I had never realized that stereoscopy and spatial judgment could be so changed after a mere three weeks in a small space. My own stereoscopy had returned, jerkily, after about two hours, but I
W wondered what happened to prisoners, con!ned for much longer periods. I had heard stories of people living in rain forests so dense that their far point was only six or seven feet away. If they were taken out of the forest, it was said, they might have so little idea or perception of space and distance beyond a few feet that they would try to touch distant mountaintops with their outstretched hands. 5 · · · hen I was a neurology resident in the early 1960s, I read the remarkable papers of David Hubel and Torsten Wiesel on the neural mechanisms of vision. Their work, which later won a Nobel Prize, revolutionized our understanding of how mammals learn to see, in particular of how early visual experience is critical for the development of special cells or mechanisms in the brain needed for normal vision. Among these are the binocular cells in the visual cortex, which are necessary to construct a sense of depth from retinal disparities. Hubel and Wiesel showed, in animals, that if normal binocular vision was rendered impossible by a congenital condition (as in Siamese cats, which are often born cross-eyed) or by experiment (cutting one of the muscles to the eyeballs, so that the subjects became walleyed), these binocular cells would fail to develop and the animals would permanently lack stereoscopy. A signi!cant number of people develop similar conditions —collectively known as strabismus, or squint—a misalignment sometimes too subtle to attract notice but
misalignment sometimes too subtle to attract notice but suKcient to interfere with the development of stereo vision. Perhaps 5 or 10 percent of the population, for one reason or another, have little or no stereo vision, though they are often not aware of this and may learn it only after careful examination by an ophthalmologist or optometrist. 6 Yet there are many accounts of stereo-blind people who nonetheless achieve remarkable feats of visuo-motor coordination. Wiley Post, the !rst person to /y solo around the world, as famous in the 1930s as Charles Lindbergh, did so after losing an eye in his mid- twenties. (He went on to become a pioneer of high- altitude /ight and invented a pressurized /ight suit.) A number of professional athletes have been blind in one eye, and so was at least one eminent ophthalmic surgeon. Not all stereo-blind people are pilots or world-class athletes, and some may have diKculty judging depth, threading needles, or driving—but by and large they manage to get along pretty well using only monocular cues. 7 And those who have never had stereopsis but manage well without it may be hard put to understand why anyone should pay much attention to it. Errol Morris, the !lmmaker, was born with strabismus and subsequently lost almost all the vision in one eye, but feels he gets along perfectly well. “I see things in 3-D,” he said. “I move my head when I need to—parallax is enough. I don’t see the world as a plane.” He joked that
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