variance of the structure of that part of the brain.” In other words, a physician could probably not see anything in the MRI that was clearly abnormal, because there is so much normal variation in that part of the brain. If, however, it were possible to use a computer to compare the patient's MRI with a probabilistic distribution calculated from 7,000 subjects, some parts of the brain might well be seen to lie outside the normal range for brains. “And if you could compare her brain with those of a well-matched population of other 19-year-old left-handed Asian women who smoke cigarettes, had 2 years of college, and had not read Gone With the Wind, you might find that there is an extra fold in the gyrus here, the cortex is a half-millimeter thicker, and so on.”

In short, because of the data that it is gathering on its subjects and the capability of isolating the brains of subjects with particular characteristics, the probabilistic brain atlas will allow physicians and researchers not only to say what is normal for the entire population, but also what is normal for subgroups with specific traits. And that is something that would not be possible without harnessing the tremendous data-handling capabilities of modern biologic databases.

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