THE VISUAL SYSTEM: EYE
TO CORTEX
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Outline
1. The Eyes
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2. The Retina
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a. Structure
b. Accommodation
c. Binocular Disparity
a. Structure
b. Completion
c. Cone & Rod Vision
d. Eye Movements
3. Visual Transduction by Rhodopsin
4. From Retina to Primary Visual Cortex
THE PERCEPTION OF
CONTRAST AND COLOR
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Outline
1. Contrast: The Perception of Edges
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2. Brightness-Contrast Detectors in the Mammalian Visual System
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a. Lateral Inhibition: The Physiological Basis of Contrast Enhancement
a. Mapping Receptive Fields
b. Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway
c. Simple Cortical Cells
d. Complex Cortical Cells
e. Hubel and Wiesel's Model of Striate-Cortex Organization
3. Seeing Color
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a. Component Theory
b. Opponent Process Theory
Receptive Fields of Neurons in the
Retina-Geniculate-Striate Pathway
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Most retinal ganglion cells, lateral geniculate
nucleus neurons, and the neurons in lower layer
IV of the striate cortex have similar receptive fields
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They are circular
They are smaller in the fovea area (larger in the periphery)
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Indicative of greater acuity in the fovea
They are monocular
They have both an excitatory and an inhibitory area
separated by a circular boundary.
Neurons in these regions have 2 patterns of
responding:
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(1) on firing
(2) inhibition followed by off firing
Simple Cortical Cells
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Respond best to bars or edges of light
in a particular location in the receptive
field and in a particular orientation
(e.g., 45 degrees)
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All simple cells are monocular
They also have on and off areas
Complex Cortical Cells
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Most of the cells in the striate cortex are
complex cells
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more numerous
like simple cells in that they respond best to
straight-line stimuli in a particular orientation
unlike simple cells in that the position of the
stimulus within the receptive field does not matter
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the cell responds to the appropriate stimulus no matter
where it is in its large receptive field
Over half of the complex cells are binocular,
and about half of those that are binocular
display ocular dominance
Hubel and Wiesel's Model of
Striate-Cortex Organization
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When you record from visual cortex using vertical
electrode passes, you find
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(1) As the electrode moves up or down through the layers of
the striate cortex it finds neurons that respond to stimulation
from about the same location on the retina
(2) simple and complex, cells that all prefer the same
orientation – the cells respond to line orientations that are at
the same degree
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Info. flows from on and off cells in lower layer IV to simple
cells and then to complex cells
(3) binocular complex cells that are all dominated by the
same eye (if they display ocular dominance)
Hubel and Wiesel's Model of
Striate-Cortex Organization
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When you record from visual cortex
using horizontal electrode passes
you find
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(1) receptive field location shifts slightly
with each electrode advance
(2) orientation preference shifts slightly
with each electrode advance
(3) ocular dominance periodically shifts to
the other eye with electrode advances
CORTICAL MECHANISMS
OF VISION
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Outline
1. The Parallel, Functionally Segregated
Hierarchical Model of Perception
2. Cortical Mechanisms of Vision
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a. Scotomas and Blindsight
b. Completion
c. Secondary Visual Cortex and Association Cortex
d. Visual Agnosia
3. Selective Attention
The Current Parallel, Functionally Segregated
Hierarchical Model of SensorySystem Organization
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Parallel: sensory systems are organized so that information
flows between different structures simultaneously along multiple
pathways
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Functionally Segregated: There are divisions of labor at the
different hierarchical levels
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Explicit –– a sensory system for consciously seeing things
Implicit –– a sensory system that helps us interact with objects in
space.
Some neurons respond to motion, some to color, and so on.
Hierarchical: information flows through brain structures in
order of their increasing neuroanatomical and functional
complexity.
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Primary
Secondary
Association
Vertemnus (1591) by
Guiseppe Arcimboldo
The Vegetable Gardener
(1590)
Neural Correlates?
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There is evidence that selective attention occurs by strengthening the
neural responses to attended-to aspects and by weakening the
response to others.
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Anticipation of a stimulus increases neural activity in the same circuits
affected by the stimulus itself.
When attempting to recognize faces - ventral stream is activated
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Does this face belong to the same person?
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face recognition activates the ventral visual pathway – is this face the same
person you saw before?
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fMRI
When attending to location - dorsal stream is activated.
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Is this face in the same position as the last face you saw?
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face position activates the dorsal visual pathway – is this face in the same
location as the face you saw before?
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fMRI
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Primates
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Recorded from neurons in the prestriate area
that is part of the ventral stream that are
particular responsive to color.
Some cells responded to red; some to green
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Monkeys were taught to perform a task that required
attention to a red cue.
The activity of red neurons was increased, whereas the
activity of green neurons was reduced