June 09, 2005
Humans Like Deer Who Freeze In Headlights

Response to threats is yet another way in which humans are like other animals.

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 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.

This suggests the threat of predators - whether human or animal - was a significant selective force even in later human evolution. Else the behavior likely would have been lost by now.

Share |      Randall Parker, 2005 June 09 09:43 PM  Brain Evolution

odograph said at June 10, 2005 7:11 AM:

I went to a gun show once, as a guest, and as we ambled past the tables in the large hall ... a shot went off! I can remeber very clearly exactly how still I stood as my eyes scanned for what was going on. As I thought about why, the idea that came back was that anyone shooting would be shooting at moving things, or that I was being invisible, .... FWIW. (The shot was from a used gun, not unloaded, and went up through the roof. No injuries.)

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