Neural correlates of explicit and implicit memory at encoding and retrieval: A unified framework and meta-analysis of functional neuroimaging studies

The extent to which explicit memory (EM) and implicit memory (IM) involve similar or differential neural substrates remains unclear. To address this issue, this study provides a direct, meta-analytic comparison of functional neuroimaging studies involving EM and IM tasks. The meta-analysis comprised two separate meta analytic comparisons. First, to compare EM and IM in terms of encoding activity, subsequent memory effects (remembered > forgotten) and repetition suppression effects (first > repeated) were directly compared. Second, to compare EM and IM in terms of retrieval activity, retrieval success effects (hit > correct rejection) and repetition suppression effects were directly compared. Based on the notion that reduced activity during repeated processing is a 'by-product' or direct consequence of the stimulus processing performed in the same regions at initial exposure, regions showing repetition suppression were thought to play an important role in both IM-encoding and IM-retrieval activities. The results indicated that subsequent memory and repetition suppression effects had extensive overlaps and no significant separations, suggesting that EM- and IM-encoding activities involve largely common regions. Retrieval success and repetition suppression effects had strong segregations and only modest overlaps, suggesting that EM- and IM-retrieval activities involve largely separate regions. Consistent with these results, Explicit/Implicit Memory Encoding and Retrieval (EIMER), a neurocognitive model of EM and IM that suggests a common-encoding, separate-retrieval hypothesis for EM and IM is proposed herein.