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Gennady Livitz, Ennio Mingolla; The Combined Effect of Chromatic Contrast and Chromatic Assimilation Produced by a Purple Surround on an Achromatic Target. Journal of Vision 2010;10(7):398. doi: 10.1167/10.7.398.
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Chromatic assimilation and chromatic contrast are two different types of spatio-chromatic interactions, which are rarely observed simultaneously. These phenomena are normally considered mutually exclusive, as they shift the chromaticity of an “induced” region in opposite chromatic directions: away and toward the chromaticity of the surround, respectively. In our displays we observed a shift of chromaticity of a target achromatic field induced by a uniform purple surround in a direction in color space that can be interpreted as the combined effect of chromatic contrast and chromatic assimilation of the surround color. We measured this combined effect by varying stimulus size, stimulus eccentricity, and binocular disparity of our stimuli. Our results show that chromatic assimilation and chromatic induction do not always cancel each other and may lead to perceptual shifts in chromaticity in a direction in color space that does not coincide with the line formed by the color of the surround and its chromatic complement. For example, due to the impact of a purple surround, a region that would look gray without chromatic surround does not look green or purplish, but is perceived as blue if viewed from a certain distance. We explain the observed effects by the structure of receptive fields of the neurons that encode spatio-chromatic interactions and by combination of individual induction effects produced by inputs representing primary chromatic signals on the output of double-opponent neurons.
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