Colour Centre Originally Proposed

V4 Colour centre originally proposed There are overlap b/w Color processing and other visual properties so not dedicated just to colour perception. Achromatopsia Color blindness Cooccur with prosopagnosia Is there a single colour center in cortex? No Regions b/w FFA and PPA Might explain overlap with prosopagnosia Monochromats color blind No functioning cones only rods = only see white grey and black But can match any WL by adjusting brightness Poor acuity Sensitive to bright light - all rods dichromats (color blindness) Missing 1 of 3 types of cones Protanopia, deuteranopia, tritanopia Unilateral dichromats : have trichromic vision in 1 eye and dichromatic in other eye —-Brian can interpret perception from both perspective Protanopia Missing long wave pigment Form of red green colourblindness Red looks more green and less bright See short wavelength as blue Above neutral point see yellow Deuteranopia missing medium wavelength pigment Form of red green colour blindness Green looks more red Short wavelengths as blue Above neural point see yellow Tritanopia missing short wavelength pigment Blue - yellow colourblindness Difficulty separating blue/green, and red/yellow Short as blue Above neutral point see red Color boosting glasses Filter out certain WL (overlap) allowing for more contrast in range of colour perceived opponent-process theory the theory that opposing retinal processes (red-green, yellow-blue, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green Cancel out perception of Color in 1 half of pair by adding WL from opposite half hue cancellation (psych evidence of opponent process) an experiment in which observers cancel out the perception of a particular color by adding light of the opponent color Eg adding some yellow takes away some of blue -at midpoint neither colour is perceived Psych evidence Physiological evidence of opponent process Single cell recording found opponent neurons respond in excitatory manner to one end of spectrum, and inhibitory to other end in the retina and LGN opponent-process theory vs trichromatic theory Both describe different physiological mechanisms in visual system for color Trichromatic explains RESPONSE OF CONESin retina Opponent process explains NEURAL RESPONSE OF CELLS connected to cones further in brain Colour constancy perception of constant surface colour despite changes in illumination and viewing conditions Sunlight = equal amounts of energy at all visible wavelengths Tungsten lighting = more long wavelength (makes it look more red) Objects reflect diff WL from these 2 sources chromatic adaptation Prolonged exposure to chromatic colours When stimulus colour selectively bleaches a cone pigment = overtime decreased sensitivity to that associated Color Response to light sources which contribute to colour constancy under different illuminations Uchkiwa color constancy experiment Observers shown sheet of green (coloured) paper under 3 lights Baseline - white light Observer not chromatically adapted - paper illumined by red light = observer by white Observer chromatically adapted - paper and obsered exposed to red light Results show adapted only slightly shifted to red but 2nd condition was more shifted to red color constancy works best When object surrounded by many colours Masked surroundings = reduces accuracy of colour perception Refers to perceptual system having more info about environment that allow it to make relate corrections when making judgments - Eg rubic cube look blue in masked surrounding when actually grey Hansen et al Showed photos of fruits against grey background When shown a spot of light which matched the grey background participants accurately perceived the spot as grey But when fruits adjusted to be grey = still perceived as being slightly coloured —-past knowledge affects Color perception lightness constancy Achromatic colors perceived as constant through diff lighting Lightness: perception of shade of achromatic colour (white, grey black) Because perception of lightness is not related to the total amount of light reflected but rather percentage Intensity of light reaching eye from object dependent on 1. Intensity of illumination 2. Objects reflectance (reflected back by object) Lightness constancy determined by reflectance not intensity Eg white squares always look white and black always look black regardless of total light there is in environment ratio principle two areas that reflect different amounts of light look the same if the ratios of their intensities are the same (Evenly illuminated) reflectance edge an edge between two areas where the reflectance of two surfaces changes Eg. Changes in material illumination edge an edge where the lighting changes Reflectance edge vs Illumination edge Distinguish on meaningfulness of objects Eg. Palm tree shape shadow Penumbra of shadow - makes it look like a different object infant color vision Perceptions of light can very on at least 2 dimensions 1.chromatic colors 2. Brightness Different in grey scale = difference in brightness Looking time technique = evidence that infants have colour vision Based on pattern of habituation and dishabituation to 3 WLs Green and yellow = no dishabituaion so look same Blue = dishabituaion so perceived as different