Memory engram stability and flexibility

Abstract

Many studies have shown that memories are encoded in sparse neural ensembles distributed across the brain. During the post-encoding period, often during sleep, many of the cells that were active during encoding are reactivated, supporting consolidation of this memory. During memory recall, many of the same cells that were active during encoding and reactivated during consolidation are reactivated during recall. These ensembles of cells have been referred to as the memory engram cells, stably representing a specific memory. However, recent studies question the rigidity of the "stable memory engram." Here we review the past literature of how episodic-like memories are encoded, consolidated, and recalled. We also highlight more recent studies (as well as some older literature) that suggest that these stable memories and their representations are much more dynamic and flexible than previously thought. We highlight some of these processes, including memory updating, reconsolidation, forgetting, schema learning, memory-linking, and representational drift.

Fig. 1. A canonical view of the stable memory engram.

Many studies have shown that memories are encoded in sparse neural ensembles distributed across the brain. After encoding, studies have shown that many of those same cells are reactivated during an offline period (e.g., sleep), stabilizing and consolidating the neural ensemble representing the memory. During recall, many of the same cells are reactivated, which have been thought to underlie the stable memory representation (i.e., the memory engram). The brain can store distinct memories through different ensembles of cells and different neural activity patterns.

Fig. 2. The memory ensemble is more dynamic than previously thought.

Recent studies have challenged the view of the stable and permanent memory engram. Several studies have demonstrated that the neuronal activity patterns representing a stable memory can change over time, even though behavior remains stable. The change in neuronal activity patterns increases as a function of time despite the behavioral memory performance being stable. This poses a big question in the field of learning and memory- how is a stable memory stored in changing neural ensembles? How stable and dynamic are memory engrams?