Last Updated: 2001-11-05 9:02:43 EST (Reuters Health)
WESTPORT, CT (Reuters Health) - Highly heritable aspects of brain structure appear to be fundamental in determining individual differences in intelligence, according to researchers in Los Angeles and Helsinki.
Dr. Paul M. Thompson, from the University of California at the Los Angeles School of Medicine, and associates obtained high-resolution T-1 weighted magnetic resonance images (MRIs) from 10 monozygotic twin pairs and 10 dizygotic twin pairs. From the MRIs, they derived three-dimensional maps of gray matter and models of cortical surface anatomy. Subjects were matched for age, gender, handedness, duration of cohabitation and parental social class.
Within-pair gray matter differences were almost zero for monozygotic twins (p < 0.001) "in a broad anatomical band encompassing frontal, sensorimotor and linguistic cortices, including Broca's speech and Wernicke's language comprehension areas," the investigators write in the online version of Nature Neuroscience for November 5.
Frontal, linguistic and parieto-occipital association cortices exhibited a genetic continuum, with monozygotic twins showing the highest within-pair correlations, followed by dizygotic twin pairs, and lowest for unrelated subjects, suggesting tight genetic control.
The subjects also underwent a neuropsychological test battery of 17 cognitive domains. Differences in frontal gray matter were significantly associated with the Spearman's g cognitive measure.
"We found a significant linkage between quantity of frontal gray matter and each individual's performance on cognitive tests," Dr. Thompson told Reuters Health. "By and large, it's a very mild effect, comparable to something that would explain 15% to 20% of variation in scores. On other hand, many genes and many experiences are involved in cognition, so it's rather remarkable that anything about it could be inherited."
Heritability was specific to the hemisphere in some respects, with Wernicke's and Broca's speech areas displaying significant heritability on the left (p < 0.0001) but not on the right (p > 0.05).
Dr. Thompson pointed out the strong familial aggregation associated with language disorders, such as dyslexia, and autism, in which those affected have profound problems with communication.
"We know that there's more tissue dedicated to language production and understanding in the left hemisphere," he said. "When we made maps of how strongly genes determine brain structure, we saw they have more control on the left side."
With this in mind, researchers can now examine differences in genetic profiles among family members, some of whom have a language or frontal cortex disorder--including schizophrenia and frontotemporal dementia--and others who do not. "It helps us pin down areas to look at, both at the DNA level and the environmental level," Dr. Thompson added.
Nature Neuroscience 2001. http://www.neurosci.nature.com.
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