Three of the five types of component patterns are defined along the three cardinal axes, termed here “S”, “L–M” (
Figure 2c), and “LUM” (not shown), which stands for luminance. The term “cardinal” implies that the colors uniquely stimulate one of the three post-receptoral mechanisms (Cole et al.,
1993; Derrington, Krauskopf, & Lennie,
1984; Krauskopf, Williams, & Heeley,
1982; Norlander & Koenderink,
1983; Sankeralli & Mullen,
1997; Stromeyer et al.,
1985). The relative cone contrast inputs to the three post-receptoral mechanisms have been estimated to be as follows:
kL c +
M c for the luminance mechanism,
L c −
M c for the mechanism that differences L and M cone contrasts, and
S c − (
L c +
M c)/2 for the mechanism that differences S from the sum of L and M cone contrasts (Cole et al.,
1993; Sankeralli & Mullen,
1997; Stromeyer et al.,
1985). The parameter
k determines the relative weightings of the L and M cone contrast inputs to the luminance mechanism, varies between observers, and was established for each subject (see below). In order to
isolate the three cardinal mechanisms from each other, the stimuli must be constructed such that the L–M stimulus does not activate either the LUM or the S mechanism, the S stimulus neither the LUM nor L–M mechanism, and the LUM stimulus neither the S nor L–M mechanism. Kingdom, Rangwala, and Hammamji (
2005) used the following combinations of
L c,
M c, and
S c to achieve this: