Effect of the S-cone mosaic and rods on red/green equilibria

The loci of unique blue and unique yellow were measured with and without a rod bleach for various test sizes in the fovea and at 1 and 8 deg nasal and superior retinal eccentricities. Test sizes and retinal positions were selected to systematically manipulate the absolute and relative numbers of S cones underlying the test stimuli. The results revealed the following: (1) The locus of unique blue shifted to longer wavelengths as the absolute number of S cones underlying the test stimulus increased, suggesting that the S-cone neural weighting factor of the red/green (R/G) opponent model is linked to the absolute number of S cones. (2) In general, the locus of unique yellow remained invariant, although changes were observed in the superior retina. This finding indicates that either the L-to-M-cone ratio may not be invariant across all retinal quadrants or that this ratio may not determine the locus of unique yellow. (3) Rod signals affected the locus of the unique hues, especially at small test sizes, demonstrating an influence of rods on the R/G opponent mechanism.