Optical phase nullification partially restores visual and stereo acuity lost to simulated blur from higher-order wavefront aberrations of keratoconic eyes

Contrast demodulation and phase distortions are exaggerated in retinal images blurred by the higher-order wavefront aberrations of keratoconic eyes. While the performance loss from the former parameter is well understood, little is known about the impact of the latter on visual functions in this disease condition. The present study investigated the impact of phase distortions on the monocular logMAR visual acuity, letter discriminability and random-dot stereoacuity of seventeen visually healthy adults (ten for visual acuity and letter discriminability; ten for stereoacuity and three common to both experiments) using images that were computationally blurred by four different higher-order wavefront aberration profiles of keratoconic eyes that showed significant distortions in the phase spectrum. Participants viewed these images through 2 mm artificial pupils to negate their native ocular wavefront aberrations. The results showed progressive losses in visual acuity and stereoacuity with increasing blur, a third of which could be recovered following phase nullification. Letter discriminability also improved following phase nullification, more so for smaller than larger optotypes. Stereoacuity loss and, consequently, its recovery following phase nullification was more prominent for profiles simulating unilateral asymmetric keratoconus than for profiles simulating bilateral symmetric keratoconus. These results agree with previous reports obtained from blur induced with lower-order aberrations and indicate that a similar trend may be observed for more complex patterns of blur like keratoconus. Overall, both contrast demodulation and misalignment of the local features of the blurred image may contribute to losses of spatial and depth vision in keratoconus. Phase nullification may partially mitigate these losses, thereby allowing the processing of finer spatial details and veridical disparity estimations for improved depth perception.