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Kaitlin Bunghardt, Melanie C. Campbell, Marsha L. Kisilak, Elizabeth L. Irving; Rapid Emmetropization Responses in Chicks Recovering from Positive and Negative Lens Wear are Due to Differing Diurnal Rhythms. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3468.
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© ARVO (1962-2015); The Authors (2016-present)
Chick eyes respond rapidly to spherical defocus in the correct direction. Here we compare the change in mean ocular refraction (MOR) in chicks in the hours following imposed myopic or hyperopic retinal blur. We analyze its linear and sinusoidal components and the contribution of the sinusoidal, diurnal component to rapid emmetropization.
On the day of hatching, 15 chicks were goggled unilaterally with -15D and 8 chicks with +10D. On day 7, 3 hours into the daylight cycle (14 hr L/10 hr D), goggles were removed permanently. Hartmann-Shack (H-S) measurements, obtained at 10 time points over the next 39 hours (minimum of 4 and maximum of 8 hours between measurements), were analyzed at the largest common pupil size across birds and across time points. Axial length measurements (cornea to the retinal surface) were taken using A-scan ultrasound (except for 7 with the -15D lenses). Refractive error was calculated from the H-S patterns. After linear fits were subtracted from the time dependent MOR data of individual birds, residual data were fit with a sinusoidal function. Apparent rates of emmetropization were assessed in daylight versus darkness and compared to the linear fits.
Both sets of treated eyes emmetropized to the imposed defocus with a linear time dependence and a residual sinusoidal variation in MOR. Amplitudes were >1D on average and not significantly different for different lens powers. For the +10D and -15D treated eyes, sinusoidal MOR variation showed the greatest reduction in defocus on average at 8 and 11 hrs after the lights turned on. Conversely, there was less compensation to defocus during the dark period. In both groups, the apparent rates of emmetropization 4 hrs after goggle removal were significantly higher than the rates predicted by the linear fits. The average rates were +0.45D/hr and -0.40 D/hr in eyes treated with a -15D and +10D lenses respectively. In contrast, linear rates were +0.18D/hr and -0.12D/hr. The apparent average rates of emmetropization in the fellow eyes at the same time point (-0.08D/hr and -0.11D/hr) were not different from their linear fits (-0.03D/hr and -0.05D/hr) nor from each other. The rates of emmetropization in eyes treated with -15D lenses and fellow control eyes differed.
In chicks recovering from lens wear of either sign, the eye responded rapidly with a reduction in spherical refractive error. Differing diurnal sinusoidal variations of MOR provided additional, immediate compensation to both positive and negative signs of imposed defocus within hours of goggle removal (on average an additional 0.2-0.25D/h over the first 4 hrs). There was no significant additional compensation in the control eyes.
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