The Story of Psychology (91 page)

The mother of each infant would stand at one side or the other of the apparatus and beckon to her child. In nearly all instances, the infant crawled readily toward her when she was on the shallow side, but only three out of twenty-seven ventured onto the deep side when their mothers were there.
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Later laboratory work by others, however, weakens the Gibson-Walk conclusion somewhat, suggesting that the fear of heights in human infants is learned through locomotor experience in general.
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But impressive evidence that depth perception is built into the nervous system came in 1960 from an unlikely source, AT&T’s Bell Laboratories, and an unlikely researcher, a young electrical engineer who was a specialist in TV signal transmission. Bela Julesz, born and educated in Hungary, came to the United States after the abortive revolution of 1956, and was hired by Bell Labs in Murray Hill, New Jersey, to develop ways to narrow the band widths used by TV signals. But Julesz was drawn to more interesting questions and from 1959, with Bell Labs’ acquiescence, devoted himself to research on human vision. Though he never acquired a degree in psychology, he became a widely known, award-winning perception psychologist, the head of visual perception research at Bell Labs, a MacArthur Fellow, and, in 1989, director of the Laboratory of Vision Research at Rutgers University.
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Julesz had barely begun vision research when he came up with the idea that made him instantly famous in psychological circles. He had been surprised to find, in reading about stereoscopic depth perception, general acceptance of stereopsis as the result of the brain’s matching cues to form and depth in each eye’s image. This was thought to lead to fusion of the images and depth perception. Julesz, who had had some experience in Hungary as a radar engineer, felt sure that this was wrong.

These consisted of randomly created patterns of black and white dots, as in this pair:

FIGURE 34
When these patterns are stereoscopically merged, the center floats upward.

There are no cues to depth in these two patterns when each is looked at alone. But although they are largely identical, a small square area in the center has been slightly shifted to one side by the computer so that when each image is seen by one eye and the patterns merged, that area produces a binocular disparity—and seems to float above the rest of the background. (To see this remarkable effect, hold a 4″×6″ card or a sheet of paper vertically in front of and perpendicular to the page so that each eye sees only one image. Focus on one corner of the pattern, and in a little while the two images will migrate toward each other and fuse. At that point the center square will appear to hover an inch or so above the page.)

The random-dot stereogram is far more than an amusing trick. It proves that stereoscopic vision does not depend on cues in each retinal image to create the experience of three-dimensionality, and that, on the contrary, the brain fuses the meaningless images and thereby reveals the hidden cues to three-dimensionality. This is not a cognitive process, not a matter of learning to interpret cues to depth, but an innate neurological process taking place in a particular layer of the visual cortex. That is where a highly organized mass of interacting cells performs a correlation of the dots in the patterns, yielding fusion and the perception of the three-dimensional effect.
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(Stereopsis is not the only way we achieve depth perception. Julesz’s work does not rule out others, including those which involve learning.)

Julesz is proud that his discovery led Hubel and Wiesel and others to turn their attention from form perception to the investigation of binocular vision, but modestly adds:

Yet another theory about depth perception was proposed several decades ago—one that was neither specifically neural nor specifically cognitive. Not that its proponent tactfully combined the two; on the contrary, he virtually ignored the neural theory and dismissed the cognitive theories as unnecessary and based on wrong assumptions.

Only a thoroughgoing maverick would reject a century’s worth of depth-perception research and claim to have found a totally different and correct approach. Only a true nonconformist would assert that we perceive depth neither by neural detection nor inference from cues but “directly” and automatically. Only a brash individualist would present a radical epistemology in which the physics of light is said to give us an accurate, literal experience of depth and that we need not interpret what we see because we see what is as it is.

Such a one was the late James J. Gibson (1904–1979), whose admirers considered him “the most important student of visual perception of the twentieth century” and “the most original theoretician in the world in the psychology of perception,” but whose theory is considered by the majority of perception specialists “extremely implausible” (one reviewer even called it too “silly” to merit discussion) and has few advocates.
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Born in a river town in Ohio and reared in various parts of the Midwest, Gibson was the son of a railroad surveyor.
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He went to Princeton University but felt out of place in a social world that revolved around clubs, and preferred to associate with what he called “the eccentrics.” For a while he vacillated between philosophy and acting (he was wavy-haired, square-jawed, and good-looking enough for leading roles), but in his senior year he took a course in psychology and at once heard the call. In 1928, he received a faculty appointment at Smith, where for some years, he was interested in relatively traditional perception research. Then, during World War II, he was asked by the Army Air Corps’s Aviation Psychology Program to develop depth-perception tests for determining who had the visual aptitudes needed for flying, particularly for making successful take-offs and landings.

