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Kaitlin Bunghardt, Marsha L. Kisilak, Elizabeth L. Irving, Melanie C. Campbell; Circadian Rhythms in Refraction have Multiple Components Including a Variation in the Power of the Eye. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6291.
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We and others have reported on circadian rhythms in eye length, choroidal thickness and mean ocular refraction (MOR). Here we study the relationship among eye length, MOR and rearing condition and deduce the circadian rhythm in ocular power.
Three groups of chicks were used. 13 chicks were allowed to develop naturally and 15 chicks were goggled unilaterally with a -15D goggle, both groups with a 14h light/10h dark cycle. 8 chicks were raised on the same light/dark cycle until the beginning of day 4 when they were placed in constant dark (DB). On day 7, 3 hours into the daylight cycle, goggles were removed permanently. In all groups, Hartmann-Shack measurements were obtained at 10 time points over the next 39 hours and images were analyzed at each time point for the largest common pupil size for components of refractive error. In most birds, lengths were measured (A-scan ultrasound) from the cornea to the retinal surface. After linear fits were subtracted from the time dependent data of individual birds, residual data were fit with a sinusoidal function.
In all rearing conditions, there was a significant sinusoidal oscillation in the JCC components of astigmatism. In all conditions, the eyes showed significant sinusoidal variations in eye length and MOR. The amplitude of MOR in previously goggled eyes (GE: 1.22D) was significantly higher than in control birds (CB: 0.85D) or control eyes of previously goggled birds (CE: 0.75D) or DB (0.77D). The position of the first minimum in length shifted significantly between GE and CB. The amplitude of length oscillation was significantly lower in DB than in CB (or CE). Changes in ocular power were calculated by taking the difference between measured MOR and MOR predicted by changes in retinal position. In all conditions, the eyes showed significant sinusoidal variation in the power of the eye with periods not different from 24 hours. Power variations had significantly larger amplitudes than MOR variations (CB: 4.6D, CE: 4.0D, GE: 4.3D and DB: 3.3D), being significantly larger in CE, GE and CB than in DB. In DB and GE, MOR, length and power were in phase but in CB and CE, power was out of phase with MOR and length.
These results demonstrate a diurnal rhythm in the power of the eye, affected by rearing in the dark. The results also demonstrate an interplay between the amplitudes and phases of oscillations in the position of the retina and the power of the eye, resulting in changes in oscillations of MOR. MOR amplitude is higher in previously goggled eyes compared to control eyes due to phase changes in its components.
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