Facilitatory mechanisms of specular highlights in the perception of depth

•Specular highlights facilitate visual search for shaded objects.•Inconsistent lighting between shading and highlights does not suppress search.•Highlights facilitate depth discrimination regardless of inconsistency in lighting. We investigated whether specular highlights facilitate the perception of shape from shading in a search paradigm and how highlights interact with shading to facilitate this perception. Our results indicated that stimuli containing highlights led to shorter searching time with the dependence on the light source direction (top lights make searching faster), suggesting that highlights indeed facilitate shape-from-shading processing. To examine how highlight processing interacts with shading processing, we tested unnatural stimuli for which the lighting directions for shading and highlights were inconsistent. The results indicated that unnatural highlights (bright spots) placed in a direction inconsistent with the shading either decrease or do not alter searching time. This suggests that highlights may facilitate, and not suppress, shading processing. With more physically plausible highlights generated from image-based lighting, we also observed facilitation with consistent highlights, but no change with inconsistent highlights. Finally, we examined whether highlights indeed work to facilitate depth perception in a discrimination task. The results showed that correct discrimination of depth increases when highlights are added to shading even when their lighting directions are inconsistent. These results indicate that specular highlights facilitate shading processing, and do not suppress it even when the highlights are placed in a direction inconsistent with shading. The results also elucidate the lighting constraints of the visual system.