We investigate the conditions under which stimuli in apparent (sampled) mot
ion are indistinguishable from those in smooth motion and compare this disc
rimination with the precision achieved by the visual system in interpolatin
g apparent motion. In an initial experiment, observers were required to dis
criminate smooth from apparent motion, at variable step sizes, contrasts, v
elocities, and stimulus types (broadband line or bar stimuli and grating pa
tches of different spatial frequency). Thresholds for discriminating smooth
from sampled motion were similar to 40 arc min under optimal conditions, c
orresponding to the diameter of foveal photoreceptors. The tolerated step s
ize between stations increased with velocity, more so for low- than for hig
h-spatial-frequency stimuli. Tolerated step size decreased with presentatio
n duration and with stimulus contrast, A separate experiment examined preci
sion of interpolation. Vernier offsets were produced through temporal delay
s along the trajectory of an apparent motion, and thresholds for the discri
mination of direction of offset were measured as a function of speed of mot
ion and of distance between stations of apparent motion. Perfect interpolat
ion was achieved for distances between stations of similar to2 arc min. A m
odel based on spatiotemporal filtering at an early stage of processing acco
unts well for the results of both types of experiments. (C) 2001 Optical So
ciety of America.