Perceived motion of contrast-modulated gratings: predictions of the multi-channel gradient model and the role of full-wave rectification.

The paper examines the perception of motion in contrast-modulated sine-wave grating patterns. These non-rigid motion patterns give rise to a spatially-structured motion percept in which perceived speed varies with spatial position. We measured the perceived motion of the low contrast regions of amplitude-modulated gratings as a function of the carrier frequency, the carrier speed, the shape of the modulation signal and the modulation depth. We found that for static carriers perceived speed was greatest in the low contrast regions of the display. The speed of the low contrast regions was underestimated and perceived speed decreased as the spatial frequency of the carrier increased. When the direction of the motion of the carrier was opposite to that of the contrast modulation, the low contrast regions could appear to be stationary. The perceived speed of the contrast modulation increased with modulation depth. The brightness contrast of the carrier grating had little effect on perceived speed of contrast-modulated patterns for average contrasts of over 10%. A motion model which had full-wave rectification as an explicit pre-processing stage followed by low-pass filtering or some other selection criterion, would predict that the motion of contrast-modulated gratings should appear rigid and that the motion of the envelope should be judged correctly. The Multi-channel Gradient Model however predicts both the structured motion field experienced when viewing these second-order motion patterns and the reductions in perceived speed as a function of carrier spatial frequency and carrier speed.