Inferential reasoning by exclusion recruits parietal and prefrontal cortices.

When one is asked to select a label for a novel object from a given group of labels that includes both novel and familiar labels, one tends to choose a novel label. In other words, people spontaneously assume that an unfamiliar label goes with an unfamiliar object, even though logically, familiar labels may also be valid choices. This may seem natural and even trivial, but the fact that nonhuman animals robustly fail to demonstrate this same tendency suggests that it is not. This tendency of choice, called "exclusion," can bias human behavior, and seems relevant to aspects of human language (e.g., word learning), although substantially more research is required to validate. In this functional magnetic resonance imaging study, we investigated the neural correlates of this bias using a matching-to-sample procedure. The subjects were first trained on two sample-to-comparison associations between abstract visual stimuli. Then, in a test of exclusion, they were shown a novel sample and were asked to choose between a trained but incorrect comparison and a novel comparison. The subjects readily chose the novel comparison and rejected the trained one, thus demonstrating exclusion. Significant activation was observed in the prefrontal cortex (PFC) and inferior parietal lobule (IPL) during exclusion. Medial frontal activation was also observed, which was related to the appearance of the novel stimuli. These results suggest that the medial frontal cortex is associated with novelty detection and that the PFC and IPL are involved in rejecting the defined comparison in favor of the novel one.