THE VISUAL SYSTEM: EYE TO CORTEX

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THE VISUAL SYSTEM EYE TO CORTEX

• Outline
• 1. The Eyes
• a. Structure
• b. Accommodation
• c. Binocular Disparity
• 2. The Retina
• a. Structure
• b. Completion
• c. Cone Rod Vision
• d. Eye Movements
• 3. Visual Transduction by Rhodopsin
• 4. From Retina to Primary Visual Cortex

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THE PERCEPTION OF CONTRAST AND COLOR

• Outline
• 1. Contrast The Perception of Edges
• a. Lateral Inhibition The Physiological Basis of
  Contrast Enhancement
• 2. Brightness-Contrast Detectors in the Mammalian
  Visual System
• 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
• a. Component Theory
• b. Opponent Process Theory

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Receptive Fields of Neurons in the
Retina-Geniculate-Striate Pathway

• Most retinal ganglion cells, lateral geniculate
  nucleus neurons, and the neurons in lower layer
  IV of the striate cortex have similar receptive
  fields
• They are circular
• They are smaller in the fovea area (larger in the
  periphery)
• 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
• (1) on firing
• (2) inhibition followed by off firing

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Simple Cortical Cells

• 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)
• All simple cells are monocular
• They also have on and off areas

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Complex Cortical Cells

• Most of the cells in the striate cortex are
  complex cells
• 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
• 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

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Hubel and Wiesel's Model of Striate-Cortex
Organization

• When you record from visual cortex using vertical
  electrode passes, you find
• (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
• 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)

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Hubel and Wiesel's Model of Striate-Cortex
Organization

• When you record from visual cortex using
  horizontal electrode passes you find
• (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

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CORTICAL MECHANISMS OF VISION

• Outline
• 1. The Parallel, Functionally Segregated
  Hierarchical Model of Perception
• 2. Cortical Mechanisms of Vision
• a. Scotomas and Blindsight
• b. Completion
• c. Secondary Visual Cortex and Association Cortex
• d. Visual Agnosia
• 3. Selective Attention

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The Current Parallel, Functionally Segregated
Hierarchical Model of Sensory-System Organization

• Parallel sensory systems are organized so that
  information flows between different structures
  simultaneously along multiple pathways
• Explicit a sensory system for consciously
  seeing things
• Implicit a sensory system that helps us
  interact with objects in space.
• Functionally Segregated There are divisions of
  labor at the different hierarchical levels
• 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.
• Primary
• Secondary
• Association

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Vertemnus (1591) by Guiseppe Arcimboldo
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The Vegetable Gardener (1590)
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Neural Correlates?

• There is evidence that selective attention
  occurs by strengthening the neural responses to
  attended-to aspects and by weakening the response
  to others.
• 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
• Does this face belong to the same person?
• face recognition activates the ventral visual
  pathway is this face the same person you saw
  before?
• fMRI
• When attending to location - dorsal stream is
  activated.
• Is this face in the same position as the last
  face you saw?
• face position activates the dorsal visual pathway
  is this face in the same location as the face
  you saw before?
• fMRI

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• Primates
• 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
• 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