Tracking seven is not the same as tracking three: The roles of parallel and serial resources in object tracking

摘要

Tracking a subset of moving targets among a group of identical items, typically studied with the multiple object tracking paradigm (MOT), has long been known to be capacity limited, usually to about three items. But recent work suggests that this limit can increase to as many as eight when objects move slowly enough. The current studies asked whether seven items are tracked in the same way as three. Participants performed MOT while also detecting transient probes that appeared on targets. In Experiment 1, participants tracked between one and five targets. Targets were always revealed one at a time, and in half the trials participants had to identify targets in the same order as they were revealed, adding serial order (SO) memory requirements. Whereas probe detection rates declined linearly as a function of load in the SO task, detection only declined in the spatial task when tracking four or five targets compared to two or three. Monotonic costs associated with additional targets in the SO task reveal the operation of a serial processing mechanism. But the absence of such costs for fewer than three targets in the spatial condition suggests that two or three targets were tracked in parallel, while tracking four or more demanded serial resources. An experiment with slow object speeds and tracking loads up to seven confirmed these intuitions. Declines in probe detection evolved for more than three targets, though there were no significant costs associated with tracking three compared to two. Further experiments used an object localization procedure and found a similar absence of per-item costs for one to three targets, compared with steep per-item costs for more than three. Overall, these experiments demonstrate that only up to three targets can be tracked in parallel, and that tracking more than three requires the allocation of serial resources.