Approaching threats elicit a freeze-like response in humans.

Freezing is one of the most widely recognized defensive reactions to approaching threats in animals. Here we tested whether the same stimuli can elicit freeze-like responses in healthy humans as well. We used a modified version of the two-frame apparent motion paradigm, in which both size and location of a stimulus within a background were manipulated; by these means, participants perceived the stimuli as approaching or receding. In Experiment 1, we showed that implicitly processed approaching threats (e.g., spiders or snakes) elicited a stronger freeze-like response (operationalized as slower reaction times) with respect to receding threats; freezing was significantly related to higher levels of participants' state anxiety. In Experiment 2, approaching/threatening animals were explicitly judged as more threatening than receding ones. Finally, in two further control experiments we observed that the same manipulation of stimuli's size and location, but in absence of apparent motion, did not affect freezing (Experiment 3) or explicit threat judgements (Experiment 4). The present findings demonstrated that approaching threats are critical to elicit freezing in humans, in line with animals' behaviour.