Contributions of Optical and Non-Optical Blur to Variation in Visual Acuity

Abstract

Purpose To determine the relative contributions of optical and non-optical sources of intrinsic blur to variations in visual acuity (VA) among normally sighted subjects. Methods Best-corrected VA of sixteen normally sighted subjects was measured using briefly presented (59 ms) tumbling E optotypes that were either unblurred or blurred through convolution with Gaussian functions of different widths. A standard model of intrinsic blur was used to estimate each subject’s equivalent intrinsic blur (σint) and VA for the unblurred tumbling E (MAR0). For 14 subjects, a radially averaged optical point spread function due to higher-order aberrations was derived by Shack-Hartmann aberrometry and fit with a Gaussian function. The standard deviation of the best-fit Gaussian function defined optical blur (σopt). An index of non-optical blur (η) was defined as: 1-σopt/σint. A control experiment was conducted on 5 subjects to evaluate the effect of stimulus duration on MAR0 and σint. Results Log MAR0 for the briefly presented E was correlated significantly with log σint (r = 0.95, p < 0.01), consistent with previous work. However, log MAR0 was not correlated significantly with log σopt (r = 0.46, p = 0.11). For subjects with log MAR0 equivalent to approximately 20/20 or better, log MAR0 was independent of log η, whereas for subjects with larger log MAR0 values, log MAR0 was proportional to log η. The control experiment showed a statistically significant effect of stimulus duration on log MAR0 (p < 0.01) but a non-significant effect on σint (p = 0.13). Conclusions The relative contributions of optical and non-optical blur to VA varied among the subjects, and were related to the subject’s VA. Evaluating optical and non-optical blur may be useful for predicting changes in VA following procedures that improve the optics of the eye in patients with both optical and non-optical sources of VA loss.

Effects of Optical Blur Reduction on Equivalent Intrinsic Blur

“…15, 21-23 The range of σ int measured through the AO system was consistent with previously reported measurements obtained with the use of a CRT monitor. 2, 3 σ int was reduced due to AO correction of wavefront error, which resulted in improvement of VA measured with an unblurred stimulus, in agreement with previous studies. 16, 24-28 The extent to which log MAR was improved due to AO depended on the level of stimulus blur.…”

“…Examples of the letter stimuli at different sizes and blur levels that are identical to the stimuli used in the current report have been presented elsewhere. 2 The blurred Es were generated by convolution with 2D low-pass Gaussian functions of 4 different standard deviations (σ stim ): 0.0 (unblurred), 0.4, 1.6, and 6.5 arcmin. The stimuli were presented on the microdisplay for duration of 100 ms and viewed through a 5 mm artificial pupil positioned in a plane conjugate to the subject's pupil.…”

“…As previously published, 2-4 equivalent intrinsic blur (σ int ) was derived from the log MAR values measured for each level of stimulus blur. In brief, the threshold log MAR values were plotted as a function of log σ stim and were fit with the log form of the following equation: 4 …”

“…2 Briefly, the wavefront aberration function was calculated for a 5.5 mm pupil diameter and included residual low-order and high-order (3rd to 7th) aberrations. From the wavefront aberration function, the two-dimensional optical point spread function (PSF) was derived according to standard transformations.…”

“…3, 4 Equivalent intrinsic blur is an estimate of the amount of blur within the visual system, 1, 4, 5 and is comprised of optical blur due to the presence of ocular aberrations and neural blur attributable to neural filtering. 2, 6 However, the extent to which optical and neural sources of blur contribute separately to equivalent intrinsic blur is presently unclear. A number of studies have examined how neural mechanisms adapt to the optically blurred retinal image, 7-10 suggesting the dependence of neural blur on optical blur.…”