Purchase this article with an account.
George A. Alvarez, Steven L. Franconeri; How many objects can you track? Evidence for a flexible tracking resource. Journal of Vision 2005;5(8):641. doi: 10.1167/5.8.641.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
The number of moving objects that can be tracked with attention is often reported to be 4, suggesting that there is a “structural limit” on tracking. We show that the tracking limit is not fixed, but depends systematically on the speed of the objects, such that at slow speeds observers can track 8 targets as well as a single target moving at a fast speed. Critically, the function relating the speed limit to the number of targets tracked is continuous, without any noticeable break in the 3-5 target range, suggesting that tracking accuracy is limited only by the amount of resources devoted to each target, not by a structural limitation. Method: Observers performed a multiple object tracking task in which they tracked 1–8 circles among a set of 16 identical moving circles. In session 1, observers adjusted the speed of the objects to find the maximum speed at which they could perfectly track the targets for 5 seconds. In session 2, we verified the accuracy of these settings by having observers track 1-8 targets moving at their “personal” speed limit for each number of targets. Results: With each increase in the number of targets, the speed limit decreased significantly. Moreover, the function was continuous, without any noticeable discontinuities in the 3–5 object range (r = .998 between speed and log of the number of targets). All speed settings were greater than zero indicating that on average observers estimated there was a speed at which they could perfectly track as many as 8 moving targets. It also appears that observers were able to accurately estimate their speed limits for tracking different numbers of targets, as tracking performance was near 100% for each number of targets in the second session and did not differ for different numbers of targets. Conclusion: These results are inconsistent with models that assume a fixed 4 object limit on tracking, and suggest that that tracking capacity is limited only by the amount of resources devoted to each target.
This PDF is available to Subscribers Only