Basic%20Processes%20in%20Visual%20Perception

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Basic Processes in Visual Perception
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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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Mapping of Visual Fields
Left visual field ? right visual cortex Right
visual field ? left visual cortex
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The Retina-geniculate-striate System

• The parvocellular (or P) pathway
• Sensitive to color and to fine detail
• Most of its input comes from cones
• The magnocellular (or M) pathway
• Most sensitive to information about movement
• Most of its input comes from rods

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A very simplified illustration of the pathways
and brain areas involved in vision. There is much
more interconnectivity within the brain (VI
onwards) than is shown, and there are additional
(not shown) brain areas involved in vision.
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Are there behavioral consequences for individual
differences in brain anatomy?
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Primary and Secondary Visual Cortex (V1 and V2)

• Retinotopic maps
• Receptive fields
• On-off cells Off-on 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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Measuring Neural Activity
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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

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On-off cell
STIMULUS RESPONSE APPROX. FIRING RATE
4 25 5 0

• Video

LGN On cell responses as shown on left
LGN Off cell opposite response pattern
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Simple Cells (bar detectors)
Video
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A wiring diagram for building simple 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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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 1) http//serendip.brynmawr.edu/7E
bbutoi/latinh.html 2) http//www.psychology.mcmast
er.ca/4i03/demos/lateral-demo.html
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Functional Specialization Theory (Zeki)

• Spatially different areas are functionally
  specialized for processing visual attributes such
  as shape, color, orientation, and direction of
  motion
• Examples
• V1 and V2
• Early stage of visual perception
• V3 and V3A
• Form, especially the shapes of objects in motion
• V4
• Responsive to colour
• V5
• Visual motion

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Evidence for Functional Specialization

• Single-cell recording
• Patient data
• Achromatopsia (damage to V4)
• Akinetopsia (damage to V5 or MT)

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Specialization for form processing in IT
(Inferotemporal-Cortex)
Kobatake Tanaka, 1994
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There is some evidence for specialization to face
processing
Bruce, Desimone Gross (1981)
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The percentage of cells in six different visual
cortical areas responding selectively to
orientation, direction of motion, disparity, and
colour.
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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
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Alternative View Hierarchical Model

• Lennie (1998)
• Visual processing is hierarchical
• Areas serve multiple functions (except for MT)

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Hierarchical Organization
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What and Where or What and How Systems

• Mishkin and Ungerleider (1982)
• Object perception (what is it?)
• Ventral pathway running from the primary visual
  area in the cortex to the inferior temporal
  cortex
• Spatial perception (where is it?)
• There is a dorsal pathway running from the
  primary visual area in the cortex to the
  posterior parietal cortex

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PerceptionAction Model

• Milner and Goodale (1995, 1998)
• Vision for perception
• Based on the ventral pathway
• Long-lasting, viewpoint-independent
  representations
• Vision for action
• Based on the dorsal pathway
• Short lasting, viewpoint-dependent
  representations

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Evidence

• Double dissociation some patients would show
  reasonably intact vision for perception but
  severely impaired vision for action, and others
  would show the opposite pattern
• Optic ataxia
• Visual agnosia

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Differential Sensitivity to Visual Illusions
Performance on a 3-D version of the Müller-Lyer
illusion as a function of task (grasping vs.
matching) and type of stimulus (ingoing fins vs.
outgoing fins).
Haart et al. (1999).
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Appropriate grasping requires theretrieval of
object knowledge from long-term memory

• Mean percentages of objects grasped
  appropriately in the control (grasping only),
  spatial imagery, and paired associate learning
  conditions.

Creem and Proffitt (2001b).