Why doesn't Viagra work for women? Today's NYTimes reports on new research that points to ways that men and women's brains differ when they become sexually aroused that may help explain this.

Emory University's Stephan Hamann and his research team had 14 male and 14 female participants view several types of sexual and social interaction images for 30 minutes. Their brain activity was then compared using functional magnetic resonance imaging (fMRI), a technology that measures neural firing through changes in blood flow. A pattern immediately emerged. The photographs set off a frenzy of brain activity, particularly in the amygdalae of men. Yet the two groups reported equal arousal most of the time.

Other studies have gone further. In a study published last year, researchers in the Netherlands recorded brain activity in men as their female partners brought them to orgasm. The amygdala, the scientists found, showed decreased activity during climax. Other studies have suggested that a larger amygdala may lead to a more robust sex drive.

Others are not so impressed with the research. "Differences between genders are boring," says Dr. Leonore Tiefer. "The big differences are within the sexes, between individuals. It is not the case that every person pays attention to the same thing. It's like everything else in life — eating, dancing, traveling. The whole experience is shaped by your history and by what you're paying attention to."

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