Why We Love (10 page)

Finally we saw the results: beautiful pictures of the brain in love. When I first looked at those brain scans, with the active brain regions lit up in bright yellow and deep orange, I felt the way I feel on a summer night when I gaze at the sparkling universe: overwhelming awe. But to understand what I saw, you must know a little bit about the furniture in your head.

The brain is composed of many parts or regions. Each has particular functions. And each communicates with other brain regions via nerve cells or neurons—some 10 billion of them. These nerve cells produce, store, and distribute neurotransmitters of different types; some, for example, synthesize dopamine, norepinephrine, and/or serotonin. When a neuron is electrically stimulated by a nearby neuron, the impulse often prompts these neurotransmitters to exit from a nerve cell, sail across a tiny gap, or synapse, and dock at “receptor sites” on the next nerve cell. This way neurotransmitters send an electrical impulse along, cell by cell.

Each nerve cell has about one thousand of these synaptic connections; and there are some 10 trillion synapses between nerve cells in the human brain. Some machine! Each nerve cell communicates only with specific others, however, producing nerve networks that connect specific brain parts and integrate our thoughts, memories, sensations, emotions, and motivations. Scientists call these networks of nerves and brain parts “circuits,” “systems,” or “modules.”

The fMRI machine that we were using only shows blood flow activity in specific brain regions. But because scientists know which kinds of nerves connect which kinds of brain regions, they can surmise which brain chemicals are active when specific brain regions begin to glow with increased activity.

Many brain parts became active in each of our love-struck subjects.
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However, two regions appear to be central to the exquisite experience of being in love.

The Reward System of the Brain

Perhaps our most important finding was activity in the caudate nucleus. This is a large C-shaped region that sits deep near the center of your brain (see diagram
here
). It is primitive; it is part of what is called the reptilian brain or R-complex because this brain region evolved long before the mammals proliferated some 65 million years ago. Our brain scans showed that parts of the body and the tail of the caudate became particularly active as a lover gazed at the photo of a sweetheart.
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I was astonished. Scientists have long known that this brain region directs bodily movement. Only recently have they come to realize that this enormous engine is part of the brain’s “reward system,” the mind’s network for general arousal, sensations of pleasure, and the motivation to acquire rewards.
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The caudate helps us detect and perceive a reward, discriminate between rewards,
prefer
a particular reward, anticipate a reward, and expect a reward. It produces motivation to acquire a reward and plans specific movements to obtain a reward. The caudate is also associated with the acts of paying attention and learning.
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Not only did our subjects exhibit activity in the caudate, but the more passionate they were, the more active their caudate was.

We discovered this in a curious way. Remember the Passionate Love Scale that our subjects filled out before entering the brain scanning machine? When we compared each subject’s responses on this questionnaire with the activity shown in their brains, we found a positive correlation: those who scored higher on the Passionate Love Scale also showed more activity in a specific region of the caudate when they looked at the picture of their sweetheart.

How remarkable. Scientists and businesspeople have long wondered whether self-report questionnaires actually reflect one’s inner feelings. In this case, the answer was yes. Our team would be among the first to show a direct link between responses to a survey questionnaire and a specific pattern of brain activation.

We also found activity in other regions of the reward system, including areas of the septum and a brain region that becomes active when people eat chocolate.
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Chocolate can be addictive. In chapter eight I will maintain that romantic love is addictive, too.

The Dopamine Mother Lode

Another striking result from our fMRI experiment was activity in the ventral tegmental area (VTA), a central part of the reward circuitry of the brain.
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This result was what I was looking for. I had hypothesized, as you know, that romantic love is associated with elevated levels of dopamine and/or norepinephrine.
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The VTA is a mother lode for dopamine-making cells. With their tentacle-like axons, these nerve cells distribute dopamine to many brain regions, including the caudate nucleus (see diagram).
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And as this sprinkler system sends dopamine to many brain parts, it produces focussed attention,
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as well as fierce energy, concentrated motivation to attain a reward, and feelings of elation, even mania
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—the core feelings of romantic love.

No wonder lovers talk all night or walk till dawn, write extravagant poetry and self-revealing e-mails, cross continents or oceans to hug for just a weekend, change jobs or lifestyles, even die for one another. Drenched in chemicals that bestow focus, stamina, and vigor, and driven by the motivating engine of the brain, lovers succumb to a Herculean courting urge.

That “inflammable matter” that Founding Father George Washington spoke of is, at least in part, dopamine churning up the caudate nucleus and other parts of the brain’s reward system—a primordial brain network that drives the lover to focus his or her attention on life’s grandest prize—a mate who may pass their DNA toward eternity.

How Love Changes

During our experiment we also discovered one way in which love changes over time. Our insight was due to a remarkable coincidence. In 2000, while we were in the middle of our project, scientists at University College, London, announced the completion of a similar experiment.
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Using an fMRI brain scanner, Andreas Bartels and Semir Zeki examined brain activity in seventeen subjects who reported being “deeply, truly and madly in love.” Eleven were women; all were between the ages of twenty-one and thirty-seven; and all looked at a photograph of their beloved, as well as photos of three friends of similar age, sex, and length of friendship.

