A map of the human brain may in fact be a two-volume edition, divided by gender, according to a new study that found significant differences between how the male and female brains are hard-wired.
Males tended to have have stronger front-to-back circuits and links between perception and action, while women had stronger left-to-right links between reasoning and intuition, according to University of Pennsylvania Perelman School of Medicine researchers who imaged the brains of 949 adolescents and young adults.
Their maps of the brain's so-called connectome, published online Monday in the journal Proceedings of the National Academy of Sciences, matched observed behavioral differences between the sexes. Women did better at tests of attention, word and face memory and cognition. Men did better on spatial processing, motor skills and sensorimotor speed.
The results lend weight to growing evidence that humans have formed strong adaptive complementarity, suggesting that biological evolution predisposes the species to divide gender roles. That implication is sure to fuel debate over the roles of nature versus nurture and the interplay of function and structure within the human brain. But they also could inform treatment of neurological disorders known to vary by age and sex, such as autism and schizophrenia.
"There is biology to some of the behavior we see among men and women," said Ragini Verma, a University of Pennsylvania biomedical imaging analyst and lead author of the study.
"In the population, men have stronger front-back connectivity, and women have inter-hemispheric or left-right connectivity more than the men. It's not that one or the other gender lacks the connectivity altogether, it's just that one is stronger than the other," Verma said.
That means men may be quicker on the perception-action path, while women better integrate the analytic side of the brain with the intuitive and social side.
"So, if there was a task that involved logical and intuitive thinking, the study says that women are predisposed, or have stronger connectivity as a population, so they should be better at it," Verma said.
"For men, it says they are very heavily connected in the cerebellum, which is an area that controls the motor skills. And they are connected front to back. The back side of the brain is the area by which you perceive things, and the front part of the brain interprets it and makes you perform an action. So if you had a task like skiing or learning a new sport, if you had stronger front-back connectivity and a very strong cerebellum connectivity, you would be better at it."
Researchers used diffusion tensor imaging, a tool that can indirectly outline the path of myelinated axons, the "wire" section of neurons that facilitate long-range conduction of electrochemical signals and are part of the brain's white matter. They looked at the brains of 428 men and 521 women, ages 8-22, who are part of a larger, long-term study known as the Philadelphia Neurodevelopmental Cohort, conducted with the Children's Hospital of Philadelphia.
In the upper parts of the brain, the men had greater connectivity within hemispheres, while women had greater connection between sides, the study found. Women also tended to have more connections among smaller-scale "modules," while men had stronger connections within those subregions.
In the lower part of the brain, the cerebellum, men had stronger connections between hemispheres, giving them a possible edge at translating perception to motor skills. Women had more interconnections across the frontal lobes, the study found.
The differences in the connectome have come to be called the hunter versus gatherer divide by two of the study's main authors, the husband-wife team of Raquel and Ruben Gur. And the data jibe with findings from a 2011 UCLA study of twins that found women had stronger inter-hemispheric connections in several subregions of the frontal cortex.
"They confirm a couple of our findings, which is very exciting," said Neda Jahanshad, lead author of the UCLA study, who is now working at USC's Keck School of Medicine. "This is interesting on a variety of levels because there have been sex differences noted among those with autism, for example."
Men outnumber women by a 4-1 proporton among those diagnosed with autism spectrum disorder.
Although such sex differences are important to the study, the Philadelphia cohort, funded by the National Institute of Mental Health, was aimed predominantly at studying how brain maturation affects psychiatric disease.
By age, differences between male and female brains become sharp around adolescence but abated somewhat in young adulthood, the study found.
Researchers cautioned that the imagery is an indirect measure of axons, not a cell-by-cell census and map. And the results are strictly statistical averages, although in a very large sample.