The visual discrimination of relative surface orientation.

In a series of three experiments, observers' discrimination thresholds for relative surface orientation were measured under full cue conditions, in which surfaces were specified by multiple sources of optical information including shading, texture, motion, and binocular disparity. The results revealed that observers' sensitivity to relative surface orientation varies for different types of structural configuration. Weber fractions were approximately 8% for planar patches connected to form a dihedral angle. They increased to 11% for planar patches that were spatially separated, and jumped to over 26% for patches that were part of a smoothly curved surface.