Color vision is trichromatic, as a result of three kinds of cone photoreceptors with different spectral sensitivities. The output of the long-wavelength cones is subtracted from the output of the middle wavelength cones to give a red/green opponent color signal in the retina, and another subtraction in different cells gives a yellow/blue opponent color signal. These signals are conveyed to the lateral geniculate nucleus without much change, then to V1, where double opponent (opponent for color in the center of the receptive field, with the opposite sign of response in the surround) cells are found in upper layers in what are called blobs. The blobs project to the thin stripes in V2, then to clusters of color-coded cells in V4, and to inferotemporal cortex. Lack of one type of cone leads to color blindness, and lesions near V4 leads to achromatopsia.
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