Evidence for suppressive mechanisms in attentional selection: feature singletons produce inhibitory surrounds.

Four experiments explored inhibitory mechanisms related to attentional selection. Observers viewed multielement displays and performed a form discrimination task involving a probe element. Also present in the stimulus display was a singleton element (possessing a unique color or orientation). In Experiments 1-3, probe discrimination performance was measured as a function of the distance between the probe and the singleton. Experiment 1 revealed that probe discriminations suffered when the probe was adjacent to the singleton, but improved as the spatial separation between the probe and attentionally salient singleton increased. Experiment 2 added a control condition, revealing that probe discriminations were inhibited near the singleton, but returned to control level performance with increased separation. Further, the amount of inhibition increased with larger stimulus onset asynchronies between the singleton and probe. Experiment 3 demonstrated that the extent of the inhibitory region is spatially mediated. In Experiment 4, the task was modified to one of probe detection. No inhibition was observed in the detection task, indicating that the decrease in probe discrimination performance observed in Experiments 1-3 was not due to observers' inability to detect the probe element.