Transcript Syllabus P140C (68530) Cognitive Science

Basic Processes in
Visual Perception
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
Mapping of Visual Fields
Left visual field 
right visual cortex
Right visual field 
left visual cortex
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
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.
Are there behavioral consequences for
individual differences in brain anatomy?
Primary and Secondary Visual Cortex
(V1 and V2)
• Retinotopic maps
• Receptive fields:
– On-off cells; Off-on cells
– Simple cells
• Lateral inhibition
Retinotopic maps in V1
Stimulus pattern
Response in monkey primary
visual cortex (V1) measured by
radio-active tracers
• 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)
Stimulus
Cortical Mapping:
Left Hemisphere
Cortical Mapping:
Right Hemisphere
Revealing retinotopic maps with fMRI
From: Geoff Boynton, SALK institute
Revealing retinotopic maps with fMRI
From: Geoff Boynton, SALK institute
Measuring Neural Activity
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:
On-off cell
STIMULUS
RESPONSE
APPROX. FIRING RATE
4
Video
25
5
LGN On cell:
responses
as shown on left
0
LGN Off cell
opposite response pattern
Simple Cells (bar detectors)
Video:
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)
Hierarchical Organization
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/%7Ebbutoi/latinh.html
2) http://www.psychology.mcmaster.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
Evidence for Functional Specialization
• Single-cell recording
• Patient data:
– Achromatopsia (damage to V4)
– Akinetopsia (damage to V5 or MT)
Specialization for form
processing in IT
(Inferotemporal-Cortex)
Kobatake & Tanaka, 1994
There is some evidence
for specialization to face
processing
Bruce, Desimone & Gross (1981)
The percentage of cells in six different visual cortical
areas responding selectively to orientation, direction of
motion, disparity, and colour.
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
Binding Problem
Alternative View: Hierarchical Model
Lennie (1998):
• Visual processing is hierarchical
• Areas serve multiple functions (except for MT)
Hierarchical Organization
“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
Perception–Action 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
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
Differential Sensitivity to Visual Illusions
Performance on a 3D 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).
Appropriate grasping requires the
retrieval 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).