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Lec 12

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Doug Mac Kay

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12 Colour VisionOctober 05 2007Colour VisionBased on 3 classes of cones each with a different visual pigment which absorbs a different band of visible spectrumoshort wavelength S conesabsorbance peak at about 420 nm uncommonevenly dispersed in the foveaomiddle wavelength M cones530 nmolong wavelength L cones552557 nmVisual pigment absorbanceRod absorptionRods fill the gap between the S and M conesMore prominent in peripheryBUT rods are roughly 10 times more sensitive to light except red than conesat the high light levels required for cone function rhodopsin is bleached and rods nonfunctionalColor selective ganglion cellsCenter and surround receptive fields receive input from different cone populations usually M and L typesoOne cone type in center different cone type in surround and compete with one anotheroLook at differences of activity between cone populationNarrows the range of response of the spectrumlateral inhibition subtracts response to light stimulus of one cone type from the otheroenhances differences in spatial sensitivityganglion cell responds to the differential effect between competing cone inputsSingleopponent receptive fieldsCombinations of M and L inputs ON or OFF in center or surround make up most color cell receptive fieldsNOTEoptimal colored stimuli give best response when they cover entire receptive fieldoUse colour stimulusganglion cells do NOT respond to color contrastothey respond to a more restricted band of spectrum than do conesothe two parts of the centersurround field will respond differentially to the given colour because of the different colour pigment in the center and surround1
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