Cone contrast sensitivity in human trichromats and dichromats

Our goal was to compare the long-wavelength (L-), middle-wavelength (M-) and short-wavelength (S-) cone contrast sensitivities of normal trichromats with those of deuteranopes (who lack M-cone function) and protanopes (who lack L-cone function). 8 trichromats and 8 dichromats (4 deuteranopes and 4 protanopes) served as observers. In a 4-AFC staircase procedure, their contrast thresholds were measured for detecting the position of a 1 or 16 Hz-flickering, 4° dia. target. The target was presented on a neutral grey background, which was bright enough (10.2 cd/m2 or 2.1 log scotopic tds) to desensitize the rods. A silent substitution technique was used to selectively modulate the target color contrast so that it stimulated only the L-, M- or S-cones. At 1 Hz, trichromats had significantly better L- and M-cone contrast sensitivities than deuteranopes and protanopes, respectively. But the S-cone contrast sensitivities of all three groups were similar. At 16 Hz, the L-cone contrast sensitivities of trichromats did not differ from those of deuteranopes, but their M-cone contrast sensitivities were significantly poorer than those of protanopes. Once again, the S-cone contrast sensitivities of all three groups were similar. These results are consistent with cone photoreceptor replacement occurring in the dichromat eye and influencing the sensitivity of the post-receptoral chromatic and luminance channels. The 1 Hz targets were selected to favour the chromatic channels, which are functionally altered in the dichromats; hence presumably the better performance of the normal trichromats. On the other hand, the 16 Hz targets were selected to favor the luminance channel, the sensitivity of which is predominantly an additive combination of the L- and M-cone inputs. Given a typical L-/M-cone ratio (1.5) in the normal eye and the full substitution of the missing by the remaining cone type in dichromats, then replacing L- by M-cones in protanopes (200 increase) may bring more benefit in contrast sensitivity than replacing M- by L-cones (50 increase) in deuteranopes