Does physical contrast affect global induction in Agostini & Galmonte's reversed lightness-induction Necker cube?  

Galmonte A., Agostini T., Righi G.
First author's affiliation: Department of Psychology and Cultural Anthropology, Verona, Italy

In 2002 Agostini & Galmonte offered a reversed-contrast display where a gray target entirely surrounded by a black area appears darker than an identical gray target entirely surrounded by white. This effect can not be attributed to assimilation phenomena; moreover, it occurs because of higher-level grouping factors: when both higher-level factors and lower-level factors affect a configuration simultaneously, the former prevail. Hence, the authors showed that the lightness induction determined by perceptual belongingness prevails against retinal lateral inhibition. The purpose of this work was to investigate whether the lightness induction produced by global grouping factors does change as a function of the physical contrast between induced and inducing elements. We systematically manipulated the relative physical contrast among the regions forming the original Agostini & Galmonte display. Observers had to judge the lightness of both the inducing and induced element on a Munsell Scale. From the results we can conclude that: 1. the global induction overcomes local induction for all the tested physical contrasts, 2. it is stronger for decrements, 3. the amount of the global induction does not depend on the size of physical contrast, 4. the global induction seems to be modulated by the physical contrast of increments only.