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(Investigative Ophthalmology and Visual Science. 2006;47:2739-2745.)
© 2006 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.05-1419
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The Letter Contrast Sensitivity Test: Clinical Evaluation of a New Design

Sharon A. Haymes,1 Kenneth F. Roberts,1 Alan F. Cruess,1 Marcelo T. Nicolela,1 Raymond P. LeBlanc,1 Michael S. Ramsey,1 Balwantray C. Chauhan,1 and Paul H. Artes1,2

1From the Department of Ophthalmology and Visual Sciences, Dalhousie University, Halifax, Canada; and the 2Faculty of Life Sciences, The University of Manchester, Manchester, United Kingdom.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. To compare the reliability, validity, and responsiveness of the Mars Letter Contrast Sensitivity (CS) Test to the Pelli-Robson CS Chart.

METHODS. One eye of 47 normal control subjects, 27 patients with open-angle glaucoma, and 17 with age-related macular degeneration (AMD) was tested twice with the Mars test and twice with the Pelli-Robson test, in random order on separate days. In addition, 17 patients undergoing cataract surgery were tested, once before and once after surgery.

RESULTS. The mean Mars CS was 1.62 log CS (0.06 SD) for normal subjects aged 22 to 77 years, with significantly lower values in patients with glaucoma or AMD (P < 0.001). Mars test-retest 95% limits of agreement (LOA) were ±0.13, ±0.19, and ±0.24 log CS for normal, glaucoma, and AMD, respectively. In comparison, Pelli-Robson test-retest 95% LOA were ±0.18, ±0.19, and ±0.33 log CS. The Spearman correlation between the Mars and Pelli-Robson tests was 0.83 (P < 0.001). However, systematic differences were observed, particularly at the upper-normal end of the range, where Mars CS was lower than Pelli-Robson CS. After cataract surgery, Mars and Pelli-Robson effect size statistics were 0.92 and 0.88, respectively.

CONCLUSIONS. The results indicate the Mars test has test-retest reliability equal to or better than the Pelli-Robson test and comparable responsiveness. The strong correlation between the tests provides evidence the Mars test is valid. However, systematic differences indicate normative values are likely to be different for each test. The Mars Letter CS Test is a useful and practical alternative to the Pelli-Robson CS Chart.

Contrast sensitivity (CS) is a fundamental aspect of vision. Its measurement provides useful independent information in relation to a patient’s visual function, which may not be revealed by visual acuity (VA).1 2 3 4 5 There is considerable evidence that it is a strong predictor of real-world performance, providing insight into a patient’s disability and quality of life.6 Specifically, studies have shown a significant relationship between CS and driving performance,7 mobility and walking speed,8 postural stability and falls,9 10 face recognition,11 reading speed,12 13 computer task accuracy,14 and ability to perform activities of daily living.15 16 Furthermore, there is evidence to suggest that CS measurement may have some value in the detection and progression of ocular diseases, such as cataract,17 glaucoma,2 18 age-related macular degeneration (AMD),19 20 diabetic retinopathy,21 and optic neuritis.22 CS tests have been useful for evaluating cataract surgery,23 YAG laser capsulotomy,24 intraocular lenses,25 26 medications and surgery for glaucoma,27 28 verteporfin and radiation therapy for AMD,29 laser photocoagulation and pharmaceutical therapeutics for diabetic retinopathy,30 31 contact lens use,32 and laser refractive surgery.33 Thus, the measurement of CS has substantive importance and value in vision research and clinical care.

Several CS tests with good psychometric properties have been developed that are easily administered in a clinical setting.6 34 They have been used in numerous clinical research studies and have become standard in low-vision care. The most widely used test is the Pelli-Robson CS Chart.35 Briefly, it is a large wall-mounted chart, with letters of a fixed size (comprising spatial frequencies appropriate for estimating peak CS) that decrease in contrast. Recently, a similar, portable test called the Mars Letter CS Test has been developed,36 facilitating convenient administration and out-of-clinic testing. Another advantage is that its termination and scoring rules are simple and unambiguous, whereas various rules have been applied to the Pelli-Robson test, with no established standard.36 Perhaps the most important new design feature of the Mars test is the use of a finer contrast scale. Contrast changes by 0.04 log units with the Mars test, compared with 0.15 log units with the Pelli-Robson test. The finer scale of the Mars test may result in lower variability,36 and hence, improved test-retest reliability,37 and accuracy.35 38 Indeed, in computer simulations, the Mars test has been shown to have lower variability than the Pelli-Robson test.36 However, these potential advantages of the Mars test have not been confirmed by sufficient empiric study. We are aware of only one recent publication in which findings for normal subjects and a heterogeneous low-vision group were reported.39

The central objectives of this study were to acquire empiric data and to evaluate the psychometric properties of the Mars test in a clinical sample. Our more specific objectives were to determine its discriminability, test-retest reliability and criterion validity for normal subjects compared to patients with glaucoma or AMD, and to determine the responsiveness of the test to cataract surgery.


