The Mind's Eye

by Oliver Sacks

  At this point, Sue wondered whether there might be any therapy by which she could acquire binocular vision, but “the doctors told me that it would be a waste of my time and money to attempt vision therapy. It was simply too late. I could only have developed binocular vision if my eyes had been properly aligned by age two. Since I had read Hubel and Wiesel’s work on visual development and early critical periods, I accepted their advice.”

  Twenty-five years passed—years in which Sue married and raised a family while pursuing an academic career in neurobiology. Though she had some difficulties with driving—merging on entrance ramps to freeways, she found it hard to estimate the speed of oncoming cars—she got along generally quite well with her monocular ways of judging space and distance. Once in a while, she even teased binocular people:

  I took some tennis lessons with an accomplished pro. One day, I asked him to wear an eye patch so that he had to hit the ball using only one eye. I hit a ball to him high in the air and watched this superb athlete miss the ball entirely. Frustrated, he ripped off the eye patch and threw it away. I am ashamed to admit it, but I enjoyed watching him flounder, a sort of revenge against all two-eyed athletes.

  But when Sue was in her late forties new problems began:

  It became increasingly difficult to see things at a distance. Not only did my eye muscles fatigue more quickly, but the world appeared to shimmer when I looked in the distance. It was hard to focus on the letters on street signs or distinguish whether a person was walking toward or away from me.… At the same time, my glasses, used for distance vision, made me far-sighted. In the classroom, I could not read my lecture notes and see the students at the same time.… I decided it was time to get bifocals or progressive lenses. I was determined to find an eye doctor who would give me both progressive lenses to improve my visual acuity and eye exercises to strengthen my eye muscles.

  She consulted Dr. Theresa Ruggiero, a developmental optometrist, who found that Sue’s eyes were developing various forms of imbalance—this sometimes happens after surgery for strabismus—so that the reasonable vision she had enjoyed for decades was now being undermined.

  Dr. Ruggiero confirmed that I saw the world monocularly. I only used two eyes together when looking within two inches of my face. She told me that I consistently misjudged the location of objects when viewing them solely with my left eye. Most importantly, she discovered that my two eyes were misaligned vertically. The visual field of my left eye was about three degrees above that of my right. Dr. Ruggiero placed a prism in front of my right lens that shifted the entire visual field of the right eye upward.… Without the prism, I had trouble reading the eye chart on a computer screen across the room because the letters appeared to shimmer. With the prism, the shimmer was greatly reduced.

  (“Shimmer,” Sue later explained, was perhaps too mild a term, for it was not like the shimmer one might see with a heat haze on a summer day—it was, rather, a rapid, dizzying oscillation of several times a second.)

  Sue got her new eyeglasses, complete with the prism, on February 12, 2002. Two days later, she had her first vision-therapy session with Dr. Ruggiero—a long session in which, using Polaroid glasses to allow a different image to be presented to each eye, she attempted to fuse the two pictures. At first, she did not understand what “fusion” meant, how it was possible to bring the two images together; but after trying for several minutes she found she was able to do it, though only for a second at a time. Although she was looking at a pair of stereo images, she had no perception of depth; nevertheless, she had made the first step, achieving “flat fusion,” as Dr. Ruggiero called it.

  Sue wondered whether, if she could hold her eyes aligned for longer, this would allow not just flat fusion but stereo fusion, too. Dr. Ruggiero gave her further exercises to stabilize her tracking and hold her gaze, and she worked on these exercises diligently at home. Three days later, something odd occurred:

  I noticed today that the light fixture that hangs down from our kitchen ceiling looks different. It seems to occupy some space between myself and the ceiling. The edges are also more rounded. It’s a subtle effect but noticeable.

