Optical correction of form deprivation myopia inhibits refractive recovery in chick eyes with intact or sectioned optic nerves

AbstractThe finding that the eyes of young chicks recover quickly from form deprivation myopia (FDM) has been interpreted as indirect evidence for active emmetropization. More direct evidence would be the demonstration that correction of FDM with spectacle lenses, thereby removing the defocus signal, prevents recovery. We investigated this issue in eyes with intact and sectioned (ONS) optic nerves. Previous studies suggest that an intact optic nerve is necessary for accurate emmetropization. Seventy day-old male chicks were monocularly deprived using velcro-mounted diffusers, which were removed after 5–6 days and in some (n=51), but not all cases, replaced by spectacle lenses (−5, −10 or −15 D). Approximately half (n=34) of the chicks also underwent ONS on day 1. Refractive errors and axial ocular dimensions were measured when the diffusers were first removed and thereafter at 2–4 day intervals over the following 1–2 weeks. In one case, measurements were continued at less regular intervals to 33 days. Lens powers were selected to either approximately correct or under-correct the refractive errors present when the diffusers were removed. Form deprivation in normal chicks produced large myopic shifts in refraction (means for groups range from −9.20 to −16.07 D). When the deprivation treatment was terminated, the myopia quickly decreased to negligible levels unless optically corrected. Correcting lenses stabilized the myopia to a level consistent with the lens power used. Interocular differences in axial length were consistent with an axial origin to the refractive changes. Results for the ONS groups exhibited similar trends although there was increased variability in the data. The findings support the interpretation that recovery from FDM is a product of active emmetropization. That ONS increased the variability of such responses implies that an intact optic nerve is required for accurate emmetropization