Looking for the brain’s g force

Nearly 100 years ago, psychologist Charles Spearman noted that individuals tend to do consistently well or poorly on challenging problems from a variety of mental tests. He used scores on those items to calculate a general intelligence factor, or g, which mental testers now view as the basic element of IQ.

Spearman argued that g taps into the brain’s fundamental intellectual capacity. Other researchers have since disagreed, splitting general intelligence into anywhere from 7 to more than 100 primary mental abilities.

A new study that combines mental testing with brain imaging takes Spearman’s side. Mental tasks that are closely allied to general intelligence engage specific parts of the brain’s frontal lobe, reports a team of neuroscientists led by John Duncan of the Medical Research Council Cognition and Brain Sciences Unit in Cambridge, England. This relatively restricted frontal-brain network may orchestrate responses to diverse mental challenges, Duncan’s group theorizes.

The researchers took positron emission tomography (PET) scans of blood flow in the brains of 13 men and women as they performed a series of tasks. Earlier testing had found that the more difficult tasks, such as discerning which of four sets of abstract shapes subtly differs from the others, were closely related to g. Easier tasks, such as identifying a shape that obviously doesn’t match three others, had only a weak association to g.

Average blood-flow increases, which indicate rises in cell activity, occurred in several frontal-brain regions as volunteers performed tasks previously linked to general intelligence, the scientists say in the July 21 Science. This overall pattern didn’t depend on individuals’ success with the tasks.

These results leave much unanswered about the nature of human intelligence, remarks psychologist Robert J. Sternberg of Yale University in the same journal. For instance, the new study doesn’t demonstrate that frontal-brain areas form the basis of intelligence, only that their increased activity accompanies certain types of problem solving. In contrast, other PET studies find that high-IQ people display drops in frontal-brain activity during problem solving (SN: 7/8/00, p. 27: Energy-efficient brains).

Conventional intelligence tests, such as those used by Duncan’s team, assess a narrow type of analytic thinking, Sternberg holds. Tests of practical and creative intelligence tap into abilities that have little to do with the g factor, in his view.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.