and decorated in a special manner.
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B A
One explanation for the Ames Room illusion is that the perceiver is in a situation of having to choose between two beliefs built up through experience - (a) rooms that look rectangular and normal, usually are just that, (b) people are usually of 'average' size. Most observers choose (a) and therefore consider the people to be 'odd'.
The interesting thing about the Ames Room illusion is that it does not disappear when you learn the true shape of the room.
PERCEPTIONS AS HYPOTHESES - R L GREGORY (B 1923)
Gregory proposes that perceiving is an activity resembling hypothesis formation and testing. He says that signals received by the sensory receptors trigger neural events, and appropriate knowledge interacts with these inputs to enable us to makes sense of the world.
Gregory has presented evidence in support of his theory, some of which is outlined below:
1. 'Perception allows behaviour to be generally appropriate to non-sensed object characteristics'.
For example, we respond to certain objects as though they are doors even though we can only see a long narrow rectangle as the door is ajar.
2. 'Perceptions can be ambiguous'
The Necker cube is a good example of this. When you stare at the crosses on the cube the orientation can suddenly change, or flip'. It becomes unstable and a single physical pattern can produce two perceptions.
3. 'Highly unlikely objects tend to be mistaken for likely objects'.
Gregory has demonstrated this with a hollow mask of a face. Such a mask is generally seen as normal, even when one knows and feels the real mask. There seems to be an overwhelming need to reconstruct the face, similar to Helmholtz's description of 'unconscious inference'.
What we have seen so far would seem to confirm that indeed we do interpret the information that we receive, in other words, perception is a top down process. However:….
EVALUATION OF THE TOP-DOWN APPROACII TO PERCEPTION
1. The Nature of Perceptual Hypotheses
If perceptions make use of hypothesis testing the question can be asked 'what kind of hypotheses are they?' Scientists modify a hypothesis according to the support they find for it so are we as perceivers also able to modify our hypotheses? In some cases it would seem the answer is yes. For example, look at the figure below:
This probably looks like a random arrangement of black shapes. In fact there is a hidden face in there, can you see it? The face is looking straight ahead and is in the top half of the picture in the centre. Now can you see it? The figure is strongly lit from the side and has long hair and a beard.
Once the face is discovered, very rapid perceptual learning takes place and the ambiguous picture now obviously contains a face each time we look at it. We have learned to perceive the stimulus in a different way.
Although in some cases, as in the ambiguous face picture, there is a direct relationship between modifying hypotheses and perception, in other cases this is not so evident. For example, illusions persist even when we have full knowledge of them (e.g. the inverted face, Gregory 1974). One would expect that the knowledge we have learned (from, say, touching the face and confirming that it is not 'normal') would modify our hypotheses in an adaptive manner. The current hypothesis testing theories cannot explain this lack of a relationship between learning and perception.
2. Perceptual Development
A perplexing question for the constructivists who propose perception is essentially top-down in nature is 'how can the neonate ever perceive?' If we all have to construct our own worlds based on past experiences why are our perceptions so similar, even across cultures? Relying on individual constructs for making sense of the world makes perception a very individual and chancy process.
The constructivist approach stresses the role of knowledge in perception and therefore is against the nativist approach to perceptual development. However, a substantial body of evidence has been accrued favouring the nativist approach, for example:
Newborn infants show shape constancy (Slater & Morison, 1985); they prefer their mother's voice to other voices (De Casper & Fifer, 1980); and it has been established that they prefer normal features to scrambled features as early as 5 minutes after birth.
3. Sensory Evidence
Perhaps the major criticism of the constructivists is that they have underestimated the richness of sensory evidence available to perceivers in the real world (as opposed to the laboratory where much of the constructivists' evidence has come from).
Constructivists like Gregory frequently use the example of size constancy to support their explanations. That is, we correctly perceive the size of an object even though the retinal image of an object shrinks as the object recedes. They propose that sensory evidence from other sources must be available for us to be able to do this.
However, in the real world, retinal images are rarely seen in isolation (as is possible in the laboratory). There is a rich array of sensory information including other objects, background, the distant horizon and movement. This rich source of sensory information is important to the second approach to explaining perception that we will examine, namely the direct approach to perception as proposed by Gibson.
A DIRECT APPROACH TO PERCEPTION - GIBSON 1966
Gibson claimed that perception is, in an important sense, direct. He worked during World War II on problems of pilot selection and testing and came to realise:
In his early work on aviation he discovered what he called 'optic flow patterns'. When pilots approach a landing strip the point towards which the pilot is moving appears motionless, with the rest of the visual environment apparently moving away from that point.
