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N. Tran, S. Chiu, C.F. Wildsoet; The Role of Retinal Image Contrast in Eye Growth Modulation in Chicks . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1975.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Myopia can be induced in young animals with diffusing lenses. Previous studies in chick and monkey suggest that form deprivation myopia is a graded phenomenon, the amount of induced myopia being directly related to the amount of retinal image degradation. This study is aimed to provide further insight into the inter–relationship between form deprivation and eye growth regulation by tracking the responses of young chicks to different levels of retinal contrast degradation over time. Methods:Different levels of spatial contrast degradation were generated by combining Bangerter diffusing filters (F) with clear plano lenses fitted monocularly via Velcro support rings to 4–day–old White Leghorn chicks (Gallus gallus domesticus; n=48). There were 5 treatment groups: plano lenses alone (no degradation), plano + 0.6 F, plano + 0.1 F, plano + <0.1 F, and plano + LP F (light perception only). Treatment effects were assessed at regular intervals over 14 days using high frequency A–scan ultrasonography to assess ocular growth, and an autorefractor to measure refractive changes. Results: Only the two densest diffusers resulted in significantly greater ocular growth than the plano lens control group. Specifically, changes in optical axial length (OAL; length from front of cornea to front of retina) for <0.1 and LP groups were similar to each other (p>0.05). The changes for these two groups were significantly different from those of the plano, 0.6 and 0.1 groups (p<0.001) which were not significantly different from each other (p>0.05). The vitreous chamber (VC) data showed similar trends, accounting for the majority of the OAL growth changes. The large increases in OAL in <0.1 and LP groups were coupled to large myopic shifts in refractive error (mean interocular difference ± SEM: –9.92D ± 1.99 and –7.26D ± 1.60, respectively), while the other three groups showed only very small myopic shifts (less than 2.00D). The <0.1 and LP groups showed steady increases in myopia over the first 9 days, refractions being relative stable thereafter. Conclusions:The growth responses elicited by our different diffuser treatments fell into two distinct subgroups, with only two treatments (<0.1 and LP) showing large myopic shifts in refractive error. These response patterns imply: 1. The spatial contrast threshold for form deprivation myopia in chicks is relatively high, and 2. That suprathreshold stimuli trigger increased eye growth as a fixed (all–or–none) response. The plateauing of the responses may represent an optical artifact tied to how refractive errors are calculated.
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