Temporal characterization of memory retrieval processes: an fMRI study of the 'tip of the tongue' phenomenon

'Tip of the tongue' (TOT) is a natural phenomenon in which people cannot re trieve a target word immediately, even though they feel confident that they know the target. This provides us an opportunity to understand the human m emory system, because cognitive components of memory retrieval such as retr ieval effort and successful retrieval are temporally dissociated from each other during the TOT states. The purpose of the present study was to reveal the neural correlates of the cognitive components of the retrieval process by separating cognitive phases of the TOT phenomenon using event-related f unctional magnetic resonance imaging with multiple regression analysis. We demonstrated that the left dorsolateral prefrontal cortex (DLPFC) and anter ior cingulate cortex were activated at the time of successful retrieval, an d the left DLPFC also showed activation when the subjects successfully retr ieved the target names as compared to when they gave up. This result sugges ts that the left DLPFC is specific to the successful retrieval process. Dur ing the TOT state, a number of regions were activated, and this suggests th at widely distributed brain regions are engaged when people make a hard eff ort to retrieve a proper name in the TOT state. Our new approach employing temporal resolution of the TOT phenomenon may contribute to the understandi ng of the mechanisms of the human memory system.