Fixing My Gaze

A Scientist's Journey Into Seeing in Three Dimensions


By Susan R. Barry

Foreword by Oliver Sacks

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A revelatory account of the brain’s capacity for change

When neuroscientist Susan Barry was fifty years old, she experienced the sense of immersion in a three dimensional world for the first time. Skyscrapers on street corners appeared to loom out toward her like the bows of giant ships. Tree branches projected upward and outward, enclosing and commanding palpable volumes of space. Leaves created intricate mosaics in 3D.

Barry had been cross-eyed and stereoblind since early infancy. After half a century of perceiving her surroundings as flat and compressed, on that day she saw the city of Manhattan in stereo depth for first time in her life. As a neuroscientist, she understood just how extraordinary this transformation was, not only for herself but for the scientific understanding of the human brain. Scientists have long believed that the brain is malleable only during a “critical period” in early childhood. According to this theory, Barry’s brain had organized itself when she was a baby to avoid double vision – and there was no way to rewire it as an adult. But Barry found an optometrist who prescribed a little-known program of vision therapy; after intensive training, Barry was ultimately able to accomplish what other scientists and even she herself had once considered impossible. Dubbed “Stereo Sue” by renowned neurologist Oliver Sacks, Susan Barry tells her own remarkable journey and celebrates the joyous pleasure of our senses.


In memory of my mother,
Estelle Florence Fisher Feinstein,
a woman who saw
in great depth.

Note to the Reader
All of the stories in this book are true, but the names of some of the people have been changed to respect their privacy.

