Competition between engrams influences fear memory formation and recall

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Science  22 Jul 2016:
Vol. 353, Issue 6297, pp. 383-387
DOI: 10.1126/science.aaf0594

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How to link and separate memories

Engrams are the changes in brain tissue that store single memories. Neuroscientists can localize and manipulate them, but until now, little was known about how multiple engrams interact to influence memories. Rashid et al. examined how neural assemblies in an area called the lateral amygdala interact. If two frightening events occurred within 6 hours, the same set of neurons was used to express fear memories for both events. However, if the events were separated by 24 hours, distinct memory traces were formed.

Science, this issue p. 383


Collections of cells called engrams are thought to represent memories. Although there has been progress in identifying and manipulating single engrams, little is known about how multiple engrams interact to influence memory. In lateral amygdala (LA), neurons with increased excitability during training outcompete their neighbors for allocation to an engram. We examined whether competition based on neuronal excitability also governs the interaction between engrams. Mice received two distinct fear conditioning events separated by different intervals. LA neuron excitability was optogenetically manipulated and revealed a transient competitive process that integrates memories for events occurring closely in time (coallocating overlapping populations of neurons to both engrams) and separates memories for events occurring at distal times (disallocating nonoverlapping populations to each engram).

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