An electrophysiological test of directed forgetting: The role of retrievalinhibition

A central issue in the research of directed forgetting is whether the diffe rential memory performance for to-be-remembered (TBR) and to-be-forgotten ( TBF) it ems is solely due to differential encoding or whether retrieval inh ibition of TBF items plays an additional role. In this study, recognition-r elated event-related brain potentials (ERPs) were used to examine this issu e. The spatio-temporal distributions of the old/new ERP effects obtained in Experiment 1 that employed a directed forgetting paradigm were compared wi th those recorded in Experiment 2 in which the level of processing was mani pulated. In Experiment 1, participants were instructed to remember or to fo rget words by means of a cue presented after each word. ERPs recorded in th e recognition test revealed early phasic frontal and parietal old/new effec ts for TBR items, whereas TBF items elicited only a frontal old/new effect. Moreover, a late right-frontal positive slow wave was more pronounced for TBF items, suggesting that those items were associated with a larger amount of post-retrieval processing. In Experiment 2, the same cueing method and the same stimulus materials were used, and memory encoding was manipulated by cueing participants to process the words either deeply or shallowly. Bot h deeply and shallowly encoded items elicited phasic frontal and parietal o ld/new effects followed by a late right-frontal positive slow wave. However , in contrast to TBR and TBF items, these effects differed only quantitativ ely. The results suggest that differential encoding alone cannot account fo r the effects of directed forgetting. They are more consistent with the vie w that items followed by an instruction to forget become inhibited and less accessible, and, therefore, more difficult to retrieve.