VISUAL-DISCRIMINATION OF LOCAL SURFACE DEPTH AND ORIENTATION

Many theoretical analyses of 3-dimensional form perception assume that visible surfaces in the environment are perceptually represented in t erms of local mappings of metric depth and/or orientation. Although th is approach is often taken for granted in the study of human vision, t here have been relatively few attempts to demonstrate its psychologica l validity empirically. In an effort to shed new light on this issue, our research has been designed to investigate the accuracy with which observers can discriminate metric depth and orientation intervals on s moothly curved surfaces. Observers were presented with visual images o f surfaces defined by shading and/or texture, on which two pairs of po ints were designated with small dots. In Experiment 1, their task was to identify which pair of points had a greater difference in depth; in Experiment 2 they were required to judge which pair had a greater dif ference in orientation. The Weber fractions obtained for these tasks w ere 10 to 100 times greater than those that have been reported for oth er types of sensory discrimination, indicating that the perception of metric structure from these displays is surprisingly coarse grained.