Even oft-repeated gender stereotypes harbor some truth: Angry men are more likely to yell or punch a wall, whereas angry women sit silently stewing. Now, a new study is tracing these distinctions in how men and women process emotion to an almond-shaped structure deep in the brain. Not only does the structure, the amygdala, function differently by gender, but its activity in men is also coupled with very different brain regions than it is in women.
The amygdala straddles both sides of the brain and helps control how emotions such as fear are processed and remembered. Several studies have found gender differences when the amygdala is stimulated--by having volunteers recall scary movies, for example. In men, the right side of the amygdala, known simply as the right amygdala, appears more likely to become active, whereas in women it's the left. Neurobiologist Larry Cahill of the University of California, Irvine, wondered whether this difference was hardwired--whether, in other words, the amygdala retained its gender-specific tendencies even when nothing was activating it. If so, this would suggest that the structure was inherently different in men than in women.
Cahill and his colleagues studied PET scans of 36 men and 36 women, all of whom were right-handed. The scans had been collected for various brain studies where volunteers were asked to close their eyes and relax while the pictures were taken. The team found that, even at rest, the amygdala worked differently in men and women. In women, blood flow to the left amygdala ebbed and flowed along with other brain structures while the right amygdala did little. In men, it was blood flow to the right amygdala that varied along with blood flow elsewhere in the brain, the researchers report 1 April NeuroImage.
Especially intriguing, says Cahill, were the regions with which the amygdala was acting in concert. In women, those tended to be the hypothalamus, which directs the body's stress response and affects feelings, and the related subgenual cortex. In men, the amygdala acted with motor and visual brain areas, which are "believed important for interacting with the external world," says Cahill. He admits he doesn't know what the volunteers were thinking while being scanned or whether that affected the results.
Still, the study suggests that "this might be a default state," says John Gabrieli, a neuroscientist at the Massachusetts Institute of Technology in Cambridge. For men, he says, "it seems that there's a stronger coupling ... [for] dealing with stuff out there in the world," while this wasn't shown for women. No one knows quite what it all means, Gabrieli admits, but the findings are "food for thought."