The London experiment was a distinct achievement. Bartels and Zeki found several brain regions that become active while subjects gaze at pictures of a sweetheart. Particularly important, they found activity in one of the same regions of the caudate nucleus. What joy. Two research teams on different continents, with different subjects from different ethnic groups and of different general ages, in somewhat different experiments, found activity in the same brain structure. The caudate nucleus—with its supercharge of dopamine—must be the furnace of human romantic love.

However, the London data also told us something about how love develops across time. We hadn’t planned to investigate how love changes. But the London study subjects were in love for an average duration of 2.3 years while ours were in love a mean duration of seven months. And their men and women showed activity in two regions—the anterior cingulate cortex and the insular cortex—whereas ours showed none (see diagram
here
). These differences provoked us to compare the subjects in our study with one another.

Sure enough, our subjects who were in
longer
relationships showed activity in the anterior cingulate cortex and the insular cortex, just like the London study.

We don’t know what this actually means. The anterior cingulate gyrus is a region where emotions, attention, and working memory interact.
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Some parts are associated with happy states; others involve awareness of one’s own emotional state and the ability to assess other people’s feelings during social interactions; and some are associated with split-second emotional reactions to a win or loss, thereby judging a reward’s value.
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The insular cortex collects data from the body regarding external touch and temperature, as well as internal pain and activities of the stomach, gut, and other viscera. With this brain part we register “butterflies” in the stomach, a pounding heart, and our many other bodily reactions. Parts of the insular cortex also process the emotions.

So we established that as a relationship lengthens, brain regions associated with emotions, memory, and attention begin to respond in new ways. Exactly what these brain parts are doing, no one knows. Is the brain laying down and consolidating emotional memories of the love relationship?
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Are we using our emotions to analyze the relationship? We all know love changes over time; when we come to understand these results, they may tell us how and why.

Our New York team also found several gender differences in romantic passion. But I will discuss these findings and their implications in chapter five.

The Drive to Love

All these data had a definite effect on me—they changed my understanding of romantic love. For many years I had regarded this wonderful experience as a constellation of related
emotions
that ranged from elation to despair. But psychologists distinguish between emotions and
motivations
—brain systems oriented around planning and pursuit of a specific want or need. And our colleague, Art Aron, was wedded to the idea that romantic love was not an emotion but a motivation system designed to enable suitors to build and maintain an intimate relationship with a preferred mating partner.
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Indeed, because of Art’s dedication to this idea, we had begun our brain scanning project with two hypotheses: my hypothesis that romantic love is associated with dopamine and/or other closely related neurotransmitters in the brain; and Art’s theory that romantic love is primarily a motivation system, rather than an emotion.

As it turns out, our results suggest that both hypotheses are correct. Romantic love does seem to be associated with dopamine. And because this passion emanates from the caudate nucleus, motivation and goal-oriented behaviors are involved.

In fact, these findings prompted me to come to an even broader consideration:
I came to believe that romantic love is a primary motivation system in the brain—in short, a fundamental human mating drive.

Neuroscientist Don Pfaff defines a drive as a neural state that energizes and directs behavior to acquire a particular biological
need
to survive or reproduce.
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We have lots of drives. They lie along a continuum. Some, like thirst and the need for warmth, cannot be extinguished until satisfied. The sex drive, hunger, and the maternal instinct, on the other hand, can often be redirected, even quelled with time and effort. I think the experience of falling in love lies somewhere along this continuum.

First of all, like drives, romantic attraction is tenacious; it is very hard to extinguish. Emotions, on the other hand, come and go; you can be happy in the morning and angry in the afternoon.

Like drives, romantic love is focussed on a specific reward, the beloved, in the same way that hunger is focussed on food. Emotions, like disgust, pin themselves to an immense variety of objects and ideas. In fact, romantic love is linked with many diverse emotions depending on whether this urge is being satisfied or frustrated.

Like drives, romantic love is not associated with any particular facial expression. All of the primary emotions—among them anger, fear, joy, surprise, and disgust—have stereotypic facial looks.

Like drives, romantic love is exceedingly difficult to control. It is harder to curb thirst, for example, than it is to control an emotion, such as anger.

Very important, all of the basic drives are associated with elevated levels of central dopamine.
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So is romantic love.

And like all the other drives, romantic love is a need, a craving. We need food. We need water. We need warmth. And the lover feels he/she
needs
the beloved. Plato had it right over two thousand years ago. The god of love “lives in a state of need.”
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Love’s Complex Chemistry

Undoubtedly many other brain systems contribute to this “pulsing rush of Longing,” as Homer called it. As you remember, I initially hypothesized that norepinephrine might be involved because it is so closely related to dopamine and produces so many of the same feelings and behaviors. I still suspect norepinephrine contributes to the passion of romance; we just haven’t yet devised the appropriate experiment to establish this.

Low levels of serotonin produce obsessive thinking—a central component of romantic love. So I think someday we may find that this chemical contributes to romantic ardor, too.
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