Ординатура / Офтальмология / Английские материалы / Fixing My Gaze_Barry, Sacks_2010

snowflakes. Overcome with happiness, I forgot all about lunch and stood quite still, completely mesmerized by the enveloping snow.

Initially, I couldn’t understand how my acquisition of stereopsis could have evoked this powerful sense of immersion in my surroundings. According to textbook descriptions, stereopsis provides an increase in depth perception only for objects at the distance at which your two eyes are aiming. But my whole sense of space had changed. Heather Fitzpatrick, strabismic since age two, began vision therapy with optometrist Carl Gruning. She echoed my feelings when she described her first experience seeing in 3D:

The coolest thing is the feeling you get being “in the dimension.” It is alive and open and you can actually see things floating by you as you walk and the depth is everywhere. . . . [I]t is ahead, but it is also down towards my feet. . . . [T]ables looked really low and walls looked really high and sitting at a desk I just wanted to put my hands all over it and push my hand in between the spaces of the objects on the desk.

To understand my new sensations, I took a careful look at some of the older scientific studies of binocular vision and stereopsis. Here I found an explanation for why I was

seeing my surroundings in layers and layers of depth. As I describe in chapter 6, objects located in front of and behind the plane at which you are looking cast their images on your retinas outside Panum’s fusional area. Thus, the images fall on regions of your two retinas too disparate to be fused. If you have normal vision, however, you unconsciously make a judgment as to whether or not the images come from the same or different objects. If the images come from the same object, then you unconsciously compare where the images fall on the two retinas and use this information to interpret how far away this object is. Although the estimate of the object’s exact location or depth is not precise, you have an impression of its “nearerness” or “furtherness.” It was this newfound sense of stereo depth for objects all around me that gave me the powerful feeling of being enveloped by the world.

But there were even more surprises in store: I started to see far more depth while moving, a phenomenon called motion parallax. As you sway, for example, to the right, objects near you appear to move to the left, while more distant objects appear to move with you to the right, and the closer objects seem to move at a faster rate than the distant ones. This difference in the relative motion of objects provides you with one of the most important depthperception cues.

Since motion parallax can be seen with one eye, I, like most other scientists, assumed that I had always obtained a good sense of depth through motion parallax. Then, one

day in the fall of 2005, I learned that I was mistaken. I had based this assumption on the erroneous premise that a strabismic sees the world like a person with normal vision who has simply closed one eye.

On that fall day, I was taking my dog for a walk along our usual route. My schnauzer felt compelled to sniff every blade of grass, leaving me bored and impatient, swaying absentmindedly underneath a dense network of tree branches. While gazing up at the trees, I was startled to see the branches in layers of depth. I saw how the outer branches captured and enclosed a palpable volume of space into which the inner branches permeated. I could make sense of the whole intricate network. From that day to the present, I have always walked to work so I can pass under the trees and get this stunning sense of three dimensions.

Since I was surprised by my improved sense of depth through motion parallax, I went to the library to read more about the topic. I learned that the same neurons and circuits that give us stereopsis may also provide us with our sensation of depth through motion parallax. So, my ability to see in stereo also translated to a heightened sense of depth through motion.

What’s more, experiments by neurobiologist Mark Nawrot and his colleagues at North Dakota State University have identified the signals coming into the brain that provide us with our sense of depth through motion parallax.

These studies reveal that pursuit movements of the eyes allow us to judge depth in this way. To keep my gaze fixed on one spot as I swayed under a tree, I unconsciously moved my eyes right when my head moved left and vice versa. My brain compared the direction of these eye movements with the way the different branches appeared to move. If a branch looked like it was moving in the same direction as my eye movements (that is, opposite to my head movement), then it was perceived as being closer to me and vice versa.

The same researchers have also reported that individuals with crossed eyes and amblyopia have a poor sense of depth through motion parallax. These results make sense to me because crossed eyes in infancy can impair pursuit movements. As my pursuit movements improved with vision therapy, I could make better use of motion parallax.

However, I don’t think these studies provide the whole explanation. The impression I had of the volume or space between the branches was a recent sensation for me, one provided by my newfound experience with stereopsis. Combine motion parallax with the sense of palpable space from stereopsis, and the effect is dramatic. For me, as well as for many of my formerly stereoblind friends, one of the greatest surprises and delights of our new vision has been this incredible sense of depth while moving.

But the visual surprises did not stop there. A few months

after my walk with the dog, my daughter and I decided to watch the film Legally Blonde on her laptop. During the opening credits, the movie camera swept across a large vase of flowers, and I immediately saw the whole collection of flowers in dramatic depth.

“Whoa,” I said to Jenny, “did you see that?”

“See what?” Jenny asked, but I had already rewound the movie to see the scene again. I would have watched the scene several more times if I’d thought Jenny would tolerate it. When the cameraman swept the camera around the vase, I saw the flowers on the screen as if I were rotating around them and this gave me a great sense of 3D. This sensation is similar to the ability to see “structure from motion,” that is, to envision the 3D form of a solid object by watching its moving shadow. So, I went back to the library to read about structure from motion and was not surprised to learn that the capacities to see depth through stereopsis and to determine structure from motion are linked. People with poor stereopsis have a poor sense of structure from motion. As I gained stereopsis, I could see more depth while watching videos. Even though movies and TV shows are displayed on flat screens, I actually enjoy watching them much more now than I did before developing stereovision.

