3D shape-contingent processing of luminance gratings.

Studies on binocular contrast sensitivity have predominantly focused on flat, two-dimensional (2D) gratings. The underlying hypothesis of such studies is that contrast sensitivity is determined at the early stages of visual processing and is not influenced by the process of three-dimensional (3D) shape recovery. However, it can be argued that contrast detection involves identifying changes in albedo of a 3D surface rather than strictly determining the presence of 2D luminance changes. In support of this hypothesis, in three experiments we found that the relative salience of oriented luminance modulations was affected by the disparity content of the stimuli, when the same luminance distribution was assigned to a flat surface or to a surface modulated in depth. In the first experiment, in particular, we found that the relative salience of an oriented luminance grating decreased when it could be interpreted as the shading produced by a Lambertian illumination, rather than a change in reflectance. In the other two experiments, moreover, we found that this effect was reduced when the frequency of the luminance modulation did not match the frequency of the stereo corrugation. These results are consistent with the idea that the appearance of a luminance distribution is affected by the perceived 3D properties of the surface on which the luminance distribution is located.