Comparing the Shape of Contrast Sensitivity Functions for Normal and Low Vision.

Low Vision Comparing the Shape of Contrast Sensitivity Functions for Normal and Low Vision Susana T. L. Chung 1 and Gordon E. Legge 2 School of Optometry, University of California, Berkeley, Berkeley, California, United States Department of Psychology, University of Minnesota, Minneapolis, Minnesota, United States Correspondence: Susana T. L. Chung, 360 Minor Hall, School of Optometry, University of California, Berkeley, Berkeley, CA 94720-2020, USA; s.chung@berkeley.edu. Submitted: August 31, 2015 Accepted: December 9, 2015 Citation: Chung STL, Legge GE. Com- paring the shape of contrast sensitivity functions for normal and low vision. Invest Ophthalmol Vis Sci. 2016;57:198–207. DOI:10.1167/ iovs.15-18084 P URPOSE . The contrast sensitivity function (CSF) provides a detailed description of an individual’s spatial-pattern detection capability. We tested the hypothesis that the CSFs of people with low vision differ from a ‘‘normal’’ CSF only in their horizontal and vertical positions along the spatial frequency (SF) and contrast sensitivity (CS) axes. M ETHODS . Contrast sensitivity for detecting horizontal sinewave gratings was measured with a two temporal-interval forced-choice staircase procedure, for a range of SFs spanning 5 to 6 octaves, for 20 low-vision observers and five adults with normal vision. An asymmetric parabolic function was used to fit the aggregate data of the normal-vision observers, yielding the ‘‘normal template.’’ Each of the 20 low-vision CSFs was fit in two ways, by using a shape- invariant version of the normal template (with the width parameters fixed) that was shifted along the log-SF and log-CS axes, and by an unconstrained asymmetric parabolic function (‘‘free-fit’’). R ESULTS . The two fitting methods yielded values of the peak CS, the SF corresponding to peak CS, and the high cut-off SF that were highly correlated and in good agreement with each other. In addition, the width parameters of the low-vision CSFs were comparable with those of the normal template, implying that low-vision CSFs are similar in shape to the normal CSF. C ONCLUSIONS . The excellent agreement of parameters estimated by the two fitting methods suggests that low-vision CSFs can be approximated by a normal CSF shifted along the log-SF and log-CS axes to account for the impaired acuity and contrast sensitivity. Keywords: contrast sensitivity function, low vision, psychophysics, spatial vision ur ability to detect the presence of an object depends on the size of the object (larger is generally easier to detect), and also on the presence of any differences, such as a luminance difference, between the object and its background. The sensitivity to the relative difference in luminance of an object from its background is referred to as contrast sensitivity. Contrast sensitivity depends on object size. A complete representation of how contrast sensitivity depends on object size is referred to as the contrast sensitivity function (CSF), where the object size is usually specified in spatial frequency (c/deg) of a sinewave pattern. As such, the CSF provides a rich description of an individual’s spatial-pattern detection capabil- ity. Knowing the CSF of a person with low vision is often informative about their ability to see shapes and recognize objects in their daily lives. The gold standard for determining a CSF is to measure contrast thresholds for detecting sinusoidal gratings across a range of spatial frequencies using robust psychophysical techniques. 1,2 However, this method is time consuming, technically demanding, and requires a carefully calibrated display, and thus is not amenable for the determination of CSF for clinical patients. Recently, a method that uses a Bayesian adaptive procedure to select the spatial frequency and contrast of each trial to maximize the expected information gain has been developed for measuring CSFs. 3,4 The efficiency of this quick CSF method relies on its assumptions about the shape of the CSF and requires 100 trials to achieve good O agreement with a CSF measured with the conventional method. 4 The determination of CSF could be made more efficient if we could reduce the number of measurements required to estimate the full CSF, and link those measurements to standard clinical measures. In fact, an important simplifica- tion would exist if low-vision CSFs are similar in shape to normal CSFs, differing only in their positions on the spatial frequency (SF) and contrast sensitivity (CS) axes. In this paper, we test this hypothesis. There are several ways in which low-vision CSFs could differ in shape from normal-vision CSFs. For example, if contrast sensitivity loss occurs primarily at high spatial frequencies, then the fall-off of contrast sensitivity with high spatial frequencies would be steeper than observed in a normal-vision CSF (Fig. 1A). In the case that contrast sensitivity is equally affected across all spatial frequencies, the resulting CSF would appear as a vertical shift of a normal-vision CSF (Fig. 1B). Alternatively, if the reduction is not uniform across frequencies, then the resulting CSF may look like the one shown in Figure 1C. In addition, notches, representing CS loss only at SFs between the peak SF and the high SF cutoff (Fig. 1D), have been reported for patients with neurologic diseases such as multiple sclerosis. 5 Notches can also be due to uncorrected refractive errors 6 or imprecision of measurements 7 ; but because they are not the etiologies of low vision, they will not be considered for the purpose of this paper. Despite the fact that some low-vision CSFs may differ in shape from normal-vision CSFs, a previous iovs.arvojournals.org j ISSN: 1552-5783 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/Journals/IOVS/934840/ on 01/21/2016