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Tired of spending hours looking for answers to your questions?
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 For almost a decade, researchers at Pfizer struggled to show that Viagra, the male impotence drug, could enhance sexual function in women. Last month, they gave up. Countless tests on thousands of women made it clear that the little blue pill, though able to stir arousal, did not always evoke sexual desire. Viagra's failure underscored the obvious: when it comes to sexuality, men and women to some extent are differently tuned. For men, arousal and desire are often intertwined, while for women, the two are frequently distinct. Scientists have recently begun to map out how this difference shows up in the brain. For example, male arousal, studies find, is strongly visual, and when men engage in sexual activity or even anticipate it, brain structures once thought to have little connection to sex spring into action. The same brain regions, however, remain relatively quiet when women are aroused. At the core of the sexual divide, some researchers say, is the amygdala, an almond-shaped nugget embedded in the limbic system, the brain's seat of emotions. Once thought to be involved exclusively with emotions like fear and anger, the amygdala is now believed to be more complicated. In one recent study, a team of researchers at Emory University had 28 men and women look at erotic photographs while an M.R.I. took snapshots of their brains. 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. "This definitely emphasizes that up until recently the amygdala has been overlooked," said Dr. Stephan Hamann, a professor of psychology and the lead author of the study, which was published online by Nature Neuroscience last week and is scheduled to appear in the journal in April. "Often, the amygdala is involved in the anticipation of positive emotions, which this parallels nicely with." Much of the insight into the amygdala's role in anticipating sex, and possibly other pleasant emotions, stems from research on animals. In a study in 1989, scientists trained caged male rats to gain access to females by pushing a lever. The researchers then destroyed part of the amygdala. The rats lost interest in pressing the lever. Yet despite this lack of motivation, they had no problem engaging in sexual intercourse when the females were placed in their cages. 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. Dr. Hamann pointed out that the amygdala is known to have intricate connections to primates' visual systems. One reason for the powerful response to visual stimuli in men, he said, could be cultural. Men tend to be inundated with sexual imagery and, possibly, are more likely to seek it out. Evolution may also have a role. Some experts argue that, over time, men naturally became more dependent than women on sight in selecting a mate. "For millions of years, men have had to size up a woman's reproductive capacity by looking for signs of youth and health that would enable them to carry a healthy baby," said Dr. Helen Fisher, an anthropologist at the Center for Human Evolutionary Studies at Rutgers, and the author of a recently published book, "Why We Love: The Nature and Chemistry of Romantic Love." Dr. Fisher has studied the brains of people in the early stages of romance. For a man, she found, pictures of a new partner light up parts of the brain involved in visual processing and arousal. But women, she noticed, show more activity in areas linked to reward, emotion and attention. "Men, despite what most people think, fall in love faster than women do, probably because they're so visual," Dr. Fisher said. Women, on the other hand, may be more attuned to the signs that a man will make a good father or provider, she added, though some other researchers find this assertion dubious. A woman's choice of a mate, Dr. Fisher argues, could involve an interplay among a number of factors, including, some experts now theorize, a man's odor or pheromones. Several years ago, Swiss scientists discovered that women could sniff out genetic differences in potential mates. When women were asked to smell T-shirts that different men had worn, they often ranked more favorably the shirts that belonged to men with dissimilar genes for major histocompatibility complex, a group of proteins involved in immunity to disease. The odors a woman preferred also tended to remind her of past and current partners. Article Suggested By: PEA editors Suggest an article. |
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