The importance of grating orientation in contrast sensitivity following refractive surgery

Higher-order aberrations degrade visual performance when the pupil is large, particularly in eyes that have abnormally high amounts of aberrations, such as those that have undergone refractive surgery. Geometrical optics predicts that asymmetrical aberrations such as coma can produce orientation selective effects, much like uncorrected astigmatism. Coma is also one of the main aberrations to increase following refractive surgery. Orientation may therefore be an important parameter when testing grating based contrast sensitivity, particularly in refractive surgery patients. Contrast sensitivity to four orientations of a 12 cycles deg(-1) sine wave grating was measured in normals (n = 34) and refractive surgery patients (n = 12). In over a third of normal subjects the higher-order aberrations produced a significant orientation-specific change in contrast sensitivity (13 out of 34 eyes, p < 0.05). No significant differences existed in the aberrations between those that displayed orientation-selective changes and those that did not. In subjects who underwent refractive surgery those that displayed orientation selective changes had significantly higher amounts of overall aberrations and also higher amounts of primary coma than those that did not. These results indicate that grating orientation is an important factor when assessing the effects of higher-order aberrations on contrast sensitivity.