Cardinal directions of color space.

Thresholds for detecting changes in color are raised following viewing a field sinusoidally modulated in color over time. This effect is highly selective. For example, thresholds for detecting reddish and greenish changes from white are raised following viewing a field varying in a reddish-greenish direction, but not after viewing one varying in a yellowish-bluish direction. Similarly thresholds for yellowish and bluish changes from white are raised following viewing a field varying along a yellowish-bluish axis but not altered by exposure to reddish-greenish variation. Thresholds for chromatic changes are not raised following viewing a field varying in luminance. Thresholds for changes in luminance are raised following viewing a field varying in luminance but not altered by exposure to purely chromatic variation. Since this selectivity is found only for these directions and not intermediate directions in color space we conclude that these directions are cardinal, that is, signals varying along these directions are carried along separate, fatiguable, second stage pathways. The results conform to the expectations of opponent process theory with the important exception that the yellowish-bluish cardinal direction is a tritanopic confusion line and not a red-green equilibrium line.