The brain works out what color it is seeing by comparing the rates at which photons are absorbed in the three different classes of cones. But there are infinitely many different spectral distributions that could give exactly the same ratios, and we cannot distinguish between them. For example, a monochromatic yellow light at wavelength 580 nm creates exactly the same absorption ratio between the cones as a combination of red light at 620 nm and green light at 540 nm, as mentioned earlier. And there are an infinite number of other such “metameric colors,” different spectral distributions that produce the same absorption ratios between the three types of cones and thus look the same to the human eye.
It is important to realize, therefore, that our range of color sensations is determined not directly by the range of monochromatic lights in the spectrum but rather by the range of possibilities of varying the ratios between the three types of cones. Our “color space” is three-dimensional, and it contains sensations that do not correspond to any colors of the rainbow. Our sensation of pink, for example, is created from an absorption ratio that corresponds not to any monochromatic light but rather to a combination of red and blue lights.
As the light fades at night, a different system of vision comes into play. The cones are not sensitive enough to perceive light in very low intensity, but there are other receptors, called rods, that are so sensitive they can register the absorption of even a single photon! The rods are most sensitive to bluish green light at around 500 nm. Our low-light vision, however, is color-blind. This is not because the light itself “forgets” its wavelength at night but simply because there is just one type of rod. As the brain has nothing with which to compare the responses from the single type of rod, no color sensation can be produced.
There are about six million cones in total in the retina, but the three types are not found in nearly equal numbers: there are relatively few short-wave (violet) cones, more than ten times as many middle-wave (green) cones, and even more long-wave cones. The far greater numbers of middle-wave and long-wave cones means that the eye is more efficient in absorbing light at the long-wave half of the spectrum (yellow and red) than at the short-wave half, so it takes lesser intensity of yellow light to be detected by the eye than blue or violet light. In fact, our day vision has a maximum sensitivity to light of 555 nm, at yellow-green. It is this idiosyncrasy of our anatomy that makes yellow appear brighter to us than blue or violet, rather than any inherent properties of the light itself, since blue light is not in itself less intense than yellow light. (In fact, wavelength and energy are inversely related: the long-wave red light has the lowest energy, yellow light has higher energy than red, but green and blue have higher energy than yellow. The invisible ultraviolet light has even higher energy, enough in fact to damage the skin.)
There is also a different type of unevenness in our sensitivity to colors: our ability to discriminate between fine differences in wavelength is not uniform across the spectrum. We are especially sensitive to wavelength differences in the yellow-green area, and the reason again lies in the accidents of our anatomy. Because the middle-wave (green) and long-wave (yellowish green) receptors are very close in their peak sensitivities, even very small variations in wavelength in the yellow-green area translate into significant changes in the ratios of light absorbed by the two neighboring cones. Under optimal conditions, a normal person can discriminate between yellow hues differing in wavelength by just a single nanometer. But in the blue and violet area of the spectrum, our ability to discriminate between different wavelengths is less than a third of that. And with red hues near the edge of the spectrum, we are even less sensitive to wavelength differences than in the blues.
These two types of unevenness in our sensitivity to color-the feeling of varying brightness and the varying ability to discriminate fine differences in wavelength-make our color space asymmetric. And as mentioned in this footnote, this asymmetry makes certain divisions of the color space better than others in increasing similarity within concepts and decreasing it across concepts.
When one of the three types of cones fails, this reduces color discrimination to two dimensions instead of three, and the condition is thus called dichromacy. The most frequent type of dichromacy is commonly called red- green blindness. It affects about 8 percent of men and 0.45 percent of women, who lack one of the two neighboring types of cones (long-wave or middle-wave). Little is known about the actual color sensations of people with color blindness, because one cannot simply “translate” the sensations of dichromats directly to those of trichromats. A few reports have been collected from the rare people with a red-green defect in one eye and normal vision in the other. Using their normal eye as a reference, such people say that their color-blind eye has the sensation of yellow and blue. But since the neural wiring associated with the normal eye might not be normal in their cases, even the interpretation of such reports is not straightforward.
Other types of color blindness are much rarer. A different type of dichromacy, called tritanopia, or in popular parlance blue-yellow blindness, arises in people who lack the long-wave (blue) cones. This condition affects only about 0.002 percent of the population (two people in a hundred thousand). A more severe defect is the lack of two types of cones. Those affected are called monochromats, as they have only one functioning cone type. An even more extreme case is that of rod monochromats, who lack all three types of cone and rely only on the rods that serve the rest of us for night vision.
Human color vision evolved independently from that of insects, birds, reptiles, and fish. We share our trichromatic vision with the apes and with Old World monkeys, but not with other mammals, and this implies that our color vision goes back about thirty to forty million years. Most mammals have dichromatic vision: they have only two types of cones, one with peak sensitivity in the blue-violet area and one with peak sensitivity in green (the middle-wave cone). It is thought that the primate trichromatic vision emerged from a dichromatic stage through a mutation that replicated a gene and split the original middle-wave (green) receptor into two adjacent ones, the new one being a little farther toward yellow. The position of the two new receptors was optimal for detecting yellowish fruit against a background of green foliage. Man’s color vision seems to have been a coevolution with the development of bright fruits. As one scientist put it, “with only a little exaggeration, one could say that our trichromatic color vision is a device invented by certain fruiting trees in order to propagate themselves.” In particular, it seems that our trichromatic color vision evolved together with a certain class of tropical trees that bear fruit too large to be taken by birds and that are yellow or orange when ripe. The tree offers a color signal that is visible to the monkey against the masking foliage of the forest, and in return the monkey either spits out the undamaged seed at a distance or defecates it together with fertilizer. In short, monkeys are to colored fruit what bees are to flowers.
It is not clear to what extent the passage from dichromacy to trichromacy was gradual or abrupt, mainly because it is not clear whether, once the third type of cone emerged, any additional neural apparatus was needed to take advantage of the signals coming from it. However, it is clear that the sensitivity to color could not have evolved continuously along the spectrum from red toward the violet end, as Hugo Magnus argued it did. In fact, if viewed over a time span of hundreds of millions of years, the development went exactly the opposite way. The most ancient type of cone, which goes back to the premammalian period, is the one with peak sensitivity in the blue-violet end of the spectrum and with no sensitivity at all to yellow and red light. The second type of cone to emerge was the one with peak sensitivity in green, thus extending the eye’s sensitivity much farther toward the red end of the spectrum. And the youngest type of cone, from some thirty to forty million years ago, had peak sensitivity slightly farther toward the red end, in yellow-green, and so increased the eye’s sensitivity to the long- wave end of the spectrum even further.
All the facts mentioned so far about the cones in the retina are correct to the best of my knowledge. But if you are under the impression that they actually explain our sensation of color, then you have been coned! In fact, the cones are only the very first level in a highly complex and still largely unknown process of normalization, compensation, and stabilization-the brain’s equivalent of the “instant fix” function of picture-editing programs.
