Visual acuity as a function of Zernike mode and level of root mean square error

Background. The coefficients of normalized Zernike expansion are orthogonal and reflect the relative contribution of each mode to the total root mean square (RMS) wavefront error. The relationship between the level of RMS wavefront error within a mode and its effect on visual performance is unknown. Purpose. To determine for various levels of RMS wavefront error how each mode of the normalized Zernike expansion for the second, third, and fourth orders affect high- and low-contrast acuity. Methods. Three healthy optimally corrected cyclopleged subjects read aberrated and unaberrated high- and low-contrast logarithm of the minimum angle of resolution acuity charts monocularly through a 3-mm artificial pupil. Acuity was defined by the total number of letters read correctly up to the fifth miss. Aberrated and unaberrated charts were generated using a program called CTView. Six levels of RMS wavefront error were used (0.00, 0.05, 0.10, 0.15, 0.20, and 0.25 mum). Each level of RMS error was loaded into each mode of the second, third, and fourth radial orders individually for a total of 72 charts. Data were normalized by subject, and the normalized data were averaged across subjects. Results. Across modes and within each mode as the level of RMS wavefront error increased above 0.05 mum of RMS wavefront error, visual acuity decreased in a linear fashion. Slopes of the linear fits varied depending on the mode. Modes near the center of the Zernike pyramid had steeper slopes than those near the edge. Conclusions. Increasing the RMS error within any single mode of the normalized Zernike expansion decreases visual acuity in a linear fashion. The slope of the best fitting linear equation varies with Zernike mode. Slopes near the center of the Zernike pyramid are steeper than those near the edge. Although the normalized Zernike expansion parcels RMS error orthogonally, the resulting effects on visual performance as measured by visual acuity are not orthogonal. New metrics of the combined effects of the optical and the neural transfer functions that are predictive of visual performance need to be developed.