Uniformity of accommodation across the visual field

Abstract

We asked the question: Does accommodation change the eye's focusing power equally over the central visual field in emmetropic and myopic adult eyes? To answer this question we modified our laboratory scanning wavefront aberrometer to rapidly measure ocular refractive state over the central 30° diameter of visual field as a function of foveal accommodative demand. On average, ocular refractive state changed uniformly over the central visual field as the eye accommodated up to 6 D. Visual field maps of accommodative error (relative to a spherical target surface of constant vergence) reveal subtle patterns of deviation on the order of ± 0.5 D that are unique to the individual and relatively invariant to changes in accommodative state. Population mean maps for accommodative error are remarkably uniform across the central visual field, indicating the retina of the hypothetical "average eye" is conjugate to a sphere of constant target vergence for all states of accommodation, even though individual eyes might deviate from the mean due to random variations. No systematic difference between emmetropic and myopic eyes was evident. Since accuracy of accommodation across the central visual field is similar to that measured in the fovea, loss of image quality due to accommodative errors, which potentially drives myopia and may affect many aspects of visual function, will be similar across the central retina.

Variation of axial and oblique astigmatism with accommodation across the visual field

“…At each level of accommodative demand, the two populations differed by about 0.2 D, which is in good agreement with the distance between the two clusters in the upper panel of Figure 4. The slight decrease (0.05 D) in mean axial astigmatism for emmetropes when the visual target was moved 1 D beyond foveal far point is probably due to the inappropriate accommodation observed in these same individuals (Liu et al, 2016) as the eye returns to a positive resting state of accommodation for negative values of accommodative demand (Hennessy, 1975). …”

“…This observation greatly simplifies the task of measuring, describing, and correcting sphero-cylindrical refractive errors for extended visual objects in the accommodating eye for clinical or research purposes. Optical modeling of the accommodating eye is also simplified when refractive state is uniform across the central field (Liu et al, 2016) and the astigmatic component of refractive state is independent of accommodation. Novel methodologies developed in this study of astigmatism are appropriate also for investigating higher order aberrations, which may greatly simplify the task of modeling aberrations and retinal image quality in the accommodating eye over a large field of view.…”

“…Astigmatism analysis in this report was based on optical measurements obtained in our recently described study of wavefront aberrometry across the visual field in the accommodating eye (Liu, Sreenivasan, & Thibos, 2016). That study employed a custom-built scanning Shack-Hartmann aberrometer to measure the left eye's wavefront aberration over the central 30° of visual field in 34 normal, healthy subjects (16 emmetropic and 18 myopic).…”

“…All measurements were obtained without the use of cycloplegic drugs. Additional methods not previously described in (Liu et al, 2016) are summarized below.…”

“…Measurements were obtained for a randomized sequence of 37 locations (eccentricities 0°, 5°, 10°, 13.5° along 12 visual meridians 0° to 360° in 30° steps) in 16 s. (Liu et al, 2016). Normal blinking was permitted, with subsequent rejection of corrupt data images by quality control procedures.…”