The Story of Psychology (86 page)

FIGURE 25
The triangle that does not exist

As we proceed, we will learn the explanations for some of these illusions; for now, the point is that perception in human beings is not simply a physiological process that transmits representations of outside stimuli to the central nervous system; it often involves higher mental processes that make sense (and sometimes nonsense) of the impulses arriving via the optic nerves.

A third interesting question—Edwin Boring, in his monumental
History of Experimental Psychology
, calls it “the first mystery of vision”
³
—is that we have two eyes yet do not see everything doubled. Galen long ago rightly hypothesized that this is because the nerve fibers from both eyes lead to the same part of the brain. But that is only a partial answer. The two retinas receive somewhat different images of all but distant objects, as is easily confirmed by alternately opening and closing each eye while looking at a nearby object. (Each eye sees more of one side of the object than the other, and sees the object in a different relationship to things in the background.) But if these somewhat different images overlap in the brain, why is the result not blurred?

Perception researchers now answer that “fusion” of the dissimilar images takes place in the visual cortex, resulting in a single three-dimensional image. By tracing the axons of the two optic nerves—which are made up of a million ganglion cells—and by using modern brain scan techniques to see what brain areas are activated by vision, perception researchers have been able to identify the intricate routing and processing of the incoming neural impulses. Omitting the bewildering details, suffice it to say that the impulses are split up and separated into thirty different pathways to areas of the visual cortex for
pattern recognition
(how things look),
place recognition
(where things are), color, and other characteristics. Then these and other arriving data are coordinated through a host of other pathways of the brain’s visual system to yield a final perception of a unified visual scene.
⁴

Another interesting question, one of the most baffling, is how the image on the retina is viewed in the brain. Nerve impulses from the retina travel to the brain’s visual cortex, but what then? No screen exists in the
brain on which they can be projected, so how is the incoming flow of data seen? And if it is displayed in some way there or elsewhere in the brain, who or what sees it? The question revives the ancient (and now thoroughly discredited) supposition that there is a homunculus or little man—the “I” of the mind—who perceives what arrives at the cortex. But if the homunculus is seeing that image, with what is it doing so? Eyes of some sort? Then who or what is looking at what arrives at the homunculus’s visual center? And so on, ad infinitum.

Allied to this puzzle is the question of visual memory. Every adult has an immense repertoire of images stored in his or her brain—familiar faces, houses, trees, leaves, cloud formations, beds slept in. They have been recorded, in some fashion, after even a single quick viewing. Though we cannot call all of them clearly to mind, it is by means of them that we recognize something we see a second time. In 1973 a Canadian psychologist, Lionel Standing, a man of great patience, showed ten thousand snapshots of miscellaneous subjects to volunteers at the rate of two thousand a day for five days. Later, when he showed them some of these pictures mixed in with new ones, they correctly identified two thirds of the old ones as pictures they had already seen.
⁵
Where had they stored all the briefly seen images and in what form? When they saw a picture the second time, how did they locate and view the image in memory to compare it with the incoming one? Not by projecting the stored one on a cerebral screen, since none exists. And however they displayed it, what inside them looked at both the stored and incoming images—ah! there’s that troublesome little man again.

(Forget the little man and the screen he’s looking at. Research done in the past two decades has come up with a more realistic but more complicated answer, based in considerable part on studies of people with specific kinds of brain damage due, usually, to strokes. One woman, for instance, when asked to describe a banana, could say that it was a fruit and grows in southern climates but could not name its color. Another patient, asked to describe an elephant, said correctly that it had long legs but incorrectly that it had a neck that could reach the ground to pick things up.

(From many such studies, plus the results of brain scans showing what areas are activated by the effort to visualize something, it has become fairly clear that mental images are not located like filed pictures in any one or several places, but that the
components
of each image—its shape,
its color, its texture, and so on—are filed away separately and that summoning up a mental image uses many of the same processes that perception itself does, calling up and coordinating these several elements into one final, more or less complete image. But not a pictorial image; just as the letters of this sentence symbolize things they don’t physically resemble, the patterns of firing of brain neurons represent objects and events in the outside world.
⁶
Why did evolution devise this scheme? Let the evolutionary psychologists figure out that one.)

