Illusory optical defocus generated by shaded surface texture

The human visual system is tasked with the problem of extracting information about the world from images that contain a conflated mixture of environmental sources and optical artifacts generated by the focal properties of our eyes. In most contexts, our brains manage to distinguish these sources, but this is not always the case. Recent work showed that shading gradients generated by smooth three-dimensional (3D) surfaces can elicit strong illusory percepts of optical defocus1,2 — the perception of illusory blur is only eliminated when the surface appears attached to self-occluding contours3, surface discontinuities1, or sharp specular reflections1,2, which all generate sharp (‘high spatial frequency’) image structure. This suggests that it should also be possible to eliminate the illusory blur elicited by shaded surfaces by altering the surface geometry to include small-scale surface relief, which would also generate high-frequency image structure. We report the surprising result here that this manipulation fails to eliminate the perception of blur; the fine texture fails to perceptually ‘bind’ to the low-frequency image structure when there is a sufficient gap between the spatial scales of the fine and coarse surface structure. These findings suggest that discontinuous ‘gaps’ in the spatial scale of textures are a segmentation cue the visual system uses to extract multiple causes of image structure. He visual system must somehow distinguish image properties caused by objects from image artifacts such as optical defocus. The authors show that gaps in spatial scale cause fine image texture to appear detached from coarse texture, which induces illusory percepts of defocus. These results suggest that scale continuity is a general grouping principle needed for texture to appear perceptually coherent.