The dynamic nature of fear engrams in the basolateral amygdala.

Great progress has been made in our understanding of how so-called memory engrams in the brain enable the storage and retrieval of memories. This has led to the realization that across the lifetime of an animal, the spatial and temporal properties of a memory engram are not fixed, but instead are subjected to dynamic modifications that can be both dependent and independent on additional experiences. The dynamic nature of engrams is especially relevant in the case of fear memories, whose contributions to an animal's evolutionary fitness depend on a delicate balance of stability and flexibility. Though fear memories have the potential to last a lifetime, their expression also needs to be properly tuned to prevent maladaptive behavior, such as seen in patients with post-traumatic stress disorder. To achieve this balance, fear engrams are subjected to complex spatiotemporal dynamics, making them informative examples of the "dynamic engram". In this review, we discuss the current understanding of the dynamic nature of fear engrams in the basolateral amygdala, a brain region that plays a central role in fear memory encoding and expression. We propose that this understanding can be further advanced by studying how fast dynamics, such as oscillatory circuit activity, support the storage and retrieval of fear engrams that can be stable over long time intervals.