The perceived speed of 1 c/deg sinusoidal gratings of contrast 0.02 wa
s measured in the presence of high contrast (0.50) 1 c/deg sinusoidal
gratings (called modifiers). The modifiers drifted or were counterphas
e modulated at various temporal frequencies. The presence of a modifie
r with temporal frequencies (0 and 3 Hz) lower than the low contrast m
oving grating decreased its perceived speed while the presence of modi
fiers with higher temporal frequencies (8, 12 and 16 Hz) increased its
perceived speed. A modifier of the same temporal frequency (6 Hz) as
the standard grating had no effect upon the perceived speed of the low
contrast gratings. Moving modifiers are more effective than counterph
ase flickering modifiers in biasing the perceived speed of low contras
t gratings if they move in the same direction as the test grating and
less effective if they move in the opposite direction. Finally, a modi
fier presented in an annulus surrounding the test grating is more effe
ctive than a modifier presented in a circular patch above or below the
test grating in raising the perceived speed of low contrast gratings.
This suggests that perceived speed depends on the ratio of low and hi
gh temporal frequency signals averaged over a significant area of the
visual field. Copyright (C) 1996 Elsevier Science Ltd