Uncertainty is often associated with subjective distress and a potentiated anxiety response. Occurrence uncertainty, or the inability to predict if a threat will occur, has rarely been compared experimentally with temporal uncertainty, or the inability to predict when a threat will occur. The current study aimed to (a) directly compare the anxiogenic effects of anticipating these two types of uncertain threat, as indexed by the eyeblink startle response, and (b) assess the relationship between startle response to occurrence and temporal uncertainty and individual differences in self-reported intolerance of uncertainty and anxiety. The findings indicated that anticipation during occurrence uncertainty elicited a larger startle response than anticipating a certain threat, but anticipation during temporal uncertainty was superior at potentiating startle blink overall. Additional analyses of the effects of order and habituation further highlighted temporal uncertainty's superiority in eliciting greater startle responding. This suggests that, while uncertainty is physiologically anxiety provoking, some level of certainty that the threat will occur enhances the robustness of the physiological anxiety response. However, self-reported anxiety was equivalent for temporal and occurrence uncertainty, suggesting that, while defensive responding may be more affected by temporal uncertainty, people perceive both types of uncertainty as anxiogenic. Individual differences in the intolerance of uncertainty and other anxiety measures were not related to anticipatory startle responsivity during any of the conditions.

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Anxiety is consistently associated with hyperactive neural responses to errors. The majority of existing research has focused on a single marker of error-elicited brain activity-the error-related negativity (ERN), an event-related brain potential (ERP) elicited 50-100ms following an erroneous response. The ERN has accumulated growing interest for its use in clinical contexts as a potential biomarker and/or endophenotype. However, it is unknown whether anxiety's effects are specific to brain activity following erroneous responses; anxiety may affect processes prior to error commission, suggesting that the ERN might reflect the output of abnormal processing that begins before an error. Here, we examined the error-preceding positivity (EPP) - an ERP time-locked to the correct response immediately before errors - that reflects a gradual disengagement of task-focused attention preceding errors. Results revealed that female worriers demonstrated significantly attenuated EPP amplitude, indicating reduced pre-error disengagement. Moreover, reduced EPP mediated the relationship between worry and the enhanced ERN following errors. These results suggest that the temporal dynamics of anxiety's impact on error processing are more nuanced than previously thought such that effects emerge prior to the actual occurrence of an erroneous response.