Colour Vision

Peter Gouras, Columbia University, New York, USA

Published online: September 2007

DOI: 10.1002/9780470015902.a0000043

Full Article on Wiley Online Library

Our daylight photoreceptors, cones, mediate colour vision but by themselves cannot distinguish wavelength from the energy of the light they absorb. By comparing the responses of two cone types, one absorbing best from one end and the other best from the other end of the visible spectrum, energy and wavelength contrasts can be determined independently, and thereby colour vision. In man and many primates, three cone mechanisms are compared; the long wavelength sensitive (L) cones are compared with the middle wavelength sensitive (M) cones to create ‘red/green’ colour vision and the short wavelength sensitive (S) cones are compared with both M and L cones for ‘yellow/blue’ form of colour vision. The latter is the only form of colour vision in most mammals and in 2% of human males. This process involves hierarchical circuits from retina to visual cortex including antagonistic interactions between these cone pairs in the same areas of visual space, ‘single opponency’ in the retina and lateral geniculate nucleus, to comparisons between adjacent areas of visual space, ‘dual opponency’ in visual cortex. By this means energy is dissociated from wavelength contrast to create colour vision where both variables are processed separately and used to sense hue, saturation and brightness, the qualities of colour vision, creating about a million varieties of colour.

Keywords: cones; cone opsins; single opponent cells; double opponent cells; successive and simultaneous colour contrast

	Figure 1. Shows the difference between colour vision using both wavelength and energy contrasts (left) and achromatic vision using only energy contrasts (right).
	Figure 2. Shows the normalized absorption spectra of the three cone opsins mediating human colour vision the responses of which are compared in pairs. One involves a comparison of S with the combined response of M and L cones, an early form of colour vision possessed by many mammals as a diviariant form of colour vision. This compares both ends of the visible spectrum. The other involves a comparison of M with L cone responses, possessed by many primates including man creating a trivariant form of colour vision. This comparison splits the yellow region of the spectrum in two. The spectral colours produced in the spectrum are illustrated by coloured lines. When these absorption spectra are plotted according to the 4th root of wavelength, they have the same shape.
	Figure 3. Shows how the cone mosaic of a divariant colour system samples the longer wavelength sensitive cones (yellow) for high spatial resolution achromatic vision and both the short-wavelength (blue) and long-wavelength (yellow) cones for chromatic contrast. Because groups of cones must be sampled for chromatic contrast, the unit area of chromatic space is larger than that of achromatic space.
	Figure 4. Shows how the on- and off-responses from a divariant colour vision system lead to the perception of different colours in which both hue and lightness and darkness affect colour vision.
	Figure 5. Shows the ganglion cell output of the primate retina that mediates a trivariant form of colour vision. One input involves a ‘blue/yellow’ opponent channel with opponent interactions between S and a combination of L and M cones (left). An S cone off-system is not shown because its circuitry is not yet clear. The other input involves the midget cell system, which transmits signals for both high resolution achromatic and lower spatial resolution chromatic vision, involving a ‘red/green’ opponent channel.
	Figure 6. Shows the receptive field organization of the S cone and the midget cell system of primate retina. The former involves coextensive inputs from S and L and M cones. The latter has concentrically organized centre/surround opponent fields, which reflect an involvement in both achromatic and chromatic vision.
	Figure 7. Shows how achromatic and chromatic signals are derived from a trivariant system in which one group of cones, the L and M cones, are used to detect both achromatic and chromatic contrast. The system mediating the signals of S cones is only used for chromatic vision. For achromatic contrast, single L and M on-cells excite a cortical cell and off-cells excite another cortical cell in linear arrays producing orientation selectivity with high spatial resolution. For chromatic contrast, a large group of cones are used to mediate chromatic vision in two paired comparisons.
	Figure 8. Shows the relationship of the on and off responses of the three cone mechanisms of trivariant human vision and the basic colours they create.
	Figure 9. Shows how white induces yellow after-images implying that the white surface strongly affects S cones and consequently weakens its responses in the after image. Stare at the image on the left and then look at the tree on the right to see the yellow after-images.
	Figure 10. Shows the synaptic targets of the midget cell and the S cone system projecting to striate cortex from the lateral geniculate nucleus. The S cone system originates in the konio-cellular layers of the geniculate and appears to project directly to ‘blob’ in layer 3. The midget cell system from the parvo-cellular layers projects to both the chromatic processing ‘blobs’ and the more extensive achromatic processing interblob areas.
	Figure 11. Shows how a single opponent cell excited by L cone on- and M cone off-inputs can be transformed into a double opponent cell specific for red/green spatial contrast. Neighbouring colour-selective cells affect the central cell in the following ways. Neighbouring ‘red’ selective single opponent cells serving adjacent areas of space inhibit the central cell; neighbouring ‘green’ selective cells excite the central cell. Similar interactions among this population of cells will transform all such single opponent cells into double opponent cells.

Further Reading
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	Conway BR, Hubel DH and Livingston M (2002) Colour contrast in macaque V1. Cerebral Cortex 12: 915–925.
	Dacey DM and Lee BB (1994) The blue on-opponent pathway in the primate retina originates from a distinct bistratified ganglion cell. Nature 367: 731–735.
	Daw N (1968) Colour-coded ganglion cells in the goldfish retina: extension of their receptive fields by means of new stimuli. Journal of Physiology 197: 567–592.
	Goldsmith TH (2006) What birds see. Scientific American (July) 295(1): 68–75.
	Kingdom FAA and Mullen KT (1995) Separating colour and luminance information in the visual system. Spatial Vision 9: 191–219.
	Land EH (1977) The retinex theory of colour vision. Scientific American 237: 108–128.
	Michael C (1978) Colour vision mechanisms in monkey striate cortex: dual opponent cells with concentric receptive fields. Journal of Neurophysiology 41: 572–588.
	Nathans J (1989) The genes for colour vision. Scientific American 260: 44–49.
	Szmajda BA, Buzas P, FitzGibbon T and Martin PR (2006) Geniculocortical relay of blue-off signals in the primate visual system. Proceedings of the National Academy of Sciences of the USA 103: 19512–19517.
	Wong-Riley M (1989) Cytochrome oxidase: an endogenous metabolic marker for neuronal activity. Trends in Neurosciences 12: 94–101.

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