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upstart writes in with an IRC submission for chromas:

Engrams emerging as the basic unit of memory:

Though scientist Richard Semon introduced the concept of the "engram" 115 years ago to posit a neural basis for memory, direct evidence for engrams has only begun to accumulate recently as sophisticated technologies and methods have become available. In a new review in Science, Professors Susumu Tonegawa of The Picower Institute for Learning and Memory at MIT and Sheena Josselyn of the Hospital for Sick Children (SickKids) and the University of Toronto describe the rapid progress they and colleagues have been making over the last dozen years in identifying, characterizing and even manipulating engrams, as well as the major outstanding questions of the field.

Experiments in rodents have revealed that engrams exist as multiscale networks of neurons. An experience becomes stored as a potentially retrievable memory in the brain when excited neurons in a brain region such as the hippocampus or amygdala become recruited into a local ensemble. These ensembles combine with others in other regions, such as the cortex, into an "engram complex." Crucial to this process of linking engram cells is the ability of neurons to forge new circuit connections, via processes known as "synaptic plasticity" and "dendritic spine formation." Importantly, experiments show that the memory initially stored across an engram complex can be retrieved by its reactivation but may also persist "silently" even when memories cannot be naturally recalled, for instance in mouse models used to study memory disorders such as early stage Alzheimer's disease.

Original Submission