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R. Ramamirtham, T.T. Norton, J.T. Siegwart, A. Roorda; Wave Aberrations of Tree Shrew Eyes . Invest. Ophthalmol. Vis. Sci. 2003;44(13):1986.
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Purpose: Because tree shrews are often used in studies of emmetropization and induced refractive error, this study examined the characteristics of wave aberrations in tree shrew eyes. Methods: A custom designed Shack-Hartmann wavefront sensor was used to obtain objective measures of aberrations. Seven juvenile tree shrews (Tupaia glis belangeri) were comfortably restrained with their head held steady while each eye (9 normal eyes, 2 control eyes in optically treated animals) was aligned to make aberration measurements in a direction perpendicular to the plane of the pupil. Five measurements were made on each eye without cycloplegia. Non-cycloplegic measures on the same axis were also made in the awake animals with a Nidek autorefractor. Results: Wavefront sensor images from all eyes showed a twin spot pattern, which indicated the presence of two distinct scattering surfaces in the retina. Analysis of the 2nd order terms from the deeper surface gave a calculated non-cycloplegic refractive error (spherical equivalent) of +2.0 ± 0.3 D, indicating that the eyes were slightly hyperopic. The calculated refractive error for the superficial surface was +3.7 ± 0.4 D. The average spherical error difference between the two surfaces was 1.9 ± 0.4 D. The RMS value of the high order aberrations at the deeper surface for all eyes was 0.17 ± 0.03 µm (4mm pupil size). The Nidek refractive error measurements were, on average 1.2 ± 0.6 D more hyperopic than the refractive error for the superficial surface. Conclusions: Juvenile tree shrew eyes are slightly hyperopic. The two surfaces corresponding to the twin spot pattern are likely the inner limiting membrane (ILM) and the tips of the inner segments of the cones. The scatter from the ILM gives rise to a spurious hyperopic component that explains, in part, the small eye artifact that is observed with other objective refractive measures (streak retinoscopy, autorefraction). The nature and magnitude of the high order aberrations are similar to human adults and young monkeys for the same 4mm pupil size. When the pupil size is scaled with the axial length (~8 mm) the aberrations in the tree shrew eye is very small in comparison to monkeys and humans.
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