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Shaw Gillispie, Myron Braunstein, George Andersen; Projected size and projected speed as indicators of change in motion path. Journal of Vision 2007;7(9):748. doi: 10.1167/7.9.748.
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Todd (1981) showed that the trajectory of an object moving in a 3D scene can be recovered from the rate of change in its projected size and speed, together with the initial projected size and speed. This suggests that changes in either the projected size or speed functions during motion should alter the perceived direction. The present study examined an observer's perception of an object's motion path as it underwent changes in the projected size or speed functions or in both functions. Observers viewed naturalistic scenes containing simulated objects that traveled along the ground on a straight path oblique to the observer's point of view. The projected path was constant across conditions. Midway through the motion path, the projected speed function was changed from linear to exponential or vice versa, or the projected size function was changed from constant projected size to constant simulated 3D size, or both functions were changed. Observers were asked to predict where an object would cross a track that had been placed within the scene by adjusting a marker along the track. We found that a change in the speed function affected observers' perception of the motion path, but a change in the size change function did not have a significant effect. This result was unexpected because a constant 3D size constraint should be more salient than a constant 3D speed constraint. A possible explanation is that size changes affected the perceived elevation of the object in the 3D scene rather than the position in depth at which the object appeared to cross the track.
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