Neural activity in human V1 correlates with dynamic lightness induction.

Two circles of the same luminance will appear to have different lightness if one is embedded in a dark and another in a light surround. Known as simultaneous lightness contrast, this phenomenon demonstrates that our perceptions are not simply a reflection of the input from the retina but instead an inference about surface properties. Using functional magnetic resonance imaging (fMRI), we investigated whether the response in primary visual cortex (V1) more closely follows retinal information or perception. We induced illusory lightness changes of a disk by temporally modulating the surround luminance. In addition, we varied the luminance of the disk in order to disambiguate the fMRI response to perceived lightness modulation from the response to luminance contrast at the border of the disk. Perceptually, the disk with the lowest luminance (and the highest border contrast) had little or no induced lightness change while the disk with luminance equal to the time-averaged luminance of the surround (and the lowest border contrast) had the strongest induced lightness change. We found that neural activity in V1 strongly correlates with perceived lightness changes of the disk, suggesting significant involvement of early visual areas in processing surface lightness information.