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Yibin Tian; Nonlinear Choroidal Responses to Imposed Defocus in Young Chick Eyes. Journal of Vision 2007;7(15):110. doi: 10.1167/7.15.110.
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Nonlinear Choroidal Responses to Imposed Defocus in Young Chick Eyes Yibin Tian (1), Widya Mulyasasmita (2), Christine F. Wildsoet (1,3) 1 Vision Science Group, University of California, Berkeley, CA 94720, USA 2 Department of Bioengineering, University of California, Berkeley, CA 94720, USA 3 School of Optometry, University of California, Berkeley, CA 94720, USA INTRODUCTION: It has previously been reported that the ocular responses to imposed defocus are sign-dependent although the range of powers studied was relatively narrow. The choroidal and vitreous chamber responses to imposed myopic defocus were further studied here. METHODS: High-frequency A-scan ultrasonography was used to monitor ocular dimensions of one-week old chicks over six days (at 0, 1, 3, 5, 24, 48, 96, 144 hours). Treatments comprised 7 lenses ranging in power from 0 to +40 D and diffusers (n=63; 7 to 9 chicks in each group), attached to the feathers around the right eyes using Velcro except during measurements and lens cleaning (less than 5 minutes each time). Devices were attached immediately after baseline measurements. The left eyes were used as controls. Differences in choroidal thickness and vitreous chamber depth between the right (experimental eyes) and left eyes (right eyes left eyes) were used as indices of the treatment responses. RESULTS: For +1, +2, +5, and +15D lenses, there were significant choroidal thickening after 1, 3 and 5 hours, although the responses of the +5 and +15D lens groups were significantly greater than those of the +1 and +2D lens groups. Over the same time, significant shortening of vitreous chambers were recorded for the +5 and +15D lens groups but not significant for the +1 and +2D lens groups. Further changes in choroid and vitreous chamber dimensions were observed in the +5 and +15D lens groups from 24 to 144 hours. A group switched from +15 to +5 D lenses after 1 hour showed similar responses to the +5 and +15 D lens groups. The +40D lens group showed minimal change initially, but vitreous chamber elongated after 48 hours, although to a lesser extent compared to the Diffuser group. The +40D lens group also showed no significant change in choroidal thickness, while the Diffuser group showed significant choroidal thinning over the first 48 hours. CONCLUSIONS: The choroidal and vitreous chamber response to imposed myopic defocus exhibits a power independence only over a mid-range of powers, this relationship breaking down both at very low and very high powers. The differences in the response patterns for the +40D lens and diffuser implies that the underlying mechanisms are different, though both caused vitreous chamber elongations.
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