Distractors in multiple object tracking can be suppressed early or late in processing: Evidence from ERPs

In Multiple object tracking (MOT) tasks participants are typically asked to keep track of a set of target items presented among identical distractors as they move around a display. We examined the role of visual attention in the MOT task by measuring event-related potentials (ERPs) to probe flashes presented on targets, distractors, or empty background areas. Across three experiments we found evidence of both enhancement of targets and inhibition of distractors. When probes were irrelevant to observers, targets were enhanced and distractors were suppressed at the level of the N1 component. However, when probes were relevant to observers (i.e., a subset of probes required a behavioral response), these early attention effects were not observed. Instead a distractor inhibition patterns was observed in the amplitude of the P300, suggesting that inhibition had been postponed to a later stage of processing. Early attention effects were reinstated, however, when the tracking task was made more difficult even though probes were still relevant. In this case, attention seemed to enhance targets early in processing (i.e., at the level of the N1) but still suppressed distractors at later levels of processing (i.e., at the level of the P300). In sum, these results suggest that attentional selection in MOT is flexibly determined by task demands and that distractor objects may be suppressed early or late in processing.