Visual Acuity With Scaled Natural And Modified Aberrations

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

Purpose: The visual system has been shown to be able to adapt to the individual’s ocular aberrations (Artal et al., J.Vis. 2004). This study explores the impact of neural adaptation to the aberrations on high contrast visual acuity (VA).
Methods: An adaptive optics visual simulator (Fernández, et al. Opt. Lett. 2001) was used to measure the high contrast VA, using SLOAN letters for a variety of aberration patterns in four normal subjects with paralyzed accommodation. Measurements were performed at the best focus position and with a 4-mm pupil diameter. The following aberration patterns were applied: the subject’s natural aberrations and a modified aberration pattern calculated to provide the same optical quality as that of the natural aberrations (equivalent Strehl ratio) but with the Zernike aberration terms modified in a randomized fashion. For both aberration patterns, natural and modified, VA was also measured when the aberrations were scaled by constant factors (M=1,2,3 and 4).
Results: Although there was individual variability, the average LogMAR VA was -0.14 for natural aberrations. This was reduced to -0.06 for the modified case. For the natural case LogMAR VA increased linearly as a function of M with a slope value of 0.06 while for the case of modified aberrations LogMAR VA increased at a higher rate (0.11 LogMAR units per M value). The variability was higher for the modified cases when compared to the natural aberration cases.
Conclusions: In the absence of aberration correction, VA was better when subjects performed testing through their natural aberration pattern than for the case when aberrations were modified. This is true even though in both cases the retinal image quality was equivalent. The relative reduction of VA, as a function of the aberration scale factor, doubled for the case of modified aberrations. These results may suggest that the neural adaptation to the high order aberrations also play a role when these aberrations are scaled. From a practical point of view, it may be advantageous to induce aberrations by scaling the natural aberrations present in an individual’s eye.