That study, published in this week’s Nature, reports that high IQ in children is highly correlated with how the cortical layer of the brain grows during childhood and adolescence. The correlation was most pronounced in the prefrontal cortex. Previous work suggested a correlation between the thickness of the cortex and intelligence, but the new findings show that rapid changes in cortical thickness, and not its absolute measure at any particular time, characterize kids with the highest IQs. An agile mind, it seems, requires an agile cortex.

Jay Giedd, Judith Rapoport, and colleagues at the National Institutes of Mental Health in Bethesda, Maryland, with collaborators at McGill University in Montreal, made the observation in a large, longitudinal MRI study of normal children between the ages of about 6 and 19. Their results suggest that one anatomical correlate of intelligence may in fact be a transient developmental process involving the plasticity of the prefrontal cortex. Pinpointing this critical event in brain development should allow a much greater understanding of how our mental abilities (and disabilities) are shaped by genetic and environmental factors.

The current work is part of a longitudinal MRI study of normal brain development in children, the largest pediatric neuroimaging study of its type going on. Over the course of 17 years, the researchers at NIMH have collected images from more than 300 children. Most of the children have had multiple scans an average of two years apart. Each child also sat for a standard IQ test at the beginning of the study. (IQ is considered quite stable, so was not retested as the study progressed.)

To look at the relationship between cortical thickness and intelligence, first author Philip Shaw first stratified the children into three groups based on IQ scores—superior (121-149), high (109-120), and average (83-108). From the scans, Shaw measured the thickness of the cortex over the entire brain and looked for correlations of thickness and IQ. With this analysis, the researchers did find some modest correlations, but none that were statistically significant. When they separately looked at different age groups, however, they found some surprising results. In the youngest children (3.8 to 8.4 years old), they observed a negative correlation between IQ and cortical thickness over the prefrontal cortex; that is, the smartest kids had the thinnest cortex. However, in the next age group (8.6 to 11.7 years), that result was reversed, and cortical thickness positively correlated with IQ. The positive correlation lessened throughout early adolescence and into adulthood, but was maintained.

When Shaw and colleagues examined changes in cortical thickness as a continuous process, the average, high, and superior IQ groups showed significantly different pictures. Children with the highest IQ started with a thinner cortex, which rapidly increased in thickness until around 11 years of age. This expansion phase was more pronounced and prolonged than in the high or average IQ groups, who started out with a thicker cortex and peaked earlier. Once cortical thickness peaked, it declined to reach similar levels in all three groups by late adolescence, but the rate of decline was faster in the group of highest IQ.

Thus, the smartest children showed a greater expansion of parts of their cortex, over a longer period of time. The most dramatic positive correlations between thickness and IQ in late childhood appeared in the prefrontal cortex. This is the same area that shows activity by fMRI during intelligence testing, and the magnitude of activation correlates highly with intelligence.

What is the connection between cortical plasticity and intelligence? As one possibility, Shaw et al. suggest that a prolonged growth phase allows an extended time for the development of higher cognitive circuits, before the pruning phase of cortical thinning begins. If this is so, then any of a number of genetic or environmental factors might determine the extent of cortical thickening at this critical age, and influence intelligence. The observations by Shaw and coworkers open up the real possibility of starting to define these factors.

Giedd and Rapoport have been leaders in the use of brain imaging to understand not only normal brain development, but also what can go wrong. They have studied children with schizophrenia, ADD, autism, or other pathologies, and others, including Paul Thompson and Art Toga at University of California, Los Angeles, have applied similar techniques to adults with Alzheimer disease (for review, see Thompson et al., 2004). In an accompanying News and Views piece, Richard Passingham of the University of Oxford points out the power of longitudinal, live imaging as performed in the present study to illuminate the subtle structural correlates of health and disease. Before this kind of study, we had only Einstein’s brain.—Pat McCaffrey.

References:
Shaw P, Greenstein D, Lerch J, Clasen L, Lenroot R, Gogtay N, Evans A, Rapoport J, Giedd J. Intellectual ability and cortical development in children and adolescents. Nature. 30 March 2006; 440:676-679. Abstract

Passingham R. Brain Development and IQ. Nature. 30 March 2006; 440:619-620. Abstract