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H.C. Howland, T. Li, C. Wahl; Comparison of Effects of Varying Diurnal Light Periods on the Growth of the Chick Eye . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1976.
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Purpose: It is known that both constant light (CL) and constant darkness (CD) cause hyperopia, shallowing of the anterior chamber and flattening of the cornea. We wished to see if these effects were symmetrical with respect to equally short periods of light or darkness. Methods: The chicks (Gallus gallus, Cornell K Strain) were raised in light regimes of 0:24 L:D, and 1:24, 4:20, 6:18, 8:16, 12:12,18:6, 20:4, 21:3, 22:2, 23:1, as well as 24:0 L:D since the first day of hatching, with 10 to 16 chicks in each group. Radius of corneal curvature, refraction, and axial lengths of the components of the eye were measured after two weeks of treatment. Results: The eyes of chicks raised in short periods of light per day resembled those of chicks raised in very short periods of darkness per day. Constant darkness had basically the same effect as constant light on the development of the chick eye. However, for other ratios of L:D the effects were not symmetrical. There were virtually no constant light effects in chicks which received 4 or more hours of darkness per day, while chicks raised with 6 hours or longer of illumination appeared to show the normal corneal curvature and refraction. Like the chicks raised under shorter than 4 hours of darkness, vitreous chamber depths of the chicks with shorter than 6 hour light periods are significantly deeper. Unlike the CL chicks, the intraocular pressures of the CD chicks still show a diurnal rhythm. Conclusions: Short periods of light daily and otherwise darkness have basically the same effects as short periods of darkness and otherwise light on the ocular development of the chick. But the effects were not symmetrical. Vitreous chamber enlargement under CD occurred even without visual input, and intraocular pressures (IOPs) showed diurnal changes under CD.
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