The Central Visual System

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The Central Visual System
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Transduction

• Photoreceptors release the neurotransmitter
  glutamate (glu) when depolarized.
• Depolarized in the dark.
• Hyperpolarized by light.
• Only ganglion cells have action potentials.
• Photoreceptors produce graded response that
  provides input aggregated by bipolar cells.
• Magno ganglion cells receive input from rods,
  parvo ganglion cells from cones

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Bipolar Cell Receptive Fields

• The receptive field is the area of the retina
  capable of changing the bipolar cells membrane
  potential
• Two kinds of receptive fields
• OFF cell excitatory
• ON cell inhibitory
• OFF and ON refers to light, not the cell
• Center and surround are opposites

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Edge Detection

• The center-surround organization of the receptive
  fields of ganglion cells exaggerates the contrast
  at borders.
• Visual processes fill in what occurs between
  borders (edges).
• Contrast effects occur because we notice
  variations, not absolute magnitudes of light.

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Color Contrast

• Cones respond to specific wavelengths of light
  that determine hue.
• Color cells have complementary surrounds that
  heighten contrast and strengthen their signal.
• Opponents are red/green, blue/yellow.

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Color Opponency

• Certain colors are never seen in combination
• Reddish green, bluish yellow.
• Red and green mix to form yellow yellow and blue
  mix to form white.
• Herings opponent process theory perceptual
  cancellation occurs because colors are processed
  as opponent pairs.
• Color cells have complementary surrounds that
  heighten contrast and strengthen their signal.

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Color Processing

• The brain compares responses of three types of
  cone cells.
• Inputs from the three types of cones are combined
  in different ways.
• The brain computes responses of specific cones
  but also all cones in the retina (background) to
  compensate for ambient light (constancy).
• Area V4 responsible for color constancy damage
  results in loss of color experience.

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Visual Fields

• Each eye has a visual field that overlaps the
  visual field of the other eye.
• Each eyes visual field is divided in half
  called a hemifield.
• The right hemifield of each eye is viewed by the
  left hemisphere of the brain.
• The left hemifield of each eye is viewed by the
  right hemisphere of the brain.

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Some Terminology

• The suffix fugal means to flee
• Retinofugal refers to where the axons of the
  optic nerve go after they leave (flee) the
  retina.
• Decussation crossing of a bundle of fibers
  (axons) from one side of the brain to the other.
• Tract a bundle of fibers going the same way

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Retinotopic Mapping

• The relationship between an image in the world,
  its impact on the retina, and the retinas
  projection to the cortex is maintained.
• This is called topographic mapping.
• Stimulation of neighboring retinal locations
  results in stimulation of corresponding areas of
  the LGN, superior colliculus, and occipital
  cortex (primary visual cortex).
• Relationships between areas are maintained.

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Types of Ganglion Cells

• Magnocellular (M cells) large cells that
  receive input from rods.
• Parvocellular (P cells) small cells that
  receive input from cones.
• Blob pathway concerned with color perception.
• Interblob pathway concerned with shape/form.
• Koniocellular (nonM-nonP) small cells involved
  in color vision (not well understood).

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Mapping Within the LGN

• Optic nerve carries information from ganglia to
  LGN. Crosses at optic chiasm.
• Separate layers are maintained for each eye and
  for each type of cell (M and P).
• Interneurons project from areas of the LGN to
  striate cortex (also called primary visual cortex
  or V1).

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Mapping in the Striate Cortex

• Separate layers from LGN to striate cortex are
  maintained in ocular dominance columns.
• M, P, non-M/P cells enter the cortex at
  different levels of layer 4 of the visual cortex.
• Information is combined by pyramidal cells that
  synapse at higher levels in the striate cortex.
• Input from both eyes is combined at layer 3.

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Stages in Edge Detection

• Retinal bipolar cells have center-surround
  receptive fields.
• LGN ganglion cells respond to contrast and change
  in visual input.
• Center-surround (on-off) receptive field.
• Neurons in the visual cortex have rectilinear
  receptive fields with excitatory and inhibitory
  zones.

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Edge Detectors

• Hubel Weisel found simple cells responding to
  edges at different orientations.
• Complex cells in the visual cortex collect on-off
  data from multiple cells to form edges.
• Complex cells provide positional invariance.
• M-channel cells are orientation and direction
  selective, for motion detection.
• P-IB channel cells analyze object shape.

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Extrastriate Pathways

• Parallel processing of visual information from
  the striate cortex.
• Three pathways
• Color processing P blob cells, goes from V1 to
  V2, then V4, then inferior temporal cortex.
• Shape processing, depth perception P interblob
  cells, go from V1 to interior temporal cortex.
• Motion spatial relations M cells, V1 to V2,
  then MT (V5), to parietal cortex.

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Equiluminance

• Holding brightness constant permits the study of
  the contribution of color to perception.
• Results
• Brightness, not color, is important to motion
  detection, perspective, relative sizes, depth
  perception, figure-ground relations, visual
  illusions.
• Motion is a cue for distinguishing among objects.
• Things that move together belong together.

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Complex Forms, Motion

• Processing of form occurs outside the visual
  cortex inferior temporal cortex.
• Not organized retinotopically.
• 10 selective for specific images (hands, faces).
• Processing of motion occurs in middle temporal
  area (MT or V5), then parietal lobe.
• Used for seeing moving objects, pursuit eye
  movements, guidance of bodily movement

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Binding Mechanisms

• How is information from the separate, parallel
  pathways brought together and associated?
• Cells may identify patterns of synchronous
  activity.
• Treisman Julesz combination requires
  attention.
• A pre-attentive process detects the major outline
  of an object.
• An attentive process notices, selects
  highlights combinations of features.

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Visual Agnosias

• Existence of distinct agnosias for aspects of
  perception suggests that these abilities are
  localized to areas selectively damaged.
• Achromatopsia good perception of form despite
  inability to distinguish hues.
• Prosopagnosia inability to recognize faces as
  particular people (identity). Can recognize that
  it is a face, and tell the parts.

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Development of the Visual System

• Pathways are developed before birth.
• Fovea develops in the first four months after
  birth ability to see detail.
• Connections between layers in visual cortex
  develop with experience, after birth.
• Visual acuity becomes adult-like by 12 months.