Abstract
Purpose :
In chick, the Hartmann-Shack aberrometer (HS) gives accurate measures of refraction and image quality with natural accommodation. Imaging with adaptive optics (AO) allows resolution of cone photoreceptors. We previously reported that chick retinal optical blur reduces to a non-zero plateau, on average, matching cone photoreceptor resolution in control eyes or slightly exceeding it in eyes goggled with lenses. We analyze potential signals to defocus after optical blur plateaus.
Methods :
Right eyes of 21 Ross Ross chicks were goggled with either +10D or -15D lenses at hatching. Left eyes grew normally. Measurements were taken on day 0 and subsequent days up to day 21. Aberrations, spherical defocus and astigmatism (Jackson cross cylinders (JCC0, JCC45)) were measured using HS. Point spread functions and contributions to blur of defocus, astigmatism and higher order aberrations (HOA) were calculated exactly and radial extents approximated from equivalent blur. Results for individual frames tracked changes in spherical defocus. Results were also averaged for individual birds, and across birds on each day. Mean ocular refraction was calculated from the most hyperopic refractions. Cone photoreceptors were imaged close to the area centralis using an AO scanning laser ophthalmoscope. Cone resolution was calculated from the images.
Results :
When retinal optical blur has reduced to a plateau, astigmatism has reduced in control eyes and increased in eyes with –15D lenses. Individual goggled eyes have fluctuating positive or negative spherical defocus. Across birds, the spherical defocus changes, DR, are larger than the astigmatism C, in both goggled (-15D: DR=4.5D, C=3.3D; +10D: DR=2.6D, C=1.8D) and control eyes (-15D: DR=4.3D, C=1.4D, +10D: DR=1.4D, C=1.1D). Thus, in individual eyes, as spherical defocus varies, (likely due to accommodation), there are two distinct line foci with a clear orientation flip that is not obscured by blur from HOA. These lines are resolvable by the cone photoreceptors. Line orientations are predominantly oblique (JCC45).
Conclusions :
Following emmetropization, total optical blur plateaus, is well matched to cone resolution and this could act as a stop signal. However, changes in spherical defocus allow the chick to resolve the astigmatic line foci. Thus, following emmetropization, oblique astigmatism is one potential signal to the sign of any drift in refractive error.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.