Studies have established that the risk of myopia progression increases with time spent reading and decreases with outdoor activity, but the reasons remain poorly understood. Results from a recent study from SUNY College of Optometry, published in Journal of Vision, support the hypothesis that reading drives myopia progression by under stimulating certain visual pathways. (Poudel, S., Rahimi-Nasrabadi, H., Jin, J., Najafian, S., & Alonso, J.-M. 2023. Differences in visual stimulation between reading and walking and implications for myopia development. Journal of Vision, 0(0):08355, 1–26, doi.org/10.1167/jov.0.0.08355.)
The study proposes that myopia progression is driven by ON and OFF visual pathways that respond differently to luminance contrast. The visual system is organized in parallel ON and OFF pathways that process light and dark objects in visual scenes. The ON pathway is faster and more sensitive for processing slow motion, bright, high-resolution images to keep a steady image on the retina during movement, which is important to maximize visual acuity. The OFF pathway has better temporal resolution and elicits cortical responses that are stronger for dark stimuli. (Think of light ON, light OFF.) The researchers believe that the activation of ON pathways provides a signal to adjust the eye size based on each visual environment.
The study compared the visual input and visuomotor activity generated by subjects performing two visual tasks that are associated with different risk of myopia progression, reading (high risk) and walking (low risk). Results showed that reading generated repetitive eye movements that oscillated between the ends of each line of text and between the top and bottom of the page. Eye movements during reading were accompanied by limited changes including fixation distance, blink rate, pupil diameter and image luminance. When compared with reading, walking generated a much more variable pattern of eye movements that were accompanied by more pronounced variations in nearly all visual parameters.
ON pathway activity may be a stop signal for eye growth. As the eye grows, retinal images become sharper, brighter and more effective at stimulating ON visual pathways. Ideally, the eye should stop growing when the retinal response of the ON pathway is as strong as for the OFF pathway, a condition that requires retinal images to be sharpest, brightest and change at the frequency that best activates ON pathways. Reading reduces ON pathway stimulation in the central retina. Under this mechanism, reading for prolonged periods of time reduces the activation of ON pathways leading to eye growth, blurring vision at distance. Researchers conclude that the best approach to control myopia progression is spending not only time outdoors, but active time.
• Linda Conlin
Pro to Pro Managing Editor
[email protected]
