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Twenty-Twenty Fingers


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Many people, scientists included, often assume that the loss of one sense strengthens others. But is there any real evidence for that assumption? Norihiro Sadato of Fukui Medical School in Japan and his colleagues at the National Institutes of Health in this country may have found some. Their studies show that in blind people, the visual cortex--a part of the brain that normally processes visual information--is instead given over to touch.

Sadato and his colleagues used positron emission tomography, or pet, to monitor the brain activity of blind people as their fingertips were engaged in three tasks: reading braille, scanning meaningless patterns of raised dots, and recognizing raised English letters and other shapes made by grooves etched in paper. The researchers also monitored sighted people (wearing eye patches) as they performed the latter two tasks.

Nothing unusual happened when either blind or sighted people passed their fingers over the meaningless dots--in both groups the pet scans showed high blood flow, and thus high nerve-cell activity, in the sensorimotor cortex, the region of the brain that routinely handles purely tactile stimuli. But when blind people read braille or scanned the letters and shapes, the blood flow increased significantly--in the visual cortex rather than the sensorimotor one. Sighted people, in contrast, actually showed a decrease in visual cortical activity while trying to recognize the letters and shapes.

In the blind, apparently, neurons normally reserved for vision were adjusting to handle information from their fingertips. Both the sensorimotor and visual cortices, says Sadato, process information on the shapes of objects (unlike, say, the auditory cortex). So it is not altogether surprising that the visual cortex in blind people, freed from its normal tasks, might augment shape recognition. Although that suggests the blind have a more acute sense of touch, Sadato stresses that his work doesn’t prove the case. We measured neuronal activity, not tactile acuity itself, he says. The point is that the same touch exercises are related to different neuronal networks in the blind compared with the sighted, says Sadato. Just how this happens is not yet understood. But Sadato believes that extensive tactile learning, such as training in braille, may rewire the brain in deeper ways than anyone had realized.

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