Responses to Lightness Variations in Early Human Visual Cortex

SummaryLightness 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, 2, 3, 4, 5, 6, 7, 8, 9 and 10. The cortical mechanisms of lightness processing are largely unknown, and the role of early cortical areas is still a matter of debate 11, 12, 13, 14, 15, 16 and 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 and 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.