Compensation to positive as well as negative lenses can occur in chicks reared in bright UV lighting

An earlier report describing a lack of compensation to imposed myopic and hyperopic defocus in chicks reared in UV lighting has led to the belief that the spatial resolving power of the UV cone photoreceptor network in chicks is not capable of decoding optical defocus. However this study used dim light rearing conditions, of less than 10 lx. The purpose of the current study was to determine if emmetropization is possible in young chicks reared under higher luminance, UV lighting conditions. Young, 4 day-old chicks were reared under diurnal near UV (390 nm) illumination set to either 20 or 200 lx while wearing a monocular defocusing lens (+20, +10, -10 or -20 D), for 7 days. Similarly treated control groups were reared under diurnal white lighting (WL) of matching illuminance. The WL and UV LED sources were set to equivalent illuminances, measured in "chick lux", calculated from radiometer readings taken through appropriate narrow band interference filters, and a mathematical model of the spectral sensitivity of the chick visual system. High resolution A-scan ultrasonography was undertaken on days 0 (before lenses were fitted), 2, 4, and 7 to track ocular dimensions and refractive errors were measured by retinoscopy on days 0 and 7. Compensation to negative lenses was unaffected by UV illuminance levels, with near full compensation being achieved under both conditions, as well as under both WL conditions. In contrast, compensation to the positive lenses was markedly impaired in 20 lx UV lighting, with increased instead of decreased axial elongation along with a myopic refractive shift being recorded with the +10D lens. Compensation under both WL conditions was again near normal for the +10D lens. However, with the +20 D lens, myopic shifts in refractive error were observed under both dim UV and WL conditions. The spatial resolving power of the UV cone photoreceptor network in the chick is sufficient to detect optical defocus and guide the emmetropization response, provided illumination is sufficiently high. However, compensation to imposed myopic defocus may be compromised, when either the amount of defocus is very high or illumination low, especially when the wavelength is restricted to the UV range.