A new chapter has been added to the continuing tale of sex differences in the brain. According to research announced at the recent American Association for the Advancement of Science (AAAS) annual meeting, the male of the species loses brain cells at a faster rate than the female.
Differences in thinking and behavior between the sexes are legendary. Little boys seemingly instinctively turn sticks into guns; girls are much less likely to do so. Later in life, men tend to say directly whatís on their minds; women give incomplete clues interspersed with silences, expecting their partner to somehow know whatís bugging them. Women remember birthdays; men wonít ask for directions. The list is long.
Sure, gender stereotypes have exceptions. And it is politically incorrect these days to claim that the sexes are anything but absolutely equal. But the behavioral quirks that fuel the persistent stereotypes of male and female behavior have at least a smidgen of basis in truth. Ask any parent of small children.
Establishing the difference
Itís hard to envision how turning sticks into firearms and oneís arguing style are rooted in a three- to four-pound whitish-gray mass of a trillion cells, but the fact is that behavior originates in the human brain. By superimposing views of anatomy provided by MRI images onto physiological portraits painted by PET scans, researchers are finding size and activity differences in particular brain regions between the sexes.
Dr. Ruben C. Gur and colleagues at the Brain Behavior Laboratory and the Center for Functional and Metabolic Imaging at the University of Pennsylvania School of Medicine in Philadelphia are superimposing MRI and PET images to compare regional brain activity in 61 healthy young people (37 men and 24 women) between the ages of 18 and 45. They obtained PET scans using labeled glucose in a dimly lit room following a night of fasting, with the subject at rest. The researchers reported in the January 27, 1995, issue of Science on differences in glucose metabolism in 17 of 36 examined brain regions, concluding that "the number of regions showing significant sex differences is larger than would be expected by chance."
Men tended to have higher metabolic activity in the cerebellum, which controls movement. The picture was more complicated in the limbic system, a group of structures in the middle of the brain that control emotional behavior, long-term memory and other functions. Menís brains were more metabolically active in the so-called "old" limbic system, responsible for active responses, such as aggression. Womenís brains were more active in the cingulate gyrus, the top portion of the limbic system concerned with symbolic communication.
Metabolic rate and cell loss
Although in the 1995 study the researchers stated that the male and female brain are more alike than they are differentñafter all, we are the same speciesñthey also made a case for a biological basis for sex differences in cognitive and emotional processing. More recent additional data seem to indicate that a fundamental difference may be in the rate of cell loss.
At the AAAS meeting in Baltimore in February, Dr. Gur updated the ongoing study on the 61 healthy young adults. Their PET scans show that in some brain regions, men lose tissue at nearly three times the rate that women do, then maintain higher metabolic rates in the remaining tissue, as if to compensate for the greater loss. "The picture you get is that women seem to be able to reduce the rate of neuronal activity in proportion to the tissue that they lose, whereas men continue to overdrive their neurons," says Gur. People begin losing brain cells in early childhood. This is a trend that continues in both sexes, but apparently according to different timetables.
The different rates of cell loss and activity level in remaining cells between the sexes may explain lifespan differences, Gur suggests. The reasonñthe frenetic neuron firing in the male brain can produce toxins that damage cells. "Women survive at least a decade longer than men. And part of the reason for that could be related to the reduced metabolism in the brains proportionate to the loss of tissue, because what happens if you overdrive cells is that you get cytotoxic effects," Gur says. "When the cells are active and the waste products of the metabolism are not cleared away efficiently enough, they build up and can further contribute to the death of cells," he adds.
The shrinking male brain
The site of the greatest difference in brain activity between the sexes, according to Gur, is the frontal lobe, the seat of "planning and inhibition." "Men lose tissue in the frontal lobe at such a rate that by the time they reach middle age, even though they start with larger brains, their frontal lobe is the same size as the frontal lobe in women. Women show almost no reduction in the brain volume of this region," he says.
It will be interesting to see what further differences in level of brain activity the ongoing study of Gur and his colleagues reveal.
By Dr. Ricki Lewis
Contributing Editor
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