679 - The Brain - IQ

High IQ

INTELLIGENCE is a slippery concept to define, so not surprisingly it has been tricky to pin it down in the brain. Several studies claim to link brain size, weight, volume or head circumference to intelligence, but no clear or consistent pattern has emerged. For example, Sandra Witelson from McMaster University in Ontario, Canada, studied the post-mortem brains of 100 people who in life had had a variety of IQ test scores. She found that while there were some positive correlations between hemisphere volume and score, the relationships varied with sex, handedness and type of test (Brain, vol 129, p 386). For example, verbal intelligence was positively correlated with cerebral volume in women and in right-handed men. And in women, visuospatial intelligence was positively linked with volume, but less strongly than verbal skills.

Certainly size is not the whole story. Women's brains are smaller than men's, even when corrected for body size, yet there is no consistent difference in men and women's IQs. Indeed, the Guinness World Records listed a woman, Marilyn vos Savant, as having the highest IQ between 1986 and 1989. Since then, incidentally, the category has not been included, partly because IQ is so hard to measure at these extreme limits - vos Savant's score varied from 186 to 228, depending on the test used, the conditions and the day.

If size does not explain all, does brain activity give any clues? In 2000, a team led by John Duncan of the MRC Cognition and Brain Sciences Unit in Cambridge, UK, identified what might be called the brain's "G spot", the area associated with general intelligence, which is what IQ tests are thought to measure (Science, vol 289, p 457). PET scans showed that puzzles and tasks that provide a good measure of general intelligence or "g" seem not to recruit vast areas of the brain as you might expect, but produce activity in a very specific region of the lateral frontal cortex. In tasks that don't measure g very well, activity is more diffuse. It is not clear exactly what this finding means or what this region does, but it hints that efficiency, connectivity and focused activity may be more important than size.

Intelligence may also be connected to working memory, located in the middle and inferior frontal gyrus, a region near the brain's G spot. It is sometimes possible to train working memory with practice, and doing so may benefit IQ, especially fluid intelligence - the ability to solve new problems. However, this may just be a short cut to better IQ test scores rather than an indication of brain structures that confer intelligence.

More recently, Philip Shaw from the National Institute of Mental Health in Baltimore, Maryland, found a developmental difference linked to IQ. His team studied more than 300 children aged 7 to 18, divided into groups with IQs that were average (up to 108), high (up to 120) and superior (above 120) (Nature, vol 440, p 676). Looking at the cerebral cortex, they found no differences in the overall thickness attained by age 18. However, children in the average group had reached peak thickness by age 8, followed by a thinning down through adolescence, whereas in the superior group, the cortex was thinner at age 7 but continued thickening until age 11 or 12 before thinning again. The high group lay in between. Shaw concludes that intelligence is a dynamic process, related to a particularly high level of plasticity during these years.