Fig. 7. The orientation differences in grating acuity for two optically corrected astigmias. A. Acuity is reduced for vertical gratings in an eye with a refractive error of + 4.50 -5.00 × 180.B. Acuity is reduced for horizontal gratings in a person withrefractive errors of -2.00 -4.25 × 180 right eye, -1.00 -4.50 × 180 left eye. The closed and open circles represent the right and left eyes, respectively. The drawing of an eye adjacent to each curve illustrates the focus of vertical and horizontal lines in the absence of optic correction and accommodation. Grating acuity for diagonal orientations is normally less than for either vertical or horizontal. (Mitchell DE, Freeman RD, Millodot M et al. Meridional amblyopia: Evidence for modification of the human visual system by early visual experience. Vis Res 1973;13:535.)