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The amount of gray matter matters
Brain maps reveal how genetic differences influence brain structure
By Birgit Reinert

American and Finnish researchers have developed a method to show how genes can influence brain structure and intelligence. They found that the volume of certain regions of the brain—especially the amount of gray matter in the frontal parts—is inherited. Differences in frontal gray matter are significantly linked with differences in intellectual function, according to the researchers.

Genetic continuum of similarity in brain structure. View larger

Paul M. Thompson, of the UCLA School of Medicine's Department of Neurology in Los Angeles, and colleagues compared the brain matter of 20 identical twins and 20 fraternal twins. They examined the brains using magnetic resonance imaging and constructed three-dimensional genetic maps that show the difference between gray and white matter. Identical twins, who share the same genes, have virtually the same amount of gray matter, whereas fraternal twins do not.

"The quantity of frontal gray matter, in particular, was most similar in individuals who were genetically alike," the researchers write. "Intriguingly, these individual differences in brain structure were tightly linked with individual differences in IQ." Gray matter refers to the areas of the brain that mainly contain nerve cells, whereas white matter forms a layer between the areas of gray matter and encloses the column of gray matter in the spinal cord. The findings were published online by Nature Neuroscience and will appear in print in the December issue.

For some time, scientists have suspected that gray matter is the key to intelligence. However, only a certain degree of intelligence is genetically predetermined before birth. "A component of our problem solving abilities is inherited, but it's only about 10 to 15 percent of the variation in that trait, which means to say that the effects of nurture, of learning—the non-genetic factors—have tremendous importance in building brain structure, intelligence and your performance in tests," Thompson was quoted as saying.

Another highly heritable region was found at the side of the left hemisphere known as Wernicke's area, which is central to language. Brain regions closely linked to language and reading skills were most similar in identical twins, according to the study.

The researchers are applying the new brain mapping method to screen individuals at genetic risk for schizophrenia and Alzheimer's disease for early brain changes.

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Thompson, P.M. et al. Genetic influences on brain structure. Nat Neurosci 4 (December 2001). Published online November 5, 2001.

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