Stop or move: Defensive strategies in humans

Abstract

Threatening cues and surrounding contexts trigger specific defensive response patterns. Potential threat evokes attentive immobility; attack evokes flight when escape is available and immobility when escape is blocked. Tonic immobility installs when threat is overwhelming and life-risky. In humans, reduced body sway characterizes attentive and tonic immobility, the former with bradycardia, and the later with expressive tachycardia. Here, we investigate human defensive strategies in the presence or absence of an escape route. We employed pictures depicting a man carrying a gun and worked with participants exposed to urban violence. In pictures simulating more possibility of escape, the gun was directed away from the observer; in those simulating higher risk and less chance of escape, the gun was directed toward the observer. Matched control pictures depicted similar layouts, but a non-lethal object substituted the gun. Posturographic and electrocardiographic recordings were collected. Amplitude of sway and heart rate were higher for gun directed-away and lower for gun direct-toward. Compared to their respective matched controls, there was a general increase in the amplitude of sway for the gun directed-away pictures; and a reduction in back-and-forth sway and in heart rate for gun directed-toward pictures. Taken together, those measures suggest that, when exposed to threat invading their margin of safety in a context indicating possible escape route, humans, as non-human species, engage in active escape, resembling the flight stage of the defensive cascade. When facing threat indicating less possibility of escape, humans present an immobile response with bradycardia.

Keywords: Body posture; Defensive responses; Escape; Freezing; Heart rate; Immobility.