Hermunculus Situates Female Genital Sensation in the Cortex

The iconic homunculus map, showing how the human body is represented in the brain, is clearly male: The drawing, published in 1950, depicts a penis deep within the medial wall of the cortex. Female anatomy, including the breasts, vagina, and clitoris, is omitted. Exactly where in the brain sensations felt in female genitalia are processed is still debated, but a study published this week (December 20) in The Journal of Neuroscience puts forward the lateral wall of the somatosensory cortex as a likely location. The study also finds that the size of the brain region that activates in response to clitoral stimulation correlates with self-reported frequency of sexual intercourse.

The paper is a step “toward understanding something unique about the female brain,” says cognitive neuroscientist Gillian Einstein of the University of Toronto who was not involved in the study.

The 1950 somatosensory homunculus was based on experiments by neurosurgeon Wilder Penfield and his team, including Theodore Rasmussen, who operated on locally-anesthetized but fully awake patients to treat epilepsy or remove tumors and asked them to describe their sensations as the researchers stimulated different areas of the cortex—the outermost layer of the brain—with electrodes. The map showed that adjacent areas in the body are for the most part represented in adjacent areas in the brain—that is, somatotopically. The genitals were an exception, as they were found below the brain region representing sensation in the foot, in the medial cortex, along the deep groove that marks the division between the brain’s two hemisphere.

Several studies have investigated where the genitalia map within the cortex, using self- or partner-delivered stimulation or electrical stimulation to try to validate the non-somatotopic location of male genitalia next to the foot and to see if female genitalia map to the same location. So far, the questions haven’t been answered conclusively, in part because how a body area is stimulated affects how the brain responds: some studies agree with the authors of the original homunculus in placing the genital field for females, males, or both in the medial cortex, while others have situated the field adjacent to the hip and knee area in the dorsolateral region, where it would fit somatotopically.

In the present study, Christine Heim and colleagues at Charité Universitätsmedizin Berlin in Germany devised a technique to stimulate the clitoris without eliciting sexual arousal. “We developed a technique to deliver an airburst via a membrane that allows for a precise stimulation while at the same time avoiding any discomfort,” writes Heim, lead author of the study, in an email to The Scientist. 

Using fMRI imaging, the researchers mapped 20 participants’ responses to those puffs of air to regions along the top and side of the cortex, known as its dorsolateral areas. Female genitalia, the authors conclude, are represented in the cortex next to the representation of hips and upper legs—where one might expect it if genitalia followed the somatotopic arrangement. Importantly, “the precise location within this region varied from woman to woman,” Heim points out. But across all the women, “there were no activations in other areas of the somatosensory cortex” beyond the region by the hips and upper legs. This observation is in line with a previous study in males using a non-arousing stimulus, which similarly placed the penis’ representation field at a somatotopic location between the legs and the trunk.

However, Rutgers University psychologist Barry Komisaruk, whose previous work mapped female genitalia to the medial cortex, questions the techniques Heim’s team used. Specifically, he “was surprised to see the crude form of mechanical stimulation that the present authors used in the study,” he writes in an email to The Scientist. As no size is specified for the membrane used to deliver stimulation, and the disc was attached above underwear, Komisaruk questions how the researchers could avoid stimulating not only the clitoris’s pudendal nerve, but also other nerves in the groin region. As a result, he postulates that the “cortical near-surface activation is most likely due to stimulation of the groin region in addition to that specifically of the genitals.”

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