More recently, several groups have pointed out that adaptation contributes significantly to the complicated pattern of results obtained by MacAdam (
1942). In MacAdam's paradigm, the observer looked at the test color for extended periods while producing the required match. Therefore, the state of adaptation was fully determined by the test color, and as a result, each measurement for each test color was made under a different state of adaptation. Recent studies have tried to decouple chromatic discrimination at a particular test color from the state of adaptation (Hillis & Brainard,
2005; Kawamoto, Inamura, & Shioiri,
2003; Kiener,
1997; Krauskopf & Gegenfurtner,
1992; Loomis & Berger,
1979; Miyahara, Smith, & Pokorny,
1993; Rinner & Gegenfurtner,
2000; Shapiro, Beere, & Zaidi,
2001,
2003; Shapiro & Zaidi,
1992; Smith & Pokorny,
1996; Smith, Pokorny, & Sun,
2000; Zaidi, Shapiro, & Hood,
1992; Zele, Smith, & Pokorny,
2006). In these studies, the state of adaptation is typically determined by a constant background of a certain color. The test stimuli are then presented on the adapting background only briefly, not to disturb the state of adaptation. A common result of these studies is that the discrimination thresholds for a fixed test color differ considerably with the state of adaptation. To a first approximation, the difference between adapting color and comparison color determines discriminability. An analysis of MacAdam's data by LeGrand (1968) revealed that some directions of color space seem to play a special role (Boynton & Kambe,
1980; MacLeod & Boynton,
1979), and these now seem to be the “cardinal directions of color space” (Krauskopf, Williams, & Heeley,
1982). These cardinal directions of color space correspond to independent mechanisms whose neuronal substrate originates in the cone-opponent cells in the retina and the lateral geniculate nucleus (Derrington, Krauskopf, & Lennie,
1984). Krauskopf and Gegenfurtner (
1992) measured chromatic discrimination in the isoluminant plane of the DKL color space under rigorously controlled adaptation conditions. Along each cardinal line, increasing the difference between adapting and standard color increased thresholds for detecting differences in the same cardinal direction, just as predicted by Weber's Law. At the same time, thresholds for differences in the other cardinal directions were unaffected. Unfortunately, if both cardinal mechanisms were activated, the pattern of color difference thresholds was still rather complex (Krauskopf & Gegenfurtner,
1992).