In 1993, Agostini and Proffitt showed that perceptual belongingness (the subsumption of some sets of elements into a perceived whole) causes simultaneous lightness contrast to be seen in configurations in which the inducing elements are not adjacent to the target. The aim of the present research was to measure the strength of belongingness in determining the contrast phenomenon when the numbers of the inducing and induced elements and their relative positions are manipulated in Agostini-and-Proffitt-type configurations. In the first experiment, by using a forced choice paradigm, naïve observers indicated which gray disks arranged to form the letter T in two rows (organized with black/white inducers) appeared lighter/darker. In the second experiment, expert observers performed two nulling tasks: 1) the lightness of gray disk(s) was adjusted until it was perceived equal to that of gray target(s) aligned with white/black inducers; 2) the lightness of target(s) organized with white/black inducers was adjusted to match the target(s) organized with black/ white inducers. We found that also when there are few inducers, perceptual belongingness causes the contrast effect to propagate spatially on all the induced elements. Spatial position does not influence the induction effect. Low-level theories cannot account for these phenomena, but higher level processes must be factored in to explain them.
