Do humans optimally integrate stereo and texture information for judgments of surface slant?

An optimal linear system for integrating visual cues to 3D surface geometry weights cues in inverse proportion to their uncertainty. The problem of integrating texture and stereo information for judgments of planar surface slant provides a strong test of optimality in human perception. Since the accuracy of slant from texture judgments changes by an order of magnitude from low to high slants, optimality predicts corresponding changes in cue weights as a function of surface slant. Furthermore, since humans show significant individual differences in their abilities to use both texture and stereo information for judgments of 3D surface geometry, the problem admits the stronger test that individual differences in subjects' thresholds for discriminating slant from the individual cues should predict individual differences in cue weights. We tested both predictions by measuring slant discrimination thresholds and stereo/texture cue weights as a function of surface slant for multiple subjects. The results bear out both predictions of optimality, with the exception of an apparent slight under-weighting of texture information. This may be accounted for by factors specific to the stimuli used to isolate stereo information in the experiments. Taken together, the results are consistent with the hypothesis that humans optimally combine the two cues to surface slant, with cue weights proportional to the subjective reliability of the cues.