Electrophysiological correlates of visual singleton detection.

Identifying a fixed-feature singleton that pops out from an otherwise uniform array of distractors elicits an ERP component called the N2pc over the posterior scalp. The N2pc has been used to track attention with millisecond accuracy, inform theories of visual selection, and test for specific attention deficits in clinical populations, yet it is still unclear what neuro-cognitive process gives rise to the component. One hypothesis is that the N2pc reflects a spatial filtering process that suppresses irrelevant distractors. In support of this hypothesis, Luck and Hillyard (1994a) showed that the N2pc is eliminated when the features of the target and distractors switch unpredictably across trials (so that participants cannot prepare to filter out irrelevant items). The present study aimed to replicate Luck and Hillyard's singleton detection experiment but with modifications to enhance the N2pc signal and to gain statistical power. We show that orientation singletons do, in fact, elicit the N2pc as well as an earlier-onset and longer-lasting singleton detection positivity over the occipital scalp when the target and distractor orientations swap randomly across trials. We conclude that spatial filtering might not play a major role in the generation of the N2pc and that the selection processes required to search for fixed-feature targets (in feature-search mode) are also engaged in the detection of variable-feature singletons (in singleton detection mode).