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Chapter 5: Perceiving Objects. Overview of Questions Why do some perceptual psychologists say “the whole differs from the sum of its parts”? How do “rules.

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Presentation on theme: "Chapter 5: Perceiving Objects. Overview of Questions Why do some perceptual psychologists say “the whole differs from the sum of its parts”? How do “rules."— Presentation transcript:

1 Chapter 5: Perceiving Objects

2 Overview of Questions Why do some perceptual psychologists say “the whole differs from the sum of its parts”? How do “rules of thumb” help us in arriving at a perception of the environment? How do we distinguish objects from their background? Why are even the most sophisticated computers unable to match a person’s ability to perceive objects?

3 The Challenge of Object Perception The stimulus on the receptors is ambiguous –Inverse projection problem: an image on the retina can be caused by an infinite number of objects Objects can be hidden or blurred –Occlusions are common in the environment

4 Inverse projection problem.

5 The Challenge of Object Perception - continued Objects look different from different viewpoints –Viewpoint invariance: the ability to recognize an object regardless of the viewpoint The reasons for changes in lightness and darkness in the environment can be unclear

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7 The Structuralist Approach Approach established by Wundt (late 1800s) –States that perceptions are created by combining elements called sensations –Structuralism could not explain apparent movement –Stimulated the founding of Gestalt psychology in the 1920s by Wertheimer, Koffka, and Kohler

8 Structuralist Approach

9 Gestalt Psychology Wertheimer, Koffka, and Kohler (1912) –Perceptions are often greater (or at least different) than the sum of the sensations –Structuralism: Purely Bottom-Up processing –Many perceptions undermine Structuralism

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11 The Gestalt Approach The whole differs from the sum of its parts –Perception is not built up from sensations but is a result of perceptual organization Principles of perceptual organization –Pragnanz - every stimulus is seen as simply as possible –Similarity - similar things are grouped together

12 Pragnanz

13 Similarity

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15 Principles of Perceptual Organization - continued Good continuation - connected points resulting in straight or smooth curves belong together –Lines are seen as following the smoothest path Proximity - things that are near to each other are grouped together Common fate - things moving in same direction are grouped together

16 Good Continuation

17 (a) Nearness and (b) Nearness competing with Similarity.

18 Gestalt Laws of Perceptual Organization Common Fate – things that move together appear to be grouped together

19 Principles of Perceptual Organization - continued Meaningfulness or familiarity - things form groups if they appear familiar or meaningful Common region - elements in the same region tend to be grouped together Uniform connectedness - connected region of visual properties are perceived as single unit Synchrony - elements occurring at the same time are seen as belonging together

20 Figure 5.20 The Forest Has Eyes by Bev Doolittle (1985). Can you find the 13 faces in this picture?

21 Figure 5.21 Grouping by (a) common region; (b) proximity; (c) connectedness; and (d) synchrony. The yellow lights blink on and off together.

22 The Gestalt Approach - continued Researchers have found neurons that respond maximally to displays that reflect: –Good continuation –Similarity Gestalt principles do not make strong enough predictions to qualify as “laws” –They are better understood as heuristics - “best guess rules”

23 Perceptual Segregation Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the background –Properties of figure and ground The figure is more “thinglike” and more memorable than ground The figure is seen in front of the ground The ground is more uniform and extends behind figure The contour separating figure from ground belongs to the figure

24 Figure 5.24 A version of Rubin’s reversible face-vase figure.

25 Figure 5.25 (a) When the vase is perceived as figure, it is seen in front of a homogeneous dark background. (b) When the faces are seen as figure, they are seen in front of a homogeneous light background.

26 Figure-Ground Segregation - continued Factors that determine which area is figure: –Elements located in the lower part of displays –Units that are symmetrical –Elements that are small –Units that are oriented vertically –Elements that have meaning

27 Figure 5.27 (a) Stimuli from Vecera et al. (2002). (b) Percentage of trials on which lower or left areas were seen as figure.

28 Figure 5.28 Examples of how displays that are (a) symmetrical; (b) small in size; c) oriented vertically or horizontally; or meaningful and more likely to be seen as figure.

29 Figure-Ground Segregation - Neural Evidence Recordings from V1 in the monkey cortex show: –Response to area that is figure –No response to area that is ground This result is important because: –V1 neurons are early in the nervous system –It reveals both a “feedforward” and “feedbackward” in the system –It demonstrates contextual modulation

30 Responses from V1 Cells (Adapted from Lamme et al., 1995.)

31 Questions Used in Modern Object Perception Research Why does the visual system respond best to specific types of stimuli? Must a figure be separated from ground before we can recognize objects? How do we recognize objects from different viewpoints? How does the brain process information about objects?

