Motion contrast and motion integration.

When a moving aperture contains a drifting grating, the perception of aperture movement is strongly affected by the grating movement. We have studied this interaction, using a moving circular patch of sinusoidal grating matched to the background in mean luminance. The circular window, or aperture, could be defined either by an abrupt transition from a full-contrast grating to the background (hard aperture) or by a two-dimensional Gaussian fall-off in contrast (soft aperture). The grating movement could be controlled independently of the aperture motion. Subjects judged the direction of the aperture movement (i.e. the movement of the patch as a whole). We find that an illusory motion of a stationary aperture can be induced depending on the direction of the grating drift. A hard aperture presented in the fovea appears to move in the direction opposite the grating movement, demonstrating simultaneous motion contrast. However, a soft aperture presented in the periphery appears to move in the same direction as the drifting grating, demonstrating motion integration (assimilation). These results are discussed in the context of interactions between short-range and long-range motion mechanisms and with respect to the significance of boundaries in determining the figure-ground relationship of motion signals.