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that it holds all the answers.
    So it is with many of the currently debated issues concerning brain function. Is language localized? Is color vision? Laughter? Once we understand these functions better, the question of "where" becomes less important than the question of "how." As it now stands, a wealth of empirical evidence supports the idea that there are indeed specialized parts or modules of the brain for various mental capacities. But the real secret to understanding the brain lies not only in unraveling the structure and function of each module but in discovering how they interact with each other to generate the whole spectrum of abilities that we call human nature.
    Here is where the patients with bizarre neurological conditions come into the picture. Like the anomalous behavior of the dog that did not
    bark when the crime was being committed, providing Sherlock Holmes with a clue as to who might have entered the house on the night of the murder, the odd behavior of these patients can help us solve the mystery of how various parts of the brain create a useful representation of the external world and generate the illusion of a "self" that endures in space and time.
    To help you get a feel for this way of doing science, consider these colorful cases—and the lessons drawn from them—taken from the older neurological literature.
    More than fifty years ago a middle−aged woman walked into the clinic of Kurt Goldstein, a world−renowned neurologist with keen diagnostic skills. The woman appeared normal and conversed fluently; indeed, nothing was obviously wrong with her. But she had one extraordinary complaint—every now and then her left hand would fly up to her throat and try to strangle her. She often had to use her right hand to wrestle the left hand under control, pushing it down to her side—much like Peter Sellers portraying Dr. Strangelove. She sometimes even had to sit on the murderous hand, so intent was it on trying to end her life.
    Not surprisingly, the woman's primary physician decided she was mentally disturbed or hysterical and sent her to several psychiatrists for treatment. When they couldn't help, she was dispatched to Dr. Goldstein, who had a reputation for diagnosing difficult cases. After Goldstein examined her, he established to his satisfaction that she was not psychotic, mentally disturbed or hysterical. She had no obvious neurological deficits such as paralysis or exaggerated reflexes. But he soon came up with an explanation for her behavior: Like you and me, the woman had two cerebral hemispheres, each of which is specialized for different mental capacities and controls movements on the opposite side of the body. The two hemispheres are connected by a band of fibers called the corpus callosum that allows the two sides to communicate and stay "in sync." But unlike most of ours, this woman's right hemisphere (which controlled her left hand) seemed to have some latent suicidal tendencies—a genuine urge to kill herself. Initially these urges may have been held in check by
    "brakes"—inhibitory messages sent across the corpus callosum from the more rational left hemisphere. But if she had suffered, as Goldstein surmised, damage to the corpus callosum as the result of a stroke, that inhibition would be removed. The right side of her brain and its murderous left hand were now free to attempt to strangle her.
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    This explanation is not as far−fetched as it seems, since it's been well known for some time that the right hemisphere tends to be more emotionally volatile than the left. Patients who have a stroke in the left brain are often anxious, depressed or worried about their prospects for recovery. The reason seems to be that with the left brain injured, their right brain takes over and frets about everything. In contrast, people who suffer damage to the right hemisphere tend to be blissfully indifferent to their own predicament. The left hemisphere just doesn't get all that upset. (More on