Yue Shi

Six just-noticeable differences in retinal image quality in 1 line of visual acuity: Toward quantification of happy versus unhappy patients with 20/20 acuity

PURPOSE: To determine the number of just‐noticeable differences in wavefront blur necessary to induce a 1‐line loss of corrected distance visual acuity (CDVA). SETTING: Visual Optics Institute, College of Optometry, University of Houston, Houston, Texas, USA. DESIGN: Evidence‐based manuscript. METHODS: The 3.0 mm wavefront error of a well‐corrected average eye was scaled to yield 9 small steps of blur quantified in units of log visual Strehl (logVS). For each logVS value, 10 unique 3‐line acuity charts were generated. Using a temporal forced‐choice paradigm, subjects compared each test chart to a reference acuity chart and indicated which chart was blurrier. The difference between 80% and 50% on the psychometric function defined a just‐noticeable difference. The CDVA was measured up to fifth‐letter miss for several aberrated logMAR charts for 6 logVS values. The number of just‐noticeable differences necessary to lose 1 line of acuity was defined as the change in logVS necessary to lose 1 line of acuity divided by the 1 just‐noticeable difference in logVS. RESULTS: Linear regression showed that logVS = −2.98 × (logMAR acuity) − 0.31 (R 2 = 0.961). The mean just‐noticeable difference was 0.049 logVS ± 0.012 (SD), resulting in a mean of 6.1 just‐noticeable differences per line of logMAR acuity. CONCLUSIONS: The retinal image quality metric logVS was highly correlated with logMAR acuity. The 6 just‐noticeable differences in logVS before 1 line of acuity was lost may provide an objective explanation for the distinction between patients with 20/20 CDVA who are happy and patients with 20/20 CDVA who are unhappy and other aberration‐related clinical complaints when acuity is near normal. Financial Disclosure: Ms. Ravikumar, Ms. Shi, Dr.Applegate, and Dr. Bedell have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Optimizing wavefront-guided corrections for highly aberrated eyes in the presence of registration uncertainty.

Dynamic registration uncertainty of a wavefront-guided correction with respect to underlying wavefront error (WFE) inevitably decreases retinal image quality. A partial correction may improve average retinal image quality and visual acuity in the presence of registration uncertainties. The purpose of this paper is to (a) develop an algorithm to optimize wavefront-guided correction that improves visual acuity given registration uncertainty and (b) test the hypothesis that these corrections provide improved visual performance in the presence of these uncertainties as compared to a full-magnitude correction or a correction by Guirao, Cox, and Williams (2002). A stochastic parallel gradient descent (SPGD) algorithm was used to optimize the partial-magnitude correction for three keratoconic eyes based on measured scleral contact lens movement. Given its high correlation with logMAR acuity, the retinal image quality metric log visual Strehl was used as a predictor of visual acuity. Predicted values of visual acuity with the optimized corrections were validated by regressing measured acuity loss against predicted loss. Measured loss was obtained from normal subjects viewing acuity charts that were degraded by the residual aberrations generated by the movement of the full-magnitude correction, the correction by Guirao, and optimized SPGD correction. Partial-magnitude corrections optimized with an SPGD algorithm provide at least one line improvement of average visual acuity over the full magnitude and the correction by Guirao given the registration uncertainty. This study demonstrates that it is possible to improve the average visual acuity by optimizing wavefront-guided correction in the presence of registration uncertainty.

Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors

We determined the degree to which change in visual acuity (VA) correlates with change in optical quality using image-quality (IQ) metrics for both normal and keratoconic wavefront errors (WFEs). VA was recorded for five normal subjects reading simulated, logMAR acuity charts generated from the scaled WFEs of 15 normal and seven keratoconic eyes. We examined the correlations over a large range of acuity loss (up to 11 lines) and a smaller, more clinically relevant range (up to four lines). Nine IQ metrics were well correlated for both ranges. Over the smaller range of primary interest, eight were also accurate and precise in estimating the variations in logMAR acuity in both normal and keratoconic WFEs. The accuracy for these eight best metrics in estimating the mean change in logMAR acuity ranged between ±0.0065 to ±0.017 logMAR (all less than one letter), and the precision ranged between ±0.10 to ±0.14 logMAR (all less than seven letters).

A Comparison of Three Methods to Increase Scleral Contact Lens On-Eye Stability.

Purpose: To quantify on-eye rotational and translational stability of three scleral contact lens stabilization methods and to model the variation in visual acuity when these movements occur in a wavefront-guided correction for highly aberrated eyes. Methods: Three lens stabilization methods were integrated into the posterior periphery of a scleral contact lens designed at the Visual Optics Institute. For comparison, a lens with no stabilization method (rotationally symmetric posterior periphery) was designed. The lenses were manufactured and lens movements were quantified on 8 eyes as the average SD of the observed translations and rotations over 60 min of wear. In addition, the predicted changes in acuity for five eyes with keratoconus wearing a simulated wavefront-guided correction (full correction through the fifth order) were modeled using the measured movements. Results: For each lens design, no significant differences in the translation and rotation were found between left and right eyes, and lenses behaved similarly on all subjects. All three designs with peripheral stability modifications exhibited no statistically significant differences in translation and rotation distributions of lens movement and were statistically more stable than the spherical lens in rotation. When the measured movements were used to simulate variation in visual performance, the 3 lenses with integrated stability methods showed a predicted average loss in acuity from the perfectly aligned condition of approximately 0.06 logMAR (3 letters), compared with the loss of over 0.14 logMAR (7 letters) for the lens with the spherical periphery. Conclusion: All three stabilization methods provided superior stability, as compared with the spherical lens design. Simulations of the optical and visual performance suggest that all three stabilization designs can provide desirable results when used in the delivery of a wavefront-guided correction for a highly aberrated eye.

Registration tolerance of a custom correction to maintain visual acuity.

Purpose To present a predictive model of the registration tolerance for wavefront-guided correction to maintain acuity within fixed limits and demonstrate the potential utility using two typical keratoconic eyes. Methods Change in log visual Strehl was plotted as a function of translation error for a series of rotations of a wavefront-guided correction. Contour lines were added at &Dgr;log visual Strehl levels predicted to induce one- and two-line losses of logMAR visual acuity. The model was validated by regressing measured acuity loss from subjects viewing acuity charts that were degraded by the residual wavefront error resulting from the movement of wavefront-guided correction against the model’s predicted acuity. Results The model’s predicted change in acuity can be substituted for measured change in acuity (R2 = 0.91) within measurement error (±0.1 logMAR). Translation and/or rotation of a wavefront-guided correction induced asymmetric optical tolerance to movement. Induced errors depended on the wavefront error being corrected, the wavefront-guided correction design, and the amount of registration error. Conclusions Change in log visual Strehl can be used to determine the registration tolerance necessary to keep the variation in acuity within user-defined limits. This tolerance is unique for each wavefront error and wavefront-guided correction design.