The use of direction and distance information in the perception of approach trajectory.

A pair of projectiles travelling on parallel trajectories produce differing patterns of retinal motion when they originate at different distances. For an observer to recognise that the two trajectories are parallel she must "factor out" the effect of distance on retinal motion. The observer faces a similar problem when physically parallel trajectories originate at different lateral positions; here direction must be "factored out". We report the results of a series of experiments designed to determine if observers can do this. The observers' task was to judge whether the direction of travel of an approaching sphere (test trajectory) was to the left or right of parallel to a previously shown trajectory (reference trajectory). In the first set of experiments the reference and test trajectories started from different lateral positions. In the final experiment they started from different distances. From the pattern of judgements we determined a set of perceptually parallel trajectories. Perceptually parallel trajectories deviated significantly from physically parallel. We conclude that under circumstances comparable to those encountered when catching a ball in flight, observers do not have access to accurate estimates of trajectory direction.