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Subjects
The sample contained 47 normal control subjects, 27 patients with open-angle glaucoma, 17 with AMD, and 17 undergoing cataract surgery. Control subjects were recruited by placement of a study information sheet on hospital notice boards and patient groups were recruited from the Eye Care Centre, Queen Elizabeth II Health Sciences Centre (Halifax, NS, Canada). Inclusion criteria for the control subjects were normal results in an ocular examination and VA better than 0.30 logMAR (20/40). For patients with glaucoma, the inclusion criteria were a glaucoma specialist’s diagnosis of open-angle glaucoma, characteristic glaucomatous optic disc (e.g., notching or progressive thinning of the neuroretinal rim), and visual field impairment detected with the Humphrey Field Analyzer (HFA). For patients with AMD, the inclusion criteria were characteristic macular changes with fluorescein angiography (e.g., drusen, retinal pigment epithelium abnormalities, choroidal neovascularization, subretinal hemorrhage, or fibrous tissue),40 and stable disease (as indicated by ophthalmoscopy and a difference in VA of < 0.20 logMAR at the first study visit compared with a clinic visit at least 1 month before participation). For patients undergoing cataract surgery, the inclusion criterion was lens opacification equal to or worse than grade II (Lens Opacities Classification System II [LOCS II]).41 To determine eligibility, a full ocular examination was performed, and the medical history was recorded for all subjects. Exclusion criteria were concomitant ocular disease, lens opacification worse than grade II41 (except for the cataract surgery group), and VA worse than 1.60 logMAR (20/800).

The study design and protocol were approved by the Institutional Ethics Review Board and adhered to the tenets of the Declaration of Helsinki. Subjects gave informed written consent before participation.

Contrast Sensitivity Measures
All subjects were tested with the Mars Letter CS Test (Mars Perceptrix, Chappaqua, NY; http://www.marsperceptrix.com/; previously supplied as the Lighthouse Letter CS Test), a portable chart measuring 23 x 36 cm, and intended for use at 50 cm.36 The Mars test has several design principles in common with the Pelli-Robson test. There are eight rows of letters, with six Sloan42 letters per row. Letters of constant size are used, which decrease in contrast across and down the chart, and the scale is in units of log10 CS (CS = 1/[contrastWeber]; contrastWeber = [Lbackground – Lletter]/Lbackground; L, luminance). The Mars test letters subtend 2° (at 50 cm), the change in contrast between successive letters is 0.04 log units (10%), and the range is from 0.04 to 1.92 log CS. To score the test, a value of 0.04 log CS is given per letter named correctly. Three chart forms are supplied, each with a different letter sequence. The charts are printed on sheets of resin-coated paper, using half-tone screening methods, and separately mounted.

Subjects were also tested with the Pelli-Robson CS Chart (Haag-Streit UK, Essex, UK). It measures 59 x 84 cm in size and at the recommended 1-m test distance, all letters subtend 2.8°. Each of the eight rows comprises two triplets of letters. The three letters within each triplet have equal contrast; however, each triplet decreases in contrast across and down the chart. The change in contrast between successive triplets is 0.15 log units (41%), and the range is from 0.00 to 2.25 log CS. The scoring rule recommended by the manufacturer is the log CS of the last triplet for which two letters (two of three), are named correctly. However, this is not an established standard, and various rules have been applied to the Pelli-Robson test.36 Assigning a value of 0.05 log CS per correct letter has been shown to improve accuracy and reliability,36 38 and this scoring rule is used regularly. Two chart forms are provided and are printed by using methods similar to those used for the Mars test.

Testing Procedures
In the normal control group, one eye was randomly selected for study. For the glaucoma and AMD groups, the eye with worse HFA mean deviation or VA, respectively, was selected. The study eye was tested twice with the Mars test and twice with the Pelli-Robson test, in random order. The median time between the test and the retest session was 7 days. As differences between available charts and forms were determined to be nonsignificant in a pilot study (P > 0.05), one chart or form of each test was used (chart 1). Background chart luminance was within the range recommended by each manufacturer (Mars test, 113 cd/m2; Pelli-Robson test, 120 cd/m2).

