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Abstract

Chapman [1] proposed that ball catchers run backwards when they see the projection of an approaching ball accelerate, and forwards when they see it decelerate. However, the threshold for
distinguishing acceleration from deceleration is too high to account for catchers' performance
in selecting the appropriate running direction. We propose an alternative cue, which is based
on a combination of estimates for the ball's horizontal and vertical speed. These estimates are
based on the rate of expansion of the ball's projection, and its angular velocity, respectively.
In order to test this alternative cue we simulated the rst 300 ms of an approaching ball on a
large screen. The rate of expansion was either simulated realistically, or else the simulated ball
either expanded too fast or too slowly. Subjects indicated whether they expected the ball to
land ahead of them or behind them. If subjects use the Chapman strategy, our manipulation
of the rate of expansion should not make a difference, because the vertical acceleration of the
projection was not varied. If they use the cue that we propose, they should be biased to respond
`ahead' when the rate of expansion is reduced and `behind' when it was increased. We did nd
such biases. However, the difference between the conditions was not as large as we predicted.
This may indicate that subjects use additional cues to estimate the ball's horizontal speed.