The Mind's Eye

by Oliver Sacks

  In the summer of 2005, Bob Wasserman and I paid Sue another visit, this time in Woods Hole, Massachusetts, where she was running a fellowship program in neurobiology. She had mentioned to me that the bay there was sometimes full of luminous organisms, mostly tiny dinoflagellates, and that she enjoyed swimming among them. When we arrived, in the middle of August, we found that our timing was perfect; the water was aflame with the luminous creatures (“Noctiluca scintillans—I love the name,” said Sue). After dark, we went down to the beach, armed with masks and snorkels. We could see the water sparkling from the shore, as if fireflies were in it, and when we immersed ourselves and moved our arms and legs in the water, clouds of miniature fireworks lit up around our limbs. When we swam, the night lights rushed past our eyes like the stars streaking past the Enterprise as it reaches warp speed. In one area, where the noctiluca were particularly dense, Bob said, “It’s like swimming into a galaxy, a globular cluster.”

  Sue, overhearing this, said, “Now I see them in 3-D—they all seemed to twinkle in a flat plane before.” Here there were no contours, no boundaries, no large objects to occlude or give perspective. There was no context whatsoever—it was like being immersed in a giant random-dot stereogram—and yet Sue now saw the noctiluca at different depths and distances, in three-dimensional space. We wanted to quiz her in more detail about the experience, but Sue, normally eager to talk about stereo vision, was mesmerized by the beauty of the scintillating organisms. “Enough thinking!” she said. “Give yourself to the noctiluca.”

  Struggling to find an analogy for her experience, Sue had suggested, in her original letter to me, that her experience might be akin to that of someone born totally colorblind, able to see only in shades of gray, who is suddenly given the ability to see in full color. Such a person, she wrote, “would probably be overwhelmed by the beauty of the world. Could they stop looking?” While I liked the poetry of Sue’s analogy, I was unsure about the thought. (My friend and colleague Knut Nordby, who was completely colorblind, thought that to be given color as an “add-on” after a lifetime without it would be grossly confusing, and impossible to integrate with his already complete visual world. Color, he felt, would be unintelligible and have no associations, no meaning, for someone like him.)

  Sue’s experience of stereoscopy, however, was clearly not a gratuitous or meaningless addition to her visual world. After a brief confusion, she embraced the new experience and felt it not as an arbitrary add-on but as an enrichment, a natural and delicious deepening of her existing vision. But terms like “enrichment” or “deepening,” Sue felt, did not begin to do justice to her acquisition of stereoscopy. It was not just a quantitative increase; it was something entirely novel. Stereoscopy, she maintains, is subjectively different.14 This difference even extends to the perception of two-dimensional representations such as photographs, movies, or paintings. Sue now finds these far more “realistic”; her now-activated stereo systems allow her to imagine space in a way she could not before.

  David Hubel has followed Sue’s case with interest and has corresponded with her and with me about it. He has pointed out that we are still quite ignorant of the cellular basis of stereoscopy. We do not know whether, even in animals, disparity-sensitive cells (the binocular cells specialized for stereoscopy) are present at birth (though Hubel suspects they are). We do not know what happens to these cells if there is strabismus and lack of binocular experience in early life or, most crucially, whether they can recover if people later learn to position their eyes for binocular fusion. With regard to Sue, he wrote, “It seems to me that [her regaining of stereopsis] occurred too quickly for it to be due to a reestablishment of connections, and I rather would guess that the apparatus was there all along, and just required reestablishment of fusion to be brought out.” But, he added, “that’s just a guess!”

  What emerges from Sue’s experience is that there seems to be sufficient plasticity in the adult brain for these binocular cells and circuits, if some have survived the critical period, to be reactivated much later. In such a situation, though a person may have had little or no stereo vision that she can remember, the potential for stereopsis is nonetheless present and may spring to life—most unexpectedly—if good alignment of the eyes can be obtained. That this seems to have happened with Sue after a dormant period of almost fifty years is very striking.

