Abstract
Emmetropization was thought to result from a genetically determined process until later discoveries found that myopia could be induced, which implied that environmental factors, even modest changes, can affect eye growth under local retinal control. Humans are born with refractive errors and emmetropize into a refractive state of no error during the developmental period. For many years, it has not yet been clearly known how the eye determines the sign of defocus during the developmental period. This review aimed to study the effects of blue light on emmetropization, especially in this digital era where children are being exposed to electronic devices that emit blue light at a very young age. In view of the possible detrimental effects of blue light on the retina, the effects of blue light need to be understood and clearly studied to achieve a balance for the healthy development of the human eyes. The mechanisms of emmetropization and development of refractive errors were studied, and articles involving a wide range of animal models, such as chicks, guinea pigs, monkeys, and tree shrews, were reviewed in this review. Numerous studies collectively revealed that emmetropization is a complicated process involving the interaction of multiple factors, such as longitudinal chromatic aberration (LCA), temporal frequency, bandwidth of light wavelength, light intensity, temporal contrast, circadian rhythm disruption, and hormones. This review focused mainly on chromatic aberration. LCA causes wavelength defocus, leading to refractive changes. It has been observed that chicks, Cichlid fish, and guinea pigs became more hyperopic when exposed to short-wavelength blue light compared to those exposed to red light. The results seemed to be conflicting in tree shrews and rhesus monkeys, but the reasons are still unclear. Experiments have also shown that LCA is not essential for emmetropization, as the eyes of many species could compensate for lens-induced myopia or form deprivation in monochromatic illumination. The measurement of ocular biometry can be influenced by the lighting conditions. For instance, under red or white light, the axial length of the eye tends to increase without any refractive changes. Conversely, when exposed to blue and white light, the eye length decreases in the presence of positive lens defocus, but this effect is not observed under red light. Furthermore, the choroidal thickness increases when a positive lens is used in red and white light, but no changes occur under blue light. It is important to note that variations in the response of ocular biometry to lighting conditions have been observed among different species. Despite these alterations, the impact of lighting conditions on emmetropization, or the process of achieving normal vision, is temporary and short-term.