Stimuli
The visual motion stimulus was a vertically oriented Gabor patch
(Gaussian standard deviation = 0.8°; grating spatial frequency = 0.8
cyc/deg at 0.75 of maximum contrast) that translated horizontally across
the screen on a background of spatially filtered noise with centre
frequency of 1.0 cyc/deg, a one-octave passband and a contrast of 0.25
of maximum. To make total duration and the start/end points unreliable
as cues for judging speed, the motion path length varied from trial to
trial by randomly jittering the start and end points. For the auditory
stimulus, this was done by randomly selecting the start point on each
trial from speaker 1, 2 or 3 and the end point from speaker 9, 10 or 11.
For the visual stimulus, path lengths varied randomly in a continuous
range between 30 and 40 degrees. The location of the translating Gabor
was randomly jittered within a 20-pixel range on every frame but
maintained a constant average speed. For both stimuli, once path length
was selected, trial duration was then varied to obtain the required
speed.