Visual acuity for vertical and horizontal lines on targets containing both was found to be unequal, though there was no detectable astigmatism in the unaccommodated eye. This difference in acuity, which tends to favor the vertical lines, is a function of accommodation and is affected systematically by changes in the vergence of the light rays. The findings suggest that the observed asymmetry in acuity resulted from a vertical-horizontal astigmatism produced by accommodation. Thus, the absence of detectable astigmatism in the unaccommodated eye is not sufficient to rule out dioptric factors as a cause of the often-reported effects of orientation on pattern acuity.
JACOB BECK, "Accommodative Astigmatism and Pattern Acuity," J. Opt. Soc. Am. 55, 1139-1142 (1965)