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.
Bibliographical noteFunding Information:
We thank Katja Doerschner for helpful discussion. This research was supported by the National Institutes of Health (NIH) grant R01 EY015261. The 3T scanner at the Center for Magnetic Resonance Research, University of Minnesota, was supported by National Center for Research Resources (NCRR) grants P41 RR008079 and P30 NS057091 and by the MIND Institute. Partial support has been provided by the Center for Cognitive Sciences, University of Minnesota.