Abstract
Purpose: :
Luminance and chromatic flicker have different effects on eye growth: Chromatic flicker produces increased eye growth and a myopic shift towards emmetropia, whereas luminance flicker produces hyperopia. We hypothesize that the reason for this difference is that changing hyperopic defocus causes a pronounced change in relative cone contrast between the S-cones and the L-cones, whereas with changing myopic defocus the cone contrast of all three cone types changes in parallel. If the eye is using relative cone contrast as an indicator of the sign of defocus, then the absence of contrast at short-wavelengths should reduce the effectiveness of this defocus signal. To test this hypothesis we exposed chicks to in-phase, luminance flicker illumination that simulates myopic or hyperopic defocus, with and without short-wavelength blue light.
Methods: :
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 in-phase modulated (2Hz) white light containing red, green and blue components (RGB) or only red and green components (RG). The lighting included a color component that simulated the chromatic effect of hyperopic defocus during week one and then myopic defocus during week two by introducing a contrast difference between the red, green and blue (if present) components. Non-flickering (NF) white light that contained only red and green components acted as a control. All conditions had a mean illuminance of 680 lux. After exposure chicks were kept in the dark overnight. Changes in the ocular components were measured with ultrasound and with a Hardinger Coincidence Refractometer.
Results: :
In the presence of blue light there was a mean hyperopic refractive shift (RGB: 0.55 D), in the absence of blue light there was a mean myopic shift towards emmetropia (RG: -0.38 D; NF: -0.95 D; p=0.005), the refractive differences were most pronounced with simulation of hyperopic defocus in week 1 (RGB v RG: p=0.01; RGB v NF: p=0.002). Consistent with the hyperopic shift, mean ocular elongation over the three days was less when blue light was present (RGB: 177 µm; RG: 237 µm; NF: 217 µm), but was only significant with the simulation of myopic defocus in week 2 (RG v RGB; p=0.035). Choroidal thinning was similar in all three conditions (RGB: -24 µm; RG: -30 µm; NF:-34 µm).
Conclusions: :
The hyperopic shift seen typically seen with luminance flicker is absent when there is no blue light. These results provide further evidence for a chromatic component of emmetropization.
Keywords: myopia • emmetropization • hyperopia