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

the image of the X is cast on corresponding retinal points. For most people, images that fall on corresponding retinal points appear in the same subjective visual direction. In other words, we interpret these rightand left-eye images as emanating from the same point in space. We see one X in one direction, in this case, straight ahead.

FIGURE 4.2. (© Margaret C. Nelson)

Most of us interpret objects that fall simultaneously on the fovea of both eyes as located in the same place in space.

vision will see. (© Margaret C. Nelson)

Most people experience the weird impression that there is a hole going right through their hand. This strange illusion occurs because our brain merges the images from the two eyes by combining inputs that fall on corresponding regions of the retinas of both eyes. Since one eye registers the image of a tube on the retina where the other eye registers an image of a hand, we interpret the tube and the hand as coming from the same point in space and see the tube as running through the hand.

One of my friends, nicknamed Red Greene, discovered for himself the intimate relationship between where his eyes were aiming and where he located objects in space. Although Red was a housepainter who climbed ladders and painted houses for many years, he was amblyopic: since early childhood, he had had very poor vision in his right eye. For most of his life, the world seen through his right eye looked like the view through the frosted glass of a shower door.

When Red was in his fifties, the vision in his left or “good” eye deteriorated, a situation that could have proved dire except that acuity improved dramatically in his weak right eye. In fact, his right eye now has 20/20 acuity. While this turn of events may seem surprising, similar cases have been documented in which the amblyopic eye regains vision following severe impairment of the “good” eye.

However, Red had a very hard time transitioning from

seeing predominantly with the left eye to seeing with the right. His world was unstable. Fluorescent lights bothered him. Reading was almost impossible. He became nervous and anxious around crowds and in busy environments. He didn’t last a single day at Disney-land. Red consulted his local ophthalmologist and traveled to Boston to consult with a neuro-ophthalmologist at the New England Eye Center of Tufts University. Even though Red went to an excellent center for vision research and care, no one could figure out how to help him or even why he was complaining. After all, he had perfect eyesight in one eye. The best they could do was to suggest that Red wear a patch over his left eye and use only his good right eye for seeing. When this didn’t work, they suggested that he add a black-tinted lens to his glasses to further reduce the sight of his left eye. He resigned himself to the fact that nothing more could be done.

But Red’s wife, a nurse, learned from her colleagues that a local optometrist treated many stroke patients and might be able to help. During Red’s first visit, the optometrist evaluated how he used his two eyes together and noticed something that none of the other doctors had. Although his left eye was no longer functioning well, it was still his dominant eye; for almost all of his life, his brain had regarded images from his left eye as the “real” images. His left eye directed where Red would look and move, a real disadvantage to him now that only his right eye saw clearly. What’s more, because he was amblyopic, his two eyes did

not work well together. The optometrist put prisms in Red’s eyeglass lenses that aligned the visual fields of his eyes. Immediately, Red felt a calmness come over him. He now saw with a steady gaze and could read comfortably. He got up from his chair and marched around the examining room in happy disbelief. Red’s vision and life were transformed.

Red’s difficulties highlight the problems with spatial localization that arise if your eyes are misaligned. If you can’t direct both eyes to the same point in space, the two images of a single object do not fall on corresponding points of your retinas. How do you interpret the unique location of an object when your two eyes present conflicting information? Throughout most of his life, Red coped with this issue by discounting, or suppressing, the input from his right eye. He used only his left eye to locate objects around him. Suppression does indeed solve the spatial-location problem, but it creates another.

When we suppress the input from one eye, we lose up to half of the visual information that’s available to us. So, some people with strabismus unconsciously develop an additional strategy that allows them to make simultaneous use of both eyes, even though the eyes are looking at different regions of space. To do this, their brain abandons the idea that the two foveas point to the same location in space. Instead, they aim one eye at a target and interpret the location of images that fall on the central retina of that eye as straight ahead. (Assume here that the head is turned neither right nor left but is oriented straight forward.)

Though aware of images that fall on the fovea of the strabismic, or turned, eye, they localize these images with reference to the “straight ahead” direction. If, for example, the strabismic eye is turned in by 10º, then images formed on the fovea of that eye are interpreted as being located 10º from straight ahead. The fovea of the straight eye no longer corresponds with the fovea of the turned eye but instead with an area of the turned retina shifted by 10º.

This is how my friend Bruce Alvarez sees. He is a computer scientist, a good ice skater, and a strabismic. His response to the hole-in-the-hand experiment is different from what you probably observed if you tried it. He sees the hand in half of his visual field and the tube of paper in the other half. Bruce does not merge the input from the two eyes but instead reports what each eye sees separately. Although this is not the response that most people have, his view of the world, in this case, is actually the more accurate one. After all, the tube of paper is to one side of the free hand. Indeed, it’s surprising to him that someone would have a different response. Bruce’s unusual way of seeing is called anomalous correspondence, an adaptation that often takes months or years of childhood strabismus to develop. It is a striking example of neuronal plasticity, a change in the way the brain handles sensory input.

