Perception of Depth and Space, Perceptual Constancy and Illusions

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Perception of Depth and Space, Perceptual
Constancy and Illusions
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Week 7 Outline

• Introduction to Depth Perception
• Pictorial Cues
• Oculomotor cues
• Disparity cues
• Perceptual Constancies and Illusions
• The Moon Illusion
• More Illusions

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Introduction to Depth Perception

• The Problem How do we perceive a 3D world from
  two 2D retinal images? How is depth recovered?

Two 2D Retinal Images (proximal)
3D Model of World
3D World (distal)
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Introduction to Depth Perception

• Two Perspectives
• Constructivist depth and space are ambiguously
  defined in the retinal images by a variety of
  depth cues, the perceptual systems must use
  inference to add information needed to resolve
  the ambiguity and construct a 3D model of the
  worldperception is indirect
• Ecological depth and space are fully defined by
  invariant information in the optic array, no
  additional information (e.g., inference) is
  neededperception is direct

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Pictorial Cues for Depth Perception

• Interposition (occlusion, overlap)

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Pictorial Cues for Depth Perception

• Aerial Perspective (atmospheric perspective or
  clearness)

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Pictorial Cues for Depth Perception

• Shading and Lighting (provide depth and shape)

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Pictorial Cues for Depth Perception

• Elevation (height in the visual field)

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Pictorial Cues for Depth Perception

• Linear Perspective

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Pictorial Cues for Depth Perception

• Texture Gradients

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Pictorial Cues for Depth Perception

• Relative Size

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Pictorial Cues for Depth Perception

• Summary
• Good for depth perception at great distances
• Monocular

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Oculomotor Cues for Depth Perception

• Accomodation
• Muscles controlling the lens of the eye cause it
  to bulge or flatten to bring the image into focus
• Feedback from the motor cortex controlling these
  muscles acts as a cue to depth out to a range of
  about 2 m
• Monocular Cue
• Dark Focus

Lens
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Oculomotor Cues for Depth Perception

• Convergence
• The eyes must converge or diverge to fixate a
  common target
• Feedback from motor cortex controlling the
  muscles that move the eyes can be used to
  estimate distance to objects within roughly 2 m

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Disparity Cues for Depth Perception

• Binocular Disparity (stereopsis)
• Because each eye sees a unique view of the world,
  images on the two retinae systematically differ
• These disparities provide a depth map relative to
  the point of fixation

Panums Fusion Area
Horopter
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Disparity Cues for Depth Perception

• Binocular Disparity (stereopsis, continued)
• Corresponding points the relative positions of 3
  green circles are the same for both eyes
• Non-corresponding points the positions of the
  red and yellow circles differ
• Red crossed disparities
• Yellow uncrossed disparities

Panums Fusion Area
Horopter
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Disparity Cues for Depth Perception

• Binocular Disparity (continued)
• Stereograms and free-fusing

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Disparity Cues for Depth Perception

• Binocular Disparity (continued)
• Stereograms and Binocular Rivalry

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Disparity Cues for Depth Perception

• Cyclopean Perception (Julesz, 1971)
• perception that occurs only from stereo vision
• Random-dot stereograms correspondence problem

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Disparity Cues for Depth Perception

• Binocular Disparity (continued)
• Magic Eye Patterns -- The Shark

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Disparity Cues for Depth Perception

• Physiology of Binocular Disparity
• Binocular depth cells
• Primarily found in the dorsal pathway (V1, V2,
  MT) Hubel and Wiesel (1970)
• 3 Categories of cells (Poggio and Fischer, 1977)
• tuned to horopter and Panums fusion area
• excited by stimuli lying in front of the horopter
  and inhibited by stimuli lying behind it
• Excited by stimuli lying behind the horopter and
  inhibited by stimuli lying in front of it
• Human stereo-blindness is consistent with
  deficiencies in one or more of these 3 cell types

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Disparity Cues for Depth Perception

• Motion disparity (parallax)
• Fundamentally the same type of disparity as that
  from binocular stereopsis
• When the observer moves, two unique images of the
  world may be compared in time, and these two
  images systematically differ
• Disparities in the images provide a depth map
  relative to the point of fixation
• Cortical cells sensitive to motion are usually
  also sensitive to disparity (e.g., MT)

Observer moving this way
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Relative Effectiveness of Depth Cues
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Perceptual Constancies

• Constancy The stability of our perception of a
  distal stimulus despite the presence of sometimes
  considerable variation in the proximal stimulus
• Types
• Lightness and color
• Shape
• Size
• Velocity

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Lightness Color Constancy

• Lightness Constancy apparent brightness and
  color of objects does not change much, even under
  very different lighting conditions that change
  the wavelengths and quanta of light considerably
• Perception is of a surface propertyalbedo
  (reflectance of a surface) not actual amount of
  light entering the eye
• Ratio Principle (Wallach, 1948) two areas that
  reflect different amounts of light will look the
  same if the ratios of their intensities to the
  intensities of the surrounds are kept constant

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Lightness Color Constancy

• Gelb (1929) A classic lightness constancy
  experiment
• Disk of low relectance (black) suspended within a
  viewing box with black interior
• Hidden spotlight within viewing box illuminates
  just the disk
• Disk appears to be white due to the dark surround
• Introducing a small piece of white paper in front
  of the disk will cause the disk to suddenly
  appear to turn black
• Shadows penumbra (fuzzy border) of shadow acts
  as a cue that the change in brightness is due to
  a change in illumination rather than a change in
  the lightness of the object

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Lightness Color Constancy

• Ratio Principle also holds for color constancy
• Perceived color of an object is influenced by the
  colors of the surrounding objects (ratio of
  wavelengths)
• Example Mondrian display
• Zeki (1983, 1984)
• Physiological mechanisms of color constancy
  located between V1 and V4

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Size Constancy

• Size and Distance
• Holway-Boring experiment (1943)
• Variation in depth cues
• Effect on size constancy
• Emmerts Law
• Ps k(RIs x Pd)
• where
• Ps is perceived size of the afterimage, k is a
  constant, RIs is the size of the reinal image,
  and Pd is the perceived distance of the surface
  on which the afterimage is projected

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Shape and Speed Constancy

• Shape Constancy Constant perceived shape despite
  changing viewpoints or depth
• multiple shapes can project the same image
• The same shape at different orientations projects
  different images
• Speed Constancy Constant perceived speed despite
  changing depth

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Illusions of Depth and Size

• The moon illusion horizon moon appears larger
  than zenith moon
• Apparent size-distance
• Relative size
• Ames room
• Size-distance illusion

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More Illusions of Depth Size

• Muller-lyer illusion ?
• Ponzo illusion
• Poggendorff illusion

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Illusions

• Contrast Illusions
• Figures 10.29-10.34
• Determinants of Illusions
• Optical-Retinal components
• curved retinal surface distorts peripheral vision
• Low-pass filtering of spatial frequencies
• Cognitive Components
• Perspective size-distance constancy
• Attention, exposure, and learning illusions
  typically lose their power with prolonged
  inspection

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End of Week 7

• Be sure to do the homework and write a journal
  entry!
• For the next Lesson read Chapters 11 and 15 of
  your text book
• Whats coming up in Week 8?
• Perceptual Development and the Orientating Sense