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VIEWS: 8 PAGES: 23

  • pg 1
									Psychology 4051


The Primary Visual Cortex
         The Primary Visual Cortex
• A multi-layered structure
  located in the occipital lobe
  (AKA, Area 17, V1, Striate
  cortex).
• Receives axons from the LGN.
• Each hemisphere represents
  the contralateral visual field.
              The Visual Cortex
• The visual cortex is topographically organized.
• A large area of the visual cortex is mapped to the
  central portion of the visual field (retina)
• Cortical magnification.
                The Visual Cortex
• In all, it contains 6 layers
  and the layers are
  distinguished by the types
  of cells they contain.
• LGN connections are sent to
  layer 4C.
                  The Visual Cortex

• Magnocellular cells input to Layer 4C alpha.
• Parvocellular cells inputs to Layer 4C beta.
              The Visual Cortex
• Layer IV projects to II, III, and IVB.
• Connections run from II and III to V and VI.
          The Primary Visual Cortex

• Hubel and Weisel discovered simple, complex and
  hypercomplex cells in the striate cortex.
• Most neurons in V1 are either
  – Simple – respond to edges at particular locations and
    orientations within the visual field
  – Complex – represent a more abstract type of visual information,
    at least partially independent of location within the visual field.
  – Hypercomplex or end-stopped – cells that are selective for a
    certain length of contour
  Receptive Fields of Simple and
 Complex Cells in the Visual Cortex

– Simple – receptive fields are rectangular with “on”
  and “off” regions

– Complex – also rectangular, larger receptive fields,
  respond best to a particular stimulus anywhere in its
  receptive field
    Receptive Field of Simple Cells
• Note that the central region is oblong and not circular as was
  the case for the center-surround receptive field of the retinal
  ganglion cells.
• Also, the surround region is now located only on the sides. In
  this particular cell, the inhibitory region is located in the
  center, not on the sides
Simple Cell Response
                                The Visual Cortex




Hubel and Weisel show that the response pattern of simple cells can
be explained by interaction of receptive fields of cells further back in
the visual system.
   Complex and Hypercomplex Cells
• Complex cells respond to orientation but
  location is less specific.
  – Do not have center surround arrangement


• Hypercomplex cells are like complex cells, but
  respond to edges that stop in the receptive
  field and might be thought of as ‘corner’
  detectors or ‘end of the line’ detectors.
           Orientation Columns
• Simple and complex cells have preferred orientation
• Electrode penetration that is exactly perpendicular to
  the cortical sheet will encounter cells with the same
  orientation preference (Columns).
             Orientation Columns
• Moving the electrode – column with a different orientation
  preference
• Preferences for neighboring columns are NOT random
   – Orientation selectivity changes only slightly as you move
     from column to column and there is a specific and
     consistent order.
         Ocular Dominance Columns
• Visual signals from the two eyes remain segregated in the LGN
  and primary visual cortex.
• As the recording electrode is moved within layer 4C, there is an
  abrupt shift as to which eye drives the unit. In layer 4C, the shift
  from one eye to the other takes place over a distance of less than
  50 microns
     Ocular Dominance Columns
• Thus, layer 4c is organized into columns based on
  ocular dominance.
   – i.e., ocular dominance columns.
Ocular Dominance Columns
                 Hypercolumns
• Together, orientation columns and ocular dominance
  columns form hypercolumns.
• 18-20 columns to represent all orientations for both eyes
   – about 1 mm square
                Hypercolumn
• Each hypercolumn encodes information about
  one small sector of the retina, and neighboring
  hypercolumns represent neighboring sectors of
  the retina
               Binocular Cells
• Although cells in layer IVC are exclusively
  monocular, cells in other layers are binocular.
  – Respond to stimulation in either eye.
  – Binocular cells
• Although these cells are binocular, some are
  driven primarily by one eye.
                Disparity Selective Cells




Binocular disparity: difference in the location between the right eye's and left eye's
image. Amount of disparity depends on the depth (i.e., the difference in distance to
the two objects and the distance to the point of fixation), and hence it is a cue that
the visual system uses to infer depth.
        Disparity Selective Cells
• We have cells that are disparity selective.
• They respond to specific amounts of disparity.
• Crucial for stereopsis (3-D vision).
           Cytochrome Oxidase Blobs
• Found primarily in Layers 2, 3,
  4. Hubel and Wiesel called
  them ‘blobs’.
• Blobs are located in the
  centers of ocular dominance
  columns.
• They are not orientation
  selective.
• Sensitive to color.

								
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