Functional magnetic resonance imaging of brightness induction in the human visual cortex.

A grey surface on a bright background appears to be darker than the same surface on a dark background. We used functional magnetic resonance imaging to study this phenomenon called brightness induction. While being scanned, participants viewed centre-surround displays in which either centre or surround luminance was modulated in time. In both cases, participants perceive similar brightness changes in the central surface. In the region of the visual cortex encoding this central surface, both modulations evoked comparable functional magnetic resonance imaging responses. However, the surround modulation signal showed a considerable delay relative to the onset of the brightness percept. This suggests that, although correlated, the functional magnetic resonance imaging signals do not bear a direct relationship with perceived brightness. We conclude that retinotopically organized visual cortex does not represent brightness per se.