Abstract
Purpose::
To investigate whether optical quality metrics (OQMs) developed for human eyes can be used as predictors of normal eye growth in young chicks.
Methods::
The ocular dimensions and monochromatic aberrations of 16 chick eyes were followed from 11 to 34 days of age, using respectively high frequency A-scan ultrasonography and a COAS aberrometer (Wavefront Sciences, NM). Aberrations were analyzed for a 2 mm pupil and represented using OSA standard Zernike polynomials. From the 2nd to 4th order Zernike coefficients, 31 OQMs, based on either the eye’s aberrations, point spread function or optical transfer function, were computed using MATLAB (MathWorks, MA). Refractive errors were derived from the 2nd order Zernike coefficients. The correlations between the developmental changes in refractive errors and vitreous chamber depth (i.e. day 34 values - day 11 values) and the OQMs were analyzed. The significance of developmental changes and correlation coefficients was tested using paired t-test and a special t-test (t-statistic = r*sqrt{[n-2]/[1-r^2]}) respectively (significance level: 0.05).
Results::
Twelve OQMs (OQM set 1) showed significant correlations with the developmental increase in vitreous chamber depth, i.e. the poorer the optical quality indicated by these 12 OQMs, the more the vitreous chamber elongated. Nine OQMs (OQM set 2) were significantly correlated with the developmental decrease in astigmatism, i.e. the poorer the optical quality indicated by these 9 OQMs, the more the astigmatism decreased. OQM set 1 and OQM set 2 represent two distinct groups athough there is some overlap. However, no significant correlations were found between OQMs and the developmental change in spherical refractive error, which showed a small yet insignificant decrease. The pattern of correlations between OQMs from these chick eyes is similar to that reported for a large-scale human study (Thibos LN et al, 2004).
Conclusions::
Optical quality metrics developed for human eyes can be applied to chick eyes. That some optical quality metrics predict the amount of vitreous chamber elongation of chicks in normal visual environment, suggest that these metrics capture optical features of importance for retinal regulation of eye growth. Whether these metrics can be applied to human myopia development warrants further investigation. However, note that chromatic aberration was not considered in analyses. Also, the results do not rule out influences of non-optical factors on normal eye growth.
Keywords: refractive error development • optical properties • emmetropization