Sex almost always gets people's attention -- whether they are consciously aware of it or not.
Some people think about it all the time: Fifty-four percent of men and 19 percent of women think about sex every day or several times a day, according to the Kinsey Institute for Research in Sex, Gender and Reproduction.
Now scientists say that some people react to erotic images that they can't even see. A study released today in the journal Proceedings of the National Academy of Sciences reports that subliminal erotic images compete for the brain's attention even if those images are hidden from our eyes.
Researchers also found an interesting connection between a people's sexual orientation and how their brain reacted to each picture. It seems that subjects were more distracted by subliminal images of the gender they found more sexually desirable.
For example, heterosexual women are attuned to images of naked men but not to images of naked women, the researchers found after analyzing their data. Homosexual men are attuned to images of naked men, and homosexual women were equally distracted by naked images of both sexes.
But this research has not convinced experts in human sexuality to jump to any conclusions about sexual orientation.
"The concept of a test to determine sexual orientation is anywhere between reductive and ludicrous," said Cory Silverberg, a sex educator and sexual media consultant in Toronto.
"Neither the medical science nor the social science community has come anywhere near producing a body of work that could concretely define something like sexual orientation."
So, these data are less about sex and more about our subconscious and how the brain reacts to subliminal information.
The idea that the brain reacts to subliminal information isn't terribly surprising -- the advertising industry has exploited this for decades. The brain can process what cannot be seen, such as subliminal presentations of erotic, violent or other emotional stimuli. But the scientists did not know whether the brain can act on this invisible information.
Researchers from the University of Minnesota wanted to see how powerfully subliminal images could affect our ability to pay attention.
Scientists gave 40 men and women a visual attention test while flashing subliminal erotic images or neutral images -- a pixilated scrambled image -- as a distracter. Scientists wanted to see how the images affected men's and women's test scores.
The erotic image was rendered "invisible" by a technique called interocular suppression. The subliminal erotic image flashed at the same time as a neutral image, on either side of a single focus point. So if the subliminal erotic image flashed on the left side of a focal point, the neutral image flashed on the right side of that point.
The visual attention task began after the erotic and neutral images disappeared.
Scientists quickly flashed a test picture of parallel lines on either the left or right side of that same focal point. They then asked the study subjects to recall on which side of the focal point the picture had appeared.
The theory is this: If the subliminal erotic image captures a subject's subconscious attention, his or her attention should focus on the side of the focal point where the subliminal image appeared. And if the test picture flashed on the same side of the focal point as the subject's now-focused attention, the subject should remember where the hologram appeared because he or she had been watching that spot closely.
The subliminal sexy pictures definitely got the subject's attention. And interestingly, the erotic images attracted or repelled attention depending on the gender depicted in the image and the sexual orientation of the subject.
But subjects didn't react the same way when they could actually see the pictures. When the scientists repeated the experiment without using interocular suppression -- so the erotic images were no longer invisible -- the same pictures did not attract the subjects' attention.
The study's authors suggest that people might consciously turn away from the erotic images if they have fears about sex or sexuality -- or this part of the finding could be caused by a flawed experimental design.
Scientists already knew that our brains collect information without our conscious awareness. "Our brain can and does process a lot of information without our conscious awareness," said the study's author, Sheng He, in the department of psychology at the University of Minnesota.
But these findings demonstrate that our brains process emotion not just information, without our conscious awareness. So people can react to something or can feel emotions without realizing where those feelings are coming from.
In particular, people can react to a sexual image without realizing that they are attracted to or repelled by this image. And in this study, that attraction seems to be related to sexual orientation.
The finding that our subliminal attentions are related to sexual orientation is an interesting idea to play with, but how far can scientists extend this finding?
It could affect how people understand their own sexuality, says Sheng. "As an extension, one could also make the inference that sexual orientation ... is not a conscious choice of the individual."
In this study, the subjects' attention "was driven by automatic mechanisms in their brain, mechanisms that are not under their control," says Sheng.
The sexual orientation findings also reflect something that scientists have seen before in a very different context.
"Visual 'reaction time' has been used in the assessment of sexual offenders to determine sexual preference, including sexual arousal to children," says Dr. Martin Kafka, a psychiatrist at McClean Hospital and professor of clinical psychiatry at Harvard Medical School.
But Sheng and other experts are extremely cautious on any interpretation about sexual orientation drawn from this data. This isn't something that could be a "test" for homosexuality, experts say.
The study results are strong, but the correlations -- the relationship between sexual orientation and attention -- are far from exact, experts say. So the test wouldn't allow you to conclude anything about any specific individuals.
"These data ... tell you about what can be predicted or was found in a group of individuals but does not predict how any one individual will behave or react," says June Reinisch, director emeritus and senior research fellow at the Kinsey Institute.
So, this is far from a test for sexual orientation. But it does paint an interesting picture of what the brain can see when we're not looking.