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Melanie C. Campbell, Kaitlin Bunghardt, Marsha L. Kisilak, Elizabeth L. Irving; Initial Rapid Response in Chicks Recovering from Experimental Myopia is Enhanced by Circadian Rhythm in MOR. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3921.
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Differential growth rates during the day and night have been reported in chicks, in response to imposed defocus blur. In addition, choroidal thickness and eye length vary on diurnal cycles, and we have reported a diurnal variation of mean ocular refraction (MOR) in normal chicks. The chick eye responds rapidly to defocus in the correct direction. Here we measure the change in MOR in chicks in the hours following imposed myopic retinal blur. We analysed its linear and sinusoidal components.
On the day of hatching, 15 chickens were goggled unilaterally with a -15D goggle. On day 7, 3 hours into the daylight cycle (14 hour light / 10 hour dark cycle), goggles were removed permanently. Hartmann-Shack (H-S) measurements were obtained at 10 time points over the next 39 hours (with a minimum of 4 and a maximum of 8 hours between measurements) and analysed at the largest common pupil size across birds and across time points. Measurements were taken for 8 of the birds using A-scan ultrasound from the cornea to the retinal surface. Components of refractive error were calculated. After linear fits were subtracted from the time dependent data of individual birds, residual data were fit with a sinusoidal function.
36 hours after goggle removal, the eyes showed on average 5.6D of emmetropization to the imposed myopic defocus with a linear time dependence. In addition, all eyes showed a sinusoidal variation in MOR with an average amplitude of +/- 1.2D. This sinusoidal variation in MOR had a period of 20 hours with most compensation for the defocus on average 11 hours after the lights turned on. Conversely, there was less compensation to the defocus in darkness. On average, the apparent rate of emmetropization at 4 hours (after goggle removal) was significantly higher (0.45D/hour) than the rate predicted by the linear fit (0.18D/hour). In control eyes, the significant linear dependence of MOR on time had the opposite sign and the sinusoidal variation in MOR had a significantly lower amplitude than in previously goggled eyes. The apparent rate of emmetropization in the control eyes at 4 hours after goggle removal was not different from the linear fit.
Following the imposition of myopic defocus, the eye responded rapidly with a reduction in refractive error. The refractive error underwent a significantly increased sinusoidal variation (relative to the control eye) which provided additional compensation to the imposed defocus after goggle removal (on average 0.25 D/hour over the first four hours). There was no additional compensation in the control eyes.
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