Stain on texture: perception of a dark spot having a blurred edge on textured backgrounds

When distinguishing illumination from reflectance edges, both edge blurriness and textural continuity across an edge are generally used as cues to promote the illumination-edge interpretation. However, when these cues were combined, i.e., when a dark spot having a blurred edge was placed on textured backgrounds, we unexpectedly found that the spot appears stained or painted rather than differently illuminated ("stain on texture" phenomenon). This phenomenon suggests a disruptive interaction between the visual processing of blurred edges and background texture. Our experiments showed that middle spatial-frequency components of background texture play a critical role in producing this interaction. Specifically, when a textured background had relatively stronger energy in middle spatial-frequency bands, the dark spot having a blurred edge on the textured background was perceived as differing in reflectance. The findings are discussed in view of multiple levels of visual processes: one mainly concerns low-level features such as spatial-frequency components and another is a higher-level process that takes into account the likelihood of spatial configurations in natural scenes, such as "spot shadow" in which the shadow is isolated and the shadow caster is out of sight.