Foreword by Oliver Sacks
I first met Sue Barry in 1996 at a launch party for her husband, Dan, an astronaut. We soon got to talking about different ways of experiencing the world—how Dan, for example, in the microgravity of spaceflight, had no direct sense of up or down, so he had to find other ways of orienting himself in space. She herself, Sue said, experienced the world in an unusual way as a consequence of having developed crossed eyes, or strabismus, in early infancy. Her eyes had been straightened by surgery, and she had 20/20 vision in both eyes, but they were still not fully aligned. Her brain had learned to suppress the image from one eye or the other so that she did not experience a confusing double vision. Normally the brain constructs a perception of depth by comparing the images from the two eyes, but in Sue’s case, where one or the other image was suppressed, no such comparison was possible. So, though she had learned to judge distance and depth by other cues, she had never experienced true “solid vision,” or stereoscopy. Her world was entirely flat.
But all in all, 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, such as perspective, occlusion, shading, or motion parallax. It was no big deal.
I asked Sue if she could imagine what the world would look like if viewed stereoscopically, and she said, yes, she thought she could—after all, she was a professor of neurobiology, and she had read plenty of papers on visual processing, binocular vision, and stereopsis. She felt this knowledge had given her some special insight into what she was missing—she knew what stereopsis must be like, even if she had never experienced it.
But in December 2004, almost nine years after our initial conversation, she wrote me a letter, which began, “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 with some conviction because she had suddenly, unexpectedly, acquired stereovision herself, and the reality of this, the actual experience, was utterly beyond anything imagination could have conceived. She was almost fifty and had been having increasing visual difficulties resulting from the misalignment of her eyes. Finally, she had embarked on an intensive course of training with a developmental optometrist, and one day, after learning to align her eyes, she suddenly saw the steering wheel of her car “popping out” from the dashboard. After having lived in a flat world for fifty years, Sue felt this sudden leap into three-dimensionality as a revelation. Her world was now full of a new sort of visual beauty and wonder so deep that three years later, when she wrote to me, she was still enraptured with it.
“My new vision continues to surprise and delight me,” she wrote.
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.
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.”
Indeed, what Sue described to me in her letter went completely against the current dogma of “critical periods” in sensory development—the notion that stereoscopy (like many other aspects of visual perception, and like language, as well)—had to be acquired in the first three or four years of life, or it could never be acquired, for the critical brain cells and circuitry needed for stereovision would fail to develop.
Long suspected by surgeons operating on children with strabismus, this notion of a critical period seemed to be confirmed by the famous experimental work of David Hubel and Torsten Wiesel, who showed that if kittens were rendered strabismic by detaching an eye muscle, binocular depth cells would fail to develop in their brains, and they would lack stereovision. It was only when Sue learned of these experiments—she was a college student at the time—that she realized she herself might be stereoblind, like the kittens. This, indeed, is the vivid opening scene in her narrative:
Stereoblind? Was I stereoblind? I looked around. The classroom didn’t seem entirely flat to me. I knew that the student sitting in front of me was located between me and the blackboard because the student blocked my view of the blackboard. When I looked outside the classroom window, I knew which trees were located further away because they looked smaller than the closer ones. The footpath outside the window appeared to narrow as it extended out into the distance. Through cues like these, I could judge depth and distance. I knew the world was in 3D. Yet, my professor implied that there was another, different way to see space and depth. He called this way of seeing stereopsis. I couldn’t imagine what he was talking about.
When Sue next went to her eye doctor for a routine check, she asked him to check whether she had stereovision. He brought out a stereoscope and test stereo pictures. Sue could not “get” any of them, could not imagine what “getting” them would be like. Would it be possible for her to gain stereovision, she asked? The doctor replied, no, it was much too late, and added, “Stereopsis is just a little fine-tuning for the visual system. You don’t need stereovision because you don’t have stereovision.”
Sue accepted that she would never have stereovision and got on with her life, becoming a teacher and researcher, marrying, and raising a family. Somewhere, at the back of her mind—for she is a scientist and incessantly curious about how the world works—was a question: what could stereovision be like? And yet, her life, visually and otherwise, was full and rich, and she did not “miss” stereo or think of it too much. So thirty years later, when she finally sought vision therapy and unexpectedly gained stereovision, this came as a bonus, a miraculous complement to her other visual improvements.
Sue exulted in her newfound sense of stereoscopic depth. She found it much more than “fine-tuning”—it was an entirely new way of seeing. “People who have always had stereopsis,” she said to me, “take it for granted. They have no idea how wonderful it is. You have to have been stereoblind for half a century and then acquire it to value it properly.”
How was Sue able to acquire, essentially, a whole new sense so long after the “critical period”? I was as puzzled by Sue’s story as she was. And I was intrigued, for I myself had never taken stereoscopy for granted. On the contrary, I was something of a stereophile, having played with 3D drawings and Victorian stereo viewers as a child and later experimented with stereo photography. So I arranged to meet Sue again, and this inspired me to write an article about her experiences in 2006.1
But that was not the end of the matter.
All that Sue intimated to me in her letters and conversations has now been expanded and deepened into a fascinating account. Fixing My Gaze is a beautiful description and appreciation of two very distinct ways of seeing—with and without the benefit of stereoscopy. But it is also an exploration of much more. Sue is at pains not only to present her story in clear, lucid, often poetic language, but also, as a scientist, to provide explanation and understanding.
She is in a unique position to do this, drawing on both her personal experience and her background as a neurobiologist. She has interviewed many eminent vision researchers and pondered the problem of critical periods with them. Her experience indicates 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 later. In such a situation, though a person may have had little or no stereovision 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.
Although Sue originally thought her own case unique, she has since found a number of other people with strabismus and related problems who have unexpectedly achieved stereovision through vision therapy. This is no easy accomplishment. It may require not only optical corrections (proper lenses or prisms and so forth) but very intensive training and learning—in effect, one must learn how to align the eyes and fuse their images, while unlearning the unconscious habit of suppressing vision, which has been occurring perhaps for decades. In this way, vision therapy is directed at the whole person: it requires high motivation and self-awareness, as well as enormous perseverance, practice, and determination, as does psychotherapy, for instance, or learning to play the piano. But it is also highly rewarding, as Sue brings out. And this ability to acquire new perceptual abilities later in life has great implications for anyone interested in neuroscience or rehabilitation and, of course, for the millions of people who, like Sue, have been strabismic since infancy. Sue’s case, together with many others, suggests that if there are even small islands of function in the visual cortex, there may be a fair chance of reactivating and expanding them in later life, even after a lapse of decades, if vision can be made optically possible. Cases like these may offer new hope for those once considered incorrigibly stereoblind. Fixing My Gaze will offer inspiration for anyone in this situation, but it is equally a very remarkable exploration of the brain’s ability to change and adapt, as well as an ode to the fascination and wonder of the visual world, even those parts of it which many of us take for granted.

