Abstract
Purpose :
The amount of the ocular coma aberration is systematically balanced between the cornea and crystalline lens in humans (Artal and Tabernero, Nature Photonics, 2008). This phenomenon is passive and has a strong genetic component (Tabernero et al, IOVS, 2017). Here we explore this effect in various mammalian species characterized by substantial differences in ocular dimensions but sharing considerable genetic resemblance with humans.
Methods :
We collected data on corneal and lens radii of curvature, thickness, axial length, and refractive index from existing literature for 14 mammalian species. These parameters were incorporated into ray tracing software to build up to 14 eye models. The pupil diameter was scaled to a 5 mm size using human data (axial length) as a reference. Rays subtended a 5-degree off-axis field. For each specie, we computed the induced coma for the entire eye and separately for the cornea. Coma values for the crystalline lens were derived by subtracting corneal values from the total values.
Results :
Among the 14 species examined, only the three eye models corresponding to primates showed coma compensation. The degree of compensation exhibited a strong correlation with the ratio of axial length to lens thickness (R2=0.85) (see figure). Eye models with a ratio falling within the range of 5 to 7, such as those of marmosets, macaques, and humans, displayed effective compensation. In contrast, the remaining mammalian models showed ratios ranging from 1.7 to 3.4 indicating much thicker lenses in comparison to axial length. The eyes of these species showed no coma compensation at all. An analytical calculation of coma in terms of corneal and lens shape factors was effective only for primates as the thin lens approach was still valid. The significantly different ratio of axial length to lens thickness in non-primates (smaller) minimized the curvature effect on coma and canceled the compensation effect.
Conclusions :
Among mammals, only primates showed compensation of ocular coma between the cornea and lens. Primates are also the only species that have a well-localized fovea. Perhaps, as an evolutionary step, such species developed an eye with a more sophisticated optical design improving optical quality at near retinal eccentricities.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.