How many objects can you track ? : Evidence for a 2 resource - limited attentive tracking mechanism

13 Much of our interaction with the visual world requires us to isolate some currently important objects from other less 14 important objects. This task becomes more difficult when objects move, or when our field of view moves relative to the 15 world, requiring us to track these objects over space and time. Previous experiments have shown that observers can track a 16 maximum of about 4 moving objects. A natural explanation for this capacity limit is that the visual system is architecturally 17 limited to handling a fixed number of objects at once, a so-called magical number 4 on visual attention. In contrast to this 18 view, Experiment 1 shows that tracking capacity is not fixed. At slow speeds it is possible to track up to 8 objects, and yet 19 there are fast speeds at which only a single object can be tracked. Experiment 2 suggests that that the limit on tracking is 20 related to the spatial resolution of attention. These findings suggest that the number of objects that can be tracked is 21 primarily set by a flexibly allocated resource, which has important implications for the mechanisms of object tracking and for 22 the relationship between object tracking and other cognitive processes. 23