Neural correlates of memory retrieval and evaluation.

Results from recent neuroimaging studies have led to a controversy as to whether right or left prefrontal regions are relatively more important for episodic retrieval. To address this issue, we recorded event-related brain potentials during two recognition tests with identical stimuli but differing retrieval demands. In both tests, participants viewed a sequence of object drawings, half of which were identical to ones viewed earlier except for a change in size and half of which were new. Instructions were to discriminate between old and new objects (general test) or to additionally decide whether old objects were larger or smaller at study (specific test). Frontal brain potentials that were more positive during the specific than during the general test for both old and new objects were interpreted as neural correlates of the process by which specific attributes of test cues are compared with information retrieved from memory. Another ERP difference between the specific and general tests, which was observed for old objects only, had a left posterior scalp topography and was interpreted to reflect the reactivation of memories for studied objects. Frontal and posterior potentials thus reflected two memory processes important for accurate episodic retrieval. Furthermore, our findings suggest that both left and right prefrontal regions were engaged when demands to retrieve and evaluate perceptual information increased.