  In her second session with Dr. Ruggiero, on February 21, Sue repeated the Polaroid exercise and tried a new one, using colored beads at different distances on a string. This exercise, known as the Brock string, taught Sue to fixate both eyes on the same point in space, so that her visual system would not suppress the images from one eye or the other but would fuse them together. The effect of this session was immediate:

  I went back to my car and happened to glance at the steering wheel. It had “popped out” from the dashboard. I closed one eye, then the other, then looked with both eyes again, and the steering wheel looked different. I decided that the light from the setting sun was playing tricks on me and drove home. But the next day I got up, did the eye exercises, and got into the car to drive to work. When I looked at the rear-view mirror, it had popped out from the windshield.

  Her new vision was “absolutely delightful,” Sue wrote. “I had no idea what I had been missing.” As she put it, “Ordinary things looked extraordinary. Light fixtures floated and water faucets stuck way out into space.” But it was “also a bit confusing. I don’t know how far one object should ‘pop out’ in front of another for a given distance between the two objects.… [It is] a bit like I am in a fun house or high on drugs. I keep staring at things.… The world really does look different.” She included some excerpts from her diary:

  February 22: I noticed the edge of the open door to my office seemed to stick out toward me. Now, I always knew that the door was sticking out toward me when it was open because of the shape of the door, perspective and other monocular cues, but I had never seen it in depth. It made me do a double take and look at it with one eye and then the other in order to convince myself that it looked different. It was definitely out there.

  When I was eating lunch, I looked down at my fork over the bowl of rice and the fork was poised in the air in front of the bowl. There was space between the fork and the bowl. I had never seen that before.… I kept looking at a grape poised at the edge of my fork. I could see it in depth.

  March 1: Today, I was walking by the complete horse skeleton in the basement of the building where I work, when I saw the horse’s skull sticking out so much, that I actually jumped back and cried out.

  March 4: While I was running this morning with the dog, I noticed that the bushes looked different. Every leaf seemed to stand out in its own little 3-D space. The leaves didn’t just overlap with each other as I used to see them. I could see the SPACE between the leaves. The same is true for twigs on trees, pebbles on the road, stones in a stone wall. Everything has more texture.

  Sue’s letter continued in this lyrical vein, describing experiences utterly novel for her, beyond anything she could have imagined or inferred before. She had discovered for herself that there is no substitute for experience, that there is an unbridgeable gulf between what Bertrand Russell called “knowledge by description” and “knowledge by acquaintance,” and no way of going from one to the other.

  One would think that the sudden appearance of an entirely new quality of sensation or perception might be confusing or frightening, but Sue seemed to adapt to her new world with remarkable ease. She was startled and disoriented at first, and had to calibrate her new visual perception of depth and distance with her actions and movements. But for the most part she felt entirely and increasingly at home with stereoscopy. Though she continues to be conscious of the novelty of stereo vision and indeed rejoices in it, she also feels now that it is “natural”—that she is seeing the world as it really is, as it should be. Flowers, she says, seem “intensely real, inflated,” where they were “flat” or “deflated” before.

  Sue’s acquisition of stereoscopy after almost half a century of being stereo-blind has also been of great practical benefit. Driving is easier; threading a needle, too. When she looks down into her binocular microscope at work, s
he can see paramecia swimming at different levels, and see this directly, rather than inferring it by refocusing the microscope up or down. And it is a continuing source of enthrallment:

  At seminars … my attention is completely captivated by the way an empty chair displays itself in space, and a whole row of empty chairs occupies my attention for minutes. I would like to take a whole day just to walk around and LOOK. I did escape today for an hour to the college greenhouse just to look at the plants and flowers from all angles.

  Most of the phone calls and letters I receive are about mishaps, problems, losses of various sorts. Sue’s letter, though, was a story not of loss and lamentation but of the sudden gaining of a new sense and sensibility and, with this, a sense of delight and jubilation. Yet her letter also sounded a note of bewilderment and reservation: she did not know of any experience or story like her own and was perplexed to find, in all she had read, that the achievement of stereoscopy in adult life was “impossible.” Had she always had binocular cells in her visual cortex, she wondered, just waiting for the right input? Was it possible that the critical period in early life was less critical than generally thought? What did I make of all this?

