chapter 5: spatial vision & form perception detection discrimination identification objects...
TRANSCRIPT
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Chapter 5: Spatial Vision & Form Perception
• Detection
• Discrimination
• Identification
• Objects & Forms– Light can vary by intensity, color, pattern.
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Analysis of Form
• Titchener & Analytic Introspection
• Campbell & Robson’s Analysis by Scale (a.k.a. the multi-channel model)
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Gratings (Gabors)
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Grating (Gabors)
• Spatial Frequency
• Contrast
• Orientation
• Spatial Phase
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Orientation
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Spatial Phase
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Spatial Phase
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An analogy between lens and vision
• The Transfer function (the lens)
• The context sensitivity function (vision)
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Gratings and the Transfer Function
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The transfer function vs. the contrast sensitivity function
• In both cases, we can have gratings as targets.
• In the eye, we don’t want the retinal image, but some perceptual fact. That is, we want both to analyze a perceptual function.
• So, we go for contrast thresholds
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Contrast thresholds
• At some levels of contrast, humans can see a grating.
• At other levels of contrast, humans cannot see a grating.
• We want to plot the line that separates the two.
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TF & CSF
• Both have a high end cut-off.
• CSF has a low-end cut-off; TF does not.
• CSF: There is a maximal spatial frequency for contrast sensitivity.
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CSF and night vision
• High spatial frequency fade first with decreasing light.
• So, at night, people can see big things, but not small things.
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How does this fit in with the “convergence trade-off?”
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How does this fit in with the “convergence trade-off?”
• What is the convergence trade off?
– The lower the convergence, the greater the resolution.
– The higher the convergence, the greater the light sensitivty.
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We can understand why fine details disappear first. The ability to see fine details is bought at the expense of light sensitivity.
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The CSF is species specific
• Different species have different CSFs.
• CSFs have been measured in over a dozen different species.
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Evidence for spatial frequency channels
• Selective adaptation
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Evidence for spatial frequency channels
• Selective adaptation
• People respond more quickly to low frequency information that to high frequency information.
• In short presentations, only low-frequency information is perceived; high-frequency information is “blotchy.”
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Metamers
• Metamers are objects that are physically different, but are perceptually indistinguishable from each other.
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Whether or not two objects are metamers depends on viewing conditions.
(See Figure 5.28 on p. 20).
For example,
Lighting
Distance
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• Form Defined by Contrast
• Form Defined by Texture
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Gestalt Principles
• Proximity
• Similarity
• Closure
• (Good) continuation
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Gestalt principles interact
• In the window, proximity indicates grouping of the face with the bars. Texture and luminance (similarity) indicate non-grouping.
• Proximity promotes perceptual grouping.
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Why do the Gestalt Principles work?
• They embody cues that help identification in the natural world. So, they are fostered by natural selection.
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Reinforcement between hypercolumns
• Bill Geisler, et al., at University of Texas studied many natural scenes.
• In short, similarity of orientation is proportional to separation.
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In other words:
• Adjacent segments of any single natural contour tend to have very similar orientations.
• Distant segments of any single natural contour tend to have dissimilar orientations.
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Perception of Symmetry (Sometimes Classified as a Gestalt
Principle)• Faces are approximately bilaterally
symmetric.
• Human body is approximately bilaterally symmetric.
• Symmetry is widely thought to contribute to attractiveness.
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Rentsler, et al.
• People prefer more symmetry.
• People prefer greater simplicity.
• But, simplicity is a function of experience, where symmetry is not.