Visual Perceptual Organization and Perceiving Objects and Form

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Visual Perceptual Organization and Perceiving
Objects and Form
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Week 6 Outline

• Bottom-up vs. Top-Down Processing
• Perceiving Figure and Ground
• Grouping based on Simplicity The Gestalt Laws
• Constructivism and Grouping based on the
  Likelihood Principle
• Schemes for Representing Objects
• Physiology of Perceiving Objects
• Attention and Perception
• Failures of object Recognition Visual Agnosia

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Bottom-up vs. Top-Down Processing

• My wife and mother-in-law
• http//psylux.psych.tu-dresden.de/i1/kaw/diverses
  20Material/www.illusionworks.com/html/perceptual_a
  mbiguity.html
• Do you see the wife (young woman) or
  mother-in-law (older woman)?
• Interpretation of Ambiguous stimuli can change
  depending on context

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Top-Down vs. Bottom-up Processing

• Top-down
• Conceptually-driven, perception based on
  information (concepts) coming from previous
  experience, stored in memory
• Bottom-up
• data-driven, perception guided by information
  (data) currently coming in through the senses
• All perception is based on some combination of
  these two factors
• Perceptual Priming

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Perception of Figure Ground

• First step in perceiving objects and forms is to
  segregate an object (figure) from the background
• Sometimes this relationship is obvious
• Sometimes this relationship is ambiguous
• face-vase figure
• Art by Escher, Dali, others

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Ambiguous Figure Ground in Art Dalis Slave
Market with the Disappearing Bust of Voltaire
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Perceptual Organization Perceiving Figure and
Ground

• Qualities of figure
• Quality of a thing
• Contours appear at and define edges
• Appears closer than and in front of the ground
• Appears more impressive, dominant, and better
  remembered (Figure 7.5)
• Suggests meaningful shape

• Qualities of ground
• Quality of a substance
• Appears relatively formless
• Appears to extend continuously behind the figure
• Unnoticed
• shapeless

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Figure-Ground, Lightness and Contrast Perception

• Do the gray circles on the right look lighter or
  darker than the gray background on the left?
• Wolff effect Figure is perceived as lighter than
  ground

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Figure-Ground, Lightness and Contrast Perception

• Koffka rings Figural properties (top-down) can
  overcome effects of simultaneous contrast
  (bottom-up)

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Other determinants of Figure-Ground

• Symmetrical areas more often seen as figure
• Convex shapes seen as figure
• Smaller areas seen as figure
• Canonical orientations (H and V) - Figure 7.4 a
• Meaningful objects - Figure 7.4b

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Grouping based on Simplicity The Gestalt Laws

• The law of Pragnanz or good figure
• We perceive the simplest organization of a
  stimulus
• goodness can be defined in terms of information
  theory (Hochberg and McAlister, 1953)
• 4 simple shapes (3 circles and 1 equilateral
  triangle) or
• 3 complex shapes (more parameters needed to
  define)
• Considered to be largely fundamental, unlearned,
  organizing principles bottom-up

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The Gestalt Laws

• nearness (proximity)
• Similarity
• Connectedness
• good continuation
• common fate ?
• Symmetry
• Closure

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Grouping based on Likelihood Constructivism

• The constraint of likelihood and Constructivist
  Approach
• We perceive the most likely (probable)
  organization of a stimulus
• Likelihood is based on context and prior
  experience - top-down
• Helmholtzs unconscious inference
• Gregory perception as hypothesis testing
• Active observation and eye movements

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Likelihood Impossible Figures
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Representing Objects

• Template Matching Models
• View-dependent or view-invariant
• View-dependent store limited number of 2D
  canonical views and interpolate between them
  (Rock, 1997)
• View-invariant 3D object templates
• Geons (Biederman, 1987)
• Structural Descriptions (Marr and Nishihara, 1978)

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Physiology of Form Perception

• Grandmother cells vs. Population (ensemble)
  coding
• Ventral (occipital-temporal) Stream
• Input from both parvo and magno systems
• Structure from dynamic information ?
• Infero-temporal cortex (IT, Desimone et al.,
  1984)
• Large receptive fields
• Receptive fields cross midline
• Receptive fields tuned to respond preferentially
  to shape

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Desimone et al. (1985) single-cell recordings
from Macaque IT
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Visual Attention

• Internal spotlight used to select or enhance
  some information at the cost of ignoring or
  degrading other information
• Generally we attend where we are looking, but
  attentional spotlight can be moved independently
  of eye
• Attentional state modifies the responses of
  visual neurons throughout extra-striate cortex
  and can modify responses in striate cortex
• Attention modulates responses of IT neurons by
  linking spatial location to object shape

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Failures of object Recognition Visual Agnosia

• Agnosia failure to recognize objects despite
  registration of visual information in cortex
• The man who mistook his wife for a hat -
  Oliver Sacks
• Sub-Types
• Apperceptive agnosia failure due to perceptual
  problems in representation often due to damage
  of right parietal lobe
• Problems with unusual views
• Problems with interpreting shadows
• Associative agnosia failure despite normal
  representation often due to damage of posterior
  left hemisphere (occipital)
• Can draw accurate pictures of viewed objects
• Cannot identify objects
• Prosopagnosia Failure to recognize faces
• Generally results from lesions of
  temporal-occipital areas
• Right-hemisphere may be more important
• Can be specific to human faces only (patient
  W.J.)