Improved phase processing following long-term adaptation to optical aberrations in keratoconus

Abstract

Purpose: Optical aberrations detrimentally affect both the amplitude and phase relationships between spatial frequencies (SFs) of visual inputs. In addition, long-term presence of optical aberrations progressively alters the way visual information is processed. Here, we investigate the basic neural substrates of altered visual processing resulting from prolonged, chronic exposure to optically degraded retinal image quality by using keratoconus (KC) as a model of long-term adaptation to visual aberrations.

Methods: An adaptive optics (AO) vision simulator was first used to measure KC patients’ habitual aberrations with their own corrective, conventional lenses. We then used these measurements to simulate KC optical quality in normal eyes using AO after correcting their native aberrations. Under this identical optically-aberrated condition, we measured tumbling E visual acuity (VA) and contrast sensitivity function (CSF) for KC eyes (n=4) with their habitual aberrations and normal eyes (n=3) with AO-induced KC aberrations, over a 6-mm artificial pupil in white light. Each control subject was tested in both experiments under each KC aberration profile.

Results: Under identical, optically-aberrated conditions (average total and higher order RMS errors in KC eyes: 2.72±0.83μm and 1.36±0.29μm, respectively; residual RMS wavefront error of induced KC aberrations in normal eyes: ~0.1μm), we found that the CSF (single SF Gabor stimuli) did not differ between KC and normal eyes. However, KC eyes showed better letter acuity (broadband SF stimuli) than normal eyes under the same conditions. Specifically, neural compensation in KC eyes accounted for ~1.2 line improvement in VA relative to normal eyes and was stronger for advanced KC conditions (~3 lines improvement for more severe cases). This difference in visual performance with broadband SF stimuli indicates that phase information plays an important role in long-term adaptation to visual aberrations.

Conclusions: Our results suggest the existence of an adaptive neural compensation mechanism in KC subjects that partially restores the phase congruency across SFs, thus benefiting the processing of optically degraded visual inputs. Altogether, our findings provide fundamental insights into the mechanisms underlying long-term neural adaptation to optical aberrations.