He considered the classical cues to depth perception, including shadows and perspective, of little worth. In his opinion they were based on paintings and parlor stereoscopes rather than on three-dimensional reality, and on static images rather than on movement. What seemed to him
much more useful and realistic were two other kinds of cues: texture gradient, like the uniformly changing roughness of the runway as seen by a pilot during the final leg of an approach; and motion perspective, or the flow of changing relationships among objects as one moves through the environment, including all that a pilot sees during take-offs and landings.
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These cues soon became, and are today, accepted components of the cue-based theory of depth perception.

Gibson’s Air Corps work held the germ of his later view. The crucial mechanism in depth perception (in all perception, according to Gibson) is not the retinal image, with all its cues, but the changing flow of relationships among objects and their surfaces in the environment that the perceiver moves through. During the 1950s and 1960s, he did a considerable amount of research at Cornell that tested his belief in texture gradients. In some experiments he placed diffusing milk-glass between an observer and textured surfaces; in others he dilated the observer’s eyes to prevent sharp focus on texture; in still others he cut Ping-Pong balls in half and made goggles of them so that what his subjects saw was foglike, without surfaces or volume.
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From these and other experiments, plus a careful consideration of his research on air-crew testing and training, Gibson came to reject texture gradients and to stress movement by the observer through the environment as the key to depth perception. However large or small the movement, it results in changes in the optic array—the structured pattern of light reaching the eye from the environment—such as is suggested in this drawing:

FIGURE 35
How optic array conveys depth

The optic array, rich in information as seen from any point, becomes infinitely richer with movement by the observer. Even minor movements of the head change the array, transforming what is seen of an object and the relationships among objects, and yielding optic flow of one kind or another. Gibson came to believe that optic array and flow convey depth and distance directly, without the need of mental calculation or inference from cues.
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This is how Gibson explained depth perception in his sweeping “ecological” theory of “direct perception.” The pity is that he felt obliged to throw out the baby with the bathwater. For it is possible to acknowledge both the neural and cognitive views of depth perception as correctly explaining different aspects of the phenomenon and the Gibsonian view as supplementary to them. But it wasn’t possible for James J. Gibson.

His name and theory have faded from view, but the cues he was so enamored of have remained accepted components of contemporary accounts of depth perception.

“Visual perception,” Bela Julesz said fifteen years ago, “is in the same state as physics was prior to Galileo or biochemistry was prior to the discovery of the double helix by Watson and Crick.”
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Since then, a good deal more has been learned, and yet it remains true that each of the two major approaches—the neural and the cognitive—explains only some of the phenomena; there is not yet a comprehensive and unifying theory of visual perception. Perhaps some great organizing concept remains undiscovered, or perhaps visual perception is so complex that no one theory can embrace all of its concepts and that the two different approaches deal with events occurring at radically different levels of complexity.

We have seen something of each of these approaches. Here, to round out the picture, are brief sketches of how each explains visual perception in general.

The neural approach
answers questions that preoccupied nineteenth-century physiologists: How can sensory nerves, though alike in structure, transmit different sensations to the brain? And how does the brain turn those incoming impulses into vision?

The answer, worked out in great detail over recent years,
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is that the nerve impulses themselves do not differ; rather, receptors that respond to specific stimuli send their messages separately to the striate or primary area of the visual cortex. The process begins on the retina, where rods are sensitive to low levels of illumination, cones to more intense levels; cones are of three types, each responsive to different wavelengths of visible light, and some, as we have already heard, sensitive to special shapes and motions.

From the rods and cones, the same kinds of nerve impulses travel along parallel pathways but end up in different areas of the brain—more than 90 percent of them in particular parts of the primary visual cortex and 10 percent in other subcortical structures. Thus the messages delivered to the brain have been analytically separated into color, shape, movement, and depth, and delivered to specialized receptive areas. By means of staining techniques that trace the neuron pathways in laboratory monkeys from retina to visual cortex, researchers have been able to identify more than thirty such distinct cortical visual areas.

What happens then? The brain puts it all together: Using single-cell recordings and two kinds of brain scans (PET, positron emission tomography, and fMRI, functional magnetic resonance imaging), perception researchers have puzzled out the extremely intricate architecture of the primary visual cortex and its wiring scheme (far too complex to take up here), which integrates the individual impulses and blends the information from the two eyes. The result is that the image cast on the retina winds up as the excitation of groups of complex neurons, but the pattern of these excitations in no way resembles the image on the retina or the scene outside the eye. Rather, as already mentioned, it is analogous to writing about a scene, which conveys what it consists of but does not in the least look like it.