Abstract: The ability to quickly and efficiently detect ecological stimuli, such as snakes, spiders, and fearful faces, is vital for one’s survival. Here we asked whether such stimuli are processed automatically. In the study, we used negative faces to examine the automatic processing of ecological stimuli with two criteria (i.e., intentionality and load-insensitivity) that are essential aspects of automaticity.
In Experiment 1, we manipulated perceptual load in a visual search task, where participants searched for a target letter (either X or N) from an array of letters (five letters, selected from O, E, F, H, K, Z, V) that together circled around a central fixation. Either a negative face (i.e., ecological stimuli) or a neutral face (i.e., non-ecological stimuli) was presented in the peripheral as a task-irrelevant distractor. To further rule out the possibility that the salience of the negative faces may account for the load-insensitive attention capture, we used the same paradigm in the Experiment 2, except that a color singleton, a salient stimulus without ecological property, was presented as the distractor in the peripheral.
A total of twenty-one college students participated in the study. As expected, in Experiment 1, the search efficiency was higher when the perceptual load was low, and the presence of the negative face captured a larger amount of attention than the neutral face. Critically, the amount of attention captured by the negative face (versus neutral face) was the same across different levels of perceptual load, suggesting that ecological stimuli are processed automatically. To further examine the possibility that the load-insensitive attention capture observed in Experiment 1 was instead due to the salience, not the ecological property, of the negative face, in Experiment 2 we used the same paradigm as that in Experiment 1, except that a salient color singleton, not the negative face, was presented in the peripheral as a task-irrelevant distractor. We found that although the presence of the color singleton captured attention, the amount of attention capture varied as a function of perceptual load, suggesting that it is the ecological property, not the salience, of the negative face that is processed automatically.
Taken together, our study provides direct evidence that ecological stimuli are processed automatically, independent of perceptual load. Further fMRI studies may help elucidate the neural basis of the automatic processing of ecological stimuli, possible via a sub-cortical pathway.

Key words: ecological significance, intentionality, load-insensitivity, perceptual load, attention capture