William-Adolphe Bouguereau's 1875 painting "Cupidon", a portrayal of the Roman god of erotic love. New research shows that even invisible erotic images are attention-grabbing.

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SYDNEY: Even when we're not aware of them, erotic images can still grab our attention, showing that the hunt for a mate takes place without our conscious control.

According to Sheng He from the Department of Psychology at the University of Minnesota and colleagues, "invisible erotic information can either attract or repel observers' spatial attention depending on their gender and sexual orientation." In other words, sex sells - even subconsciously.

The researchers used an experimental technique known as interocular suppression, in which subjects are presented with images without being aware of them. The reason subjects do not notice the images - placed in the visual field of one eye - is because they are suppressed by high-contrast noise patches (something like television static) presented to the other eye.

Previous research has shown that our brains can still respond to these 'invisible' images. For example, the amygdala, which lies deep within the brain and plays an important role in processing emotion, has been shown to respond to emotional cues without our being consciously aware of them.

The researchers combined the technique of interocular suppression with another experimental tool, the Posner cuing paradigm, which measures the direction of a subject's gaze after they have been presented with a visual cue.

He and colleagues reasoned that if an invisible cue played upon a subject's emotions, the brain might redirect visual attention to it. "Because we were interested in the potential effect on attention from the emotional system's response to invisible images, we chose highly arousing erotic images," explain the researchers in a paper published today in the Proceedings of the National Academy of Sciences.

These erotic images of naked men and women were selected from a scientific catalogue known as the International Affective Picture System. The researchers enlisted both heterosexual and homosexual men and women for their tasks.

They found that the subjects all paid attention when these invisible nude images passed in front of their eyes, looking harder at the place where an image had just been shown.

Heterosexual men were more likely to look in the place where a nude female image had just been displayed, and straight women paid attention to nude men.

Gay men showed responses that were similar to those of heterosexual women, whereas gay and bisexual women responded in a manner somewhere between straight female and male observers, but closer to the females.

Evolutionary principles suggest that important stimuli such as food, threats, or potential mating partners should be effective at capturing our attention. These results confirm that "even in the absence of awareness, the emotional system processes information in a very specific fashion, both in terms of representing the spatial location and in terms of coding the gender information of the image content," according to the researchers.

"This level of specificity makes it possible to orient attention to rewarding opportunities or away from aversive events before conscious perception occurs."

He and colleagues point out that the group differences were measures of central tendencies and were not guaranteed at the individual level. "In other words, we do not foresee that the pattern of results reported here could be used to determine an individual person's sexual orientation."