In the mind's eye, disturbing or pleasant images can't hold a candle to the erotic, says a recent study out of the Washington University School of Medicine. One of the first functions the brain performs with an image is classifying the picture into one of two categories: "hot" or "not."
While the study's finding that we're wired to recognize sex will come as no surprise to anyone affiliated with the multi-billion dollar pornography industry, its subject pool may throw convention for a loop: All 264 subjects were women.
The report, published online in the journal Brain Research, indicates that a person's brain determines whether or not an image is erotic long before the viewer is even aware they are seeing the picture. The study's researchers also discovered that sexy shots induce a uniquely powerful reaction in the brain.
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"Reproduction is, obviously, biologically, a very significant function. It is essential for survival," said Washington University psychiatry professor Andrey Anokhin, the study's lead author. "Therefore we can speculate that there may be some specialized neural circuits that process signals that relate to reproduction."
Anokhin and his colleagues showed a series of images varying in content and emotional significance to their female subjects while their brain activity was being monitored.
The subjects' brains reacted 20% faster to erotic images than to all other pictures.
While the researchers could not determine precisely where in the brain the erotica-specific signal originated, Anokhin said it came from somewhere in the frontal area, which contains structures responsible for processing visual images related to tasks or emotions.
Anokhin added that his team's results show the brain discriminates pictures based on overall scene content, not on any specific physical feature.
"There were humans in other scenes—also interacting humans and also half-naked humans—and still those images did not produce that effect," he said. "It's only the images that had an erotic content."
When the researchers tested 18 men, Anokhin said, they found the guys' immediate neural response was no greater than that of the female sample.
Studies on subjective experience (and countless personal testimonials) conclude that men are more "visual" than women— that they become more aroused when they look at sexy pictures. Several studies of brain activity also show greater neural response in men when they watch erotic films.
While Anokhin said his results are interesting because they don't seem to support our notions of how the sexes differ, authors of earlier studies said the new work is perfectly compatible with older findings.
"The dissimilarities that we found—using erotic film excerpts instead of static images—were related mostly to a deep brain structure (the hypothalamus) with patterns of brain activation being otherwise virtually identical between genders," said an e-mail from McGill psychiatrist Sherif Karama, the lead author of a 2002 paper on brain activation during erotic film watching.
Emory psychology professor Stephan Hamann authored a study on how the sexes differ with respect to brain response to visual sexual stimuli and found that men experience more activity in an area linked to arousal. Hamann said that while the immediacy of the response observed in Anohkin's study is interesting, the brain activity recorded might be completely unrelated to arousal.
"I don't think that the data necessarily show anything other than the fact that the brains of men and women can differentiate between erotic stimuli and non-erotic stimuli really pretty quickly," Hamann said. "It doesn't necessarily mean that anything going on in the brain is actually now linked to arousal at that early stage."
"I'd be willing to guess, though," he continued, "that there might be some other areas having to do with emotion that might be related to this."