The human visual system has a remarkable ability to accurately estimate the relative brightness of adjacent objects despite large variations in illumination. However, the lightness of two identical equiluminant gray regions can appear quite different when a light-dark luminance transition falls between them. This illusory brightness "filling-in" phenomenon, the Craik-Cornsweet-O'Brien (CCOB) illusion, exposes fundamental assumptions made by the visual system in estimating lightness, but its neural basis remains unclear. While the responses of high-level visual cortex can be correlated with perception of the CCOB, simple computational models suggest that the effect may originate from a much lower level, possibly subcortical. Here, we used high spatial resolution functional magnetic resonance imaging to show that the CCOB illusion is strongly correlated with signals recorded from the human lateral geniculate nucleus. Moreover, presenting the light and dark luminance transitions that induce the CCOB effect separately to each eye abolishes the illusion, suggesting that it depends on eye-specific signals. Our observations suggest that the CCOB effect arises from signals in populations of monocular neurons very early in the human geniculostriate visual pathway
