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Alan Busby; Luminance variations can reduce or reverse plus lens compensation in chicks. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5181.
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© ARVO (1962-2015); The Authors (2016-present)
Luminance varies significantly in the natural environment, often by 5-6 log units or more. Published experiments have shown that bright lights can alter the expected emmetropization responses in animals (Ashby & Schaeffel, IOVS, 2010; Smith et al, IOVS, 2012). It was hypothesized that a visual scene’s brighter areas would affect emmetropization more than its dimmer areas. The experiments below tested whether an environment’s luminance variations could alter chick lens compensation.
1-week-old Chicks (8 birds/group with 2 birds/cage) wore a +3D lens on one eye and an occluder on the other in a cylindrically shaped cage (18cm x 12cm high), so that all objects inside the cage had hyperopic defocus while objects outside had myopic defocus. 50% of the cage’s walls were covered with 4 equidistant 7cm wide paper strips, and 4 equal open sections permitted distance views of 0.5 to 3m. The experimental cage had a 5000 lux near light (a 50 watt desk lamp 12cm from the cage floor), while the outside was illuminated with ~60 lux, ~140 lux, or typical overhead fluorescent lighting (~600 lux). The control cage had no near light but equal outside illumination. One eye, with the lens, viewed the experimental cage for 1hr. The lens and occluder were then switched, and the fellow eye viewed the control cage through the lens for 1hr. Chicks had 2 exposures per condition on one day, and 2 more the following day over 24hrs.
The control eyes became hyperopic as expected with +3D lenses. However, the experimental eye’s hyperopic responses were statistically significantly reduced or reversed in the experimental eye for all three background light levels (~60, ~140, ~650 lux), based on a thinner choroid (-69±12 vs. 6±17, -53±12 vs. -4±20, 31±16 vs. 64±16µm), larger vitreous chamber depth (86±18 vs. 2±29, 55±23 vs. -11±17, -27±19 vs. -99±12µm), and more negative refraction (-1.59±0.53 vs. +1.19±0.64, -1.85±0.58 vs. +0.12±0.9D, not tested). Axial changes were not expected due to the short time period.
A visual scene’s brighter areas seem to affect lens compensation more than darker areas, even when light levels are comparable to common environments. Plus lens compensation was inhibited or even reversed when the brightest areas had hyperopic defocus. This phenomenon may help explain some of the seemingly contradictory results between human epidemiological and animal emmetropization studies.
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