The female sexual brain has been charted. For the first time, the brain regions that respond to stimulation of the clitoris, cervix, vagina and nipples have been mapped in a new study.
Not surprisingly, the brain region associated with sensation from the clitoris was distinct from those that respond to stimulation of the cervix or vagina. Each area of the genitals showed up in its own spot, clustered in one region of the brain — the same region associated with genital stimulation in men — overlapping but separate.
What the researchers did not anticipate was that sensation from the nipples also excited the brain within the same region, right alongside the areas corresponding to genital stimulation, according to lead author Barry Komisaruk, a professor of psychology at Rutgers University. “That was completely unexpected,” he says. It would help explain why the nipples are erogenous zones in women, he says.
The sensory regions of the brain were first mapped in the 1950s by Canadian surgeon Wilder Penfield. During brain surgery on patients with epilepsy, which required that patients remain awake to ensure that necessary brain regions were not excised, Penfield electrically stimulated various brain regions and asked the patients — all men — to report what they felt. Based on this procedure, Penfield mapped the “sensory homunculus”: a strip of cortex that runs roughly from one ear, up and over the top of the brain, to the other ear.
Homunculus means “little man” in Latin, and the well-known diagram of the sensory homunculus shows an array of a man’s body parts arranged along the brain region corresponding to sensations produced from each area of the body. Since the mouth, lips, hands and genitals are rich with nerves, they appear larger on the homunculus than other body parts, giving it the look of a grotesque. More nerves require more brain territory.
What about the “femunculus”? The map of the erotic female sensory brain had to wait until 2011, when Komisaruk and his colleagues undertook their research. “In doing the mapping, we wanted to see if our map was consistent with the classical map generated by electrical stimulation,” Komisaruk says.
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His group studied 11 women who were instructed to stimulate their clitoris, vagina and cervix, manually or with a sex toy, while their brain activity was scanned in an fMRI machine. As a control, researchers also mapped the brain regions activated by sensation from the hand, foot and nipples. “The hand and foot were exactly where the classical map shows them and nipple self-stimulation got activation in the chest area, which was consistent,” Komisaruk says. “But in addition, we saw activation in the genital region.”
There are two possible explanations for the nipple finding. The first is related to the fact that nipple stimulation releases oxytocin, the same hormone that is released during orgasm, bonding and breast feeding, which also causes uterine contractions. So stimulation of the nipples may indirectly also stimulate the genitals and account for the activation in the same brain region.
The second explanation is that the wiring from the nipples to the brain is direct; perhaps the sensory pathways from the nipples converge with those from the genitals in the cortex, and simultaneously stimulate cells in another brain region that produces oxytocin. Komisaruk is studying whether men’s nipples are wired to the same brain regions as women’s. Since men do not have a uterus or cervix, if their nipples also map to the genital areas of the brain, it would suggest the connection is direct.
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That the clitoris, cervix, uterus and vagina each mapped to distinct spots in the brain was not surprising, not only because women report different experiences in response to stimulation of these areas but also because they use separate nerves to carry signals to the brain.
“There are four different nerves, so they can’t all go to the same spot,” Komisaruk says. “My hypothesis was that there would be distinct areas of activation because there are distinct nerves. We did see differences in the distribution of the sensory responses, but they are all clustered in same genital region.”
This may help explain why women describe experiencing distinct types of orgasm related to vaginal and clitoral stimulation. But what makes a sensation erotic remains mysterious. Komisaruk notes that many people find foot stimulation to be erotic, for instance. That may be because the region associated with sensation from the genitals is right next to the region for the feet.
Indeed, neuroscientist V.S. Ramachandran reported two cases in which people who had had a foot amputated described their orgasms as being experienced not just in the genitals but also in “phantom” feet. “It enlarged their sense of orgasm, and they felt it in their foot as well as their genitals,” Komisaruk says.
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The research was published in the Journal of Sexual Medicine.
Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland‘s Facebook page and on Twitter at @TIMEHealthland.