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S Diether, CF Wildsoet; Stimulus Requirements for the Decoding of Myopic and Hyperopic Defocus in Chickens . Invest. Ophthalmol. Vis. Sci. 2002;43(13):188.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate if high spatial frequencies and high image contrast are fundamental to the decoding of defocus. Methods: Cone-lens-devices were monocularly applied to 5-day-old chicks for 4 days (7 D myopic defocus: 3 cm cone, +40D lens; 7 D hyperopic defocus: 2.7 cm cone, +30D lens). One of 2 Maltese cross targets covered the ends of the cones (target 1: 1.2 cyc/deg striped filler pattern, target 2: no filler pattern). Contrast was 100 or 16%. Refractive error (RE) was measured by retinoscopy; vitreous chamber depth (VCD) and choroidal thickness (CHT) were measured by A-scan ultrasonography under anesthesia. Changes in interocular difference are presented. Unless otherwise stated, results measured after 4 days of treatment are shown. Results: View OriginalDownload SlideView OriginalDownload Slide With imposed myopic defocus and high contrast, target 1 yielded better compensation than target 2, e.g. VCD showed a progressive decrease with target 1, but an early decrease only with target 2 (day 2 to 4: p=0.012, t-test; see also table). With imposed hyperopic defocus, the opposite result was obtained, i.e. better compensation with target 2 than target 1 (VCD: +0.160.02 vs. +0.060.03 mm; p=0.002; CHT: -0.020.01 vs. +0.030.04; p=0.033; RE: -2.51.7 vs. -1.31.1D; n.s.; t-tests; n = 4+4). Reducing the contrast to 16% precluded compensation to myopic defocus; eyes became myopic instead of hyperopic (target 1, 2: RE: -5.26.1D, -6.22.9D; VCD: +0.130.20 mm, +0.280.10 mm; contrast effect significant, p<0.05, Tukey/Kramer post hoc test; n = 8+8). For some chicks poor compensation reflects a change in the direction of eye growth during the treatment period. Conclusion: Middle to high spatial frequencies appear to be more fundamental to the decoding of myopic defocus than to the decoding of hyperopic defocus. There is also a threshold contrast requirement.
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