Clitoral, vaginal, and cervical self-stimulation differentially activated regions of the sensory cortex, but all were clustered in the medial paracentral lobule.
Because the perineal (groin) region is also stimulated incidentally during the clitoral, vaginal, and cervical self-stimulation, its corresponding sensory cortical region -- i. e, the dorsal convexity of the paracentral lobule, immediately lateral to the midline -- was also activated.
The present findings may help to resolve a discrepancy in the literature that claims that the location of the genital sensory cortical representation is on the dorsolateral paracentral lobule, rather than the medial paracentral lobule [8
]. That is, based on the present findings, the discrepancy in the literature may be due to responses to indirect stimulation of the perineal (groin) region rather than to adequate stimulation of the genitals per se
It is likely that the clitoris is indirectly stimulated by self-stimulation of the cervix or vagina. Under the conditions of the present study, it is not possible to discern whether the overlap among regions of the sensory cortex activated in response to self-stimulation of each of these three genital regions is due to true overlap of the brain regions that would be activated by “pure” stimulation of each of these three genital regions separately, or whether the overlap is due to incidental stimulation of one genital region (e.g., vagina) during self-stimulation of a different genital region (e.g., cervix). What is clear, however, is that to some extent, the sensory cortical regions activated by each of these three genital regions are to some extent separable and distinct. Unexpectedly, nipple/breast self-stimulation activated not only the (expected) thoracic sensory homuncular region, but also the region of the paracentral lobule that overlaps with the region activated by clitoral, vaginal, or cervical self-stimulation. This finding is consistent with many women's reports that nipple/breast stimulation is erotogenic and can elicit orgasms ([16
] and personal communication).
The present finding of convergence between nipple and genital input in the genital sensory cortex is supported by an intriguing observation by Penfield and Rasmussen ([3
], p.26): “One patient, Case E.D., a woman of 27 years who had a small glioma in the right postcentral gyrus next to the falx [i.e., the dura mater in the midline, separating the two cerebral hemispheres], experienced spontaneous sensory seizures that involved the left labium and left breast. At times…[the sensation] began in the left labium, spread to the left breast and continued to tingle in the labium and nipple. On one occasion this sensory aura was followed by twitching of the left foot…there was nothing in the sensation that resembled sexual excitement. But the description does suggest that the labium and nipple have a neighboring localization in the contralateral sensorimotor area near the motor representation for foot.”
The ability of nipple stimulation to activate genital sensory cortex could have an indirect basis. Thus, nipple/breast self-stimulation-induced oxytocin secretion could stimulate uterine contractions that in turn generate afferent activity that projects to the paracentral lobule. However, it is also possible that nipple/breast and genital sensory activity converge directly not only on oxytocinergic neurons of the hypothalamic paraventricular nucleus [19
], but also on paracentral lobule neurons of the genital sensory cortex.
The cerebellum activation observed in the present study during vaginal and cervical self-stimulation is a common observation during genital stimulation, especially during orgasm [12
]. It is likely that it is involved in controlling muscle tension during genital stimulation [14
]. Two other brain regions that were seen to be activated in the present study are the supplementary motor area and SII (Secondary somatosensory cortex). Other brain regions activated more variably were thalamus, frontal and parietal cortices.
Regarding the observation of bilateral activation of the hand representation in sensory cortex in response to unilateral hand-applied self-stimulation that was noted in the Results section for clitoral and vaginal self-stimulation, it is likely that the sensory stimulation emanating from that single hand, by utilizing the corpus callosum, generates contra- as well as ipsi-lateral activation of the hand representation in sensory cortex. This observation is supported by substantial evidence in the literature of bilateral sensory cortical response to unilateral hand stimulation [20
]. A more curious observation was the activation of the hand representation in sensory cortex during investigator-applied toe stimulation. One speculation to account for this observation is that subtle muscle-induced contractions of the hand in response to toe stimulation (a compensatory response preparatory to breaking the fall in the “stumble” response: [22
]) activates the hand representation area in the sensory cortex, although no obvious hand movement was observed. Another possibility is that the response is among the class of atypical forms of referred sensation (e.g., [23
The present findings provide evidence that, rather than vaginal stimulation being just an indirect means of stimulating the clitoris [17
], vaginal and cervical stimulation per se
activate specific sensory cortical regions that are distinct from the clitoral sensory projection. These differential routes of entry into the brain are undoubtedly of significance in activating the diverse and differential consequences of clitoral, vaginal or cervical stimulation; they include differential physiological effects, e.g., on prolactin secretion [25
], analgesia [26
], and blood pressure reactivity to stress [27
], and differential behavioral effects, e.g., on orgasm [28
], sexual satisfaction [29
], and intimate relationship quality [30
While the present study mapped the primary sensory field of genital input to the sensory cortex, it would be of interest in future studies to extend this analysis to brain fields beyond the sensory cortex that are activated when genital stimulation is perceived as ‘erotic’ versus when it is perceived as ‘just pressure’.