Velocity discrimination at constant multiples of threshold contrast.

Velocity discrimination was measured in the fovea and at 7 deg eccentricity. Stimuli were gaussian bars and sinusoidal gratings, presented in a gaussian temporal window. Stimulus contrasts were low, fixed multiples (2.25x or 4x) of the contrast threshold. This procedure was intended to: (1) Eliminate the use of perceived contrast as a cue for velocity. (2) Stimulate only a small number of channels which are most sensitive to the test velocity. Under these conditions velocity discrimination retained a unimodal curve over velocity similar to that of Orban et al. [Vision Res. 24, 33-39 (1984)], but with steeper high and low velocity falloff. Peak discriminability shifted to higher velocities outside the fovea. Another experiment measured flicker-frequency discrimination, which was very different from grating velocity discrimination below 8 Hz.