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Groping in the Dark By Julia McNamee
Neenan WESTPORT, Conn. (HealthScout)-You wake up in the middle of the night, and you need to go to the bathroom. It's pitch black, and yet you navigate your way there, feeling your way among the bedposts, dressers and doorframes. Are you using your sense of touch? Or sight? A little bit of both, according to a study reported in a recent Nature. Even though you can't see, your "mind's eye" can. "The nerve fibers in your hand signal to your brain what they're feeling, but in order for you to make sense of the information, you have to compare what you're feeling with what you know of objects and the layout of the room," said Dr. Krishnankutty Sathian, author of the article and associate professor of neurology and joint associate professor of rehabilitation at Emory University in Atlanta. "Your brain actually makes a mental picture." Sathian's study in Nature showed that, in fact, even when we can't see, the part of our brain that has to do with touch is not the only part used to figure out where objects are in space. "The part of the brain involved in processing sight is also involved in processing certain aspects of touch," Sathian said. "We feel things on our fingers and then imagine in our mind's eye -- and then we compare what we feel and what we see." Surrounding the white matter of our brains is a layered, enfolded shell a few millimeters thick called the cerebral cortex. In the past, scientists thought that certain areas of the cortex were responsible for different senses. The visual cortex was thought to be responsible for sight and the somatosensory cortex for touch, for example. But Sathian's study suggests the senses may act in concert, instead. "The senses are constantly interacting and calibrating with each other to remove the confusion any single sense might convey," he said. In Sathian's experiments, 14 subjects either closed their eyes or were blindfolded and then were presented with a 2 centimeter by 2 centimeter plastic square engraved with 3-millimeter-wide grooves. The subjects were asked whether the grooves were parallel or at right angles to their index fingers. They were correct 85 percent of the time. But when the visual cortex was temporarily distracted, they were correct just 50 percent of the time, suggesting that some kind of internal vision is critical to picturing orientation even when sight is blocked. The visual cortex was distracted when a magnetic field crossed the skull, inducing an electric current in that cortex, temporarily making it unable to function normally. In another part of the study, Sathian said, the experiment was set up identically except that the unseeing subjects were asked to tell the difference between squares with larger and smaller grooves. Even when the visual cortex was blocked, Sathian said, the subjects could tell if the grooves were larger or smaller, suggesting that the "mind's eye" is used only in certain tasks, such as orientation, and not others, such as spacing. Previous experiments using a new kind of MRI-type technology had already shown that the orientation task uses both the visual and somatosensory cortexes -- even when subjects could not physically see. It looks as if the somatosensory cortex is used first, about 30 milliseconds after the square is touched, Sathian said. About 150 milliseconds later, the visual cortex is employed -- enough later that apparently the brain has had time to process the touch data before opening its "mind's eye." Next up is a study that looks at whether the somatosensory cortex is employed when subjects look at richly textured art -- the kind that makes you want to break the security barrier to touch the painting, Sathian said. Perhaps, he said, when we look at that kind of art, we are feeling within our brains how we imagine the surface of the painting would feel. What to Do This study is small but well designed. And while it might
not help you find your glasses on the nightstand in the middle of the night, at
least you'll understand what's happening inside your brain. Interestingly, it
adds to a previous study that showed blind people reading Braille with their
fingers also use their visual cortex. |