In an earlier functional MRI study of romance, published in 2000, researchers at University College London monitored brain activity in young men and women who had been in relationships for about two years. The brain images, also taken while participants looked at photos of their beloved, showed activation in many of the same areas found in the new study — but significantly less so, in the region correlated with passionate love, she said.

In the new study, the researchers also saw individual differences in their group of smitten lovers, based on how long the participants had been in the relationships. Compared with the students who were in the first weeks of a new love, those who had been paired off for a year or more showed significantly more activity in an area of the brain linked to long-term commitment.

Last summer, scientists at Emory University in Atlanta reported that injecting a ratlike animal called a vole with a single gene turned promiscuous males into stay-at-home dads — by activating precisely the same area of the brain where researchers in the new study found increased activity over time.

"This is very suggestive of attachment processes taking place," Brown said. "You can almost imagine a time where instead of going to Match.com you could have a test to find out whether you're an attachment type or not."

One reason new love is so heart-stopping is the possibility, the ever-present fear, that the feeling may not be entirely requited, that the dream could suddenly end.

In a follow-up experiment, Fisher, Aron and Brown have carried out brain scans on 17 other young men and women who recently were dumped by their lovers. As in the new love study, the researchers compared two sets of images, one taken when the participants were looking at a photo of a friend, the other when looking at a picture of their ex.

Although they are still sorting through the images, the investigators have noticed one preliminary finding: increased activation in an area of the brain related to the region associated with passionate love. "It seems to suggest what the psychological literature, poetry and people have long noticed: that being dumped actually does heighten romantic love, a phenomenon I call frustration-attraction," Fisher said in an e-mail message.

One volunteer in the study was Suzanna Katz, 22, of New York, who suffered through a breakup with her boyfriend three years ago. Katz said she became hyperactive to distract herself after the split, but said she also had moments of almost physical withdrawal, as if weaning herself from a drug.

"It had little to do with him, but more with the fact that there was something there, inside myself, a hope, a knowledge that there's someone out there for you, and that you're capable of feeling this way, and suddenly I felt like that was being lost," she said in an interview.

And no wonder. In a series of studies, researchers have found that, among other processes, new love involves psychologically internalizing a lover, absorbing elements of the other person's opinions, hobbies, expressions and character, as well as sharing one's own. "The expansion of the self happens very rapidly, it's one of the most exhilarating experiences there is, and short of threatening our survival it is one thing that most motivates us," said Aron, of SUNY, a co-author of the study.

To lose all that, all at once, while still in love, plays havoc with the emotional, cognitive and deeper reward-driven areas of the brain. But the heightened activity in these areas inevitably settles down. And the circuits in the brain related to passion remain intact, the researchers say — intact and capable in time of flaring to life with someone new.