When perceptual belongingness over-rules lateral inhibition

It is well known that a gray surface surrounded by a light background appears darker than an identical gray surface surrounded by a dark background. This phenomenon is called simultaneous contrast and it is mostly explained by low-level mechanisms. Other examples of simultaneous contrast have been offered where higher level processes must be taken into account (Benary, 1924; White, 1979; Agostini & Proffitt, 993). Recently, Agostini & Galmonte (submitted for publication) reported a novel display of simultaneous contrast where, for the first time, perceptual belongingness and local contrast directly compete one against the other. They found that the effect is largely due to perceptual belongingness. In the present work we manipulated local and global factors to measure their relative strength on the size of simultaneous contrast. Observers were asked to perform a matching task on a gray scale placed on the same background of the tested displays. We found that when global factors are weak contrast is induced by the background intensity while when they are strengthened contrast is entirely determined by belongingness relationships. When low- and high-level mechanisms are directly competing, the latter overrule the previous. In order to account for these results, contrast models would have to factor in also higher level processes computing perceptual belongingness relationships.