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Single subject and group whole-brain fMRI mapping of male genital sensation at 7 Tesla 79
INTRODUCTION
Human sexual behaviour is characterized by inter- and intrapersonal intimacy. This in- cludes tactile stimulation of the external genitalia, giving rise to both discriminative and affective sensations.1 Despite the evident role of the brain in the perception of touch, it remains unclear how tactile input from the external genitalia is centrally processed in the healthy individual.
The primary and secondary somatosensory cortices (S1 and S2) are well-recognized as the main cortical areas involved in processing the discriminative properties of tactile in- put. Over the years, studies investigating genital touch in women and men have generally focused on S1 due to the counterintuitive location of the genitalia beneath the feet in the sensory homunculus.2,3 Some confirm this location, recording cortical evoked responses deep in the medial wall of S1 during electrical stimulation of the dorsal nerve of the penis (DNP; principal sensory nerve of the penis).4-6 Later functional magnetic resonance imag- ing studies (fMRI) studies, however, demonstrated significant brain activation dorsolateral in the groin region of S1 in response to passive tactile stimulation of the external genita- lia.7,8 Findings regarding the involvement of S2 in processing tactile penile input show more consistency, demonstrating bilateral activation during both electrical6 and tactile stimulation.7,8 Other brain regions have also been associated with processing the affec- tive properties of touch. This includes not only the insula,9 but also the anterior cingulate (ACC) and medial prefrontal cortex (mPFC) during tactile stimulation of the forearm.10 Bilateral activation of the insula has been demonstrated in response to both electrical6 and tactile penile stimulation.8 Activation of the midcingulate cortex (MCC), but not ACC, and mPFC have only been reported following tactile stimulation of the male genitalia.8
Recently, high field (7 Tesla) fMRI has emerged as a superior neuroimaging technique to study human brain function in vivo. Increases in signal-to-noise ratio (SNR) and blood-oxy- genation-level dependent (BOLD) sensitivity at 7T11,12 have made it possible to acquire data with high spatial acuity and demonstrate robust activation in individuals. Recent studies showcasing these advantages include mapping the representation of individual digits13 and the lower limb14 in S1 at the single subject level. Given the substantial intersubject variability of neural representations as well as the need to treat patients as individuals, it has become eminently important to study human brain function at the level of individuals.13-15 Moreover, signal advantages gained at 7T can not only be observed at a limited field of view such as in S1, but also in more extended whole-brain acquisitions16 including the cerebellum.17
In the present study, we used 7T fMRI to acquire high-resolution neural representations of male genital sensation in the whole-brain at both single subject and group level. In order to do so, subjects underwent passive tactile stimulation of the genitalia in a way that minimized sexual arousal. Passive tactile stimulation of the medial aspect of the feet, performed in an equivalent manner as genital stimulation, served as a control task. The feet were deliber-
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