We Are Our Brains (16 page)

The activity that sex causes in the brain's reward circuitry is visible on scans. In Groningen, the neuroscientist Gert Holstege somehow persuaded people to induce orgasms in their partners while the latter were lying with their heads in a brain scanner. The scans revealed activity in the dopamine-producing system in the ventral tegmental area (
fig. 16
) that was identical to the response to a heroin injection. That makes sense, because in addition to the dopamine system, the brain's opiate system is also involved in orgasm. Patients who take naloxone, a substance that inhibits the production of opiates in the brain, experience less pleasure from orgasms. Holstege's scans showed that different areas of the brain are activated in men and women during sexual arousal. Activity in the female brain is concentrated in the motor and sensory areas of the cerebral cortex; in the male brain it's mainly in the occipito-temporal cortex (
fig. 1
) and the claustrum, a thin sheet of gray matter just under the insular cortex (
fig. 27
). So in contrast to the theory put forward by the Nobel Prize–winning neuroscientist Sir Francis Crick, namely that the claustrum plays a role in our highest brain function, that is, consciousness, it turns out that this structure, at least in men, is in fact occupied with the very down-to-earth activity of sex. In men, sexual stimulation also activates the insular cortex, a brain area that regulates heart rate, breathing, and blood pressure. It's striking that the brains of both sexes turn out to approach the same goal, orgasm, via different routes. The amygdala, one of the structures that inhibits sexual behavior when we're supposed to be concentrating on something else, becomes less active in both men and women during orgasm.

While the scans revealed different routes to orgasm in men and women, they showed largely identical patterns of activity and inhibition during orgasm, with a considerable increase of activity in the cerebellum. It appears that in both sexes the cerebellum regulates muscle contractions during orgasm. The prefrontal and temporal
cortices become less active during orgasm, ensuring that sexual behavior is even less inhibited during this stage. So at such moments you're effectively of unsound mind. In the case of men, moreover, an area in the brain stem known as the periaqueductal gray matter is activated during orgasm in exactly the same way as when drug addicts inject themselves with heroin.

Holstege's scanner studies were met with predictable outrage. In an interview, he mentioned that one of his presentations in the United States caused an American colleague to blush and mumble as he walked away, “Is nothing sacred anymore?”

Sexuality and Hormones

Hormones are involved in every aspect of sexual behavior. Sexual arousal is influenced by the male sex hormone testosterone. In older men, testosterone levels can decline, lowering their sex drive and causing depression. (Testosterone replacement therapy alleviates both symptoms.)

Testosterone is also responsible for promoting women's sex drive. In their case, testosterone is produced in the adrenal gland and the ovaries. A woman who had a testosterone-producing tumor found that she missed it after it was surgically removed, because one of its side effects had been an exceptionally intense sex life.

The monthly fluctuations in women's hormone levels send signals to their brains indicating their degree of fertility. A study showed that, without being aware of it, American female students dressed more fashionably around the time of ovulation. They were more
inclined to wear skirts than pants, wore more jewelry, revealed more skin, and were more sexually active. The conclusion was that they were picking up signals from their hormones that they were at their peak fertility and subconsciously chose clothing and behavior that flaunted it. The unconscious signals that women transmit during their fertile period are also picked up by the outside world. Another study has shown that strippers' tips around the time of ovulation averaged $335 an evening, whereas they “only” netted $195 an evening during the rest of their cycle. Around ovulation, women also show a preference for more masculine faces, voices, and behavior in the males around them. This 2007 study by Geoffrey Miller and Brent Jordan won the Ig Nobel Prize, a parody of the Nobel Prize.

Indeed, the brain's response to erotic images depends not only on gender and age but also on hormone levels. Erotic images provoke greater sexual arousal and more activity in the brains of young men than of young women. In general, in the early stage of sexual stimulation, the hypothalamus and amygdala are more strongly activated in men than women. For women, the degree of activation also depends on the stage of the menstrual cycle. Around ovulation, women react comparatively more strongly to such stimuli than during menstruation. In middle-aged men (ages 46–55) certain brain areas like the thalamus and hypothalamus cease to display activity, a sign that aging reduces arousal in response to erotic stimuli.

Oxytocin is a hormone produced by brain cells in the hypothalamus and released into the bloodstream via the pituitary gland (see
chapter 1
). It affects muscle fibers in the reproductive organs but also influences behavior when it's released in the brain itself. In men, the release of oxytocin in the hypothalamus also plays an important role in erection. During sexual arousal, high levels of oxytocin are found in the blood of both sexes, and oxytocin is involved in both the male and the female orgasm. In men, oxytocin stimulates contractions of the smooth muscle fibers, facilitating sperm transport. In women, it sends sperm transport one way and egg transport the other way, ensuring that an encounter is inevitable.

Oxytocin's effect on sperm transport means that the female orgasm also affects partner choice, because a partner who can ensure orgasm gains an evolutionary advantage by increasing the likelihood of fertilization. Furthermore, a genetic component has been found in the female orgasm, so there is every reason to regard it as an adaptive mechanism that has developed through natural selection. What's more, the release of high levels of oxytocin, not only in the blood but also in the brain itself, promotes pair forming. The blissful feeling of orgasm is heightened because oxytocin causes opiates to be released from other brain cells. That explains why chronic pain sufferers report a reduction in pain after making love. The peak values of oxytocin in the plasma that occur during sexual arousal inhibit the stress system, which has a relaxing effect. So oxytocin would at first sight appear to be the neurobiological substrate for the credo of the 1960s: “Make love, not war.” However, oxytocin doesn't just inhibit aggression within one's own group—it also stimulates aggression toward others. So it certainly isn't an entirely harmless substance.

