The Hidden Snake in the Grass: Superior Detection of Snakes in Challenging Attentional Conditions

Abstract

Snakes have provided a serious threat to primates throughout evolution. Furthermore, bites by venomous snakes still cause significant morbidity and mortality in tropical regions of the world. According to the Snake Detection Theory (SDT Isbell, 2006; 2009), the vital need to detect camouflaged snakes provided strong evolutionary pressure to develop astute perceptual capacity in animals that were potential targets for snake attacks. We performed a series of behavioral tests that assessed snake detection under conditions that may have been critical for survival. We used spiders as the control stimulus because they are also a common object of phobias and rated negatively by the general population, thus commonly lumped together with snakes as "evolutionary fear-relevant". Across four experiments (N = 205) we demonstrate an advantage in snake detection, which was particularly obvious under visual conditions known to impede detection of a wide array of common stimuli, for example brief stimulus exposures, stimuli presentation in the visual periphery, and stimuli camouflaged in a cluttered environment. Our results demonstrate a striking independence of snake detection from ecological factors that impede the detection of other stimuli, which suggests that, consistent with the SDT, they reflect a specific biological adaptation. Nonetheless, the empirical tests we report are limited to only one aspect of this rich theory, which integrates findings across a wide array of scientific disciplines.

Figure 1. Example of the circular stimuli display (set size 6) used in Exp. 1 and 3.

Figure 2. Experiment 1: Mean Reaction Times (RTs) in milliseconds (ms) to locate a discrepant target stimulus that could be a snake, a spider, or a mushroom, in displays exposed for 300 ms, 600 ms, and 1200 ms.

Figure 3. Experiment 1: Mean accuracy proportions to locate a discrepant target stimulus that could be a Snake, a Spider, or a Mushroom, in displays exposed for 300 ms, 600 ms, and 1200 ms that included eight (a) or four items (b).

Figure 4. The visual display in Exp. 2 was presented in a grid, with the pictures arranged on an imaginary rectangle that was divided into a 6×6 grid (i.e., 36 cells).

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.

Figure 5. Experiment 2: Attentional efficiency reflected in slopes across different set sizes (3, 6, 12, 18) (expressed as the mean search time [in milliseconds]/searched item) for locating the target picture (snake, spider, mushroom) as a function of eccentricity (fovea, parafovea, periphery).

Figure 6. Experiment 2: Reaction times (RTs) in milliseconds (ms) to detect the target picture - snake (left panel), spider (middle panel), and mushroom detection (right panel), as a function of set size (3, 6, 12, 18) and location in the visual field (fovea, parafovea, periphery).

Figure 7. Mean Reaction Times (RTs) in milliseconds (ms) to locate a discrepant target (bird) in the different type of distractor conditions (snake, spider, mushroom, and no distractor), in Experiment 2.

The upper panel refers to the homogeneous displays, whereas the lower panel illustrates the heterogeneous displays, both as a function of the set size (4; 6). Longer RTs indicate larger interference scores.

Figure 8. Example display of Experiment 4, depicting a high perceptual load trial with a snake distractor.

Note that stimuli are not drawn to scale.

Figure 9. Experiment 4: Mean Reaction Times (RTs) in milliseconds (ms) to discriminate the target letter (X or N) in the different type of distractor conditions (snake, spider, mushroom, flower, and no distractor), as a function of the perceptual load condition (low; high) and the sex of the participant (women; men).

Longer RTs indicate larger interference scores.