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The Visual System: Color Vision

Learn about the Trichromatic Theory, Opponent Process Theory, Retinex Theory, and Color Processing in the brain, including V1 organization and perception nuances. Explore how we perceive colors and the complexities of visual processing.

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The Visual System: Color Vision

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  1. The Visual System:Color Vision Lesson 17

  2. The Trichromatic Theory • Young-Helmholtz (1802) • 3 types of color receptors • Cones • Differential sensitivity to light wavelengths • red (long) • green (medium) • blue (short) ~

  3. The Trichromatic Theory • Perceived color • overall pattern of stimulation • Like mixing paint • Negative After-image?

  4. 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? ~

  5. Hering Opponent Process Theory • Competing theory • Center-Surround organization • BP, RGC, & LGN • Antagonistic for color • Red-Green • Blue-Yellow • Black-White

  6. Hering Opponent Process Theory • Center- surround antagonistic • Each color can be excitatory (+) or inhibitory (-) • 12 combinations total ~

  7. Which Theory? • Both are correct in retina • Photoreceptors: trichromatic • Higher levels: Opponent Process • BP, RGC, & LGN • More complex at cortical level • Retinex Theory ~

  8. 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 ~

  9. 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 ~

  10. 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 ~

  11. Color Processing:Primary Visual Cortex

  12. 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 ~

  13. Modular V1 Organization L R L B L O B 2 & 3 4 LGN 5 & 6

  14. 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 ~

  15. Double-Opponent Process Blob Cells R-G+ R-G+ R+G- R+G-

  16. 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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