Figure 2.

Depicted is an invalid trial, with a masked shape-singleton prime in the upper left corner (the one red diamond among the three different, shape-homogenous square primes) in the first display (depicted in the background) followed by square-shaped masks at all four positions (depicted in the foreground), with one of the masking squares serving as a target. In the depicted example trial, participants have to search for a black target square (in the upper right corner of the depicted display), and have to respond to its location (i.e., whether it is left or right). Thus, the trial is invalid because the masked singleton prime is presented at a position away from the target. Note that the masked shape-singleton prime is task-irrelevant in almost every respect. It has a color and a shape different from that of the target. Thus, participants have no reason to intentionally search for the shape or for the color of the shape-singleton prime. Furthermore, under the depicted conditions, participants have also no incentive to search for a singleton by intention, because the target is not a singleton either (neither with respect to its shape nor its color). Would the masked shape-singleton prime still capture attention away from the target? (Under the depicted conditions this prediction is made by theories assuming that attention is captured to locations containing the largest feature differences relative to the rest of the display.) The corresponding attentional effect would be reflected in posterior ERP laterality indices (compared to conditions with masked shape-singleton prime and target being presented on the same side, e.g., both being on the right). Note that under the depicted conditions, stimulus intensity in the priming display is the same at all positions. Therefore, any index of attentional capture by the masked shape-singleton prime cannot be attributed to stimulus intensity. (Arrow: direction of time.)