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F. J. Rucker, J. Wallman; Chromatic and Luminance Flicker Have Different Effects on Emmetropization. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1730.
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Experiments by Crewther et al., (2006) and Schwahn & Schaeffel (1997) found opposite effects of flickering light on lens compensation. Crewther et al (2006) found that modulating light at 1 - 4 Hz with an asymmetric waveform increased the myopic shift in refraction with negative lens compensation, but had no effect on refraction with positive lenses, or in the control eye. Schwahn & Schaeffel (1997) exposed chicks to periodic pulses at 6 Hz (duty cycle ≤ 50%) and found decreased myopic shifts in refraction with negative lens compensation, and a hyperopic shift in the control eye, but positive lens compensation was normal. We now examine the effects on the chick eye of 2 Hz sine wave luminance and chromatic (equiluminant) flicker and compare them to a control condition of steady illumination.
Chicks were exposed daily (10am to 5pm), for three days, without lenses, to 2 Hz sinusoidally modulated illumination, on two consecutive weeks. Chicks were exposed to either white light, counterphase-modulated equiluminant red/green light (peak wavelengths 619 nm and 515 nm, respectively), or a steady yellow light (619 nm + 515 nm). All conditions had a mean illuminance of 680 lux. Equiluminance was determined as the minimal pupil response to counterphase red/green flickering light (2 Hz). Chicks were kept in the dark overnight during the three day exposure period and otherwise in a brooder. Changes in the ocular components were measured with ultrasound and with a Hartinger Coincidence Refractometer.
Luminance flicker produced a small hyperopic shift in refraction (+0.8 D), a slight thinning of the choroid (-22 µm) and a slightly reduced ocular elongation (161 µm increase over 3 days). In contrast, equiluminant counterphase flicker produced a shift in the myopic direction (-2.12 D) towards emmetropia, and almost twice as much eye growth (277 µm; p<0.001). Steady yellow light also produced a myopic shift (-1.73 D), again bringing the eye towards emmetropia, but with less ocular elongation (144 µm) and some choroidal thinning (-64 µm).
We show that the properties of temporally modulated illumination affect refraction: chromatic flicker causes increased ocular elongation and a myopic shift towards emmetropia, whereas luminance flicker does not. These results provide further evidence for a chromatic component of emmetropization.
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