Neuropsychiatric Disorders and Sexuality

Brain Damage and Disorders of the Brain

Damage to the brain or spinal cord can impair sexual functioning. The type of disorder depends on the site of the lesion, rather than its cause. Damage to the prefrontal cortex can cause apathy and a reduced sex drive or, conversely, a lack of inhibition and an increase in sex drive. In the case of dementia, the loss of inhibition caused by degeneration of the cerebral cortex can lead to abnormal behavior ranging from inappropriate sexual allusions to exhibitionism and sexual offenses. Lesions in the temporal lobe can cause people to
develop Klüver-Bucy syndrome, the symptoms of which are hypersexuality and hyperorality (constantly putting things in one's mouth). Considerable loss of inhibition has also been reported in individuals with lesions of the thalamus or the subthalamic nucleus. The majority of MS patients experience sexual dysfunction. This can take any form, depending on the site of the lesions (MS plaques). Two months before her death from MS, one patient developed an extremely rare complication that combined hypersexuality with multiple forms of paraphilia, such as pedophilia, zoophilia (desire for sex with animals), and incest. She had many lesions in her hypothalamus, basal prefrontal cortex, septum, and temporal lobe, so it's impossible to say which lesion was responsible for what behavior. A thirty-four-year-old man became aroused at the sight of sleeping women and at the prospect of painting their nails, especially those of the right hand. He lost control of this paraphilic urge, sedating his wife so that he could indulge in it. She was outraged when she found out, but he proved unable to control this obsession. When he tried to knock her out with pepper spray, she called the police and he was examined by a psychiatrist. A scan showed atrophy in the frontoparietal region, along with severe white matter injury and lesions in the subcortical fiber systems. He had sustained a head injury at the age of ten, after which he had been in a coma for four days. He also proved to have a body image disorder (see
chapter 2
) with an incomplete mental image of his right hand. Studies have shown that around 50 percent of people who have paraphilia or commit sexual offenses have experienced severe skull trauma causing unconsciousness.

Depression is associated with a loss of libido. That is because the dopamine reward system is inhibited by higher levels of cortisol, the stress hormone, which can deprive patients of all their pleasure in life (a condition known as anhedonia). Depression also causes testosterone levels to decline, further lowering mood. On top of that, antidepressants lower libido and inhibit orgasm. Conversely, patients with bipolar disorder can have a heightened sex drive during manic periods.

Nearly all patients with a disorder of the hypothalamus or the pituitary gland as a result of bleeding, trauma, or infection also experience sexual problems, caused partly by the impairment of the autonomic nervous system and partly by hormonal factors. Diabetics can suffer from sexual dysfunction as a result of damage to nerve fibers. In fact, diabetes is the most common cause of erectile dysfunction in men and painful intercourse in women.

Chronic diseases and certain forms of medication (for instance, for high blood pressure, depression, schizophrenia, and epilepsy) also impair sexual functioning.

Spinal Cord Injuries

A paraplegic whose wife was expecting a baby heard, through the grapevine, reactions like “How could she do that to the poor man, getting pregnant by someone else—as if his life wasn't tough enough already.” Indeed, one wouldn't expect someone with paraplegia to be able to perform sexually. Men with a full spinal cord injury (paraplegia or quadriplegia) involving the loss of sensation from the navel downward can't have erections induced by the brain (psychogenic erections), which result from arousal caused by seeing, feeling, and smelling a partner. But they retain the ability to have erections caused by stimulation of the penis (reflex erections), because that reflex travels through the lowest part of the spinal cord, which is still intact. In able-bodied men, psychogenic erections start in the brain and erotic impulses from the sex organs travel up the spinal cord. When you think how much traffic goes up and down the spinal cord during sexual activity, all of it necessary for ejaculation, it's surprising that 38 percent of men with a complete spinal injury can still experience orgasm. There are three explanations for this, none of them obvious. First, some paraplegics find that the skin near the site where the loss of feeling starts becomes so hypersensitive that it turns into a new erogenous zone, stimulation of which can lead to orgasm. This can be in places like the breasts or mouth or even the ears or shoulders. Second, functional brain scans carried out by the American psychologist Barry Komisaruk have shown that in women with spinal cord injuries who had no feeling up to the navel or higher, the impulses from the vagina are diverted around the damaged spinal cord via a cranial nerve (vagus nerve,
fig. 17
), which then activates many brain areas, making orgasm possible. A third explanation came to light through studies of men with complete quadriplegia, like the movie star Christopher Reeve, famous for his role as Superman, who fell from his horse and was paralyzed from the neck down. Fifty percent of men with this kind of injury, in which all four limbs are paralyzed, can nevertheless experience orgasm and sometimes even ejaculate, despite having no feeling in the genital region. Nerve cells low down in the spinal cord take over the function of the ejaculation center. So when they made their well-meaning but insensitive comments, the friends of the paraplegic man's pregnant wife failed to take into account how resourceful the nervous system can be when it comes to reproduction.

FIGURE 17.
Psychogenic erections arise in the brain, and erotic impulses from the sex organs travel up the spinal cord to the brain. Although a great many stimuli travel in both directions along the spinal cord during sexual intercourse, individuals with a full spinal cord injury who have no feeling below the level of the navel can nevertheless experience orgasm. In such cases, stimuli from the sex organs are diverted from the damaged spinal cord and sent to the brain via a cranial nerve (the vagus nerve). Moreover, the function of the ejaculation center can be taken over by nerve cells lower down the spinal cord.