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Study Offers New Insight Into Why Learning Disorders Are Genetic

Scientists analyzing new images of the brain have discovered that structures associated with language are heavily influenced by genetics. The finding 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 IQ.

Twin brains

Brainy Findings

In these brain images, the areas in red show regions that are heavily influenced by genetics. The scientists compared the regions labeled with letters.

Photograph courtesy of Paul Thompson

More images of comparative brain scans of twins

"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 wiring that connects the brain cells. The gray matter is thought to be the most important part of the brain for cognition and emotion.

Thompson and colleagues in Finland set out to determine which brain structures are controlled predominantly by genes. They used magnetic resonance imaging (MRI), which can distinguish between white and gray matter, to produce brain images of identical and fraternal 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 highly heritable.

"But the volume of gray matter alone cannot be used to gauge an individual's IQ," Thompson cautioned. 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 sought 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.

Thompson's study is part of much broader effort to understand which regions of the brain are associated with brain diseases such as Alzheimer's, schizophrenia, and Parkinson's.

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