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D.Y. Tse, J.W. Chan, C.H. To; Competing Defocus Introduced by Concentric Bifocal Lenses Results in Summated Emmetropization Response in Chicks . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1974.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: While it is well documented that the chick eye can compensate a wide range of single optical defocus accurately, it is unknown how the emmetropization is affected in the presence of multiple defoci. We sought to investigate the effect of competing defoci on emmetropization of chick eyes and quantify their relationship. Methods: Concentric bifocal lenses manufactured based on the Fresnel’s principle were applied to one eye of 8–day–old chicks. The power of the lens was manufactured to produce competing defocus stimuli of various combinations: (a) +10/–10D, (b) +20/–10D, (c) +5/–10D, (d) plano/–10D. And for a subset of (a), the relative strength of the stimuli was varied from typical ratio of (a) 50:50 to (e) 33:67 and (f) 25:75 by manipulating the pitch width and relative pitch numbers of annulus on the lenses. Refractive error and axial dimensions were measured by retinoscopy and A–scan ultrasonography respectively under isofluorane anaesthesia before the treatment, and at the third and sixth day after treatment applied. Treatment effect was defined by the interocular difference of the parameters. Results of each treatment group were compared with control groups which corresponding single defocus stimuli were introduced by single vision lenses of matched optical powers as the competing defocus stimuli. Results: The competing defocus stimuli induced intermediate emmetropization responses in all treatment groups except group (f). Refractive error at day 6 after lens wear: (a) +1.80D, (b) +13.74D, (c) –0.86D, (d) –4.50D, (e) –7.83D, (f) –10.50D. Refractive error and vitreous chamber depth of those groups were significantly different from their corresponding single defocus control group at the 0.05 level (n= 8–13). A positive correlation between the resultant response and the relative strength of the competing stimuli was showed in the experiment subset comprising of group (a), (e) and (f). Conclusions: Our data show that competing defocus stimuli induce intermediate emmetropization responses. The emmetropization set point depends on the relative strength of the competing defocus. These findings suggest that an integrative process of multiple spatial defocus may be employed in the decoding of visual stimuli for emmetropization. These results may also contribute to an understanding of how multiple defocus in the visual environment influence emmetropization and refractive error development.
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