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Y. Cohen, M. Belkin, I. Avni, U. Polat; Dependency Between Light Intensity and Chick's Refractive Development Under Light Dark Cycle . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3325.
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© ARVO (1962-2015); The Authors (2016-present)
Light intensity modulates the chicks' refractive error induced by continuous light rearing, mainly via fine regulation of corneal curvature (Cohen et al., ARVO 2005). The emmetropization process is a fine tune of the refractive state by altering the refractive components toward near zero refraction. In chicks, the emmetropia is reached after 8 weeks. We provided cyclic illumination conditions and examined the effect of light intensity on the progression of emmetropization.
Forty chicks were reared under 12h/12h dark/light cycle at three different light intensities: 10 000 lux (n = 13, high intensity group), 500 lux (n = 14, medium intensity group), and 50 lux (n = 13, low intensity group). Their eyes were followed for 90 days by repeated retinoscopy, keratometry, as well as measurements in vivo by ultrasound and, after enucleation, by caliper.
The time to emmetropization is light intensity dependent and reached after 50 days for the low intensity group and 60 days for the medium intensity group. The high intensity group had stable hyperopia of +1.1±0.05 D from day 60 onwards and did not become emmetropic even on day 90. The low intensity group developed myopia of –2.4±0.26 D on day 90. Corneal curvature differ significantly between the groups and directly correlates with light intensity (R²=0.185, P<0.000), the higher the intensity the steeper is the cornea. The various light intensities also produced significant other changes in the components of the anterior and posterior segments .
Light intensity is an environmental factor that modulates the process of emmetropization, the lower the intensity the faster emmetropia is reached. Thus, ambient light under light/dark cycle is a risk factor for myopia development. It seems that the dependency between visual experience and the development of refractive errors is determined at least by three parameters, the age of the chick (residual plasticity), it's current refractive error (residual modifiable power) and the environmental inducer (in this case –light intensity).
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