, Volume 37, Issue 1, pp 3-23

Constant velocity — Not perceived as such

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Summary

Most students of visual velocity perception implicitly assume that constant velocity is seen as such. This notion was challenged and shown to be inadequate in four experiments. A CRT-generated bright spot moved in a linear path according to different velocity time functions. Observers compared these movements pairwise as to their perceived constancy of velocity, or drew graphs describing perceived velocity along the track. Objects moving with constant velocity appear to move fast in the beginning and suddenly slow down to a much lower velocity. There is also a qualitative difference between the start phase and the rest. To be seen as constant, a movement must start smoothly with a certain acceleration and later level off to a constant velocity (natural motion). Such movements also look slower than others with the same average velocity. The results are taken to imply the existence of a perceptual concept of velocity which includes or presupposes a natural start. Consequences for velocity scaling, acceleration thresholds, and other motion perception studies, as well as relations to eye movements and conceptual problems, are discussed.

This study was conducted under the supervision of Professor Gunnar Johansson, Uppsala University, Sweden, and the author is indebted to him for encouragement and valuable discussions. Economic support was provided by the Swedish Council for Social Science Research and the Tricentennial Fund of Bank of Sweden through grants to Professor Johansson. The author is also indebted to Professor J. J. Gibson, Cornell University, USA, and to Professor R. M. Gottsdanker, University of California, USA, for comments on earlier versions of this paper.