  Though Sue originally thought her own case unique, she has found, on the internet, a number of other people with strabismus and related problems who have unexpectedly achieved stereo vision through vision therapy. Their experiences, like Sue’s, suggest that if one has even small islands of function in the visual cortex, there may be a fair chance of reactivating and expanding them in later life, despite a lapse of decades.

  Whatever its neurological basis, the augmentation of Sue’s visual world has effectively granted her an added sense, a circumstance that the rest of us can scarcely imagine. For her, stereopsis continues to have a quality of revelation. “After almost three years,” she wrote, “my new vision continues to surprise and delight me. One winter day, I was racing from the classroom to the deli for a quick lunch. After taking only a few steps from the classroom building, I stopped short. The snow was falling lazily around me in large, wet flakes. I could see the space between each flake, and all the flakes together produced a beautiful three-dimensional dance. In the past, the snow would have appeared to fall in a flat sheet in one plane slightly in front of me. I would have felt like I was looking in on the snowfall. But now, I felt myself within the snowfall, among the snowflakes. Lunch forgotten, I watched the snow fall for several minutes, and, as I watched, I was overcome with a deep sense of joy. A snowfall can be quite beautiful—especially when you see it for the first time.”

  Postscript

  Seven years after acquiring stereoscopy, Sue still delights in her “new” sense and finds her visual world infinitely richer for it. Since writing to me in 2004, she has continued to think about her own experiences and to reach out to many people in similar situations, as well as to vision researchers. In 2009, she published a beautiful and profound book about her experiences, Fixing My Gaze: A Scientist’s Journey into Seeing in Three Dimensions.

  1. Wheatstone’s name is more commonly associated with the invention of the Wheatstone bridge, an instrument used to measure electrical resistance. But like several other eminent nineteenth-century scientists, Wheatstone was also deeply interested in the physical basis of perception. All of these “natural philosophers” (we would now call them physicists), using ingenious experiments, contributed to our understanding of how the eye and brain construct our perceptions of depth and movement and color, as they also contributed to the technological development of stereo, cinematic, and color photography.

  Michael Faraday, in addition to his electromagnetic studies, played a part in devising zoetrope-like instruments that presented a series of still drawings to the eyes in rapid succession, demonstrating that at a critical rate these could be fused by the brain to create a sensation of motion.

  James Clerk Maxwell was intrigued by Thomas Young’s hypothesis that there were three—and only three—distinct types of color receptors in the retina, each responsive to light of a certain wavelength (roughly corresponding to red, green, and blue). He devised an elegant test of this by photographing a colored bow through red, green, and violet filters and then projecting the three photographs through their corresponding filters. When the three monochromatic images were perfectly superimposed, the picture burst into full color.

  2. By the mid-1850s, a subspecialty of stereo photography, stereo pornography, was already well established, though this was of a rather static type, because the photographic processes used at the time required lengthy exposures.

  3. There is one situation, as I learned by painful experience, when two eyes do not help. When I was growing up, we always had a clothesline strung across the garden, and since it traversed the entire visual field horizontally, it appeared exactly the same t
o both eyes, and I could never judge how far away it was. I had to approach it cautiously, since it was strung rather low, at about the height of my neck. Sometimes, forgetting this, I would run straight into it, almost garroting myself.

  4. Richard Gregory, who studied visual illusions for many years, insisted that perceptions were, in fact, perceptual hypotheses (as, in the 1860s, Hermann von Helmholtz called them “unconscious inferences”). Gregory was a stereo enthusiast—he often sent his friends stereoscopic Christmas cards—but when I spoke to him about seeing faces as hollow masks, he was very surprised. With something as familiar and crucial as a face, he thought, probabilities and context would weigh the odds heavily against such a radical misperception. I agreed, but could not gainsay my own experience, and Gregory had to concede that such an improbable phenomenon might indeed occur in someone who is strongly biased towards binocular cues.

