Neural correlates of shape from shading.

Psychophysical studies have shown that human observers resolve shape-from-shading ambiguities by assuming that light is coming from above-left. Using event-related potentials (ERPs), we measured the processing time of the perception of an ambiguous shaded pattern. We found that the N2 component followed the change of perceived shape with stimulus orientation. We also found that the P1 component in occipital and temporal areas was correlated with the observers' idiosyncratic bias for light source position. The precocity of the correlated ERP components suggests that the light source is represented early in the visual system. Altogether, our results indicate that shape-from-shading is a mostly bottom-up mechanism.