Sheena A. Josselyn, Susumu Tonegawa. Memory engrams: Recalling the past and imagining the future. Science, 03 Jan 2020:Vol. 367, Issue 6473, eaaw4325. DOI: 10.1126/science.aaw4325

“The ability to form memory is an essential trait that allows learning and the accumulation of knowledge. But what is a memory? There has been a long history of searching for the neuronal substrate that forms memory in the brain, and the emerging view is that ensembles of engram cells explain how memories are formed and retrieved. In a Review, Josselyn and Tonegawa discuss the evidence for engram cells as a substrate of memory, particularly in rodents; what we have learned so far about the features of memory, including memory formation, retrieval over time, and loss; and future directions to understand how memory becomes knowledge.”

Abstract
“In 1904, Richard Semon introduced the term “engram” to describe the neural substrate for storing memories. An experience, Semon proposed, activates a subset of cells that undergo off-line, persistent chemical and/or physical changes to become an engram. Subsequent reactivation of this engram induces memory retrieval. Although Semon’s contributions were largely ignored in his lifetime, new technologies that allow researchers to image and manipulate the brain at the level of individual neurons has reinvigorated engram research. We review recent progress in studying engrams, including an evaluation of evidence for the existence of engrams, the importance of intrinsic excitability and synaptic plasticity in engrams, and the lifetime of an engram. Together, these findings are beginning to define an engram as the basic unit of memory.”

Sheena A. Josselyn, Susumu Tonegawa. Memory engrams: Recalling the past and imagining the future. Science, 03 Jan 2020:Vol. 367, Issue 6473, eaaw4325. DOI: 10.1126/science.aaw4325