For both tests, subjects were instructed to begin reading the letters at the top of the chart and to continue reading across and down the chart. The Mars test was terminated when two consecutive letters were named incorrectly,36 and the Pelli-Robson test when two of three letters were named incorrectly.35 Subjects were encouraged to observe letters for at least 20 seconds, as this is often necessary for perception at threshold.38 Subjects were also encouraged to guess. Although accepting a response of "O" for a presented "C" has been suggested,43 this method was not applied in our study. Both tests were scored using the letter-by-letter method,38 44 where a value of 0.04 log CS and 0.05 log CS was given per correct letter for the Mars and Pelli-Robson tests, respectively.

The responsiveness of the Mars and Pelli-Robson tests to cataract surgery was evaluated by testing patients once before surgery and once after surgery (median time before surgery, 2 days; median time after surgery, 8 weeks). The tests were administered and scored for this group in the same manner as described earlier. All cataract patients underwent small-incision phacoemulsification in the study eye, with implantation of a monofocal posterior chamber intraocular lens, by the same surgeon.

For all subjects, distance VA was also tested at each study session, using the Early Treatment Diabetic Retinopathy Study (ETDRS) logMAR chart,45 with background luminance in the recommended range,46 47 a termination rule at four of five letters named incorrectly,48 and letter-by-letter scoring. All tests were performed with optimal spectacle refractive error correction.

Data Analysis
Data were analyzed on computer (SPSS, 12.0 for Windows; SPSS Inc., Chicago, IL). Mars test and Pelli-Robson test descriptives were calculated, and analysis of variance (ANOVA) was used to evaluate the significance of group differences. Linear regression analysis was used to evaluate the relationship between age and each CS test. Spearman’s rank correlation coefficient was used to determine the association between Pelli-Robson CS and Mars CS. All analyses were two-tailed and P < 0.05 was considered statistically significant.

Test-retest reliability was determined by Bland-Altman analysis.49 Specifically, we evaluated plots of the difference between the test-retest CS against the mean of the test-retest CS, and the test-retest 95% limits of agreement (LOA; where 95% LOA = mean test-retest difference ±1.96 SD). Differences in 95% LOA between tests were evaluated using F-tests. Responsiveness was investigated by comparing the mean change in scores (difference in pre- and postsurgery CS) and effect size (ES) statistics for the Mars and Pelli-Robson tests. ES statistics are expressions of the magnitude of change in terms of standard units of test variability (SD units) and thereby facilitate comparisons between tests. We selected the Cohen’s d ES statistic for this study,50 as it is well-established and there are guidelines for comparing results.51 Cohen’s d ES was calculated as follows: ES = (mean CSpostsurgery – mean CSpresurgery)/SDpooled. Cohen has suggested that ES statistics of 0.2, 0.5, and 0.8, represent small, medium, and large effects, respectively.50


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Subject Characteristics
Descriptive statistics for the characteristics of each subject group are given in Table 1 .


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  		TABLE 1. Subject Characteristics by Group

 
Descriptives and Normative Data
The Mars and Pelli-Robson tests were both appropriate for use with all subject groups, with no upper or lower end-of-scale limitations. For the normal control group, mean results with the Mars and Pelli-Robson tests were 1.62 log CS (0.06 SD) and 1.79 log CS (0.11 SD), respectively. There was a significant decrease in both Mars CS and Pelli-Robson CS with age (slope of fitted regression line = 0.012 and 0.028 log CS units per decade [R2 = 0.17 and 0.22, P = 0.004 and 0.001], for Mars CS and Pelli-Robson CS, respectively; Fig. 1 ). The Mars test prediction interval (Fig. 1) suggests that, for a person aged 25 years, the upper and lower limits of normal Mars CS were approximately 1.72 and 1.56 log CS, respectively. For a person aged 60 years, the upper and lower limits of normal Mars CS were approximately 1.68 and 1.52 log CS, respectively.


Figure 1
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  		FIGURE 1. Relationship between age and CS, with the Mars (top) and the Pelli-Robson (bottom) tests, for the normal control group (n = 47). Solid line: fitted linear regression (Mars log CS = –0.0012 · age + 1.68 [R2 = 0.17, P = 0.004]; Pelli-Robson log CS = –0.0028 · age + 1.92 [R2 = 0.22, P = 0.001]); dashed lines: 90% prediction interval.

 
Mean CS with the Mars test and the Pelli-Robson test for each group is given in Table 2 . The difference between groups was statistically significant with both tests (Mars test ANOVA F2,88 = 56.5, P < 0.001; Pelli-Robson test ANOVA F2,88 = 59.0, P < 0.001).


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  		TABLE 2. Mean Contrast Sensitivity with the Mars Test and Pelli-Robson Test by Group

 
Test-Retest Reliability
Measures of test-retest reliability are presented in Table 3 . The Mars mean test-retest difference for the normal control group was 0.02 log CS (0.07 SD), indicating a small learning effect. Similarly small mean test-retest differences were observed with both the Mars and Pelli-Robson tests for all groups (mean test-retest difference ≤ 0.02 log CS, P > 0.05); the only exception being for the AMD group with the Mars test (mean test-retest difference = 0.11 log CS, 95% CI, 0.04–0.17 log CS; t = 3.66, P = 0.002).


