Image Quality on the Retina of the Chick Eye During Emmetropization: Goggled vs Control Eyes

Abstract

Abstract: : Purpose: Higher order monochromatic aberrations decrease in untreated chick eyes during growth for a constant pupil size. Higher order aberrations in goggled and control eyes are significantly different. However, these aberrations and their changes with age are small and variable. We wish to test whether the average and the best image quality on the retina available to the visual system varied in a manner similar to average root mean square (rms) wavefront error. We investigate whether the change in monochromatic aberrations with age and with lens induced myopia produces overall changes in retinal image quality. Methods: On the first day post-hatching, chicks were unilaterally fitted with minus 15D goggles. Goggles were removed for brief time periods for Hartmann-Shack wavefront measurements and retinoscopy on days 0, 1, 2, 4, and 7. Results for 17 chicks that developed refractive errors, consistent with the goggle power, were analysed. Hartmann-Shack images analysed corresponded to larger pupils and relaxed accommodation. Point spread functions (PSFs) on the retina were calculated for the measured aberrations and defocus and for only the higher order aberrations. Strehl ratios were used to quantify retinal image quality. For each eye, the Strehl ratios of the PSFs corresponding to the Hartmann-Shack images were calculated. Both the overall average and the best Strehl ratios for each bird were determined. These retinal image quality measures were then analysed for both control and goggled eyes as a function of age. Results: For higher order aberrations alone and with astigmatism for a 1.6 mm pupil size, the retinal image quality averaged within control eyes improved significantly between days 0 and 7. This retinal image quality in control eyes was significantly better than in goggled eyes at day 7 for higher order aberrations alone. The retinal image quality averaged within birds for treated eyes did not change significantly between days 0 and 7. For higher order aberrations alone, the best retinal image quality available to the eye showed the same dependence as the average quality. Conclusions: Image quality on the retina due to higher order aberrations alone or to higher order aberrations and astigmatism improves with age in control eyes and does not change significantly for goggled eyes. For higher order aberrations alone, goggled eyes have significantly worse image quality at day 7 than controls. Spherical defocus present in the eyes overpowered the changes in Strehl ratio due to higher order aberrations and astigmatism.

Keywords: optical properties • emmetropization • animal model