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Melanie C W Campbell, Zheng Shao; Rates of change of ocular properties during emmetropization in the developing chick eye vary linearly with retinal blur. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5841.
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© ARVO (1962-2015); The Authors (2016-present)
Emmetropization is an active process involving retinal feedback. We examine changes in ocular properties during emmetropization in the normal chick eye to determine which changes are proportional to the retinal blur due to defocus.
Mean ocular refraction (spherical equivalent, MOR), dioptric length (K′), eye power, pupil radius and cornea and lens powers were calculated from literature values of ocular components (including our results). They and their rates of change were then fitted versus MOR, rate of change in optical axial length (OAL-cornea to retina) and calculated retinal blur due to defocus. The relationships of corneal radius, corneal and lens powers to other parameters and to angular (EB) and linear retinal blur (LRB) were examined.
Because of its exponential decrease with age during emmetropization, the rate of change of MOR (D/day) varies linearly with MOR. However, it also varies approximately linearly with retinal blur (both EB and LRB; p<0.001) before emmetropization is complete (up to day 30). During emmetropization, the rate of increase of OAL decreases, as a linear function of decreasing retinal blur (EB p=0.004 and LRB p<0.02). This relationship breaks down around the time that emmetropization is complete (~day 30). Similarly, during emmetropization, the rate of increase in corneal radius varies linearly with retinal blur (EB p=0.002 and LRB p<0.01). The rate of change in lens power is almost proportional to the rate of change of retinal blur (p<0.0001) before the completion of emmetropization. As expected from the above results, the rate of change of corneal radius (p<0.0001) and lens power (p<0.0001) are proportional to the rate of change of OAL during emmetropization.
In chick, emmetropization occurs with a rate of ocular elongation proportional to the size of the hyperopic defocus blur on the retina until about day 30. When retinal blur approaches cone spacing, it no longer decreases, MOR is stable and many relationships above no longer apply during slower eye growth to day 75. Thus emmetropization appears to be driven by the angular defocus blur directly causing proportional changes in OAL, corneal radius and lens power until blur reduces sufficiently and emmetropization is complete. Many of these results are consistent with findings during emmetropization in children but here we show a direct link to retinal blur.
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