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Snakes and primates have coexisted in mutual predator-prey relationships for about 100 mya , but the extent of exposure has varied across continents, as the result of migration and the breaking up of the southern supercontinent Gondwana . Isbell  argues that this variation has produced correlations between evolutionary snake exposure, on the one hand, and fear of snakes and advanced vision in different primate species, on the other. Thus, African monkeys and apes have been continuously exposed to snakes for about 100 my, are uniformly afraid of snakes, and have the most advanced visual system among primates. In contrast, the lemurs of Madagascar, who have a history with very little snake exposure, do not fear snakes and have poor vision. The New World monkeys, finally, who were given a reprieve from snakes that lasted for about 30 my when emigrating from Africa to South America, show variable visual systems and fear of snakes, suggesting less consistent long-term predatory pressure from snakes.
The work of Isbell ,  outlines the evolutionary contingencies that underlie the effect of snakes on primate vision and attention. In this article we exploit the evolutionary ideas presented by Isbell ,  in seeking to understand the shaping of human perceptual capabilities for efficient detection of snakes as part of the behavioral defense systems.
Currently, few studies have been directly designed to test predictions from the SDT. Van Le et al.  showed through single cell recordings in the Macaque pulvinar nucleus that many cells were specifically responsive to snakes, compared to control stimuli. Moreover, Van Strien and colleagues  demonstrated earlier capture of visual attention for snakes (compared to spiders and birds), reflected in larger early posterior negativity for this stimulus. However, the experimental paradigm serving as basis for these findings only included passive viewing of the stimuli without any measures of overt or covert visual attention. Two studies from our laboratory have manipulated specific factors depicted from the SDT , . The results showed that snakes more potently capture attention than spiders (and mushrooms) under high perceptual load conditions . However, those preliminary studies were designed to open the avenue for the extensive behavioral testing presented in the present study. In the present study, we further increased the perceptual complexity of stimulus displays under several conditions known to deplete attentional resources, to delimitate the conditions where snakes reliably capture attention. Because the SDT holds that snake detection is part of a defense system, with snake avoidance as its evolutionary function, understanding how snakes affect attention on the behavioral level with systematic and robust experimental testing is paramount for examining the SDT  predictions.
The objective of the experiments reported in this article was to systematically contrast pictures of snakes and spiders in their effects on human attention. We were particularly interested in examining the modulating effect of ecological factors on the difference between snakes and spiders in capturing attention. The specific factors that we examined were derived from the SDT on the premise that perceptual abilities to detect camouflaged snakes have been more consistently selected for than detection of spiders. Hence, our experimental work focused on factors such as stimulus duration, foveal versus peripheral vision, the complexity of the display as indicated by number of distracting items, and top-down versus bottom-up control of attention, which correspond to ecological conditions thought to be important for snake detection .
In Experiment 4 a two-choice reaction time task was used and participants were asked to indicate, as quickly and accurately as possible, the identity of a target letter (X or N) while grayscale images of snakes, spiders, flowers and mushrooms were presented simultaneously as distractors on 20% of the trials.
Because snakes have a short reactive distance within which they provide serious danger, snake detection should be fast and require only a quick glimpse in order to activate defense. In Experiment 1, therefore, we examined the modulating effect of stimulus duration on detection of snakes in complex scenes. In a previous experiment from our lab , the attentional efficiency to detect snakes, compared to spiders and mushrooms, was relatively independent of the perceptual load (set size: 4, 6, 8 items). However, this effect was not influenced by the stimulus exposure durations (150 ms vs. 300 ms). The restricted time for processing targets resulted in lower rates of correct responses and, therefore, in fewer trials on which RTs could be measured, which could have obscured the interaction effects with exposure duration. Thus, it remains unseen if differences between snakes and spiders emerge with a wider manipulation of the exposure duration of the stimulus displays. In the present experiment, participants were exposed to a visual search paradigm in which the duration of the displays varied between 300, 600, and 1200 ms, and the set size varied between 4 and 8 items.
The 4 stimuli categories (snakes, spiders, mushrooms, and fruits) each consisted of 18 different color picture exemplars, which displayed the object in center of the picture against a background of its typical ecology. More details about the stimuli are given in S1 File.
After providing written informed consent, the participants were asked to find a position in the chair where they could comfortably reach the two response keys with their right and left index fingers. Written instructions were self-paced and emphasized that the participant's task was to determine, as quickly and accurately as possible, whether a deviating target stimulus (a snake, a spider, or a mushroom) was present or absent among the background stimuli (fruits). The specific identity of the target stimulus was not specified. A target stimulus was presented on half of the trials in the experiments, whereas the other half only contained distractor displays, where no deviating target stimulus was presented. Target location and order of presentation was randomized for each subject.
This experiment tested whether the detection of snakes is especially superior to that of spiders and mushrooms in peripheral vision, given the advantage of an enlarged field of view for detecting nearby snakes and the hypothesis that snakes may stimulate mainly the magnocellular system. The spatial distribution of targets and distractors in this experiment changed randomly from trial to trial, and the location of the target was systematically varied between foveal (
Upper left: Arrangement of the images in the display in the four foveal locations (A), twelve parafoveal locations (B), and twenty peripheral locations (C) (1.2, 3.4, and 5.7, respectively) in Exp.2. Upper right: Example of a display with 3 items and a target picture (mushroom) in the periphery. Bottom left: Example of a display with 12 items and a target picture (snake) in the parafovea. Bottom right: Example of a display with 18 items and the target picture (spider) in the fovea.
Target location was randomized and could occur at one of 4 (fovea), 12 (parafovea) or 20 (periphery) positions (Fig. 4). Blank spaces occupied the locations where no pictures were presented. Participants were exposed to 288 trials (displays with or without target), three types of target (snakes, spiders, and mushrooms), three possible spatial locations for the target (eccentricities of 1.2, 3.4, and 5.7), four set sizes (3, 6, 12, 18), and four replications of both target present and target absent trials.
Participants were presented with 192 trials (96 target present and 96 target absent trials), randomly assigned to each participant. On 36 of both target present and target absent trials a distractor picture replaced one of the background stimuli for both display sizes, with the equal probability of appearance for each of the distractor categories. On the remaining 60 trials no distractor stimulus was presented. Both targets and distractors positions were counterbalanced across trials. Within displays, the background stimuli could be homogeneous or heterogeneous, which was manipulated as a between-participants factor. In addition, the participants could be exposed to two different stimulus set sizes (4; 6), manipulated as a within-participants factor. The additional within-participants factor was the type of distractor stimuli (snake, spider, or mushroom), with displays with no distractor picture as the control condition.
Finally, the results showed that, in general, there was a larger interference (longer RTs) for displays including a fear-relevant stimulus (snake and spider distractors), than a neutral one (mushroom) (p
Thus, consistent with an evolutionary perspective, attention appeared to be automatically reoriented to suddenly appearing snakes in the immediate environment, which goes in line with the preliminary findings from .
Because the subject population in the previous three experiments was dominated by females, the majority of participa