Edwin Land emphasized the importance of global influences in color vision. He demonstrated that objects stimulating the retina identically could have totally different colors depending on their surrounding spectral illumination. He proposed a model to handle such global effects called the Retinex model, implying that it depended on both the retina and the cerebral cortex. The model corrects for changes in the spectral characteristics of the illuminant and therefore facilitates color constancy. Color constancy allows us to see a green plant as green in sunlight or artificial light, usually tungsten light, which emits many more long wavelengths than sunlight. The key element in his model is that the signals from each cone mechanism have to be normalized across the visual scene before being compared with another cone mechanism to establish color. It implies that Hering's paired comparisons occur after a stage in which each cone mechanism's response is normalized over a large part of the visual field. Such wide field interactions are quite possible in visual cortex as or before the paired comparisons of cone signals occur.
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