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Abstract

Chapman (1968) proposed that ball catchers run backwards when they see the projection of an approaching ball accelerate, and forwards when they see it decelerate. However, we suggested that the threshold for distinguishing acceleration from deceleration is too high to account for catchers' performance (Brouwer et al., 2002). This work has been criticized for generalizing thresholds as measured on small screens to a ball catching situation (Zaal et al., 2003). In real ball catching, subjects track the ball by making head movements. This allows for the possibility that signals from the vestibular system reduce the acceleration threshold. We simulated the first 319 ms of approaching balls on a large screen and asked 10 subjects to judge whether the balls would land behind or ahead of them. Subjects did not perform better when they tracked the balls with their head compared to when they kept their head stationary. It is thus unlikely that tracking enables subjects to use acceleration. We propose an alternative cue, which is based on a combination of the rate of expansion of the ball's projection, and its angular velocity. We found some (tentative) evidence for the use of rate of expansion as proposed by our cue.