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Abstract
Lightness is the apparent reflectance of a surface, and it depends not only on the
actual luminance of the surface but also on the context in which the surface is viewed
[1-10]. The cortical mechanisms of lightness processing are largely unknown, and the
role of early cortical areas is still a matter of debate [11-17]. We studied the cortical
responses to lightness variations in early stages of the human visual system with
functional magnetic resonance imaging (fMRI) while observers were performing a demanding
fixation task. The set of dynamically presented visual stimuli included the rectangular
version of the classic Craik-O'Brien stimulus [3, 18, 19] and a variant that led to
a weaker lightness effect, as well as a pattern with actual luminance variations.
We found that the cortical activity in retinotopic areas, including the primary visual
cortex (V1), is correlated with context-dependent lightness variations.