Extended amygdala function is governed by threat probability, not uncertainty
Anxiety is a universal experience that arises as we anticipate negative events that may or may not happen. When extreme, anxiety is a major cause of distress, suffering, and disability-adjusted life years, with nearly one in four individuals experiencing clinically relevant anxiety in their lifetime1-3. Elucidating the biological basis of anxiety has the potential to help people understand, cope with, and ultimately receive improved treatments for their anxiety. The canonical neuroscientific model of anxiety posits that distinct regions of the extended amygdala (EA) elicit distress that is conditional on threat uncertainty4. Here, we refute this model using a precise computational framework to disambiguate specific dimensional facets of threat uncertainty in a racially diverse sample of 104 individuals. We show that self-report, psychophysiological, and neural correlates of threat anticipation are much more strongly associated with threat probability than uncertainty per se. Importantly, EA regions, including the central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST), were most reliably associated with threat probability--contrary to the canonical model. Re-examination of the pioneering experiments that motivated the canonical model reveal a previously unappreciated confound between probability and uncertainty, suggesting the novel hypothesis that EA regions differ in their sensitivity to low-probability threats. Our fMRI data support this, showing that BST is more sensitive to low-probability threats than Ce. Moreover, lesion experiments in rhesus monkeys (Macaca mulatta) demonstrate that BST causally contributes to defensive behavior in low-probability threat contexts. Together, these data reconcile previous empirical inconsistencies, provide a fresh theoretical perspective on the neurobiology of anxiety, and underscore the theoretical importance of a formal lingua franca for coordinated cross-species emotion research.