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The Visual System: Color Vision. Lecture 16. 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

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the trichromatic theory
The Trichromatic Theory
  • Young-Helmholtz (1802)
  • 3 types of color receptors
    • Cones
  • Differential sensitivity to light wavelengths
    • red (long)
    • green (medium)
    • blue (short) ~
slide3
The Trichromatic Theory
  • Perceived color
    • overall pattern of stimulation
    • Like mixing paint
  • Negative After-image?
the trichromatic theory1
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
Hering Opponent Process Theory
  • Competing theory
  • Center-Surround organization
    • BP, RGC, & LGN
  • Antagonistic for color
    • Red-Green
    • Blue-Yellow
    • Black-White
hering opponent process theory1
Hering Opponent Process Theory
  • Center- surround antagonistic
    • Each color can be excitatory (+) or inhibitory (-)
    • 12 combinations total ~
which theory
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
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
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
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 ~
v1 organization
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
Modular V1 Organization

L R L

B

L

O

B

2 & 3

4

LGN

5 & 6

blob cells color perception
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 ~
higher level color processing
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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