Constructing threat probability, fear behaviour, and aversive prediction error in the brainstem.

When faced with potential threat we must estimate its probability, respond advantageously, and leverage experience to update future estimates. Threat estimates are the proposed domain of the forebrain, while behaviour is elicited by the brainstem. Yet, the brainstem is also a source of prediction error, a learning signal to acquire and update threat estimates. Neuropixels probes allowed us to record single-unit activity across a 21-region brainstem axis during probabilistic fear discrimination. Against a backdrop of widespread threat probability and behaviour signaling, a dorsally-based brainstem network rapidly signaled threat probability. Remapping of neuronal function following shock outcome gave rise to brainstem networks signaling prediction error on multiple timescales. The results reveal construction of threat probability, behaviour, and prediction error along a single brainstem axis.