Man has a penis, but no clitoris. At least if you start from the so -called homunculus, the "human clever": those areas of the brain that map your own body as well as on a map point by point. But generally only the male anatomy - female parts of the body: none. Something is slowly changing. Researchers also take a look at the neural image of the female body.
Homunkulus was discovered in the 1950s by Wilder Penfield (1891-1976).
In order to treat epilepsy or remove tumors, the neurosurgeon and his colleagues operated locally, but wake up patients.
These should describe their sensations, while different areas of their cerebral cortex (cortex) - the extreme layer of the brain - were stimulated with electrodes.
When the electrodes were sitting in a brain twist of the parting of the parting of the central furrow, twitching or tingling it in different areas of the body in the patients.
Based on his experiments, Penfield had a card drawing. The card showed that neighboring areas of the body are largely represented in neighboring areas of the brain - brain research calls the principle of "somatotopic". In the cerebral cortex, more sensitive body parts such as the tongue that have many receptors are enlarged. Hence the strange proportions of homunculus: the skinny arms go into huge hands, and the face is dominated by a tongue and huge lips that stand out far. Homunkulus shows how a person looks like when his body proportions correspond to those of the brain areas that represent them.
Gender differences are often neglected
But although Penfield examined at least 107 women, there is no trace of the female anatomy. It is unclear why women were not represented in Penfield's original illustration. The history of science shows that differences between the sexes are often neglected. It is often the male anatomy or physiology that should stand for both genders.
"It's remarkable that we still don't really know what the difference is between the female and male homunculus," says Gillian Einstein of the University of Toronto. "That's because we simply haven't mapped the female somatosensory cortex." In a paper, the Canadian psychologist coined the term »Hermunculus« for the female version of the little human being.
Neuroscientist Susana Lima from the Champalimaud Centre for the Unknown in Lisbon assumes that there must be differences in the brain maps of male and female bodies. If only because of the obvious discrepancies in the anatomy of the genital organs. And of course there are other differences: think of the reproductive cycle and the hormonal influences on neural circuits. "Pregnancy and motherhood are also known to change the female brain," says Lima. "All these processes influence development and lead to gender differences." This must also have an effect on the neural image of the body.
But what do you actually know beyond such assumptions? Even if the female homunculus has never been systematically created: individual parts of the map of the female brain are already known. However, comparisons between men and women have so far promoted similarities than differences. Susana Lima himself refers to a study on mice. Researchers around the neurobiologist Michael Brecht from the Bernstein Center for Computational Neuroscience Berlin found similar anatomical cards from penis and clitoris in the somatosensory cortex of rodents.
Controversy over the location of the genitals
Where exactly the genitals in humans in the brain are shown, there were and are controversial. "Penfield has located the male genitals on the homunculus map below the representation of the toes," says Christine Heim, psychologist and neuroscientist from the Berlin Charité. Others, on the other hand, represent the hip, in analogy to the anatomy of the body, because there are sexual organs and hips side by side. The fact that there are contradictory findings could be related to genital stimulation methods of different precise methods. There are also similar controversy around the location of the female genitals.
To end the controversy, Christine Heim and her colleagues recently examined 20 test subjects. With a puff of air, a membrane was set in vibration, which lay over the underwear in the region of the clitoris. The stimulation should not be perceived as unpleasant or sexually arousing in any way. "This is important not only for ethical reasons," explains Christine Heim, "but also to prevent other brain regions from being activated that do not reflect the purely sensory-tactile stimulus." In this way, Heim's team was able to show that the genitals are indeed located in a somatotopic sequence next to the hip, analogous to the anatomy of the body. The observation is consistent with a previous study in men. Here, researchers similarly located the representation field of the penis in the brain at a location between the legs and the trunk.
But there are even more similarities between the sexes. Namely, in how the human breast is represented in the brain. Jop Beugels, a plastic surgeon at the Maastricht University Hospital, and his colleagues pushed ten men and ten women into the functional magnetic resonance tomographs and stimulated various areas on their breasts with vibrations. As expected, this activated parts of the somatosensory cortex, with the representation of the nipples in the cerebral cortex being enlarged. The researchers found no statistically significant differences between the sexes. However, this could also have been due to the small sample.
Space zone changes
However, there are measurable differences in women during pregnancy. In one study, a team led by Flavia Cardini of Anglia Ruskin University in England looked at women in the second and third trimesters: at week 20, when the belly begins to grow, and at week 34, when the belly is clearly visible. Cardini's team was able to show that at a late stage of pregnancy, the representation of the distance zone in the brain, the area that directly surrounds the body at "arm's length", increases.
According to the researchers, the results suggest that the brain adapts the representation of the surrounding space to the new circumstances. The expansion of space around the body in the third trimester may be related to the need to monitor a larger area, Flavia Cardini explains on demand. And this is due to the increased vulnerability of the abdomen: "Not only because of its larger dimensions and thus a less mobile body, but above all because of the fetus.«
Experiences form the female homunkulus
However, changes in the hermunculus do not only occur with physical changes. In the already mentioned study on the representation of the female genitals in the brain, Christine Heim showed that the more sexual contacts women had in the past year, the thicker the genital field in the cerebral cortex turned out to be. This fits in with other results from neuroscience. It has long been known that experiences can have a long-term effect on the brain. Thus, in people who played the violin in childhood, the hand in the somatosensory cortex is depicted larger in adulthood than in peers without comparable musical experience.
In an earlier study, Christine Heim met an opposite effect in women who were sexually abused as children. A common consequence of sexual abuse in childhood is the development of sexual dysfunction, including orgasm disorders and the inability to feel sexual pleasure. In Heim's examination with 51 adult women, the brain bark was thinned out by tests with such trauma in the places that represent the clitoris and the surrounding genital area in the brain.
"We assume that this is a protective mechanism that reduces the sensory processing of the aversive experience," says Christine Heim. In the long term, this thinning can contribute to the development of behavioral problems and sexual dysfunction in later life. But Heim also has an alternative explanation. "It is possible that women with experiences of abuse are less sexually active, and the genital field is therefore thinner.«
The mapping of the somatosensory cortex is still pretty at the beginning in women. But there is currently hope that the homunculus of the future will also have a clitoris.
