Sex Sleep Eat Drink Dream (22 page)

Why is it that my husband's hand just dimpling the skin of my back like a water strider keeps me blissfully in the present? It's not anyone's caress I savor. Grasping the mechanics of loving touch is one thing, but getting a handle on the nuts and bolts of love itself is quite another. Still, science is trying.

Italian researchers probing the hormonal changes that come with courtship found that both men and women in the first mad bloom of love have more Cortisol pulsing through their blood, suggesting that the state is both stressful and arousing. Perhaps more noteworthy are studies showing that love-struck men have reduced levels of testosterone compared with controls; smitten women have raised levels. This could be a simple result of stepped-up sexual activity, say researchers, or it could be that this hormonal convergence somehow facilitates the courtship game. More testosterone would make women more sexually assertive; less of the hormone would make men less aggressive—good beginnings for the formation of a strong pair bond, notes the anthropologist Helen Fisher.

Fisher, for her part, has peered inside the head to see what brain systems might be activated when we're in the throes of lust, romantic involvement, and long-term attachment. She and her team at Rutgers University conducted brain scans on young adults who had just fallen head over heels in love, as measured by the so-called Passionate Love Scale. This lab standard (a sexual counterpart to the Stanford Sleepiness Scale) asks what the subject feels in the presence of a loved one—trembling, pounding heart, accelerated breathing, or excessive energy. It also inquires what percentage of waking hours is spent musing about the "love object" and then rates intensity of feeling, from tepid to madly in love.

The team chose those subjects deemed wildly, recklessly in love, then used fMRI to look at the brain circuitry activated when the lovesick subjects viewed photos of their beloved as compared with photos of mere acquaintances. The picture of the sweetheart, it turns out, ignited neurons in the dopamine-rich reward system of the brain, the caudate nucleus and ventral tegmental area—the very regions that light up during alcohol and drug use. The love-struck also showed raised levels of noradrenaline and low serotonin levels resembling those of people with obsessive-compulsive disorder.

That the neurochemistry of new love is entwined with the reward system of the brain is not so surprising from an evolutionary point of view. What's interesting is this: When Fisher and her colleagues compared the brain activity of the newly infatuated subjects with those involved in longer-term relationships, they found a difference. In long-term lovers, the view of a loved one sparked lots of activity in the brain regions devoted to emotion. But for those newly in love, the images triggered little activity in these areas. This result confirmed earlier findings by researchers studying volunteers who professed to have recently fallen "truly, deeply and madly" in love. Those scientists were surprised by how small an area of the brain (just that dopamine-rich region) was activated when beholding the image of the loved one: "It is fascinating to reflect," they wrote, "that the face that launched a thousand ships should have done so through such a limited expanse of cortex."

The whistling, wing-beating fury of early love is more like a compulsion than an emotion, Fisher suggests, a motivational drive so powerful that it resembles the urge induced by addictive drugs, making the brain focus solely on the longing for and pursuit of reward—in this case, the love object.

As love progresses from lust to crush to commitment, different sorts of biology and brain chemistry kick in, posits Fisher. Simple lust, which motivates people to seek sex with a range of partners, engages androgen circuits. Romantic love, which guides the pursuit of preferred partners, is linked strongly with dopamine systems. And attachment, which ensures that individuals remain with their mates long enough to support a child and to parent well, is associated with a web of neurochemical networks involving two hormones: vasopressin, which boosts male attachment, and oxytocin, which seems to regulate all manner of positive social interactions, including trust. (One widely touted study in 2005 of Swiss students playing an investment game reported that oxytocin delivered as a nasal spray increased the students' willingness to trust one another.)

Of course nothing in human biology is so neat. These systems may operate independently or they may overlap, says Fisher, and their activity differs in men and women. The pattern of neuronal firing in your besotted brain does not necessarily match that of your lover's.

"A man falls in love through his eyes, a woman through her ears," wrote one British politician. Indeed, studies of sex differences in the processing of sexually arousing images show that men display extra firing in visual areas of the brain, in the amygdala and hypothalamus, and, as Fisher puts it, in regions "associated with penile turgidity." Women are more sexually aroused by romantic words and themes in films and stories than by images, she says. Women in love also generally display more activation in brain areas linked with attention and memory earlier in a relationship than do men; later on, they show more activity in regions associated with emotions.

