Investigators in the tradition of Kay and Kempton (
1984) generally attempt to assure that their results are due to a perceptual rather than a sensory difference between their stimuli by choosing stimuli that are separated by a constant distance in some presumably uniform metric color space. However, the rigorous assumption of uniformity of any color space is unwarranted. Uniform chromaticity spaces differ from one another in several ways, including in their design criteria. For example, the color samples that define the Munsell color order system were chosen to be perceptually uniformly spaced but not necessarily to be uniform with respect to sensory color discrimination. Indeed, stimuli that are separated by constant distance in Munsell space are not generally equally discriminable from one another, as specified by CIELAB (e.g., Kuehni,
1999). More generally, the separation of two colors in JND units (by Fechnerian scaling) is known to be an unreliable guide to their perceived difference (Wyszecki,
1972), and the perceptual uniformity of the Munsell color system, even with respect to perceived hue, saturation, and value, is at best an approximation (Indow,
1988). The spacing of the colors in CIELAB and CIELUV uniform chromaticity spaces was intended to represent approximately constant-sized JNDs in all locations and in all directions of color space, based on MacAdam's ellipses (Wyszecki & Stiles,
1982), but even these spaces are both non-uniform and non-isotropic (e.g., Figure 5.6 in Shevell,
2003 and Figure 5.4.1 in Wyszecki & Stiles,
1982). The designs of CIELUV and CIELAB were heavily influenced by the desire for the uniform chromaticity spaces to be computationally simple transformations of CIE color matching functions. This means that the discriminability of equally spaced colors can vary substantially, even when they are close together. The problem is even more severe when the “within-category” stimuli are located in a different region of color space than the “between-category” stimuli (e.g., Daoutis et al.,
2006; data reanalyzed and discussed in depth in Drivonikou, Davies, Franklin, & Taylor,
2007). Furthermore, all color spaces, including the Munsell color system, CIELAB, and CIELUV, were designed based on average data, with considerable smoothing, so they will not apply to any single observer. This makes them inadequate benchmarks against which to assess categorical perception of individuals. Finally, the general model that RT should be monotonic with separation in color space is incorrect in the limit, because there is a maximum separation in color space beyond which RT is fast and constant (Nagy & Sanchez,
1990). In view of the modest sizes of some of the effects reported in this literature, it is striking that so little attention has been paid to the metric that quantifies the differences between the stimuli.