N2pc and attentional capture by colour and orientation-singletons in pure and mixed visual search tasks.

The capture of attention by singleton stimuli in visual search is a matter of contention. Some authors propose that singletons capture attention in a bottom-up fashion if they are salient. Others propose that capture is contingent upon whether or not the stimuli share task-relevant attributes with the target. This study assessed N2pc elicited by colour and orientation singletons in a mixed task (the singleton defined as target changed block-to-block), and a pure task (the target was the same across the whole task). Both singletons elicited N2pc when acting as targets; when acting as non-targets, orientation singletons elicited N2pc only in the mixed task. The results suggest that the singletons were not salient enough to engage attention in a purely bottom-up fashion. Elicitation of N2pc by non-targets in the mixed task should be attributed to top-down processes associated with the current task. Stimuli that act as targets in part of the blocks become not completely irrelevant when non-targets.