The male urogenital system has been comprehensively mapped on to the sensory cortex in the first study of its kind. These data can be used to improve our understanding of the sexual response in men.

Investigators used functional MRI to map primary somatosensory cortical responses to self-stimulation of the penis shaft, glans, testicles, scrotum, rectum, urethra, prostate, perineum and nipple. They also compared the neural responses to erotic and nonerotic touch of the penile shaft.

The study included 21 men with no urogenital or sexual health issues. These men were trained to rhythmically squeeze their penile glans, penile midshaft, scrotum, left testicle, right testicle and left nipple to achieve nonerotic touch. They were also asked to stimulate their penile shaft in a way they perceived as erotic, as erotic touch is subjective. A custom-made rod was used to stimulate the perineum, rectum and prostate; urethral stimulation was achieved using a thin intermittent urinary catheter. Participants were able to leave out any procedures they did not wish to perform and not all men completed each procedure. A post-scan questionnaire was administered to ascertain the participants’ perception of each procedure.

The results showed that the response to mild stimulation of the penile glans or shaft was superficial in the paracentral lobule, whereas the location of the response to forceful squeezing was deep in the paracentral lobule. Direct stimulation of the urethra using a catheter caused a response that was distinct from forceful penile stimulation. Stimulation of the corpus cavernosum also seemed to cause a unique response in the deepest portion of the paracentral lobule.

The dorsomedial region of the paracentral lobule near the cortical surface was activated by mild rectal self-stimulation, whereas activation of a more posterior region of the dorsomedial paracentral region was evident when the anterior wall of the rectum was stimulated.

Activation of the posterior dorsomedial region of the paracentral lobule was observed after subtraction of the response to rectal stimulation from combined rectal–prostate self-stimulation, indicating that the rectum has a different homuncular projection region from the prostate.

The paracentral lobule near the cortical surface was activated on self-stimulation of the perineum and rectum. A region deep in the midline of the paracentral lobule was activated on stimulation of the scrotum or either testicle. The deep medial paracentral lobule region was activated on nipple stimulation, which partially overlapped with the region activated by penile or testicular stimulation.

The response pattern of intentional erotic self-stimulation of the penile shaft was markedly different from the response to nonerotic self-stimulation. Further analysis suggested that different physical touch does not account for differences between erotic and nonerotic stimulation.

The observations of the neural response to urogenital self-stimulation in this study increase our understanding of the sexual response in men and could help identify targets for treating urogenital and sexual arousal disorders.