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  		TABLE 3. Test-Retest Reliability of the Mars Test and Pelli-Robson Test by Group

 
The 95% LOA were the same or narrower (lower test-retest variability) with the Mars compared to the Pelli-Robson test, for all groups (Table 3) . For the normal control group, the 95% LOA were ±0.13 log CS with the Mars test compared with ±0.18 log CS with the Pelli-Robson test (F-statistic = 1.91, P = 0.03). Comparing the subject groups, the 95% LOA were narrowest for the normal control group, followed by the glaucoma group and widest for the AMD group (Table 3) . The increased test-retest variability of patients with AMD compared with the normal control subjects, was statistically significant with both the Mars and the Pelli-Robson tests (F-statistic = 3.62 and 3.39, respectively, P < 0.005). The test-retest difference as a function of the test-retest mean is presented in Figure 2 , for each CS test and each group. The mean test-retest differences and 95% LOA from Table 3 are indicated on each plot. In all cases, test-retest differences did not vary in a systematic manner over the CS range measured (Fig. 2) .


Figure 2
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  		FIGURE 2. Difference between the test-retest CS plotted against mean of the test-retest CS, for the Mars test (top row) and the Pelli-Robson test (bottom row), for the normal control (left; n = 47), glaucoma (middle; n = 27) and AMD (right; n = 17) groups. All differences are test log CS subtracted from retest log CS. A small amount of noise was applied to allow overlapping data points to be differentiated. Solid line: mean of the test-retest differences; dashed lines: test-retest 95% limits of agreement (LOA, where 95% LOA = mean test-retest difference ±1.96 SD). To facilitate comparisons, the y-axis scale is the same in all graphs. However, because of widely different ranges, the x-axis for each group is scaled independently to show the data with clarity.

 
Comparison of Mars Contrast Sensitivity and Pelli-Robson Contrast Sensitivity
The correlation between Mars and Pelli-Robson CS for subjects in the normal control, glaucoma, and AMD groups combined was strong (Spearman’s r = 0.83, P < 0.001). Even so, there were systematic differences between the tests, as indicated by a plot of the difference between the tests as a function of Pelli-Robson CS (Fig. 3) . The data do not form a horizontal band across the measurement range. In particular, the difference between Mars CS and Pelli-Robson CS was greater at the upper "normal" end compared with the lower end of the measurement range, where Mars CS was less than Pelli-Robson CS. Both CS tests correlated moderately with ETDRS VA (Spearman’s r = –0.64 and –0.68, with the Mars test and Pelli-Robson test, respectively; P < 0.001).


Figure 3
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  		FIGURE 3. Comparison between Mars and Pelli-Robson CS. Difference between Mars and Pelli-Robson CS as a function of Pelli-Robson CS for normal control (n = 47), glaucoma (n = 27), and AMD subjects (n = 17). All differences are Mars log CS minus Pelli-Robson log CS. Each data point was calculated by using the mean of the test and retest CS scores. Dashed line: equality.

 
Test Responsiveness
After cataract surgery, mean improvement in best spectacle-corrected VA was 0.39 logMAR (0.33 SD; range, 0.00–1.28 logMAR), with 15 (88%) of 17 patients improving by >0.10 logMAR. Change scores after cataract surgery and ES statistics for the Mars and Pelli-Robson tests are given in Table 4 . Mean CS change after cataract surgery was 0.21 (0.27 SD) and 0.24 log CS (0.31 SD) with the Mars and Pelli-Robson test, respectively. Although the mean change was slightly smaller with the Mars than with the Pelli-Robson test, lower variability resulted in a slightly larger ES statistic (0.92 and 0.88, respectively).


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  		TABLE 4. Responsiveness of Mars Test and Pelli-Robson Test to Cataract Surgery

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
CS is important because it provides valuable information, independent of VA. Furthermore, it is an important predictor of real-world performance and may be useful for monitoring ophthalmic treatment and detecting disease.6 Therefore, it is imperative that we have convenient tests with good psychometric properties for measuring CS. A new test, the Mars Letter CS Test, has been designed to improve on the reliability and practicality of current tests, in particular the well-established Pelli-Robson test. In this study, we evaluated the properties of the Mars test (discriminability, test-retest reliability, criterion validity, and responsiveness) in a sample of normal control subjects and patients with glaucoma, AMD, or cataract. Data from the normal control subjects were also used to establish reference values.

