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

Abstract

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

Representation of different exact numbers of prey by a spider-eating predator

“…Although this limit is the same as the limit normally attributed to subitizing, there is also evidence that the proposed working-memory limit at three or four can be exceeded (e.g. [9,59]). Moreover, there are divergent arguments over whether the limits that apply to working memory are best characterized as being based on a limited availability of object files or instead as being based on a limited continuous resource, interference from multiple bindings or some combination of these different factors [60][61][62][63][64].…”

Bayesian models of eye movement selection with retinotopic maps

“…For the particular experimental design we used, the maximum number of targets consistently tracked was 5, which justifies our choice of the number of target variables. Other experimental studies suggest that this maximum number of target is not fixed and seems to depend on factors such as speed and spacing of the objects (Alvarez and Franconeri 2007). In addition, each of our target variables cover the whole visual field (encoded in the logcomplex mapping) although there are works indicating that some representation capacities are separated across the hemifields .…”

Interference and memory capacity limitations.

“…With respect to multiple object tracking, a traditional view held that observers can attentionally follow 3 or 4 moving objects on a display (e.g., Pylyshyn & Storm, 1988;Scholl & Pylyshyn, 1999), a capacity limitation that seems to coincide with traditional working memory and subitizing limitations. However, it is now clear that the number of objects that can be tracked depends on factors such as their speed and their spacing (Alvarez & Franconeri, 2007), and the main factor that limits how many item we can follow appears to be inter-item interference (Franconeri, Alvarez, & Cavanagh, 2013): objects interfere with each other when their receptive fields come too close. However, while such tracking abilities and WM might both be limited by interference, the current proofs do not apply to multiple object tracking, because memory interference depends on the number of objects in memory, while tracking interference depends on the number of objects present in a display, and not just those that are tracked (Bettencourt & Somers, 2009; see also Franconeri, Jonathan, & Scimeca, 2010), and, more generally, because tracking limitations are due to occasional interactions between objects (i.e., when they come to close), requiring a somewhat different model.…”