We then compared the questionnaire responses with the brain data, and here’s what we found:

1. Voters sense both peril and promise in party brands. When we showed subjects the words “Democrat,” “Republican” and “independent,” they exhibited high levels of activity in the part of the brain called the amygdala, indicating anxiety. The two areas in the brain associated with anxiety and disgust — the amygdala and the insula — were especially active when men viewed “Republican.” But all three labels also elicited some activity in the brain area associated with reward, the ventral striatum, as well as other regions related to desire and feeling connected. There was only one exception: men showed little response, positive or negative, when viewing “independent.”

2. Emotions about Hillary Clinton are mixed. Voters who rated Mrs. Clinton unfavorably on their questionnaire appeared not entirely comfortable with their assessment. When viewing images of her, these voters exhibited significant activity in the anterior cingulate cortex, an emotional center of the brain that is aroused when a person feels compelled to act in two different ways but must choose one. It looked as if they were battling unacknowledged impulses to like Mrs. Clinton.

Subjects who rated her more favorably, in contrast, showed very little activity in this brain area when they viewed pictures of her.

This phenomenon, not found for any other candidate, suggests that Mrs. Clinton may be able to gather support from some swing voters who oppose her if she manages to soften their negative responses to her. But she may be vulnerable to attacks that seek to reinforce those negative associations.

3. Hillary Clinton and Rudy Giuliani are on opposite sides of the gender divide. We found indications that Mrs. Clinton and Mr. Giuliani represent two sides of the same coin: Men show little interest in Mrs. Clinton initially but after watching her video they react positively. Women respond to her strongly at first, but their interest wanes after they watch her video.