Abstract
Purpose :
Time outdoor is a critical parameter for emmetropization and to prevent the onset of myopia. Here, the purpose is to analyze whether oblique ray angles at the retina may cause photoreceptor light leakage and contrast reduction in low luminance conditions. If so, this will give an optical understanding to mechanisms that halt myopia and why dim light may trigger excessive eye growth.
Methods :
Reported age-dependent changes in axial length for emmetropes and myopes (Jones et al., IOVS 2005), cone inner and outer segments (Lee et al., IOVS 2015) and cone densities (Yuodelis and Hendrickson, Vision Res. 1986) are used to evaluate the degree of photoreceptor light leakage, as well as cross-coupling, in terms of visual contrast as a function of pupil size doe typical outdoor and indoor conditions. The impact of age-dependent changes in axial length, cone photoceptor density and geometry is evaluated using ray optics in a scalable eye model to determine which parameters matter most for infants, children and adolescents from 0 to 20 years.
Results :
The analysis reveals two distinct phases: (i) In the infant, post-natal photoreceptor parameters change significantly in the first 4 years of life while the foveal pit matures with denser cone packing. (ii) For the child and adolescent, photoreceptor dimensions remain stable while axial length and foveal cone density increase at a balanced pace in the emmetropic eye. A typical outdoor pupil of 2.4 mm is small enough to ensure that rays cannot leak from foveal cones whereas a larger indoor pupil increases the likelihood of leakage before traversing the entire outer segment. For the infant eye, the reduction in calculated contrast is less than 20% when the pupil increases to 4 mm. In turn, the dropoff is faster for the adult eye with denser cone packing and a corresponding contrast reduction by up to 70%.
Conclusions :
The study shows that leakage of light presents a serious challenge for emmetropization when foveal cone density reaches that of the adult eye. Myopia-induced eye growth reduces the ray angle in ideal conditions and lowers the foveal cone density thereby limiting leakage-associated contrast loss. The same concept may also explain why form-deprivation myopia with a diffuser stimulates open-loop eye growth as an ocular mechanism to search for improved contrast as the retina is moved further from the pupil.
This is a 2021 ARVO Annual Meeting abstract.