In the study, Fisher, Dr. Lucy Brown of Albert Einstein College of Medicine in the Bronx and Dr. Arthur Aron, a psychologist at the State University of New York at Stony Brook, led a team that analyzed about 2,500 brain images from 17 college students who were in the first weeks or months of new love. The students looked at a picture of their beloved while an MRI machine scanned their brains. The researchers then compared the images with others taken while the students looked at picture of an acquaintance.

Functional MRI technology detects increases or decreases of blood flow in the brain, which reflect changes in neural activity.

In the study, a computer-generated map of particularly active areas showed hot spots deep in the brain, below conscious awareness, in areas called the caudate nucleus and the ventral tegmental area, which communicate with each other as part of a circuit.

These areas are dense with cells that produce or receive a brain chemical called dopamine, which circulates actively when people desire or anticipate a reward. In studies of gamblers, cocaine users and even people playing computer games for small amounts of money, these dopamine sites become extremely active as people score or win, neuroscientists say.

Yet falling in love is among the most irrational of human behaviors, not merely a matter of satisfying a simple pleasure, or winning a reward. And the researchers found that one particular spot in the MRI images, in the caudate nucleus, was especially active in people who scored highly on a questionnaire measuring passionate love.

This passion-related region was on the opposite side of the brain from another area that registers physical attractiveness, the researchers found, and appeared to be involved in longing, desire and the unexplainable tug that people feel toward one person, among many attractive alternative partners.

This distinction, between finding someone attractive and desiring him or her, between liking and wanting, "is all happening in an area of the mammalian brain that takes care of most basic functions, like eating, drinking, eye movements, all at an unconscious level, and I don't think anyone expected this part of the brain to be so specialized," Brown said.

The intoxication of new love mellows with time, of course, and the brain scan findings reflect some evidence of this change, Fisher said.