Abstract
Purpose:
The role of the autonomic nervous system in the color and luminance emmetropization mechanisms is unknown. This study analyzed the response to the non-selective, parasympathetic antagonist, atropine, and the sympatholytic, beta-adrenergic antagonist, timolol, in chicks subjected to illumination conditions that selectively stimulate the color and luminance emmetropization mechanisms.
Methods:
Chicks were binocularly exposed eight hours each day, for four days, to one of three illumination conditions: 2Hz sinusoidal luminance flicker (LUM), 2Hz sinusoidal color flicker (B/Y), or steady light. Mean illuminance was 680 lux. Eyes received daily injections of either 20μl atropine (18nmol) (N=8), 2 drops of 0.5% timolol (N=8), 20μl phosphate-buffered saline (N=8), or no injection (N=8). Measurements of the axial dimensions of ocular components and refraction were performed using A-scan ultrasonography, photorefraction and a Hardinger Refractometer. In each illumination condition, the saline effect was subtracted from the drug effect [Drug (ΔX-ΔN) - Saline (ΔX-ΔN)].
Results:
LUM flicker demonstrated opposite effects on eye growth and refraction with atropine and timolol treatment. Atropine caused a reduction in eye growth (-0.08 ± 0.02 mm, p=0.01) and a reduction in vitreous chamber depth (-0.10 ± 0.02 mm, p=0.004), evoking a hyperopic shift in refraction (3.40 ± 1.77 D), despite an antagonistic increase in lens thickness (0.14 ± 0.05 mm, p=0.004). In contrast, timolol elicited a myopic shift in refraction (-4.07 ± 0.92 D, p=0.001), due to an increase in eye length (0.045 ± 0.030 mm). Color flicker induced choroidal compensation for eye growth, preventing refractive shifts with atropine and timolol. With atropine, hyperopia was not observed, because a reduction in eye length (-0.05 ± 0.02 mm, p=0.01) was compensated for by choroidal thinning (-0.05 ± 0.02 mm, p=0.03). With timolol, myopia did not occur because a reduction in eye length (-0.05 ± 0.018 mm, p=0.02) was also compensated for by choroidal thinning (-0.052 ± 0.015 mm, p=0.01).
Conclusions:
The opposing growth and refractive effects of atropine and timolol with luminance flicker, and the compensatory choroidal compensation with color flicker, suggest a precise balancing mechanism between the parasympathetic and sympathetic nervous system, and the visual environment, in achieving emmetropization.