Abstract

Velocity discrimination is not affected by random trial-to-trial variations in contrast, indicating that observers do not confuse changes in contrast with changes in velocity. By analogy with color vision, perceived velocity could be based on the ratio of the contrast energies in two or more temporal mechanisms, producing a velocity percept invariant with contrast changes.¹ The velocity Weber fraction (ΔV/V) would then be inversely proportional to the derivative of this ratio. Velocity discrimination for sinusoidal gratings is a simple U-shaped function of temporal frequency for all velocities. Using Anderson and Burr's² estimate of the two temporal mechanisms for motion, we accurately predicted the shape of this empirical function at low contrasts (<6%) from the ratio model. At higher contrasts, the velocity function flattens because performance is limited to a Weber fraction no better than 0.03–0.05. Thus the velocity function depends on a ratio of contrast energies plus another source of noise which sets a limit on best performance.

© 1988 Optical Society of America