Visual Cognition I basic processes

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Visual Cognition Ibasic processes
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What is perception good for?

• We often receive incomplete information through
  our senses. Information can be highly ambiguous
• Perceptual system must resolve ambiguities by
  drawing inferences from a large set of perceptual
  cues and conceptual knowledge of the world

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Perceptual system is always looking for
meaningful patterns
Slide from Rob Goldstone
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Object Recognition System must be flexible to
recognize variations in object shapes
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Basic processes in the Visual System
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Hierarchical Organization
Visual Input
Oriented bars/edges, Motion, Texture,
Depth Object/Face/Scenerecognition
Low Level Vision
High Level Vision
Knowledge
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Functional Specialization

• Spatially different areas are functionally
  specialized for processing visual attributes such
  as shape, color, orientation, and direction of
  motion
• Achromatopsia (damage to V4)
• cortical color blindness all color vision is lost
  and the world appears in shades of gray. And in
  achromatopsia, unlike as in blindness caused by
  damage to the eyes or optic nerve, even memory of
  color is gone
• Akinetopsia (damage to V5 or MT)
• or motion blindnessthe loss of the ability to
  see objects move. Those affected report that they
  perceive a collection of still images.

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Primary and Secondary Visual Cortex (V1 and V2)

• Retinotopic maps
• Receptive fields
• On-off cells Off-on cells
• Orientation sensitive cells (simple cells)
• Lateral inhibition

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Retinotopic maps in V1
Response in monkey primary visual cortex (V1)
measured by radio-active tracers
Stimulus pattern

• Retinotopic mapping locations on retina are
  mapped to cortex in orderly fashion. Note more
  of visual cortex is dedicated to foveal vision

Tootell, R. B., M. S. Silverman, et al. Science
(1982)
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Stimulus
Cortical Mapping Left Hemisphere
Cortical Mapping Right Hemisphere
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Revealing retinotopic maps with fMRI
From Geoff Boynton, SALK institute
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Revealing retinotopic maps with fMRI
From Geoff Boynton, SALK institute
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Single Cell Recording(usually in animal studies)
Measure neural activity with probes. E.g.,
research byHubel and Wiesel
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Hubel and Wiesel (1962)

• Studied LGN and visual cortex in the cat. Found
  cells with different receptive fields different
  ways of responding to light in certain areas

• What are cells 1, 2, and 3 doing ?
• detecting edges
• detecting oriented bars
• detecting movement in particular direction
• detecting cat faces
• What are likely locations for cells 1, 2, and 3?
• LGN
• V1 (primary visual cortex)
• V5

Cell 1
Cell 2
Cell 3
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Receptive Fields

• The receptive field (RF) of a neuron is the area
  of retina cells that trigger activity of that
  neuron

On-off cells and off-on cells
Simple cells (bar detectors)
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A wiring diagram for building orientation-sensitiv
e cells out of on-off cells
Hierarchical organization of the brain by
aggregating responses over several on-off cells,
the brain can detect more complicated features
(e.g. bars and edges)
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Hierarchical Organization
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What is this cell coding for?a) any faceb)
monkey facec) human faced) eyese) hands
spike train each individual line represents a
neuron firing. The axis represents time
Bruce, Desimone Gross (1981)
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Mach Bands and Lateral Inhibition
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Lateral Inhibition

• Lateral inhibition sets up competition between
  neurons so that if one neuron becomes adept at
  responding to a pattern, it inhibits other
  neurons from doing so.

Light
On-Off Cells with lateral inhibition
Response ? Edge detection
DEMO APPLETS http//www.psychology.mcmaster.ca/4
i03/demos/lateral-demo.html http//serendip.brynma
wr.edu/bb/latinhib_app.html
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Lateral Inhibition enhances edges
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Craik-Cornsweet-OBrien Illusion
Left part of the picture seems to be darker than
the right one. In fact they have the same
brightness.
The same image as above, but the edge in the
middle is hidden. Left and right part of the
image look to be equally dark
How is this different from mach bands?
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Another demo of the same effect
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Sensory Binding Problem

• If spatially different areas are functionally
  specialized for processing visual attributes such
  as shape, color, orientation, and direction of
  motion.
• then how does the brain then bind together the
  sensory attributes of an object to construct a
  unified perception of the object?
• ? Binding Problem

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Binding Problem