  5. In The Forest People, Colin Turnbull described driving with a Pygmy man who had never left the jungle before:

  He saw the buffalo, still grazing lazily several miles away, far down below. He turned to me and said, “What insects are those?” At first I hardly understood; then I realized that in the forest the range of vision is so limited that there is no great need to make an automatic allowance for distance when judging size.… When I told Kenge that the insects were buffalo, he roared with laughter and told me not to tell such stupid lies.… As we got closer, the “insects” must have seemed to get bigger and bigger. Kenge kept his face glued to the window, which nothing would make him lower. I was never able to discover what he thought was happening—whether he thought that the insects were changing into buffalo, or that they were miniature buffalo growing rapidly as we approached. His only comment was that they were not real buffalo, and he was not going to get out of the car again until we left the park.

  6. More rarely, stereopsis may be lost, sometimes suddenly, with a stroke or other damage to the visual cortex. Macdonald Critchley, in his book The Parietal Lobes, also refers to the opposite condition as a rare consequence of cerebral lesions in the early visual cortex: an enhancement of stereo vision “whereby near objects seem to be abnormally close, and distant objects seem to be much too far away.” Enhancement or loss of stereo vision can also occur transiently in a migraine aura or with certain drugs.

  7. A number of people with misaligned eyes may not only lack stereo vision but have double vision or shimmering effects, which can cause them problems with daily activities generally, and especially with reading or driving.

  8. Photographers and cinematographers, concerned to create an illusion of three-dimensionality on a flat plane, must deliberately renounce their binocularity and stereoscopy, confining themselves to a one-eye, one-lens view, to better frame and compose their pictures.

  In a 2004 letter to the editor of the New England Journal of Medicine, Harvard neurobiologists Margaret Livingstone and Bevil Conway suggested, after an examination of Rembrandt’s self-portraits, that the painter was so walleyed as to be stereo-blind, and that “stereoblindness might not be a handicap—and might even be an asset—for some artists.” Subsequently they proposed, after looking at photographs of other artists, that many of them—de Kooning, Johns, Stella, Picasso, Calder, Chagall, Hopper, and Homer, among others—also seemed to have significant misalignment of the eyes and were perhaps also stereo-blind.

  9. Walleyed people enjoy an unusually wide field of vision due to the divergence of their eyes and may hesitate to sacrifice this for an operation that might align their eyes cosmetically but fail to give them stereoscopy. Intriguingly, several such people have written to me that they are able to converge their eyes and achieve stereo vision briefly.

  10. Together, the three of us had collaborated on several cases, including that of the “colorblind painter,” who suddenly lost all ability to see in color, and that of Virgil, a man blind nearly from birth whose sight had been restored after almost fifty years of blindness. (Both of these case histories, “The Case of the Colorblind Painter” and “To See and Not See,” were published in An Anthropologist on Mars.)

  11. If a stereo photograph is flashed on a screen for as little as twenty milliseconds, a person with normal stereoscopy can perceive some stereo depth straightaway. But what one sees in a flash is not the full depth; the perception of this requires several seconds, even minutes, in which the picture seems to deepen as one continues to gaze at it—it is as if the stereo system takes a certain time to warm up, to come to its full capacity. Such a deepening seems peculiar to the stereo system (colors, by contrast, do not normally become more pronounced as one looks at them). The underlying cause for this is unknown, though it has been suggested that it entails the recruitment of additional binocular cells in the visual cortex.

  (There is, additionally, a clear practice effect, so that people who exercise their stereo powers—for example, by working with a binocular microscope—may experience striking improvements in stereo acuity and stereo depth over a longer period. Here, too, the underlying mechanism is unknown.)

  12. Bela Julesz, the remarkable researcher who studied random-dot stereoscopy, spoke of “cyclopean vision,” and regarded it as entailing neural mechanisms over and above those employed in ordinary stereo vision. This too is suggested by the fact that it may take a minute or more to “get” random-dot stereograms, where ordinary stereograms can be seen instantly.