Dr. Frederick W. Brock, an optometrist practicing in the mid1900s, was an expert in treating strabismus and wrote many papers on the subject. He called patients with good acuity in both eyes and anomalous correspondence “fully

adapted strabismics.” Although a strabismic can suppress information in order to have a single view of the world, a great deal of data is lost. But with anomalous correspondence, a strabismic can effectively make simultaneous use of the information from both eyes. In fact, Dr. Brock described a cross-eyed truck driver who gave up driving his truck after his eyes were surgically aligned. Prior to surgery, he could look straight ahead at the road with one eye and look at a sign at the side of the road with the other. But once his eyes were straightened, he lost this useful panoramic view. He found it much more difficult to drive when he had to turn both eyes to read a road sign instead of reading the sign with one eye and keeping the other eye on the center of the road.

While investigating the ways his patients adapted to visual problems, Dr. Brock studied the work of neurologist Kurt Goldstein. In his book, The Organism, Goldstein stresses that a patient’s symptoms are often a response to, or coping mechanism for dealing with, his or her disorder. With Goldstein’s research in mind, Brock noted that a strabismic “speaks a different language.” Eye misalignment hinders a person’s ability to know where objects are located; as a result, strabismics develop different ways of seeing. Indeed, we cannot understand strabismus or any other visual disorder unless we consider how we use our vision on an everyday basis just to move from place to place.

Like cross-eyed individuals, people with many different

deficits or disorders find surprising ways to adapt. Before becoming an astronaut, my husband was a physician specializing in physical medicine and rehabilitation. He took care of many patients with neuromuscular problems. He recalls one day as a new doctor when he was asked to examine a man with polio. Dan introduced himself to the man sitting on the examination table and proceeded to test the strength of the patient’s leg muscles, quadriceps and hamstrings. He found them to be far weaker than normal and, after leaving the examining room, reported his results to the attending physician.

“Can the patient walk?” asked the senior doctor.

“I didn’t test that,” Dan said—but he assumed that the man could not.

So, Dan and the attending physician went back into the examining room and asked the patient to stand up and try to take some steps. To Dan’s astonishment, the man got up off the table and confidently walked across the room. What’s more, to Dan’s unschooled eye, the patient’s gait looked pretty normal. By any textbook description of locomotion, this patient should not have been able to walk —he simply didn’t have the necessary strength in his quadriceps and hamstrings. But he had adapted; he had learned to recruit additional muscles in his legs because he needed to find a way to move in everyday life. Dan had been so focused on testing each individual muscle that he didn’t ask the patient how he managed day to day. To Dan,

this experience was particularly ironic because his own father had suffered from polio as a boy. His father’s leg muscles were also weak, and Dan knew that, but until this incident, he had never wondered how his father had relearned to walk or climb stairs.

Adaptations, however, can come at quite a cost. No one knows this better than my friend Rachel Hochman. Rachel is a botanist, educator, and naturalist. She has accomplished all of this despite having been born premature and with a cataract, a cloudiness of the lens, of her right eye. When Rachel was a baby, she couldn’t see clear images with her right eye, so she depended upon her left eye for vision. When she was four years old, her surgeon removed the clouded lens and replaced it with a contact lens, but by this time, her right eye’s vision was seriously compromised. For the next four years, Rachel’s left, or “good,” eye was patched for part of each day in an attempt to strengthen the connections between the right eye and the brain. The visual acuity in her right eye never achieved normal levels, although it improved further with later surgery that included a lens implant. Unfortunately, Rachel had never used her two eyes together, so her brain still ignored much of the input coming from her right eye.

At age thirty-six, Rachel started optometric vision therapy with optometrist Dr. Hans Lessmann in Pittsburgh, Pennsylvania. Dr. Lessmann, an intense and compassionate doctor, has a keen interest in the relationship of vision to development, movement, and

behavior. He noticed right away that Rachel’s tendency to rely on her left eye for vision translated to the way she moved her body. During one therapy session, Dr. Lessmann tossed a ball to her and was flabbergasted by her reaction. Although Rachel was a rock climber and skilled kayaker, she was completely inept at catching a ball with two hands.

Catching a ball with both hands requires simultaneous mirror-image movements of the right and left side, but Rachel’s left hand reached out too quickly while her right hand moved too slowly. Suspecting that her one-sided vision had disrupted her coordination, Dr. Lessmann assigned her an additional task—to move her right and left limbs in alternation, all to the beat of a metronome. Rachel overreacted with her left side and underreacted with her right. Her left side dominated the movements even though Rachel is right-handed. What’s more, her hips were out of alignment with her right hip rotated toward the right side.

Dr. Lessmann now understood the source of the problem. Rachel used her left eye to see, so she turned her head—in fact her whole body—to the right in order to point her left eye straight ahead. Just as her left eye compensated for her right eye, her left arm and leg compensated for her right side. Rachel’s adaptation to her one-sided way of seeing had distorted her posture and affected the way she moved. Her vision affected her entire body.