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V Sharda, D Troilo, DL Nickla; Temporal Integration Characteristics of Responses to Myopic Defocus Induced by Spectacle Lenses Versus Form Deprivation in Chicks . Invest. Ophthalmol. Vis. Sci. 2002;43(13):191.
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Purpose: In chicks, the compensatory growth response to myopic defocus induced by positive spectacle lenses requires only short daily episodes of lens-wear (Schmid & Wildsoet, 1996), while responses to form deprivation and negative lenses require much longer exposures. These findings suggest that the emmetropization system "weights" the signals differently. To further explore how the eye integrates opposing visual signals we examined the responses to myopic defocus induced by prior form deprivation versus positive spectacle lenses. Methods: (1) Form deprivation was induced by monocular occluders worn for 7 days (d). For the subsequent 7 d, occluders were removed daily for 12 hrs (n=16), 4 hrs (n=8), 2 hrs (n=8), or 0 hrs (n=7). (2) Birds were fit with an occluder on day 7. For the subsequent 7 d, occluders were replaced with a +10D lens for 2 hrs per day (n=7). (3) Birds received 2, 3 or 6 hrs daily +10D lens wear alternating with unrestricted vision for 5 d. Eyes were measured using A-scan ultrasonography and retinoscopy at various intervals over the experimental period. Results: (1) In eyes recovering from form deprivation myopia, 2 hrs/day of myopic defocus is as effective as 12 hrs/day (change in refractive error: +10D vs +13D, respectively; n.s.). In eyes receiving 2 hrs of defocus per day, the choroid shows transient increases in thickness of about 130 µm (ttest: p<0.0001), for a total increase of 175 µm over the recovery period; this increase is significantly less than that in eyes receiving 12 hrs of defocus (450 µm; p=0.0001). (2) In contrast, 2 hrs/day of lens-induced myopic defocus alternating with form-deprivation is not sufficient to induce compensation to the defocus (change in RE: -1.7D). (3) When myopic defocus alternates with unrestricted vision, 6 hrs/day are required to induce nearly full compensation (2 hrs vs 6 hrs: 4.4 D vs 8.2 D; p<0.0005). The choroid shows transient thickening and thinning in response to the alternating myopic and hyperopic defocus, with 6 hrs of defocus but not 2 hrs producing a significant overall change in choroidal thickness (252 µm vs -53 µm). Conclusion: The response to myopic defocus induced by prior form deprivation is more robust than that induced by positive lenses, suggesting that the underlying mechanisms differ. Presumably, this difference is non-visual. Finally, the decrease in growth rate occurs without the "full" compensatory choroidal thickening.
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