As much as I loved my newfound ability to see the empty space around me, my new sense of immersion could be unnerving and downright frightening at times. On a trip to Hawaii with my husband and children in the early stages of my stereo life, I visited a scenic viewing spot overlooking a beautiful canyon. I went right up to the protective railing to take in the view. Suddenly, I felt like I was floating unsupported high above the incredibly deep canyon. I quickly backed away from the railing, moving to a safer spot behind other tourists and the view below. Later on that day while hiking, I felt panicked every time my children or husband approached the cliff’s edge. I had to use all of my self-control not to scream at them to get away from the cliff.

When I returned home, I realized that my stereovision contributed to this new fear of heights. I was standing on a familiar bridge overlooking a waterfall on the Mount Holyoke campus when, again, I felt this new and unnerving sensation of floating unmoored, this time above the water. Since the bridge was not high and the scene very familiar to me, I could control my panic, relax, and eventually enjoy the sensation.

Despite my experience in Hawaii, my adjustment to a world in stereo went fairly smoothly because my worldview changed gradually. First, I saw more depth only for close objects and then gradually for objects further away. I had a chance to savor each new sight and time to learn how to move through an increasingly depth-filled world. For some people, however, stereopsis emerges suddenly and in full

measure, and this abrupt change can be very disorienting and frightening.

When Cyndi Monter, a forty-eight-year-old woman with strabismus and amblyopia, was attempting to eat her lunchtime salad, her 3D vision “clicked in.” Suddenly, she was looking at a salad in which tomatoes popped out at her and the lettuce leaves appeared to lie in very distinct layers of depth. Cyndi was so alarmed that she couldn’t eat her food and headed straight to her optometrist instead. As she drove, roadside trees and signs seemed to loom threateningly toward her. Her optometrist, Carl Hillier, ran some tests and informed her that she was now seeing in stereoscopic depth. He had tried to warn her that her work in vision therapy might bring about these changes, but without any prior experience with stereopsis, Cyndi couldn’t imagine what he meant.

Both Jennifer Clark and Tracy Gray experienced an abrupt onset of stereovision as well—and with it a wideangled view of the world. Tracy wrote to me about the moment when her vision suddenly transformed:

I was sitting on the sofa in my living room about midnight, and I immediately saw the whole room and all the objects in it in 3D. I was able to take in so much more of the room than I did before (previously, I would generally fixate on whatever object I was looking at; I didn’t have tunnel vision . . . but I don’t think I really took it all in). When I got up from the

sofa and walked around, for the first time I was walking through the three-dimensional space, clearly walking among the objects. It was so overwhelming and a little disorienting, that I went straight to bed. When I woke up, I was wondering if it was still going to be there, and when I opened my eyes, I saw the whole room, and my ceiling fan was sticking out.

Jennifer Clark, who underwent vision therapy with optometrist Caroline Hurst of St. Neot’s in England, notes the following:

I had about four months of eye exercises with initially only some small effect. Then suddenly my vision changed to 3D completely, in one shot, whilst walking down the corridor at work one day. It was very dramatic as my peripheral vision suddenly filled i n on both sides and the corridor ceiling changed completely from a parallelogram to a rhombus.

For weeks, Jennifer struggled to integrate her new views with her old ways of interpreting the world. She would run her hands along the TV set to reconcile its threedimensional feel with its new three-dimensional look. For many months, she was plagued with motion sickness. Buildings appeared to lurch up and down as she walked along the road. With more therapy, Jennifer’s worldview stabilized, and she began to relish her new way of seeing.

Similarly, Tracy initially felt disoriented and overloaded:

It took a long time to integrate it [stereovision] and

have it not be overwhelming. [My view of the world] was too bright, and my eyes watered for several days. [My optometrist] recommended no television, no reading, no driving, and as little work at the office as possible, and to work to integrate it by walking outside in short segments and trying to take in the whole scene. For the first several days, I was only able to stay at work a few hours a day. I had to spend a fair amount of time each day just lying down with my eyes closed. It was about a month before I drove a car again. . . . With my vision like it is now though, I can shut one eye and still see more than I used to with two.

Tracy’s comments helped me to understand my own experiences, for there were many days when I felt both exhilaration and exhaustion. My old way of seeing had given me the world in smaller doses. With my new outlook, all of my senses were awakened. The world was keenly present. While I used to play the piano once a week, I started to play every day, pausing often in the middle of a piece, captivated by a given theme or combination of musical intervals. Like my view of the coats mentioned at the beginning of this chapter, I saw much more texture and shape in the objects around me. I felt as if I could touch and manipulate everything with my vision alone and finally understood what philosopher Maurice Merleau-Ponty meant when he said, “Vision is the brain’s way of touching.”

But on many days, I also experienced sensory overload. I

used to come home from work and immediately tune in to the radio to catch up on the news. In the early months of my vision therapy, I avoided the news altogether, listening to familiar music instead. I worried that I had lost concern for world affairs but later realized that I was perceptually tired. Now, my ordinary day was like a day in Disney World, a strange foreign city, or a tropical forest.

Once my vision changed, I was so preoccupied by my new sense of the world that I took on fewer responsibilities at work. I justified this by reminding myself that I was also caring for aging parents and growing children. Yet, in the past, I had willingly juggled many demands. What I was experiencing now was different. Most people learn to see when they are infants, at a time in their lives when they are cared for, are free to get cranky, and enjoy lots of naps. I was relearning how to see as a responsible, contributing adult. While I went through all the motions at work, I desperately wanted to be left alone, to be quiet and reverent, to take in one long, delicious look after another. I disappeared on long, solitary walks. I was at a loss as to how to explain this to my colleagues and friends.

Most surprising to me was that the change in my vision affected the way that I thought. I had always seen and reasoned in a step-by-step manner. I saw with one eye and