Neural correlates of immediate and delayed word recognition memory: an MEG study

We investigated the neural correlates of immediate and delayed word recognition memory using whole head magnetoencephalography (MEG). Event-related potential (ERP) and event-related field (ERF) were simultaneously recorded from 10 subjects while performing a continuous recognition memory task. Subjects were required to determine whether the word was "new" (never before presented) or "old" (presented previously). Old items were presented either immediately or delayed with five intervening words, and the words presented only once were referred to as new words. The grand average ERP waveform showed that immediate repetition was associated with early P300 and absence of N400, while delayed repetition and new word were associated with late P300 and presence of N400. Three ERF components were observed for immediate repetition, while four components were observed for delayed repetition and new word conditions. M1 and M2 were observed across all stimulus-presentation conditions. M3 with medial temporal generators was observed only in delayed repetition and new word conditions. M4 was observed earlier in the immediate repetition than in the delayed repetition and new-word conditions. For M4 generators, cingulate gyrus activations were observed for all stimulus-presentation conditions. In addition, activations in the medial temporal areas were observed for both repetition conditions. These results suggest that immediate and delayed recognition memory have similar neural networks except for the additional involvement of medial temporal areas in delayed recognition memory. The functional roles of M3 and M4 peaks were discussed in terms of functional significance of N400 and early/late P300 ERP peaks, which correspond to M3 and M4, respectively.