Scale-Invariance in Brightness Illusions Implicates Object Level Visual Processing

That the perceived brightness of an object depends on its visual context has been documented in many well-known brightness illusions, such as simultaneous brightness contrast and Knill and Kersten's (1991) illusion. Such illusions have led to a divergent set of explanatory theories ranging from simple lateral inhibition to theories that take into account the influence of knowledge of, and experience with, the world. Here we show that the perceived brightness of objects embedded in natural scenes is independent of the viewing distance (scale invariant) --- it does not depend on the absolute sizes of the objects on the retina (retinal size), but on their sizes in the images (image size). Moreover, a simple model that removes low spatial frequency information determined by the image size of the object accounts for the perceived brightness of the object in photographs of natural scenes. Our results suggest that brightness perception involves scale invariant computations of object properties; once object scale is determined, mid-level visual system responses can account for perceived brightness.