March 8, 2004 - It's no mystery: Men are more interested and respond to erotic pictures more than women do. But now, with sophisticated new imaging, scientists can now see what's happening in the brain that makes this happen -- and explain men's sex drive.
Their report appears in this week's issue of Nature Neuroscience.
It's a subject that has long fascinated researchers. In animal studies, several sex differences have been identified in areas of the brain called the limbic regions -- those that regulate reproductive behavior, writes researcher Stephan Hamann, PhD, a psychologist at Emory University in Atlanta.
For example, male rats respond to sexual stimulating smells and visuals -- a response that occurs in two structures within the limbic area of the brain called the amygdala and hypothalamus. Female rats don't show this brain activity pattern, he writes.
But do these same "sex drive brain patterns" exist in humans?
Sex and College Students
To answer that question, Hamann and his Emory colleagues used sophisticated MRI brain imaging to look into the brain activity of 28 male and female college students.
The students looked at arousing photographs of heterosexual couples engaged in sexual activity and sexually attractive opposite sex nudes and at "neutral" photographs -- pleasant pictures of men and women in nonsexual situations.
"We found that the [brain areas] amygdala and hypothalamus were more activated in men than in women when viewing identical sexual stimuli, even when females reported greater arousal," writes Hamann.
In fact, men and women both rated the erotic photographs as sexually attractive and physically arousing. Both groups also found the photos of the couples more attractive and arousing than the photographs depicting a nude person of the opposite sex.
However, in a closer look at responses to the photographs of couples -- which got the biggest arousal from both men and women -- men had greater activity in the amygdala and hypothalamus than did women, writes Hamann. Women showed no significant activation in these regions.
Men also revealed a "greater propensity" for finding even the "neutral" photographs arousing -- although to a lesser degree, reports Hamann. This pattern may reflect what other sex drive studies have found, that across all cultures men prefer sexual variety more than women, he says.
Inside the Amygdala
The amygdala has multiple functions, although its prime role is processing emotions, Hamann explains.
The amygdala plays an important role in survival -- in quickly determining what's good and what's threatening, Hamann writes.
This study, among others, shows that the amygdala is also related to a stronger sex drive, not just emotional arousal. Even though women were aroused by certain photographs, they didn't experience the activation of this brain structure.
However, whether men's and women's brain activity in sexual scenarios is a function of inherent brain differences -- or whether they stem from life experiences -- is a matter for further study of the human sex drive, he concludes.
SOURCE: Hamann, S. Nature Neuroscience, March 7, 2004, advance online publication.