A, Schematic of the observational fear paradigm. Demonstrator mice received tone-shock pairings, while observer mice witnessed these events through a transparent wall from the adjacent compartment. B, Schematic of in vivo neural recordings combined with optogenetic strategies to monitor the activity of either ACC-to-BLA projectors or BLA neurons during the task. C, ACC-to-BLA projectors acquired prominent responses to the shock-predicting cue. D, BLA neurons also acquired prominent responses to the cue, but such responses were abolished by photoinhibition of the ACC input.