Transcript primary visual cortex

Psychology 304:
Brain and Behaviour
Lecture 28
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The Visual System
1. What are the major areas of the brain that are
associated with the perception of light?
2. What features of the visual system facilitate edge
perception?
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What are the major areas of the brain that are associated
with the perception of light?
• The thalamic neurons that receive visual information
subsequently project the information to the primary
visual cortex.
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Primary Visual Cortex
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Primary Visual Cortex
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• The primary visual cortex is organized into functional
vertical columns.
• Information received by the primary visual cortex is
segregated into distinct pathways that project to areas of
the secondary visual cortex and, then, the association
visual cortex.
• Two main pathways from the primary visual cortex
have been identified: The dorsal stream and the ventral
stream. The dorsal stream is associated with movement;
the ventral stream is associated with colour and form.
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The Dorsal and Ventral Streams
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Secondary Visual Cortex
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What features of the visual system facilitate edge
perception?
• Three features of the visual system facilitate the
perception of edges
1. Lateral Inhibition
 The impact of lateral inhibition is illustrated by the
following image:
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Contrast Enhancement
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 Contrast enhancement results from lateral inhibition
among the receptors of the retina.
 The axons of adjacent receptors form a lateral
neural network.
 Each receptor in the network fires at a rate
proportional to the intensity of the light striking it.
Moreover, when a given receptor fires, it inhibits the
firing of adjacent cells.
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 A receptor that fires rapidly produces greater
inhibition of adjacent cells than a receptor that fires
slowly.
 The differential rates of firing and, hence, differential
rates of lateral inhibition produced by receptors
surrounding an edge account for contrast
enhancement:
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Lateral Inhibition and Contrast Enhancement
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 Receptor D receives less lateral inhibition than
Receptors A, B, and C and, thus, fires more rapidly
than these receptors.
 Receptor E receives more lateral inhibition than
Receptors F, G, and H and, thus, fires less rapidly
than these receptors.
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2. On-center and off-center cells of the retinageniculate-striate system
 The neurons that comprise the retina-geniculatestriate system (i.e., retinal ganglion cells, lateral
geniculate neurons, and neurons of lower layer
IV of the primary visual cortex) are characterized
by circular receptive fields.
 These fields are comprised of an excitatory area
and an inhibitory area separated by a circular
boundary.
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 On-center cells respond to illumination in the
central region of its receptive field by firing more
rapidly, and illumination in the peripheral region
of its receptive field by firing less rapidly.
 Off-center cells respond to illumination in the
peripheral region of its receptive field by firing
more rapidly, and illumination in the central
region of its receptive field by firing less rapidly.
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The Receptive Fields of On-Center and
Off-Center Cells
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 The firing of on-center and off-center cells is most
affected by contrasting levels of illumination
between the “on” and “off” regions of the receptive
field.
 Diffuse light of a constant intensity shone on both
the on and off regions does not effect the firing rate
of on-center and off-center cells.
 Thus, on-center and off-center cells facilitate edge
perception.
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On-Center Cells, Off-Center Cells, and
Edge Perception
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3. Simple Cortical Cells
 Comprise all layers of the primary visual cortex,
except lower layer IV.
 Characterized by rectangular receptive fields.
 These fields are comprised of excitatory areas
and inhibitory areas separated by straight lines.
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Light bar in
dark field.
Dark bar in
light field.
Straight edge
between dark
and light areas.
The Receptive Fields of Simple Cortical Cells
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 The firing of simple cortical cells is most affected
by contrasting levels of illumination between the
“on” and “off” regions of the receptive field.
 Diffuse light of a constant intensity shone on both
the on and off regions does not effect the firing rate
of simple cortical cells.
 Thus, simple cortical cells facilitate edge
perception.
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 Simple cortical cells are distinct from complex
cortical cells.
 Like simple cortical cells, complex cortical cells
comprise all layers of the primary visual cortex,
except lower layer IV. They are also found in the
secondary visual cortex.
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 Like simple cortical cells, complex cortical cells
are characterized by rectangular receptive fields.
However, these fields are larger and have no
inhibitory areas; thus, the firing of complex
cortical cells is affected by illumination in any
region of the receptive field.
 Complex cortical cells respond to movement of
stimuli across the receptive field and, thus, are
referred to as “movement detectors.”
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The Visual System
1. What are the major areas of the brain that are
associated with the perception of light?
2. What features of the visual system facilitate edge
perception?
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