New love can look for all the world like mental illness, a blend of mania, dementia and obsession that cuts people off from friends and family and prompts out-of-character behavior — compulsive phone calling, serenades, yelling from rooftops — that could almost be mistaken for psychosis.

Now for the first time, neuroscientists have produced brain scan images of this fevered activity, before it settles into the wine and roses phase of romance or the joint holiday card routines of long-term commitment.

In an analysis of the images appearing in The Journal of Neurophysiology, researchers in New York and New Jersey argue that romantic love is a biological urge distinct from sexual arousal.

It is closer in its neural profile to drives like hunger, thirst or drug craving, the researchers assert, than to emotional states like excitement or affection. As a relationship deepens, the brain scans suggest, the neural activity associated with romantic love alters slightly, and in some cases primes areas deep in the primitive brain that are involved in long-term attachment.

The research helps explain why love produces such disparate emotions, from euphoria to anger to anxiety, and why it seems to become even more intense when it is withdrawn. In a separate, continuing experiment, the researchers are analyzing brain images from people who have been rejected by their lovers.

"When you're in the throes of this romantic love it's overwhelming, you're out of control, you're irrational, you're going to the gym at 6 a.m. every day — why? Because she's there," said Dr. Helen Fisher, an anthropologist at Rutgers University and the co-author of the analysis. "And when rejected, some people contemplate stalking, homicide, suicide. This drive for romantic love can be stronger than the will to live."

Brain imaging technology cannot read people's minds, experts caution, and a phenomenon as many-sided and socially influenced as love transcends simple computer graphics, like those produced by the technique used in the study, called functional MRI.

Still, said Dr. Hans Breiter, director of the Motivation and Emotion Neuroscience Collaboration at Massachusetts General Hospital, "I distrust about 95 percent of the MRI literature, and I would give this study an A; it really moves the ball in terms of understanding infatuation love."

He added: "The findings fit nicely with a large, growing body of literature describing a generalized reward and aversion system in the brain, and put this intellectual construct of love directly onto the same axis as homeostatic rewards such as food, warmth, craving for drugs."