Curvilinear shapes and the snake detection hypothesis: An ERP study

Consistent with the snake detection hypothesis, previous ERP studies have established a larger early posterior negativity (EPN) in response to pictures depicting snakes than to pictures depicting other creatures. Here, we examined to what extent the curvilinear shape of the snake's body drives the larger EPN. To this end, we employed pictures of threatening and nonthreatening species with or without typical curvature. Participants watched a random rapid serial visual presentation of snake, worm, spider, and beetle pictures. The EPN was scored as the mean activity (225-300 ms after picture onset) at occipital and parieto-occipital electrodes. Across electrodes, the EPN was significantly larger for snake pictures than for spider, worm, and beetle pictures, and for spider and worm pictures than for beetle pictures. The results suggest that curvilinear body shapes may partly drive the enhanced EPN. However, the unique cortical response to snakes is not fully explained by this mechanism, and is most probably also determined by other threat-relevant cues.