But yield who will to their separation,
My object in living is to unite
My avocation and my vocation
As my two eyes make one in sight.
—“Two Tramps in Mud Time,” by Robert Frost
I was twenty years old and a college student before I learned that I did not see the way other people did. This surprising news came to me as I listened to a lecture on vision in my college neurobiology class. On that gray November morning, I felt sleepy and sluggish, but something my professor said jolted me out of my inattentive state. He was describing the development of the visual system, highlighting experiments done on walleyed and cross-eyed kittens. Cats, like people, have two forward-facing eyes that they move together in coordinated ways. But the kittens in these studies had strabismus, or misaligned eyes. My professor mentioned that vision in these kittens had not developed normally. They probably couldn’t see in 3D. In fact, many scientists and doctors assumed that the cats would never acquire stereovision, even if their eyes were later straightened, because this ability could develop only during a “critical period” in early life. What was thought to be true for cats was also believed to be true for people.
I was floored. My eyes had crossed when I was about three months old. When I looked at an object with my left eye, my right eye turned in, and when I looked with my right eye, my left eye moved noseward. But I had three eye-muscle surgeries at ages two, three, and seven, and these operations had aligned my eyes so that my eyes looked normal almost all the time. Surely, I saw normally too. Throughout childhood, I had 20/20 acuity with each eye and assumed that I had perfect vision.
Yet, I had just learned that people like me were missing a fundamental way of seeing. Fully alert now, I listened carefully to the professor’s explanation. We have two eyes, he said, but only one view of the world. Since our eyes are separated on our face by our nose, they see from a slightly different perspective. It is in the brain that the images from the two eyes merge into one. For most people, this happens effortlessly. Both eyes are aimed at the same point in space, and the information from each is combined in the brain. The result is a sharply outlined, detailed, and depth-filled view of the world.
My professor added that a strabismic (or person with misaligned eyes) is not so lucky. Since a strabismic’s eyes are not aimed at the same point in space, the difference between the left- and right-eye views is too great for the brain to combine the images into a single picture. The strabismic is confronted with a serious perceptual problem: she must somehow create a single, coherent world view from conflicting input from the two eyes. To solve this problem, many strabismics suppress the information from one eye and look through the other. Some always use the same eye, while others continually switch between the two eyes, but in either case, they may never see normally through the two eyes together. As a result, most strabismics have reduced or absent stereovision. The professor concluded the lecture by saying that many strabismics don’t see in 3D. They’re virtually stereoblind.
Stereoblind? Was I stereoblind? I looked around. The classroom didn’t seem entirely flat to me. I knew that the student sitting in front of me was located between me and the blackboard because the student blocked my view of the blackboard. When I looked outside the classroom window, I knew which trees were located further away because they looked smaller than the closer ones. The footpath outside the window appeared to narrow as it extended out into the distance. Through cues like these, I could judge depth and distance. I knew the world was in 3D. Yet, my professor implied that there was another, different way to see space and depth. He called this way of seeing stereopsis. I couldn’t imagine what he was talking about.
After the lecture, I went directly to the college library and struggled through the scientific papers on vision. I spent the rest of the semester studying the subject and wrote my term paper on changes to the visual system of cats that started out life with misaligned eyes. I learned that the brain processes vision in a region in the back of the cerebral cortex called the visual cortex. Neurons from the retina in the back of the eye communicate over several synaptic connections with neurons in the visual cortex, and these cortical neurons are either “monocular” or “binocular.” I learned that monocular neurons respond with nerve impulses to light stimuli coming from only the right or left eye, while binocular neurons respond to input from either eye. The majority of neurons in the visual cortex are binocular. However, strabismic infants have neurons that respond to the right or the left eye, but not both. The loss of binocular neurons results in a loss of normal binocular vision and stereopsis.
As I stayed up late reading through all these papers, I realized that I too might have a “monocular brain.” Most of the neurons in my visual cortex probably responded to input from either my right or left eye, but not both. Although I no longer looked grossly cross-eyed as I had as a child, my eyes still wandered out of alignment on occasion, especially when I was tired. So, I always avoided looking people directly in the eye. Now I suspected that I was not only a little cross-eyed but also stereoblind.
On my next trip to the eye doctor for a routine eye exam, I asked about stereovision. The doctor was surprised by my concern and interest but got out his stereo tests. I flunked them all. He shrugged his shoulders and explained that I did not fuse the images provided by my two eyes. I saw the input from only one eye at a time and switched rapidly between them.
“Don’t worry,” he told me. “Stereopsis is just a little fine-tuning for the visual system.” Then, he added, “You don’t need stereovision because you don’t have stereovision,” a statement whose logic escapes me to this day.
Was stereopsis just a little fine-tuning for the visual system, or was it an important component of everyday seeing? Indeed, the answer to this question has eluded scientists for years. In fact, the entire phenomenon of stereopsis escaped scientists for centuries. Many of the great students of optics, including Euclid, Archimedes, da Vinci, Newton, and Goethe, never figured out how we see in stereoscopic depth. This role fell to a brilliant, but reticent, inventor by the name of Charles Wheatstone.
Wheatstone, a British scientist working in the early and mid- 1800s, became the first person to measure the speed of electricity and was instrumental in the development of the first telegraph. He also discovered that the difference in viewing perspective between our two eyes is not an imperfection in our vision. Instead, this difference provides us with stereopsis, or a depth-filled way of seeing the world.
FIGURE 1.1: Your eyes turn in, or converge, to fixate, or look directly at, a near object; they turn out, or diverge, to fixate a more distant target. The straight lines in the figure indicate the lines of sight for each eye. (© Margaret C. Nelson)