Mean Mars CS was 1.62 log CS (0.06 SD), for our sample of normal subjects aged 22 to 77 years. As expected from other studies of the CS function52 53 and letter CS,3 4 54 55 we found a small, but statistically significant decrease in Mars CS with age, of 0.012 log CS units per decade (P = 0.004). This decline was less than that for Pelli-Robson CS (0.028 log CS units per decade, P = 0.001). Also, the Mars test was able to discriminate between different patient groups. Compared with normal subjects, Mars CS was lower for patients with glaucoma or AMD (mean Mars CS = 1.62 [normal], 1.56 [glaucoma], and 1.03 log CS [AMD]; P < 0.001). These group differences with the Mars test were comparable to those obtained with the Pelli-Robson test (mean Pelli-Robson CS = 1.79 [normal], 1.64 [glaucoma], and 0.98 log CS [AMD]).

To evaluate the consistency of our findings, there are several studies that have provided normative data for the Pelli-Robson test,1 3 4 5 39 54 56 57 58 59 60 with which we can make comparisons. In general, our results are consistent with those that used similar methods. For example, Elliott and Bullimore found mean Pelli-Robson CS was 1.83 log CS (0.14 SD) for normal subjects,57 slightly greater than our finding of 1.79 log CS (0.11 SD). The small difference is likely to be because on average, their sample was younger than our sample. Lovie-Kitchin and Brown3 found the reverse, a slightly lower value (mean Pelli-Robson CS = 1.74 log CS), possibly because the test was administered at 3 m rather than at 1 m and because the habitual rather than the optimal refraction was used. During the review of this article, a recent clinical study comparing the Mars test and Pelli-Robson test was also published.39 Compared with our results, Dougherty et al.39 found a lower mean Pelli-Robson CS of 1.70 log CS for their sample of normal subjects. Again, this may be because the habitual refraction was used, whereas we used the optimal refraction. For the Mars test, mean CS was 1.72 log CS (0.07 SD) in their study,39 somewhat higher than our finding of 1.62 log CS (0.06 SD). The reason for this difference is unclear. We would have expected mean Mars CS to be lower rather than higher when using the habitual refraction. A possible explanation is that they accepted "C" and "O" miscalls,39 whereas we did not accept any miscalls.

Mars test-retest 95% LOA were ±0.13 log CS for the normal control group in this study, suggesting that a significant change based on actual scale values would be ±0.16 log CS (four letters). As hypothesized, the 95% LOA for the normal subjects indicate that the Mars test was somewhat less variable (or more reliable) than the Pelli-Robson test (95% LOA = ±0.13 and ±0.18 log CS, with the Mars and Pelli-Robson test, respectively; P = 0.03). This finding is supported by previous studies of the Pelli-Robson test, where test-retest reliability has been found to be in the range of ±0.15 to ±0.20 log CS in normal subjects.3 38 54 57 61 Furthermore, Dougherty et al.39 found a similar difference between Mars and Pelli-Robson test-retest reliability in their sample of normal subjects (95% LOA = ±0.14 and ±0.18 log CS, with the Mars and Pelli-Robson test, respectively; after correction for the differences in the Mars chart forms used). We also found that the reliability of the Mars test was equal to or better than that of the Pelli-Robson test in the patient groups (glaucoma and AMD; Fig. 2 ), which is consistent with results for a heterogeneous group of low-vision patients.39 We suggest that the improved reliability of the Mars test over the Pelli-Robson test is most likely due to the incorporation of a finer contrast scale.

The 95% LOA found in this study also suggest that test-retest reliability is worse in those with glaucoma or AMD than in normal subjects (Fig. 2) , with patients with AMD having the lowest reliability of the three groups (Mars test 95% LOA = ±0.24 log CS; Pelli-Robson 95% LOA = ±0.33 log CS). This finding is supported by several other studies of CS,39 58 62 63 and studies of VA,63 64 65 66 in which poorer test-retest reliability has been found for samples comprising vision-impaired patients. For example, Haymes and Chen62 found Pelli-Robson test-retest 95% LOA were ±0.18 log CS and ±0.25 log CS for normal subjects and low-vision patients, respectively.

The strong correlation found between the Mars test and the Pelli-Robson test in this study provides evidence that the Mars test is valid (Spearman’s r = 0.83, P < 0.001). However, systematic differences between the Mars test and the Pelli-Robson test were observed (Fig. 3) . At the upper-normal end of the range, Mars CS was less than Pelli-Robson CS and, conversely, Mars CS was greater than Pelli-Robson CS at the lower end of the range. Contrary to this, a difference between the Mars test and Pelli-Robson test was not observed in Arditi’s Monte Carlo computer simulation study of the upper range, in which the Mars test had almost negligible bias relative to the Pelli-Robson test.36 Indeed, our clinical findings do not support his suggestion that scores between the two tests and with published norms for the Pelli-Robson test are directly comparable. With regard to the lower range, there is evidence to support our finding that Mars CS measures were greater than Pelli-Robson CS measures for subjects with poorer CS.39 However, it should be noted that the samples of subjects investigated with poorer CS have been small.

