Neural compensation for long-term asymmetric optical blur to improve visual performance in keratoconic eyes

Abstract

Purpose. To investigate whether long-term visual experience with irregular optical blur compensates for the impact of higher-order aberration on visual performance in keratoconic (KC) eyes. Methods. The aberrations and high (100%)- and low (20%)-contrast tumbling E visual acuity (VA) were measured in four moderate KC eyes in which the subjects were wearing their own prescribed soft toric contact lenses over a 6-mm pupil. VA was measured in three emmetropic normal eyes for comparison with each of the four KC eyes. An adaptive optics system was used to correct the aberration of the normal eye and to induce the aberration of the KC eye simultaneously during vision testing. The magnitude of neural compensation was defined as improvement in VA in each KC eye compared with the normal eyes with KC aberrations. Results. Mean total and higher-order root mean square errors in the KC eyes with contact lenses were 2.72 +/- 0.83 mum and 1.36 +/- 0.29 mum, respectively, for a 6-mm pupil. Residual RMS wavefront error in induction of KC aberrations on normal eyes was approximately 0.1 mum in all cases. Each KC eye had statistically better high (P < 0.02)- and low (P < 0.03)-contrast VA than the three normal eyes. Mean compensation for high-contrast VA in logMAR was 0.12 +/- 0.09, corresponding to an improvement of 23.8%. A similar result was obtained for low-contrast VA. The magnitude of compensation increased with the severity of KC aberrations. Conclusions. In KC eyes, the neural visual system compensates for long-term visual experience with an asymmetrically blurred retinal image, resulting in improved visual performance.

Figure 1.

Wavefront map of a moderate KC eye with its routine prescription soft contact lens and of a normal eye that has the same aberration imposed on it. Both wavefront maps are on the same color bar. The residual error of aberration induction of was 0.09 μm in RMS.

Figure 2.

Time-course of RMS for one normal eye when a moderate KC eye's aberration is imposed on it during VA testing. Curve: evolution of RMS with time. At approximately 6.5 seconds, the KC aberration is imposed. Dashed line: RMS for the moderate KC eye. Peaks in the time-course correspond to blinks during the VA test, at which points correction was suspended.

Figure 3.

Comparison of visual performance between KC eyes and normal eyes with KC aberrations. Each data point represents the average of three normal eyes for each KC eye. Dashed line: equal acuity line; shaded region: average 1.2-line improvement in VA in KC eyes.

Figure 4.

Neural compensation (logMAR) versus theoretical everyday retinal image quality. Retinal image quality is represented as the area under the MTF calculated up to 60 cyc/deg. The coefficient of correlation for a linear fit for high- and low-contrast neural compensation is also indicated.