Color Ontology and Color Science

12. What Do the Color-Blind See?

12 What Do the Color-Blind See? Justin Broackes 12.1 Color Blindness: A Guide and Test for Theories of Normal Vision What do the color-blind see? Can we tell? Does it matter? The theory of normal human color vision is one of the triumphs of nineteenth-century science, emerging in Helmholtz and J. C. Maxwell in the 1850s, out of suggestions from Tobias Mayer and Thomas Young and others half a century and more earlier.1 The trichromatic theory— the view that a normal perceiver can, in light mixing, match any given color with some combination of three suitably chosen primaries, and that this is a sign of our having three main receptor types at work in human color vision—has constant con- firmation in the fact that color photography with three emulsions works (cp. Maxwell 1855, 136–137), as does color television with three phosphors. The psychophysics and theory of color measurement have been marvelously systematized;2 the “opponent theory” first proposed by Ewald Hering as a radical alternative to Helmholtz’s theory has been modified and in some manner joined up with it—notably by Hurvich and Jameson, who in the mid-1950s took the outline of G. E. Müller’s model of color vision, devised an experimental “hue cancellation” procedure as a quantitative measure of certain features in the structure of our experienced color space, and found a physiological counterpart for that structure in the findings of the new electrophysiology of Granit and Hartline (and Kuffler and Svaetichin), and later in the reports of De Valois and De Valois on the macaque visual system. There are other stories to be told about the physiology of the receptor pigments of the eye and the genes that code for them, about the various kinds of cells in the retina and elsewhere in the visual system, and now increasingly about color processing in the brain. It is a topic on which psychologists , physicists, biologists, and neurophysiologists—not to mention paint manufacturers , dyers, and makers of photographic equipment—have reason to be proud and glad of the convergence of interests and views. Color blindness might at first seem just a peripheral abnormality. But it has often been both a guide to the nature of normal color vision and a test application for theories of it. It promises to provide cases where the various components of a complex 292 Justin Broackes process that are either hard or impossible to separate artificially are found already separated in nature.3 The great physicist J. C. Maxwell, having just graduated in mathematics from Cambridge in 1854, did his first research comparing the color matching of ten normal perceivers with that of two color-blind people (whom we would now call protanopes), to identify the sensation that was “wanting” in the color-blind but present in the normal perceivers (see Maxwell 1855). And the color-blind are often a test of a particular theory—in both its more empirical and more theoretical or philosophical aspects. If normal vision involves (as Helmholtz and Maxwell thought) three basic sensations, say, of red, green, and violet,4 produced by three receptor, or nerve, types, then we might expect a person missing one of these types to have sensations corresponding merely to the other two. Someone missing the “red” receptor, or nerve, type (“red-blind,” in Helmholtz’s terms) would have sensations only of green and violet (along with the blue that results from combining them, which would—surprisingly , indeed—be experienced on looking at white things)—all at high saturation, since there would be no third color sensation ever to desaturate the other two. On the other hand, if (as Hering believed) color vision involves some kind of distinct red-green, yellow-blue, and light-dark processes, then we might expect color blindness to involve the loss of one of those three dimensions. And we might hope to find some kind of empirical confirmation—though it would no doubt be indirect—of one or other of these views of the color-blind, and hence in turn of some or all of Helmholtz’s or Hering’s general view. For 150 years and more, the main camps, whatever their many differences, have agreed on one thing: that the main groups of color-blind people—those who confuse reds and greens and browns, and are today classed as dichromats5 —have in some sense no perception of red and green at all, but only of...