Texture-shading flow interactions and perceived reflectance

The appearance of surface texture depends on the identification of edge contours in an image generated by local variations in reflectance. These edges in the image need to be distinguished from diffuse shading gradients caused by the interaction of light with surface relief. To understand how the brain performs this separation, we generated textures with orientation flows that were initially congruent with the diffuse shading flow of planar surfaces. We found that rotating textures relative to shading increased the appearance of surface pigmentation, which was well explained by an increase in the variation of local orientation fields with increasing offset of texture gradients (Experiment 1). We obtained similar findings when rotating texture flow relative to the diffuse shading of spherical surfaces with global curvature (Experiment 2). In a second set of experiments, we found that perceived pigmentation of spherical surfaces depended on the perceived orientation of the light field; rotating images of spherical surfaces reduced both perceived pigmentation (Experiment 3) and perceived global texture contrast in an objective task (Experiment 4). The dependence of perceived texture on image orientation suggests that the separation of texture flow from shading depends on an assumed light source from above bias. These findings support the view that separation of texture flow from shading, and thus perceived pigmentation, depend not only on the local structure of orientation fields in an image, but also on midlevel representations of shading and illuminance flow.