  I mulled over Sue’s letter for a few days and discussed it with several colleagues, including Bob Wasserman, an ophthalmologist, and Ralph Siegel, a vision physiologist.10 A few weeks later, in February of 2005, the three of us went to see Sue at her home in Massachusetts, bringing along ophthalmological equipment and various stereoscopes and stereograms.

  Sue welcomed us and, as we chatted, showed us some childhood photos, since we were interested in trying to reconstruct her early visual history. Her childhood strabismus, prior to surgery, was quite clear in the photographs. Had she ever been able to see in three dimensions? we asked. Sue thought for a moment and answered yes, perhaps—very occasionally, as a child, lying in the grass, she might suddenly see, for a second or two, a blade of grass stand out from its background; she had almost forgotten about this until we quizzed her. The grass would have to be very close to her eyes, within inches, requiring her (like any of us) to cross her eyes. So there was a suggestion that the potential for stereopsis was there and could be brought out if she moved her eyes into the proper position for stereo viewing.

  Sue had written in her letter, “I think, all my life, I have desired to see things in greater depth, even before I knew I had poor depth perception.” This strange, poignant remark made me wonder whether she had retained some dim, barely conscious memory of having once seen things in greater depth (for she would have no sense of loss or nostalgia for something she had never had). It was important to test her with special stereograms that had no cues or clues as to depth—no perspective or occlusion, for example. I had brought one stereogram with lines of print—unrelated words and short phrases—that, if viewed stereoscopically, appeared to be on seven different planes of depth but, if viewed with one eye or without true stereo vision, appeared to be on the same plane. Sue looked at this picture through the stereoscope and saw it as a flat plane. It was only when I prompted her by telling her that some of the print was at different levels that she looked again and said, “Oh, now I see.” After this, she was able to distinguish all seven levels and put them in the correct order.

  Given enough time, Sue might have been able to see all seven levels on her own, but such “top-down” factors—knowing or having an idea of what one should see—are crucial in many aspects of perception. A special attention, a special searching, may be necessary to reinforce a relatively weak physiological faculty. It seems likely that such factors are strongly operative with Sue, especially in this type of test situation. Her difficulties in real life are much less, because every other factor here—knowledge, context, and expectation no less than perspective, occlusion, and motion parallax—helps her experience the three-dimensional reality around her.

  Sue was able to see depth in the red-and-green drawings I had brought. One of these images—an impossible three-pronged tuning fork such as M. C. Escher might have drawn, with three tines of increasing heights—Sue found “spectacular”; she saw the top of the uppermost prong as three or four centimeters above the plane of the paper. Yet Sue had spoken of herself as having only a “shallow” stereoscopy, and indeed, Bob and Ralph both saw the uppermost prong as about twelve centimeters above the plane of the paper, while I saw it as five centimeters higher still.

  I found this surprising, because we were all the same distance from the drawing, and I had imagined that there would be, by a sort of neural trigonometry, a fixed relationship between the disparity of the images and their perceived depth. Puzzled by this, I wrote to Shinsuke Shimojo, at Caltech, an expert on many aspects of visual perception. He brought out, in his reply, that when one looks at a stereogram, the computational process in the brain is based not solely on the binocular cue of disparity but also on monocular cues such as size, occlusion, and motion parallax. The monocular cues may work against the binocular ones, and the brain must balance one set of cues against the other to arrive at a weighted average. This final result will be different in different individuals, because there is huge variation, even in the normal population: some people rely predominantly on binocular cues, others on monocular cues, and most use some combination of both. In looking at a stereo picture such as the tuning fork, a strongly binocular person will see unusual stereo depth; a monocularly oriented person will see much less depth; and others, relying on both binocular and monocular cues, will see something in between. Shimojo’s formulation gave substance to the obstinate belief held by many of us in the New York Stereoscopic Society that we lived in a “deeper” world, visually, than the majority of people.11

  · · ·

  Later in the day, we paid a visit to Sue’s optometrist, Dr. Theresa Ruggiero, who described how Sue had first consulted her, in 2001. Sue had complained then of eyestrain, especially when driving, impaired clarity, and a disconcerting jumping or flickering of images—but had not mentioned her lack of stereoscopy.

