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KL Schmid, DR Iskander, D Brinkworth, T Ainsworth; Do Lens-induced Myopic Defocus and Defocused Printed Material Produce Similar Eye Growth Responses in Chick? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):187.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:While we know that ocular development is dependent on visual feedback, we don’t fully understand the nature of the visual information required for emmetropization. We sought to determine whether lens-induced defocus and defocus produced within printed material had similar or different effects on eye growth, i.e. were these stimuli equivalent or not to the emmetropization system Methods:The visual environments of young chicks were controlled using high powered positive lenses and cones closed with a printed complex Maltese cross target (MX). Lens-induced blur was produced, using a positive lens too highly powered for the target distance, such that 0, 2, 5, 10 or 15 D of defocus was experienced at the target plane. An image processing technique was used to approximate the defocus experienced from an out-of-focus retinal image, and computer generated printed MX targets with a similar defocus range produced. The imaging devices were applied monocularly to 8-day-old chicks (n=7 to 9 per treatment group) and worn for 4 days. Refractive errors (RE) and ocular dimensions were measured using retinoscopy and A-scan ultrasonography under isoflurane anaesthesia. Results:Regardless of how the defocus was produced, defocus greater than 2 D in magnitude produced myopia in most animals to a similar level observed with a blank target, i.e. -8.26 D (5.06 D). For lens-induced defocus measured [mean diff. (SD)] REs were 1.44 D (1.51 D), -7.44 D (6.52 D), -5.94 D (7.02 D), -6.42 D (7.91 D), and -10.72 D (5.42 D) respectively for 0, 2, 5, 10 and 15 D of defocus. Printed target defocus produced a similar result; RE data were 0.80 D (1.38 D), -5.43 D (3.33 D), -8.19 D (5.56 D), -7.85 D (3.03 D), -6.38 D (2.11 D). In all cases the myopia was caused by axial elongation. The response to lens-induced defocus was the more variable (some animals went long-sighted), presumably because the magnitude of the induced defocus could not be as accurately determined as that of printed targets (varying slightly with lens to target distance and the initial refractive error of the eye). Conclusion:An out-of-focus retinal image whether produced by optical defocus or printed defocus results in myopia when emmetropization cues are limited. Defocus greater than 2 D produced myopia in some animals regardless of how it was produced.
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