  13. Brewster also invented, around 1844, a simple handheld stereoscope using lenses (Wheatstone’s mirror stereoscope was large and heavy and had to sit on a table). While Brewster was at first full of admiration for Wheatstone, he subsequently became jealous of his younger colleague and began publishing vindictive articles about him, pseudonymously. Finally, in 1856, in his otherwise charming book The Stereoscope: Its History, Theory and Construction, he attacked Wheatstone openly and denied him any claim to priority in the realm of stereoscopy.

  14. This view, which I share, seems to be in contradiction with the views of the great visual pioneer J. J. Gibson. In his 1950 book The Perception of the Visual World, he wrote, “If the gradient theory is correct, binocular vision simply takes its place as a determinant, but only one determinant, of visual space.” Several eminent contemporary vision researchers hold similar views. Thus Dale Purves and R. Beau Lotto, in their book Why We See What We Do, write of “a seamless relationship” between the three-dimensional world we construct with one eye and its “augmentation” by stereopsis. Such views, while wholly consistent with a behavioral or empirical theory of vision, give no weight to the qualitative and subjective aspects of stereoscopy. Here one needs inside narratives, personal accounts of what it is like to suddenly gain stereo vision after a lifetime of stereo blindness (as Sue describes) or to suddenly lose it after a lifetime of seeing in stereo (as I describe in the following chapter).

  Persistence of Vision

  A Journal

  ON DECEMBER 17, 2005, a Saturday, I had my usual morning swim and then decided to go to the movies. I arrived a few minutes early and took a seat in the back of the cinema—I had no intimation of anything unusual until the previews started. Then I immediately became conscious of a sort of fluttering, a visual instability, to my left. At first I thought it was the start of a visual migraine, but I soon realized that whatever it was affected only the right eye and must therefore be arising in the eye itself, and not in the visual cortex, the way a migraine would.

  When the cinema screen went dark after the first preview, the spot that had been quivering to my left flared up like a white-hot coal, with spectral colors—turquoise, green, orange—at its edges. I was alarmed: was I having a hemorrhage into the eye, a blockage of the central retinal artery, a retinal detachment? I then became conscious of a blind spot within the incandescent area, for using just my right eye and looking to the left, where a line of little lights along the floor indicated a way out of the cinema, I found that all the forward ones were now “missing.”

  I felt panic rising. Would the dark area continue t
o enlarge until the right eye was completely blind? Should I leave at once? Go to an emergency room? Call my ophthalmologist friend, Bob? Or should I sit tight and see if the disturbance spontaneously resolved? The film started, but I paid little attention to it; I was entirely preoccupied with checking my vision every few seconds.

  Finally, after about twenty minutes, I burst out of the cinema—perhaps everything would look fine once I got into daylight, the real world. But it didn’t. The flaring had died down a little, but when I used only my right eye, a pie-shaped chunk of my visual field was still missing to the left. I walked, almost ran, back to my apartment and phoned Bob. He asked a few questions, suggested a couple of instant tests, then told me to get myself to an ophthalmologist immediately.

  A couple of hours later I was in the ophthalmologist’s consulting room. I told my story again, indicated the quadrant of blindness in my right eye. He listened carefully, looked noncommittal, and, after checking my visual fields, took his ophthalmoscope and peered into the eye. Then he put down the instrument, leaned back, and gazed at me, I thought, with different eyes. There had been a certain lightness or casualness in him before—we were not exactly friends, but we were colleagues, both medical men. Now, suddenly, I was in a quite different category. He spoke carefully, picking his words; his demeanor was one of seriousness and concern. “I see pigmentation,” he said, “something behind the retina. It could be a hematoma, or it could be a tumor. If it’s a tumor, it could be benign or malignant.” He seemed to take a deep breath. “Let’s look at the worst-case scenario,” he continued. I cannot be sure what he said next, for a voice had started up in my head, shouting, “CANCER, CANCER, CANCER …” and I could no longer hear him. He said he would make arrangements for me to see Dr. David Abramson, a great expert on ocular tumors, as soon as possible.