A Neuroscientist Explains Why We Look At Porn

David J. Linden, Ph.D. is a Professor of Neuroscience at The Johns Hopkins University School of Medicine and the author of Touch: The Science of Hand, Heart and Mind.

After a long struggle involving many different medical procedures my children were conceived by in vitro fertilization. My wife bore nearly the entire indignity of this process with countless injections, tests and surgery. My own minor contribution consisted of masturbating in the decidedly unsexy environs of the doctor's office.

I was handed a plastic cup by a scowling nurse and directed to the designated room. It had amateurish seascapes on the walls, furniture fitted with thick plastic slipcovers and a stack of dirty magazines—Penthouse, Hustler and the like. This was 1996, before the full-on internet porn era, and those who preferred a special flavor were out of luck. It was a struggle to get in the mood but, in truth, it helped to have the magazine. In time, I dutifully pulled my pud to Penthouse and handed over my small, warm cup with a sheepish smile.

While survey numbers are almost certainly underestimates, a recent poll revealed that 66 percent of all men and 41 percent of American women view pornography at least once a month. One analysis of data packets flowing through Internet servers indicated that about half of all web traffic is porn; some call that myth, insisting the number is much smaller, more realistically like 4%. The actual numbers remain clouded in the obscurity of the internet's data traffic as much as in cultural taboo, but there's little questioning the popularity of porn.

Why is porn so enormously popular? There are several types of answers to that simple question. While some porn is used in the service of getting in the mood for sexual activity, the vast majority of porn use, by both women and men, functions to help the user achieve orgasm through masturbation. In fact, many people find it difficult to achieve orgasm through masturbation without the use of porn.

But why does porn help? On a psychological level, one can say that porn facilitates self-induced orgasm by increasing sexual arousal, by promoting fantasies and sexual escapism. These answers are correct but I suspect that they are also incomplete and fail to capture an essential neurobiological aspect of self-stimulation.

We are wired to pay less attention to touch signals that result from our own movements as compared to those that originate in the outside world. For example, when we walk down the street, we barely notice the sensations of our clothing moving against our skin. However, if we experienced these identical sensations while we were standing still, they would be very conspicuous and would demand our immediate attention: Who or what is rubbing up against us? This makes sense: sensations that come from the outside world are the ones that are most likely to call for our attention because they are potentially threatening or otherwise salient (flirtatious, delicious, puzzling, etc.).

This phenomenon is clearly illustrated in the case of tickling. Most people can't tickle themselves effectively; the tactile sensation from self-tickling is much weaker than that which results from being tickled by another person. Sarah-Jayne Blakemore and her colleagues at the Institute of Neurology in London have performed experiments in which subjects were either tickled or instructed to self-tickle while they were in a brain-scanning machine.

Tickling resulted in activation of regions of the brain involved in decoding the precise location and quality of touch (called the primary and secondary somatosensory cortices), as well as the parts of the brain that are responsible for the positive emotional tone of touch (the anterior cingulate cortex and the posterior insular cortex). When the experiment was repeated with self-tickling, activation of these touch centers was reduced when compared with conventional tickling.

However, at the same time, self-tickling produced a strong stimulation of the cerebellum, a brain structure that receives both touch signals and instructions from other brain regions that initiate motion—like the electrical signals that flow through neurons to control muscles in the hand and arm during self-tickling. The cerebellum is activated when instructions for motion are specifically correlated with sensory feedback from the skin's touch sensors. It then sends inhibitory signals to the brain's touch centers (and other regions) that dampen their activation and thereby attenuate the ticklish sensation during self-tickling. Our brains can tell when our hand is tickling our body, and reduces how much it feels that tickling accordingly.

The parallels between self-tickling and masturbation are obvious. In both cases, the cerebellum will be activated that this will serve to suppress neural activity in those parts of the brain that respond to both the sensory and emotional components of touch. To intensify the touch-driven signals during masturbation, one can engage in daydreaming fantasy or, even more effectively, one can use porn. Both of these practices produce activation of visual and other sensory regions of the brain. Even if you're reading a sexy story, rather than watching images, this will activate higher visual regions of your brain as you create a mental image of the storyline.

(Note that, to my knowledge, this theory has not been explicitly tested in a lab—and is unlikely to be given the priorities of government funding. But it could easily be done so by having men and women self-stimulate in a brain scanner, with and without porn. One other challenge to overcome, however: it's hard to self-stimulate without jiggling your brain a bit.)

That we can thrill to the touch of performers on the screen as we might react to a caress on our own skin results from the neural information received by the posterior insula, the main cortical center activated by caresses and a crucial node in the emotional brain. In addition to the nerve fibers activated by caresses, the posterior insula receives highly processed visual information as well. Amazingly, merely watching a movie of someone's body being caressed will activate the posterior insula of a subject in a manner that's similar to her receiving a real caress. The activation of emotional touch centers by porn counteracts the cerebellum's action to blunt self-initiated touch sensations. The end result is that increased activity of the emotional touch centers will boost the intensity and pleasantness of the self-touching sensation, making it easier to achieve orgasm.

So there you have it. If using porn didn't counteract the cerebellar circuits designed to detect and suppress the sensory consequences of self-motion, it would be much less effective as an aid to masturbation, and it would not have such a prominent role in our cultural and private lives.

Check out David's website and get your hands onTouch: The Science of Hand, Heart and Mind (Viking, 2015). You can also follow him on Twitter.

This story is part of Motherboard's Sex Ed Week, a series of sex-focused science and technology stories. Check out more stories here.