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A. Ravikumar, Y. Shi, H. E. Bedell, R. A. Applegate; How Many JNDs of Blur Before One Line of Best Corrected VA is Lost?. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3956.
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In refractive surgery "20/20 happy" vs "20/20 unhappy" is a common clinical criterion to differentiate success and failure. Our purpose was to determine the number of JNDs for blur that corresponds to a one line loss of best corrected VA in a normal eye.
The 3mm wavefront error (WFE) through the 5th radial order of a well corrected average young eye from the Thibos et al 2002 data set served as the underlying WFE to determine the JND. The coefficients of each Zernike mode were scaled up and down to yield 9 log VSOTF(Visual Strehl ratio computed in frequency domain) values ranging from 0.00 to -0.40 in steps of 0.05 log units. For each of the 9 test conditions, 10 unique letter arrangements of a 100% contrast, three line acuity chart (0.1, 0.0 and -0.1 logMAR) were convolved with PSFs resulting from the scaled WFEs. Three normal subjects had one eye dilated (1% tropicamide) and viewed 300 cd/m2 test charts at 10 ft through a 3mm pupil with the other eye occluded. Using a temporal forced choice paradigm, subjects compared each test chart to a unique reference test chart (log VSOTF value of -0.20) and indicated which chart was blurrier. The difference between 80% and 50% on the psychometric functions fit to 3 replications defined a JND in log VSOTF for each subject. To determine the change in log VSOTF necessary to induce a one line loss in VA, VA was measured up to the 5th letter miss for a series of logMAR charts constructed as above for log VSOTF between 0 to -1.5 in steps of 0.3 log units. Log VSOTF was linearly regressed against logMAR acuity.
The linear regression revealed a best fit line (y = -3.06x - 0.30), with R2 = 0.987. Consequently, a change of 0.31 in log VSOTF induced a loss of one line of logMAR acuity (e.g., -0.1 to 0.0 logMAR). The average JND was 0.049 ±0.014 log VSOTF. Thus, ~6 JNDs comprise 1 line of log MAR acuity.
Different outcomes within the range of 6 JNDs in log VSOTF that correspond to a line of best corrected acuity provide a plausible explanation for the distinction between "20/20 happy" and "20/20 unhappy".
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