The ghosts of brain states past: remembering reactivates the brain regions engaged during encoding

Abstract

There is growing evidence that the brain regions involved in encoding an episode are partially reactivated when that episode is later remembered. That is, the process of remembering an episode involves literally returning to the brain state that was present during that episode. This article reviews studies of episodic and associative memory that provide support for the assertion that encoding regions are reactivated during subsequent retrieval. In the first section, studies are reviewed in which neutral stimuli were associated with different modalities of sensory stimuli or different valences of emotional stimuli. When the neutral stimuli were later used as retrieval cues, relevant sensory and emotion processing regions were reactivated. In the second section, studies are reviewed in which participants used different strategies for encoding stimuli. When the stimuli were later retrieved, regions associated with the different encoding strategies were reactivated. Together, these studies demonstrate not only that the encoding experience determines which regions are activated during subsequent retrieval but also that the same regions are activated during encoding and retrieval. In the final section, relevant questions are posed and discussed regarding the reactivation of encoding regions during retrieval.

Figure 1

The prototypical design for exploring the reactivation of encoding regions during retrieval. STUDY. Neutral items are studied in one of multiple possible contexts. The different contexts could be different kinds of stimulus associations (e.g., pictures or sounds) or different encoding strategies (e.g., verbal vs. imagery). TEST. At test, brain activity is measured while retrieval cues are presented. Participants make some sort of memory decision about the cue such as old/new decisions, remember/know decisions, or context decisions. Strong support for reactivation requires: 1) The engagement of different brain regions during the retrieval of information studied in different encoding conditions, and 2) The engagement of the same brain regions during the different encoding conditions. Many of the reviewed studies provide only the first form of evidence but some, such as Wheeler et al. (2000, data shown, copyright © by the National Academy of Sciences), provide both forms.