To assess the speed limit for tracking moving objects with attention, first some blobs are designated as targets, then they and other identical blobs travel about. After a variable tracking period, participants must indicate which blobs had been designated as targets. If more blobs are designated for tracking, the maximum speed yielding accurate performance decreases. However, Franconeri et al. (2008, 2010) suggested that the decrease in high-speed performance with more targets is entirely attributable to crowdinga??in most studies, at higher speeds objects pass near each other more frequently. We assessed the speed limit for tracking one and for tracking two targets. In each of two concentric circular trajectories, two blobs traveled. Within a trajectory, the two blobs were always on opposite sides of fixation. One blob in one trajectory (one-target condition) or one blob in each trajectory (two-target condition) was precued. Separation between the trajectories was varied to assess any effect of crowding. RESULTS. The average speed limit (68% threshold) of six participants was substantially higher for tracking one target (1.9 rps) than for tracking two targets (1.5 rps), even when crowding was avoided with large separation. The slowness of the two-target limit found is similar to that predicted (1.6 rps) if each participant tracked only one target at high speeds, guessing when they picked the wrong one to track. To further investigate what causes the speed limits, we exploited the finding of hemisphere-specific tracking resources (Alvarez & Cavanagh, 2005). Two targets were in either the same hemifield or different hemifields. The speed limit was significantly lower (six participants) for targets in the same hemifield than in opposite hemifields, consistent with the involvement of independent resources. Availability of such resources may set the severe speed limits on tracking documented here.
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