Dissociating explicit and implicit category knowledge with fMRI.

Neuroimaging of healthy volunteers identified separate neural systems supporting the expression of category knowledge depending on whether the learning mode was intentional or incidental. The same visual category was learned either intentionally or implicitly by two separate groupsof participants. During a categorization test, functional magnetic resonance imaging (fMRI) was used to compare brain activity evoked by category members and nonmembers. After implicit learning, when participants had learned the category incidentally, decreased occipital activity was observed for novel categorical stimuli compared with noncategorical stimuli. In contrast, after intentional learning, novel categorical stimuli evoked increased activity in the hippocampus, right prefrontal cortex, left inferior temporal cortex, precuneus, and posterior cingulate. Even though the categorization test was identical in the two conditions, the differences in brain activity indicate differing representations of category knowledge depending on whether the category had been learned intentionally or implicitly.