32 Why Does the Visual System Respond Best to Specific Types of Stimuli? Regularities in the environment –There is a preponderance of verticals and horizontals Oblique effect - people are more sensitive to these orientations Occurs due to biology and experience –Gestalt heuristics are reflected in environmental objects

33 Figure 5.30 Left: Photographs like the ones taken by the participants in Coppola et al.’s (1998) experiment as they walked around the Duke University campus. The results of a computer analysis of the orientation in each type of scene (indoor campus, outdoor campus, and in the forest).

34 Must a Figure Be Separated from Ground Before We Can Recognize Objects? Research has shown that objects may be recognized before or during the separation of figure from ground –Stimuli with a standing woman and a less meaningful shape were used –The meaningful stimulus (the woman) was recognized more often than the other –When the picture of the woman was turned upside down, this effect disappeared

35 How Do We Recognize Objects From Different Viewpoints? Structural-description models –3-D objects are based on 3-D volumes called volumetric features that are combined for a given shape Marr’s model proposed a sequence of events using simple geometrical features The sequence begins with identifying edges and proceeds to recognition of the object

36 Structural-Description Models (Think 3D) Recognition-by-components theory by Irving Biederman –Volumetric features are called geons –Theory proposes there are 36 geons that combine to make all 3-D objects –Geons include cylinders, rectangular solids, and pyramids

37 Structural-Description Models - continued Properties of geons –View-invariant properties - aspects of the object that remain visible from different viewpoints –Accidental property - a property that appears rarely and from certain viewpoints –Discriminability - the ability to distinguish geons from one another –Principle of componential recovery - the ability to recognize an object if we can identify its geons

38 Can you Recover the Components?

39 Image-Description Models Ability to identify 3-D objects comes from stored 2-D viewpoints from different perspectives –For a familiar object, view invariance occurs –For a novel object, view invariance does not occur This shows that an observer needs to have the different viewpoints encoded (stored) before recognition can occur from all viewpoints

40 How Does the Brain Process Information About Objects? Perceiving an object - sunburst or butterfly? –Experiment by Sheinberg & Logothetis Monkey was trained to pull a lever for a sunburst or a butterfly Binocular rivalry was used - each picture shown to one eye Neuron in the IT cortex was monitored Firing was vigorous for only the butterfly

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42 Identifying an Object: Is That Harrison Ford? Grill-Spector experiment –Region-of-interest approach: the FFA for each person was determined first by: Showing participants faces and non- faces Finding the area that responded preferentially to faces

43 Grill-Spector Experiment FFA in each participant was monitored On each trial, participants were shown either: –A picture of Harrison Ford’s face –A picture of another person’s face –A random texture –All stimuli were shown for 50 ms followed by a random-pattern mask –Participants were to indicate what they saw 60 pictures of each type were presented

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45 Grill-Spector Experiment - continued For trials that only included Harrison Ford’s face, results showed that FFA activation: –Was greatest when picture was correctly identified as Ford –Was less when picture was identified as other object –Showed little response when there was no identification of a face Neural processing is associated with both the presentation of the stimulus and with the response (recognition) to the stimulus

46 Activity in FFA is not passively processing info, It is influenced by attention and the task at hand.

47 Perceptual Intelligence Theory of unconscious inference –Created by Helmholtz (1866/1911) to explain why stimuli can be interpreted in more than one way –Main Principle - perceptions are result of unconscious assumptions about the environment –Likelihood principle - objects are perceived based on what is most likely to have caused the pattern –Your visual system is “making decisions” about what you see before you know what you are looking at!

48 Figure 5.44 The display in (a) looks like (b) -- a blue rectangle in front of a red rectangle -- but it could be (c), a blue rectangle and an appropriately positioned 6-sided red figure.

49 Modern Ideas on Perceptual Intelligence Palmer experiment –Observers saw a context scene flashed briefly followed by a target picture –Results showed that: Targets congruent with the context were identified 80% of the time Targets that were incongruent were only identified 40% of the time –When we are not sure, we “decide” to see things that fit the scene and “decide” not to see things that are not

50 Stimuli used in Steven Palmer’s (1975) experiment. The scene at the left is presented first, then one of the objects on the right is presented. The observer is then asked to identify if one of the objects on the right was presented.

51 Modern Ideas about Perceptual Intelligence - continued Light-from-above-heuristic –Objects are generally perceived with the assumption that illumination comes from above –This is consistent with our experiences from the environment

52 Figure 5.47 Why does (a) look like indentations in the sand and (b) look like mounds of sand? See text for explanation.

53 Figure 5.46 (a) Some of these discs are perceived as jutting out, and some are perceived as indentations. (b) Light coming from above will illuminate the top of a shape that is jutting out, and (c) the bottom of an indentation.

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