When less means more: deactivations during encoding that predict subsequent memory

In event-related functional MRI (fMRI) studies, greater activity for items that are subsequently remembered (R-items) than for items that are subsequently forgotten (F-items), or Dm effect (Difference in memory), has been attributed to successful encoding operations. In contrast, regions showing a reverse DM effect (revDM = F-items > R-items) have been linked to detrimental processes leading to forgetting. Yet, revDMs may reflect not only activations for F-items (aFs) but also deactivations for R-items (dRs), and the latter alternative is more likely to reflect beneficial rather than detrimental encoding processes. To investigate this issue, we used a paradigm that included a fixation baseline and could distinguish between the two types of revDMs (aF vs. dR). Participants were scanned while encoding semantic associations between words or perceptual associations between words and fonts, and their memory was measured with associative recognition tests. For both semantic and perceptual encoding, dR effects were found in dorsolateral prefrontal, temporoparietal, and posterior midline regions. In contrast with a prior study that attributed revDMs in these regions to detrimental processes, the present results suggest that these effects reflect beneficial processes, that is, the efficient reallocation of neurocognitive resources. At the same time, aF effects were found in other regions, such as the insula, and these are more consistent with an interpretation in terms of detrimental processes. Whereas most fMRI studies of encoding have focused on activation increases, the present study indicates that activation decreases are also critical for successful learning of new information.