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D.P. Crewther, S.G. Crewther, M.J. Murphy, D.G. Simpson; Duty Cycle Affects Refractive Compensation to Low Frequency Temporal Modulation Myopia and Hyperopia . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3330.
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High frequency flicker has been reported to induce a hypermetropic shift in refractive compensation to optical defocus with duty cycle also affecting the suppression of hypermetropic defocus. By comparison low temporal frequency non–square–wave flicker profiles induce a myopic shift on top of a sign dependent refractive compensation response. Thus this study sought to reconcile these effects by varying duty cycle of a 1Hz square wave flicker.
Sixty–seven chickens were raised from Day 5–9 with monocular +/–10D spectacle goggles under a 1 Hz flicker produced by a LED light source. Illuminance fluctuated between 2 and 100 lux and duty cycle was varied from 5% to 50% to 95% darkness. Retinoscopy to determine refractive error and ultrasonography for axial length were performed prior to sacrifice and results analyzed in terms of differences in refraction and axial length between the experimental and fellow eyes.
5% darkness led to a 4–5D myopic shift in all lensed chicks on top of a further suppression of compensation to positive lens wear. 50% duty cycle suppressed refractive compensation to both lens types by about 8D whereas 95% darkness led to less refractive suppression but greater suppression of negative lenses than positive lenses. Axial dimensions especially anterior chamber were significantly longer in 95% compared to 5% duty cycle group.
Irrespective of duty cycle, 1Hz temporal modulation induces a suppression of defocus induced hypermetropia. These results also indicate that the extent of refractive compensation to optical defocus is highly vulnerable to the total amount of light experience but is not linearly related.
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