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

Abstract

Purpose To determine the number of JND's of wavefront blur necessary to induce a one line loss of best corrected visual acuity (VA). Setting Visual Optics Institute, College of Optometry, University of Houston, Houston, Texas, USA. Methods The 3mm wavefront error of a well corrected average eye was scaled to yield 9 small steps of blur quantified in units of log Visual Strehl (log VS). For each level of log VS, 10 unique three line acuity charts were convolved with the resulting point spread functions. Using a temporal forced choice paradigm, subjects compared each test chart to a reference test chart indicating which chart was blurrier. The difference between 80% and 50% on the psychometric functions defined a JND. VA was measured for 6 log VS values. The number of JND's necessary to lose one line of acuity was defined as the change in log VS necessary to lose one line of acuity divided by the 1 JND in log VS. Results Linear regression revealed log VS = −2.98 * (logMAR acuity) - 0.31; R2 = 0.961. The average JND in log VS is 0.049 ±0.012 resulting in an average of 6.1 JND's per line of log MAR acuity. Conclusions The RIQ metric, log VS, is highly correlated with logMAR acuity. The 6 JND's in log VS before 1 line of acuity is lost may provide an objective explanation for the distinction between “20/20 happy” and “20/20 unhappy” and other aberration related clinical complaints when acuity is near normal.

Change in visual acuity is highly correlated with change in six image quality metrics independent of wavefront error and/or pupil diameter

“…Image quality has to change even more before a change in acuity is detected. More specifically, Ravikumar, et al 2011 demonstrated that there are on average 6 just noticeable differences in blur for every 5 letter change of logMAR acuity (one line). This “dead zone” shifts the linear regressions to the right and as a consequence, shifts the intercept of the best fitting linear regression to a slightly better acuity.…”

“…3 out of 4 subjects were naive to the psychophysical experiments and the design of the experiment. One subject was familiar with the approach from a previous study (Ravikumar et al, 2011). …”

“…Studies to date using letter acuity as the outcome measure have typically used individual aberrations or combinations of aberrations e.g., (Cheng et al, 2010; Cheng et al, 2004; Legras & Rouger, 2008; Marsack et al, 2004; Rouger et al, 2010; Watson & Ahumada, 2008) or natural aberration structures for a fixed pupil diameter (Nestares et al, 2003) and natural pupil diameters for young adult eyes having a limited range of pupil diameters (Dalimier & Dainty, 2008) and scaled wavefront errors from normal eyes for a fixed pupil diameter (Ravikumar, Applegate, Shi, & Bedell, 2011). …”

Factors Accounting for the 4-Year Change in Acuity in Patients Between 50 and 80 Years

“…3-6 More specifically, research reveals the visual Strehl (the optical Strehl ratio weighted by the neural contrast sensitivity function for interference fringes) 5 is a particularly good metric, accounting for about 80% of variance in the resulting induced change in high contrast logMAR acuity. 4,7 Despite these gains, it is not known if the naturally occurring change in optical quality over years 8-14 within an individual is as well correlated with change in acuity and the extent such measures can help identify individuals on a fast track to an acuity loss.…”

Predicting Through-Focus Visual Acuity with the Eye’s Natural Aberrations

“…26 A more recent study completed by Ravikumar et al found high correlation to distance visual acuity when taking the log of image quality metrics presented by Thibos et al; specifically logNS, logVSX, logVSMTF, and logPFSt gave R 2 Q0.86. 19,20,24 These previous studies show good correlation to distance visual acuity but still present a need for a through-focus image quality metric that takes into account the eye's natural aberrations, pupil diameter, and residual accommodative amplitude. The goal of this work was to develop a new image quality metric that is highly correlated with through-focus visual acuity and can be easily related to clinical measurements.…”