Assessment of Just-Noticeable Differences for Refractive Errors and Spherical Aberration Using Visual Simulation

Purpose. The aim of this study was to evaluate the threshold levels of aberration change that a typical reference eye is able to detect. Methods. The method involved simulation of the foveal vision of a typical eye in polychromatic light through optics affected by different levels of the various chosen monochromatic aberrations. The reference eye had the following monochromatic wavefront characteristics based on the aberrations of a population of young adults: no spherical defocus, astigmatism −0.37 D oriented at 0°, coma −0.17 D/mm oriented at 270°, and spherical aberration −0.12 D/mm2. Average amounts of longitudinal and transverse chromatic aberration were assumed, and allowance was made for the Stiles-Crawford effect. The pupil diameter of the simulated eye was kept fixed at 6 mm. Three observers each compared, 100 times, a simulated image as seen through the standard reference eye with a variant “aberrated” image. The varying parameter was the value of a chosen additional aberration affecting the variant image in the reference eye. The test was repeated for varying amounts of spherical defocus, astigmatic defocus, and spherical aberration. For each of these aberrations and each observer, the discrimination probability as a function of the aberration level in the variant image was determined. The just-noticeable difference in aberration (JNDA) was derived from each discrimination curve as the difference between the aberrations corresponding to discrimination probabilities of 75% and 25%. The JNDA values obtained were expressed in the form of root mean square (RMS) wavefront error thresholds. Results. It was found that 0.04 &mgr;m of RMS aberration should be considered as the threshold of just-noticeable image change, in good agreement with the Maréchal criterion. Conclusions. The results imply that in normal viewing conditions (e.g., a 3-mm pupil size), optical corrections should be in the range of ±0.15 D in sphere and cylinder from the target prescription if perceptible change in the quality of the perceived images is to be avoided. The design of conventional soft contact lenses of high negative power or positive power should aim to produce −0.07 D/mm2 of spherical aberration, with a tolerated interval between −0.15 to +0.01 D/mm2 for a 6-mm pupil size.