This Is Your Brain on Sex (20 page)

“It may not be good enough for some, though. Of course, if I was going to check anyway, I wouldn’t even know what to check for. What is a good oxytocin level?”

“Basically people range between fifty and one thousand five hundred picograms per milliliter,” she told me.

“That is a huge range,” I replied with surprise.

“It is,” she agreed with a smile.

I could get myself tested. I could make a potential suitor do the same. But I can’t help but feel that there is no point to doing so. Scientists don’t know if the oxytocin is driving the behavior or if the behavior is driving the oxytocin.
Furthermore it’s not just oxytocin at work in the formation or maintenance of a bond. Scientists have identified many other chemicals that also play important roles.

Changes over Time

As discussed in chapter 3, Donatella Marazziti found that romantic love affects one’s levels of testosterone, follicle-stimulating hormone (FSH), and cortisol.
⁷
Being passionately in love also alters nerve growth factor levels in the brain.
⁸
And all the variations observed during that initial romantic period were no longer present when the couples were retested one to two years later. The chemistry changes as you move from being in the first throes of passionate love to a committed relationship.

Helen Fisher and her colleagues proposed that three distinct yet overlapping systems in the reptilian brain are implicated in love. Sexual attraction has its seat in the hypothalamus. Romantic love resides in the ventral tegmental area and the caudate nucleus, and deep emotional attachment activates the ventral pallidum. All these areas work with dopamine, oxytocin, and vasopressin—they just do it a little differently depending on the type of loving relationship you happen to be in. When Fisher and her colleagues conducted their study, all participants reported feeling passionately in love for one to seventeen months. They were measured only once. One might assume the length of time two people are in a relationship would be one variable that transforms activation from the romantic love system to the one underlying deep attachment.

A group of researchers at the Catholic University of Korea used fMRI to compare cerebral blood flow in five heterosexual couples claiming to be passionately in love, both at the start of their relationship (within one hundred days) and six months later. They used a paradigm similar to Fisher and her colleagues’ original romantic love study: study participants were scanned as they passively viewed a photo of their loved one and photos of a close friend of the same sex as their partner, blurred faces, and a gray background without a face. They also filled out a standardized questionnaire to measure just how passionately in love they were.

The researchers found, unsurprisingly,
that brain activations did change over time. The study participants were found to be less passionate about their partner after six months. In terms of cerebral blood flow, activation in the caudate nucleus, a reward area that has been implicated in passionate love, was significantly reduced. The authors of the study argued that this reduction in activity demonstrates that romantic love makes dynamic changes as the relationship evolves over time.
⁹

But these researchers stopped at six months. What kinds of changes might one find after one year? Ten years? Twenty years? When I asked Fisher if she thinks brain activations will change over time, she answered affirmatively. “Relationships change over time,” she said. “I would expect to see that reflected in brain activation too.”

Might they change in a way that can be generalized? After all, it may be that not all relationships are doomed to the quiet, warm attachment designated by ventral pallidum activation. Perhaps high-quality relationships, like Dirk and Lola’s, are so because the partners somehow manage to stay passionately in love no matter how much time passes.

Recently Fisher and her colleagues looked at the cerebral blood flow of individuals who claimed to still intensely love their partner after decades of togetherness. The group scanned ten women and seven men in the fMRI while they were looking at photos of their long-term significant other, a close long-term friend, a familiar acquaintance, and a not-so-familiar acquaintance. The group found that the brain activation of these long-term lovers closely resembled that of those who were newly in love: there was a lot of overlap in this study and the group’s first romantic love study that was published in 2005. Fisher and her colleagues observed that dopamine-rich reward areas like the ventral tegmental area (VTA) and the dorsal striatum were activated in long-term love, as were the globus pallidus, substantia nigra, thalamus, insula, and cingulate cortex. It would appear that it is possible to remain madly in love, even after decades together.

Following the study, the group compared brain activation with scores from a love and relationship questionnaire and found some interesting correlations. The ventral tegmental area and caudate nucleus activation was highly correlated with romantic love scores. Sexual frequency, on the other hand, was linked to hypothalamus and posterior hippocampus activation. The results suggest
that love—intense, passionate love—can be sustained over time. It would seem one can, indeed, keep the love alive.
¹⁰

It’s a great, inspiring conclusion. But this study, no matter how optimistic it may seem, does not offer us any indication of
why
these couples were able to stay in love after all this time. Though the researchers say there may be specific brain mechanisms to assist in sustaining passionate love, the identities of these mechanisms are yet to be discovered.

Less Vasopressin, More Marital Problems

Recent research done at Sweden’s Karolinska Institute suggests that vasopressin has an impact on the quality of relationships. A team of researchers led by Hasse Walum showed that men with a particular variant of the gene thought to be responsible for vasopressin processing are more likely to experience relationship discord when in a monogamous relationship than those with a different genotype.

Walum and his colleagues examined the DNA of hundreds of individuals in long-term, cohabitating relationships (some married, some not, but together for at least five years). In addition the researchers gave the study participants and their significant others questionnaires about the state of their relationship. They then compared those answers to the participants’ genetic makeup. The researchers were particularly interested to see if there was any interaction between relationship strength and genes related to vasopressin and its receptors. “We hoped to identify variation in how close these individuals were bonded to their partner,” said Walum. “Our main intent was to see if variation in the specific gene that had been shown to be so important for pair-bonding in voles would influence human behavior as well.”

