Previous theories had suggested the brain first acquires information about sexual attractiveness from visual cues, which are then sent upward through the emotional centers and finally on to regions of more complex thought.



But Ortigue's study shows that the sophisticated mind may be first sending information downward, providing the visual cortex with a blueprint for who is attractive while prepping our emotional centers with prejudged responses.



"We've found the brain knows who we desire and when we desire before we are aware of it," she said. "It's very unconscious."



Self-Image Key to Attraction?



For their study, published in 2008 in the journal NeuroImage, Ortigue and Bianchi-Demicheli wired a group of 13 healthy adults so the scientists could record brain activity via a technique called high-density EEG neuroimaging.



The volunteers were asked to look at pictures of people in swimsuits and decide whether the subjects were hot or not. (Related: "Bikinis Make Men See Women as Objects, Scans Confirm.")



Most people made a decision in well under half a second, the study authors report. But during that short amount of time, several different regions of the brain lighted up electrical activity, including higher brain regions.



One of the areas most active in desire is intimately involved with people's self-awareness and self-image, the researchers found.



"Basically what that means is that people who have disorders of their self-image might also have disorders of sexual desire," Ortigue said.



In addition, test subjects took slightly longer to identify attractive people than they did to discount the unattractive—a finding that makes sense to evolutionary psychologist Donald Symons of the University of California, Santa Barbara.



"For a stimulus image to be highly attractive, all the observable attractiveness cues must fall into the highly desirable range," said Symons, who was not involved in the study. (Related: "Symmetrical Bodies Are More Beautiful to Humans.")



But just one unattractive quality, such as obesity or extreme acne, could be enough of a turnoff for someone to instantly judge a person as undesirable, he said.



"So detecting ugliness would be, on average, an easier perceptual problem, and one that could be made more quickly then detecting great beauty."



Evolution of Desire



Ultimately, Ortigue and Bianchi-Demicheli's research could provide new insight into why humans have evolved to desire others the way we do.



"What I would expect, and what Stephanie has begun to demonstrate, is that complex, specialized cortical adaptations exist that were designed to solve the problem of mate choice faced by our ancestors," Symons said.



(Related: "Ardi's Secret: Did Early Humans Start Walking for Sex?")



In other words, the brain's current wiring could be a product of the way ancient humans gathered information about a potential mate's value based on specific physical characteristics.



In the days before Proactiv and Oil of Olay, for example, skin condition might have been a good indicator of a person's age or larger health problems.



"Therefore," Symons said, "we'd expect natural selection to have created psychological mechanisms specialized to detect and use reliable information about mate value."



For future work, study author Ortigue said she would like to move beyond photographs and wire live couples to record their mutual brain activity, allowing her to map the dynamics of desire "in the flesh."