How Long Must Defocus Persist to Induce Changes in Eye Growth?

The kinetics have been extensively studied by Winawer and Wallman [87] in chickens. Their finding that the temporal summation of defocus is highly nonlinear was not totally unexpected, as this was indicated by the experiments of Schmid and Wildsoet [64], who showed that the response of refraction to brief periods of normal vision in lens-reared chicks varied greatly with the sign of the lens. Winawer and Wallman found that multiple daily periods of defocus produce much larger changes in eye growth than one single period of the same total duration. If the single periods of lens treatment were shorter than 20 s, the lenses had no effect on eye growth. The most compelling result was, however, that the effects of positive and negative lenses did not cancel each other out: if negative lenses were worn all day, but were replaced with positive lenses for only 2 min, four times a day, the refractive state shifted still in the hyperopic direction [91]. Similarly, if monkeys wore negative lenses all day except for 1 h, the refraction remained in the range of normal animals [25]. These results suggest that the eye normally has a built-in protection against myopia develop ment. It is also striking that the time constants for inhibition of deprivation or negative lens-induced myopia by interruption of treatment are very similar among different animal models [70]. A difference between chicks and monkeys was that interruption of negative lens wearing with positive lens wearing did not inhibit myopia more than interruption without lenses. However, the positive lenses used in the rhesus monkeys were +4.5 D and may have been too strong, given that the linear range of compensation is narrower in monkeys, compared to chickens.

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