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The Visual System_ Color Vision

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									The Visual System:
   Color Vision
      Lesson 18
      The Trichromatic Theory
 Young-Helmholtz (1802)
 3 types of color receptors
    Cones

 Differential sensitivity to light
  wavelengths
    red (long)

    green (medium)

    blue (short) ~
    The Trichromatic Theory
 Perceived color
   overall pattern of stimulation

   Like mixing paint

 Negative After-image?
    The Trichromatic Theory?
 What colors do you see?
 How can you see colors that weren’t
  there?
 Negative After-image
    Does it fit Trichromatic Theory? ~
Hering Opponent Process Theory
  Competing theory
  Center-Surround organization
    BP, RGC, & LGN

  Antagonistic for color
    Red-Green

    Blue-Yellow

    Black-White
Hering Opponent Process Theory
    Center- surround antagonistic
      Each color can be excitatory (+) or

       inhibitory (-)
      12 combinations total ~
             Which Theory?
 Both are correct in retina
 Photoreceptors: trichromatic
 Higher levels: Opponent Process
       BP, RGC, & LGN
   More complex at cortical level
     Retinex Theory ~
          Retinex Theory
 Perception of visual stimulus
   subjective

   includes context & past experiences

   color & brightness constancy

 Color constancy
   e.g., grass in sun vs. grass in shade

   perceived as same color green

   e.g., Rubic’s cube ~
        Lateral Geniculate Nucleus
   6 layers
      dorsal  ventral

         6  1
   RFs center-surround
   Input from each eye
      monocular

      2, 3, 5 from the ipsilateral eye

      parallel processing ~
       LGN: Parallel Processing
   Parvocellular system
     layers 3-6

     small RFs

     info from cones

     color & form

   Magnocellular system
     Large RFs

     layers 1 & 2

     info from rods

     form only, no color ~
  Color Processing:
Primary Visual Cortex
         V1 Organization
 6 layers
    Most input  Layer 4

 1st binocular receptive fields
    cells get input from both eyes

    ocular dominance

 Modular organization
 Blob cells: color processing
    Wavelength specific ~
      Modular V1 Organization
              L        R        L
          B
          L
2&3       O
          B

  4
LGN

5&6
Blob cells & Color Perception
 Same color perceived differently
 Double Opponent Process Cells
   Different from RGC & LGN

   center-surround

 Center:
   red  excitation

   green  inhibition

 Surround:
   red  inhibition

   green  excitation ~
Double-Opponent Process
       Blob Cells

   R-G+        R-G+

   R+G-        R+G-
     Higher Level Color Processing
   V1  V2  V4
   V4 in medial occipital cortex
   V4 damage
     disrupts color constancy

     achromatopsia

     unilateral – 1 visual field

     bilateral – no color perception ~

								
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