Chick eyes compensate for chromatic simulations of hyperopic and myopic defocus: Evidence that the eye uses longitudinal chromatic aberration to guide eye-growth

Abstract

Longitudinal chromatic aberration (LCA) causes short wavelengths to be focused in front of long wavelengths. This chromatic signal is evidently used to guide ocular accommodation. We asked whether chick eyes exposed to static gratings simulating the chromatic effects of myopic or hyperopic defocus would “compensate” for the simulated defocus. We alternately exposed one eye of each chick to a sine-wave grating (5 or 2 cycle/deg) simulating myopic defocus (“MY defocus”: image focused in front of retina; hence, red contrast higher than blue) and the other eye to a grating of the same spatial frequency simulating hyperopic defocus (“HY defocus”: blue contrast higher than red). The chicks were placed in a drum with one eye covered, with one grating, and then switched to another drum with the other eye covered, with the other grating. To minimize the effects of altered eye-growth on image contrast, we studied only the earliest responses: first, we measured changes in choroidal thickness 45 min to 1 hour after one 15-min episode in the drum, then we measured glycosaminoglycans (GAG) synthesis in sclera and choroid (by the incorporation of labeled sulfate in tissue culture) after a day of four 30-min episodes in the drum. The eyes compensated in the appropriate directions: The choroids of the eyes exposed to the HY simulation showed significantly more thinning (less thickening) over the course of the experiment than the choroids of the eyes exposed to the MY simulation (all groups mean: −17 μm; 5 c/d groups: −24 μm; paired t-test (one-tailed): p=0.0006). The rate of scleral GAG synthesis in the eye exposed to the HY simulation was significantly greater than in the eye exposed to the MY simulation (HY/MY ratio = 1.20; one sample t-test (one-tailed): p=0.015). There was no significant interaction between the sign of the simulated defocus and either the spatial frequency or the presence of a +3 D lens used to compensate for the 33 cm distance of the drum. Although previous work has shown that chromatic cues to defocus are not essential for lens-compensation, in that chicks can compensate in monochromatic light, our evidence implies that the eye may be able to infer whether the eye is myopic or hyperopic from the different chromatic contrasts that result from different signs of defocus.

Myopia, Light and Circadian Rhythms

“…The contribution of the chromatic component of light to refractive development has been investigated in a number of animal models including chicks (Rucker and Wallman 2009), fish (Kroger and Wagner 1996) and guinea pigs (Long, Chen et al 2009). Guinea pigs raised under long-wavelength light (peak at 760 nm) reportedly develop a significant degree of myopia, associated with a significant increase in vitreous chamber depth, over a period of 4 weeks when compared to animals raised under mixed-wavelength light (unfiltered halogen lamps) (Long, Chen et al 2009).…”

The Influence of Light Exposure and Seasonal Changes on Short-Term and Longer-Term Changes in Axial Length of the Human Eye

“…Since the spectral content of natural outdoor light is different to that of artificial indoor lighting, this leaves open the possibility that the spectral composition of light exposure may be an additional factor contributing to the longitudinal changes in axial length. Animal studies also indicate that the spectral composition of light may be important in the regulation of eye growth, since animals reared under various monochromatic lighting conditions develop refractive error, albeit with substantial interspecies differences in the apparent responses to different wavelengths (Long et al, 2009;Rucker & Wallman, 2009;Liu et al, 2011;Wang et al, 2011;Liu et al, 2014;Smith et al, 2015). Animal studies tend not to support a major role for ultraviolet (UV) light in eye growth regulation however, since compensatory eye growth in response to defocus can be achieved with or without the presence of UV light (Ashby et al, 2009;Ashby & Schaeffel, 2010;Hammond & Wildsoet, 2012;Smith et al, 2012;Smith et al, 2013;Wang et al, 2015).…”

“…However, the white light captured using the light sensors in this study cannot completely explain the variations in axial length changes. Given that animal studies suggest a role for different monochromatic light sources on eye growth (Rohrer et al, 1992;Rucker & Wallman, 2009;Rucker, 2013), further research using light sensors capable of recording different spectral sensitivities is required to better understand the influence of indoor/outdoor light on both the short-term and longer-term changes occurring in axial length.…”

“…The strength of this association suggests that light exposure is not the only factor influencing the seasonal changes in axial length. Since animal studies suggest that the spectrum of light can significantly influence eye growth (Rucker & Wallman, 2009;Smith et al, 2015), and since the spectral characteristics of outdoor light are also known to vary seasonally (Thorne et al, 2009), future studies assessing the spectral characteristics of personal light exposure may help to further explain the nature and magnitude of the association between light exposure and axial length changes.…”

“…However, we did not assess near-work in this study, but assumed that the near-work demands would have been similar for all the participants, since all of the participants were university students and all measurements were taken during the academic semesters. The spectral content of indoor artificial Chapter 6: Conclusion 169 lighting could be another factor involved in the association between less time outdoors and greater axial elongation, since animal studies have shown that different monochromatic light sources can influence eye growth (Long et al, 2009;Rucker & Wallman, 2009;Liu et al, 2011;Wang et al, 2011;Liu et al, 2014;Smith et al, 2015).…”