How Temporal Predictability of Threat and Action Preparation Affect Defensive Freezing Responses

In the face of threat, defensive circuits are activated to ensure survival. In this context, freezing-like behavior has been linked to action preparation. However, the disruption of action preparation by aversive unpredictable stimuli that are difficult to learn, discriminate, or predict remains poorly understood. Here we present a paradigm which is intended to capture the action preparation mechanism under conditions of (temporal) threat predictability, unpredictability and safety. In a multimethod approach, we combine human postural sway and cardiac responding as proxies for freezing-like behavior in humans. Additionally, we use skin conductance levels, electromyographic startle responses, behavioral task performance as well as subjective ratings as further outcome measures. Results from a large sample (n = 235) revealed, on a behavioral level, faster reaction times at the expense of accuracy in threat conditions as compared to no threat conditions. During action preparation under threat, parasympathetically-driven postural freezing-like behavior but also sympathetically-driven increased skin conductance levels as compared to safety were observed. Crucially, our results suggest that the (un)predictability of a threat does not modulate threat-induced freezing-like behavior or skin conductance levels during action preparation. Moreover, we observed startle inhibition during all conditions (threat, safety) as compared to startle responses in the inter-trial interval. In sum, action preparation under threat is associated with the increased co-activation of the parasympathetic (postural freezing) and sympathetic nervous system (skin conductance), converging with animal models on the defensive mechanism of freezing-like behavior. Our findings suggest that threat predictability does not modulate freezing-like behavior, as temporal unpredictability did not trigger distinct defensive responses from those triggered by predictable threats. This implies that preparatory mechanisms may serve as a generalized defensive response, prioritizing readiness over specificity in the face of potential threats in humans. These findings challenge existing models that emphasize differential responses based on predictability and contribute to a better understanding of how humans adapt to unpredictable threats, which is crucial for refining theories of defensive behavior and informing clinical approaches to anxiety and threat-related disorders.