Abstract
According to the snake detection hypothesis (Isbell, 2006), fear specifically of snakes may have pushed evolutionary changes in the primate visual system allowing pre-attentional visual detection of fearful stimuli. A previous study demonstrated that snake pictures, when compared to spiders or bird pictures, draw more early attention as reflected by larger early posterior negativity (EPN). Here we report two studies that further tested the snake detection hypothesis. In Study 1, we tested whether the enlarged EPN is specific for snakes or also generalizes to other reptiles. Twenty-four healthy, non-phobic women watched the random rapid serial presentation of snake, crocodile, and turtle pictures. The EPN was scored as the mean activity at occipital electrodes (PO3, O1, Oz, PO4, O2) in the 225–300 ms time window after picture onset. The EPN was significantly larger for snake pictures than for pictures of the other reptiles. In Study 2, we tested whether disgust plays a role in the modulation of the EPN and whether preferential processing of snakes also can be found in men. 12 men and 12 women watched snake, spider, and slug pictures. Both men and women exhibited the largest EPN amplitudes to snake pictures, intermediate amplitudes to spider pictures and the smallest amplitudes to slug pictures. Disgust ratings were not associated with EPN amplitudes. The results replicate previous findings and suggest that ancestral priorities modulate the early capture of visual attention.
Highlights
As snakes were probably the first predators of primates, snakes will phylogenetically be more fear-relevant to humans than other reptiles
In our previous eventrelated potentials (ERPs) study (Van Strien et al, 2014), we found that the early posterior negativity (EPN) amplitude was largest for snake pictures, intermediate for spider pictures and smallest for bird pictures
GENERAL DISCUSSION In the present work, we examined by means of rapid serial visual presentation (RSVP) whether the preferential early attentional capture of snake stimuli as reflected in the EPN is limited to snakes or extends to other reptiles (Study 1), whether the EPN is modulated by disgust, and whether the modulation of the EPN by phylogenetically fear-relevant snake stimuli is found both in women and men (Study 2)
Summary
As snakes were probably the first predators of primates, snakes will phylogenetically be more fear-relevant to humans than other reptiles. An evolved and specialized visual monitoring system for the detection of animals posing deadly threat would be highly adaptive from an evolutionary perspective. Such a fear module is activated automatically by fear-relevant stimuli, and is largely independent of conscious cognition. Van Le et al (2013) measured neuronal responses in the medial and dorsolateral pulvinar of macaque monkeys These laboratory animals had no chance to encounter snakes before the experiment. The study demonstrated the existence of pulvinar neurons that respond selectively to visual images of snakes. These neurons responded faster and stronger to snakes stimuli than to (angry) monkey faces, monkey hands, or geometrical shapes. In support of an evolved fear module, an fMRI study with non-phobic participants demonstrated that the amygdala responds to threatening animals such as snakes and spiders but not to threatening objects (i.e., weapons) with comparable valence and arousal levels (Yang et al, 2012)
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