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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
• Retinotopic mapping: locations on retina are mapped to cortex in orderly fashion. Note: more of visual cortex is dedicated to foveal vision
Response in monkey primary visual cortex (V1) measured by
radio-active tracers
Stimulus pattern
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
Video
STIMULUS RESPONSE APPROX. FIRING RATE
4
25
5
0
LGN On cell:responses
as shown on left
LGN Off cellopposite 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.
DEMO APPLETS: 1) http://serendip.brynmawr.edu/%7Ebbutoi/latinh.html2) http://www.psychology.mcmaster.ca/4i03/demos/lateral-demo.html
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Light:
On-Off Cells with lateral inhibition:
Response Edge detection
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 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).
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).