Abstract
Purpose: :
To determine the effects of astigmatism on contrast sensitivity (CS).
Methods: :
Twelve normal volunteers (31.2 ± 5.2 [mean ± SD] years) with spherical equivalent refraction between 0 and -6.00 diopters (D) and refractive astigmatism up to 0.75 D were recruited. After correcting each refractive error by spectacles, against-the-rule (ATR) or with-the-rule (WTR) astigmatism of +1.0, +2.0 and +3.0D was intentionally produced in both eyes, and then binocular CS was measured. The cylindrical addition of different powers (+1.0 ~ +3.0D) was compensated with spherical lenses so that the spherical equivalent refraction became zero for each eye. Subsequently, the above cylindrical addition was monocularly induced (i.e.; no astigmatism in fellow eye), and binocular CS was measured again. The order of cylindrical induction with different powers and axes was randomly determined. CS was assessed at five spatial frequencies (1.5, 3, 6, 12 and 18 cycles per degree [cpd]). We investigated CS in relation to astigmatic power, axis, and monocular or binocular astigmatism.
Results: :
Under +3.0D astigmatic defocus, the area under the log CS function (AULCSF) in binocular ATR astigmatism was 1.15 ± 0.55 and significantly worse than that in binocular WTR (1.84 ± 0.15), monocular ATR (1.95 ± 0.22), and monocular WTR (1.96 ± 0.14) astigmatism (p < 0.0001, Fisher’s PLSD). Under binocular ATR and WTR astigmatism, AULCSF decreased as astigmatic power increased (r = -0.734, p < 0.0001 for ATR, r = -0.591, p < 0.0001 for WTR, Spearman correlation test). In contrast, astigmatic power did not affect AULCSF under monocular astigmatic defocus. In binocular high-power (+2.0 and +3.0D) astigmatic defocus, ATR astigmatism resulted in significantly worse AULCSF than WTR (p < 0.01, Mann-Whitney U test). When compared between binocular and monocular astigmatic defocus, CS under binocular conditions was significantly worse than that under monocular conditions at higher spatial frequencies (especially at 12 and 18 cpd) regardless of astigmatic power (p < 0.05).
Conclusions: :
Binocular astigmatic defocus caused CS reduction depending on increases in astigmatic power. The influence of ATR astigmatism on CS was larger than that of WTR astigmatism. In addition, binocular astigmatic defocus affected CS more severely than monocular astigmatic defocus especially at high spatial frequencies.
Keywords: contrast sensitivity • astigmatism • refraction