Our findings indicate that there are differences in CS measurements obtained with the Mars test and the Pelli-Robson test. Given the similarity of the test designs, we suggest that the differences are most likely due to discrepancies in the actual contrast levels. Letter contrast in the midrange has been measured and found to be on average 0.07 log units higher than the stated value for the Mars test, but within 0.02 log units of the stated value for the Pelli-Robson test,39 providing support for this hypothesis. However, it is difficult to verify the contrast levels of letters in the normal CS range.67

As expected, Mars CS improved after cataract surgery (mean change score = 0.21 log CS; 0.27 SD). The ES statistic for the Mars test was 0.92 (95% CI, 0.20–1.61), suggesting a large effect and good responsiveness.50 In comparison, the Pelli-Robson ES statistic was slightly lower, 0.88 (95% CI, 0.16–1.56). The ES statistic is equal to the magnitude of change divided by the test variability, and although CS with the Mars test changed less, lower variability resulted in a larger ES statistic compared with the Pelli-Robson test (SDpooled = 0.23 and 0.28 log CS with the Mars and Pelli-Robson test, respectively). Nevertheless, the small difference between tests is unlikely to be clinically important.50

This clinical study has shown that the reliability, validity, and responsiveness of the new Mars Letter CS Test are at least equal to those of the Pelli-Robson CS Chart. However, we found systematic differences between Mars CS and Pelli-Robson CS, indicating that normative values are likely to be different for each test. Although we provide data from a group of normal control subjects for reference, we propose that normative values may have to be established from a larger sample. The Mars Letter CS Test is a useful and practical alternative to the Pelli-Robson CS Chart, with broad applicability in clinical research, low vision care, disease monitoring, and outcomes research.


   Footnotes
 
Presented in part at the annual meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, May 2005.

Supported by a grant from the Capital Health Research Fund.

Submitted for publication November 2, 2005; revised January 4 and 18, 2006; accepted March 14, 2006.

Disclosure: S.A. Haymes, None; K.F. Roberts, None; A.F. Cruess, None; M.T. Nicolela, None; R.P. LeBlanc, None; M.S. Ramsey, None; B.C. Chauhan, None; P.H. Artes, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Sharon A. Haymes, Department of Ophthalmology and Visual Sciences, Dalhousie University, Room 2109, Centennial Building (VG site), 1278 Tower Rd, Halifax, Nova Scotia, Canada B3H 2Y9; sharon.haymes{at}dal.ca.


   References
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 Abstract
 Methods
 Results
 Discussion
 References
 

