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J.J. Hunter, M.C. Campbell, M.L. Kisilak, E.L. Irving; Signals to the Direction of Defocus From Monochromatic Aberrations in Chick Eyes That Develop Lens Induced Myopia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4341.
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Purpose: Chick eyes develop myopia in response to goggles of powers up to minus 15D. Goggle defocus induces much larger amounts of root mean square (rms) wavefront error than the amount of rms error contributed by higher order aberrations in the normal eye. We wish to determine if these small monochromatic aberrations could provide a signal to the direction of defocus initially imposed by the goggles. Methods: On the first day post-hatching (day 0), chicks were unilaterally fitted with -15D goggles. Goggles were removed for brief periods for Hartmann-Shack wavefront measurements (633nm light) and retinoscopy performed on days 0 and 7. We analysed, for the day of hatching, the aberrations for 7 chicks that went on to develop refractive errors consistent with the goggle power. Hartmann-Shack images chosen for analysis corresponded to larger pupils (1.6mm) and relaxed accommodation. Point spread functions (PSFs) were calculated on the retina for the aberrations measured alone and combined with +15D or -15D of defocus. Contributions from different aberration types were assessed. The resulting retinal intensity patterns were analysed using metrics from Matrox Inspector©. Results: In the absence of monochromatic aberrations, the PSFs for +15D or -15D defocus are identical symmetric blurs. The defocused PSFs for all birds differed from the PSF produced by only diffraction and defocus. In the presence of the measured astigmatism and higher order aberrations, 6 of the 7 birds showed a clear change in symmetry of the PSF with the direction of defocus. This was confirmed by the change in principal axis of the PSFs between -15D and +15D from vertical to horizontal (3 birds), from horizontal to vertical (2 birds) or from 45 degrees to 135 degrees (1 bird). This orientation signal is a low spatial frequency signal. In the absence of astigmatism, this orientation signal remained present in some birds, but was reduced. The average spherical aberration (SA) alone produced a difference in the PSFs with defocus quantified by zero crossings. Amounts of SA and coma of the average magnitudes of those found in the chick eyes explained a high spatial frequency signal that was asymmetric with defocus. Conclusions: The higher order monochromatic aberrations in combination with the small amount of astigmatism present in the chick eyes on the day of hatching could provide signals to the direction of defocus induced by optically perfect 15D goggles. This may explain part of the response exhibited by these eyes to the -15D goggles used.
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