Reprinted from “The Aesthetic Brain”

Despite the fact that sex takes up a huge  part of our cognitive and emotional  mental  space, scientific  research on sex has  been  limited. Breakthrough  reports, such as those by Kinsey  or Master’s and Johnson, remain unusual. Perhaps overzealous notions  of propriety  have historically inhibited such research and investigators are easily branded  as perverts. Recently, Ogi Ogas and Sai Gaddam examined sexual desires on the basis of search terms that more than 2 million people use on the Internet. In their sample of 400 million, more than a quarter of all search terms were about sex. Their fascinating book, A Billion Wicked Thoughts, generated controversy, ranging from being heralded as providing  new and unprecedented insights into human sexuality to reifying cultural stereotypes and simplifying  gender differences in sexual desire. Despite  wariness in this research, knowledge about the neurobiology of sex is growing. Some themes that emerge from this research will be familiar  from our rumination on food.

We can think of sex as a play with different acts. The first act is desire, the next one is sexual stimulation and pleasure, and the final act is the aftermath, the languorous glow of the sexually sated. Most of what we know about how the brain responds in these acts comes from studies of young heterosexual men. These specimens are found in abundance on college campuses and are quite willing to volunteer for sex studies.

We approach things that we desire and, as we saw before, the amygdala helps us do that. In the last chapter, we saw that the amygdala plays this role in our approach to food, and it seems to be true for sex as well. In animals, the amygdala activates their sexual response, a pattern  also seen in humans. When  young men look at short arousing video clips, their amygdalas are active. We think such activity arouses them to move toward objects of their desire. After a successful approach, when the penis or clitoris is stimulated, the amygdala becomes less active. Thus, amygdala activation is critical in getting us to act on our desires and then settles down when we receive them.

The neurotransmitter dopamine plays an important role in our desires. The brainstem sends dopamine  to many areas of our reward systems, like the ventral striatum  (especially  its major  subcomponent,  the nucleus accumbens), the amygdala, the hypothalamus, the septum, and the olfactory tubercle.  As we saw earlier, these areas are involved when we desire food. They are also involved  when we desire sex. The neuroscientist Itzhak Aharon and his colleagues showed that heterosexual men will exert extra effort to view pictures of attractive women, and that this effort is associated with more neural activity in the nucleus accumbens. Cocaine and amphetamines amplify the effects of dopamine and enhance the desire for sex. Neural activity in the hypothalamus that increases during sexual arousal is enhanced with the drug apomorphine, which works on dopamine receptors. Conversely, antipsychotic  medications and some antidepressants that block dopamine receptors inhibit sexual desire.

Dopamine lets us anticipate sex but does not itself cause the intense peak of sexual pleasure. When men with erectile dysfunction  are given apomorphine, they have more neural activity in their brains in response to sexually arousing  images, without increasing their pleasure. Neuroscientists can conduct studies of how dopamine regulates the anticipation of sex in rats with a detail not possible in humans. By inserting very small catheters, they measure the chemical environment in areas important for rewards. When a male rat is separated from a receptive female by a barrier, his nucleus accumbens is flooded by dopamine. If the male rat is then allowed to copulate with the female rat, dopamine levels plummet. However, if the rat then  sees a new female,  his arousal and the dopamine levels rise again.

Given how engaging sexual experiences can be, it is no surprise that many parts of the brain are active when people are sexually aroused. The insula, the anterior cingulate, and the hypothalamus get into the act. The insula monitors the internal state of the body and regulates our autonomic nervous system, including  heart rate, blood pressure, and sweat responses. The anterior  cingulate monitors for mistakes to guide future behavior. The hypothalamus regulates the secretion of hormones such as prolactin and oxytocin into our bloodstream. In addition to the usual reward systems, parts of sensory cortex also get engaged.

