The following is an excerpt from “The Aesthetic Brain” by Anjan Chatterjee with permission from Oxford University Press USA, © 2013 by Oxford University Press. Click here to buy the book.

Before I realized what was happening, the patient reached down between my legs and grabbed my genitals. It was 1985, in the middle of the night during my medicine internship. I was working about 110 hours a week. Every third night I was on call and felt lucky if I got a couple of hours of sleep. That night, I was taking care of this patient for another intern. On my endless “to do” list was the task of placing an intravenous line. When I got to her room it was dark. I didn’t know what her medical condition was. I was focused on starting her IV and then moving on to my next task. I turned on the soft light over her hospital bed and gently woke her. She seemed calm. I loosened her restrained arm to look for a good vein. That was when she grabbed me.

Even in my sleep-, food-, and sex-deprived state, I recognized that my charms were not the reason for her attention. She acted indiscriminately. She grabbed nurse’s breasts and students’ buttocks with the same enthusiasm. I had not yet started my neurology residency and did not know that she was suffering from a human version of KlÃ¼ver-Bucy syndrome. The syndrome is named after Heinrich KlÃ¼ver, a psychologist, and Paul Bucy, a neurosurgeon, who observed that rhesus monkeys changed profoundly when their anterior-medial temporal lobes were removed. They became placid. They were no longer fearful of objects they would normally avoid. They became “hyper-oral,” meaning they would put anything and everything in their mouth. They also became hypersexual. A similar syndrome occurs in humans. The patient I encountered that night had an infection affecting parts of her brain analogous to those parts in monkeys that Paul Bucy removed. All the cultural and neural machinery that puts a check on such behavior was dissolved by her infection. She displayed sexual desire, the deep-rooted instinct that ensures the survival of our species, in its most uninhibited form.

People are preoccupied by sex. In an American national survey from the mid-1990s, over half the men and a fifth of the women reported thinking about sex at least once a day. In an earlier survey from the 1970s, people were called at different times of the day and asked if they had thought of sex in the last 5 minutes. For people between 26 and 55, 26% of men and 14% of women said yes. Sex sells. Pornography is one area that has not had trouble surviving commercially on the Internet. By some accounts, over $3,000 are spent every second on Web-based pornography. Lest you think this propensity is a peculiarly human obsession, it turns out that male rhesus monkeys also watch pornography. Researchers at Duke University found that male monkeys choose to watch pictures of aroused female monkey behinds even if it means foregoing juice rewards. As an aside, they also look a lot at high-status males. Our preoccupation with sex and power is built into the hardware of our simian brains.

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, ABillion 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 LÃ©ger. 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. Click here to buy the book.