Transcript Session 3

Perception and VR
MONT 104S, Spring 2008
Lecture 3
Central Visual Pathways
1
The Eye
2
Receptive Fields
The receptive field of a neuron is the region of the visual
field to which a cell responds.
Receptive fields can be mapped for neurons in the primate
visual cortex by recording electrical signals from the cell
while showing the animal a visual stimulus (e.g. a bar of
light).
The cell will respond by increasing or decreasing the rate
of action potentials when the light is in the cell's receptive
field.
Show Movie!
3
Response of Retinal Ganglion
Cells
Retinal Ganglion cells and bipolar cells have receptive fields that
exhibit a center-surround structure.
Discussion
Question: What is
the centersurround structure
useful for?
4
After-images
When photoreceptors absorb light, they are temporarily
"bleached", and rendered less sensitive to light.
If we are exposed to a bright light, such as a camera flash, we will
see a dark after-image in the location of the bleaching. The image
appears to drift as we move our eyes.
If we bleach the region of the fovea, it becomes difficult to read or
perform tasks that require high visual acuity.
5
Central Visual Pathway
Overview
Processing of visual information in primates is accomplished in
stages along the visual pathway:
Retina
Lateral Geniculate Nucleus
Striate Cortex
Extrastriate cortex
6
Projections from the Retina
Axons from the retina project to three areas:
1) Pretectal area: a midbrain area that controls pupillary reflexes.
2) Superior colliculus: controls saccadic (rapid) eye movements.
3) Lateral geniculate nucleus (LGN) of the thalamus: This is the
principal pathway, which projects to visual cortex.
4) Some of the axons from the retinal ganglion cells cross over to
the opposite side of the brain before reaching the LGN (at the
optic chiasm).
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Projection from the Retina
8
The Optic Chiasm
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The Lateral Geniculate Nucleus
6 layers of cells
4 parvocellular
2 magnocellular
Input from the
two eyes largely
separate.
Projection from
retina is
retinotopic.
10
Primary Visual Cortex
Primary Visual Cortex (also known as Striate Cortex or V1) is the
first cortical area in the visual pathway.
Hubel and Wiesel (1950's and 60's) were the first to describe
properties of V1 cells.
They described 3 types:
Simple cells: Elongated Receptive fields. Orientation selective.
Defined regions of excitation and inhibition.
Complex cells: Also orientation selective. No well defined
regions of excitation and inhibition.
Hypercomplex cells: End-stopped.
11
Columns in the cortex
The cortex is a folded sheet of cells, about 2 mm thick.
The cells form layers (6 layers in primary visual cortex).
If move perpendicular to the surface of the cortex, cells will
respond primarily to input from one eye (ocular
dominance).
The pattern of responses forms columns of ocular
dominance.
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Ocular Dominance Columns
13
Orientation Columns
Preference for a given orientation also has a columnar structure:
14
Retinotopic organization
The visual field maps onto
visual cortex in a
systematic way.
More of V1 is devoted to
processing the central
visual field than to the
periphery.
15
Mapping of Visual Areas
This map shows a flattened cortex with the
known visual areas mapped onto it.
There are a large number of
distinct visual areas (probably
at least 20).
Each area appears to have a
specific function.
The areas show a roughly
hierarchical organization
(although most areas have
reciprocal connections).
16
Two Major Processing Streams
There appear to be 2 major processing streams (although there
are cross connections between them):
1. The Dorsal Stream:
Includes areas MT, MST, VIP, 7a, etc.
Processes motion, stereo, spatial relationships
The "where" pathway.
2. The Ventral Stream:
Includes areas V4, IT, etc.
Processes color, form, objects.
The "what" pathway.
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