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Simon K. Rushton, Philip A. Duke; Perceived trajectory direction of an approaching object. Journal of Vision 2006;6(6):1050. doi: 10.1167/6.6.1050.
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© ARVO (1962-2015); The Authors (2016-present)
Pairs of projectiles travelling on parallel trajectories produce differing patterns of retinal motion when they originate from different distances. For an observer to recognise that the two trajectories are parallel she must “factor out” the effect of distance on retinal motion. A similar problem occurs when physically parallel trajectories originate at different lateral positions; here direction must be “factored out”. We report the results of two 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 experiment the reference and test trajectories started from different lateral positions. In the second experiment they started from different distances. From the pattern of judgements we determined a set of physical trajectories that were perceptually parallel. Perceptually parallel trajectories deviated significantly from physically parallel. The results can be explained by assuming that observers did not use explicit estimates of either distance or direction in this task. We identify a retinal speed ratio on which observers appear to be basing their judgements. We propose that the brain is sensitive to this ratio and show that it is potentially useful information for the visual guidance of action.
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