WASHINGTON — Like any young woman in love, Bianca Acevedo has exchanged valentine hearts with her fiance.

But the New York neuroscientist knows better. The source of love is in the head, not the heart.

She's one of the researchers in a relatively new field focused on explaining the biology of romantic love. And the unpoetic explanation is that love mostly can be understood through brain images, hormones and genetics.

That seems to be the case for the newly in love, the long in love and the brokenhearted.

"It has a biological basis. We know some of the key players," said Larry Young of the Yerkes National Primate Research Center at Emory University in Atlanta. There, he studies the brains of an unusual monogamous rodent to get a better clue about what goes on in the minds of people in love.

In humans, there are four tiny areas of the brain that some researchers say form a circuit of love. Acevedo, who works at the Albert Einstein College of Medicine in New York, is part of a team that has isolated those regions with the unromantic names of ventral tegmental area (VTA), the nucleus accumbens, the ventral pallidum and raphe nucleus.

The hot spot is the teardrop-shaped VTA. When people newly in love were put in a functional magnetic resonance imaging machine and shown pictures of their beloved, the VTA lit up. Same for people still madly in love after 20 years.

The VTA is part of a key reward system in the brain.

"These are cells that make dopamine and send it to different brain regions," said Helen Fisher, a researcher and professor at Rutgers University. "This part of the system becomes activated because you're trying to win life's greatest prize — a mating partner."

One of the research findings isn't so complimentary: Love works chemically in the brain like a drug addiction.

"Romantic love is an addiction; a wonderful addiction when it is going well, a horrible one when it is going poorly," Fisher said. "People kill for love. They die for love."

The connection to addiction "sounds terrible," Acevedo acknowledged.

"Love is supposed to be something wonderful and grand, but it has its reasons. The reason I think is to keep us together."

But sometimes love doesn't keep us together. So the scientists studied the brains of the recently heartbroken and found additional activity in the nucleus accumbens, which is even more strongly associated with addiction.

"The brokenhearted show more evidence of what I'll call craving," said Lucy Brown, a neuroscientist also at Einstein medical college. "Similar to craving the drug cocaine."

The team's most recent brain scans were aimed at people married about 20 years who say they are still holding hands, lovey-dovey as newlyweds, a group that is a minority of married people. In these men and women, two more areas of the brain lit up, along with the VTA: the ventral pallidum and raphe nucleus.

The ventral pallidum is associated with attachment and hormones that decrease stress; the raphe nucleus pumps out serotonin, which "gives you a sense of calm," Fisher said.

Those areas produce "a feeling of nothing wrong. It's a lower-level happiness and it's certainly rewarding," Brown said.

While the recent brain research is promising, University of Hawaii psychology professor Elaine Hatfield cautions that too much can be made of these studies alone. She said they need to be meshed with other work from traditional psychologists.

Brain researchers are limited because there is only so much they can do to humans without hurting them. That's where the prairie vole — a chubby, short-tailed mouselike creature — comes in handy. Only 5 percent of mammals more or less bond for life, but prairie voles do, Young said.

Scientists studied voles to figure out what makes bonding possible. In females, the key bonding hormone is oxytocin, also produced in both voles and humans during childbirth, Young said. When scientists blocked oxytocin receptors, the female prairie voles didn't bond.

In males, it's vasopressin. Young put vasopressin receptors into the brains of meadow voles — a promiscuous cousin of the prairie voles — and "those guys who should never, ever bond with a female, bonded with a female."

Researchers also uncovered a genetic variation in a few male prairie voles that are not monogamous — and found it in some human males, too.

Those men with the variation ranked lower on an emotional bonding scale, reported more marital problems, and their wives had more concerns about their level of attachment, said Hasse Walum, a biology researcher in Sweden. It was a small but noticeable difference, Walum said.

Scientists figure they now know better how to keep those love circuits lit and the chemicals flowing.

Young said that romantic love theoretically can be simulated with chemicals, but "if you really want, you know, to get the relationship spark back, then engage in the behavior that stimulates the release of these molecules and allow them to stimulate the emotions," he said. That would be hugging, kissing, intimate contact.

"My wife tells me that flowers work as well. I don't know for sure," Young said. "As a scientist it's hard to see how it stimulates the circuits, but I do know they seem to have an effect."