In this work, Refaeli et al. investigate recent and remote hippocampal representations (engrams) of a given memory in the mouse brain, and find that the engrams are stable. They show that memory is impaired when recent recall engram cells are inhibited during remote recall. Additionally, they show that new cells are added to the hippocampal remote engram during maturation of the e ngram, and that this process is based on the cells’ anterograde and retrograde connections.

Figure 1:
Dual labeling of active cells of engram neurons in the mouse hippocampus. Active cells for recent memories are labeled red, while active cells for remote memories are labeled white.

Figure 2:
A mouse brain hemisphere with the engram cells (red) in the hippocampus, projecting to their targets.