Object frequency characteristics of visual acuity

Abstract

Purpose. To examine the extent to which visual acuity (VA) for broadband optotypes is scale invariant by determining whether the same object frequencies mediate VA for individuals with different levels of VA. Methods. LogMAR (minimum angle of resolution) VA for briefly presented tumbling E's was measured in 10 visually normal individuals and in five patients with VA loss. The E's were either unblurred or blurred with Gaussian low-pass filters that had cutoff frequencies spanning a 1.2-log unit range. The data were fit with a standard equivalent intrinsic blur model to estimate each subject's unblurred VA (MAR 0 in minutes of arc) and equivalent intrinsic blur (σ int in minutes of arc). From these estimates, the high-frequency cutoff of the band of retinal frequencies (cpd crit in cycles per degree) and object frequencies (cpl crit in cycles per letter) mediating VA were derived. Results. LogMAR 0 was related linearly to log σ int with a slope of 1.47, which is steeper than that predicted by scale invariance. Log cpd crit was related linearly to logMAR 0 by a slope of -0.64, which is shallower than that predicted by scale invariance. This lack of scale invariance is due to a linear relationship between log cpl crit and logMAR 0 that had a slope of 0.36. Conclusions. The overall pattern of results is not consistent with the expectation of scale invariance underlying the MAR scale. Optotypes that conform to the expectations of scale invariance are needed to improve vision assessment and to provide equivalency of VA defined in terms of MAR and cpd. © 2011 The Association for Research in Vision and Ophthalmology, Inc.