Extrapolation occurs in multiple object tracking when eye movements are controlled

Tina Luu, Piers D. L. Howe

There is much debate regarding the types of information observers use to track moving objects. Howe and Holcombe (Journal of Vision 12(13): 1-10, 2012) recently reported evidence that observers employ extrapolation while tracking. However, their study is potentially confounded because it did not control for eye movements. As eye movements can aid extrapolation, it is unclear whether extrapolation can still occur in multiple object tracking (MOT) when eye movements are eliminated. In the current study, we addressed this question using an eye tracker to ensure that fixation was always maintained on a central fixation point while observers performed a tracking task. In the predictable condition, objects always travelled along linear paths. In the unpredictable condition, objects randomly changed direction every 300-600 ms. If observers employ extrapolation, we would expect performance to be greater in the former condition than in the latter condition. Our results showed that observers did indeed perform better in the predictable condition than in the unpredictable condition, at least when tracking just two objects (Experiments 1, 3, and 4). Extrapolation occurred less when tracking loads increased or when the objects moved more slowly (Experiment 2).