Melé

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Colour

‘Color-perception by V1, V2 and V4 indicates some remarkable properties of the visual cortex. The three types of cones are sensitive to a range of wavelengths, achieving maximum sensitivity at about 440, 520 and 580 nanometers (corresponding to blue, green and red). Nonetheless, what we see as blue, green or red can correspond to different wavelengths depending on whether a room is lit by tungsten light, fluorescent light or daylight. The property of color constancy has survival value insofar as it assists in the identification of objects. It turns-out that neurons in V1 are actually sensitive to wavelength, whereas neurons in V4 are sensitive to color. Between V1 and V4 an automatic processing occurs to interpret each color in the context of surrounding colors and thereby “discount the illuminant”. An analogous kind of processing can even occur in a black, white and grey picture. Notice that the horizontal band in the illustration looks lighter on the right than on the left. Yet it is easy to prove that the shade is uniform — by covering the background.’

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Learning, Memory and Plasticity