Download Session 4

Perception, Illusion and VR
HNRS 299, Spring 2008
Lecture 4
Central Visual Pathways
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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
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The Eye
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Processing in the Retina
Light is absorbed by the photoreceptors in the retina.
Some processing is performed in the retina itself.
The retinal ganglion cells are the final stage in the retinal
processing. They send axons out of the eye to the LGN.
Retinal ganglion cells have receptive fields with center-surround
spatial structure.
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On center/off surround
Off center/on surround
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
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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.
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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.
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Listening to Neurons
Hubel and Wiesel, Recording from a Complex Cell in Cat Visual Cortex
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
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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
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Orientation Columns
Preference for a given orientation also has a columnar structure:
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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.
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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).
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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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