StarTrek illusions demonstrate general object constancy

Abstract

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 new illusions of depth further supporting this hypothesis. Pairs of disks moved across the screen in a pattern of radial optic flow. A pair comprised either a small black disk floating in front of a large white disk, creating the percept of a pencil tip viewed head on (thus called the "pencil" stimulus), or a white disk floating upper left to a black disk, creating the percept of a white disk casting a shadow (thus called the "shadowed disk" stimulus). For the "pencil" stimulus, as the "pencils" moved away they appeared to grow in contrast, in diameter, and to be getting sharper; for the "shadowed disk" stimulus, as the disks moved away they also appeared to grow in contrast, and to be separating farther away both laterally (size illusion) and in depth. The contrast and size illusions replicated our previous findings, while the depth gradient (sharpness) illusion and the depth separation illusion manifested a depth constancy phenomenon. We discovered that depth and the size constancies were related, e.g., the size illusion and the depth gradient/separation illusions were strongly correlated across observers. On the one hand, the illusory diameter/separation increase could not be canceled by any degree of depth modulation. On the other hand, decreasing the diameter of the pencils during optic flow motion (thus increasing their disparity gradient) could affect the illusory depth gradient increase; decreasing the separation between the disks and their shadows during optic flow motion could cancel or even reverse the illusory depth separation increase. These results are explained by the General Object Constancy model: besides using the same scaling factor to account for size, contrast, and depth variations with viewing distance, the brain uses the apparent object size to additionally scale contrast and depth signals to yield the perceived contrast and depth.