Intercepting moving targets: why the hand's path depends on the target's velocity

In order to intercept a moving target one must reach some position at the same moment as the target. Considering that moving towards such a position takes time, it seems obvious that one must determine where one can best intercept the target well in advance. However, experiments on hitting moving targets have shown that the paths that the hand takes when trying to intercept targets that are moving at different velocities are different, even if the targets are hit at the same position. This is particularly evident at high target velocities, which seems strange because the benefit of considering the target’s velocity should be largest for fast targets. We here propose that the paths’ curvature may intentionally differ for different target velocities in order to maximise the chance of hitting the target. Arriving at the target with a velocity that matches that of the target can reduce the consequence of certain temporal errors. In particular, if the path curves in a way that makes the component of the hand’s final velocity that is orthogonal to the hitting direction exactly match the velocity of the target, then no additional error will arise from arriving at the target slightly earlier or later than expected. On the other hand, moving along a curved path is likely to increase the spatial errors. We argue that a compromise between these two influences could account for the differences between paths towards fast and slow targets.