Apparent speed of cycloidal motions

Abstract

Purpose: The apparent speed of cycloidal motions is often misperceived. The speed of a single cycloid can be overestimated by as much as 25%. Configurations with multiple spots tracing cycloids yield varying degrees of speed error. Here we have examined several factors related to the perception of speed. Methods: Test stimuli were one or two white spots (0.3 deg, 5 cd/m2) on a dark background, moving as though attached to the rim of an invisible wheel of 3.7° diameter, or along another trajectory with cycloid-like qualities. Angular velocity varied from 2 to 10 rad/sec. Subjects compared the forward speed of the invisible wheel (the speed of the horizontal component of the motion) to that of a comparison stimulus seen simultaneously. The comparison spot moved linearly in the opposite direction. An interleaved 2AFC staircase was used to determine the comparison stimulus speed that appeared to match the forward speed of the invisible wheel. Direction of motion (left/right) and spatial position (upper/lower) of test and comparison stimuli were randomized on each trial. The distance traveled and starting location of each stimulus varied pseudorandomly. Results and Conclusions: As the difference in phase between multiple points increases from 0° through 180°, the degree of speed overestimation decreases. We also tested whether it was the cycloidal trajectory, per se, or just the fluctuation in forward speed that caused the overestimation of cycloid speed. The apparent speed of stimuli containing the horizontal motion component of cycloids but not the vertical component was overestimated. The cycloid path per se is not necessary for speed overestimation.