Effects of luminance contrast and phase difference on motion assimilation for sinusoidal gratings

When a sinusoidal (test) grating is displaced horizontally by a phase angle of 180 deg in a two-frame apparent motion display, the perceived direction of motion is ambiguous; the test grating appears to move either to the left or to the right (or to both directions). On the other hand, when the test grating is displaced by 180 deg synchronously with the inducing gratings which, presented above and below the test grating, jump unambiguously in one direction (e.g. displaced by 90 deg), the test grating always appears to move in the same direction as the inducing gratings (motion assimilation). In the present study, the effects of luminance contrast and phase difference on motion assimilation were examined. The proportion of perceived direction of motion (left or right) was measured as a function of phase difference between the test grating in the first and the second frame. The magnitude of motion assimilation was evaluated as the change in the phase difference for which the proportions of observers' response were equal (50%) for both directions. The magnitude of motion assimilation increased with increase in the contrast of the inducing gratings or with decrease in the contrast of the test grating. Also, the magnitude increased as the phase difference of the inducing gratings departed from 180 deg. Based on these results, a quantitative formulation between the magnitude of motion assimilation, and the contrast and the phase difference of the stimulus gratings was derived. Further, a model was proposed which explains the stimulus dependences of motion assimilation in terms of response-integration among local motion detectors.