The role of depth stratification in the solution of the aperture problem

When a plaid pattern composed of a stationary vertical grating and a horizontally drifting diagonal grating is shown behind a circular aperture, the pattern appears to move coherently in a vertical direction. When the bars of the stationary grating are narrower than those of the moving grating, only the latter is seen to move, in a direction orthogonal to its orientation (ie diagonal); but when the bars of the stationary grating are wider than those of the moving grating, vertical motion of the whole plaid predominates. It is argued that, in the absence of occlusion information, the motion of a plaid within an aperture depends on the unambiguous displacement of inner line terminators at the crossings of the two gratings. Relative motion and differences in bar width between the two gratings provide information about which set of bars is in front of the other. When these sources of information are consistent with each other, separation of the two gratings in depth occurs: inner line terminators no longer perceptually exist and the direction of motion becomes determined only by terminators at the edges, which causes a shift from vertical to orthogonal motion. Differences in luminance also provide (asymmetrical) information about depth relationships: when darker bars occlude lighter bars, the probability of orthogonal motion increases as a function of the difference in luminance, whereas when lighter bars are over darker bars, vertical motion prevails.