Peripheral optical anisotropy in refractive error groups

Introduction: This study investigated differences in peripheral image quality with refractive error. Peripheral blur orientation is determined by the interaction of optical aberrations (such as oblique astigmatism) and retinal shape. By providing the eye with an optical signal for determining the sign of defocus, peripheral blur anisotropy may play a role in mechanisms of accommodation, emmetropisation and optical myopia control interventions. This study investigated peripheral through-focus optical anisotropy and image quality and how it varies with the eye's refractive error. Methods: Previously published Zernike coefficients across retinal eccentricity (0, 10, 20 and 30 degrees horizontal nasal visual field) were used to compute the through-focus modulation transfer function (MTF) for a 4 mm pupil. Image quality was defined as the volume under the MTF, and blur anisotropy was defined as the ratio of the horizontal to vertical meridians of the MTF (HVRatio). Results: Across the horizontal nasal visual field (at 10, 20 and 30 degrees), the peak image quality for emmetropes was within 0.3 D of the retina, as opposed to myopes whose best focus was behind the retina (-0.1, 0.4 and 1.5 D, respectively), while for hyperopes it lay in front of the retina (-0.5, -0.6 and -0.6 D). At 0.0 D (i.e., on the retina), emmetropes and hyperopes both exhibited horizontally elongated blur, whereas myopes had vertically elongated blur (HVRatio = 0.3, 0.7 and 2.8, respectively, at 30 degrees eccentricity). Conclusions: Blur in the peripheral retina is dominated by the so-called "odd-error" blur signals, primarily due to oblique astigmatism. The orientation of peripheral blur (horizontal or vertical) provides the eye with an optical cue for the sign of defocus. All subject groups had anisotropic blur in the nasal visual field; myopes exhibited vertically elongated blur, perpendicular to the blur orientation of emmetropes and hyperopes.