When it comes to mental processing in men and women, the list of studies suggesting distinctions is growing: language processing, spatial skills, navigation, sense of smell. Functional MRI studies show that during reading, men and women use certain language areas of the brain differently. In navigating the physical world, men are better at mentally rotating maps and tend to think in terms of cardinal directions, while women excel at remembering landmarks and use relative directions. (This disparity emerges only at puberty; before then, girls and boys use the same navigational style, which suggests that steroid hormones may bring out the divergence.) We're only beginning to fathom the nature of these gender differences in brain activity, especially when it comes to sex, but the gulf seems real. So it's somewhat unexpected to learn that orgasm is processed in the brain in a similar way for both sexes—even if thinking or reading about it isn't.

Called at once the "supreme ecstasy" and "
la petite morte,
" the intense wave of pleasure known as orgasm has been the subject of abundant literature; nonetheless it remains an abiding mystery In his classic studies on human sexuality, Alfred Kinsey described orgasm as an explosive release of built-up neuromuscular tension—so intense in some individuals that it may cause a man (or woman) to "throw his whole body into continuous and violent motion, arch his back, throw his hips, twist his head, thrust out his arms and legs, verbalize, moan, groan, or scream, in much the same way as a person who is suffering the extremes of torture."

We know that orgasm is the result of contractions in the pelvis and the perception of pleasure in the brain. But until lately, we haven't grasped how the two phenomena are linked.

In men, orgasm usually coincides with ejaculation, but the one can be experienced without the other. Erection, the necessary prelude to ejaculation and orgasm, often begins with tactile stimulation, especially of the glans penis, which sports a high density of tactile pressure receptors. The sensation of touch travels along sensory nerves to the lower spinal cord, which causes blood vessels in the penis to dilate and blood to rush through hundreds of corkscrew-like vessels to the organ's spongy tissues at fifty times the normal rate.

All of this can happen without conscious control. In fact, most of the erections experienced by young men, totaling around three hours over the course of a day, occur primarily during sleep. As Leonardo da Vinci wrote, with characteristic irreverence, the penis "sometimes displays an intelligence of its own; where a man may desire it to be stimulated it remains obstinate and follows its own course; and sometimes it moves on its own without permission or any thought by its owner. Whether one is awake or asleep, it does what it pleases; often the man is asleep and it is awake; often the man is awake and it is asleep; or the man would like it to be in action but it refuses; often it desires action and the man forbids it. That is why it seems that this creature often has a life and intelligence separate from that of the man."

Some years ago, scientists at Johns Hopkins University School of Medicine discovered a controlling factor in the complex physiology of erection: Contributing to the blood flow that initiates and sustains an erection is nitric oxide, the same gas formed during a lightning storm and so essential to the heavy breathing that accompanies exertion. In the penis, nitric oxide acts as a potent muscle relaxant on the smooth muscles that surround the blood vessel walls, allowing the vessels to dilate. An erotic thought or tactile stimulation brings about an initial surge of nitric oxide from nerve endings in the region, which triggers the erection; then the blood vessels release more of the gas to sustain it. Eventually, an enzyme kicks in to break down the nitric oxide, the arteries constrict, and the animation fades. The drug Viagra works by interfering with this breakdown enzyme, allowing the nitric oxide to hang around longer and maintain the pressure.

So, too, we've learned something about what controls ejaculation. It's no simple knee-jerk reflex, as once imagined, but rather the upshot of complex coordinated actions of the prostate, seminal vesicles, urethra, and pelvic floor muscles. What triggers it is still little understood. A study by Lique Coolen, a neuroscientist at the University of Western Ontario, has revealed that a small cluster of nerve cells buried in the spinal cord of the lower back may generate the action. Rats in which this so-called ejaculation generator is destroyed are able to find their mate, mount her, and achieve an erection, but can't ejaculate. Coolen suspects that the ejaculation generator serves as a kind of way station, processing sensory cues from the genitals and erotic perceptions from the brain. It then sends out signals that control the muscular spasms of ejaculation and also informs the brain of its occurrence. Work by Coolen also suggests that the cells in the spinal ejaculation generator may form synapses with cells in the brain's ventral tegmental area—a pleasure region activated during orgasm.

