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X Zhu, JA Winawer, JW Choi, J Wallman; The Effect of Defocusing Lenses Depends on the Temporal Integration Characteristics of the Emmetropization Mechanism . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2929.
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Purpose: In normal viewing, retinal regions experience periods of myopic defocus (from objects beyond the point of regard) interleaved with periods of hyperopic defocus (from objects in front of the point of regard). To understand how these periods of defocus are integrated, we alternated myopic and hyperopic defocus. Methods: Chicks wore +6 D lenses while restrained for 30 min 4 times/day in a small drum (10-cm; -14 D hyperopic defocus if no accommodation), made of a patterned perforated scrim, which was centered in a large drum (60-cm; +2.7 D myopic defocus if no accommodation). When the inside wall of the inner drum was illuminated, it appeared opaque and imposed hyperopic defocus; when, instead, the outer drum was illuminated, the inner drum appeared transparent and myopic defocus was imposed by the outer drum. Results: As expected, when only the inner or outer drum was illuminated, eyes changed in opposite directions: Hyperopic defocus imposed by the small drum doubled the normal increase in vitreous chamber depth (from 80 to 190 µm) and induced 3 D of myopia in the experimental eyes; myopic defocus imposed by the large drum stopped the normal increase in vitreous chamber depth (-20 µm) and induced a small hyperopia relative to the untreated fellow eye. If lenses were manually switched every 30 min, all measures went strongly in the hyperopic direction, as though only plus lenses had been worn. If the illumination was alternated 10 times/sec, eyes were intermediate in refraction, choroidal thickness and vitreous chamber depth (a significant 52 µm increase in vitreous chamber depth). Conclusion: These preliminary results suggest that the response to episodes of myopic or hyperopic defocus depends on the episode duration. If episodes last minutes, the eye compensates for the myopic defocus and ignores the hyperopic defocus; if intervals are very brief, the two signs of defocus tended to cancel. These temporal integration effects may help explain (1) why brief accommodation events may not influence lens-compensation and (2) a child's total reading time may be a poor predictor of myopic progression.
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