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The Visual System: Color Vision

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

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  1. The Visual System:Color Vision Lecture 16

  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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