Monochromatic and white light and the regulation of eye growth

Abstract

Experiments employing monochromatic light have been used to investigate the role of longitudinal chromatic aberration (LCA) as possible signals for emmetropization for many years. LCA arising from the dispersion of light, causes differences in the focal length at different wavelengths and can impose defocus (wavelength defocus). Short-wavelength light focuses with a shorter focal length than long-wavelength light and, as such, would be expected to produce a smaller, more hyperopic eye. Emmetropization can respond to wavelength defocus since animals reared in monochromatic light adjust their refractive state relative to that measured in white light. In many species, animals reared in monochromatic light respond as predicted by wavelength defocus, becoming more hyperopic in blue light and more myopic in red light. However, tree shrews and rhesus monkey become more hyperopic in red light, and while tree shrews initially become more hyperopic in blue light, they later become more myopic. This review examines the experiments performed in monochromatic light and highlights the potential differences in protocols affecting the results, including experiment duration, circadian rhythm stimulation, light intensity, bandwidth, humoral factors and temporal sensitivity.

Keywords: Blue light; Emmetropization; Longitudinal chromatic aberration; Monochromatic light; Myopia.

Copyright © 2019 The Author. Published by Elsevier Ltd.. All rights reserved.

Figures

Figure 1:

When the eye is emmetropic, defocus caused by LCA causes red light to focus behind and blue light to focus in front of the retina. When the eye is hyperopically defocused, blue light is focused closer to the retina than red light, but both red and green light are focused behind the retina. When the eye is myopically defocused, blue and green light are focused in front of the retina, while red light is focused closer to the retina. Drawings are not to scale.

Figure 2.

Refractive error (best sphere) versus threshold for the S+ increment and S− decrement blob conditions. Each point is an observer's threshold, the line is the best-fitting regression line, and the gray shaded region is the 95% confidence interval of the fit. A negative correlation with RE was observed for both S+ (r = −0.28; p = 0.06; BCI: r = 0.5, 0.04) and S− (r = 0.23; p = 0.13; BCI: r = 0.46, 0.01) blobs. Republished from Taylor et al. (2018) [25] with permission from IOVS.

Figure 3.

One-day-old chicks (n = 16) raised in red light became myopic at 14 days, more myopic at 21 days (n = 6), and still more myopic at 28 days (n = 6), while those (n = 19) raised in blue light became hyperopic at 14 days, more hyperopic at 21 days (n = 6), and still more hyperopic at 28 days (n = 6). The differences in mean (±SD) myopic and hyperopic refractive errors were significant at each time interval (*P ≤ 0.001). At 14 days, induced myopia or induced hyperopia were each significantly different from emmetropia (zero refractive error) (P

Figure 4.

Choroidal and eye-length changes (ΔX-ΔN) to positive lens defocus and negative lens defocus under low-intensity illumination (0.67 "chick lux") in animals exposed from 9 am to 5 pm for 3 days. Under red monochromatic illumination the choroid thickened in response to positive lenses and thinned in response to negative lenses, but there was little change in eye length. Under blue monochromatic illumination the ocular elongation was inhibited in response to positive lenses and increased in response to negative lenses, but there was no change in choroidal thickness. The sign of lens defocus is indicated as + or −. Error bars show the standard error of the mean. Republished from Rucker and Wallman (2008) [21] with permission from Vision Research.

Figure 5.

Diurnal changes from the mean (mean ± SE, μm) with fitted cosine functions over 24 hours for 17 human emmetropic subjects (closed symbols, solid lines) and 25 myopic subjects (open symbols, dashed lines) for axial length (AL, black lines) and choroid thickness (square symbols, red lines. The amplitude of the diurnal changes in the central 1mm of choroid was correlated with axial length (p = 0.05), though not with spherical equivalent refraction. Republished from Burfield et al. (2018) [57] with permission from IOVS.