Basic Processes in Visual Perception What is perception good for? We often receive incomplete...

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).