When humans play dead

When a rabbit or other animal is trapped by a predator, it will freeze and assess the situation. It might then flee or attack, what we usually call the "fight or flight response". If that fails, a last-ditch defence mechanism is to go completely immobile, to play dead.

Researchers in Brazil now say that in times of grave danger, this same automatic last resort is also exhibited by humans and is experienced as a terrifying feeling of being "locked-in". The team led by Eliane Volchan performed what they describe as the first lab-study of "tonic immobility" in humans, and they argue that greater awareness of the response could help our understanding of people's reactions in real-life situations. For example, rape victims often experience shame after not resisting physically, and in some jurisdictions their passive response is interpreted as a sign of consent. Similarly, police officers and related professionals may be condemned for not reacting proactively in danger situations.

Volchan and her colleagues recruited 33 trauma survivors (15 women), including 18 with a dignosis of Post-Traumatic Stress Disorder (PTSD). They were asked to describe their ordeals in minute detail and these accounts were transformed into a 60-second audio narrative presented by a male voice in the second-person, present tense (e.g. "You are walking home and a man appears ..."). Each participant's account was played back to them over head-phones while they stood on a platform that records body sway. Their heart rate was also monitored and afterwards they were asked questions about how they felt as they listened to the recording.

The results provided physiological evidence of "tonic immobility" in humans. Participants who reported a strong sense of being paralysed, frozen, unable to move or scream, tended to show less body sway, higher heart rate and less heart rate variability. This was true across both PTSD and non-PTSD patients, but it was the PTSD patients who were more likely to report feelings of paralysis whilst listening to the recording of their ordeal.

"We succeeded in experimentally inducing tonic immobility in humans and recording its biological correlates, indicating that tonic immobility is preserved in humans as an involuntary defensive strategy to life-threatening events," the researchers said.

"Tonic immobility still remains largely unrecognized in humans," they added. "Thus, essential steps to alleviate entrapment symptoms, guilt and prejudice in the aftermath of tonic immobility are the recognition of tonic immobility and dissemination of this knowledge to the public."
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Volchan, E., Souza, G., Franklin, C., Norte, C., Rocha-Rego, V., Oliveira, J., David, I., Mendlowicz, M., Coutinho, E., Fiszman, A., Berger, W., Marques-Portella, C., and Figueira, I. (2011). Is there tonic immobility in humans? Biological evidence from victims of traumatic stress. Biological Psychology, 88 (1), 13-19 DOI: 10.1016/j.biopsycho.2011.06.002

Post written by Christian Jarrett for the BPS Research Digest.