We propose and evaluate a source of information that ball catchers may use to determine whether a ball will land behind or in front of them. It combines estimates for the balls horizontal and vertical speed. These estimates are based respectively on the rate of angular expansion and vertical velocity. Our variable could account for ball catchers data of Oudejans et al. (1997), but those data could also be explained by the use of angular expansion alone. We therefore asked subjects where simulated balls would land under conditions in which both angular expansion and vertical velocity must be combined for obtaining a correct response. We found evidence for the use of angular velocity but hardly any indication for the use of angular expansion. Thus if catchers use a strategy that involves combining vertical and horizontal estimates of the balls speed, they do not obtain their estimates of the horizontal component from the rate of expansion alone.
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