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Alan C. Lu, Manbir Nagra, David S. Hammond, Christine F. Wildsoet; Effect of Extended Photoperiod on Myopia in Young Chicks. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3444.
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The question of whether the degree of myopia increases or decreases with increased photoperiod (such as with students studying at night) remains unresolved. In this study, we investigated the effect of photoperiod length on ocular growth and emmetropization to lens-induced myopia in young chicks. Responses to imposed optical defocus were compared in young chicks reared under extended day or normal diurnal lighting cycles.
Young chicks (n = 27) were reared from 3 days of age under 2 different lighting regimes: 18 h light: 6 h dark or 12 h light: 12 h dark. White light illumination was provided by phosphor-coated white LED lamps and maintained at 800 lux. Chicks were treated monocularly with a -10 D lens, with a monocular plano lens group serving as a control. Biometric (high resolution A-scan ultrasonography) and refractive error (retinoscopy) data were collected on days 0, 4, and 7 after lens treatment. Unpaired student’s t-tests were used to compare optical axial length, choroidal thickness, and refractive error (RE) of lens-treated eyes between the lighting regimes.
There were no significant differences between the optical axial length (-10 D: -0.026 mm, p = 0.825), choroidal thickness (-10 D: -0.077 mm, p = 0.112) or the RE (-10D: -0.04 D, p =0.984) of the birds reared under extended light compared to those reared under normal light. In contrast to previous photoperiod studies involving form-deprived chicks, extended day conditions did not result in significant anterior chamber depth shallowing (-10 D: -0.28 mm, p = 0.247) or vitreous chamber depth elongation (-10 D: 0.170 mm, p = 0.716).
Increasing the photoperiod by 6 h at the end of the day did not significantly affect emmetropization to imposed hyperopic defocus. This finding is in contrast to previous studies involving photoperiod manipulations in which rearing chicks in constant light disrupted compensation to lens-imposed hyperopia, suggesting that a period of dark is required for emmetropization.
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