Sure enough, that is just what Walum and his colleagues found. Genes are not static; they can evolve and change by interacting with different environmental variables. It is part and parcel of the whole idea of evolution: our genes must change by natural selection, by favoring the mutations that make us more likely to survive in our ever-changing environment. In Walum’s study men who had a particular genetic variant, called the 334 variant
of the
AVPR1A
gene, were more likely to express dissatisfaction in their relationship. The
AVPR1A
gene is the human equivalent to the so-called monogamy gene in prairie voles; it has been linked to issues with vasopressin receptor expression in the human brain and, intriguingly, autism, a disorder characterized by severe difficulties forming social attachments.
¹¹

If the individual had a glitch in his DNA that resulted in more than one copy of that varied gene in the chromosome, it actually doubled the chance that he reported a relationship crisis within the past year. A mutation to a single gene—the same gene that seemingly turns monogamy on and off in the prairie and meadow voles—was linked to more problematic monogamous relationships in human males. Interestingly women did not show this same gene-relationship correlation.

As these various lines of neuroscientific research converge, it’s clear that oxytocin, vasopressin, and dopamine play important roles both in the formation of a bond and in its maintenance over time. But they aren’t the only chemicals involved. As I said in chapter 3, love, in all of its forms, serves up a complex combination of neurochemicals in the brain. Aragona and his lab recently presented new findings demonstrating that a specific type of opioid receptor is also important to maintaining a bond.
¹²
There are likely more chemicals and receptor subtypes with important roles when it comes to social attachment just waiting to be discovered.

When one considers the unknowns involved in these chemicals and how they influence each other, the problem with so-called love-promoting brain chemistry supplements becomes readily apparent. As these pills modulate oxytocin, testosterone, or vasopressin in the body and the brain (if indeed they even have the power to do so), they may very well be altering other chemical and receptor types that are just as important to social bonds. They may change the body’s endogenous production and regulation of these chemicals and receptors over time. Without a better understanding of all the ingredients of this complex neurobiological cocktail, one can never know if those changes are to our benefit or detriment.

Alas, our current understanding of the brain’s neurochemistry has not offered all that much to our practical knowledge of how to make your relationship last. There is no one chemical,
no magic formula that is responsible for making love stay. Although Snowdon’s work with tamarins suggests that a physical connection is important to maintaining a quality bond over time, it’s doubtful that this is the only factor involved in human relationships—though, as he said, upping your efforts in that department certainly couldn’t hurt.

Chapter 9

Mommy (and Daddy) Brain

Have I mentioned how much I love
my son? I mean, I really love him—I am head over heels, utterly besotted, crazy in love with that boy. Pick whatever “in love” cliché you like; each and every one fits the way I feel about my kid. If you get me started on all the ways my son is brilliant, charming, and pretty darn perfect, I am likely to continue talking even when your eyes glaze over and you try to force a change of subject. I am that far gone. As I said earlier, my kid may just be “sexy” enough to merit banning—not in the creepy way, but in that irresistible way Nicolas Read quipped about at the first “Is There a Neurobiology of Love?” meeting.
¹

Bruce McEwen, the author of the 1996 Wenner-Gren Symposium meeting report, said he included Read’s sexy baby–banning quote to illustrate the power of the mother-child bond and the importance of studying such a phenomenon in a mechanistic way. After I mentioned that the line was one of personal significance to me, McEwen laughed. “That line brings to mind an amazing paper by Craig Ferris,” he told me. “He presented pups and cocaine to mother rats and found that the nucleus accumbens was much more activated by the pups than the drug. It’s a remarkable finding and justifies the Read quote, in a very practical sense, by showing just how strong the bond between the mother and her pups really is.”
²

And strong it is. I’m not afraid to admit that the way I feel about my son is like nothing else I’ve ever experienced. It is all-encompassing and unalterable, a true force to be reckoned with. These are feelings that surprise me. Though I had always
liked children, I wasn’t that girl who wanted to have babies above everything else. I was actually fairly ambivalent about the idea of being a mom. But something changed in me after having the boy. Pretty profoundly too, I might add.

You see these same changes in rats. Virgin rats aren’t so keen on the youngsters. In fact they find youngsters pretty aversive and often try to eat or kill them. But something happens after females give birth and then start to nurse offspring: a serious reworking of the reward processing circuitry in the brain that results in moms’ finding pups more tantalizing than cocaine. Who knew?

Craig Ferris, a neuroscientist at Northeastern University, noted several studies demonstrating that rat pups are quite the positive reinforcement for moms. Mother rats work hard to be near their pups while they are lactating. They will even, when forced by experimental paradigm, press a bar like crazy to collect pups from a dispenser.

Ferris wondered what was going on in the maternal brain to account for such effects. In an fMRI study he and his colleagues observed that the act of nursing pups lit up the reward processing circuitry in the mothers. Those brain areas, including the olfactory system, nucleus accumbens, ventral tegmental area, cortical amygdala, and hypothalamus, among others, showed a similar pattern of activation in the brains of virgin females after a hit of cocaine. “We knew cocaine lit up this dopamine reward and motivation circuitry in virgin females. And in the moms, pups were lighting up the same circuits,” said Ferris. “We wanted to see if there might be a conflict. So we took the moms and gave them cocaine to see what would happen.”