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Computational Vision
CSCI 363, Fall 2012
Lecture 3
Neurons
Central Visual Pathways
See Reading Assignment on "Assignments page"
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The Neuron
Major parts of a neuron:
1) Cell body
2) Axon
3) Terminal
4) Synaptic cleft
5) Dendrites
Action Potentials
1. The resting potential is an electrical
Action potential
potential (from -40 to -110 mV) across
Na+ current
the cell membrane.
K+ current
2. Excitatory input causes the potential to
decrease (depolarize).
3. This causes a voltage-sensitive
mechanism to set off a rapid change in
electrical potential in the membrane.
4. Action potentials move rapidly down the axon to the axon
terminal.
5. Action potentials require energy.
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Neural Signaling
1.
2.
3.
4.
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6.
7.
Input to the cell causes depolarization of the cell body to threshold.
An action potential propagates down the axon to the terminal.
Transmitter is released, diffuses across the synaptic cleft to the postsynaptic
cell and binds to receptors on the postsynaptic cell.
The transmitter causes an electrical (positive or negative) response in the
dendrite of the postsynaptic cell.
The electrical signal in the dendrite spreads passively to the cell body.
Electrical signals from many dendrites combine at the cell body. If excitation
outweighs inhibition, the cell may reach threshold causing another action
potential.
The cycle begins again in the next cell.
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Complexity of Neural
Processing
Things to bear in mind:
•One neuron may connect to 1000 post-synaptic neurons.
•One neuron may receive 10,000 inputs from other cells.
•Lots of computation takes place locally within the dendrites.
•There are approximately 1011 neurons in the brain.
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Simplified Model Neurons
Typical neural network models use very simplified model
neurons:
1) The response is modeled as the rate of action potential firing.
This is a graded response with a threshold and saturation.
2) The neurons are generally homogeneous. There are no
differences in shape or transmitter effects.
3) A single neuron can have both excitatory and inhibitory effects
on the postsynaptic cell.
4) There is no systemic modulation of neurons (e.g. from
hormones).
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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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The Optic Chiasm
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The Lateral Geniculate Nucleus
The LGN is
layered.
Input from the
two eyes largely
separate.
Projection from
retina is
retinotopic.
Cells respond like
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RGCs.
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.
(Movie)
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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Ocular Dominance Columns
•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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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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Extrastriate Visual Areas
•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).
This map shows a flattened cortex with the known visual areas
mapped onto it.
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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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