Chapter 5 Perceiving Objects and Scenes The

Chapter 5: Perceiving Objects and Scenes

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

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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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Perceptual Organization • Approach established by Wundt (late 1800 s) – States that perceptions are created by combining elements called sensations – Structuralism could not explain apparent movement – Stimulated the founding of Gestalt psychology in the 1920 s 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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Perceptual Organization - continued • Illusory contours- contours that appear real but have physical edge

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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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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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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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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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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 figure-ground can affected by meaningfulness of a stimuli.

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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

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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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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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

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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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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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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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.

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

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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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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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. – f. MRI showed an increase in activity in • Parahippocampal place area for the house • Fusiform face area for the face

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Reading the Brain • Experiment by Kamitani & Tong – Gratings with different orientations were presented to participants. – Responses from f. MRI 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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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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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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