Home Mind & Brain Human Freezing-Like Responses Linked to Faster Reactions to Threats

Human Freezing-Like Responses Linked to Faster Reactions to Threats

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A new study has shed light on the freezing-like motion patterns exhibited by humans in response to avoidable threats. The research, published in the journal Psychophysiology, provides valuable insights into the translational indicators of defensive behaviours, previously studied extensively in rodents, and their occurrence in humans.

Freezing is a well-documented defensive behaviour in rodents, characterised by a cessation of movement except for respiration. It has been primarily studied using video tracking and force transducers. In humans, similar behaviours have been observed, typically measured through reductions in body sway and gaze movements during anticipation of a threat. The study aimed to elucidate the relationship between body and eye movements and autonomic dynamics in response to avoidable threats

The study involved 50 participants who were exposed to naturalistic images preceded by coloured fixation crosses indicating whether they should expect an inevitable shock, no shock, or a potential shock that could be avoided by pressing a button quickly. Body sway, eye movements, heart rate, and skin conductance were recorded to measure the participants’ responses.

Participants stood on a stabilometric platform while viewing images and responding to different cues. The researchers hypothesised that reduced body and gaze movements, heart rate deceleration, and increased skin conductance would be observed during flight trials, where participants could avoid the shock.

The study confirmed several key observations. Participants exhibited reduced body sway and less dispersed, fewer, and longer fixations when anticipating an avoidable shock. These findings suggest that both body and gaze metrics reflect a defensive state akin to freezing behaviour in rodents. There was a noticeable deceleration in heart rate and an increase in skin conductance during flight trials, indicating a coordinated autonomic response to the perceived threat.

Stronger reductions in gaze dispersion were found to predict faster motor reactions on a trial-wise basis. This highlights the functional role of gaze control in action preparation during threat anticipation. While body sway and gaze movements did not show a trait-like relationship across participants, their temporal profiles were positively correlated within individuals, suggesting that both metrics may partly reflect the same underlying defensive construct.

The findings of this study contribute to a better understanding of human defensive behaviours and their physiological underpinnings. The positive correlation between gaze and body movement profiles supports the notion of an integrated defensive state in humans, similar to that observed in rodents. However, the researchers emphasise the need for further studies to assess these response patterns in more naturalistic settings to enhance the ecological validity of the findings.

The study also highlighted several challenges and limitations. The experimental setup, which involved standing in a fixed position and viewing two-dimensional images, may not fully capture the complexity of naturalistic threat scenarios. Future studies should aim to explore defensive behaviours in more dynamic and three-dimensional environments. There is a need for standardised methodologies to assess body sway and other motion metrics in human research. Variations in experimental designs and measurement techniques can impact the comparability of findings across studies. While the study observed bradycardia in response to threats, other studies have reported different autonomic patterns, such as tachycardia. This indicates that multiple defensive states may exist, each with distinct neuroanatomical profiles.

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