Ординатура / Офтальмология / Английские материалы / Fixing My Gaze_Barry, Sacks_2010
.pdfI did, and, in a single moment, everything changed. The rope circle shrank in diameter and appeared to float in front of the wall. I had made a perceptual shift. I knew intellectually that the rope circle was projected onto the wall and that the wall was a stable, unmoving object. But in order to fuse the rightand left-eye images, my eyes were aimed not at the wall but at a location in front of the wall. When I tapped the wall and used my sense of touch to gauge its distance from me, I learned that it was not at the distance where my eyes were directed, not at the depth of the fused image of the rope circle. I had to come up with a new interpretation—the rope circle was floating in space in front of the wall.
A normal binocular viewer makes this sort of interpretation all the time. But this way of seeing is new to someone still learning how to use stereopsis cues to see in depth. As Dr. Brock noted in several of his articles, “A strabismic speaks a different language.” With procedures like this, I learned to speak the language of stereovision.
FIGURE 7.9: As the fused image of the rope circle appears to recede in space, it looks larger. As it appears to float forward, it looks smaller. (© Julia Wagner)
I experienced another curious effect with the rope circle vectogram that optometrists call the small in, large out (SILO) phenomenon (Figure 7.9). When I first saw the rope circle float forward in Dr. Lessmann’s office, the rope circle also appeared to shrink in size. When the rope circle appeared to recede away from me, it also looked larger. This was counterintuitive as I knew that something coming closer to me should appear larger, while something moving away should get smaller.
A phenomenon called size constancy helps to explain this sensation. Size constancy allows us to judge accurately the size of people and objects located at different distances. If you look at a person across the room from you, he doesn’t look tiny; he still appears life-size. Yet, if you use
your fingers to outline your view of the person in order to determine the amount of space that he takes up in your visual field, you will notice that the amount of space is really quite small. Somehow, you have adjusted for the fact that the image of the individual will take up less and less retinal space as the individual moves away from you.
FIGURE 7.10: The corridor illusion. (Deregowski J in Gregory RL, Gombrich EH (ed), 1973. Art and Illusion, London: Duckworth)
The phenomenon of size constancy altered the way I saw the fused image of the rope circle in the Polaroid vectogram. As Dr. Lessmann slid the two sheets toward each other, it appeared, thanks to stereopsis, that the rope circle was coming toward me. But I was fooled. The rope circle was still projected onto the wall a fixed distance away. The image of the rope circle on my retinas had not changed size. It was only my newfound ability to use retinal disparity cues that gave me the illusion that the rope circle was closer. If the fused image had been a real object actually moving toward me, I would have unconsciously made an adjustment for the size of that image so that the object wouldn’t appear to enlarge as it approached. So, when the rope circle appeared to move closer to me, I saw it as smaller as well.
The “corridor illusion” also makes use of the phenomenon of size constancy. The two poles in Figure 7.10 are actually the same height. However, we judge the pole on the right to be further away due to the decreasing size and converging lines of the floor, wall, and ceiling tiles. Since we interpret the right pole as further away, we see it as taller. Even if we know the cylinders are the same height, it’s incredibly difficult to view them as such.
The SILO phenomenon helps optometrists to know whether a patient is seeing with stereopsis. The doctor will slide the sheets of the vectogram apart and ask the individual to report any size changes that occur, along with any sensation that the image is floating toward or away from them. If the patient reports the appropriate size changes, then the optometrist knows that he or she is using retinal disparity cues to see.
When I got back to Massachusetts after my meeting with Dr. Lessmann, I purchased a set of vectograms for my own use and worked at seeing the rope circle float in space. Initially, this was quite difficult. Most people learn to integrate these cues as infants. They begin to see in stereo at a time when they first start swiping at those brightly colored toys hanging from their cribs and strollers. With their first reaching movements and initial attempts to crawl, they learn to combine monocular and binocular cues to depth and then hone these skills throughout the first ten years of life. For me to see the “float,” I had to learn to trust my new stereopsis cues and combine them with my old monocular ways of inferring depth.
As I made increasing use of my stereopsis cues, I was often surprised by new sights. In the past, any view framed by a window or mirror always appeared at the plane of the
frame. Now, when looking out a window, I can see the space between the window pane and a tree outside. At first I was startled when looking in the mirror to see my own reflection not at the plane of the mirror but some distance behind it. I still get a kick out of walking back and forth in front of the mirror, watching my own reflection recede and come forward in the reflected space.
