The Visual System: Color Vision Lesson 18

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

The Trichromatic Theory n Perceived color l overall pattern of stimulation l Like mixing paint n Negative After-image?

The Trichromatic Theory? n What colors do you see? n How can you see colors that weren’t there? n Negative After-image l Does it fit Trichromatic Theory? ~

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

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

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

Retinex Theory n Perception of visual stimulus l subjective l includes context & past experiences l color & brightness constancy n Color constancy l e.g., grass in sun vs. grass in shade l perceived as same color green l e.g., Rubic’s cube ~

Lateral Geniculate Nucleus n 6 layers l dorsal  ventral l 6  1 n RFs center-surround n Input from each eye l monocular l 2, 3, 5 from the ipsilateral eye l parallel processing ~

LGN: Parallel Processing n Parvocellular system l layers 3-6 l small RFs l info from cones l color & form n Magnocellular system l Large RFs l layers 1 & 2 l info from rods l form only, no color ~

Color Processing: Primary Visual Cortex

V1 Organization n 6 layers l Most input  Layer 4 n 1st binocular receptive fields l cells get input from both eyes l ocular dominance n Modular organization n Blob cells: color processing l Wavelength specific ~

Modular V1 Organization 2 & & 6 LGN L R L BLOBBLOB

Blob cells & Color Perception n Same color perceived differently n Double Opponent Process Cells l Different from RGC & LGN l center-surround n Center: l red  excitation l green  inhibition n Surround: l red  inhibition l green  excitation ~

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

Higher Level Color Processing n V1  V2  V4 n V4 in medial occipital cortex n V4 damage l disrupts color constancy l achromatopsia l unilateral – 1 visual field l bilateral – no color perception ~