S-cones differ from LM-cones in their ORG sensitivity and kinetics.

Abstract

Purpose : Electrophysiology and psychophysics suggest that S-cones have different light response properties - sensitivity & kinetics - compared to LM-cones. Here, we examine these features of the ORG response in S-cones and compare them against LM-cones.

Methods : A line scan AO-OCT system was used to map the ORG response of S-cones to a broad range of bleaching stimuli in two subjects. Cone spectral types were first mapped at 1.5° eccentricity by recording OCT volumes at 20 Hz with 660±10 nm (10.3 x 10⁷ photons/µm²) stimuli. The S-cone response family was obtained with 428 ± 12 nm stimuli, varied in photon density between 0.046 to 14.7 x 10⁶ photons/µm². OCT volumes were processed to obtain changes in optical path length (ΔOPL) in each cone. The spectral type was assigned based on the differential ΔOPL to 660 nm stimulus. S-cone ΔOPL for different bleach strengths was subjected to componential analysis, as described in Pandiyan et al. 2022, and compared with LM-cones.

Results : Similar to LM-cones, S-cones exhibit the three ORG components: a rapid shrinkage denoted by Comp0 and two slower elongation components, Comps 1 & 2, that differ in their time constants and sensitivity. The S-cone maximum ΔOPL was saturated at ~220 nm; however, it was 60% lower than that of LM-cones ΔOPL, which reaches at least 556 nm in response to 530 nm stimuli at the same eccentricity. The outer segment length in S-cones was ~ 6 - 14.4% smaller than LM-cones and cannot account for the lower ΔOPL saturation. In S-cones, Comp1 had a time constant of 143 ms and an amplitude of 275 nm, while Comp2 had a time constant of 750 ms and an amplitude of 200 nm. The time constant of Comp1 was ~2-fold longer, and of Comp2 was 25% shorter in S-cones compared to LM-cones. Comp2 in S-cones appeared to be more sensitive to light than in LM-cones and was observed at a relatively low stimulus strength (0.46 x 10⁶ photons/µm²). In contrast, Comp2 was observed in LM-cones at a higher light level (23.9 x 10⁶ photons/µm²). Surprisingly, Comp2 disappeared at the highest light level in S-cones, contrary to its behavior in LM-cones. The disappearance may owe to a novel shrinkage component that only appears in response to the strongest stimuli.

Conclusions : S-cone ORG components differ quantitatively in their saturating amplitudes and kinetics compared to LM-cones. These results provide a basis for understanding mechanistic differences between LM and S-cones in living humans.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.