Is there tonic immobility in humans? Biological evidence from victims of traumatic stress

Tonic immobility, characterized by profound motor inhibition, is elicited under inescapable threat in many species. To fully support the existence of tonic immobility in humans, our aim was to elicit this reaction in a laboratory setting and measure it objectively. To mimic exposure to life-threatening events in the lab, trauma-exposed participants with PTSD (n = 18) and without PTSD (n = 15) listened to the script of their autobiographical trauma. Posturography and electrocardiography were employed. Reports of script-induced immobility were associated with restricted area of body sway and were correlated with accelerated heart rate and diminished heart rate variability, implying that tonic immobility is preserved in humans as an involuntary defensive strategy. Immobility reports seemed more evident in PTSD, suggesting that, in some patients, tonic immobility may be elicited during re-experiencing episodes in daily life. This study provided a measure of tonic immobility, a peritraumatic reaction for which cumulative clinical evidence had linked to the severity of PTSD.

► Tonic immobility, the last-ditch of defense cascade, occurs in traumatized humans.
► Tonic immobility-like reaction is induced through script-driven symptom provocation.
► Reports of immobility predict smaller amplitudes of body sway.
► Reports of immobility correlate with tachycardia and low heart rate variability.
► Tonic immobility may be elicited during PTSD re-experiencing symptoms.