It's All In The Mind... Brain Mapping Of Sexual Arousal

The complex cerebral bases of human sexual behavior have rarely been investigated. Most studies have been done on animals and patients, especially subjects with certain rare forms of epilepsy characterized by sensations of uncontrolled genital pleasure. Serge Stoleru (INSERM research unit 292 specialized in public health, directed by Alfred Spira) and colleagues at CERMEP have unveiled one of the biological aspects of this supposedly abstract phenomenon we call desire. They have not only found that the activity of the brain is related to sexual desire, but have also identified the precise brain areas concerned.

Visual stimuli play a major role in human sexuality. The researchers therefore selected eight healthy young men aged between 21 and 25, all of whom were right-handed (emotions are lateralized in the brain, meaning that subjects in this type of study must all be either right-handed or left-handed), and showed them a series of three radically different six-minute films, in the following order: an emotionally neutral sequence (a geographic documentary); an extract from a comedy film (to provoke positive emotions) and a sexually explicit sequence (eliciting sexual emotions). The volunteers watched the films while being monitored by a positron emission tomograph, a device which provides extremely detailed images of the brain and can be used to identify brain regions that are activated during different mental operations.

Five brain areas were found to be more active during the sexually explicit film than during the neutral and/or comic film.

• The first was the inferior temporal cortex, a region also corresponding to the visual associative zone. The researchers inferred that the subject was assessing and analyzing the visual stimulus when this zone was activated, corresponding to the perceptive-cognitive component of sexual arousal.

• The second region was the right orbitofrontal cortex, which might be related to emotional and motivational phenomena.

• The third area was the left anterior cingulate cortex, which appears to control primary physiological responses (endocrine and autonomic), but also affective responses, to sexual stimuli. In other words, it would govern physical and psychological preparation for sexual activity.

• The fourth region was the right insula, that could be involved in subjective perception of physiological modifications associated with arousal (heart rate acceleration, penile erection, etc.)

• Finally, the right caudate nucleus probably controls whether sexual arousal is followed by sexual activity.

The researchers also measured various physiological parameters, especially the blood concentration of testosterone, the principal male hormone. The testosterone level rose very strongly during the sexually explicit film; it also increased slightly during the comedy sequence.

These results may throw light on the neurophysiological bases of sexual arousal dysfunctions, which may take the form of sexual apathy or, on the contrary, pathological hyperactivity leading to deviant behavior such as rape. Serge Stoleru and colleagues have already started to study men with a marked and durable loss of sexual desire.

Why so little research on female sexual desire?
Female sexual desire and its cerebral correlates are more difficult to study, for two reasons. Positron emission scanning involves the use of a low dose of radioactive markers, which could be dangerous for the fetus in women with an early, undiagnosed pregnancy. In addition, women's hormone cycles are not synchronous, and a methodological bias might be introduced if all the women in such a study were not in the same phase of their monthly cycle.