Active suppression in the mediotemporal lobe during directed forgetting.

The aim of the present study was to investigate whether forgetting is merely a passive process or whether it can also be caused by active suppression of memory contents. We investigated effects of directed forgetting by intracranial event-related potentials (ERPs) in 12 patients with mesial temporal lobe epilepsy. In a single-item directed forgetting paradigm, the patients were presented with words either followed by the instruction that this word has to-be-remembered (TBR) or to-be-forgotten (TBF). All patients were implanted with multicontact depth electrodes along the rhinal cortex and hippocampus as part of their presurgical evaluation. Patients recognized significantly less TBF than TBR words in a subsequent recognition test. In the hippocampus, TBF cues that caused subsequent forgetting were associated with decreased negative ERPs. In the rhinal cortex, TBF cues elicited a generally prolonged positivity, as compared to TBR cues. We interpret the decreased hippocampal ERPs following the TBF cues as an indication for an active suppression of hippocampal functions. The increased rhinal activity in response to the TBF cue might indicate an active involvement of this structure in the suppression of hippocampal memory formation.