PSYC 441 Cognitive Psychology II

Transcription

1 PSYC 441 Cognitive Psychology II Session 5 Theories of Perception Lecturer: Dr. Benjamin Amponsah, Dept., of Psychology, UG, Legon Contact Information: bamponsah@ug.edu.gh College of Education School of Continuing and Distance Education 2014/ /2017

2 Session Overview In this section, we will try to answer whether we see reality or what we see is guided by expectations. We will be studying three important theories of visual perception such as Gibson s theory of Direct Perception and Gregory s Constructivist theory of perception. A third theory of historical importance that will also be looked at is the Gestalt theory. Slide 2

3 Session Objectives At the end of the session, the student will be able to Discuss Gibson s Direct perception theory Discuss the Greggory s Constructivist theory Attempt to contrast the two theories Gestalt contribution to perception Slide 3

4 Session Outline The key topics to be covered in the session are as follows: Topic One: Gibson s Direct Perception Theory Topic Two: Greggory s Constructive Theory of Perception Topic Three: Contrast of the Two Theories Topic Four: Gestalt Contribution to Perception Slide 4

5 Reading List Ashcraft, M. H. (2013). Cognition (6 th edn.), London: Pearson Education Int. Eysenck, M. W. (2012). Fundamentals of Cognition (2ed). Psychology Press, Sussex. Galotti, K. M. (2017). Cognitive Psychology: In and out of the laboratory (6 th edn.). Belmont, CA: Wadsworth. Hunt, R. R. & Ellis, H. C. (2004). Fundamentals of Cognitive Psychology (7 th edn.), New York: McGraw-Hill. Willingham, D, B. (2013). Cognition: The thinking animal (International edn. NJ: Prentice-Hall). Slide 5

6 Topic One GIBSON S DIRECT PERCEPTION THEORY Slide 6

7 Theories of Perception Do we see reality or what we see is guided by expectation? Three traditional theories of how we perceive the world will help us to answer the question. 1. Gibson s theory of Direct Perception 2. Gregory s Constructivist Theory of Perception 3. Gestalt theory Slide 7

8 Gibson s Theory of Direct Perception Direct perception proposes that information in the stimuli is the most important element in perception and that learning and cognition are unnecessary in perception. The leading proponent James Gibson (1966,1979) and his followers at Cornell University stated: Direct perception assumes that the richness of the optic array just matches the richness of the world. Slide 8

9 Optic Array is the pattern of light reaching the eye, which is thought to contain all the visual information available on the retina. This implies that much of what we perceive about the world is directly available in the stimulus information that strikes the sensory system. The idea of direct perception has gained support among ecologically minded psychologists because it is assumed that the stimulus contains enough information for correct perception and does not require internal representations for perception. The perceiver does minimal work in perception because the world out there offers so much information, leaving little demand to construct perception and draw inferences. Slide 9

10 According to Gibson our nervous system is perfectly attuned for detecting necessary information in the environment. Evidence Evidence for Gibson s direct perception theory can be found on how we detect distances - Depth perception. Depth perception can be detected in two main ways Using either: Monocular or Binocular cues Slide 10

11 Monocular and Binocular Vision

12 MONOCULAR CUES Monocular cues are any visual cues for depth or distance perception that operates with one eye. Example Cover one eye with your hand and look around. Hold one finger in front of the opened eye. Bring it very close to the eye. What do you see? (shadow and blurred figure) The shadowing and accommodation produces a strong monocular depth cue. (Accommodation how the eye maintains a clear image for objects at different distances). Slide 12

13 Example: Shadow Monocular cue - gives rise to the awareness that one object is in front of another object. Shadow Monocular Depth cue has several rules. If an object is solid, it will not allow light to pass through and will therefore cast a shadow. If there is only one light source, then all shadows will fall in the same direction and the shadow will be opposite from the source of light. With a hole in the ground, the shadow appears on the same side as the source. Objects with shadows falling on them are farther away than objects casting the shadow. Slide 13

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15 More Examples of Monocular Depth Cues Relative Size - If an image contains an array of similar objects that differ in size, we interpret the smaller objects as being farther away (smaller retinal image). Superposition/Interpositioning which occurs when a close object obscures parts of a more distant object (one object overlaps the other, which causes us to perceive depth). Also Occlusion. Relative Height the object closer to the horizon is perceived as farther away and the object farther away from the horizon is perceived as closer. Linear Perspective - When parallel lines appear to converge, they are perceived as vanishing in distance.

16 Texture Gradient, which can be observed by comparing the details of near and distant objects: objects at a distance appear to be smoother in appearance and more grey in colour, while near objects have clear, sharp colour and are more detailed. Motion Parallax is another important cue to depth perception and can be observed when looking out of the window on a moving vehicle: objects in the distance appear to move slowly than do near objects.

