The Mind's Eye

by Oliver Sacks

  Photography was invented only months after Wheatstone’s 1838 article describing his stereoscope, and stereo photographs quickly became popular.1 Queen Victoria herself was presented with a stereoscope after admiring one in the Great Exhibition at the Crystal Palace, and soon no Victorian drawing room was complete without one. With the development of smaller, cheaper stereoscopes, easier photographic printing, and even stereo parlors, there were few people in Europe or America who did not have access to stereo viewers by the end of the nineteenth century.

  With stereo photographs, viewers could see the monuments of Paris and London or great sights of nature like Niagara Falls or the Alps in all their majesty and depth, with an uncanny verisimilitude that made them feel as if they were hovering over the actual scenes.2

  In 1861, Oliver Wendell Holmes (who invented the popular handheld Holmes Stereo Viewer), in one of several Atlantic Monthly articles on stereoscopes, remarked on the special pleasure people seemed to derive from this magical illusion of depth:

  The shutting out of surrounding objects, and the concentration of the whole attention … produces a dreamlike exaltation … in which we seem to leave the body behind us and sail into one strange scene after another, like disembodied spirits.

  There are, of course, many other ways of judging depth besides stereo vision: the occlusion of distant objects by closer objects, perspective (the fact that parallel lines converge as they recede, and that distant objects appear smaller), shading (which delineates the shape of objects), “aerial” perspective (the blurring and blueing of more distant objects by the intervening air), and, most important, motion parallax—the changing appearance of spatial relationships as we move around in the world. All these cues, acting together, can give a sense of reality and space and depth. But the only way to actually perceive depth—to see it rather than judge it—is with binocular stereoscopy.3

  In my boyhood home, in London during the 1930s, we had two stereoscopes: a large, old-fashioned wooden one, which took glass slides, and a smaller handheld one, which took cardboard stereo photographs. We also had books of bicolor anaglyphs—stereo photographs printed in red and green, which had to be viewed with a pair of glasses with one red and one green lens, which effectively restricted each eye to seeing only one of the images.

  So when, at the age of ten, I developed a passion for photography, I wanted, of course, to make my own pairs of stereo photos. This was easy to do, by moving the camera horizontally about two and a half inches between exposures, mimicking the distance between the two eyes. (I did not yet have a double-lens stereo camera, which would take simultaneous stereo pairs.)

  After reading how Wheatstone explored stereoscopic effects by exaggerating or reversing the disparity between the two images, I began experimenting with this, too. I started taking pictures with greater and greater separations between them, and then I made a hyperstereoscope, using a cardboard tube about a yard long with four little mirrors. With this, I could turn myself, in effect, into a creature with eyes a yard apart. I could look through the hyperstereoscope at a very distant object, like the dome of St. Paul’s Cathedral, which normally appeared as a flat semicircle on the horizon, and see it in its full rotundity, projecting towards me. I also experimented with making a “pseudoscope,” which transposed the views of the two eyes to reverse the stereo effect to some extent, making distant objects appear closer than near ones and even turning faces into hollow masks. This, of course, contradicted common sense, as well as all the other depth cues of perspective and occlusion—sometimes the images would rapidly shift back and forth from convex to concave, a bizarre and disorienting experience as the brain struggled to reconcile two rival hypotheses.4

  After the Second World War, new techniques and forms of stereoscopy became popular. The View-Master, a little stereoscope made of plastic, took reels of tiny Kodachrome transparencies that one flicked through by pressing a lever. I fell in love with faraway America at this time, partly through View-Master reels of the grand scenery of the American West and Southwest.

  One could also get Polaroid Vectographs, in which the stereo images were polarized at right angles to each other; these were viewed through a special pair of Polaroid glasses with the polarization of the lenses also at right angles, insuring that each eye saw only its own image. Such Vectographs, unlike the red-and-green anaglyphs, could be in full color, which gave them a special appeal.

  Then there were lenticular stereograms, in which the two images were printed in alternating narrow vertical bands covered by clear, ridged plastic. The ridges served to transmit each set of images to the proper eye, eliminating the need for any special glasses. I first saw a lenticular stereogram just after the war, in the London Tube—an advertisement, as it happened, for Maidenform bras. I wrote to Maidenform, asking if I could have one of their advertisements, but got no reply; they must have imagined I was a sex-obsessed teenager, rather than a simple stereophile.

  Finally, in the early 1950s, there were 3-D films (like the Madame Tussauds horror film, House of Wax), which one would look at through red-and-green or Polaroid glasses. As cinema, some of these were awful—but a few, like Inferno, were very beautiful and used stereo photography in an exquisite, delicate, unintrusive way.

