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Abstract

It has been 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 backward or forward. We questioned whether subjects can detect this information well enough to use it for this task.

We determined how well subjects could detect acceleration for short presentation times. About 25% change of the average velocity was needed to detect acceleration with reasonable confidence, independent of presentation time. Has the optical velocity changed that much at the time that ball catchers start to run?

We examined the results of a real ball-catching task (Oudejans, Michaels & Bakker, 1997) and found that subjects reacted before optic acceleration could have been detected. Thus, acceleration is not used to initiate locomotion in catching balls. This raises the question as to what alternative information can be used.

The combination of horizontal and vertical speed determines the distance that a ball is going to cover. Subjects can estimate this distance by using information about angular velocity and the ball's visual size divided by its rate of expansion (tau). We estimated a discrimination threshold for this ‘perceptual predicted distance’ from discrimination thresholds for angular velocity and tau as described in the literature. The perceptual predicted distances of the balls in the experiment of Oudejans et al. (1997) can be discriminated well enough to be a possible cue for determining the initial running direction.