Swelling detected at the genitals appears to add to desire arising from the stimulus of a potential partner (Georgiadis et al., 2012) and might even provide the principal stimulus to desire (Laan and Both, 2008). Positive feedback seems to be involved (Mouras et al., 2008); in response to a visual stimulus, desire would contribute to genital arousal and, by means of signals from the genitals to the brain, there would be an amplification of desire.

The bits that make up the whole

Knowledge of the brain bases of desire comes from several sources (Chapter 2):

1. research on non-humans and cautiously extrapolating to humans;

2. looking at changes in the sexual desire of people following brain damage or disease;

3. using neuroimaging to examine activity in the brains of people exposed to erotic images.

Neuroimaging reveals a network of interacting brain regions, parts of which are excited and others of which are inhibited by erotic visual stimuli (Georgiadis et al. 2010; Redouté et al. 2005). Researchers distinguish regions serving some closely related but nonetheless somewhat conceptually distinct roles:

1. to make an initial assessment of the sexual value of the content of the image;

2. to produce a signal that is sent to the genitals to trigger swelling;

3. to receive feedback from the genitals on their arousal – this signal is thought to contribute to the conscious awareness of the state of the body, particularly the dimension of eroticism;

4. to create a motivational signal, having unconscious and conscious aspects, which tends to direct behaviour towards the sexual stimulus – a feature of this is conscious desire.