FIGURE 7.11: A non-random dot stereogram. If you are able to cross your eyes and “free-fuse” these two images, the inner circle will appearto pop out. If you fuse the images by looking “though” the page, the inner circle will appear to recede behind the paper.
Although I saw the steering wheel of my car “pop out” in 3D in 2002, I did not see depth or hidden images in random dot stereograms until much later. I was quite happy
about “getting” some random dot stereograms because many scientists believe that seeing images in these stereograms, images that cannot be seen with monocular cues alone, is the ultimate proof that a person has stereopsis. In the non-random dot stereogram shown in Figure 7.11, it is easy to see how the fused image will consist of two circles. However, in a random dot stereogram, the stereo pair consists of an array of confusing dots that do not form an image unless they are seen by both eyes. If you cross your eyes to free-fuse the stereo pair in Figure 7.12, a central square will appear, as if by magic, to float in front of the background. (Free-fusing a random dot stereogram is difficult to do even for people with normal vision.)
Once I was able to “get” some of the easiest random dot stereograms, I found that I could also see some of the hidden images in the popular Magic Eye books. To see the hidden images, I had to override monocular cues that told me I was looking at a flat drawing and place ultimate trust instead in what my stereopsis was telling me.
FIGURE 7.12: A random dot stereogram. (Stereogram by Benjamin Backus)
Learning to see with stereopsis changed more than my sense of empty space. It altered my perception of being in space and moving through it. Since I had had imperfect vision, I relied more on the way my body felt and less on what I saw in order to get around. This became clear to me several years ago when on vacation with my family at a tropical resort. Every day, we would walk along a landscaped and terraced path from our cabins to the dining hall. One night, the electricity went out, and we had to make our way back to the cabins by the dim light of a crescent moon. I was surprised to discover that everyone but me
was hesitant about finding their way back. Even though I was the one who usually got lost, I confidently led my family back to the cabins. I realized that without knowing it, I had memorized the route, not by counting my steps but by learning the timing and rhythm of my body’s movements as I made my way from one location to the next. When we came to an outside staircase, I knew how many steps we needed to descend because of the familiar rhythm of my own steps as I went down the stairs. Back home from vacation, I understood why my children always turned on the light when they climbed up the stairs.
What was true for me was also true for my stereoblind friends. Many have told me that they are dedicated dancers and ice skaters, but very few described a passion for playing ball. Indeed, individuals with binocular vision disorders tend to enjoy activities in which they feel their body move through space rather than watch and interact with things in motion. Not surprisingly, my favorite activities in the past had been swimming and snorkeling, activities in which I felt myself enveloped in the water and had a strong sense of my body in motion.
When I gained stereopsis, I felt like I was immersed in a medium more substantial than air, a medium on which tree branches, flower blossoms, and pine needles floated. I wondered if this sense of the air was what Monet spoke about in the quote at the beginning of this chapter: “I want the unobtainable. . . . I want to paint the air.” Or perhaps Eric Woznysmith, a strabismic, echoed Monet’s thoughts
when he described what it was like for him to see with stereopsis. Eric had studied drawing and learned that artists pay attention not just to the objects they will draw but also to “negative space,” that is, the space, or the air, to the sides, in front of, and behind objects. When he gained stereovision, he told me that he could see one hundred times more negative space. Indeed, when I walk in a forest these days, I pay more attention to the pockets of space between the branches and trees than to the trees themselves. I seek out particularly beautiful volumes of space and like to immerse myself in their pockets.
This new sense of immersion in space is completely captivating and enchanting. Indeed, Rachel Cooper, who had a form of amblyopia (lazy eye), notes that before gaining stereopsis, “It felt like I was here and everything I was looking at was over there. I couldn’t visually perceive or measure the space between me and other objects.” Now, Rachel sees in 3D, and she says, “It feels like I am in the world. Empty space looks and feels palpable, tangible— alive!”
Rachel’s words remind me of a wonderful sight I experienced one late winter day in the early stages of gaining stereopsis. I rushed out of the classroom building to grab a quick lunch, and I was startled by my view of falling snow. The large wet flakes were floating about me in a graceful, three-dimensional dance. In the past, snowflakes appeared to fall in one plane slightly in front of me. Now I felt myself in the midst of the snowfall, among all the