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Gokhan Malkoc, Michael A. Webster, Paul Kay; Individual differences in color categories. Journal of Vision 2002;2(7):144. https://doi.org/10.1167/2.7.144.
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Color-normal observers show large and reliable differences in the stimuli they choose as unique hues, yet the variations across hues are uncorrelated, suggesting that they are constrained by independent factors. We compared the patterns of variation for both unique hues (red, green, blue, and yellow) and intermediate 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 hue angles corresponding to each of the 8 colors were estimated by varying successive stimuli in two randomly-interleaved staircases. Measurements were made for 35 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 (roughly half the standard deviation of other hues and centered near the −L pole of the LM axis), even though focal green and blue show large individual differences. This pattern of results does not support a perceptual organization in which the dimensions of red-green and blue-yellow have superordinate status.
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