INFREQUENT EVENTS TRANSIENTLY ACTIVATE HUMAN PREFRONTAL AND PARIETAL CORTEX AS MEASURED BY FUNCTIONAL MRI

P300 is an event-related potential elicited by infrequent target event s whose amplitude is dependent on the context provided by the immediat ely preceding sequence of stimuli, suggesting its dependence on workin g memory. We employed magnetic resonance imaging sequences sensitive t o blood oxygenation level to identify regional changes evoked by infre quent visual target stimuli presented in a task typically used to elic it P300. Targets evoked transient event-related activation bilaterally in the middle frontal gyrus, in the inferior parietal lobe, and near the inferior aspect of the posterior cingulate gyrus beginning within 1.5 s of target onset and peaking between 1.5 and 6 s. These regions h ave been identified in previous neuroimaging studies in humans, and in single-unit recordings in monkeys. as components of a neural system m ediating working memory, which suggests that this system may be activa ted by the same events that evoke P300.