The development of eye shape and the origin of lower field myopia in the guinea pig eye.

In a variety of species, the refractive state of the eye differs in different parts of the visual field (VF) with greater myopia in the region that views the ground ("lower field myopia"). We studied the refraction and eye shape of the normal guinea pig eye to determine what feature(s) underlie this visual adaptation. Guinea pigs (n=67) were either newborn or raised under incandescent light until 14, 37 or 45 days of age (20, 44, 20 and 11 eyes respectively). Refractive error was measured on-axis and 30° off-axis in the superior (SVF), inferior (IVF), temporal (TVF) and nasal (NVF) visual fields. Eye shape was analyzed from images of frozen hemisections in both the horizontal and vertical mid plane in 14 day animals, and in the vertical plane at 0, 14 and 45 days of age. Axial distances in vitro were correlated with in vivo high frequency ultrasound (r(2)=0.90). In the horizontal plane, asymmetry was caused by a ± 6° conical zone surrounding the optic nerve (12° off-axis in NVF), suggesting significant myopia in this zone. At 30°, there was no asymmetry in eye length, but the NVF was +1.7D more myopic due to asymmetry in corneal power. In the vertical plane at 30°, the IVF was more myopic than the SVF by -3.8D at 0 days, -5.9D at 14 days and -6.0D at 37 days. It resulted from vertical asymmetry in the distance of the retina from the lens center, which was longest in the mid IVF. This non-linear ramp retina was present at birth. In older animals, the peak of the ramp shifted more centrally, and the eye developed longer lengths in the extreme upper periphery (SVF) which may have been caused by the low position of the room ceiling. The vertical asymmetry in eye shape was mirrored by changes in choroid thickness, suggesting a mechanism by which eye shape was refined by vision during development. In early life, ocular growth in the vertical plane was 1.7 times higher in the center relative to the periphery, a pattern that reversed in the following month. Since emmetropization was achieved over this period, local visual cues related to clear vision may provide a switch to change ocular growth from a central to a peripheral emphasis.