Abstract
Purpose :
It has been found that the neural visual system exhibits orientation-dependent sensitivity in the peripheral visual fields, where sensitivity is higher for radially-oriented stimuli compared to tangentially-oriented stimuli. However, the cause of neural anisotropy remains unclear. Adaptation to the optics of the eye, which cause asymmetrical blur in the periphery due to an increase in assymetrical aberrations such as astigmatism and coma, provides a plausible explanation for this phenomenon. Furthermore, asymmetrical peripheral blur could provide a cue for axial elongation during developmental stages of eye growth. The goal of this study is to examine the effects of astigmatism on neural anisotropy in the periphery.
Methods :
An adaptive optics (AO) visual simulator was used to bypass ocular optics by simultaneously measuring and correcting low- and higher-order aberrations of seven healthy astigmatic subjects (cyl ≥ 0.5 D) while they performed monocular psychophysical tasks (contrast sensitivity, two alternative forced-choice) displayed through the optical system by a projector. Subjects were dilated with tropicamide, and foveal and peripheral aberrations, including vertical astigmatism (z5), were measured using a 6 mm pupil. Contrast thresholds for vertical and horizontal gratings were measured at 15° nasal visual field for the same pupil size. Data are presented as mean ± SD.
Results :
Subjects with against-the-rule (ATR) astigmatism had positive z5 (0.92 ± 0.86 µm, 1.53 ± 1.52 µm) at fovea and periphery, respectively, while with-the-rule (WTR) subjects had negative z5 (-0.81 ± 0.82 µm, -0.36 ± 0.33 µm) at both visual field locations. Contrast thresholds were the same or lower for horizontal gratings (0.042 ± 0.018), compared to vertical gratings (0.061 ± 0.031), for all seven subjects in the 15° nasal visual field.
Conclusions :
The axis of cylindrical refractive error at fovea impacts the magnitude of z5 in the periphery, with ATR subjects having a relatively higher magnitude of z5 and WTR subjects having a lower magnitude of z5 in the nasal peripheral field. Likewise, the intrinsic ATR astigmatism in the nasal periphery can be amplified or compensated by foveal ATR and WTR astigmatism, respectively. Under AO conditions, all seven subjects preferred radially-oriented gratings in the periphery, regardless of their on-axis cylinder. This finding suggests that astigmatism may not be the underlying cause of meridional anisotropy.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.