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J Jason McAnany, Kenneth R. Alexander, Mahnaz Shahidi; Factors Affecting Visual Acuity for Broadband Optotypes as a Function of Duration. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1902.
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© ARVO (1962-2015); The Authors (2016-present)
Visual acuity (VA) for broadband letter optotypes is known to improve with increasing exposure duration. The purpose of this study was to determine the effect of duration on three factors that are related to VA: equivalent intrinsic blur, object spatial frequency, and equivalent retinal spatial frequency.
VA of eight visually normal individuals (ages 22 to 59) was measured for a set of tumbling E optotypes presented on a computer display for 59 ms and 590 ms. The Es were either unblurred or blurred through convolution with Gaussian functions of different widths (σ = 0.2, 0.8 or 3.2 arcmin). Equivalent intrinsic blur (σint) was estimated with a standard model: MAR = MAR0[1 + (σstim/ σint)2 ]0.5, where σstim is the width of the Gaussian convolved with the stimulus, MAR is VA for a given value of σstim, and MAR0 is VA for the unblurred stimulus. The object spatial frequencies mediating VA (cycles per letter; cpl) were derived from plots of log MAR vs. log σstim. Object spatial frequency was converted to equivalent retinal spatial frequency (cycles per degree; cpd) based on target angular subtense.
The increase in duration produced a statistically significant improvement in unblurred VA (decrease in log MAR0), as expected (p<0.01). In comparison, the change in duration did not significantly affect equivalent intrinsic blur (σint) (p=0.25), as a result of similar temporal integration for the different values of stimulus blur. The increase in duration produced a statistically significant decrease in object spatial frequency (cpl) (p<0.01), but not equivalent retinal spatial frequency (cpd) (p=0.16).
The decreases in log MAR0 and object spatial frequency together with the constant values of equivalent intrinsic blur and equivalent retinal spatial frequency for these two durations do not support scale invariance. Instead, visual acuity is based on different object spatial frequency information at different durations.
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