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Guang Zeng, Hannah E. Bowrey, Jun Fang, Yanhua Qi, Sally A. McFadden; The Development And Source Of Lower Field Myopia In The Guinea Pig Eye. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3462. doi: https://doi.org/.
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In a wide variety of species, including humans, the refractive state of the eye varies in different regions with greater myopia in the zone which views the ground ("lower field myopia"). This asymmetry has been proposed as an adaptation for viewing near objects with varying theories as to its underlying source. We studied the development in refractive error and eye shape of the normal guinea pig eye to determine what feature(s) underlie this visual adaptation.
Guinea pigs (n=43) were either newborn (within 12 hrs of birth) or raised under incandescent light until 14 or 37 days of age. In each group, the refractive error was measured off-axis in the superior (S), inferior (I), temporal (T) and nasal (N) visual fields under cycloplegia. Eyes were rapidly frozen and sectioned in either the horizontal or vertical plane. The freezing method was evaluated by comparing distances measured in vivo with high frequency ultrasound on the optical axis with the same distances measured in frozen sections in 14 day-old animals. Eye shape was analyzed from high resolution images of the section with maximum lens thickness and relative to the center of the lens axes. Eye length was defined as the distance from the front of the cornea to the back of the retina through the lens center.
Axial distances from frozen sections were highly correlated with those from ultrasound (r2 = 0.90). Lower field myopia was present within 12 hrs of birth (I-S difference in refractive error at 0, 14 and 37 days of: -3.8 D; -5.9 D; -6.0 D respectively) and was caused by asymmetry in eye length (I-S difference at 0, 14 and 37 days of: 78 µm; 87 µm; 87 µm respectively). Asymmetry in the thickness of the choroid was present from birth and contributed ~44% to this ramp shape. The cornea was symmetrical and little astigmatism was present, even at birth.
In the guinea pig, significant lower field myopia is due to a ramped retina and asymmetrical choroidal thickening, both of which were present within 12 hours of birth and maintained during emmetropization. Therefore, lower field myopia is not a direct consequence of close vision of the ground during development as it is likely present at birth, but has evolved as a useful adaption to the ground plane.
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