This Is Your Brain on Sex (6 page)

“When people say this or that area of the brain is activated in love, my standard reply is, ‘So what?’” Ortigue said emphatically as we discussed what her analysis of love and the brain really tells us. “People think it’s easy to look at these activations and know what is happening. But there’s so much more beyond that. It’s very dangerous to try and simplify this kind of network. There is not a single brain area at work here. We need to understand how all these areas work together
before we can say anything about the nature of love.”

We are not there yet—nowhere close—but headway has been made in understanding how the ventral tegmental area, caudate nucleus, putamen, and other areas work. Before I can explain how they may be interacting to get us into some of love’s most blessed (and thorny) situations, I need to discuss a powerful neurotransmitter called dopamine and other chemicals that help fuel this love-related brain network.

Chapter 3

The Chemicals between Us

Each distinct region of the brain is made
up of specialized cells called neurons. There are many of them—billions, in fact—and every few years someone attempts to get an accurate count of these bad boys. It is a daunting task, but the latest estimate tallied the number at about eighty-six billion.
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That is a lot of cells.
A lot
.

No cell, however, works alone. In order for the b Frain to perform its magic, these neurons need to signal each other by releasing a variety of chemical messengers called neurotransmitters. A stimulated cell releases the messenger, which then enters the synapse, which is the minuscule space between that brain cell and its neighbors. There the neurotransmitter may be picked up by an adjacent neuron, partner up with another chemical, or float in the ether for a while before it is taken back by the initiating cell. That is an oversimplification of the process, but it hits the basics.

We often discuss the function of these various brain chemicals as if there were only one molecule traveling between two cells—just a lonely little passenger traveling along a simple, direct path. In truth a single synapse during real-time neurotransmission looks more like holiday weekend rush hour on the Cross Bronx Expressway. Hundreds, if not thousands of different neurochemicals, hormones, and proteins move around the synapse at any given moment, and Demolition Derby rules are in place; these compounds not only stimulate bordering cells but they can modulate fellow travelers
as well. They can transform, cleave, or even block chemicals from communicating with other cells as they float in the synapse. Or they may just hang out in the synapse until they are taken back by the releasing cell, changing which chemicals are released the next time the cell fires. They can even help fellow neurochemicals get to their destinations faster and talk to different kinds of receptors.

Here is a situation where the old adage about elephants works all too well. How do you eat an entire elephant? You can do it only one way: a single bite at a time. How else could you handle millions of neurons, trillions of synapses, and the thousands of chemicals and proteins roaming within each synapse? By observing one reaction, one neurotransmitter, one receptor at a time, scientists have uncovered several interesting chemicals related to love.

The Dope on Dopamine

At the base of the forebrain, the large frontal cortex that differentiates humans from other mammals, are the basal ganglia. Hundreds of millions of years ago, when our ancestors climbed out of the water and started spending time on dry land, they possessed brains not unlike the current incarnation of our basal ganglia, composed of the corpus striatum, a striped structure made up of the caudate nucleus, putamen, and nucleus accumbens; the pallidum, with its pale globe, the globus pallidus, and neighboring ventral pallidum; the substantia nigra, or “black substance,” the region in the basal ganglia that appears darker than the rest; the ventral tegmental area; and the subthalamic nucleus, a small area adjacent to the thalamus, the region responsible for relaying sensations. As we developed larger frontal lobes, our basal ganglia maintained their formidable sway over behavior and learning by forming strong connections both to and from the prefrontal cortex and other key brain areas. The basal ganglia region is linked to almost every aspect of cognition and is also part of an important brain pathway called the mesocortical limbic system, which is involved with motivation and reward processing.

Our “reptilian brain,” the major structures of the basal ganglia.
Illustration by Dorling Kindersley.

The basal ganglia are home to many
different neurotransmitters, but these areas, as well as the mesocortical limbic system, are fueled primarily by the dopamine released from a group of neurons in the substantia nigra and VTA. It is this influential chemical that allows for profound changes to the brain that result in learning, memory, and movement. Dopamine is as old as the reptilian brain itself—and quite potent stuff.
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Dopamine has been studied extensively in individuals who have Parkinson’s disease. The death of critical dopamine-producing neurons in the substantia nigra results in Parkinson’s hallmark symptoms: tremor, rigidity, and dementia. Schizophrenia has been characterized as an overabundance of dopamine production in the brain. And with a hand in Tourette’s syndrome, anorexia nervosa,
obsessive-compulsive disorder (OCD), attention-deficit/hyperactivity disorder (ADHD), and drug addiction, this is a neurotransmitter with some serious pull.

