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Until recently, Zeki believed that without the area known as V1, the part of the brain that first receives input from the retina, conscious visual perception would be impossible. V1 is a sort of clearinghouse, a place where incoming signals are split up and sent to the sites where they can be processed. But one patient, a 38-year-old man whose V1 for one eye was wiped out in an automobile accident, is also quite clearly aware of motion seen by the "blind" eye even when the good eye is covered. "We find," says Zeki, "that he is consciously aware of moving stimuli and of their direction. He will tell you that the bars on a TV screen are moving left or right, toward or away, and he gets it 100% correct every time." Furthermore, notes Zeki, PET scans show that the patient's perception of motion is accompanied by the appropriate activation of V5. So how does the signal travel? Zeki is convinced the answer lies in a secondary pathway, a kind of back road created to get around the damaged area.
Indeed, the brain abhors a vacuum, observes neuroscientist Dr. Vilayanur Ramachandran of the University of California at San Diego; it craves information, and when it can't come by the data honestly, it does the best it can with what it has. One of his patients, for instance, a physical-therapy professor from San Antonio, Texas, suffered a brain hemorrhage that left a huge blank spot in her otherwise normal field of vision-or, rather, it would be blank if her brain allowed it. First, she saw a drawing of a cat, presumably supplied by her visual memory. "Then," says M.J. Blaschak, "I started to see flowers." Soon cartoon characters like Mickey Mouse began to appear. "I've got to the point where I think they're pretty funny," she says.
Another well-documented example of the brain's need to fill in the blanks is the phenomenon of phantom limbs. When an arm or a leg is amputated, the victim almost invariably "feels" sensations like pain or itching, often very strongly, in the missing limb. What's happening? The brain carries within it a mental map of the body, a well-formed sense of where every part is in relation to every other. That's why it's possible for you to extend your arm and then, with your eyes closed, bring it in to touch the tip of your nose. (Drunkenness distorts your perception of the map; thus the nose-touching sobriety test often administered by cops.)
Even when a limb is gone, its place on the mental map remains, and the neurons formerly responsible for processing sensations from it occasionally fire at random-the sensory equivalent of Mickey Mouse hallucinations. The brain also attempts to make up for the deficit physically, perhaps, suggests Ramachandran, by sprouting new sets of connections. Because neurons that process information from the arm are near those that handle the face, for example, these new connections can cause a blindfolded patient to think a gentle touch on the face is really a touch on a missing fingertip. Says Ramachandran: "Reorganization can occur in a period of weeks."