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

6

THE SPACE BETWEEN

It must be repeated here that, before stereopsis is actually

experienced by the patient, there is nothing one can do or say

which will adequately explain to him the actual sensation

experienced. . . . Once the patient has experienced this new

sensation, he is only too anxious to use it again and again

until it is surelyand definitelyestablished.

—Frederick W. Brock

For most people, seeing in stereoscopic depth happens effortlessly. When we look at an object, our brain automatically compares the images seen by the right and left eyes. If the position of the image on the right fovea is a little different from that of the image on the left fovea, the brain commands the eyes to turn in or out to minimize this disparity. When I looked at something before I acquired

stereovision, the object cast an image on the fovea of one retina but on a noncorresponding, nonfoveal point of the other retina. The disparity between these two images was too great for me to automatically make corrective convergence or divergence eye movements. I needed to learn very consciously how to aim my two eyes. To do this, I needed feedback that showed me where my eyes were pointing in space so that I could redirect them. Fortunately, Dr. Ruggiero had a solution, a simple device invented by optometrist Frederick W. Brock.

Dr. Brock was born in Switzerland in 1899 and came to the United States in 1921 to attend the Columbia School of Optometry. He was an inventive and playful man. When, one summer, he found himself without an outdoor grill, he simply fashioned one out of a garbage can lid. When his daughter went to summer camp, he sent her letters peppered with little drawings and visual puns. His lifetime passion was helping people with their vision, and in the mid-1900s, he helped many strabismics learn to see in stereo. Not content to test merely what his patients could not see, he performed many thoughtful experiments to determine what and how his patients did see. From these investigations, and with his typical resourcefulness, he built some of his own vision aids out of everyday items. In so doing, he taught us that you can learn a lot from a simple piece of string.

When I learned to use the “Brock string,” I received the feedback that I needed to know where my eyes were

pointing and then to redirect them so that they were aiming simultaneously at the same point in space. During one vision therapy session, Laurie took a five-foot-long piece of ordinary string on which were threaded three brightly colored beads. She attached one end of the string to a doorknob and gave the other end to me. She told me to walk backward until the string was taut and then to put the free end of the string up to the bridge of my nose (Figure 6.1). She slid one of the beads along the string so it was only a few inches from my face. Then she asked me to look at the bead and tell her how many strings I saw.

I thought that Laurie had asked a pretty ridiculous question. I looked down the string at the bead, saw one string, and told her so. Laurie nodded and then, while gently moving the string up and down, asked me to look again at the bead and tell her what I saw. To my surprise, I momentarily saw two strings emerging in front of the bead and two strings receding from the bead as in Figure 6.2.

FIGURE 6.1: Using the Brock string. (© Rosalie Winard)

Why did I see four images of the string, two in front and two behind the bead, when only one string existed? To help me understand, Laurie drew the picture shown in Figure 6.3.

FIGURE 6.2. (© Julia Wagner)

In this picture, the two foveas are pointing at the same place in space, the fixation point, which in my case was the bead on the string. The image of the bead fell on corresponding points on my two retinas, and my brain fused the bead images from the right and left eye into one. There is a small area right in front of and behind the fixation point where the images fall on almost corresponding retinal points. This area is called Panum’s fusional area, and I could also fuse the images of those parts of the string that fell within this area. All other parts of the string fell outside Panum’s fusional area and were seen as double. So, I saw four images of the string, two in front of and two behind the

bead.

FIGURE 6.3: Panum’s fusional area. Each dotted line represents the line of sight for each eye. (© Margaret C. Nelson)

This was fantastic feedback. Now I knew if I was suppressing the input from one eye because then I would see only one image of the string in front of and behind the bead. I could tell when my eyes were both aimed at the bead because then the string images would appear to emerge symmetrically from its center. With the Brock string, I could tell where in space my two eyes were looking.

Initially, I could “get” the four string images only when I

brought the bead very close to my nose. If I looked at a more distant bead, I would lose one of the string images behind the bead. Getting the second string image into consciousness was a strange experience. I would sometimes “feel” that the second string image had just been there, that I could almost reach out and touch it. If, at that moment, I made a slight eye movement, I might get the second string image to appear. This was incredible, a way of controlling what I consciously saw.

At other times, I would see all four string images, but they would meet not at the bead but in front of it. Then I knew I was aiming my eyes in front of the bead. I would touch the bead with my fingers so that my arm movements could help me to judge how far away the bead was and thus to direct my eyes accordingly.

Once I developed a range of distances over which I could properly aim both eyes at a bead, Laurie started me on a task involving two beads. She slid one bead close to my face and the other two feet away. She told me to look with both eyes first at the close bead, then at the further one, then back at the close one, and so on. She was asking me to converge my eyes to see the close bead and diverge my eyes to see the distant one. With a determined and conscious effort, I could turn in my eyes to point them at the closer bead and turn out my eyes to aim them at the more distant one. I knew that I had performed the procedure correctly if I could see the four string images emanating from the fixated bead. But, above all, I could feel my eyes

long session with the Brock string. I got into my car, sat down in the driver’s seat, placed the key in the ignition, and glanced at the steering wheel. It was an ordinary steering wheel against an ordinary dashboard, but it took on a whole new dimension that day. The steering wheel was floating in its own space, with a palpable volume of empty space between the wheel and the dashboard. Curious and excited, I closed one eye and the position of the steering wheel looked “normal” again; that is, it lay flat just in front of the dashboard. I reopened the closed eye, and the steering wheel floated before me.

I wondered if I was seeing in stereoscopic depth but reminded myself that this should not be possible. It was one day after my forty-eighth birthday. I was more than forty years beyond the critical period for the development of stereovision. Since the sun was low in the sky and shining light into the car at an odd angle, I told myself that the fading light must have created this unusual illusion.

The next morning, I practiced the Brock string procedure for ten minutes before getting in the car to begin my routine drive to work. As I looked up to adjust the rearview mirror, the mirror popped out at me, floating in front of the windshield. I was transfixed. Throughout the day, my stereovision would emerge—intermittently, fleetingly, unexpectedly—bringing me moments of absolute wonder and delight. The most ordinary objects looked so beautiful. A large sink faucet reached out toward me, and I thought I had never seen such a lovely arc as the arc of the faucet.

The grape in my lunchtime salad was rounder and more solid than any grape I had ever seen before. I could see, not just infer, the volume of space between tree limbs, and I loved looking at, and even immersing myself in, those inviting pockets of space.

This new way of seeing was confusing as well, as I soon discovered while walking to my office one day. On the ground floor of the biology department where I work, we have a skeleton of a large horse displayed along with skeletons of smaller creatures and cases full of stuffed birds. I have passed these rather spooky displays every morning for the past ten years, giving them little attention or thought. But just eight days after I began to see in stereo depth, I was walking to my office and happened to glance directly at the horse’s head. The horse’s skull, with its large teeth and two empty eye sockets, loomed so far out in front of its body that I thought it was moving toward me. I jumped backward and cried out. Fortunately, no one was around to witness my panic.

This sense of objects projecting straight toward me was novel. Objects had always appeared a little to one side, depending upon whether I was paying attention to the input from my right or left eye. Now that I was developing stereovision, I saw the horse’s head projecting straight toward me in the direction of a virtual “cyclopean eye.” The same was true for my view of car bumpers, open doors, light fixtures, tree limbs, and outside corners of large buildings.