It's a vicious circle: Women who pay too much attention to their performance during sex could be inhibiting sexual desire — and that lack of desire increases self-consciousness.

The idea's not new, of course, but now it's got scientific data to back it up. A new study by the Stanford University School of Medicine suggests that women with hypoactive sexual desire disorder (characterized by a continual lack of sexual interest or fantasies) use more brainpower than sexually healthy women in monitoring their reactions and performance during sex.

Researchers used functional magnetic resonance imaging to track brain response to sexual stimuli in 16 HSDD women and 20 women who don't suffer the condition. Participants were shown clips of erotic films, women's sporting events, and relaxing nature scenes. In addition to the brain scans, the women subjectively rated their levels of arousal while an instrument objectively measured vaginal response. Their brains lit up in very different ways.

"Many of the HSDD subjects spent their time monitoring their experience, or lack thereof," said Leah Millheiser, one of the lead researchers of the study, which was published in the journal Neuroscience.

"For example, they may have been asking themselves, 'Am I responding correctly?', 'Is this how I am supposed to be feeling?', 'Should I be experiencing more arousal than I currently am?', instead of actually allowing themselves to integrate the information being presented to them in the erotic video."

These women, Millheiser says, may be cheating themselves out of the ability to associate positive emotional memories with sex.

In addition, the results pointed to a disconnect between both groups' subjective ratings and the arousal measurements taken by the vaginal instrument — results that are consistent with those found in other studies.

"Women can experience little subjective arousal, but still have a genital response, meaning vaginal engorgement and lubrication," Millheiser said. "The reason behind this phenomenon is not well understood. There is a popular theory that women could have developed this response during the evolutionary process to protect the pelvic floor during forced sexual acts for procreation or childbirth."

The study suggests a useful strategy:

"As a female sexual medicine practitioner, I encourage women to 'be in the moment,'" Millheiser said. "That means to focus on the pleasure they are experiencing either by themselves or with their partner.

What I discourage is for women to focus on what they 'think' they should be experiencing. If certain expectations are not met during a sexual situation, this may lead to a negative sexual experience."

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Sally Law has written about health and sexuality for the Cleveland Clinic, and has appeared regularly as a guest host on Sirius Radio. Her column, The Science of Sex, appears weekly on LiveScience.