Abstract
Snakes have proven to drive early attentional capture due to their evolutionary importance, as reflected by the early posterior negativity (EPN). The EPN snake effect might be partly driven by the proximity of the animal. In this study, by employing full-body (medium shot) and head-focused (close-up) pictures, we investigated whether the relative nearness (proximity) of the animal on the picture affects the snake EPN effect. We presented thirty participants with medium shot and close-up snake, spider and bird pictures in a rapid serial presentation paradigm at a presentation rate of three frames per second. We extracted the mean EPN activity from the 225–330 ms time frame after stimulus onset at the parietal–occipital cluster (PO3, O1, Oz, O2, PO4). The results indicate enhanced EPN for snake pictures as compared to spider and bird pictures. In addition, medium-shot snake pictures elicited higher EPN amplitudes than close-up snake pictures, suggesting that the EPN is higher when local, high spatial frequency attributes are visible. Spatial frequency analysis of the stimuli indicated that medium-shot snake pictures possess more power in the high spatial frequency bands, compared to medium-shot spider and bird pictures.
Highlights
Snakes have been proven to trigger fast detection and early attentional capture in humans and primates, due to their evolutionary importance (Öhman et al 2001)
Mann–Whitney tests for each category and frequency band revealed significantly lower energy for close-up as compared to medium-shot snake pictures in the two highest spatial frequency bands (> 26.3 cycle/degree, p = 0.002; 13.2–26.3 cycle/degree, p = 0.007)
Energy was significantly lower for close-up as compared to medium-shot bird pictures in the 13.2–26.3 cycle/ degree frequency band (p = 0.041) and the 3.3–6.6 cycle/ degree frequency band (p = 0.049)
Summary
Snakes have been proven to trigger fast detection and early attentional capture in humans and primates, due to their evolutionary importance (Öhman et al 2001). An evolutionary forced readiness of the visual system seems to be active in the presence of phylogenetic fear (Mühlberger et al 2006). This suggests a visual attentional mechanism responsible to rapidly detect dangerous stimuli in order to avoid danger and promote survival. The “Snake Detection Theory” (SDT) (Isbell 2006), suggests that snakes, as evolutionary agents, have shaped the development of the primate visual system and have set vision as the primary sense for threat detection in humans and primates.
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