VCL | Publications

Grouping and Attention

In dealing with a dynamic world, people have the ability to maintain selective attention on a subset of moving objects in the environment. To explore our ability to maintain attention on multiple objects in a laboratory setting, researchers often rely on the multiple-object-tracking (MOT) task. This task requires observers to mentally track a set of target objects moving among featurally identical distractor objects.

Performance in tracking multiple objects is limited by three primary factors—a capacity on the number of objects that one can track, the speed at which one can track the objects, and how close together the objects can be. We argue that this last limit of proximity, or object spacing, can explain the limits of capacity and speed.

Within this framework, why have previous studies shown increased speed leading to lower tracking capacity? We hypothesized that the speed effect can be explained by an interaction between speed and proximity. As objects move faster, they travel more distance during a given trial. Traveling greater distance means more opportunities to come close to other objects, creating competitive interactions that can impair performance.

We addressed these questions using two paradigms. First, participants tracked target objects among featurally identical distractor objects in an "orbiting moons" display. We presented the same tracking task animation in "slow motion" and "fast forward." On some trials the objects traveled at a slow speed but for a longer time, and on other trials the objects traveled at a fast speed but for a shorter time, such that each object traveled an identical distance (i.e., speed x time) in each trial. With this de-confounding of speed and proximity, speed changes no longer affected performance.

In a second series of experiments, we replicated these results using a traditional stimulus display with random object motion, instead of more constrained planets-and-moons motion. The effect of speed was observed in separate 4-target and 2-target tracking conditions. Like the first experiment, when object speeds were increased across trials, we offset the effect of this speed increase by a proportional decrease in time such that each object traveled an identical distance in each trial.

In this design, increased speed does lead to lower tracking performance. However, speed increases led to no more impairment in 4-target tracking tasks than 2-target tracking tasks. In other words, while there was a drop in accuracy with increasing speed, this impairment did not increase when the number of targets being tracked increased. Together, these experiments suggest tracking multiple objects occurs in a parallel fashion and that speed, per se, has no effect on tracking performance.