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 hav e studied this interaction, using a moving circular patch of sinusoida l grating matched to the background in mean luminance. The circular wi ndow, or aperture, could be defined either by an abrupt transition fro m a full-contrast grating to the background (hard aperture) or by a tw o-dimensional Gaussian fall-off in contrast (soft aperture). The grati ng movement could be controlled independently of the aperture motion. Subjects judged the direction of the aperture movement (i.e. the movem ent of the patch as a whole). We find that an illusory motion of a sta tionary aperture can be induced depending on the direction of the grat ing drift. A hard aperture presented in the fovea appears to move in t he 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, demonst rating motion integration (assimilation). These results are discussed in the context of interactions between short-range and long-range moti on mechanisms and with respect to the significance of boundaries in de termining the figure-ground relationship of motion signals.