Perspective: How Might Emmetropization and Genetic Factors Produce Myopia in Normal Eyes?
Review
Siegwart, John T. Jr; Norton, Thomas T.
Optometry & Vision Science:
March 2011 - Volume 88 - Issue 3 - pp E365-E372
This article discusses the process the eye goes through to become emmetropic, that is, with good vision at near and far. This process is very active during the first year of life, and then it slows down. There's a strong genetic component, and children who have one or two myopic parents, usually have longer eyes. Animal studies have shown that visual stimuli play an important role in the development of myopia and hyperopia, and that the impact is greater the younger the animal is. Studies have shown than hyperopic defocus may cause the eye to grow longer and become more myopic. Minus lenses at near cause hyperopic defocus; but if it they are removed for 2 hours a day, at least in animals, the elongation might be prevented.
From the text:
"The animal data on which this speculative article is based suggest that there may be a normal decrease in the ability of the emmetropization mechanism to use myopia to slow axial elongation and that this may impart a natural bias toward developing myopia. Throughout evolution, it presumably was rare for juvenile or adult vertebrate eyes to encounter significant periods of hyperopia, so that once eyes had achieved emmetropia it was not an issue whether myopia became less effective in controlling eye growth. However, when humans developed environments with extensive and long-duration nearwork demands, if the accommodative mechanism did not sufficiently remove the hyperopia associated with nearwork, and if myopia, except in infants, is unable to slow the axial elongation rate, it would be natural that at least some eyes develop myopia for distant "
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