Abstract
Blackness and whiteness are not symmetrical in figure-ground organization. Figures are usually light, they stand out against a darker background and they are cohesive. When contrast polarity is reversed, what used to be figure becomes a cut-out or hole. Holes are typically dark. They have no figural properties, are part of the ground and appear to lie behind. The shape of a hole is defined by the border around it, but the border belongs to the surround. Take a light blue annulus on a black background: the enclosed area looks co-planar and has no surface quality of its own. Now take an Ehrenstein figure with gray radial lines: the central area looks darker than the background and stands out like a disk. We demonstrate that when the blue annulus and the gray Ehrenstein figure are combined, the central disk assumes an emergent property: the blackness becomes blacker and its mode of appearance changes to a void. Method: An 4 × 4 array of Ehrenstein figures was used in which a light blue (or gray) annulus on a black background bounded the inner tips of the grey radial lines. Subjects rated the blackness in terms of self-luminosity, density and surface color when the number, length, width, and contrast of the radial lines were varied. Results: The illusory disks of the standard Ehrenstein pattern appeared as figure, darker and denser than the background. When blue annuli were added, the central areas appeared even darker and could be perceived as holes without any density. Conclusion: Radial lines are crucial to the blacker-than-black effect, attesting to the importance of long-range effects on the mode of blackness appearance. The results suggest an explanation in terms of brightness and darkness systems involving long-range cortical mechanisms.
Supported by Senior Award from Alexander von Humboldt-Foundation, Fondazione Banco di Sardegna, ERSU, Banca di Sassari, German Research Council grant SP 67/8–2