By Shankar Vedantam
Washington Post Staff Writer
Thursday, March 30, 2006
The brains of very intelligent children appear to develop in a distinctive and surprising way that distinguishes them from less intelligent children, a federal study reported yesterday.
The study is the first to try to measure whether differences in brain development are linked to intelligence, said researchers at the National Institute of Mental Health, who did several brain scans on 309 healthy children between the ages of 6 and 19.
The scans showed that children with the highest IQs began with a relatively thin cortex -- the folded outer layer of the brain that is involved in complex thinking -- which rapidly grew thicker before reaching a peak and then rapidly becoming thinner, said Philip Shaw, the lead investigator. Children of average intelligence had a thicker cortex around age 6, but by around 13 it was thinner than in children of superior intelligence.
The study is the most definitive finding to date of a relationship between the physical characteristics of the brain and intelligence. Such a relationship has long been something of a holy grail for scientists.
"Studies of brains have taught us that people with higher IQs do not have larger brains," said National Institutes of Health Director Elias A. Zerhouni in a statement, but "thanks to brain imaging technology, we can now see that the difference may be in the way the brain develops."
"I was surprised that the relationship between intelligence and brain structure changed so much as a child grows up," said Shaw. "In early childhood, the smartest children had a thinner cortex -- this is the opposite of what you'd expect. By late childhood, the pattern had changed completely."
The cortex has long been known to get thinner in late adolescence, presumably because the brain prunes cells, neurons and connections that are not being used. The new study found that the cortex continued to thicken in gifted children until around age 11 or 12, much later than in children of average intelligence, whose cortex thickening peaked by age 8. "It's almost like the most agile minds have the most agile cortex," Shaw added.
The study, being published today in the journal Nature, does not suggest any particular interventions that might boost a child's intelligence. But Richard J. Davidson, a brain imaging expert at the University of Wisconsin at Madison, said the fact that the region of the brain being studied is highly malleable suggests that experience and environmental cues may play a very important role in shaping intelligence.
Although the difference between smart kids and not-so-smart kids is often obvious in social, academic and psychological settings, it has been extremely difficult to identify the brain mechanisms that allow some children to excel, for example, at reading, writing or arithmetic.
The advent of technology that allows noninvasive imaging of the brain has shown that many complex mental skills associated with intelligence involve the part of the cortex near the front of the brain. The newest scans measure changes in the working brain in real time, but scientists still have only a dim understanding of how the more obvious changes observed in scans relate to underlying changes at the cellular level and in nerve cell networks.
The new study does not address what is happening at those more subtle levels, but Shaw said the study does suggest that, whatever those processes are, "they are happening in different ways in children of different intellectual ability."
The researchers at the NIMH and McGill University divided the children -- a large number for such a study -- into three groups. They were categorized as having superior, high or average intelligence based on IQ tests. Most of them were scanned two or more times. By comparing brain images from the different groups, the study teased out developmental differences associated with intelligence.
In a commentary accompanying the study, Richard E. Passingham, an experimental psychologist at the University of Oxford, said that although there is a strong link between genes, brain development and intelligence, the research should not lead people to assume that the observed changes in brain structure are genetically predestined.
"One should be very wary of such a conclusion," he said. "The body's development is intimately linked to interactions with its environment. . . . It could be that people with superior intelligence also live in a richer social and linguistic environment, and that it is this that accounts for the sharp increase in the thickness of their prefrontal cortex in late childhood."
Children of smart parents tend to be smart, but that may not be because there are smart genes and dumb genes, scientists say. That view reflects the trend in modern neuroscience to largely discard the age-old nature-vs.-nurture debate.
"Children might inherit certain genes that incline them to interact with their environment in very 'stimulating' ways," Shaw said. "This stimulation could then influence brain development. . . . What is less likely is the idea that we are born with a certain set of genes which 'fix' at birth our intelligence and the trajectory of our brain development."
Elizabeth Sowell, a neuroscientist at the University of California at Los Angeles whose research has linked brain changes to verbal ability, said it is unclear which is cause and which is effect. "We don't know if kids are super smart because their brain matures at the rate it does, or their brain matures at the rate it does because they are really smart," she said.