Excitability-Independent Memory Allocation for Repeated Event.

How memory is organized in cell ensembles when an event is repeated is not well-understood. Recently, we found that retraining 24 h after the initial fear conditioning (FC) event induces turnover of neurons in the lateral amygdala (LA) that encodes fear memory. Excitability-dependent competition between eligible neurons has been suggested as a rule that governs memory allocation.

Turnover of fear engram cells by repeated experience.

A sparse population of neurons active during a learning event has been identified as memory engram cells. However, cells that are recruited to support memory when experience is repeated have been scarcely explored. Evidence from previous studies provides contradictory views.

Synaptic plasticity-dependent competition rule influences memory formation.

Memory is supported by a specific collection of neurons distributed in broad brain areas, an engram. Despite recent advances in identifying an engram, how the engram is created during memory formation remains elusive. To explore the relation between a specific pattern of input activity and memory allocation, here we target a sparse subset of neurons in the auditory cortex and thalamus.

Optogenetic activation of presynaptic inputs in lateral amygdala forms associative fear memory.

In Pavlovian fear conditioning, the lateral amygdala (LA) has been highlighted as a key brain site for association between sensory cues and aversive stimuli. However, learning-related changes are also found in upstream sensory regions such as thalamus and cortex. To isolate the essential neural circuit components for fear memory association, we tested whether direct activation of presynaptic sensory inputs in LA, without the participation of upstream activity, is sufficient to form fear memory in mice.