Enhancing simultaneous lightness contrast by increasing surround articulation: when and why it works

It has been informally shown [Gilchrist et al, 1999 Psychological Review 106 795 - 834; Adelson, 2000, in The New Cognitive Neurosciences Ed. M Gazzaniga (Cambridge, MA: MIT Press)] that simultaneous lightness contrast (SLC) is enhanced when the dark and light surrounds of the classic display are replaced by articulated fields of equivalent average luminances. We explored this effect systematically, by comparing dark and light homogeneous and articulated surrounds in all possible combinations. Relative to their surrounds, the two targets could be double increments in luminance, double decrements, or one increment and one decrement. Observers adjusted the luminance of one target to match the achromatic colour of the other. Gilchrist et al's (1999) anchoring model accounts for increased SLC on articulated surrounds by assuming that (i) SLC occurs because in the local framework (target plus immediate surround) the incremental target is white rather than grey, and (ii) increasing the articulation of such a surround causes a stronger weighting of the local framework, thus magnifying the difference between the two targets. Our results are consistent with the first but not with the second assumption, and are discussed in the light of a modified version of the anchoring model.