Photoreceptor directional sensitivity in normals, dichromats and S-cone monochromats

The Stiles-Crawford function (SCF) of foveal and 10 degrees peripheral photoreceptors was examined in five normal trichromats, three protanopes, two deuteranopes and two S-cone monochromats on a three-channel Maxwellian-view system. The isolating conditions for L-, M- and S-cones were confirmed by heterochromatic flicker photometry. The magnitude of the SCF was found to be concordant with the anatomical variations of the photoreceptors due to receptor type and retinal eccentricity. At the different locations, the SCF was similar in trichromats, dichromats and S-cone monochromats, but varied considerably between observers, regardless of phenotype. Large differences in the number and/or type of functioning cones - as found in normal and colour blind observers - do not seem to influence cone directional properties and as a consequence cone morphology - the SCF being mainly influenced by anatomical changes.