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Gokhan Malkoc, Paul Kay, Michael A. Webster; Individual differences in unique and binary hues. Journal of Vision 2002;2(10):32. doi: 10.1167/2.10.32.
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We compared the pattern of inter-observer variation in unique hues (red, green, blue, and yellow) and binary hues (orange, purple, blue-green, and yellow-green), to test the relative status of different color categories. Stimuli were moderately-saturated, equiluminant pulses on a gray (30 cd/m2) background and fell along a circle within a threshold-scaled version of the LM vs S chromatic plane. The chromatic angle corresponding to a subject's “best example” of each color was estimated by varying successive stimuli in two randomly-interleaved staircases. Measurements were made for 73 observers, all screened for normal color vision. If orange and purple represented color categories derived from more fundamental primaries, the focal choices for these hues might plausibly be correlated with an observer's unique hue settings (with red and yellow or red and blue, respectively). Instead, clear correlations did not emerge for any of the hue pairs. Settings of yellow-green and blue-green presumably reflected judgments of category boundaries rather than focal colors. These boundaries were again uncorrelated with the loci of the focal components. Moreover, the variance in settings did not distinguish unique and intermediate hues. Surprisingly, the lowest variance occurred for blue-green even though focal green and blue show large individual differences. In further measurements subjects selected focal colors from a palette of stimuli that varied in both hue and lightness (over a range of contrasts). Luminance contrast was a significant component of some focal hues. Individual differences in luminance were not strongly tied to the variations in hue angle, and showed stronger interobserver correlations, suggesting that they may depend in part on a common influence. The pattern of results does not support a perceptual organization in which the dimensions of red-green, blue-yellow, and bright-dark have superordinate status.
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