Ideal observer perturbation analysis reveals human strategies for inferring surface orientation from texture

Optical texture patterns contain three quasi-independent cues to planar surface orientation: perspective scaling, projective foreshortening and density. The purpose of this work was to estimate the perceptual weights assigned to these texture cues for discriminating surface orientation and to measure the visual system's reliance on an isotropy assumption in interpreting foreshortening information. A novel analytical technique is introduced which takes advantage of the natural cue perturbations inherent in stochastic texture stimuli to estimate cue weights and measure the influence of an isotropy assumption. Ideal observers were derived which compute the exact information content of the different texture cues in the stimuli used in the experiments and which either did or did not rely on an assumption of surface texture isotropy. Simulations of the ideal observers using the same stimuli shown to subjects in a slant discrimination task provided trial-by-trial estimates of the natural cue perturbations which were inherent in the stimuli. By back-correlating subjects' judgments with the different ideal observer estimates, we were able to estimate both the weights given to each cue by subjects and the strength of subjects' prior assumptions of isotropy. In all of the conditions tested, we found that subjects relied primarily on the foreshortening cue. A small, but significant weight was given to scaling information and no significant weight was given to density information. In conditions in which the surface textures deviated from isotropy by random amounts from stimulus to stimulus, subject judgements correlated well with the estimates of an ideal observer which incorrectly assumed surface texture isotropy. This correlation was not complete, however, suggesting that a soft form of the isotropy constraint was used. Moreover, the correlation was significantly lower for textures containing higher-order information about surface orientation (skew of rectangular texture elements). The results of the analysis clearly implicate texture foreshortening as a primary cue for perceiving surface slant from texture and suggest that the visual system incorporates a strong, though not complete, bias to interpret surface textures as isotropic in its inference of surface slant from texture. They further suggest that local texture skew, when available in an image, contributes significantly to perceptual estimates of surface orientation.