Effect of myopia on visual acuity measured with laser interference fringes

The aim was to determine how visual acuity is affected by myopia when optical factors of the eye are controlled. Grating acuity was measured with interference fringes to avoid the effects of aberrations, and ocular biometry was used to compensate for differences in retinal image size among subjects. Distance spectacle refractions ranged from +2.25 to −14.75D. The retinal magnification factor (RMF) in mm/deg was computed for each eye from the distance refraction, central corneal power and ultrasound biometry. A forced-choice orientation discrimination method was used to measure acuity for high-contrast 543nm laser interference fringes in three retinal locations: the fovea, and at 4deg and 10deg eccentricity in the temporal retina. Acuity, expressed in c/deg and adjusted for spectacle magnification, was not significantly correlated with refraction at any of the three retinal locations. When acuity was converted to retinal spatial frequency units (c/mm) via the RMF, acuity decreased with increasing myopia at all three retinal locations (significantly at the fovea and at 10deg eccentricity). Retinal acuity values in highly myopic subjects (>6D) are consistent with retinal sampling distances that are larger than published values of human cone or ganglion cell spacing. The results imply that a highly myopic eye has retinal neurons that are more widely spaced than normal, but the increased axial length enlarges the retinal image enough to compensate for the retinal stretching. The data are consistent with a retinal stretching model that primarily affects the posterior pole.