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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 visual system color vision

The Visual System:Color Vision

Lecture 16


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) ~


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 ~


Color processing primary visual cortex

Color Processing:Primary Visual Cortex


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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