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.

Two-criterion threshold techniques: evidence for separate spatial and temporal mechanisms?

Contrast thresholds were determined for counterphase flickering and drifting spatial gratings using pattern and flicker/motion criteria. In contrast to previous reports, the two criteria yielded contrast sensitivity functions (CSF) of similar form in the counterphase condition. However, moving gratings yielded CSF's of different form for the two criteria. These differences are probably due to eye movements.