“Opponent” processing of cone photoreceptor signals in neural substrates located early in the visual pathways is sufficient to generate two, polarity-sensitive signals that describe the red-green and yellow-blue “cardinal directions” in colour space. Numerous studies support the implication that the same neural substrates generate polarity sensitive signals both in the retina and in the lateral geniculate nucleus. Studies in patients with diseases of the retina and the optic nerve tend to produce symmetric loss of either red-green or / and yellow-blue chromatic sensitivity. The processing of opponent colour signals and the generation of perceived primary colours in extra striate areas of the cortex is less well understood. We have investigated 20 subjects with cortical damage using visual psychophysical techniques that isolate the use of colour signals. The results reveal a variety of specific colour discrimination losses that range from severe red-green loss with normal yellow-blue sensitivity to loss of “red” but not “green” sensitivity.

The results suggest that the processing of different colour categories is carried out in distinct neural substrates and that selective damage to brain tissue can result in loss of chromatic sensitivity that affects selectively the perception of red-green, yellow-blue or single colour categories.