Similarities between visual processing of shear and uniform motion.

A number of papers have claimed that at moderate to high contrasts, sensitivity is higher for shear motion than for uniform motion. We show in a 2 x 2AFC task, designed to minimize any potential artefacts due to criterion level or response bias, that sensitivities are essentially equal for shear and uniform motion under general conditions. It has also been claimed that position tracking enhances sensitivity for shear motion. We added moving sinusoidal gratings to stationary sinusoidal gratings of the same spatial frequency and orientation, to create stimuli in which position changes and motion energy have opposite directions, to show that shear and uniform motion are both subserved by motion-energy mechanisms at speeds above 2.0 deg/s and by position tracking at slower speeds.