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

Defensive responses to threat are critical for survival and often involve freezing-like behavior, which has been linked to action preparation. However, it remains unclear how this preparatory mechanism is influenced by unpredictable aversive stimuli that are difficult to discriminate or anticipate. In this study, we introduce a modified paradigm to examine how freezing-like behavior is affected by action preparation under temporal threat predictability, unpredictability, and safety. Using a multimodal approach in a large sample (n = 235, 148 female), we assessed postural sway and heart rate as markers of freezing-like behavior, alongside skin conductance levels, electromyographic startle responses, behavioral performance, and subjective ratings. Behaviorally, participants responded faster but less accurately under threat compared to safety, replicating previous findings. Physiologically, threat conditions were associated with both parasympathetically mediated postural freezing and sympathetically driven increases in skin conductance. Importantly, the temporal (un)predictability of threat did not modulate freezing-like behavior or skin conductance responses. Startle responses were generally inhibited during all conditions (threat and safety) relative to the inter-trial interval. These findings indicate that action preparation under threat involves concurrent activation of both parasympathetic (postural freezing) and sympathetic systems (skin conductance), consistent with animal models of freezing. The absence of modulation by temporal threat unpredictability may be indicative of a generalized defensive mode that prioritizes readiness over specificity.