This demo illustrates the effect of differential depth ordering on perceived direction of motion. A clear version of a similar effect using a grating can be found at Gene Stoner’s web site (here). The demo below is an illustration of the stimuli used in: van der Smagt MJ & Stoner GR (2008). Occlusion and the solution to visual motion ambiguity: Looking beyond the aperture problem. Journal of Vision, 8, 2, 4 (doi: 10.1117/8.2.4), where we show that classification of ‘line terminators’ in intrinsic and extrinsic can only be part of the story explaining these depth-ordering effects.

Use red-green anaglyph glasses (red over the right eye). The motion information in the movie is at zero disparity. The flanking quadrants switch from crossed disparity (‘NEAR’ flankers) to uncrossed disparity (‘FAR’ flankers).

Half the dots in the central aperture oscillate from top-left to down-right and back, the other half from top-right to down-left and back. Which axis of motion do you see most? This depends upon the depth ordering of the flankers. It is much easier to see the dots oscillate along the axis of the NEAR flankers.