Blurring attenuates the early posterior negativity in response to snake stimuli

Previous event-related potential studies have reported enhanced Early Posterior Negativity (EPN) in response to snake pictures compared to pictures of other animals. This EPN snake effect may be partly driven by specific snake skin patterns. In this study, by using blurred pictures to make these patterns less visible, we explored whether the relative absence of such local features will attenuate the EPN snake effect. Non-blurred and blurred pictures of snakes, spiders, and birds were presented in a rapid serial visual presentation paradigm with a rate of three pictures per second. The EPN mean activity was extracted from the 225-330 ms time frame after stimulus onset at the parietal-occipital cluster (PO3, O1, Oz, O2, PO4). The results show an enhanced EPN in response to snake pictures compared to spider and bird pictures. Non-blurred snake pictures elicited much larger EPN amplitudes than blurred snake pictures, suggesting that the EPN is larger for snake pictures when the local features of the snake skin are clearly visible. Yet, blurred snake pictures elicited higher EPN amplitudes when compared to blurred spider and bird pictures, suggesting a complementary role for the more global features of snakes. Spatial frequency analysis of the stimuli indicated excess energy for high spatial frequencies in non-blurred snake compared to spider and bird pictures.