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.