Chapter 5: Perceiving Objects and Scenes

Transcription

1 Chapter 5: Perceiving Objects and Scenes

2 The Puzzle of Object and Scene 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.

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7 Why Is It So Difficult to Design a Perceiving Machine? 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.

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11 Why Is It So Difficult to Design a Perceiving Machine? - continued Objects can be hidden or blurred

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14 Why Is It So Difficult to Design a Perceiving Machine? - continued Objects look different from different viewpoints Viewpoint invariance: the ability to recognize an object regardless of the viewpoint This is a difficult task for computers to perform

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16 Perceptual Organization 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 The whole differs from the sum of its parts. Perception is not built up from sensations, but is a result of perceptual organization.

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22 Perceptual Organization - continued Illusory contours- contours that appear real but have physical edge

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24 Gestalt Organizing Principles Principles of perceptual organization. Good continuation - connected points resulting in straight or smooth curves belong together Lines are seen as following the smoothest path Pragnanz - every stimulus is seen as simply as possible Similarity - similar things are grouped together

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28 Gestalt Organizing Principles - continued Proximity - things that are near to each other are grouped together Common fate - things moving in same direction are grouped together Common region - elements in the same region tend to be grouped together Uniform connectedness - connected region of visual properties are perceived as single unit

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35 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 (border ownership).

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38 Perceptual Segregation - continued Factors that determine which area is figure: Elements located in the lower part of displays Convex side of borders

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41 Subjective Factors That Determine Which are is Figure Gestalt psychologists believed that experience and meaning play a minor role in perceptual organization. Gibson Experiment showed that figureground can affected by meaningfulness of a stimuli.

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45 Perceiving Scenes and Objects in Scenes A scene contains: background elements. objects organized in meaningful ways with each other and the background. Difference between objects and scenes A scene is acted within An object is acted upon

46 Perceiving Scenes and Objects in Scenes - continued Research on perceiving gists of scenes Potter showed that people can do this when a picture is only presented for 1/4 second Fei-Fei used masking to show that the overall gist is perceived first followed by details.

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48 Perceiving Scenes and Objects in Scenes - continued Global image features of scenes Degree of naturalness Degree of openness Degree of roughness Degree of expansion Color Such features are holistic and perceived rapidly

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50 Regularities in the Environment: Information for Perceiving Physical regularities - regularly occurring physical properties Oblique effect - people perceive horizontals and vertical more easily than other orientations Uniform connectedness - objects are defined by areas of the same color or texture

51 Regularities in the Environment: Information for Perceiving continued Physical regularities - regularly occurring physical properties Homogenous colors and nearby objects have different colors Light-from-above heuristic - light in natural environment comes from above us

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55 Regularities in the Environment: Information for Perceiving - continued 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.

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58 Role of Inference in Perception Theory of unconscious inference Created by Helmholtz (1866/1911) to explain why stimuli can be interpreted in more than one way Likelihood principle - objects are perceived based on what is most likely to have caused the pattern Modern researchers use Bayesian inference that take probabilities into account

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60 Connecting Neural Activity and Object Perception 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.

61 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 nonfaces Finding the area that responded preferentially to faces

62 Connecting Neural Activity and Object Perception - 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 to the stimulus.

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65 Brain Activity and Seeing Experiment by Sheinberg & Logothetis Monkey was trained to pull two levers: one for a sunburst one for a butterfly Binocular rivalry was used - each picture shown to one eye at the same time Neuron in the IT cortex that responded only to the butterfly was monitored. Firing was vigorous for only the butterfly

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67 Brain Activity and Seeing - continued Experiment by Tong et al. Binocular rivalry used again with people Picture of a house shown to one eye and a face to another Participants pushed button to indicate perception. fmri showed an increase in activity in Parahippocampal place area for the house Fusiform face area for the face

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69 Reading the Brain Experiment by Kamitani & Tong Gratings with different orientations were presented to participants. Responses from fmri voxels were measured. Activity patterns across voxels varies by grating orientation. An orientation decoder was used to analyze the voxel activity. The decoder could accurately predict which orientation had been presented.

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73 Are Faces Special? Fusiform face area (FFA) - responds only to faces Amygdala (AG) - activated by emotional aspects of faces Superior temporal sulcus (STS) - responds to where the person is looking and to mouth movements Frontal Cortex (FC) - activated when evaluating facial attractiveness

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77 Infant Face Perception Understanding what an infant sees using preferential looking effect. Human faces are among the most important stimuli in an infants environment.

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