Memories are not made in a vacuum and typically carry an emotional value or valence, a quantity that need not necessarily be fixed. However, the neural mechanisms underlying memory-valence associations or valence switching are not known. Here, Susumu Tonegawa and colleagues labelled specific fear- (negative valence) or reward-based (positive valence) memory traces or engrams with optogenetic tools, allowing for later artificial memory reactivation. Memory engram ensembles could be re-associated with the opposite valence following a second round of association training combined with engram activation. These changes were apparent within the engram cells located within the dentate gyrus. Thus, dentate gyrus memory engram cells exhibit a plasticity in valence associations, and these data suggest that emotional memory associations can be changed at the circuit level.
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