Response bias contributes to distractor suppression in Gaspelin et al.’s (2015) probe letter task

Visual attention is often inadvertently captured by salient stimuli. It was suggested that it is possible to prevent attentional capture in some search tasks by suppressing salient stimuli below baseline. Evidence for suppression comes from a probe task that was interleaved with the main search task. In the probe task of Gaspelin et al. (2015), letters were shown on the stimuli of the search display and participants had to identify as many letters as possible. Performance was found to be worse for letters shown on the distractor compared to non-salient nontarget stimuli, suggesting that distractor processing was suppressed below baseline. However, we propose that participants may have reported letters on the distractor less frequently than letters on nontargets. This response bias may have degraded performance for letters on distractor compared to nontarget stimuli. After replicating the original findings, we conducted an experiment where we avoided response bias by cueing only a single letter for report. Consistent with the response bias account, we found that the difference between distractor and nontarget stimuli disappeared. In contrast, the difference between target and nontarget stimuli persisted, showing that the modified probe task was sensitive to the deployment of attention induced by the search task. In sum, the results suggest that the probe letter task may not provide convincing evidence for attentional suppression.