Cortical mechanisms of colour vision
Nature Reviews Neuroscience, 4 (7):
563--572 (July 2003)DOI: 10.1038/nrn1138
Abstract
Colour vision is an integral part of the human visual system. It relies on the presence of three types of cone photoreceptor in the retina, which have different but overlapping wavelength tuning curves.Colour information is sent in three colour-opponent channels from they eye to the brain. These 'cardinal' mechanisms, which are usually termed black–white, red–green and blue–yellow, have been characterized psychophysically, physiologically and computationally to be independent and efficient.In the primary visual cortex (area V1), a large proportion of neurons respond selectively to colour information. Most of these neurons also respond to variations in the brightness of visual stimuli. The colour combinations that are preferred by these neurons are no longer constrained to the three cardinal directions. In higher visual areas, neurons become more selective in their colour tuning and respond only to a small range of colours. About a third of neurons in area V2 combine their inputs in such a nonlinear manner.Colour is processed jointly with other visual attributes, such as orientation, depth and motion. Neurons in different anatomically defined compartments of the second visual area, for example, show joint selectivity for these different attributes. For most psychophysical tasks, the visual system works just as well for coloured stimuli as it does for black and white stimuli, once the magnitude of the different stimuli is made comparable.Colour constancy describes the ability of the visual system to discount large changes in illumination, so that objects look the same colour even under different illuminations. Both retinal and cortical factors contribute to colour constancy. Cells in V1 with both spatial and chromatic opponency ('double-opponent' cells) might be important for this achievement.People with an acquired colour vision deficiency (achromatopsia) often have damage to a small region in the lateral occipital cortex. The same region is typically highly active in neuroimaging experiments when subjects view coloured scenes. The residual abilities of achromatopsic patients show that their main deficit seems to be the assignment of colours to objects, rather than the perception of colours per se.Our understanding of the cortical processing of colour is still far from complete. A better understanding of the relationships between areas of monkey visual cortex and apparently homologous areas in the human brain will help us to address remaining questions, such as the degree to which colour information is segregated from other visual attributes. In the long run, more emphasis has to be given to the computations that are performed on the colour signals (and visual signals in general), rather than to the localization of regions that are important for the analysis of colour.