The interactions between bright light and competing defocus during emmetropization in chicks

Abstract

PURPOSE. The environment comprises multiple optical signals that affect eye growth. We aimed to determine if the inhibitory effects of myopic defocus and bright light (BL) against myopia are additive in the presence of the myopia-genic hyperopic defocus. Methods. In experiment 1, three groups of 24 chicks each were fitted with the following multizone dual-power lenses (pl): pl/-10 D (50: 50 area), +10/-10 D (50: 50 area), and +10/-10 D (33: 67 area) monocularly for 6 days. Half of each group were raised under normal illumination of 500 lux, 12/12-hour light/dark cycle, whereas the remainder were exposed to 6-hour BL of 40 klx and 6-hour 500 lux during the light cycle. In experiment 2, 38 chicks wore +10/-10 D (33: 67 area) lenses monocularly for 8 days and were exposed to one of four light intensities for 6 hours per day-500 lux, 10 klx, 20 klx, or 40 klx-and received 500 lux for the remainder of the light cycle. Results. In experiment 1, interocular difference in refractions after 6 days for the three groups were -3.6 D, +2.0 D, and -4.2 D, respectively, under normal light and were -0.9 D, +4.2 D, and +0.67 D under BL, manifesting as a shorter anterior segment and vitreous chamber. In experiment 2, the effect of BL increased with light intensity in the +10/-10 D (33:67) group, with a significant difference in refraction between the 10 klx and 20 klx groups (interocular difference -2.75 +/- 2.76 D vs. 1.70 +/- 2.40 D, P < 0.01), but plateaued between 20 klx and 40 klx (1.70 +/- 2.40 D vs. 1.70 +/- 0.35 D, P > 0.05). Conclusions. The protective effects of myopic defocus and BL against experimental myopia were additive. The inhibitory effect of BL against myopia was dose dependent at 10 klx and above but plateaued at 20 klx.