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Brazilian Study Shows That Humans Kept Animal Freezing Instinct

Standing still when a threat is detected is a defensive, protective reaction. This ancestral and automatic behavior allows the prey to stay unnoticed by a potential predator.

A new study done by a team of  Brazilian scientists and published in Psychophysiology finds that humans, like many other complex animals, freeze when encountering a threat. The mere picture of an injured or mutilated human induces this reaction.


When viewing these unpleasant images, the study’s participants froze as their heart rate decelerated and amount of their body sway reduced. The authors found that this abrupt reaction, so critical for the survival of some animals, has stayed with humans. 


Forty-eight male volunteers stood barefoot on a stabilometric platform, to measure balance and body sway, and viewed twenty-four pictures from three different categories.


They were: pleasant (sports), neutral (objects), and unpleasant (injured or mutilated humans). Posturographic and electrocardiographic recordings were collected.


The author found a significant reduction in body sway along with increased muscle stiffness following the unpleasant/mutilation block of pictures compared to the neutral pictures.


The number of heartbeats per minute was also lower after viewing the mutilation pictures than after looking at the others.


“This pattern resembles the ‘freezing’ and ‘fear bradycardia’ seen in many species when confronted with threatening stimuli, mediated by neural circuits that promote defensive survival,” author Eliane Volchan explains.


The study is authored by Tatiana M. Azevedo, Eliane Volchan, Erika C. Rodrigues, Luiz G. Lutterbach, and Claudia D. Vargas from the Institute of Biophysics Carlos Chagas Filho, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.


And Luiz A. Imbiriba, José M. Oliveira, and Liliam F. Oliveira from the School of Physical Education, Federal University of Rio de Janeiro, also from Rio de Janeiro.


Eliane Volchan is an associate professor and work with a team of students and collaborators in the Institute of Biophysics Carlos Chagas Filho at the Federal University of Rio de Janeiro, Brazil.


In 2005, Dr. Volchan was awarded the Comendador da Ordem Nacional do Mérito Cientí­fico, by the Brazilian Presidenet Luiz Inácio Lula da Silva.


This study is published in the current issue of Psychophysiology. Psychophysiology reports on new theoretical, empirical and methodological advances in: psychology and psychiatry, cognitive science, cognitive and affective neuroscience, social science, health science and behavioral medicine, and biomedical engineering.


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