The outflow of the optic array in a landing glide.
According to Gibson such optic flow patterns can provide pilots with unambiguous information about their direction, speed and altitude.
Three important components of Gibson's Theory are 1. Optic Flow Patterns; 2. Invariant Features; and 3. Affordances. These are now discussed.
1. Light and the Environment - Optic Flow Patterns
Changes in the flow of the optic array contain important information about what type of movement is taking place. For example:
2Any flow in the optic array means that the perceiver is moving, if there is no flow the perceiver is static.
3The flow of the optic array will either be coming from a particular point or moving towards one. The centre of that movement indicates the direction in which the perceiver is moving. If a flow seems to be coming out from a particular point, this means the perceiver is moving towards that point; but if the flow seems to be moving towards that point, then the perceiver is moving away. See above for moving towards an object, below is moving away:
The Optic Flow pattern for a person looking out of the back of a train.
2. The role of Invariants in perception
We rarely see a static view of an object or scene. When we move our head and eyes or walk around our environment, things move in and out of our viewing fields. Textures expand as you approach an object and contract as you move away. There is a pattern or structure available in such texture gradients which provides a source of information about the environment. This flow of texture is INVARIANT, ie it always occurs in the same way as we move around our environment and, according to Gibson, is an important direct cue to depth. Two good examples of invariants are texture and linear perspective.
3. Affordances
Are, in short, cues in the environment that aid perception. Important cues in the environment include:
OPTICAL ARRAY The patterns of light that reach the eye from the environment.
RELATIVEObjects with brighter, clearer
BRIGHTNESSimages are perceived as closer.
TEXTUREThe grain of texture gets
GRADIENTsmaller as the object recedes. Gives the impression of surfaces receding into the distance.
RELATIVE SIZEWhen an object moves further away from the eye the image gets smaller. Objects with smaller images are seen as more distant.
SUPERIMPOSITION If the image of one object blocks the image of another, the first object is seen as closer.
HEIGHT IN THEObjects further away are
VISUAL FIELDgenerally higher in the visual field.
EVALUATION OF GIBSON'S DIRECT APPROACH TO PERCEPTION
Visual Illusions
Gibson's emphasis on DIRECT perception provides an explanation for the (generally) fast and accurate perception of the environment. However, his theory cannot explain why perceptions are sometimes inaccurate, eg in illusions. He claimed the illusions used in experimental work constituted extremely artificial perceptual situations unlikely to be encountered in the real world, however this dismissal cannot realistically be applied to all illusions.
For example, Gibson's theory cannot account for perceptual errors like the general tendency for people to overestimate vertical extents relative to horizontal ones.
Neither can Gibson's theory explain naturally occurring illusions. For example if you stare for some time at a waterfall and then transfer your gaze to a stationary object, the object appears to move in the opposite direction .
Bottom-up or Top-down Processing?
Neither direct nor constructivist theories of perception seem capable of explaining all perception all of the time. Gibson's theory appears to be based on perceivers operating under ideal viewing conditions, where stimulus information is plentiful and is available for a suitable length of time. Constructivist theories, like Gregory's, have typically involved viewing under less than ideal conditions.
Research by Tulving et al manipulated both the clarity of the stimulus input and the impact of the perceptual context in a word identification task. As clarity of the stimulus (through exposure duration) and the amount of context increased, so did the likelihood of correct identification. However, as the exposure duration increased, so the impact of context was reduced, suggesting that if stimulus information is high, then the need to use other sources of information is reduced. One theory that explains how top-down and bottom-up processes may be seen as interacting with each other to produce the best interpretation of the stimulus was proposed by Neisser (1976) - known as the 'Perceptual Cycle'.
PERCEPTUAL SET
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 set works in two ways: (1) The perceiver has certain expectations and focuses attention on particular aspects of the sensory data: This he calls a 'Selector'. (2) The perceiver knows how to classify, understand and name selected data and what inferences to draw from it. This he calls an 'Interpreter'.
It has been found that a number of variables, or factors, influence set, and set in turn influences perception. The factors include:
• Expectations
• Emotion
• Motivation
• Culture
1. EXPECTATION
(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.
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:
- 'The Cat Sat on the Map and Licked its Whiskers'.
(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.
2. MOT1VATION AND EMOTION
Allport, 1955 has distinguished 6 types of motivational-emotional influence on perception:
(i) bodily needs (eg 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 favoured 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.
3. CULTURE
(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.
Elephant drawing split-view and top-view perspective. The split elephant drawing was generally preferred by African children and adults.
(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.