  Over the years, I amassed a collection of stereograms and books about stereoscopy. I became an active member of the New York Stereoscopic Society, and at our meetings I encountered other stereo buffs. We stereo enthusiasts subscribe to stereo magazines, and some of us attend stereo conventions. The most ardent take their stereo cameras and go on “stereo weekends.” Most people are not particularly conscious of what stereoscopy adds to their visual world, but we revel in it. While some may not notice any big difference if they close one eye, we stereophiles are sharply aware of a great change, as our world suddenly loses its spaciousness and depth and becomes as flat as a playing card. Perhaps our stereoscopy is more acute; perhaps we live, subjectively, in a deeper world; or perhaps we are simply more aware of it, as others may be more attuned to color or shape. We want to understand how stereoscopy works. The problem is not a trivial one, for if one can understand stereoscopy, one can understand not only a simple and brilliant visual stratagem but something of the nature of visual awareness, and of consciousness itself.

  One has to lose the use of an eye for a substantial period to find how life is altered in its absence. Paul Romano, a sixty-eight-year-old retired pediatric ophthalmologist, recounted his own story in the Binocular Vision & Strabismus Quarterly. He had suffered a massive ocular hemorrhage, which caused him to lose nearly all sight in one eye. After a single day of monocular vision, he noted, “I see items but I often don’t recognize them: I have lost my physical localization memory.… My office is a mess.… Now that I have been reduced to a two-dimensional world I don’t know where anything is.”

  The next day he wrote, “Things are not the same at all monocularly as they were binocularly.… Cutting meat on the plate—it is difficult to see fat and gristle that you want to cut away.… I just don’t recognize it as fat and gristle when it only has two dimensions.”

  After almost a month, though Dr. Romano was becoming less clumsy, he still had a sense of great loss:

  Although driving at normal speed replaces the loss of depth perception with motion stereopsis, I have lost my spatial orientation. There is no longer the feeling I used to have of knowing exactly where I am in space and the world. North was over here before—now I don’t know where it is.… I am sure my dead reckoning is gone.

  His conclusion, after thirty-five days, was that “even though I adapt better to monocularity every day, I can’t see spending the rest of my life in this way.… Binocular stereoscopic depth perception is not just a visual phenomenon. It is a way of life.… Life in a two-dimensional world is very different from that in a three-dimensional world and very inferior.” As the weeks passed, Dr. Romano became more at home in his monocular world, but it was with enormous relief that, after nine months, he finally recovere
d his stereo vision.

  In the 1970s, I had my own experience with losing stereoscopy when I was put in a tiny windowless room in a London hospital, following surgery for a ruptured quadriceps tendon. The room was scarcely bigger than a prison cell, and visitors complained of it, but I soon accommodated and even enjoyed it. The effects of its limited horizon did not become apparent to me until later, as I described in A Leg to Stand On:

  I was moved into a new room, a new spacious room, after twenty days in my tiny cell. I was settling myself, with delight, when I suddenly noticed something most strange. Everything close to me had its proper solidity, spaciousness, depth—but everything farther away was totally flat. Beyond my open door was the door of the ward opposite; beyond this a patient seated in a wheelchair; beyond him, on the windowsill, a vase of flowers; and beyond this, over the road, the gabled windows of the house opposite—and all this, two hundred feet perhaps … seemed to lie like a giant Kodachrome in the air, exquisitely colored and detailed, but perfectly flat.

  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 wondered what happened to prisoners, confined 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

  · · ·

  When 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 significant number of people develop similar conditions—collectively known as strabismus, or squint—a misalignment sometimes too subtle to attract notice but sufficient 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 first person to fly 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 flight and invented a pressurized flight 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 difficulty 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 filmmaker, 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 he considered stereopsis no more than a “gimmick” and found my interest in it “bizarre.”8

  I tried to argue with him, to expatiate on the special character and beauty of stereopsis. But one cannot convey to the stereo-blind what stereopsis is like; the subjective quality, the quale, of stereopsis is unique and no less remarkable than that of color. However brilliantly a person with monocular vision may function, he or she is, in this one sense, totally lacking.

