Engrams

An engram is a theoretical physical substrate of a memory in the brain. The term, introduced by the early 20th‑century scientist Richard Semon, refers to the lasting changes in neural tissue that encode and store information from experience, sometimes called a memory trace. In contemporary neuroscience, engrams are considered to be patterns of neural activity and synaptic changes that represent a specific memory rather than a single identifiable structure.

Engrams are thought to arise from activity-dependent plasticity that alters synapses and neural circuits. Memory formation often involves coordinated changes across brain regions, with the hippocampus supporting initial encoding and consolidation to cortical networks over time. Consolidation and reconsolidation allow memories to become more stable and sometimes re-entrant when recalled.

Modern research identifies engram cells as discrete ensembles of neurons that are active during learning and can be reactivated to evoke memory recall. Techniques such as genetic tagging and optogenetic manipulation in animals have shown that activating the learned ensemble can induce recall and silencing it can impair retrieval. These findings support a view of memory as a distributed network with identifiable, manipulable components rather than a single localized site.

Historically, efforts to locate one specific brain locus for all memories (as proposed by early localization experiments) faced mixed results, leading to a more nuanced understanding: memories are stored across interconnected circuits. The engram remains a working concept, reflecting the idea that memories have physical traces in brain networks whose precise composition varies by memory type and individual.