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Abstract

To investigate whether acceleration can be used in interception tasks, we determined how well subjects could detect acceleration at short presentation times. Two different tasks, in which differential or absolute judgments had to be made, were used to find thresholds. In the differential- judgment task, two dots were presented successively; one accelerated and the other decelerated. Subjects had to indicate which of the two accelerated. In the absolute-judgment task, subjects had to adjust the motion of a dot such that it appeared to be moving at a constant velocity. The results were similar. Most subjects could do these tasks even when the presentation time was only 300 ms. About 25 change of the average velocity was needed to detect acceler- ation with reasonable confidence. It is claimed that, in order to catch a ball, one uses the distinction between acceleration and deceleration of the optic projection of the ball to determine whether one has to run back- ward or forward. We examined the results of a real ball-catching task (Oudejans, Michaels, and Bakker, 1997 Journal of Sports Sciences 15 587 ^ 595) and found that subjects reacted before optic acceleration could have been detected. We conclude that acceleration is not used to initiate locomotion in catching balls.