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Teaching Cross-eyed People to See in 3D

A new study indicates that stereovision can develop in adulthood.

I have been cross-eyed (strabismic) and stereoblind since early infancy. Since the first months of life, I looked at things with one eye and turned in the other. Three childhood surgeries made my eyes look straighter but did not change the way I used them. I continued to look with one and turn away the other. My crossed eyes may have been less noticeable after the operations, but I still suppressed the input from the turned eye and saw an abnormally flat, less detailed, and less vibrant view of the world. I did not see in 3D; I was stereoblind.

As I grew older, my vision became more troublesome. So at age 48, I consulted a developmental optometrist. I went to see her not to gain stereovision; that possibility was not even on my radar. Instead, I spoke to her about my unstable gaze. My view of distant objects was jittery which made driving difficult and frightening. I wanted to gain more competence and confidence for chauffeuring my children around town.

Over the next year, my developmental optometrist, Dr. Theresa Ruggiero, taught me something that most infants learn within the first months of life - how to aim both eyes at the same place in space at the same time. This required a great deal of practice using a variety of elegantly-designed vision therapy tools. To my astonishment, I began to see in 3D. Ordinary things looked extraordinary. Sink faucets reached out toward me, hanging light fixtures seemed to float in mid-air, and I could see how the outer branches of trees captured whole volumes of space through which the inner branches penetrated. Borders and edges appeared crisper; objects seemed more solid, vibrant, and real. I was overwhelmed by my first stereo view of a snowfall in which I could see the palpable pockets of space between each snowflake.

Yet, for almost three years, I told no one, outside of family and friends, about my new vision. I did not think anyone would believe me. For more than half of a century, the conventional wisdom among scientists and physicians maintained that stereopsis, or the ability to see in 3D, must develop during a critical period in early life. After that time-frame, the brain would rewire itself in a way that permanently prevented the ability to see in stereo . It was impossible, so most believed, for an adult like me, cross-eyed since infancy, to develop stereopsis. As a college neurobiology professor, I had described to my students the experiments supporting this conventional wisdom and even used my own stereoblindness as a way to illustrate how well the theory of critical periods fit reality.

But, finally, one night in December of 2004, while washing the dinner dishes, I decided I had to put my story down in narrative form. As luck would have it, my children and husband got involved that evening in a very long Monopoly game, and I retired to a quiet room in my house where I wrote a detailed letter to the neurologist and author, Oliver Sacks. I did not know Dr. Sacks personally, but I had read his books in which he stressed the importance of listening carefully to his patients. Although I was not his patient, I hoped he would listen to my story as well. Dr. Sacks responded to my letter immediately and came to visit me and my optometrist. He wrote a story about me, "Stereo Sue" which appeared in the June 19, 2006 issue of The New Yorker, and in 2009, I published my own book, Fixing My Gaze: A Scientist's Journey into Seeing in Three Dimensions.

Most scientists were intrigued by my story, but some maintained that I must be "one in a million." Yet, I knew that I had a very standard case of infantile strabismus and that my story was not unique. What set me apart from others was the optometric vision therapy that I received. Most people with strabismus are told that, if they do not receive surgery within the first year of life, they can never attain stereovision. If you are given this pronouncement by experts in authority, then the chances of you seeking optometric vision therapy are minimal. Who are you to question fifty years of scientific wisdom? Yet, I had learned through my own experience that the only way for someone like me to gain stereovision was to be taught how to coordinate the eyes. I had to undertake vision therapy or training to change the way I saw. A Catch-22 indeed.

Recently a paper appeared in the Proceedings of the National Academy of Sciences that supports my claim. In this paper, several stereoblind people with a longstanding strabismus underwent vision training using a digital stereoscope. First they learned how to fuse or merge the images coming from their two eyes, and then they practiced fusing images that evoked a 3D view. This experience is similar to the stereo experience you enjoy when wearing special glasses and watching a 3D movie. Afterward, the subjects in this study reported the remarkable sensation of seeing 3D for the first time in real life.

The title of this paper is "Recovery of stereopsis through perceptual learning in human adults with abnormal binocular vision." Perceptual learning refers to an improvement in performance and perception brought about by practice of specific tasks. Exactly. Sometimes, practice comes naturally, as when we are infants and first teach ourselves to see and move. As adults, however, our habits have become highly individualized and entrenched. For a cross-eyed individual to see in stereo, lifelong viewing habits (looking with one eye and turning and suppressing input from the other) have to be replaced with more efficient ones (aiming both eyes at the same place in space and merging their input). This takes training and practice with specific tasks. If I was "one in a million," it's only because I was given the relatively uncommon opportunity to undertake optometric vision therapy and the encouragement and tools to practice.