We studied the cortical responses to illusory luminance variations induced by the Cornsweet effect, as well as actual luminance variations. Both the illusory and actual luminance variations were composed of two flanking territories and a contrast border between them. In the Cornsweet effect, two equiluminant flanking territories appear to have different luminances due to a region composed of two opposite sign luminance gradients and a contrast border between them. In the "actual" condition the luminance on both sides of the border were uniform and counter-phase modulated in time around the mean luminance (square-wave modulation, 0.5Hz). In the "illusory" condition, the luminance of the flanking territories remained constant while the gradients within the Cornsweet region reversed their signs in time (square-wave modulation, 0.5Hz.) This modulation induced the illusory percept of an animated luminance change across the flanking territories, which were the regions of interest. We measured reliable BOLD modulation in early visual cortical regions of interest, particularly in V2 and V3, in response to both illusory and actual luminance variations. The magnitude of the BOLD response to the illusory luminance variation was similar to the response to the actual luminance variation in those areas. Our results suggest that neurons in early visual cortex are sensitive to illusory luminance changes engendered by distant scene cues.

We also addressed the role of perceived 3D configuration on the fMRI signal, where the Cornsweet-like luminance pattern is induced either by surface pigmentation, or by surface curvature and spatial layout of light sources (Knill and Kersten, 1991).