Multiple-object tracking (MOT) is a widely used paradigm in investigating the relationship between object and attention. Recent research on tracking of unique objects has shown that target features can be processed during tracking, and object uniqueness can produce varying effects on tracking, from enhanced to impaired tracking performance. In this study, we proposed that object uniqueness brings both benefits and costs to tracking task, and investigated the effect of uniqueness on MOT and its underlying mechanism. Study 1 manipulated the working memory (WM) capacity of tracking stimuli, and compared tracking performance for unique and identical objects. We found that unique objects with large WM capacity facilitate tracking, whereas unique objects with small WM capacity impair tracking. We suggested that the effect of uniqueness is set by the amount of attentional resources demanded for encoding and storing object identities in WM. In study 2, the nameability of targets was manipulated. Effect of uniqueness was only observed for nameable targets but not for unnamable targets with surface features. We suggested that during tracking object identities are stored in verbal code in WM system. The current study provided an explanation for seemingly contradictory findings on effect of uniqueness on MOT with a unified theory.
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