  1. Haegerstrom-Portnoy G. The Glenn A. Fry Award Lecture 2003. Vision in elders: summary of findings of the SKI study. Optom Vis Sci. 2005;82:87–93.[Medline][Order article via Infotrieve]
  2. Hawkins AS, Szlyk JP, Ardickas Z, Alexander KR, Wilensky JT. Comparison of contrast sensitivity, visual acuity, and Humphrey visual field testing in patients with glaucoma. J Glaucoma. 2003;12:134–138.[CrossRef][ISI][Medline][Order article via Infotrieve]
  3. Lovie-Kitchin JE, Brown B. Repeatability and intercorrelations of standard vision tests as a function of age. Optom Vis Sci. 2000;77:412–420.[CrossRef][ISI][Medline][Order article via Infotrieve]
  4. Rubin GS, West SK, Munoz B, et al. A comprehensive assessment of visual impairment in a population of older Americans. The SEE Study. Salisbury Eye Evaluation Project. Invest Ophthalmol Vis Sci. 1997;38:557–568.[Abstract/Free Full Text]
  5. Hirvela H, Koskela P, Laatikainen L. Visual acuity and contrast sensitivity in the elderly. Acta Ophthalmol Scand. 1995;73:111–115.[ISI][Medline][Order article via Infotrieve]
  6. Owsley C. Contrast sensitivity. Ophthalmol Clin North Am. 2003;16:171–177.[Medline][Order article via Infotrieve]
  7. Owsley C, Ball K, McGwin G, Jr, et al. Visual processing impairment and risk of motor vehicle crash among older adults. JAMA. 1998;279:1083–1088.[Abstract/Free Full Text]
  8. Marron JA, Bailey IL. Visual factors and orientation-mobility performance. Am J Optom Physiol Opt. 1982;59:413–426.[ISI][Medline][Order article via Infotrieve]
  9. Lord SR, Dayhew J. Visual risk factors for falls in older people. J Am Geriatr Soc. 2001;49:508–515.[CrossRef][ISI][Medline][Order article via Infotrieve]
  10. Lord SR, Menz HB. Visual contributions to postural stability in older adults. Gerontology. 2000;46:306–310.[CrossRef][ISI][Medline][Order article via Infotrieve]
  11. Bullimore MA, Bailey IL, Wacker RT. Face recognition in age-related maculopathy. Invest Ophthalmol Vis Sci. 1991;32:2020–2029.[Abstract/Free Full Text]
  12. Whittaker SG, Lovie-Kitchin J. Visual requirements for reading. Optom Vis Sci. 1993;70:54–65.[CrossRef][ISI][Medline][Order article via Infotrieve]
  13. Crossland MD, Culham LE, Rubin GS. Predicting reading fluency in patients with macular disease. Optom Vis Sci. 2005;82:11–17.[ISI][Medline][Order article via Infotrieve]
  14. Scott IU, Feuer WJ, Jacko JA. Impact of visual function on computer task accuracy and reaction time in a cohort of patients with age-related macular degeneration. Am J Ophthalmol. 2002;133:350–357.[CrossRef][ISI][Medline][Order article via Infotrieve]
  15. Haymes SA, Johnston AW, Heyes AD. Relationship between vision impairment and ability to perform activities of daily living. Ophthalmic Physiol Opt. 2002;22:79–91.[CrossRef][ISI][Medline][Order article via Infotrieve]
  16. West SK, Rubin GS, Broman AT, et al. How does visual impairment affect performance on tasks of everyday life? The SEE Project. Salisbury Eye Evaluation. Arch Ophthalmol. 2002;120:774–780.[Abstract/Free Full Text]
  17. Elliott DB, Hurst MA. Simple clinical techniques to evaluate visual function in patients with early cataract. Optom Vis Sci. 1990;67:822–825.[CrossRef][ISI][Medline][Order article via Infotrieve]
  18. Ansari EA, Morgan JE, Snowden RJ. Psychophysical characterisation of early functional loss in glaucoma and ocular hypertension. Br J Ophthalmol. 2002;86:1131–1135.[Abstract/Free Full Text]
  19. Midena E, Degli AC, Blarzino MC, Valenti M, Segato T. Macular function impairment in eyes with early age-related macular degeneration. Invest Ophthalmol Vis Sci. 1997;38:469–477.[Abstract/Free Full Text]
  20. Bellmann C, Unnebrink K, Rubin GS, Miller D, Holz FG. Visual acuity and contrast sensitivity in patients with neovascular age-related macular degeneration: results from the Radiation Therapy for Age-Related Macular Degeneration (RAD) Study. Graefes Arch Clin Exp Ophthalmol. 2003;241:968–974.[CrossRef][ISI][Medline][Order article via Infotrieve]
  21. Stavrou EP, Wood JM. Letter contrast sensitivity changes in early diabetic retinopathy. Clin Exp Optom. 2003;86:152–156.[Medline][Order article via Infotrieve]
  22. Trobe JD, Beck RW, Moke PS, Cleary PA. Contrast sensitivity and other vision tests in the optic neuritis treatment trial. Am J Ophthalmol. 1996;121:547–553.[ISI][Medline][Order article via Infotrieve]
  23. McGwin G, Jr, Scilley K, Brown J, Owsley C. Impact of cataract surgery on self-reported visual difficulties: comparison with a no-surgery reference group. J Cataract Refract Surg. 2003;29:941–948.[CrossRef][Medline][Order article via Infotrieve]
  24. Tan JC, Spalton DJ, Arden GB. The effect of neodymium: YAG capsulotomy on contrast sensitivity and the evaluation of methods for its assessment. Ophthalmology. 1999;106:703–709.[Medline][Order article via Infotrieve]
