Presentation on theme: "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."— Presentation transcript:
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 ~
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 & 3 4 5 & 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 ~
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