Here's what they did (Science Daily):
NYU's David Amodio, a professor of psychology and the study's lead author, and his colleagues recorded electrical activity from the brain using electroencephalograms (EEGs) in people who rated themselves as either conservative or liberal. During these recordings, subjects had to press a button when they saw a cue, which was presented often enough that the button-press became habitual.
However, subjects occasionally saw another, infrequent cue signaling them to withhold their habitual button press. When such response inhibition was required, liberals had significantly greater neural activity originating in the anterior cingulate cortex, a portion of the brain known to be involved in conflict monitoring. Liberals were also more likely to withhold their habitual response when they saw the infrequent cue.
Previous studies have found that conservatives tend to be more persistent in their judgments and decision-making, while liberals are more likely to be open to new experiences. These differences are related to a process known as conflict monitoring-a mechanism for detecting when a habitual response is not appropriate for a new situation.