As you can imagine, it is hard to study what happens in the brain during orgasm. From the little information we have, the ventral striatum is active in men and in women. That activity is to be expected, since so many studies link the nucleus accumbens, a major subcomponent of the ventral striatum, to pleasure. Interestingly, activity in many parts of the brain decreases during orgasm. The ventromedial prefrontal cortex, the anterior cingulate, the parahippocampal gyrus, and the poles of the temporal lobes decrease their  activity. The ventromedial prefrontal cortex is engaged when we think  about ourselves and about our fears. The anterior cingulate is engaged when we monitor mistakes. The ends of the temporal lobes organize our knowledge of the world, and as we saw in the discussion of landscapes, the parahippocampus represents our external environment. What could a drop in neural activity in these areas mean? Perhaps it means that the person is in a state without fear and without thought of themselves or their future plans. They are not thinking  about anything in particular and are in a state in which the very boundaries that separate them from their environment have disappeared. This pattern of deactivation could be the brain state of a purely transcendent experience enveloping a core experience of pleasure.

In French literature, the release from orgasm is famously referred to as la petite mort, the little death. Freud thought that orgasms opened the way for Thanatos (the death instinct) after Eros had departed. These death images capture the lassitude that follows orgasm, but not the emotionally satisfied feeling. The satisfied state probably results from release of a combination of beta-endorphins, prolactin, and oxytocin. The hypothalamus regulates the production of prolactin and oxytocin. Prolactin, a hormone that  helps women  produce  milk when breastfeeding,  contributes  to the sense of sexual satiety. At least in men, prolactin  plays an important role in the refractory period after orgasm during which men have little further sexual desire. Given the blockbuster sales to men of drugs like Viagra, it is no surprise that prolactin-inhibiting drugs are being researched with the hope of minimizing this refractory period. Oxytocin  is a hormone associated with trust and a sense of affiliation. In sex, it is the “cuddling” hormone. Users of the death metaphor for the post-orgasmic state simply ignore the warm glow of endorphins and oxytocin, unless they know something about death that the rest of us do not.

When people are sexually satisfied, they have more neural activity in the lateral OFC. This is the same pattern of increased neural activity seen in people who are sated with food. Neural activity in this area suppresses our reflexive tendency to act on urges. Damage to this area as well as to the anterior and medial temporal lobe can produce hypersexuality. These areas that regulate behavior, either because desires have been satisfied or because acting on desires could get us into trouble, were almost certainly damaged in the patient that made a grab for me.

Pleasures are more than simple reflexive reactions to desirable things. We saw this principle with food, and the same applies to sex. The context in which  we encounter  objects makes a big difference in our subjective experiences. For example, pain can topple into pleasure. Women have higher thresholds for pain when sexually aroused. These thresholds increase on average by 40% with vaginal stimulation and by 100% near and during orgasm. Despite these changes in what counts as pain, the sensation itself is not dulled and is no less arousing. Rather, the same intense sensation is not experienced  as pain. In the brain, the insula and anterior cingulate are active during arousal. These same areas are active when people feel pain. Curiously, people’s faces take on similar  contortions when experiencing intense pain as when experiencing  orgasms. Here the sensations producing  pain are still experienced, but they are not unpleasant.

Why  should  brains have a mechanism to keep the arousing properties of pain and discard their unpleasant ones? The adaptive significance of this mechanism is probably to reframe the pain of childbirth. Minimizing pain during the “vaginal stimulation” of childbirth is a good thing  if women are to repeat the event. This adaptive mechanism explains why otherwise painful stimulation can be pleasurable during  sex. The sensations remain intense and during sexual arousal are not aversive. An adaptive mechanism that evolved for procreation got co-opted for recreation.

Pleasures help us learn. In animals, food or juices are commonly  used as rewards. In the same way that food can be paired with something neutral to make Pavlov’s dogs salivate to bells and whistles, sex can be associated with neutral objects. This association is one way that fetishes develop. In the 1960s, researchers exposed young men to sexually arousing images along with knee-high boots. After the exposure, these men found boots sexually arousing. Linking sex to neutral things may be especially powerful during adolescence when our brains and behaviors are being molded by sex hormones. This phenomenon explains in part why fetishes can seem bizarre to people who do not share the fetish. It is the intrinsic neutrality of the fetish object that makes it seem so strange if you have not had the experience of pairing it with the pleasure of sex.

