Story Highlights • MRI scans show activity in caudate area of the brain at the sight of one's beloved

• When you're in love, caudate area flooded with dopamine, a pleasure chemical

• Researcher: "Exactly the same system becomes active as when you take cocaine"

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(CNN) -- Close your eyes for a minute and envision all the romantic parts of the human body.

Her beautiful eyes. His strong shoulders. We'll stop there, but you go right ahead and think about all the body parts you want.

Bet you didn't think about the caudate and the ventral tegmental areas, did you?

These areas of the brain, while little known to most people, are helping scientists explain the physiological reasons behind why we feel what we feel when we fall in love.

By studying MRI brain scans of people newly in love, scientists are learning a lot about the science of love: Why love is so powerful, and why being rejected is so horribly painful.

In a group of experiments, Dr. Lucy Brown, a professor in the department of neurology and neuroscience at the Albert Einstein College of Medicine in New York, and her colleagues did MRI brain scans on college students who were in the throes of new love.

While being scanned, the students looked at a photo of their beloved. The scientists found that the caudate area of the brain -- which is involved in cravings -- became very active. Another area that lit up: the ventral tegmental, which produces dopamine, a powerful neurotransmitter that affects pleasure and motivation.

Dr. Brown said scientists believe that when you fall in love, the ventral tegmental floods the caudate with dopamine. The caudate then sends signals for more dopamine.

"The more dopamine you get, the more of a high you feel," Dr. Brown says.

Or as her colleague, Dr. Helen Fisher put it: When you fall in love, "exactly the same system becomes active as when you take cocaine. You can feel intense elation when you're in love. You can feel intense elation when you're high on cocaine."

Is it love -- or sex?

Scientists then wondered: Does a brain in love look much like a sexually stimulated brain? After all, we associate love and sex and sometimes confuse them.

The answer is: Brains in love and brains in lust don't look too much alike.

In studies when researchers showed erotic photos to people as they underwent brain scans, they found activity in the hypothalamus and amygdala areas of the brain. The hypothalamus controls drives like hunger and thirst and the amygdala handles arousal, among other things.

In the studies of people in love, "we didn't find activity in either," according to Dr. Fisher, an anthropologist and author of "Why We Love -- the Nature and Chemistry of Romantic Love."

"We now have physiological data that suggests there are different brain systems for sex and love," says Dr. Fisher.

At some point, the two do become linked. People in love have elevated levels of dopamine. Lots of dopamine, in turn, triggers the production of testosterone, which is responsible for the sex drive in both men and women.

This helps explain why falling in love can make someone all of a sudden seem sexy.

"Three weeks ago he was just another nice guy in the office and now everything about him is sexual," says Dr. Fisher.

All this research into sex and love got the researchers thinking: Most other mammals don't have this drive for romantic love and attachment. Why do humans have it? After all, we could easily propagate the species just with our sexual urges.

Dr. Fisher thinks it has a lot to do with how difficult it once was to raise children.

"Go back millions of years to the grasslands of Africa. A woman was carrying the equivalent of a 20-pound bowling ball in one arm, and sticks and rocks in another arm to protect herself in this dangerous environment. She needed a partner to help her. She couldn't do it alone," Dr. Fisher says.

And even today, when we have strollers and the environment isn't quite as dangerous, having a mate still helps. "There are women who raise a baby by themselves, but it's a lot harder," she says.

Male brain - female brain

In their work with the lovestruck, the scientists found brain differences between men and women.

"The men had quite a bit more activity in the brain region that integrates visual stimuli. This isn't surprising considering that men support the porn industry and women spend their lives trying to look good for men," says Dr. Fisher.

But she adds there's probably a more anthropological reason at work. Simply put: A man's evolutionary mission is to spread his seed. That won't work if he mates with an 80-year-old grandmother.

"Men have to be able to size up a woman visually to see if she can bear babies," says Dr. Fisher.

The women's brain activities were a bit more puzzling.

The scientists found that women in love had more activity than men in the areas of the brain that govern memories. Dr. Fisher theorizes that this is a "female mechanism for mate choice." There are no visual clues for whether a man is fertile, but if a woman really studies a man and remembers things about his behavior, she can try to determine whether he'd make a reliable mate and father.

Thus, if it sometimes seems like a woman remembers everything -- good and bad -- about a man, "it's not just her being picky. It's an old Darwinian evolutionary strategy."

What's love got to do with it?

In the end, Drs. Fisher and Brown say what they learned from lovers' brains is that romantic love isn't really an emotion -- it's a drive that's based deep within our brains, right alongside our urges to find food and water.

"This helps explain why we do crazy things for love," says Dr. Brown. "Why did Edward VIII give up the throne for Wallis Simpson? The systems that are built into us to find food and water are the things that were also active when he renounced the throne of England."

Now their research is centered on the flip side of love. They've recruited college students who'd just been rejected by their sweethearts. Again, the scientists performed MRI's while these students looked at photos of the objects of their affection.

This time, the results were different, Dr. Brown says. The insular cortex, the part of the brain that experiences physical pain, became very active.

"People came out of the machine crying," she said. "We won't be doing that experiment again for a long time."

Elizabeth Cohen is a correspondent with CNN Medical News. Producer Amy Burkholder contributed to this report.