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

## TL;DR

This study explores how humans prepare for threats and finds that freezing behavior involves both parasympathetic and sympathetic systems, regardless of whether the threat is predictable or not.

## Contribution

The study introduces a modified paradigm to examine human action preparation under varying threat predictability using multimodal measures.

## Key findings

- Threat conditions activated both parasympathetic (postural freezing) and sympathetic systems (skin conductance).
- Temporal unpredictability of threat did not modulate freezing-like behavior or skin conductance responses.
- Startle responses were inhibited during all conditions, indicating a generalized defensive mode.

## Abstract

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.

This study introduces a modified paradigm to investigate action preparation under varying threat predictability conditions using a large human sample and multimodal psychophysiological measures. Contrary to prominent theories emphasizing differential responses to predictable versus unpredictable threats, we found no modulation of freezing‐like behavior or sympathetic system activation by threat predictability. Our findings suggest that in humans, action preparation may function as a generalized defensive mechanism, co‐activating parasympathetic and sympathetic systems irrespective of temporal threat predictability.