For much of the twentieth century, psychophysical models of color vision consisted of two stages: three linear transducers (the S, M, and L cones) followed by two opponent combinations and one additive combination of cone responses (for a review, see Kaiser & Boynton,
1996). Two-stage models of color vision, however, cannot account for multiple orthogonal directions in color space (Clifford, Spehar, Solomon, Martin, & Zaidi,
2003; D'Zmura,
1991; D'Zmura & Knoblauch,
1998; Gegenfurtner & Kiper,
1992; Hansen & Gegenfurtner,
2006; Krauskopf & Gegenfurtner,
1992; Krauskopf, Williams, Mandler, & Brown,
1986; Krauskopf, Wu, & Farrell,
1996; Krauskopf et al.,
1986; Li & Lennie,
1997; Lindsey & Brown,
2004; McGraw, McKeefry, Whitaker, & Vakrou,
2004; Webster & Mollon,
1991,
1994; Zaidi & Shapiro,
1993). Multiple orthogonal directions in color space are found when noise, distracters, modulating surround lights, or adaptation lights are presented along a color line intermediate to the cardinal directions. So, for instance, noise presented along a 45° color line will create maximal disruption along a 45° line, but minimal or no disruption for lights along a 135° line. Classic two-stage theories can not account for such findings because such models always predict maximal disruption along the cardinal axes.