Mind the snake in the shadows: Preattentive processing and autonomic responses to subliminal evolutionary threats

In the course of evolution, human survival has largely depended on the ability to rapidly detect and respond to potential threats. Accordingly, the human visual system has evolved mechanisms that enable us to detect danger before conscious awareness. Previous studies have reported enhanced detection abilities and increased electrodermal activity in response to subliminal presentations of snake and spider images, with stronger effects in individuals with specific phobias. However, this advantage in the unconscious processing of evolutionarily threatening animals has not been consistently supported in the literature. Across two experiments, we subliminally presented spider and snake images, in both intact and scrambled versions, using the breaking Continuous Flash Suppression (bCFS) technique. In both experiments, we recorded the time required for the stimuli to break through suppression, as well as several physiological indices, including skin conductance, pupil dilation, and facial EMG. We also assessed participants’ levels of specific phobias using the Spider Fear Questionnaire (SPQ-12) and the Snake Anxiety Questionnaire (SNAQ-12) to investigate potential interindividual differences in physiological and behavioural responses. Additionally, in the second experiment, participants were asked to report how clearly they saw the stimulus and to categorize it. In both experiments, we observed shorter breakthrough times for snakes than for spiders, and faster responses to intact images than to their scrambled counterparts. Moreover, both animals were associated with higher electrodermal responses even before reaching visual awareness. In the first experiment, pupillometry analyses further revealed a clear preconscious advantage of threatening animals. When taking inter-individual differences in phobia level into account, the results suggest an avoidant response in arachnophobics compared to non-phobic participants. Together, our findings support the existence of a subcortical pathway that is preattentively activated to prepare the organism to react to potential threats. Moreover, this network appears to respond differently depending on individuals’ levels of phobia.