Abstract
Implicit probability cueing of target locations is a robust phenomena in which participants respond faster to targets that appear in predictive locations. Probability cueing differs from traditional endogenous cueing paradigms in that there is no explicit directional cue and participants are largely unaware of biases in their behavior. Sudden onsets, on the other hand, have been shown repeatedly to be an effective exogenous cue. We explored how implicit probability cueing of target location and exogenous cueing by an onset stimulus interact in a visual search task. Our findings suggest that these two cueing conditions produce strong main effects: Responses were faster to targets in high- than low-probability locations and faster when the exogenous cue preceded the target compared to a distractor. Furthermore, when the target location was preceded by the exogenous cue, response was facilitated compared to a random-probability control when that location was also the high-probability location and inhibited when it was a low-probability location. However, when targets were in the high-probability location, an exogenous cue at the target location did not facilitate response whereas an exogenous cue at a distractor location inhibited response relative to a no-onset control. That is, the exogenous cue did not increase the competitiveness of a location for processing when attention, due to the probability cue, was already drawn there, but it did increase the competitiveness of distractor locations. This suggests that the effect of probability cueing, even when participants are unaware of it, is extremely powerful in orienting attention to a location, but is not impervious to bottom-up attentional capture by salient stimuli. The results are interpreted within an attentional framework in which mechanisms sensitive to different properties such as spatial regularities and sudden onsets interact in competitive and cooperative ways.