Biology: Your Brain In Love

What goes on in your head when you fall madly in love? One scientist decided to find out. In an exclusive book excerpt, she lays bare the physiology of passion

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Martin Parr / Magnum

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At this point, I interviewed each candidate. My first question was always the same: "How long have you been in love?" My second question was the most important: "What percentage of the day and night do you think about your sweetheart?" Because obsessive thinking is a central ingredient of romantic passion, I sought only participants who thought about their beloved almost all their waking hours. I also looked for men and women who laughed and sighed more than usual during the interview.

If a potential subject showed signs of passion, I invited him or her to participate. We acquired two photographs: one of the beloved and one of an emotionally neutral individual. Generally the latter was someone the subject had known casually in high school or college. Then we set a date to put each subject into the brain scanner.

The Brain-Scanning Procedure
The procedure was simple but not easy. First Mashek and I made the participant as comfortable as possible in the scanner — a large, horizontal, cylindrical, cream-colored plastic tube that is open at both ends and extends from above the head to about the waist. After taking preliminary scans to establish basic brain anatomy, the 12-min. experiment started. First the subject looked at the photograph of the beloved on the screen for 30 sec. as the scanner recorded blood flow in various brain regions. Next the subject was shown a large number and asked to count backward for 40 sec. The participant then looked at the neutral photograph for 30 sec. while the brain was scanned again. Finally the subject was shown another large number and asked to count backward again, this time for 20 sec.

This cycle (or its reverse) was repeated six times — enabling us to collect 144 scans of different brain regions for each participant. After the experiment was over, I interviewed all the subjects again, asking how they felt and what they were thinking about during all parts of the test. To express our gratitude, we gave each participant $50 and a picture of his or her brain.

The Brain in Love
Before we could understand the results of our scanning, we had to make an in-depth analysis of the brain pictures. The fMRI machine that we were using shows only blood-flow activity in specific brain regions rather than the chemicals involved. But because scientists know which kinds of nerves connect which kinds of brain regions, they can often surmise which brain chemicals are active when specific regions begin to glow.

Many brain parts became active in our love-struck subjects when they focused on their beloved. However, two regions appear to be central to the experience of being in love. Perhaps our most important finding concerned activity in the caudate nucleus. This is a large, C-shaped region that sits deep near the center of your brain. It is very primitive — part of what is called the reptilian brain because it evolved long before 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.

I was astonished. Scientists have long known that this brain region directs bodily movement. Only recently have they come to realize that it is also a key part of the brain's "reward system," the mind's network for general arousal, sensations of pleasure and the motivation to acquire rewards. Not only did our subjects exhibit activity in the caudate, but also the more passionate they were, the more active their caudate was.

We discovered this in a curious way. Before our subjects entered the brain scanner, we asked each to fill out several questionnaires, including a survey designed by psychologist Elaine Hatfield and sociologist Susan Sprecher called the Passionate Love Scale. We wanted to compare the brain activity of each subject to what that subject reported on a questionnaire. 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.

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. Chocolate can be addictive. I maintain that romantic love is addictive too.

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