This study shows that nonvisual mechanism(s) can guide chick eyes to recover from myopia or hyperopia bidirectionally to regain their age-matched length. Because eye growth control is phylogenetically conserved across many species, it is possible that, in general, emmetropization mechanisms are not exclusively based on a local visual feedback system.
Across species, growing eyes compensate for imposed defocus by modifying their growth, showing the visual controls on eye growth and emmetropization. When the spectacle lens is removed, the eyes rapidly recover back to a normal size similar to that in the untreated eyes. We asked whether this recovery process was dependent on visual feedback or whether it might be guided by intrinsic nonvisual mechanisms.
Chicks wore either a +7 (n = 16) or −7 D (n = 16) lens over one eye for 4 to 7 days; the fellow eye was left untreated. After lens removal, half were recovered in darkness and half in white light. Refractive error and ocular dimensions were measured before and after lens treatment and after recovery with a Hartinger refractometer and A-scan biometer, respectively.
Whereas chick eyes completely recovered from prior lens treatment under normal light after 2 days, they also partially recovered from prior hyperopia (by 60%) and myopia (by 69%) after being kept in darkness for 3 days: a +7 and −7 D lens induced a difference between the eyes of +7.08 and −4.69 D, respectively. After recovery in darkness, the eyes recovered by 3.18 and 2.88 D, respectively.
In the absence of visual cues, anisometropic eyes can modify and reverse their growth to regain a similar length to their fellow untreated eye. Because eye growth control is phylogenetically conserved across many species, it is possible that nonvisual mechanisms may contribute more generally to emmetropization and that recovery from anisometropic refractive errors may not be wholly visually controlled.