Wheatstone knew that we turn in our eyes to look at nearby objects and turn them out to look at targets further away (Figure 1.1). You can determine this for yourself by asking a friend to follow the tip of an upright pencil held straight in front of him. As the pencil is brought closer to his face, he will turn in (converge) his eyes. As the pencil moves away, his eyes will turn out (diverge). These vergence movements cause the image of the pencil to fall on corresponding points of the two retinas where the light-sensing cells are found.
In order for you to see an object, light rays bouncing off it enter your eye and travel to the back of the eyeball where they land on the retina (Figure 1.2). In the retina, the rod and cone cells sense the light and pass this information on to other retinal cells and ultimately to neurons deeper in your brain.
FIGURE 1.2: The human eye including the pupil, lens, and retina. The central region of the retina is called the macula, and the center of the macula is the fovea. (© Margaret C. Nelson)
We can divide each retina into three regions: the fovea, or central region, the right side, and the left side. When you look directly at an object, its image falls on corresponding points on the central (foveal) region of both retinas. Other objects that cast their images on regions that are the same distance and in the same direction from each fovea also project to corresponding retinal points. Imagine that you are looking directly at the toy block in figure 1.3. The teddy bear located to your left casts its image on corresponding points on the right side of both your retinas, while the rattle, to the right, casts its image on corresponding points on the left side of both retinas.
In 1838, Wheatstone explained how the relative position of the images on the two retinas allows us to see in 3D. He published a paper quaintly titled “Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision.” He explained how stereopsis works by introducing a model of the first stereoscope, so named because “stereo” is the Greek word for “solid,” and images seen through the stereoscope look solid and real.
FIGURE 1.3: F refers to fovea. (© Margaret C. Nelson)
In the drawing of a stereoscope in Figure 1.4 on page 8, the two A’s in the center represent mirrors oriented at 90º to each other. To use the stereoscope, you place your nose right at the juncture between the two mirrors. In this way, the right eye can see only the reflected version of a photograph placed at E in the figure, while the left eye can see only the reflected version of the image at E on the other side. Wheatstone placed into right slot E a mirror-image picture of an object as it would be seen by your right eye and into left slot E a mirror-image picture of the same object as it would be seen by your left eye. If you were to look into the stereoscope, your brain would fuse the two images into one, and you’d see the image in stereoscopic depth.
Wheatstone highlighted several figures to be used in his stereoscope, including the one illustrated in figure 1.5 on page 9. Each member of the pair shows a small square surrounded by a larger one. The figures look flat. If you were to cut out these two drawings and put one on top of the other, the large squares would overlap perfectly but the small squares would not. Now, imagine that the figures were placed into the E slots in the stereoscope. When you looked into the stereoscope, your left eye would see only the reflected image of figure A, the left-hand figure, while your right eye would see only the reflected image of figure B. If you can see in 3D, your brain will fuse figures A and B, causing you to see just one small and one large square. As you look into the stereoscope, the edges of the larger, outer squares will fall on corresponding points of your two retinas, while the edges of the smaller, inner squares will not. This difference will cause the fused image of the large and small square to appear in different depth planes.
FIGURE 1.4: Wheatstone’s illustration of his stereoscope. (Wheatstone C. 1838. Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomena of Binocular Vision. Philosophical Transactions of the Royal Society of London 128: 371-94)
Using the stereoscope, Wheatstone demonstrated that two flat images, like the two in Figure 1.5, fuse in your brain and magically appear three-dimensional. Here was a beautiful example of how the visual system combines 2D images cast on our retinas and transforms them into one figure seen in 3D. Shortly after Wheatstone invented his stereoscope, the first 3D cameras were developed. They took photographs from two different perspectives, mimicking the perspectives seen by the two eyes. When these photographs were put into a stereoscope, the scenes appeared in realistic and vivid depth.
FIGURE 1.5: A stereo pair used by Wheatstone in his stereoscope. (Wheatstone C. 1838. Contributions to the Physiology of Vision.—Part the First. On some remarkable, and hitherto unobserved, Phenomenaof Binocular Vision. Philosophical Transactions of the Royal Society of London 128: 371-94)
Soon, stereoscopes were all the rage in Europe; 3D movies followed in the 1890s and draw great crowds to this day. Just as stereoscopes provide two different views of the world, a 3D movie is made by filming the scenes using two cameras taking pictures from slightly different perspectives. When you put on your 3D glasses in the movie theater, each eye sees the pictures shot by only one of the cameras. Your brain does the rest of the work, fusing the two images into one scene seen in depth. The View-Master, a common toy sold even in supermarkets, works in the same way: it presents to each eye a scene drawn or photographed from a slightly different point of view.
The stereoscope and the View-Master make it easy to fuse two images by allowing each eye to see only one member of the stereo pair. But many people are able to bring images together without the help of a stereoscope by either crossing their eyes or looking “through” the page. See if you can “free-fuse” the right and left pictures in figure 1.5 and get the inner square to recede into or pop out from the page. If you fuse the two images by crossing your eyes, the center square will pop out, while if you fuse the images by looking “through” the paper, the center square will recede behind the outer one.
As a child, I had always wondered why other people seemed so entertained when they looked through a View-Master. I didn’t see Disney characters or Superman popping out at me through the toy. All I saw was a flat photograph. Now, in college, I understood at least theoretically what other people experienced. But could I actually imagine what they saw? My newfound knowledge made me wonder if people could imagine a quality, a sensation, that they have never experienced. I thought about people who were totally colorblind. They see no colors at all but live instead in a black, gray, and white world. Could they imagine what the color red looks like? What if they knew all about the science behind color vision? With this knowledge, could they see in their mind’s eye what they couldn’t see in the real world? I wanted to know the answer to these questions, but I didn’t think that I could ever find out. From all that I had read and learned in class about stereovision development, it was not possible for me, cross-eyed since early infancy, to gain stereopsis as an adult.


