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A. Ayotte, D.A. Rushforth, J. Tao, K.A. Vessey, W.K. Stell; The Effects of Stroboscopic Flicker on Form–Deprivation Myopia in Chicks . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1981.
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Purpose: Stroboscopic illumination has been reported to attenuate form–deprivation myopia (FDM) in chicks. The aim of the present studies was to determine the critical stimulus parameters (frequency, duration) for this effect and to determine whether it was mediated locally within the eye. Methods: White Leghorn cockerels were kept on a 12:12 hr light:dark cycle (on at 07:00). 7 days after hatching (P7) one eye was fitted with a diffuser; the other eye remained uncovered. Each experiment included (all N=6–7) one control group, which spent 12 hr under 64 lux "daylight" fluorescent light, and several treated groups, which were exposed to stroboscopic flicker (Grass PS–2, setting 2) as follows: (1) Effect of treatment duration. Treatment for (t)=1, 3, 6, or 12 hr, with the remaining (12–t) hr under "control" lighting, for 7 da. (2) Effect of flicker frequency. Treatment for 12 hr/da at 2, 10, or 20 Hz, for 7 da. (3) Role of connection to the brain. Treatment at 10 Hz, 12 hr/da for 6 da, + intravitreal injection of deprived eye with tetrodotoxin (TTX, 1.1x10–4M; blocks optic nerve conduction for >48 hr) and control eye with vehicle, 20 µL each, on P8, P10, and P12. The day after the final treatment, eyes were refracted, measured by 30 MHz A–scan ultrasound, removed and weighed. Results: (1) 12 hr/da of 10 Hz flicker in the dark strongly inhibited vitreal elongation in form–deprivation myopia (FDM); 6 hr/da was only marginally effective, whether early or late in the light phase; and 1 or 3 hr/da was totally ineffective. (2) 12 hr/da of 20 Hz flicker was slightly but significantly more effective than 10 Hz flicker of equal mean intensity. In contrast, 2 Hz was ineffective; eyes treated at 2 Hz were indistinguishable from eyes of chicks kept in darkness for 24 hr/da. (3) TTX had no effect on the inhibition of FDM by 10 Hz flicker, 12 hr/da. Conclusions: 10 Hz stroboscopic flicker suppressed the development of form–deprivation myopia, only when delivered in the darkness for 6–12 hr/da, much longer than the <3 hr/da required for inhibition of FDM by good "form" vision. 20 Hz was at least as effective as 10 Hz, suggesting that even higher frequencies might be effective. We conclude that flickering illumination prevents FDM by a retinal mechanism different from the one(s) by which in–focus and plus–defocused images promote emmetropization.
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