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We measured phase shifts between Long-wavelength cone (L-cone) and Middle-wavelength cone (M-cone) signals as well as sensitivity in the luminance pathway either following a cone-silent substitution of colored background or on a steady colored background. In background substitution, the phase shifts between L- and M-cone signals varied only slightly depending on the substituted color, whereas marked elevation of the threshold following the substitution of colored background was found. In contrast, the phase shifts, as well as threshold, varied largely, depending on the background color in the steady background. These facts suggest that suppression by the cone-opponent process for background color substitution is different from the one for a steady colored background.

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.