Determination of Customized Aberration Thresholds

Investigative Ophthalmology & Visual Science
Carmen Canovas; Patricia A. Piers; Silvestre Manzanera; Christina Schwarz; Pedro M. Prieto; Henk A. Weeber; Pablo Artal

Purpose: Different aberration terms may have lower or higher impact on vision. This could be used to optimize correction strategies. We studied the customized level of spherical aberration (SA) and vertical coma (VC) that can be induced while maintaining acceptable high contrast visual acuity (VA) and to then determine whether these levels of aberration affect subjects’ through focus vision.
Methods: An adaptive optics visual simulator (Fernández, et al. Opt. Lett. 2001) was used to correct individual aberrations up to the fifth order and to induce a common higher order aberration baseline (0.15 µm RMS for a 5mm pupil) corresponding to that of an average pseudophakic eye, in five normal subjects. High contrast VA was measured with SLOAN letters at the best focus position with a 4mm physical pupil. Letter size was then increased to correspond to a decrease in VA by 0.1LogMAR. Thresholds for SA (positive and negative) and positive VC were then subjectively determined by increasing its value in steps of 0.02 µm until the letter could not be read.
Results: The threshold value for negative SA was the smallest (-0.16±0.04 µm at 5mm pupil), followed by positive SA (0.25±0.05 µm). The custom threshold for VC was consistently the highest (0.78±0.12 µms), ranging from 0.70 to 0.98 µms. Monocular through focus VA was measured, both for the baseline aberration level and after inducing the individual threshold for coma. In this case, VA for the best focus position decreased by 0.16LogMAR with respect to the baseline, reaching an average absolute VA of 0.04± 0.04LogMAR. VA decreased linearly as a function of defocus in both cases, but has a flatter slope when the individual threshold for VC was induced (0.17LogMAR per diopter of defocus versus 0.40LogMAR, respectively).
Conclusions: Customized aberration thresholds for individual Zernike modes were similar for the aberrations studied for all subjects. The high contrast VA threshold for positive VC was consistently higher than that for SA demonstrating the robustness of our visual system to this aberration and indicating that disturbances resulting in VC, such as decentration of optical surfaces that correct spherical aberration, may play a less impact on vision than originally believed. The induction of VC may be advantageous for extending depth of focus.