The use of sexual pleasure for learning has a dark side. The annals of medical  therapy  include  the use of this kind of learning for deeply disturbing purposes. The episode that I am about to recount is a detour  from  the main points of this chapter, but I feel compelled to tell it, perhaps as a confessional in shame for my profession. Anhedonia is a medical word for the lack of pleasure. It is a common symptom in mental illnesses like depression and schizophrenia. In the 1950s and 60s, researchers were making great strides in mapping  the neural  bases of emotions. They discovered that electrically or chemically activating deep parts of the limbic system produced  intense pleasure. The researchers were probably  stimulating the nucleus accumbens. In people, such stimulation produced multiple orgasms. Robert Heath, a psychiatrist, worked with these stimulation techniques to alleviate anhedonia in patients. He was an early advocate of biological psychiatry, believing that most psychiatric illness had a physical basis, before this was a popular  idea. He also thought  that the stimulation technique could treat homosexuality.

In 1972, Heath published a study with Charles Moan that used deep brain stimulation  in a man  referred  to as B-19.  This 24-year-old man had a troubled  psychological and social background. His father was abusive and drank excessively. His mother  was withdrawn and rigid. B-19 had no memory of ever being embraced by her. He was expelled from schools three times by the age of 11. He then dropped out of school and had a few short-lived jobs. Then he enlisted, but was discharged because of “homosexual tendencies.” He was described as being  hypochondriacal and paranoid. He became addicted to alcohol and drugs, but said that he did not receive pleasure from them or from sex. Heath and his team placed electrodes throughout  B-19’s brain, including  frontal, parietal, septal, and hippocampal regions. Only electrical activity in the deep limbic regions produced pleasure. Dr. Heath saw an opportunity to “cure” homosexuality, which at the time  was labeled as a disease by the American  psychiatry establishment. B-19 was shown 15-minute  “stag” films of a man and woman having sex while his brain was stimulated. To “test” the effectiveness of his treatment,  a 21-year-old prostitute  was brought  to his room. B19 was able to have sex with her. After this treatment,  he had a short-lived affair with a married  woman.  He continued to have sex with men because (according to the researchers’ report) hustling was a quick  way to make money. However, the doctors concluded that an important  part of the study was the “effectiveness of pleasurable stimulation in the development of new and more adaptive sexual behavior.” The next year, in 1973, homosexuality was removed from the list of diseases in the Diagnostic and StatisticalManual of Mental Disorders. To my knowledge, studies such as those done with B-19 did not continue.

What can we say about  sex and pleasure? Clearly, sexual pleasure is adaptive in the most basic of ways. Enjoyment in sex guaranteed  that our Pleistocene ancestors begat us. They did not have the option of making babies in a lab. This pleasure system, like that of food, has components of desire, components of actions to satisfy those desires, and components that revel in pleasure itself. There are systems that put breaks on our sexual behavior. Pleasures help us learn and develop emotional  bonds to objects that are not inherently  pleasurable. Finally, the pleasure of sex can change depending on its context. Painful things can become pleasurable and pleasurable things can become painful  if doused with guilt and shame. Like food,  the basic pleasure of sex is malleable. The fact that these experiences are so supple is critical to understand  when we consider our responses to beauty and to art. Aesthetic encounters too can change radically depend-ng on the context and the experiences we bring to the encounter.

Pleasures are promiscuous. The fetish example shows us that pleasures attach easily to other objects. These other objects include money. Some time ago, I was eating a fine dinner at an upscale Italian restaurant in West Palm Beach, Florida. West Palm Beach is one of the richest communities in the United States. I was there as part of a fund-raising effort on behalf of the University of Pennsylvania’s School of Medicine. A few professors gave brief talks about science to an ultra-rich audience in the hopes that they would feel good about learning and then feel good about writing big checks to support the institution. This fancy dinner topped off the event. To my left was a dapper  man in his late 70s. His date, a woman  about 25 years younger,  was wearing  jewelry  that oozed money. My dapper companion turned out to be a charming conversationalist. Our discussion took off when he found out about my interests in aesthetics. He talked about his own dabbling  with  painting  over the years and past interactions with the painter Fernand Léger. I mentioned to him that I was planning to write a book  on the science of aesthetics.  As he listened to me, coddled by food and wine, he drew me in close to share his wisdom. “If you want the book to sell,” he said, “make sure you include a lot of sex.”

Reprinted from “The Aesthetic Brain” by Anjan Chatterjee with permission from Oxford University Press USA, © 2013 by Oxford University Press