The visual discrimination of bending.

The sensitivity of observers to nonrigid bending was evaluated in two experiments. In both experiments, observers were required to discriminate on any given trial which of two bending rods was more elastic. In experiment 1, both rods bent within the same oriented plane, and bent either in a frontoparallel plane or bent in depth. In experiment 2, the two rods within any given trial bent in different, randomly chosen orientations in depth. The results of both experiments revealed that human observers are sensitive to, and can reliably detect, relatively small differences in bending (the average Weber fraction across experiments 1 and 2 was 9.0%). The performance of the human observers was compared to that of models that based their elasticity judgments upon either static projected curvature or mean and maximal projected speed. Despite the fact that all of the observers reported compelling 3-D perceptions of bending in depth, their judgments were both qualitatively and quantitatively consistent with the performance of the models. This similarity suggests that relatively straightforward information about the elasticity of simple bending objects is available in projected retinal images.