Image: Courtesy of PAUL THOMPSON

Scientists are finally beginning to understand how common genetic differences among individuals underlie differences in the structures that make up their brains. In the first attempt to actually map these variations, neurologist Paul Thompson and colleagues at the University of California at Los Angeles have discovered that brain structures related to cognitive ability and language seem to be under tight genetic control. The group's findings, which could help explain how diseases like schizophrenia are passed on, will appear in a report in the December issue of Nature Neuroscience.

To construct their so-called genetic brain maps, the researchers scanned the brains of 20 sets of twins (ten fraternal and ten identical) with magnetic resonance imaging and combined the results to construct an average brain map for each kind of twin. In the brain map of identical twins pictured at the right, for example, brain areas exhibiting more variation appear in blue, whereas those showing less variation are red. These pairs of twins showed almost no differences in the amounts of gray matter in the frontal, sensory-motor and language-related parts of their cortexes. Fraternal twins, who share half of each other's genes, showed more variation in these structures than did identical twins and less than unrelated individuals did, suggesting that "some areas of the brain are under tight genetic control¿language in particular," Thompson explains. This genetic control may also extend partly to cognitive ability: study participants with more gray matter in the front of their brains scored higher on a common test designed to measure Spearman's g, which is similar to IQ. "But this is quite a mild correlation," Thompson says. "You can't predict an individual's IQ from a brain scan, and I think that's quite a relief."

The kind of brain mapping employed in this study could help scientists determine why dementias such as schizophrenia, which affects the frontal cortex, are often passed down between generations. By "building a mosaic, or jigsaw, which shows each individual part of the brain and to what extent genes influence it," Thompson says, "we can begin to point to why there's an inherited risk to brain disease.

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