  And stereopsis, as a biological strategy, is crucial to a diverse array of animals. Predators, in general, have forward-facing eyes, with much overlap of the two visual fields; prey animals, by contrast, tend to have eyes at the sides of their heads, which gives them panoramic vision, helping them spot danger even if it comes from behind. The hammerhead shark is a fearsome predator, partly because its bizarre head shape allows its forward-facing eyes a greater separation—a hammerhead is a living hyperstereoscope. Another astonishing strategy is found in the cuttlefish, whose wide-set eyes normally permit a large degree of panoramic vision but can be rotated forward by a special muscular mechanism when the animal is about to attack, giving it the binocular vision it needs for shooting out its tentacles with deadly aim.9

  In primates like ourselves, forward-facing eyes have other functions. The huge, close-set eyes of lemurs serve to clarify the complexity of dark, dense foliage, which, if the head is kept still, is almost impossible to sort out without stereoscopic vision—and in a jungle full of illusion and deceit, stereopsis is indispensable in breaking camouflage. On the more exuberant side, aerial acrobats like gibbons might find it very difficult to leap from branch to branch without the special powers conferred by stereoscopy. A one-eyed gibbon might not fare too well—and the same might be true of a one-eyed shark or cuttlefish.

  Stereoscopy is highly advantageous to such animals, despite its costs: the sacrifice of panoramic vision, the need for special neural and muscular mechanisms for coordinating and aligning the eyes, and, not least, the development of special brain mechanisms to compute depth from the disparities of the two visual images. Thus, in nature, stereoscopy is anything but a gimmick, even if some human beings manage, and may even enjoy certain advantages, without it.

  In December of 2004, I received an unexpected letter from a woman named Sue Barry. She reminded me how we had met, in 1996, at a shuttle-launch party in Cape Canaveral (her husband, Dan, was an astronaut). We had been talking about different ways of experiencing the world—how, for example, Dan and other astronauts would lose their orientation, their sense of “up” and “down,” in the microgravity conditions of outer space and had to find ways of adapting. Sue then told me of her own visual world: since she had grown up cross-eyed, her eyes did not work in tandem, and so she viewed the world with one eye at a time, her eyes rapidly and unconsciously alternating. I asked if this was any disadvantage to her. No, she said, she got along perfectly well—she drove a car, she could play softball, she could do whatever anyone else could. She might not be able to see depth directly, as other people could, but she could judge it as well as anybody, using other cues.

  I asked Sue if she could imagine what the world would look like if viewed stereoscopically. Sue said yes, she thought she could—after all, she was a professor of neurobiology, and she had read Hubel and Wiesel’s papers and much else on visual processing, binocular vision, and stereopsis. She felt that this knowledge had given her a special insight into what she was missing—she knew what stereopsis must be like, even if she had never experienced it.

  But now, nearly nine years after our initial conversation, she felt compelled to write to me about this question:

  You asked me if I could imagine what the world would look like when viewed with two eyes. I told you that I thought I could.… But I was wrong.

  She could say this because now she had stereopsis—and it was beyond anything she could have imagined. She went on to give me details of her visual history, starting with her parents noticing that she was cross-eyed a few months after she was born:

  The doctors told them that I would probably outgrow the condition. This may have been the best advice at the time. The ye
ar was 1954, eleven years before David Hubel and Torsten Wiesel published their pivotal papers on visual development, critical periods, and cross-eyed kittens. Today, a surgeon would realign the eyes of a cross-eyed child during the “critical period” … in order to preserve binocular vision and stereopsis. Binocular vision depends on good alignment between the two eyes. The general dogma states that the eyes must be realigned in the first year or two. If surgery is performed later than that, the brain will have already rewired itself in a way that prevents binocular vision.

  Sue did have operations to correct her strabismus, first on the muscles of the right eye, when she was two, and then of the left eye, and finally of both eyes, when she was seven. When she was nine, her surgeon told her that she could now “do anything a person with normal vision could do except fly an airplane.” (Wiley Post, apparently, had already been forgotten by the 1960s.) She no longer looked cross-eyed to a casual observer, but she was half aware that her eyes were still not working together, that there was still something amiss, though she could not specify what it was. “No one mentioned to me that I lacked binocular vision, and I remained happily ignorant of the fact until I was a junior in college,” she wrote. Then she took a course in neurophysiology:

  The professor described the development of the visual cortex, ocular dominance columns, monocular and binocular vision, and experiments done on kittens reared with artificial strabismus. He mentioned that these cats probably lacked binocular vision and stereopsis. I was completely floored. I had no idea that there was a way of seeing the world that I lacked.

  After her initial astonishment, Sue began to investigate her own stereo vision:

  I went to the library and struggled through the scientific papers. I tried every stereo vision test that I could find and flunked them all. I even learned that one was supposed to see a three-dimensional image through the View-Master, the toy stereo viewer that I had been given after my third operation. I found the old toy in my parents’ home, but could not see a three-dimensional image with it. Everyone else who tried the toy could.