Dependence of speed and direction perception on cinematogram dot density

In the present experiments, we find that with abrupt decreases in dot density of random-dot cinematograms, perceived speed decreases, while with abrupt increases in dot density, perceived speed increases. Further, in steady-state conditions, perceived speed is also affected in the same way, but to a lesser degree, by the dot density of cinematograms. Direction discrimination of random-dot cinematograms is enhanced when dot density increases abruptly from one stimulus to the next, but is degraded when dot density decreases abruptly. Finally, speed discrimination remains constant even when density changes abruptly. The perceived-speed and direction-discrimination data are consistent with the Motion Coherence theory which motivated this study, and with models that include a smoothing stage similar to this theory. Of the other models that we consider, most predict that increasing dot density reduces perceived speed. The speed-discrimination data could not distinguish between the different theories.