These are but a few of the mysteries of visual perception; perhaps no area of psychology has produced as much research data and as relatively few definitive answers. Some years ago, James J. Gibson, a controversial but noted perception theorist, flatly asserted that most of what perception researchers had learned in the past hundred years was “irrelevant and incidental to the practical business of perception.”
⁷
A trifle more moderately, the perception psychologists Stephen M. Kosslyn and James R. Pomerantz said in 1977 that, despite all the accumulated data, perception is still poorly understood.
⁸
Still, they added, “we do know some things about it.” And, they could add today, they now know a good deal more about it. Indeed, enough to begin to understand it, to answer at least some of the interesting questions, and to discard others in favor of more cogent ones.

For centuries, philosophers debated about whether we are born with mental equipment that makes sense of what we see (the Kantian or nativist view), or must learn from experience to interpret what we see (the Lockean or empiricist view). When psychology became experimental, the findings of perception research not only failed to answer the question but added to the evidence for each side. Although today the terms have been redefined and the hypotheses have become more sophisticated, the debate continues.

Locke, Berkeley, and other philosophers and psychologists sometimes fantasized a test case that would definitively resolve the issue: a person blind from birth who, through an operation or some other intervention, suddenly gains sight. Would he know, without touching what he was looking at, that the object was a cube rather than a sphere, a dog rather
than a cat? Or would his perceptions be meaningless until he learned what they meant? Such a person’s experiences might hold the key.

In recent centuries a handful of such cases have, in fact, turned up. The most carefully reported was that of an Englishman with opaque corneas who, in the early 1960s, at the age of fifty-two was able to see for the first time.
⁹
S.B., as he is called by Richard L. Gregory, a British psychologist and perception expert who studied him closely, was an active and intelligent man who had made a good adaptation to his blindness: He was skilled at reading Braille, made objects with tools, often chose to walk without the customary white cane even though he sometimes bumped into things, and would go bicycling with a friend holding his shoulder to guide him.

In S.B.’s middle years, corneal transplants became possible, and he underwent an operation. According to Gregory’s report, when the bandages were removed from his eyes, he heard the surgeon’s voice and turned toward what he knew must be a face. He saw only a blur.

Experience, however, rapidly clarified his perceptions: Within days he could see faces, walk along a hospital corridor without touching the walls, and recognize that the moving objects he saw through the window were cars and trucks. Spatial perception, however, came to him more slowly. For a while, he judged the distance to the ground below his hospital window to be such that he could touch it with his toes if he hung by his hands from the windowsill, although it was ten times that distance.

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. Late in the nineteenth century, however, brain localization—not of the phrenological type, and only of certain functions—gained new credibility after Wernicke and Broca discovered that speech functions are carried out in two small areas in the left half of the brain. This inspired researchers to look for an area where visual messages are received and understood, and through autopsies of brain-damaged human beings and operations on monkeys they identified it, in general terms, as the rear of the brain.

More precise pinpointing of the visual cortex was a byproduct of weaponry used in the Russo-Japanese War of 1904–1905.
¹¹
In that conflict the Russians introduced a new rifle, the Mosin-Nagant Model 91, which fired bullets of smaller diameter and higher velocity than the rifles of earlier wars. The bullets often penetrated the skull without shattering it, and in some cases destroyed, partly or totally, the victim’s vision without killing him. Tatsuji Inouye, a young Japanese army doctor who worked with wounded soldiers, plotted the extent to which each patient’s visual field had been lost by each eye, determined from the site of the bullet’s entry and exit which parts of the brain had been damaged, and, putting these data together, identified the precise location and extent of the visual cortex.

Among his findings was that the areas of the visual cortex that receive the retinal messages are grossly disproportionate to the areas of the retinal image. A very large part receives impulses coming from the fovea, the small central area of the retina where vision is sharpest, and only a small part from the larger peripheral area. (Later research showed the disparity in proportions to be about 35 to 1.
¹²
) That settled one great issue: what arrives at the brain is in no way an image corresponding in layout to the image on the retina.