David Linden - Touch

The brain has an evolutionarily ancient dedicated circuit for pleasure. It’s a complicated one, but the core of it involves activation of a region in the midbrain called the ventral tegmental area and the consequent release of the neurotransmitter dopamine in a number of brain areas that are the target of ventral tegmental area axons, most notably the nucleus accumbens and the dorsal striatum. When we’re hungry, eating delicious food activates this dopamine-using pleasure circuit; it is also activated when we have an orgasm. This makes sense: The pleasure circuit is engaged by behaviors that keep us alive and get our genes into the next generation, like eating food, drinking water, and having sexual intercourse. Many psychoactive drugs, including alcohol, nicotine, cocaine, amphetamines, and heroin, can artificially activate this brain pleasure circuit.

We know, however, that the experience of orgasm is more complex than just a feeling of pure pleasure. To help us understand the subjective experience of sexual bliss, it’s useful to examine the complete pattern of brain activity during orgasm. In relevant experiments men and women have their heads placed in a brain-scanning machine. While the scientists in the room stand around with their clipboards, trying not to make everyone feel too uncomfortable, the subjects are brought to orgasm by manual stimulation from their partners. In this way, one can compare three conditions: unstimulated, genital stimulation before orgasm, and genital stimulation during orgasm. 35

The pattern of brain activation during genital stimulation and orgasm is mostly similar in men and women. When the genitals are stimulated but orgasm has yet to occur, patches of activation are seen in the primary somatosensory cortex: one in the contiguous body map and another beyond the toe representation in the mesial postcentral gyrus. Additional patches of activation are seen in the corresponding parts of the body map of higher somatosensory processing areas, such as the secondary somatosensory cortex. As genital stimulation continues, one sees a reduction in the ongoing activity in the amygdala, a region that processes emotional signals related to fear. 36The interpretation of this finding is that continued genital stimulation is associated with a reduction of fear and a consequent loss of vigilance toward potential threats—in other words, relaxation.

When orgasm occurs, in addition to activation of the somatosensory cortex and deactivation of the amygdala, we also see activation of key portions of the pleasure circuit: the ventral tegmental area, the nucleus accumbens, and the dorsal striatum. Another activated region is the cerebellar nuclei, an area involved with motor coordination as well as certain aspects of emotional cognition. It’s possible that cerebellar activation is related to the involuntary nature of thrusting and writhing movements as well as involuntary facial expressions during orgasm. Certain brain regions are also deactivated during orgasm. These include the lateral orbitofrontal cortex and the anterior temporal pole, areas involved in thoughtful decision making, self-control, moral choice, and social evaluation. Clearly, the moment of orgasm is not the time you want to be making careful decisions about your life, as many of the most important brain regions for this function have temporarily gone offline.

Most brain-scanning machines have poor temporal resolution: They must average activity that occurs over the course of several seconds. As a consequence we don’t have a detailed sense of the temporal progression of brain-region activation or deactivation during genital stimulation and orgasm. Presumably, stimulation of touch sensors in the genitals leads to activation of the genital areas on the somatosensory map. Beyond that, the details are unclear. It could be that the genital areas of the somatosensory cortex, when repeatedly stimulated in the right conditions, send signals to activate regions like the ventral tegmental area and deactivate others like the amygdala and lateral orbitofrontal cortex. The flow of signals between brain regions during orgasm remains poorly understood.

Here’s the miracle: When we have an orgasm it feels like a transcendent, unified moment, not merely a collection of disparate sensations. We experience orgasms as intrinsically pleasurable and emotionally positive. Why is this? If one were simply to sum up the brain scanner results, we’d get a sort of recipe.

For an orgasm, mix together the following ingredients:

Activation of touch sense from the genitals (somatosensory cortex)

Deactivation of fear and vigilance (amygdala)

Activation of the pleasure circuit (ventral tegmental area, nucleus accumbens, dorsal striatum)

Activation of a motor control center (cerebellar nuclei)

Deactivation of areas involved in slow decision making (lateral orbitofrontal cortex and anterior temporal pole)

Serves: 1

If this is really the recipe for an orgasm, then we can imagine a fanciful thought experiment: Could artificial activation and deactivation of this constellation of brain regions result in an experience that effectively simulates natural orgasm? We don’t know, and the closest thing we do have to such a scenario is a natural experiment: In some people, epileptic seizures can produce an orgasm. 37It’s unlikely that any of these seizures produce the particular pattern of activation and deactivation included in our orgasm recipe, and so it is not surprising that the patients who experience them point out the differences between their seizure-evoked orgasms and those resulting from sexual activity. For example, it’s unlikely that the vigilance and cognitive effects produced by deactivation of the amygdala and decision-making cortical areas are found in seizure-evoked orgasms.

Seizure-evoked sexual sensations are not all identical. When seizures are restricted to the parietal lobe, they can produce the sensation of touch to the genitals and perigenital areas, but they do not result in the intense pleasurable sensation that is the hallmark of orgasm. When seizures are restricted to the midbrain/temporal lobe region, they can result in a wave of pleasure, but that pleasure has no particular genital or even tactile focus. It is more akin to the diffuse rush of pleasure one gets from opiate drugs. It is only when both the somatosensory cortex in the parietal lobe and the pleasure circuit in the midbrain/temporal lobe are activated together that a seizure can produce a sensation that approaches that of a natural orgasm.

Although we experience orgasms as intrinsically pleasurable, they are really just a trick our brains are playing on us through simultaneous activity in many brain regions. The gestalt feeling of an orgasm is divisible into its component parts. The discriminative sense of genital touch is mediated by activation of the somatosensory cortex, but that is not pleasurable in and of itself. It has no emotional valence. That pleasurable emotional feeling is produced only when the ventral tegmental area dopamine neurons are activated. And the accompanying relaxation (loss of vigilance) and cognitive disinhibition during orgasm result from their own regional deactivation effects. The great thing is that we don’t have to think about all these disparate ingredients that go into producing an orgasm. We just get to enjoy the unified sensation that the brain cooks up.

CHAPTER FIVE

Hot Chili Peppers, Cool Mint, and Vampire Bats

Here’s the plan: I’m going to give you a backpack filled with Ziploc bags—some will be filled with fresh mint leaves and others with juicy habanero chili peppers. You’ll also get a clipboard, a pencil, a spare pair of socks, and a round-the-world airplane ticket. Your job, should you choose to accept it, is to travel around the world and visit all sorts of places, from the biggest cities to the most remote jungle encampments. In each location you will seek out a wide variety of people—young and old, rich and poor—and then rub chopped-up mint leaves or diced chili peppers on their skin, ask them to describe the sensation, and record their responses. Try applying the samples on both the glabrous skin of the lips and the hairy skin of the forearm. (These substances don’t have to touch the tongue for their effects to be experienced.)