Large study of MRI scans, seventeen years in the making, done by National Institute of Mental Health (NIMH), has shown that brain cortex of highly intelligent people matures differently than of their peers:
The researchers found that the relationship between cortex thickness and IQ varied with age, particularly in the prefrontal cortex, seat of abstract reasoning, planning, and other “executive” functions. The smartest 7-year-olds tended to start out with a relatively thinner cortex that thickened rapidly, peaking by age 11 or 12 before thinning. In their peers with average IQ, an initially thicker cortex peaked by age 8, with gradual thinning thereafter. Those in the high range showed an intermediate trajectory (see below). While the cortex was thinning in all groups by the teen years, the superior group showed the highest rates of change.
“Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age,” explained Rapoport [Dr.Judith Rapoport at the NIMH and McGill University in Montreal -ed.]. “Rather, IQ is related to the dynamics of cortex maturation.”
The observed differences are consistent with findings from functional magnetic resonance imaging, showing that levels of activation in prefrontal areas correlates with IQ, note the researchers. They suggest that the prolonged thickening of prefrontal cortex in children with superior IQs might reflect an “extended critical period for development of high-level cognitive circuits.” Although it’s not known for certain what underlies the thinning phase, evidence suggests it likely reflects “use-it-or-lose-it” pruning of brain cells, neurons, and their connections as the brain matures and becomes more efficient during the teen years.
“People with very agile minds tend to have a very agile cortex,” said Shaw [Dr. Philip Shaw at McGill University in Montreal and NIHM -ed.]. The NIMH researchers are following-up with a search for gene variants that might be linked to the newly discovered trajectories. However, Shaw notes mounting evidence suggesting that the effects of genes often depends on interactions with environmental events, so the determinants of intelligence will likely prove to be a very complex mix of nature and nurture.