Perceptual belongingness and spatial propagation of lightness contrast on the unified elements

In the domain of lightness perception, Galmonte & Agostini (1998) have demonstrated that perceptual belongingness over-rules retinal lateral inhibition: When the two explanations directly compete one against the other to account for simultaneous lightness contrast phenomenon, the former one wins. The aim of this work was to further investigate the relationships between perceptual belongingness and lightness contrast by measuring the strength of perceptual belongingness in determining simultaneous lightness contrast when 1. the number of inducing and induced elements and 2. their relative spatial inclusion is systematically manipulated in Agostini & Proffitt-type configurations (1993). Observers had to perform a nulling task. They adjusted the luminance of the middle reflectance elements spatially arranged (according to specific laws of perceptual organization) with the low reflectance inducing elements to cancel the induction effect out. It has been found that even when the number of induced elements is considerably larger than that of inducing elements, perceptual belongingness causes simultaneous lightness contrast to spatially propagate on all the unified elements, independently from the inclusion factor. This finding has important implications for lightness perception theories. Furthermore, it suggests that perceptual belongingness can be indirectly measured by testing the size of simultaneous contrast.