Men and women who have equivalent IQ scores get to those levels of intelligence using different areas of the brain, Richard J. Haier, PhD, and colleagues reported in the January 20 online edition of NeuroImage. Using voxel-based morphometric analysis of MRI, the researchers found that:

• Women have more white matter and fewer gray matter areas related to IQ than men do.

• In men, the strongest correlations between gray matter and IQ are in the frontal and parietal lobes.

• In women, the strongest correlations between gray matter and IQ are in the right frontal lobe and in Broca’s area.

Dr. Haier, Professor of Psychology in the Department of Pediatrics at the University of California, Irvine, told Neurology Reviews that these differences may have some important clinical implications.

BASIC QUESTION

The investigators studied two samples of volunteers—14 women and nine men with a mean age of 27, and 13 men and 12 women with a mean age of 59. All were in good physical and mental health, with no history of head injury or clinical signs of dementia.

“The basic question we are asking is, Where in the brain is intelligence?” Dr. Haier said. “Ultimately, we also want to find out what makes some brains smarter.”

Intelligence was assessed using the Wechsler Adult Intelligence Scale battery. Participants underwent MRI, results of which were then analyzed with voxel-based morphometry to identify brain areas where gray matter and white matter were correlated to IQ for males and females. Statistical analysis controlled for age and intrasex brain size as nuisance variables.

UNEXPECTED ANSWER

The researchers were surprised at the clear divergence in brain patterns. Dr. Haier commented, “Imaging studies of intelligence have consistently shown multiple areas throughout the brain related to intelligence and that these have been identified mostly, but not exclusively, in the frontal lobe. Because the males and females in our study did not differ in intelligence, we did not expect to find much in the way of sex differences.”

Instead, the MRI analyses revealed that male and female brains are anatomically different with respect to intelligence. “There appear to be two different brain architectures related to intelligence,” Dr. Haier said.

In men, the gray matter volume was correlated to IQ most strongly in bilateral frontal lobes (Brodmann areas [BA] 8 and 9) and in left parietal lobe (Wernicke’s area, BA 39 and 40). In women, the strongest gray matter volume correlation to IQ was in the right frontal lobe (BA 10), and the largest cluster was in Broca’s area (BA 44 and 45). White matter volume correlations with IQ were more extensive in women than in men.

MEN GO GRAY, WOMEN GO WHITE

Men had 6.5 times more gray matter linked with intellectual functioning, and women had about nine times more white matter linked with intellectual functioning, Dr. Haier reported. “In women, general intelligence is related largely to the white matter, while in men general intelligence is related to gray matter. Perhaps when you get away from general intelligence, to such things as verbal or mathematical ability, there might also be different brain patterns,” he commented.

This study may partly explain the puzzling results of previous work by this group which showed sex differences in mathematical reasoning ability between 22 men and 22 women, all of whom had scores of at least 500 to 800 on the Scholastic Aptitude Test mathematics section. Subjects underwent positron emission tomography (PET) scans while solving math problems, which showed a significant correlation between mathematical reasoning and temporal lobe activity in men but no such correlation in women.

“The women were solving these problems as well as the men—but however they were doing it, we couldn’t see on the PET scans,” Dr. Haier remarked. “This latest work may explain why: White matter activity does not show up on PET scans. MRI spectroscopy is the method for looking at white matter function.”

CLINICAL IMPLICATIONS

The differences in brain architecture may have major clinical implications. “When women have brain damage to the frontal lobes, they are likely to be much more cognitively impaired than men with the same degree of damage,” Dr. Haier noted. “There also might be a difference in how best to approach cognitive rehabilitation in men versus women. This gives us at least a hint of what to look for.”

There are also implications for Alzheimer’s disease, he noted. “Frontal lobe changes might not be seen in women until the disease is farther along than in men, so women might not be getting early intervention at a comparable stage of disease.”

NR

—Janis Kelly

Suggested Reading
Haier RJ, Benbow CP. Sex differences and lateralization in temporal lobe glucose metabolism during mathematical reasoning. Dev Neuropsychol. 1995;11:405-414.
Haier RJ, Jung RE, Yeo RA, et al. The neuroanatomy of general intelligence: sex matters. NeuroImage. 2005;25: 320-327.

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