Dynamic and heterogeneous neural ensembles contribute to a memory engram

In the century since the notion of the 'engram' was first introduced to describe the physical manifestation of memory, new technologies for identifying cellular activity have enabled us to deepen our understanding of the possible physical substrate of memory. A number of studies have shown that memories are stored in a sparse population of neurons known as a neural ensemble or engram cells. While earlier investigations highlighted that the stability of neural ensembles underlies a memory representation, recent studies have found that neural ensembles are more dynamic and fluid than previously understood. Additionally, a number of studies have begun to dissect the cellular and molecular diversity of functionally distinct subpopulations of cells contained within an engram. We propose that ensemble fluidity and compositional heterogeneity support memory flexibility and functional diversity.