Electrophysiological indices of target selection and distractor suppression under varying perceptual load: Evidence for spreading suppression

摘要

Target selection and distractor suppression mechanisms consume limited cognitive resources. The N2pc and the Pd event-related potential components index target selection and distractor suppression respectively. These two mechanisms overlap in time and are polarity inverses of each other. It is unclear whether these mechanisms compete with each other for cognitive resources, or alternatively, draw from separate resource pools. To distinguish between these competing hypotheses, we orthogonally manipulated perceptual load and the presence of a salient distractor in a visual search task. Low perceptual load displays had homogeneous distractors whereas high load displays had heterogeneous distractors. For half of all trials, one distractor was rendered salient in color. If target selection and distractor suppression mechanisms compete for resources, then searching for the target under high load will leave fewer resources to suppress the salient distractor. However, if the mechanisms are separable, then more difficult target selection will not hinder distractor suppression. Participants were slower to respond when the distractor was present, but less so for high than low load. These behavioral results replicate the load effect; however, the electrophysiological results reveal a more nuanced account. We observed no difference in the N2pc or Pd components for low and high load trials, but the mean amplitude for the combined N2pc/Pd components was larger under high load when the target and distractor were in opposite hemifields. This indicates target selection does not outright compete with distractor suppression. Rather, our results suggest that target selection is separable from distractor suppression, but that these mechanisms are not necessarily independent. Furthermore, this outcome offers an alternative interpretation to the load effect. Whereas load theory invokes attentional resource 'spill-over' to account for increased distraction in low load, our data suggest that heterogeneous distractor search involves local suppression that spreads to the distal salient distractor.