Abstract
Irrelevant transient stimuli can speed responses to visual targets that appear soon after at the same location (relative to other locations). How do these stimuli speed target processing? Traditionally, they are thought to act as cues that reflexively capture visuospatial attention, a mechanism that provides processing priority to specific regions of the visual field. Here we report behavioral and electrophysiological evidence of the limits of this explanation. In the first experiment we show that while targets are identified faster at a cues locations (the classic cueing effect), this effect is increased when the cue and target are visually similar. Thus, the reflexive cueing effect is not a general attentional enhancement of all visual processing within a region of space; rather, some component of the effect is related to the identity of the cue. In a second experiment we used attentional control settings to manipulate whether cues captured attention or not and measured event-related potentials. Cues that captured attention produced a posterior contralateral positivity between 200 to 400 ms after their onset that was absent when they did not capture attention. This component resembles the Ptc, which has been associated with the resolution of perceptual competition between proximal stimuli. More importantly, a similar component was observed time-locked to the target onset, except when the target appeared at a cued location. Thus cues may speed target processing by inducing competition resolution, making this process unnecessary when the target subsequently appears at that location. These results do not fit well with the notion that reflexive attention is a mechanism deployed to enhance visual processing within regions of space. Instead, the present results suggest that transient stimuli initiate perceptual processing, and subsequent targets can exploit these ongoing processes if, for example, they appear at the same location or are visually similar.
Natural Sciences and Engineering Research Council of Canada.