Making room for new memories

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Science  30 Mar 2018:
Vol. 359, Issue 6383, pp. 1461-1462
DOI: 10.1126/science.aat1493

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What are our memories made of? Plato suggested imagining a block of wax in our soul, where perceptions and thoughts leave impressions that we can remember as long as they have not been erased. This historic metaphor captures the transience of some memories and the stability of others, and it illustrates the brain's plasticity. The mechanisms of memory formation and retention remain a key question in neuroscience. Groundbreaking work on the rodent hippocampus (a network in the temporal lobe) revealed that certain neurons form transiently stable representations of places (1). Hence, this brain region has become an important focus for studying spatial memory (or engram) formation. It also serves as an experimentally accessible proxy for declarative (knowledge) and episodic (experience) memory in humans, which involves the same brain structures and mechanisms. However, how can we remember an almost infinite number of items with the limited storage capacity of the hippocampus? There is good evidence that relevant representations are transferred to neocortical networks before forming long-lasting engrams. The hippocampus is then reset for acquisition of new memories. Studies in animals (2) and humans (3) show that neuronal activity during sleep plays a major role in these processes. The underlying mechanisms, however, have remained mostly enigmatic. On page 1524 of this issue, Norimoto et al. (4) show how sleep-associated activity patterns induce “negative” neuronal plasticity in the hippocampus, erasing remote memories. A previous, related paper by Khodagholy et al. (5) reveals similar activity patterns in the neocortex, which, hence, may mediate long-term consolidation of transient engrams at their final location.