Functional Architecture of the Visual Cortex

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Functional Architecture of the Visual Cortex
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Emergent Properties of Cortical Neurons

• Thalamic neuron activity is altered by Visual
  stimulus
• position
• eye input
• Cortical neuron activity is altered by Visual
  stimulus
• position
• orientation
• eye input

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The Geniculocortical Projection

• Neurons in the retina and the LGN have a center
  surround receptive field organization

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Orientation Receptive Field of layer IV in
Primary Visual Cortex
Visual Neural Stimulus Response


This one neuron responds preferentially to
vertical oriented bars centered at 0 deg
eccentricity.
0 deg 0.5 deg
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Arrangement of Orientation Selectivity

• An orderly arrangement of orientation
  selectivity's was hinted at in single unit
  recordings.

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Retinotopic Preservation

• Neighboring points of the retina are represented
  by neighboring points of cortical tissue
• Anatomical and physiologic support
• 14C-2-deoxy-d-glucose
• Activity dependant labeling

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Proline Injections to Show Anatomic Organization
of Ocular Dominance Columns
Radioactive Proline injected into the eye
Tran-synaptic transport through the LGN
terminates in visual cortex
Cortical Layers I - III
Optic Radiation
Lateral Geniculate Nucleus
Optic Nerve
Optic Tract
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Ocular dominance columns were highlighted by
injecting radio-actively labeled amino-acids into
the right eye of an anaesthetized animal
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• Ocular dominance stripes reflect the merging of
  two retinotopic maps (one from each eye) onto the
  same cortical space

Hubel and Wiesel, 1977
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Normal
Tangential
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Functional Modules

• The primary cortex (V1) is organized by at least
  three basic principles.
• Retinotopic position
• Ocular (Eye) dominance
• Orientation preference

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A single function model, or hyper column, is then
thought to be replicated across the cortical
space so that all positions on each retina has a
corresponding hypercolumn
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Take Homes for modular function
Cortex is organized according to three basic
features

• Retinotopic Position Columns
• Ocular Dominance Columns
• Orientation specificity Columns

Hyper-column model proposes that all three (or
more) columns represent a single functional unit
in cortex

• All three features are equally represented in
  cortex for every retinal position.
• Each layer IV neuron is has characteristic
  position, ocular dominance, and orientation
  response preference.

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Problems with this model
Is the Cortex only organized by three basic
properties?
Other possible candidates include color
selectivity (CO Blobs), spatial frequency,
velocity of the image
How are functional units separated from one
another
Can we determine what features mark the edges of
one functional unit and the beginning of another?
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Normal developmental course for monkey
Modified from LeVay, Stricker, Shatz, 1978
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Whats so critical about a critical Period?