Abstract
More than a century after Richard Semon's theoretical proposal of the memory engram, technological advancements have finally enabled experimental access to engram cells and their functional contents. In this review, we summarize theories and their experimental support regarding hippocampal memory engram formation and function. Specifically, we discuss recent advances in the engram field which help to reconcile two main theories for how the hippocampus supports memory formation: The Memory Indexing and Cognitive Map theories. We also highlight the latest evidence for engram allocation mechanisms through which memories can be linked or separately encoded. Finally, we identify unanswered questions for future investigations, through which a more comprehensive understanding of memory formation and retrieval may be achieved.
Highlights
In order for learning to occur, an experience must result in enduring changes in anatomical connections and physiological processes within the brain
We have reviewed an intuitive function of the engram in the hippocampal CA1, where it is proposed to index experiences stored in upstream networks as memories
Because the content of the engram is heavily influenced by the perception of Euclidean space, how the encoding of space contributes to formation of episodic memory remains an enigma
Summary
In order for learning to occur, an experience must result in enduring changes in anatomical connections and physiological processes within the brain. The sparse ensemble of neurons across multiple brain regions manifesting these learning-induced changes is called an engram This concept that learning induces persistent alterations in a subset of neurons was first proposed by the German scholar and theorist Richard Semon, who defined the engram as “...the enduring though primarily latent modifications in the irritable substance produced by a stimulus...” (Semon and Simon, 1921). Based on his “Law of Engraphy,” Semon postulated that “All simultaneous excitations...within our organisms form a connected simultaneous complex of excitations which, as such, acts engraphically, that is to say leaves behind it a connected and, to that extent, unified engram-complex” (Semon et al, 1923). We begin by contrasting two theories of how the hippocampus supports episodic memory, and how new discoveries in the engram field reconcile the theories
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