Recent evidence indicates that distractor objects may be actively inhibited or suppressed during multiple object tracking (MOT; Doran & Hoffman, in press; Pylyshyn, 2006; Pylyshyn et al., 2008) however the mechanism of suppression is currently unclear. In one view, zones of suppression may surround tracked targets so that objects that are near targets (and therefore within the suppressive region) would be suppressed (Franconeri et al., Psychonomics 2009). Alternatively, it may be that suppression is only applied to distractors that are likely to be confused with targets and therefore interfere with tracking performance (Pylyshyn et al., 2008). We examined this issue by measuring the amplitude of the N1 component of the event-related potential (ERP) elicited by probe flashes presented on targets, nearby distractors, and distant distractors. Critically, some of the distractors were a??confusablea?? with the targets (i.e., they were the same color and shape) while others were not (i.e., they were a different color and shape). If distractors are suppressed via an inhibitory region surrounding targets then confusability shouldn't matter and both confusable and nonconfusable distractors should be suppressed when they are near targets. Alternatively, if suppression is a??intelligenta?? or selective then only the confusable distractor objects should be suppressed perhaps because they are more likely to interfere with accurate tracking. The results of this experiment support intelligent suppression as N1 amplitude for probes appearing on nearby distractors was suppressed only when they were confusable with the target. In sum these data suggest that suppression of distractors during MOT is a??intelligenta?? as it is applied only to distractors that are potentially confusable with targets.
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