  • "Essential reading for people interested in the brain."—Temple Grandin
  • "[A] powerful account.... [Barry's] journey to attain the type of vision that most take for granted is inspirational and instructive."
  • "[Barry's] buoyant journey into stereovision is an eye-popping ride."
  • "[A] fascinating account.... In addition to recounting her personal triumph, Barry clearly explains the visual and clinical science needed to understand the significance of this achievement.... [T]his engaging book will leave both readers knowledgeable in the field, as well as those just looking to understand something about the visual process, pondering what else there is left to see."
    The Journal of Clinical Investigation
  • "Enticing.... [Barry] combine[s] a vivid and poetic account of her recovery with a detailed description of her treatment and the underlying science."
    Nature Neuroscience
  • "Fixing My Gaze provides a fascinating, informative, and beautifully written account of [Barry's] acquisition of stereopsis after vision therapy at the age of 48 years.... Barry's insights about her own vision provide wonderful insights into what it means to not have stereopsis, and the profound, life-changing effect of acquiring it."
    Optometry and Vision Science
  • "Readers of this book will be enriched by the experiences that Sue Barry recounts on her marvelous journey.... Part memoir and part science, Fixing My Gaze is a fitting tribute to the determination of a patient and her optometrist in challenging conventional wisdom and dogma."—Journal of Behavioral Optometry
  • "One axis of [Barry's] book is a graceful and grateful appreciation of a newly acquired ability to see the volume of space between objects and to see each object as occupying its own space - revelations that allowed her to live among and in the things of this world and gave her first movements of snow falling, trees branching, and a faucet arcing out of the sink.... The book's main contribution, however, is exposing the wrong-headed dogma that acuity and binocular vision can be restored only during a critical developmental period."
    New England Journal of Medicine
  • "[A] testament both to human physiology and spirit that permits someone to live with - and then change - a uniquely altered view of the world.... This book opens up the possibility that people can change their physical limitations, and that it is never too late to try."
  • "[An] exemplary and informative testimony to the probably lifelong plasticity of the brain."
  • "[Barry] tells a poignant story of her gradual discovery of the shapes in flowers in a vase, snowflakes falling, even the folds in coats hanging on a peg.... Recommended for all readers who cheer stories with a triumph over seemingly insuperable odds."
    Publishers Weekly

On Sale
May 26, 2009
Page Count
272 pages
Basic Books

Susan R. Barry

About the Author

Susan R. Barry is professor emeritus of biology and neuroscience at Mount Holyoke College, where she researched stereovision, plasticity, and coordination. She's written for and been covered by the New York Times, LA Times, Big Think, NPR's Morning Edition and Fresh Air, and elsewhere. You might know Barry as "Stereo Sue," a nickname bestowed by Oliver Sacks when he wrote about her for a New Yorker essay that was later anthologized in The Mind's Eye. She lives in Massachusetts.

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