This study demonstrates that optogenetic stimulation of a sparse population of hippocampal neurons activated during fear conditioning is sufficient to induce freezing behavior, a signature of fear memory recall. The researchers labeled a specific subset of dentate gyrus (DG) neurons with channelrhodopsin-2 (ChR2) using an AAV9-TRE-ChR2-EYFP construct in c-fos-(TA transgenic mice. After fear conditioning, optogenetic stimulation of these labeled neurons in a different context led to increased freezing, indicating light-induced fear memory recall. This effect was context-specific, as stimulation of neurons labeled in a different context did not evoke freezing. The findings suggest that activating a specific ensemble of hippocampal neurons can recapitulate the behavioral expression of a memory, providing a method for mapping cellular populations involved in memory engrams.This study demonstrates that optogenetic stimulation of a sparse population of hippocampal neurons activated during fear conditioning is sufficient to induce freezing behavior, a signature of fear memory recall. The researchers labeled a specific subset of dentate gyrus (DG) neurons with channelrhodopsin-2 (ChR2) using an AAV9-TRE-ChR2-EYFP construct in c-fos-(TA transgenic mice. After fear conditioning, optogenetic stimulation of these labeled neurons in a different context led to increased freezing, indicating light-induced fear memory recall. This effect was context-specific, as stimulation of neurons labeled in a different context did not evoke freezing. The findings suggest that activating a specific ensemble of hippocampal neurons can recapitulate the behavioral expression of a memory, providing a method for mapping cellular populations involved in memory engrams.