Building Blocks of Memory in the Brain

Modern memory research identifies “engrams”—sparse subsets of neurons whose activity encodes particular experiences—using clever tag-and-manipulate experiments in rodents, especially in fear-conditioning paradigms.

Only a small fraction of neurons in regions like amygdala and dentate gyrus are recruited into a given engram, with their selection governed by intrinsic excitability and local inhibitory competition.

Overlapping engram populations mediate links between memories when experiences occur close together in time, creating shared neural substrates that couple their recall and extinction.

Memories that were originally encoded with non-overlapping engrams can later become linked through repeated co-retrieval, creating new “link engrams” that encode associations between them.