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Alex Holcombe, Wei-Ying Chen; Dividing attention reduces both speed and temporal frequency limits on object tracking. Journal of Vision 2014;14(10):352. doi: https://doi.org/10.1167/14.10.352.
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Low-level motion processing is limited by temporal frequency, not speed: for a broad range of spatial frequencies, the fastest speeds at which motion is perceived correspond to a single temporal frequency, meaning that the threshold speed is proportional to the inverse of spatial frequency (Burr & Ross, 1982). Like low-level motion, attentional tracking also has a temporal frequency limit, but one that is much lower (7 Hz) and declines progressively with more targets to track. Also unlike low-level motion, attentional tracking additionally has a speed limit (Holcombe & Chen, 2013 and Verstraten et al., 2000), supporting a qualitative difference between tracking processes and low-level motion. Here, one tracking target revolved about the fixation point, along with a distractor or distractors sharing the same circular trajectory. When the number of distractors is higher than about four, and speed is increased, performance falls to threshold when a particular temporal frequency is reached. But for fewer distractors, the limiting factor instead appears to be speed. Like Verstraten et al. (2000) and Holcombe & Chen (2013), for one target we observe a limit of about 2 revolutions per second (rps). Here we varied the number of targets by presenting three concurrent, concentric displays. A second target reduced the speed limit by 0.4 rps, and a third target by a further 0.5 rps. Thus, dividing attention reduces tracking's speed limit substantially, just as it does its temporal frequency limit. Unlike the temporal frequency limit, the speed limit was not robust to changes in the number of distractors sharing the trajectory. When two rather than one distractor shared each target's trajectory, the speed limits fell by 0.5 rps. Theories of tracking must be modified if they are to explain how dividing attention reduces both limits, and additionally explain the decrease in speed limits caused by an additional distractor.
Meeting abstract presented at VSS 2014
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