Uniformity of Accommodation across the Visual Field

Abstract

Purpose: Optical blur due to accommodative error in peripheral vision has been implicated in myopia development and progression. We investigated the uniformity of focusing errors over central retina as a function of accommodation demand in young adult and juvenile subjects.

Methods: A laboratory scanning wavefront aberrometer (Wei & Thibos, Opt Express. 2010;18/2:1134-43) uses rotating mirrors to pivot a laser probe beam (850 nm) about the pupil center to place a “retinal beacon” at a random sequence of 37 retinal locations over the central 27° of visual field. For each retinal location, beacon light reflected out of the eye is de-scanned by the same mirrors and then directed to a conventional Shack-Hartmann wavefront sensor for analysis. The visual stimulus was displayed on an achromatic micro-display in a Badal configuration. Aberrations were measured at 8 levels of accommodative demand (1D steps of target vergence starting 1D beyond the estimated far point). Accommodative error is the difference between target vergence and the eye’s refractive state as measured by defocus Zernike coefficient C₂⁰ (in diopters).

Results: For most adult subjects, ocular refractive state changed uniformly over the central visual field as the eye accommodated. Visual field maps of accommodative error 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. This result confirms and extends previous work on adults (Mathur, Atchison, & Charman, 2009. J Vis 9, 20 1-11). Similar results were observed also for children. No systematic difference between emmetropic and myopic eyes was evident.

Conclusions: Ocular refractive state changes uniformly over the central visual field as the eye accommodates. Accuracy of accommodation across the visual field is similar to that measured in the fovea. Thus loss of image quality due to accommodative errors, which potentially drives myopia, will be similar across the central retina.