PURPOSE. To measure the diurnal variation of spherical equivalent refractive error (mean ocular refraction or MOR) and to investigate factors contributing to it in chick, an important animal myopia model. METHODS.Nine chicks developed naturally on a 14-hour light/ 10-hour dark cycle. Optical axial length (OAL) and HartmannShack wavefront error (HSWE) measurements, including pupil size, were taken starting on day 7, at eight times during the following 32 hours. MOR was calculated for a constant pupil size from HSWE measurements.RESULTS. MOR, OAL, and pupil size showed significant diurnal variation (P < 0.0001). Most eyes showed significant sinusoidal variations in MOR and in pupil size with periods close to 24 hours. On average, MOR oscillated 60.84 diopters. OAL varied with a period not different from 12 hours. Diurnally varying MOR and OAL were correlated (P ¼ 0.0003, R 2 ¼ 0.62). However, as previously reported, the variation in OAL did not account for the variation in MOR. From these results, we derived the diurnal variation in ocular power necessary to give the measured MOR variation.CONCLUSIONS. We confirmed a diurnal variation in OAL and found diurnal variations in pupil size and MOR. Although changes in OAL explain the MOR previously observed in response to lenses and diffusers, they do not completely account for the observed diurnal variation of MOR nor for the reduction in hyperopia during normal development. We infer that the diurnal variation in MOR and normal emmetropization both result from small differences in the relative changes of OAL and ocular power. (Invest Ophthalmol Vis Sci. 2012;53:6245-6253)

Cones can be imaged longitudinally in vivo in the awake chick. The packing arrangement of cones is 40% hexagonally packed. Although AO is not necessary to visualize the cones, including the subarrays of like cones, some closely spaced cones of different types may not be resolved. Most importantly, there is a need to use a larger pupil with growth to maintain the same linear resolution in the larger eye. Novel longitudinal imaging techniques and methods in animal models are shown here to give insights into normal development and, in future, will give insights into visual disorders and diseases, including myopia.

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