|Cortex Matures Faster in Youth with Highest IQ
Youth with superior IQ are distinguished by how fast the thinking part of their
brains thickens and thins as they grow up, researchers at the National Institutes
of Health’s (NIH) National Institute of Mental Health (NIMH) have discovered.
Magnetic resonance imaging (MRI) scans showed that their brain’s outer mantle,
or cortex, thickens more rapidly during childhood, reaching its peak later than
in their peers — perhaps reflecting a longer developmental window for high-level
thinking circuitry. It also thins faster during the late teens, likely due to
the withering of unused neural connections as the brain streamlines its operations.
Drs. Philip Shaw, Judith Rapoport, Jay Giedd and colleagues at NIMH and McGill
University report on their findings in the March 30, 2006 issue of Nature.
“Studies of brains have taught us that people with higher IQs do not have larger
brains. Thanks to brain imaging technology, we can now see that the difference
may be in the way the brain develops,” said NIH Director Elias A. Zerhouni, M.D.
While most previous MRI studies of brain development compared data from different
children at different ages, the NIMH study sought to control for individual variation
in brain structure by following the same 307 children and teens, ages 5-19, as
they grew up. Most were scanned two or more times, at two-year intervals. The
resulting scans were divided into three equal groups and analyzed based on IQ
test scores: superior (121-145), high (109-120), and average (83-108).
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
“Brainy children are not cleverer solely by virtue of having more or less gray
matter at any one age,” explained Rapoport. “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.
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.
Also participating in the study were Drs. Dede Greenstein, Liv Clasen, Rhoshel
Lenroot, and Nitin Gogtay, Child Psychiatry Branch, NIMH; and Drs. Jason Lerch
and Alan Evans, Montreal Neurological Institute, McGill University.
|The developmental trajectory of waxing and waning in cortex thickness differs as the brain matures in different IQ groups. Thickness of the area at the top/front/center, highlighted in MRI brain maps at left, peaks relatively late, at age 12 (blue arrow), in youth with superior intelligence, perhaps reflecting an extended critical period for development of high-level cognitive circuits.
Source: NIMH Child Psychiatry Branch
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