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Fractured Vision Reveals Brain Functions

In 1985, neurologist Oliver Sacks published a bestseller called The Man Who Mistook His Wife for a Hat. The subject of the title essay, a music professor called Dr. P., indeed one day grabbed his wifeís nose, mistaking her for his hat. Unable to perceive faces, the perpetually bewildered Dr. P. could only recognize his students by their voices, and he took to listening to people by turning his ears toward them, rather than his eyes. Yet he saw faces where there were noneñmistaking such objects as fire hydrants for children. His brain could not assemble eyes, nose and mouth into the image and concept of "face." Only after Dr. P.ís death from a brain disorder was damage noted in the visual processing centers in his cerebral cortex.

Dr. Sacks had thought that Dr. P.ís distorted world was most unusual, but he eventually found other cases reported in medical literature. He compiled a bizarre list: a farmer who lost the ability to recognize his cowsí faces; a man who, after a head injury, could no longer recognize his family or even himself in the mirror; and a man who worked in a natural history museum who would see the image of an ape when he looked in the mirror.

Patients with unusual talents or deficits have long been the backbone of experimental neurology, indirectly highlighting normal brain function by exhibiting what happens when they are impaired. Coupled with todayís scanning technologies, such rare patients are even more helpful, as researchers pinpoint precise brain regions that are responsible for, or at least involved in, particular deficits. Now, a decade after Oliver Sacks reported on his interesting music professor patient, a 58-year-old man with a similar but subtler problem has captured neurologistsí attention.

The importance of space

The patient known as R.M. suffered strokes in June 1991 and March 1992. Shortly thereafter, he began experiencing poor balance and disorientation, losing sense of where he was in space relative to other objects. He would stare intensely at a single object, and if shown a scene, would report seeing only one object in it. R.M. sought help, and eventually came under the scrutiny of Lynn Robertson, a research scientist at the Veterans Administration and the Center for Neuroscience at the University of California, Davis.

Robertson, UC Davis graduate student Stacia Friedman-Hill and Princeton University professor Anne Treisman report R.M.ís "feature binding problem" in the August 11 issue of Science magazine. "Feature binding" describes what the brain does with visual information.

Seeing is an enormous task, requiring about half of the brainís cerebral cortex. Incoming visual stimuli activate about 100 million photoreceptor cells (rods and cones) in the retina, and they send messages along a million nerve fibers to about 30 visual areas in the cerebral cortex.

There, the information is funneled into two pathways: One processes "object features" such as color, shape and size, and the other addresses "spatial features" such as location and direction of motion. Researchers had suspected that both types of information were necessary for forming a perception of an object in the visual field but, until R.M. came along, were lacking the "abnormal" case to reveal the role of the normal.

And abnormal he surely is. In the words of Lynn Robertson: "Often it takes a long time for this patient to tell us what he sees. He says things like. ëIt keeps shifting color,í or ëThe color is floating off to one side.í My perception of what he sees is that it is total chaosñthings moving in and out of view, colors appearing and disappearing, colors and shapes changing in size."

R.M. demonstrated his perceptual difficulty in experiments that showed him various combinations of letters and colors. If shown a red X and a blue O, for example, he would report seeing a red O and a blue X. Normal individuals make this type of mistake, but only when they are given excess information in rapid-fire displays. R.M. was given ample time and concentrated only on the images, but he had a very high error rate.

In another type of test, if shown two shapes, one of which was taller, and asked to tell the taller one, he could do so only if one shape at a time was shown. He had great difficulty telling the taller apart when both shapes appeared at the same time. This is precisely opposite the normal situation.

It is as if R.M.ís brain compartmentalizes shape and color, and is missing whatever it is that integrates these characteristics into an overall image. To try to localize the origin of the problem, the researchers took serial MRIs every 3 mm and reconstructed images of R.M.ís brain.

Fragmented images

Like the man who mistook his wife for a hat, R.M. will, unfortunately, have to continue perceiving the world in a fragmented way. But his interesting case has shown researchers that there is more to a perception than just color and shape. Knowing an objectís spatial location is important too.

In addition to clarifying a longheld question about brain function in perception, the researchers hope that this discovery will help neurologists identify stroke patients with such subtle but intensely disturbing systems.

"One of the more perplexing problems for people with spatial deficits is that they do not typically receive rehabilitation," says Robertson. "Unlike people with language deficits caused by stroke who may receive years of speech therapy, there has been little appreciation for rehabilitation for people with spatial problems."

Friedman-Hill sums it up this way: "People have known for a long time that this area of the brain is involved in spatial processing. Weíve shown the connection that, in order to see an object totally, you need to know where the features are located in space. This study is important because it confirms what vision scientists have long theorizedñif the neural mechanisms that bind objects together are damaged, then vision can become unglued."

By Dr. Ricki Lewis
Contributing Editor

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