Modern Day Understanding of Color Vision

It was not until almost 100 years after Young and Helmoltz originally proposed the trichromatic theory that the existence of three kinds of photopigment in cones was confirmed (Brown and Wald, 1964). Using a technique known as microspectrophotometry, evidence has been found for a short-wave receptor (blue) which absorbs light best at 419 nm, a medium-wave receptor (green) which absorbs best at 531 nm and a long-wave receptor (red) which absorbs best at 558nm (Dartnall et al, 1983). The three types of cones differ in their overall numbers and their distribution over the retina. It is estimated that the normal retina contains 54 percent medium-wavelength sensitive cones, 38 percent long-wavelength sensitive cones and 13 percent short-wavelength sensitive cones. Of particular interest is the relatively small proportion of blue cones at the fovea, where they account for just 3-4 percent of the receptors. Although Hering's opponent theory was supported by numerous psychophysical observations (perhaps most notably the experiments by Hurvich and Jameson, 1957), it was not until convincing physiological evidence became available that the opponent theory was fully accepted. Svaetichin (1956) recording from cells in the fish retina, was the first to provide physiological evidence for color opponency. A few years later, De Valois et al (1966) demonstrated that cells in the monkey's lateral geniculate nucleus show color opponency. Subsequent research has uncovered a number of different types of opponent cells but they all share the property of being excited by some wavelengths and inhibited by others.

In summary, at the level of the photoreceptors, vision is trichromatic and mediated by three different types of cone. The responses from the three types of photoreceptor are transformed by the complex neural network in the retina so that at the level of the ganglion cells, color information is coded into two opponent color channels (red vs green and blue vs yellow) and a brightness channel.