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Martin Giesel, Thorsten Hansen, Karl R. Gegenfurtner; Chromatic discrimination of textured stimuli. Journal of Vision 2007;7(9):673. doi: 10.1167/7.9.673.
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We have previously investigated chromatic discrimination for chromatic distributions (Hansen & Gegenfurtner, 2005) and observed that discrimination contours are shaped in a manner that matches the chromatic distributions of the input. Interestingly, this is only the case for conditions when discrimination is measured at the adaptation point. At the same time, overall thresholds for color discrimination are lowest at the adaptation point and increase linearly with increasing distance of the test color from the adaptation point. Here we investigate the interplay between the effects of chromatic distributions and adaptation systematically. Discrimination thresholds were measured at various distances from the adaptation point for stimuli whose chromaticities were modulated around a test color symmetrically on a line in the DKL color space. These stimuli had a spatial frequency characteristic with an amplitude spectrum of 1/f. A spatial four-alternative forced-choice procedure was employed to measure discrimination thresholds along eight comparison directions. The four stimuli were presented against an equal-energy white background. Three of the stimuli were identical while the chromaticity of the fourth one (comparison stimulus) was varied by shifting its normalized chromatic distribution into one of the eight comparison directions. Thresholds were defined as just-noticeable shifts of the chromatic distribution. Discrimination ellipses were fitted to the thresholds. Thresholds were measured in this way at the adaptation point and at different distances from the adaptation point for eight test directions. We found that the shape of the discrimination ellipses is determined by the amplitude and direction of both the chromatic distribution and the shift away from the adaptation point. Increasing the distance between the adaptation point and the test color, induces an additional modulation of chromaticities causing an increase in thresholds. Depending on the amplitude of the shift, these elevated thresholds outweigh the effect of the chromatic distribution.
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