As for women: Estrogen plays little part in arousal, despite the derivation of its name from the Greek
estrus,
or "intense desire." It does prepare the vagina for sex, elongating and widening the vaginal tract and triggering the cells lining the passage to secrete droplets of lubricating fluid. But it's a weak version of the "male" hormone testosterone, made in a woman's adrenal glands and ovaries, that heightens the sensitivity and responsiveness of touch receptors in her clitoris, labia, and nipples. From these receptors and a rich variety of specialized nerve endings in the genital area—clitoral shaft, glans, urethra, and the so-called G spot, a zone of acute sensitivity—come the sparks of arousal.

Yes, the G spot is real, at least according to Italian researchers. The spot is thought to reside a couple of inches inside the vagina, on the front wall behind the pubic bone. Buried in the flesh here are glands comparable to the male prostate gland. The Italian team reported that the same enzyme markers of nitric oxide activity found in the erectile tissue of penises also abound in most women in the G spot region. Gentle pressure on the spot raises pain thresholds by 40 percent and causes oxytocin levels to surge up to five times higher than normal. Some scientists speculate that this rush of oxytocin may explain sex's calming effect. In 2006, British researchers found that having sex before a stressful event such as public speaking lowers blood pressure, an effect that may last for as long as a week.

The nerves responsible for communicating stimulating sensation from the genital areas to the brain issue from the spinal column. But scientists at Rutgers University studying women with spinal cord injuries say they have discovered a novel sensory pathway outside the spinal cord that may also convey sensations from the vagina and cervix directly to the brain. This pathway travels via the vagus nerve, which wends its long way from the brain stem through organs in the neck, thorax, and abdomen ("vagus" means wanderer), bypassing the spinal cord altogether. Thanks to this pathway, say the researchers, women who have suffered "complete" spinal cord injuries may nonetheless experience orgasm.

Why some women reach orgasm during intercourse and others do not has been a persistent riddle. One new study points an intriguing finger at heredity. A team at St. Thomas' Hospital in London asked thousands of female twins about how often they achieved orgasm during intercourse. Most of the women said only infrequently; a small percentage reported always experiencing it; and an equally small percentage said they never reached it at all. By examining differences in results among identical and nonidentical twins, the team found a clear genetic influence accounting for 35 to 45 percent of the variation. The nature of this influence, however, is far from obvious. It could reside in anything from personality traits to the anatomy of sexual organs to levels of enzymes and circulating hormones.

Most of us are surprised to learn that orgasm actually takes place not in the genitals but in the brain. A case report in the
Lancet
entitled "Unwelcome orgasms" shed light on this odd phenomenon. A forty-four-year-old woman reported having recurrent episodes of orgasm unrelated to any sexual activity, once every couple of weeks. "They had no definite triggers," the doctors wrote, "and were neither particularly pleasurable nor satisfying because they were out of her control. On several occasions she experienced an episode while driving and had to stop the car." It turned out that the woman had a vascular abnormality in her right temporal lobe.

Orgasm is, in reality, a cerebral experience, as the neuroscientist Jean-Pierre Changeux once said, "and it is in the brain we must look for it."

Dutch scientists shocked the neuroscience community by doing just that. Gert Holstege and his colleagues at the University of Groningen in the Netherlands used a
PET
scanner to view the brain regions activated in men who were manually stimulated to orgasm by a spouse or lover. A year later, they did the same for women. The results showed that women's and men's brains display roughly the same widespread pattern of neural firing—about a 95 percent overlap. (The main difference was in a midbrain area called the periaqueductal gray; this region, which plays a role in modulating pain, fired up only in women.) Most of the activity occurred in the caudate nucleus and ventral tegmental areas of the brain, the same dopamine circuits triggered by romantic love and by drug use. In fact, brain activation during orgasm closely resembles the pattern seen during a heroin or cocaine "rush." This may explain why heroin addicts have a suppressed sex drive—because the drug already heavily stimulates this region.