Now think about falling in love. You may show some obsessive tendencies when it comes to your intended, not so unlike an individual with OCD. You may be distracted at work or at home, a little love-related ADHD. You may start to attribute significant meaning to minor traits you have in common with your mate: your mutual adoration for horror films and the fact that you are both addicted to pistachios, not to mention the way your competitive natures both come out during March Madness. Taking trivial connections and turning them into matters of great importance can often be seen in schizophrenic patients (like John Nash’s psychotic code-breaking methods in
A Beautiful Mind
). A person in love may have impaired decision-making ability; Parkinsonian patients often have great difficulty making even the simplest choices.

Addiction? That leap is not hard to make. It is all too easy to remember those first few months of love, when you just couldn’t get enough of your significant other. I am not saying love is a disease (though more than a few writers have poetically done so), nor that love is part and parcel of these disorders. Rather, when you look closer at the behavioral effects resulting from too much or too little dopamine in the basal ganglia, you can see why Helen Fisher thought it must underlie the neurobiology of love. “I read descriptions of romantic love from the last forty years so I could get a feel for what the elements were,” Fisher explained. “And when I looked at those elements—the focus of love, the energy, the motivational factors—I came up with the hypothesis that dopamine had to be behind it.”

She was right—many areas in the basal ganglia, particularly those that send or receive dopamine projections, lit up in response to romantic love in the fMRI scanner. Animal studies have suggested dopamine plays a remarkable role in the forming of pair-bonds too. Prairie voles are a monogamous species; more than 80 percent of these small rodents pair up with just one mate for life. The dopamine released after the initial mating session, likely the source of its pleasure, primes the brain to make that bond. When scientists gave prairie voles drugs that inhibit the production of dopamine, these über-monogamous rodents were unable to form pair-bonds after mating.
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By contrast, prairie voles that were injected with drugs that increased the amount of dopamine in their brains formed bonds even without the precursory mating session. Dopamine matters. Not just in the positive reinforcement from sex to form the original bond but also in the continued matings that allow the animals to maintain that bond over time.
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How does one little chemical have such reach? It comes down to the processing of risks and rewards. Recent research into the study of reward processing and decision making has shown that dopamine plays a big role. Using animal models, scientists have learned that when a rat is making a choice that leads to an unexpected reward, like food or a positively stimulating drug, there is a notable increase in dopamine release in the basal ganglia. After being trained to expect a reward with a paired stimulus, if the rat does not receive it or gets something less than stellar (some plain old food pellets instead of a hit of cocaine, for example), the cells in the basal ganglia release less dopamine. The same kinds of effects have been observed in people during neuroimaging studies.

Michael Frank, a neuroscientist at Brown University who studies the basal ganglia, believes that this is how dopamine facilitates learning. After all, when you receive a reward, you want to figure out what you did to earn it so you can go out and do it again. Depending on how good the treat is, you might even perform that task a few more times after that. On the other side of the coin, if a certain behavior results in punishment or a negative consequence, you’ll want to avoid that situation in the future. “The basal ganglia circuit is perfectly structured to help execute reward-based learning,” Frank told me. “It allows you to learn about positive and negative outcomes of your choices. When you have that increase in dopamine, you are more likely to pursue the same rewarding outcome in the future.”

In prairie voles that dopamine flood that results from mating primes the brain for creating a lasting pair-bond with a member of the opposite sex. Without that dopamine rush, the bond might never happen. This is true in humans too—and not just with sex. Any social interaction promotes the release of dopamine. Laughing with friends, cuddling with your kids, holding hands with your significant other, even petting your dog—simply being with others is a reward in its own right.

The basal ganglia facilitate more than just
pleasurable feelings, though. This collection of regions sends signals to areas that help filter out unnecessary information so you can focus on the most important aspects of the task at hand. They enable you to form a memory of an experience to retrieve when you later find yourself in a similar situation, and they help you organize your movements so you can act in response to a stimulus. The basal ganglia are a critical circuit that informs all manner of cognitive activities, and dopamine fuels it, that is, when we are talking about any kind of learning that stems from a reward—love included.

Neuroscientists are very interested in how we assess both reward and risk, and how this valuation plays out in many types of behaviors. This means they’re fascinated by sex and love; after all, is there any greater reward (or risk, for that matter) than finding your one true love (or, in his absence, getting a little something-something while you wait for him to show up)? Is there any other stimulus with such profound and far-reaching effects on behavior? Very few, I’d wager.

Oxytocin, Oxytocin Everywhere

Just above the brain stem, below the thalamus and basal ganglia, is the hypothalamus. This little almond-shaped brain region has the critical function of mediating communication between the brain and the endocrine system. In other words, this area has a say on which hormones, those great behavioral primers, are released into the bloodstream and into the brain. It is directly linked to the pituitary gland, the brain’s so-called master gland and secretor of hormones into the body, as well as to the thalamus, the nucleus accumbens, and the ventral tegmental area. Specialized areas within the hypothalamus, like the paraventricular nucleus (PVN) and the supraoptic nucleus (SON), do some of that mediating with the release of oxytocin.