He lay inside an M.R.I. machine, watching commercials playing on the inside of his goggles as neuroscientists from the University of California, Los Angeles, measured the blood flow in his brain. Instead of asking the subject, John Graham, a Democratic voter, what he thought of the use of Sept. 11 images in a Bush campaign commercial, the researchers noted which parts of Mr. Graham's brain were active as he watched. The active parts, they also noted, were different from the parts that had lighted up in earlier tests with Republican brains.

The researchers do not claim to have figured out either party's brain yet, since they have not finished this experiment. But they have already noticed intriguing patterns in how Democrats and Republicans look at candidates. They have tested 11 subjects and say they need to test twice that many to confirm the trend.

"These new tools could help us someday be less reliant on clichés and unproven adages," said Tom Freedman, a strategist in the 1996 Clinton campaign, later a White House aide and now a sponsor of the research. "They'll help put a bit more science in political science."

In the experiment with Mr. Graham, researchers exposed him to photographs of the presidential candidates, commercials for President Bush and John Kerry, and other video images, including the "Daisy" commercial from 1964. In that advertisement, promoting Lyndon B. Johnson against Barry Goldwater, images of a girl picking petals from a daisy were replaced by images of a nuclear explosion."...

"It seemed so last century," Professor Freedman said. "Consultants were quoting Freud as if it was cutting edge. It was all about interpretation instead of using new technology to measure what's actually happening in the mind."

But then, after the Bush campaign commercial is shown, the subjects respond in a partisan fashion when the photographs are shown again. They still respond emotionally to the candidate of their party, but when they see the other party's candidate, there is more activity in the rational part of the brain, the dorsolateral prefrontal cortex. "It seems as if they're really identifying with their own candidate, whereas when they see the opponent, they're using their rational apparatus to argue against him," Professor Iacoboni said.

The neuroscientists warned against drawing conclusions until the experiment was over. They said the results would mainly point the way for future research, and other neuroscientists echoed their caution.