PERCEIVED MOTION OF CONTRAST-MODULATED GRATINGS - PREDICTIONS OF THE MULTICHANNEL 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 w ith spatial position, We measured the perceived motion of the low cont rast regions of amplitude-modulated gratings as a function of the carr ier frequency, the carrier speed, the shape of the modulation signal a nd the modulation depth, We found that for static carriers perceived s peed was greatest in the low contrast regions of the display, The spee d of the low contrast regions was underestimated and perceived speed d ecreased as the spatial frequency of the carrier increased, When the d irection of the motion of the carrier was opposite to that of the cont rast modulation, the low contrast regions could appear to be stationar y, The perceived speed of the contrast modulation increased with modul ation 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 rectificatio n as an explicit pre-processing stage followed by low-pass filtering o r some other selection criterion, would predict that the motion of con trast-modulated gratings should appear rigid and that the motion of th e envelope should be judged correctly, The Multi-channel Gradient Mode l however predicts both the structured motion field experienced when v iewing these second-order motion patterns and the reductions in percei ved speed as a function of carrier spatial frequency and carrier speed .