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G.L. DeVetten, D.B. LeBaron, A.G. Ayotte, W.K. Stell; Effects of Stimulus Orientation and Defocus on Prevention of Form–Deprivation Myopia in Chicks by Gratings in Lens–Cones . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3329.
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© ARVO (1962-2015); The Authors (2016-present)
Form–deprivation myopia (FDM) in chicks is prevented by ≤3 hours/day of unrestricted vision. We showed previously (ARVO 2005) that in–focus 1.0 cy/deg square–wave gratings, mounted vertically in lens–cones (Nevin et al., 1998), substitute effectively for unrestricted viewing of the cage environment. The purpose of the present study was to determine the effects of grating orientation and defocus on FDM–prevention, thereby defining further the tuning properties of retinal circuits that mediate this action.
The left eyes of White Leghorn cockerels were form–deprived with a removable diffuser on day 7 after hatching (P7), leaving the right eyes uncovered as controls. On P8, P9 and P10, the diffuser was replaced for 2 hr/day with a lens–cone, presenting either (1) a horizontal (H), vertical (V) or oblique (OB) 1.0 cy/deg square–wave grating, focused at 3.3 cm by a +30D spheric PMMA contact lens, or (2)the same vertical grating, in focus (+30D lens) or +7D or –7D defocused (+37D or +23D lens, respectively), all without cycloplegia. 5–12 chicks were treated in each group. On P11 all eyes were refracted, ultrasonographed with 30 MHz A–scan and weighed. Data, presented as mean (treated–control) ± SD, were analyzed by one–way ANOVA.
(1) Prevention of FDM was independent of grating orientation. Refractive error (RE) = (V) 4.9 ± 1.7 D, (H) 5.9 ± 1.6 D, (OB) 5.4 ± 1.5 D, vs (FD) 14.2 ± 3.5 D; vitreous depth (VCD) = (V) 104 ± 132 µm, (H) 229 ± 107 µm, (OB) 181 ± 53 µm, vs (FD) 365 ± 136 µm [p>0.05 among V,H,OB; p<0.05 between these and FD]. (2) Defocus by –7D, 0, or +7D (+23D, +30D, +37D lens) inhibited FDM significantly and about the same. RE = (+23D) 8.5 ± 1.3 D, (+30D) 5.8 ± 2.1 D, (+37D) 4.5 ± 1.5 D, vs (FD) 14.0 ± 2.1 D; VCD = (+23D) 104 ± 132 µm, (+30D) 229 ± 107 µm, (+37D) 181 ± 53 µm, vs (FD) 365 ± 136 µm [p>0.05 among +23D,+30D,+37D; p<0.05 between these and FD].
FDM–preventing retinal circuits are not selective for stimulus orientation. FDM–prevention with gratings defocused by –7D is likely due to their being focused by accommodation. FDM–prevention with gratings defocused by +7D confirms that even under very restricted imaging conditions such as these, plus–defocus is a powerful "stop" signal. These data are consistent with previous evidence that both in–focus and plus–defocused images drive "stop" mechanisms.
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