Story of Psychology - Morton Hunt [74]
As a physicist, Helmholtz knew that three specific colors—particular hues of red, blue-violet, and green—could, mixed in the proper proportions, reproduce any other color; these are the primary colors.* He reasoned that this meant human vision can detect those three colors and hypothesized that the retina must have three different kinds of receptor cells, each furnished with a chemical sensitive to one of the primary colors. Relying on Müller’s doctrine of specific nerve energies, he suggested that the nerves leading from each receptor to the brain conveyed not just visual messages but specific color messages.
An English scientist, Thomas Young, had advanced somewhat the same theory in 1802, but without experimental evidence; it had been generally ignored. Helmholtz, however, amassed a variety of supportive evidence, including that of the colors we experience when lights of different hues are mixed, the afterimage of a complementary color that we see after staring at a strong color for a while, the kinds of color blindness that exist in some people and animals, the influence of particular brain lesions on color vision, and so on. He generously acknowledged Young’s priority, and his account of color vision has been known ever since as the Young-Helmholtz theory (or the trichromatic theory).
The color theory, a testable mechanistic explanation of how the mind perceives colors, was a stunning achievement. Link by link, from the outside world to the receptive area of the brain, Helmholtz had forged a chain of causal events that replaced the guesses and fantasies of philosophers and physiologists. It is still the reigning theory of color vision, though in more complex form and stripped of the notion that the nerves from each kind of receptor carried different kinds of energy.
As for the profoundly troubling question about perception asked by Democritus, Berkeley, Hume, and others—whether what we see is a true representation of what is out there—Helmholtz, far more mechanistic than Müller, dismissed it as being without meaning or value:
In my opinion, there can be no possible sense in speaking of any other truth of our ideas except a practical truth. Our ideas of things cannot be anything but symbols, natural signs for things that we learn how to use in order to regulate our movements and actions. Having learned how to read those symbols correctly, we are able by their help to adjust our actions so as to bring about the desired result; that is, so that the expected new sensations will arise… Hence there is no sense in asking whether vermilion [mercuric sulfide], as we see it, is really red or whether this is simply an illusion of the senses. The sensation of red is the normal reaction of normally formed eyes to light reflected from vermilion… The statement that the waves of light reflected from vermilion have a certain length is something different; that is true entirely without reference to the special nature of our eye.32
Thus the mechanist physiologist was, after all, a philosopher of psychology, and one to reckon with.
Helmholtz’s color vision research was only one facet of a comprehensive inquiry into visual perception that he carried on for a number of years. The fruits of this labor, his Handbook of Physiological Optics (1856–1867), ran to half a million words and covered all previous research in the field as well as his own; for several generations it remained the standard authority on the optical and neural properties of the eye. He also performed a similar service for hearing in another, not quite so massive, work.
In Optics Helmholtz dealt chiefly with the physics and physiology of vision and made some keen observations about the psychological processes by which the mind interprets messages from the optic nerves. He drew an invaluable distinction that had eluded earlier psychologists between sensation (the excitation of the retina’s rods by light of whatever color, and the resultant impulses of the optic nerves) and perception (the meaningful interpretations the mind makes of the arriving impulses). He made the