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S. T. L. Chung, H. E. Bedell; Tolerance of Letter Acuity to Defocus and Blur Discrimination in Observers With Low Vision. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2349.
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Previously we found that the tolerance of letter acuity to defocus parallels the ability to discriminate blurred images in observers with normal vision. In this study, we investigated whether this relationship between the tolerance of letter acuity to defocus and blur discrimination holds for people with low vision, who have a higher tolerance to blur than people with normal vision.
We measured monocular letter acuity and blur discrimination in the presence of blur, in seven observers with low vision (age: 62 to 90; logMAR acuity: 0.2 to 1.0). Letter acuity was determined for 4-orientation letter Ts, in the presence of various amount of dioptric blur, up to between +4 and +11 D for different observers. Observers were required to identify the orientation of the Ts. Blur discrimination was determined using pairs of edges, each represented by a cumulative Gaussian luminance profile. One edge was a reference with base blur B; the other comparison edge had blur B+delta B. The task of the observer was to indicate which of the sequentially presented edges was more blurred. Five levels of base blur, with standard deviations ranging from 0 to 33.8 arc min, were tested.
As expected, optimal blur discrimination thresholds were worse (range: 3 - 12 arc min) in our group of low vision observers than at the fovea of the normal observers (average = 0.27 arc min). However, the shapes of the blur-discrimination, and acuity-vs.-defocus functions remain similar to those of normal observers. In contrast to normal observers, when dioptric and Gaussian blur were converted to a common spatial frequency scale, the acuity and blur-discrimination functions of the observers with low vision do not superimpose on each other. In these observers, the estimates of "critical blur" (the largest amount of blur that can be tolerated without loss of performance) from the acuity and blur-discrimination functions were uncorrelated.
The amount of dioptric blur that can be tolerated without significantly affecting visual acuity is used clinically to represent patients' sensitivity to blur. Unfortunately, the tolerance to defocus for letter acuity is a poor predictor of how well people with low vision can discriminate blurred images.
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