Story of Psychology - Morton Hunt [305]
—Binocular disparity: When we look at something relatively close to us, its image falls on the fovea—center of the retina—of each eye, and the images of other objects equally far away fall on corresponding parts of both retinas. The images of objects either nearer or farther away, however, fall on different parts of the two retinas, as this diagram indicates:
FIGURE 33
How binocular disparity conveys depth
The disparity between the retinal images is interpreted by the brain to indicate which object is farther from us. Binocular disparity is most effective from close up to somewhere between eight hundred to nineteen hundred feet.60 Some perception theorists regard it as the most important of all cues to depth.
All the foregoing redundant cues to depth can be explained in terms of innate mechanisms or of learned behavior. But the innate aspect of depth perception is supported by other and more convincing evidence.
A historic series of experiments indicating that depth perception is instinctive was performed at Cornell in the late 1950s and early 1960s by Eleanor Gibson, whose work on high-speed reading of pronounceable and unpronounceable words we saw earlier, and a colleague, Richard Walk. Gibson, who had a lifelong aversion to cliffs, and Walk, who during World War II had trained paratroopers to jump off a high platform, jointly conceived of and created a “visual cliff” to determine whether rats learn depth perception or are born with it. The visual cliff was a thick sheet of glass with tile-patterned wallpaper on the underside of half of it and the same paper under the other half but several feet below. The question was whether creatures that had had no experience of depth—that had never tumbled off a high place of any sort—would automatically shun what looked like a drop-off.
The researchers reared chicks, rats, and other animals in the dark, depriving them of any experience of depth, then placed them on a board that crossed the glass between the shallow side and the seemingly deep one. The results were dramatic. The animals, though they had never experienced depth, almost always avoided the deep side and stepped off the board onto the shallow side.
Gibson and Walk then tried human infants. As Gibson recalled later:
We couldn’t very well rear the infants in the dark, and we had to wait until they could locomote on their own to use avoidance of the edge as our indicator of depth discrimination, but infants of crawling age did avoid the “deep” side. They may have learned something in the months before they could crawl; but whatever it was, it could not have been externally reinforced, since the parents never reported that the babies had fallen from a height.61
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.62
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.63 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