17 Binocular Depth Cues BINOCULAR DEPTH CUES Binocular cues are visual cues for depth perception that arise from the two retinal images obtained with both eyes. Example: Hold a pen at arms length. Close each eye alternatively. The pen seem to swing back and forth. 1. The difference between the two retinal images, known as binocular disparity can give rise to distance. 2. Another binocular cue is binocular convergence - the movement of the eye muscles as we focus on near and distant objects can give rise to distance. Slide 17

18 When an object is close to the observer, the two eyes must rotate inward (toward the nose) convergence. NB Convergence (eyes turn-in) is effective for distance less than 10 meters. When the target is farther away, the eyes rotate outward divergence. The two eyes are horizontally separated by the nose. The separation results in a distance of up to 6.5 cm between the pupils. Merging into one common percept is called fusion. Slide 18

19 Accommodation and Convergence Accommodation and Convergence allow us to see objects clearly both near and far without diplopia (double vision)

20 Summary The visual cues discussed (monocular and binocular) are evidence of direct perception, and they are not dependent upon top-down processes. Perception in this sense can be considered as direct decoding of information. In general, direct perception assumes that the distal stimulus contains enough Information Apprehension. Slide 20

21 Topic Two GREGGORY S CONSTRUCTIVE THEORY OF PERCEPTION Slide 21

22 Constructive theory of perception is based on the assumption that during perception we form and test hypotheses concerning percepts based on both what we sense and what we know (perceptual hypothesis). Perception then reflects combined effect of what comes in through our sensory system and what we have learnt about the world through experience. Think about how you recognize a familiar person his features, nose, hair, etc. These features are registered by the eyes and also because you know the person to be in that locality. It s possible to recognise him even if his features have changed. Slide 22

23 From the perspective of the constructivists, though there may be changes in the pattern of original stimuli, you may still recognise your friend accurately because of unconscious inference a process by which we spontaneously integrate information from several sources to construct a percept. When we look at something, we develop a perceptual hypothesis, which is based on prior knowledge. The hypotheses we develop are nearly always correct. In a few cases though, our perceptual hypotheses are disconfirmed by the data we perceive. Slide 23

24 Support for Gregory s Theory SUPPORT FOR GREGORY S THEORY Several studies using visual illusions provide support for Gregory s constructive theory of perception. Visual illusion or an optical illusion is characterized by visually perceived images that differ from objective reality. This information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. See examples below: Slide 24

25 How many legs does this elephant have? 4, 5, 6 maybe 7?

26 Is this window on the right or left of this building?

27 Let us focus specifically on the Müller-Lyer illusion. It consists of two equal length parallel lines, which appear to be different in length when one line has inwardly pointing fins and the other has outwardly pointing fins. Müller-Lyer Illusion

28 Gregory argued that the illusion occurs because it brings in cues of our visual knowledge (experience) of the world. Again Gregory argued that many visual illusions occur due to misapplied constancy scaling (applying 3-D characteristics to 2-D figure). An example is the Ponzo illusion, in which two horizontal lines appear to be of unequal length when enclosed between two converging lines.

29 Remember Gibson s Direct Perception theory, argued strongly against the idea that perception involves top-down processing and criticized Gregory s discussion of visual illusions on the grounds that they are artificial examples and not images found in our normal visual environments. The argument is interesting and we may probably consider illusions as exceptions and might not be that informative towards the debate. Slide 29

30 Are there top-down influences in Gibson s Direct perception? When we consider Gibson s Direct perception theory, there is the argument that his theory of affordances can be influenced by top-down processes such as:» expectation,» motivation and» emotions. According to Gibson, affordances are acts or behaviours permitted by objects, places and events. The things offered by the environment to the organism. For humans, chairs afford sitting, a handle or knob affords grasping, a window affords looking through. Slide 30

31 Gibson claimed that affordances of an object are also directly perceived; that is, we see a chair is for sitting, just made of wood. Can we really see sitting in a chair? (We can t see except by imagination) We avoid crashing into walls and closed doors because such surfaces do not afford passing through and we perceive this as we move toward them. Lets look at ways in which affordances can be influenced by top-down processes such as expectation, motivation and emotion. Slide 31

32 Expectation Bruner and Minturn (1955) presented either letters or numbers to their participants, and then showed them an ambiguous figure that was a cross between B and 13. Participants who were shown letters perceived the figure as B, whereas those who were shown numbers perceived 13. Thus, perception of an ambiguous objects can be influenced by what one expects or anticipates. Motivation Research has shown that the longer individuals are deprived of food, the more likely they are to perceive ambiguous pictures as food-related. Slide 32

33 Gilchrist and Nesberg (1952) showed that food deprivation was associated with rating pictures of food as being visually brighter than other pictures. Emotion Lazarus and McCleary (1951) have demonstrated that there could be emotional associations with stimuli and that events can be influenced by our perception of them Slide 33

34 Topic Three CONTRAST BETWEEN THE TWO THEORY Slide 34

35 The two theories work well to explain perception but they focus on different phases of the process. The direct-perception view is important for our understanding of perception because it calls to attention the significance of sensory stimuli, suggesting that the processing of visual input is simple and direct and that cognition and perception are natural and ecologically based. Slide 35

36 The constructive perception theory on the other hand is useful in understanding how sensory impressions are comprehended by the thinking brain. The theory also emphasises prior knowledge, past experiences which are consistent with top-down theories of recognition. The two theories must be seen as complementary. Slide 36