  Dr. Ruggiero herself was greatly pleased, she said, when, immediately after achieving flat fusion, Sue experienced stereoscopy. The conscious effort and act of moving her eyes into position for binocular fusion, Ruggiero speculated, may have been critical for Sue’s breakthrough. And she stressed, over and above the initial achievement of stereoscopy, Sue’s adventurous and positive reaction to this and her fierce determination to hold on to it and enhance it, however much work it might entail.

  And it did indeed entail, and still entails, a great deal of work—taxing fusion exercises for at least twenty minutes every day. With these exercises, Sue found that she was starting to perceive depth at greater and greater distances, where at first she had seen depth only close up, as with the steering wheel. She continued to have jumps of improvement in her stereo acuity, so that she was able to see depth with smaller and smaller disparities—but when she stopped the therapy for six months, she quickly regressed. This upset her deeply, and she resumed the eye exercises, working on them every day, “religiously.”

  Sue uses a kinetic metaphor for her learning to use stereo vision, comparing it to learning to walk again. “I had to develop a new choreography for my own eye movements,” she recently wrote, “how to move my eyes in harmony, before I could tap into latent binocular circuits and see in stereo depth.”

  Sue has continued to work very hard on her stereo perception and stereo acuity, and her perception of stereo depth is again on the increase. Moreover, she has developed a skill she did not have when we initially visited her: the ability to see random-dot stereograms. At first glance, these do not appear to contain any images at all. But as one continues to gaze at them through the stereoscope, one becomes aware of a strange sort of turbulence among the dots, and then a startling illusion—an image, a shape, whatever—suddenly appears far above or far below the plane of the paper. This illusion takes some practice, and many people, even those with normal binocular vision, are not able to get it. But it is the purest tes
t of stereoscopic vision, for there are no monocular cues whatever; only by stereoscopically fusing thousands of seemingly random points as seen by two eyes can the brain construct a three-dimensional image.12

  David Brewster, a nineteenth-century scientist who was inspired by Wheatstone’s work, observed a related form of stereo illusion. Gazing at wallpaper with small repetitive motifs, he observed that sometimes, with the proper convergence or divergence of gaze, the patterns might quiver or shift and then jump into startling stereoscopic relief, seeming to float in front of or behind the wallpaper.13 Brewster wrote about these stereoscopic illusions, and believed he was the first to observe them—although it seems likely that such “autostereograms” have been experienced for millennia, with the repetitive patterns of Islamic art, Celtic art, and the art of many other cultures. Medieval manuscripts such as the Book of Kells or the Lindisfarne Gospels, for example, contain exquisitely intricate designs done so exactly that whole pages can be seen, with the unaided eye, in stereoscopic relief. (John Cisne, a paleobiologist at Cornell, has suggested that such stereograms may have been “something of a trade secret among the educated élite of the seventh- and eighth-century British Isles.”)

  In the past decade or two, elaborate autostereograms have been widely popularized in Magic Eye books. The illusions are single images that one views without a stereoscope; but they contain horizontal rows of repeating “wallpaper” patterns that are slightly different. At first glance, all the patterns seem to be on the same level, but if one learns how to diverge or converge the eyes, letting each eye focus on a different row, then striking stereoscopic illusions appear. Sue loves these, and they have added another dimension to her newfound life in stereo: “I find these wallpaper autostereograms easy (and quite thrilling),” she recently wrote, “probably because I practice convergent and divergent fusion regularly.”