Size constancy is a well-known example of perceptual stabilization accounting for the effect of viewing distance on retinal image size. In a recent study (Qian & Petrov, 2012), we demonstrated a similar stabilization mechanism for contrast perception and suggested that the brain accounts for effects of perceived distance on various other object features in a similar way, a hypothesis that we called General Object Constancy. Here we report a new illusion of depth further supporting this hypothesis. Pairs of disks moved across the screen in a pattern of radial optic flow. A pair comprised a small black disk floating in front of a large white disk, creating the percept of a pencil tip viewed head on. As these "pencils" moved away, they appeared to grow in contrast, in diameter, and also appeared to be getting "sharper." The contrast and size illusions replicated our previous findings, while the depth gradient (sharpness) illusion revealed a depth constancy phenomenon. We discovered that depth and size constancies were related, e.g., the two illusions were strongly correlated across observers. Whereas the illusory diameter increase could not be canceled by any degree of depth modulation, decreasing the diameter of the "pencils" during optic flow motion (thus increasing their disparity gradient) weakened the illusory depth gradient increase. This paradoxical result, as well as our other results, is explained by the General Object Constancy model: Besides using the same scaling factor to account for size, contrast, and depth variations with distance, the brain uses the apparent object size to additionally scale contrast and depth signals.
Keywords: depth constancy; psychophysics; size constancy.