Contrast Sensitivity for Lime Purple and Cyan Orange Gratings

Measurements of contrast sensitivity are well established for modulations in luminance, red-green and yellow-blue color directions in color space. Relatively less work has been carried out, however, to measure contrast sensitivity in other color directions or to assess the effect of the average color of the stimulus has been largely ignored. In this study we have measured conventional contrast-sensitivity functions for iso-luminant red-green and yellow-blue gratings and for gratings in two other color directions that we have nominally called lime-purple and cyan-orange (selected so that they bisect the red-green and yellow-blue directions in Boynton-MacLeod cone space). The measurements were repeated for modulations of cone contrast on chromatic fields (for example, we measured sensitivity to modulations in yellow-blue for gratings whose means colors were either yellow, neutral or blue). Our data show that the contrast-sensitivity functions for all chromatic directions are consistently low-pass irrespective of the average colour of the stimulus. However, we find an interesting asymmetry in that sensitivity to yellow-blue contrast is reduced for blue gratings relative to yellow gratings. This asymmetry is not observed for the red-green colour direction and asymmetries in the lime-cyan and purple-orange colour directions are consistent with an effect of S-cone adaptation. If sensitivity to chromatic contrast depends upon the mean color of the image then sophisticated models of the contrast-sensitivity function (that include, for example, parameters to describe local mean color of the stimulus or image) may be required.