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Frans W. Cornelissen, Just J. van Es, Tony Vladusich; FMRI of relational color constancy in human visual cortex. Journal of Vision 2006;6(6):389. doi: 10.1167/6.6.389.
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Human subjects are able to judge colors relatively independently of illumination, a phenomenon called color constancy. Psychophysical studies indicate that color constancy is not a unitary phenomenon, and that processes of adaptation and spatial comparison may take place at multiple sites in the visual system. To explore the nature of cortical computations underlying different aspects of color constancy, we performed a functional magnetic resonance imaging (fMRI) study. Subjects judged whether a chromatic or achromatic target embedded in an array of patches (a) stayed the same across a simulated illumination change (local surface judgment), or (b) changed in a manner consistent with the illuminant change (relational surface judgment). Psychophysical evaluation of subjects? performance indicated that constancy was low in the local judgment task and high in the relational task. Color-sensitive cortical areas were localized through their preferential response to chromatic versus achromatic stimuli. Compared to a control task in which colors were viewed passively, the local judgment resulted in equally large increments in BOLD activation of color-sensitive areas. Similar results were obtained for chromatic and achromatic patterns. Earlier visual areas showed a similar pattern of results. These data suggest that task-related differences in color-area activation were aspecific and primarily associated with the use of surface color, rather than computations specifically associated with color constancy. Moreover, relational judgments were associated with additional activation of parietal and frontal regions, relative to local judgments, indicative of the spatial nature of the task.
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