Faster Changes in Cortex Size Linked to Higher IQ


by Thomas S. May

July, 2006

Neuroscientists have long suggested that intelligence is related to some aspects of brain anatomy, with the size of the cortex (the outer part of the brain) thought to be the most likely feature to correlate positively with intelligence. However, according to a recent study, the way the cortex develops over time is a much better predictor.

Smart People Have Agile Brains

  In a longitudinal study published in the March 30 issue of Nature, Philip Shaw and colleagues at the U.S. National Institutes of Health and the Montreal Neurological Institute in Canada used magnetic resonance imaging (MRI) to measure changes in cortical thickness in the brains of more than 300 children and teenagers over a period of about 10 years. The scientists obtained brain scans several times during this period from the participants, whose intelligence also was measured using standardized intelligence quotient (IQ) tests. 

  Although IQ is but one of many measures used to quantify higher brain function, there is some evidence that it is a valuable index that can predict future academic performance and occupational success.

  There were no significant differences in the size of the cortex among subjects with various IQ levels, and everyone showed the same basic pattern of cortical growth throughout most of the brain, Shaw says: “The cortex initially got thicker, peaked, and then got thinner.”

  However, the researchers also found that the rate of these changes differed markedly in the children studied, depending on how intelligent they were.

  “In the most intelligent kids, the cortex started out very thin; got thicker very quickly, reaching its peak thickness relatively late; and then also got thinner very quickly,” Shaw says. “Children of average intelligence had the same pattern, but all of the changes were slower,” he adds.

  Shaw says these results indicate that intelligence is related not to the size of the cortex but to the dynamics of how the brain develops. “It’s [as though] those with the most agile minds have the most agile cortex,” he says.

Greater Plasticity = Higher IQ?

  The investigators are not sure what underlies the observed changes in cortical thickness at the level of individual cells, but they hypothesize that a ‘use it or lose it’ pruning of neurons (and the connections between them) is involved in the process.

  “The basic one-two punch of brain development seems to be massive overproduction of cells and synapses (called arborization) followed by competitive elimination (or pruning),” says Jay Giedd, chief of brain imaging at the Child Psychiatry Branch of the National Institute of Mental Health and a co-author of the study.

  “The ‘use it or lose it’ hypothesis suggests that connections that are used will survive and flourish, while those that are not used will perish,” Giedd explains. If the changes in cortical thickness reflect the degree of arborization and pruning, then rapid changes might reflect a high degree of neural plasticity, he says. 

  The idea that greater neural plasticity may manifest itself in higher intellectual ability was first proposed by Dennis Garlick, a postdoctoral fellow in psychology at the University of California, Los Angeles. In a 2002 paper, which appeared in Psychological Review, Garlick (while at the University of Sydney, Australia,) argued that having a high degree of neural plasticity may enable humans to better adapt to the demands of their environment—and that it may also be the key to having a superior IQ.

  Garlick says the study by Shaw and colleagues provides some support for his theory. “Specifically, it shows that changes in cortical thickness are related to intelligence, and it is possible that changes in cortical thickness reflect the adaptation of the neural circuits to environmental demands,” he says.

  He adds a word of caution, however: “While the study is suggesting that differences in neural plasticity could be related to differences in intelligence, much still needs to be discovered about how the neural connections adapt to the environment. Understanding this is critical to understanding how the human brain is able to achieve intelligence.”?