Immobility reactions under threat: A contribution to human defensive cascade and PTSD

- Stabilometry is a successful methodology to study the defensive cascade in humans.
- Human defensive reactions: attentive immobility, immobility under attack and tonic immobility parallel those proposed for other mammals.
- Symptom severity in PTSD is associated with peritraumatic tonic immobility, an involuntary reaction under extreme threat.
- Biological indicators of tonic immobility in humans are characterized by reduced body sway, tachycardia and low heart rate variability.

Violence exacts a burden on public health. Gun violence is a major trigger for motor defensive reactions in humans and post-traumatic stress disorder (PTSD) is its main psychiatric sequela. However, studies of the human defensive cascade, especially the motor reactions, are at an early stage. This review focuses on studies that employ stabilometry, a methodology that assesses whole body motor reactions, to address defensive behaviors to violence-related threats. Special attention is given to three reactions: “attentive immobility”, “immobility under attack” and “tonic immobility”, with emphasis on the latter - a peritraumatic reaction which has been strongly associated with the severity of PTSD. These reactions are characterized by reduced body sway and bradycardia, except tonic immobility that presents robust tachycardia. The advances made by investigations into the immobility reactions of the human defensive cascade contribute to helping to bridge the gap between human and non-human species. Furthermore, progresses in basic research to objectively monitor motor defensive reactions under threat can help to develop a dimensional, trans-diagnostic approach to PTSD.