Abstract
Purpose :
According to the Beer-Lambert Law, visual pigment optical density (OD) should increase linearly with increasing photoreceptor outer segment (POS) length. Mammalian studies have confirmed that visual pigment concentration increases with POS length, however this relationship may be affected by several factors including pigment concentration and retinal eccentricity. In this study, the aim was to establish the relationship between POS length and visual pigment OD in humans.
Methods :
Photoreceptor outer segments were imaged using spectral domain optical coherence tomography and visual pigment OD was measured using imaging retinal densitometry in 19 healthy participants (age range 25-82 years). POS length and OD measurements were extracted manually from 23 corresponding locations over the central 9° of the retina using ImageJ and were averaged across participants. Models based on the Beer-Lambert law were fitted to the data on a least squares basis to establish the relationship between visual pigment OD and POS length. Photoreceptor distribution was estimated using publicly available retinal topography data, and the final model assumed extinction coefficients of 4.72 m2/mol and 4.05 m2/mol for cones and rods respectively.
Results :
Visual pigment OD increased monotonically with POS length, but the relationship was nonlinear. The best fitting model not only included a parameter for POS length but also variables for separate rod and cone visual pigment extinction coefficients and molar concentrations. Specifically, the data was well described by a model where the molar concentration of visual pigment in cones was 13.9 mol/L and in rods, 8.0 mol/L.
Conclusions :
Consistent with the Beer-Lambert law, the relationship between visual pigment OD and POS length is directly proportional. However, the precise relationship is dependent on photoreceptor type, where differences between the spatial location, extinction coefficients and molar concentration of rods and cones are respected. Our results indicate that the visual pigment concentration in rods is approximately 57% that of cones. This may be due to the ubiquitous nature of artificial light that works to reduce the concentration of rhodopsin in rod photoreceptors.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.