Smarty brains: high-IQ kids navigate notable neural shifts.

The road to exceptional intelligence is paved with dramatic neural alterations, a new brain-imaging study finds.

Critical parts of the brain's outer layer, or cortex, thicken more rapidly during childhood and thin more drastically during adolescence in individuals with extremely high IQ scores compared with peers of average or moderately above-average intelligence, say neuroscientist Philip Shaw of the National Institute of Mental Health in Bethesda, Md., and his colleagues.

The scientists propose that distinctive brain growth in superior-IQ youth reflects prolonged development of neural circuits that contribute to reasoning, planning, and other facets of analytical thinking.

"Cortical thickness at any one age tells you next to nothing about intelligence," Shaw says. "What's important is that cortical development occurs differently in extremely clever kids, possibly as a result of particularly efficient sculpting of the brain."

The report appears in the March 30 Nature.

The researchers used a magnetic resonance imaging scanner to track brain changes in 307 children and teenagers deemed free of psychiatric or neurological disorders. Most volunteers submitted to two or more brain scans at intervals averaging 2 years. Participants also completed a verbal-and-nonverbal IQ test upon entering the study as children or teenagers.

Earlier research had indicated that IQ scores don't change much over time, so it wasn't necessary to administer intelligence tests more than once, Shaw says.

The sample was divided into three groups: 91 youngsters with "superior" IQ scores of 121 to 149,101 with "high" scores of 109 to 120, and 115 with "average" scores of 83 to 108. Each group yielded about 200 brain scans.

The researchers used a computer program to analyze average patterns of cortical development, from age 7 to 18, in each group.

To Shaw's surprise, the 7-year-olds with the highest IQ scores displayed slightly thinner cortices than their peers did. However, the superior-IQ children then experienced particularly rapid cortical thickening, which peaked at age 11 to 12. The same children's cortices thinned dramatically thereafter.

In average-IQ kids, cortical thickness peaked at age 8 and then declined, but to a lesser extent than was observed among superior-IQ kids.

High-IQ youngsters displayed a pattern of brain development more closely resembling that of average-IQ peers than that of the superior-IQ group. The three groups ended up with about the same cortical thickness.

The most prominent cortical changes, especially in superior-IQ children, occurred in the prefrontal cortex, an area already implicated in analytical thinking. It's not known what cell processes mediate childhood thickening and thinning of cortical tissue, Shaw notes.

Although IQ measures a narrow set of analytical abilities, the new findings highlight how neural development cultivates intelligence, remarks psychologist Elena Grigorenko of Yale University. "Children are not born 'clever' or 'not clever,'" she says. "Cognitive functioning is related to the changing dynamics of a complex cascade of brain-maturation processes."

Kids in Shaw's study who came from the wealthiest families tended to have the superior IQ scores, Grigorenko notes. This raises the possibility that environmental factors, from diet to social activities, stimulate the pattern of brain development exhibited by superior-IQ kids.

Psychologist Richard Passingham of the University of Oxford in England agrees but notes that past studies have shown that genes also substantially influence brain structure and individual differences in general intelligence (SN: 2/8/03, p. 92).