Fear commands attention : Snakes as the archetypal fear stimulus?

Sammanfattning: Information regarding successful solutions to environmental hazards has accumulated in the gene pools of species, as a result of evolution. Therefore, from an evolutionary viewpoint, fear has played a central role in shaping mammalian genotypes. The goal of the present research was to elucidate the role of fear in the control of attention by investigating meaningful differences in the attentional processing of evolutionary-relevant animal stimuli and different categories of neutral stimuli. In study I we used a visual search task to examine attentional selectivity to a class of fear-relevant animal stimuli (snakes and spiders), compared to a different animal category, that of non-threatening animal stimuli presumably lacking evolutionarily derived fear-relevance (cats and fish). The results showed no asymmetry in reaction time and accuracy data between fear-relevant and neutral animals when they served either as targets or distractors. Instead, there was an increased distraction effect when the fear-relevant categories were presented simultaneously in the visual displays. In studies II-IV we did not collapse snakes and spiders into the same category of evolutionarily fear-relevant stimuli, but compared these carefully matched stimuli in terms of their association with danger. The comparison was predicated on the notion that snakes carry a considerable more heavily evolutionary baggage to be feared by humans (Isbell, 2006; 2009) than do spiders (e.g., Davey, 1994). In order to avoid potential differences in variability among fear-relevant and neutral animal stimuli, we compared snakes and spiders with an ecologically valid stimulus, i.e., mushrooms, and presented these stimuli of interest against an emotionally neutral background composed by pictures of fruits. Moreover, we intended to study whether the perceptual load (e.g., increments in set size) modulated the attentional processing of such stimuli (Lavie, 1995; 2005). The results from studies II-IV consistently showed that snakes (compared to spiders and mushrooms) were preferentially processed, particularly under the most demanding perceptual conditions. Specifically, the privileged attentional processing of snake stimuli was most evident among many distractors (studies II-IV), in peripheral vision (study III Experiment 1), at brief exposure times (< 300ms) (study IV), and when unexpectedly presented among the background stimuli (study III Experiment 2). The evidence demonstrated that snakes are special and do not, like spiders, influence attention according to expectations from standard theory (Lavie, 2005). Rather this specificity of snake processing invites an evolutionary explanation, such as the one offered by Isbell s (2009) Snake Detection Theory. Finally, our set of results relating the effects of prior fear on attention showed somewhat inconsistent results. In study I, where snake and spider fearful participants were collapsed into one single group, participants were specifically sensitized to detect their feared stimulus, with the emotional ratings mirroring this effect. However, this result did not enable examination of potential differences in responses between snake and spider fearful individuals. Indeed, there are indications in the literature pointing to the relevance of such differentiation, showing that while snake fear is associated with the predatory defense system (e.g., Öhman, 2009), spider fear is more likely to be mediated by disgust (Matchett & Davey, 1991). Therefore, in studies II and IV, we examined potential differences between the two groups of participants. In study II there was a clear dissociation between the two types of animal fear, reflected in attention and emotion measures, indicating that spider fear was highly specific, whereas snake fear was associated with a more generalized enhanced evaluation of all negative stimuli. However, and given that in study IV the findings were not consistent, further research is clearly needed in order to clarify the potential moderators in the effects of prior fear on attention.