Abstract
Shortly after birth, the eyes of most animals (including humans) are hyperopic because the short axial length places the retina in front of the focal plane. During postnatal development, an emmetropization mechanism uses cues related to refractive error to modulate the growth of the eye, moving the retina toward the focal plane. One possible cue may be longitudinal chromatic aberration (LCA), to signal if eyes are getting too long (long [red] wavelengths in better focus than short [blue]) or too short (short wavelengths in better focus). It could be difficult for the short-wavelength sensitive (SWS, "blue") cones, which are scarce and widely spaced across the retina, to detect and signal defocus of short wavelengths. We hypothesized that the SWS cone retinal pathway could instead utilize temporal (flicker) information. We thus tested if exposure solely to long-wavelength light would cause developing eyes to slow their axial growth and remain refractively hyperopic, and if flickering short-wavelength light would cause eyes to accelerate their axial growth and become myopic. Four groups of infant northern tree shrews (Tupaia glis belangeri, dichromatic mammals closely related to primates) began 13 days of wavelength treatment starting at 11 days of visual experience (DVE). Ambient lighting was provided by an array of either long-wavelength (red, 626 ± 10 nm) or short-wavelength (blue, 464 ± 10 nm) light-emitting diodes placed atop the cage. The lights were either steady, or flickering in a pseudo-random step pattern. The approximate mean illuminance (in human lux) on the cage floor was red (steady, 527 lux; flickering, 329 lux), and blue (steady, 601 lux; flickering, 252 lux). Refractive state and ocular component dimensions were measured and compared with a group of age-matched normal animals (n = 15 for refraction (first and last days); 7 for ocular components) raised in broad spectrum white fluorescent colony lighting (100-300 lux). During the 13 day period, the refraction of the normal animals decreased from (mean ± SEM) 5.8 ± 0.7 diopters (D) to 1.5 ± 0.2 D as their vitreous chamber depth increased from 2.77 ± 0.01 mm to 2.80 ± 0.03 mm. Animals exposed to red light (both steady and flickering) remained hyperopic throughout the treatment period so that the eyes at the end of wavelength treatment were significantly hyperopic (7.0 ± 0.7 D, steady; 4.7 ± 0.8 D, flickering) compared with the normal animals (p < 0.01). The vitreous chamber of the steady red group (2.65 ± 0.03 mm) was significantly shorter than normal (p < 0.01). On average, steady blue light had little effect; the refractions paralleled the normal refractive decrease. In contrast, animals housed in flickering blue light increased the rate of refractive decrease so that the eyes became significantly myopic (-2.9 ± 1.3 D) compared with the normal eyes and had longer vitreous chambers (2.93 ± 0.04 mm). Upon return to colony lighting, refractions in all groups gradually returned toward emmetropia. These data are consistent both with the hypothesis that LCA can be an important visual cue for postnatal refractive development, and that short-wavelength temporal flicker provides an important cue for assessing and signaling defocus.