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New Insight Into Why Learning Disorders Are Genetic
Scientists analyzing new images of the brain have discovered that structures used in language processing are strongly influenced by genetics. This begins to explain why learning disorders such as dyslexia and autism can run in families. The same study also revealed that the volume of gray matter is strongly linked with intelligence. "Our study reveals there is a heritable component to intelligence," said neuroscientist Paul Thompson of the School of Medicine at the University of California at Los Angeles, who led the research. "The finding is particularly surprising," he said, "because you wouldn't think something as general as the volume of gray matter could effect something as complex as intelligence." The brain consists of two layers: gray matter and white matter. Gray matter is a one-quarter-inch layer of brain cells surrounding a ball of white matter. The white matter occupies the inner core of the brain and contains the links that connect the brain cells. The gray matter is thought to be the most important part of the brain for knowledge and feelings. Thompson and colleagues in Finland set out to determine which brain structures are controlled mainly by genes. They used magnetic resonance imaging (MRI), which can distinguish between white and gray matter, to produce brain images of identical and non-identical twins. A comparison of the MRI scans revealed that the volume of gray matter in the frontal lobe, the area just behind the eyes, is strongly heritable. "But the volume of gray matter alone cannot be used to gauge an individual's IQ," Thompson warned. The study found that differences in the volume of gray matter account for only 10 to 15 percent of the variation in intelligence. "That's good news," said Thompson. "This shows how important the 'nurture' part is." Science has long been divided about whether levels of intelligence are shaped mainly by "nature" (genetics) or by "nurture," which refers to non-genetic factors such as education, environment, diet, rest, and overall health. The brain is a highly modular structure, with different sections handling discrete tasks—reading, speaking, risk assessment, and visual processing, for example. Thompson and his colleagues aimed to find out whether the size of these modules was influenced by genetics. The MRI scans indicated that two areas of gray matter that control reading comprehension and speaking (known respectively as Wernicke's area and Broca's area) were highly similar in size in identical twins, which share an identical set of genes. The Broca's and Wernicke's areas were also similar in non-identical twins, who on average share about half of their genes. But these differences were greater than in the comparison of identical twins, and fewer than in two unrelated individuals. The study shows that the more closely related two people are, the more likely they are to share similar brain structure in regions heavily controlled by genetics. They are also more likely to share vulnerabilities to specific diseases affecting these areas. While these ideas are not new, Thompson's work is the first detailed study showing how strongly brain structure is determined by genes and inheritance. The results are described in the November issue of the journal 'Nature Neuroscience'. Full story in the highly recommended National Geographic News. 
National Geographic main News Page.
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