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. Furth
er, in steady-state conditions, perceived speed is also affected in th
e same way, but to a lesser degree, by the dot density of cinematogram
s. Direction discrimination of random-dot cinematograms is enhanced wh
en dot density increases abruptly from one stimulus to the next, but i
s degraded when dot density decreases abruptly. Finally, speed discrim
ination remains constant even when density changes abruptly. The perce
ived-speed and direction-discrimination data are consistent with the M
otion Coherence theory which motivated this study, and with models tha
t include a smoothing stage similar to this theory. Of the other model
s that we consider, most predict that increasing dot density reduces p
erceived speed. The speed-discrimination data could not distinguish be
tween the different theories.