by DAVID J. LINDEN
What makes touch on some parts of the body erotic but not others? Cutting-edge biologists are arriving at new answers
Growing up in Los Angeles in the 1970s, I gradually came to realise that my father was not the stereotypical psychoanalyst. Yes, he had an office with enigmatic modern art on the walls, copies of The New Yorker in the waiting room and the requisite analytical couch. It’s true that said couch had a wedge-shaped pillow designed for the client to assume the supine posture so frequently portrayed in the cartoons from those same issues of The New Yorker. And, during psychoanalytic sessions, my father did indeed perch in a black leather Eames chair, notebook in hand. But beyond those trappings, he had the sceptical and logical mind of a physician (in those days, nearly all psychoanalysts were, like my father, MDs).
Starting when I was a small child and continuing until I left for university, my father and I would eat dinner together at one of several local restaurants every Wednesday night. Over matzoh ball soup at Zucky’s Delicatessen, we’d discuss anything and everything, including the progress of his psychoanalytic clients (with names and identifying details omitted of course). It was an odd way to grow up and I loved it. In our Wednesday night case studies, there would be the expected psychodynamic talk of dream interpretation and early childhood experiences, but it was all tempered by what would come to be known as neuroscience. He would say that, when the talking cure worked (as it did for most of his clients), it did so not in the nebulous realm of id, ego and superego, but rather by changing the cellular and molecular structure of the brain.
In some cases, he dispensed, not just with psychoanalytic theory, but with any form of talking cure. I remember one particular night when I was in middle school when he was delighted to realise that a client’s persistent and unrelenting depression was not rooted in a tormented childhood or conflicted sexuality. Instead, it was a direct consequence of thyroid hormone deficiency and hence best treated with a simple pill. As I grew older, I came to admire his flexibility and breadth of intellect in these matters. He encouraged me to follow in his footsteps and become a doctor, maybe even a psychiatrist, but I knew that would be a poor choice for me.
‘It wouldn’t work. I’m not as empathetic as you, Dad. In fact, sick people kind of annoy me.’
‘Then you could be a pathologist,’ was his reply.
Like my father, I longed to understand human mental life. But, unlike him, I came of age in an era when biological understanding of brain function was beginning to gain some traction. And so, I took a different path towards the same goal and became a neuroscientist, working at the level of molecules and cells in the brain.
Nothing in psychoanalytic theory evoked more scepticism in my father than Sigmund Freud’s conception of sexual sensation and orgasm. Yes, father and son would discuss Freud’s key theory of immature clitoral orgasms transitioning to mature vaginal orgasms in female development. ‘It’s unmitigated bullshit!’ he’d say, his voice rising. ‘No medical evidence exists for this idea at all!’ It’s not surprising that the servers at Zucky’s would give us a wide berth. To make up for it, he tipped well.
Sexual sensation is absolutely central to both our shared human experience and our individual quirks and kinks. It’s exactly the sort of topic that has traditionally had lots of behavioural observation and theorising attached to it (including by psychoanalysts). There’s no question that sexual sensation is sculpted by early life experience. Yet it’s also embedded in a biological matrix, with brain regions and nerve bundles and special molecular machines specialised to transduce pressure on the skin into what is, ultimately, erotic sensation. It’s one of many places where psychology and biology meet.
Until very recently, biology had rather little to bring to the table, and our understanding of sexual sensation had been shockingly incomplete. We have known for some time that touch sensations from the genital region pass through four different nerves on their way to the spinal cord and, ultimately, the brain. Of these, the pudendal nerve is the most important for sexual sensation, carrying signals from the clitoris in cisgendered women and the penis in cisgendered men. In women, the pelvic nerve conveys touch signals from the labia minora, the vaginal walls, the anus and the rectum. In men, the pudendal nerve carries information from the anus and the scrotum as well as the penis. In women, sensations from the cervix and the uterus can also be conveyed by the hypogastric nerve as well as the vagus nerve, which travels directly to the brain stem, thereby bypassing the spinal cord entirely.
Touch signals from the pelvis ultimately arrive at the outer rind of the brain, a region called the neocortex, where they are represented in a distorted and fragmented body map in the primary somatosensory region. You’ve probably seen drawings of these body maps with giant hands and feet and relatively small torsos, corresponding to the density of certain types of touch sensor in the skin. In one study, women with their heads in a brain scanner were given a handheld dildo and asked to self-stimulate various genital regions – the external clitoris, the vagina and the cervix.
Stimulation of the female genitals activated two completely different spots in the neocortical body map: one just where you might expect, at the groin; and another, a bit oddly, adjacent to the representation of the toes. It turns out that men also have this dual mapping of the genitals. More generally, it raises the issue of whether this particular toe-adjacent bit of the primary somatosensory cortex (called the mesial surface of the postcentral gyrus) has a special role in sexual touch. Some people have even speculated that this particular toe/genital adjacency in the brain somehow underlies foot fetish sexual behaviour (I’m not convinced).
Disappointingly, if you were to examine a microscope slide prepared from postmortem tissue of this area of the brain, you’d find nothing unusual about it. The individual neurons and their overall layered structure look almost the same as those in regions processing touch information from less erotic parts of the body. So, despite some tantalising hints, the biology of what makes touch on some parts of the skin feel sexy while other parts not has remained a mystery. Only now, with a recent publication from David Ginty’s and Stephen Liberles’s groups at Harvard Medical School, are some real answers emerging.
If you ask around or do a few spicy internet searches, you will find that there are people who feel a sexual sensation from being touched on nearly any part of the body. Still, while recognising the solid supporting roles of ears and feet and so on, the genitals are clearly the star of this show. In the right context, almost everyone – no matter their sex assigned at birth, their gender identity, their sexual orientation or the particulars of the early life experiences they might recount to a psychoanalyst – finds stimulation of the clitoris or the glans (head) of the penis to be sexually exciting. But how, through the course of evolution, have these body parts become sexual ground zero? Is there something special about the structure of the nerve endings in the penis and the clitoris and the way they ultimately convey electrical messages to the brain?
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