(cm) The brain is often also considered as a sexual organ (1). This view now gains further support from a study carried out by two scientists from ETH Zurich and the University of Zurich in the latest edition of the science magazine "Current Biology" (2).

Felicitas Kranz and Alumit Ishai from the Institute of Biomedical Technology have discovered that when shown a face that corresponds to their sexual preference, men and women show increased cerebral activity in the orbitofrontal cortex that lies just behind the forehead. Heterosexual women and gay men thus reacted more strongly to men's faces and vice versa.

In all, 40 test subjects took part in the experiment, which included heterosexual and homosexual men and women, four groups with ten subjects in each category. The subjects were each asked to study a hundred pictures of faces of men and women. While they were doing so the researchers measured cerebral activity in various areas of their brains using functional magnetic resonance imaging (fMRI).

Although, as stated, sexual preference activates a specific area of the brain, the four groups of test subjects all agreed on the attractiveness of the faces. It also transpired that the attribute "unattractive" was assigned more readily than "attractive".

A closer look at the fMRI results reveals that the difference in the level of activity in the orbitofrontal cortex is greater in men than in women. The smallest difference was found in the group of heterosexual women. Generally speaking, the area of the brain with the observed differences is integrated in the brain's reward circuitry.

As the observed differences occurred in the brains of both the heterosexual and homosexual subjects of the study, the scientists conclude that it is sexual preference and not reproductive fitness that modulates the neuronal response to pertinent stimuli in the brain of an adult. In other words, the brain reacts to possible sex and not to possible progeny.