37 Topic Four GESTALT CONTRIBUTION TO PERCEPTION Slide 37

38 Gestalt Approach to Perception Gestalt Approach to Perception Gestalt Theory In the 1930s the Gestalt psychologists, the most noted such as Koffka, Köhler and Wertheimer investigated how we perceive objects and visual forms. They argued that perceivers constantly search for a good fit between the visual image and stored memories of visual objects. Gestalt psychology maintains that psychological phenomena can only be understood if they were viewed as organized and structured wholes. Slide 38

39 To understand their contribution to perception and pattern recognition we will discuss two of their principles: Form perception Gestalt Principles of Perceptual Organization. FORM PERCEPTION Form perception helps us understand how perceivers come to recognize objects or forms. One important concern is to understand how we interpret stimuli (as consisting of objects that we see) from their background. Slide 39

40 Figure above shows a reversible figure-ground picture

41 Generally, there are two distinct ways of seeing the pictures in the Figure above: a black vase against a white background or two white faces against a black background. This segregation of the whole display into objects (figure) and the background (ground) is an important process known to cognitive psychologists as form perception (the process by which the brain differentiates objects from their background). The separation of objects from their background has important consequences. These are: Slide 41

42 1. The part of the display recognized as the figure will be better remembered than whatever is interpreted as background. 2. The background will be seen as shapeless, less formed and farther away in space. 3. Perception is taken for granted but it s more complex, because what we perceive is not completely determined by the stimulus display that we see, it requires the perceiver s active participation.

43 Figure-ground separation is something we do everyday. E.g., we abstract the voice of a speaker from the background sound of a noisy party. When people recover their sight after many years of blindness they commonly experience many difficulties in seeing the world as it is, but almost without exception, the figure-ground separation is achieved right from the onset of perception. This makes us take form perception for granted but the Gestaltists use of ambiguous figures as we have seen above show the complexity of perception. Slide 43

44 Gestalt Principles of Perceptual Organization Gestalt Principles of Perceptual Organization Gestalt Psychologists also believed that perceivers follow certain laws or principles of organization in coming to their interpretation. They believed the brain imposes meaning on the discrete elements and that the whole is greater than the sum of its parts. Hence, they rejected the claim that perceivers recognize objects by identifying individual elements (bottom-up), instead, we see and recognize each object or unit as organized wholes. They vehemently opposed the structuralists who considered elements as the building blocks of conscious experience. Slide 44

45 Example In one experiment by Navon (1977), the assumptions of the Gestalts were confirmed that in perceptual processing the most important features are identified first before the fine-grained details. E.g., when the letter sssssss was used to write the letter H, subjects analyzed the H first before turning to the contents or the elements in the letter H which were the sssssss. What then are the Gestalt principles of perceptual organization that allow us to perceive objects and events as organized wholes? Let us look briefly at 4 major principles: Slide 45

46 Principle of proximity The closer two figures are to each other, the more they will tend to be grouped together perceptually. Proximity may operate in time just as it does in space.

47 Principle of similarity There is the tendency for our perceptual system to group figures based on their similarity.

48 Principle of Good Continuation Our visual system seems to prefer contours that continue smoothly along their original course. This principle of grouping is called good continuation. This principle is used by the military for camouflage. It also helps to camouflage animals against their natural predators. It helps to conceal various insects from predators who tend to see parts of the insect s body as continuations of the twigs on which it stands.

49 In A all subjects see two complex patterns intersected by a horizontal line. Hardly anyone sees the hidden 4 contained in that figure and shown in B despite the fact that we have encountered 4 s much more than the two complex patterns which are completely new. The line segments in the figure will generally be grouped so that the contours continue smoothly. As a result, segment A will be grouped with D, and C with B, rather than A with B and C with D.

50 Principle of Closure There is the tendency for our visual system to complete figures that have gaps in them. The figure is seen as a triangle despite the fact that the sides are incomplete. There is the tendency to complete or close figures that have a gap in them, as in the incomplete triangle shown above.

51 Summary Proximity, similarity, good continuation and closure are among the factors that determine whether we see portions of the visual world as belonging together or apart. Although the Gestalt theory has intuitive appeal, certain questions are raised. E.g., we do not know how these principles are transformed or translated into cognitive or physiological processes. Slide 51

52 Their concepts are also very difficult to define. Despite the concerns, we still owe a lot to the Gestaltists for helping us to understand some processes in visual perception. Slide 52

53 Sample Questions Describe the two theories of perception Gibson s theory of direct perception and Greggory s constructive theory of perception Define Form perception and discuss why it is an important concept in visual perception. Slide 53

54 References Gibson, J.J. (1979). The ecological approach to visual perception. Boston: Houghton Mifflin. Brunner, J. S. & Minturn, A. L. (1955). Perceptual identification and perceptual organization. Journal of General Psychology, 53, Gilchrist, J. C. & Nesberg, L. S. (1952). Need and perceptual change in need-related objects. Journal of Experimental Psychology, 44, Navon, D. (1977). Forest before trees: The precedence of global features in visual perception. Cognitive Psychology, 9, Slide 54

55 THANK YOU Slide 55