Story of Psychology - Morton Hunt [292]
S.B. was soon able to identify at first sight articles he had known by touch, such as toys, but many objects that he had never touched were mysteries to him until he was told or discovered what they were. Gregory and a colleague took him to London, where he recognized most of the animals at the zoo because he had petted cats and dogs and knew how other animals differed from them. But in a science museum S.B. saw a lathe—a tool he had always wanted to use—and could make nothing of it until, with his eyes closed, he ran his hands over it. Then, opening his eyes and looking at it, he said, “Now that I’ve felt it, I can see.”
Interestingly, when Gregory showed S.B. some illusions, he failed to be misled by them; he did not, for instance, perceive the straight lines of the Hering illusion as curved or the parallel ones of the Zöllner as divergent. Such illusions evidently depend on one’s having learned cues that denote perspective, and those cues, given by the other lines in the illusions, meant nothing to S.B.
The conclusions one can draw from his case are thus disappointingly mixed; some of the evidence favors innateness, some, experience. Besides, the evidence is contaminated: S.B. had had a lifetime of sensory experiences and learning with which to interpret his first visual perceptions, and his story does not reveal the extent to which the mind, before experience, is prepared to understand visual perceptions. Nor is the question answered by developmental research with infants, since it is unclear how much the development of an infant’s perceptual abilities at any juncture is due to maturation and how much to experience. Only impermissible experiments that would deprive an infant of perceptual and other sensory experience could tease the two apart and measure their relative influence.
Making a still worse muddle of the matter is the question of whether perception is primarily a physiological function or a mental one.
The founders of scientific psychology in the nineteenth century and the early decades of the present one tried to evade this issue by asserting that mind was unobservable and perhaps illusory, and by limiting themselves to the study of physical realities. Those who were interested in perception investigated the physiology of the sensory systems, especially the visual one, and over the course of more than a century, a number of them in Europe and America assembled a mass of data on the mechanics of that system. By the early years of the twentieth century they had determined that the retina of each eye, a thin sheet of specialized neural tissue, contains about 132 million photoreceptor cells of two types, rods and cones, both of which convert light into nerve impulses; that the rods, more common in the periphery of the retina, are more sensitive and respond only to very low levels of illumination; that the cones, more common in the center, respond at higher levels of illumination; and that there are three species of cone, one containing primarily chemicals that absorb light of short wavelengths (and thus react to blue and green), another, of middle wavelengths (green), and a third, of longer wavelengths (yellow, orange, and red).10
They had also traced out much of the complex wiring scheme by which the rods and cones send their impulses to the brain. From the retinas, the bundles of optic nerve fibers make their way to the visual cortex, an area at the lower part of the back of the brain. En route, the fibers carrying messages from the left half and from the right half of each eye’s field of vision are sorted out and redirected; the messages from each eye’s right-hand field of vision end up at the left visual cortex, the left-hand field of vision at the right visual cortex. (To this day, no one has the least idea why evolution arranged this crisscross.)
Many psychologists were long reluctant to accept the evidence that visual functions are centered in the visual cortex; such localization smacked of phrenology.