Long-wavelength (red) light produces hyperopia in juvenile and adolescent tree shrews
In infant tree shrews, exposure to narrow-band long-wavelength (red) light, that stimulates long-wavelength sensitive cones almost exclusively, slows axial elongation and produces hyperopia. We asked if red light produces hyperopia in juvenile and adolescent animals, ages when plus lenses are ineffective. Animals were raised in fluorescent colony lighting (100–300 lux) until they began 13 days of red-light treatment at 11 (n = 5, “infant”), 35 (n = 5, “juvenile”) or 95 (n = 5, “adolescent”) days of visual experience (DVE). LEDs provided 527–749 lux on the cage floor. To control for the higher red illuminance, a fluorescent control group (n = 5) of juvenile (35 DVE) animals was exposed to ˜975 lux. Refractions were measured daily; ocular component dimensions at the start and end of treatment and end of recovery in colony lighting. These groups were compared with normals (n = 7). In red light, the refractive state of both juvenile and adolescent animals became significantly (P < 0.05) hyperopic: juvenile 3.9 ± 1.0 diopters (D, mean ± SEM) vs. normal 0.8 ± 0.1 D; adolescent 1.6 ± 0.2 D vs. normal 0.4 ± 0.1 D. The fluorescent control group refractions (0.6 ± 0.3 D) were normal. In red-treated juveniles the vitreous chamber was significantly smaller than normal (P < 0.05): juvenile 2.67 ± 0.03 mm vs. normal 2.75 ± 0.02 mm. The choroid was also significantly thicker: juvenile 77 ± 4 μm vs. normal 57 ± 3 μm (P < 0.05). Although plus lenses do not restrain eye growth in juvenile tree shrews, the red light-induced slowed growth and hyperopia in juvenile and adolescent tree shrews demonstrates that the emmetropization mechanism is still capable of restraining eye growth at these ages.