Approximation, torsion, and amodally-completed surfaces

In a stereo display simulating two rectangles, the lower frontoparallel and the upper slanted around the vertical axis, amodally completed behind a frontoparallel surface, either real or illusory, relative slant is underestimated with respect to a separate-rectangle condition. We compared with- vs. without-occluder displays. In Exps. 1 and 2 we found slant assimilation in with-occluder displays and slant contrast in without-occluder displays. In Exp. 3 we evaluated the effect of the mere presence of the occluder. Twist classification was impaired even when edge geometry hindered amodal completion, but the performance loss was larger when the patches were amodally completed. To minimize torsion, fragments are misperceived, indicating that in limiting conditions completion is mediated by approximation rather than interpolation. Slant assimilation decreases as twist increases, up to the limit for the formation of a smooth surface with torsion.