  25. Kamlesh Dadeya S, Kaushik S. Contrast sensitivity and depth of focus with aspheric multifocal versus conventional monofocal intraocular lens. Can J Ophthalmol. 2001;36:197–201.[ISI][Medline][Order article via Infotrieve]
  26. Nejima R, Miyata K, Honbou M, et al. A prospective, randomised comparison of single and three piece acrylic foldable intraocular lenses. Br J Ophthalmol. 2004;88:746–749.[Abstract/Free Full Text]
  27. Sponsel WE, Paris G, Trigo Y, Pena M. Comparative effects of latanoprost (Xalatan) and unoprostone (Rescula) in patients with open-angle glaucoma and suspected glaucoma. Am J Ophthalmol. 2002;134:552–559.[CrossRef][ISI][Medline][Order article via Infotrieve]
  28. Gandolfi SA, Cimino L, Sangermani C, et al. Improvement of spatial contrast sensitivity threshold after surgical reduction of intraocular pressure in unilateral high-tension glaucoma. Invest Ophthalmol Vis Sci. 2005;46:197–201.[Abstract/Free Full Text]
  29. Monès J, Rubin GS. Contrast sensitivity as an outcome measure in patients with subfoveal choroidal neovascularisation due to age-related macular degeneration. Eye. .doi:10.1038/sj.eye. 6701717. Published online October 1, 2004
  30. Talwar D, Sharma N, Pai A, et al. Contrast sensitivity following focal laser photocoagulation in clinically significant macular oedema due to diabetic retinopathy. Clin Exp Ophthalmol. 2001;29:17–21.[Medline][Order article via Infotrieve]
  31. Verma LK, Vivek MB, Kumar A, Tewari HK, Venkatesh P. A prospective controlled trial to evaluate the adjunctive role of posterior subtenon triamcinolone in the treatment of diffuse diabetic macular edema. J Ocul Pharmacol Ther. 2004;20:277–284.[CrossRef][Medline][Order article via Infotrieve]
  32. Ozkagnici A, Zengin N, Kamis O, Gunduz K. Do daily wear opaquely tinted hydrogel soft contact lenses affect contrast sensitivity function at one meter?. Eye Contact Lens. 2003;29:48–49.[Medline][Order article via Infotrieve]
  33. Yamane N, Miyata K, Samejima T, et al. Ocular higher-order aberrations and contrast sensitivity after conventional laser in situ keratomileusis. Invest Ophthalmol Vis Sci. 2004;45:3986–3990.[Abstract/Free Full Text]
  34. Woods RL, Wood JM. The role of contrast sensitivity charts and contrast letter charts in clinical practice. Clin Exp Optom. 1995;78:43–57.
  35. Pelli DG, Robson JG, Wilkins AJ. The design of a new letter chart for measuring contrast sensitivity. Clin Vision Sci. 1988;2:187–199.
  36. Arditi A. Improving the design of the letter contrast sensitivity test. Invest Ophthalmol Vis Sci. 2005;46:2225–2229.[Abstract/Free Full Text]
  37. Raasch TW, Bailey IL, Bullimore MA. Repeatability of visual acuity measurement. Optom Vis Sci. 1998;75:342–348.[ISI][Medline][Order article via Infotrieve]
  38. Elliott DB, Bullimore MA, Bailey IL. Improving the reliability of the Pelli-Robson contrast sensitivity test. Clin Vision Sci. 1991;6:471–475.
  39. Dougherty BE, Flom RE, Bullimore MA. An evaluation of the Mars Letter Contrast Sensitivity Test. Optom Vis Sci. 2005;82:970–975.[CrossRef][Medline][Order article via Infotrieve]
  40. The Age-Related Eye Disease Study system for classifying age-related macular degeneration from stereoscopic color fundus photographs: the Age-Related Eye Disease Study Report Number 6. Am J Ophthalmol. 2001;132:668–681.[CrossRef][ISI][Medline][Order article via Infotrieve]
  41. Chylack LT, Jr, Leske MC, McCarthy D, et al. Lens opacities classification system II (LOCS II). Arch Ophthalmol. 1989;107:991–997.[Abstract]
  42. Sloan LL. New test charts for the measurement of visual acuity at far and near distances. Am J Ophthalmol. 1959;48:807–813.[ISI][Medline][Order article via Infotrieve]
  43. Elliott DB, Whitaker D, Bonette L. Differences in the legibility of letters at contrast threshold using the Pelli-Robson chart. Ophthalmic Physiol Opt. 1990;10:323–326.[ISI][Medline][Order article via Infotrieve]
  44. Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effects of scaling. Invest Ophthalmol Vis Sci. 1991;32:422–432.[Abstract/Free Full Text]
  45. Ferris FL, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol. 1982;94:91–96.[ISI][Medline][Order article via Infotrieve]
  46. World Health Organization. Consultation on Development of Standards for Characterization of Vision Loss and Visual Functioning. 2003;5–6. World Health Organization Geneva, Switzerland. WHO/PBL/03.91
  47. Sheedy JE, Bailey IL, Raasch TW. Visual acuity and chart luminance. Am J Optom Physiol Opt. 1984;61:595–600.[ISI][Medline][Order article via Infotrieve]
  48. Carkeet A. Modeling logMAR visual acuity scores: effects of termination rules and alternative forced-choice options. Optom Vis Sci. 2001;78:529–538.[CrossRef][ISI][Medline][Order article via Infotrieve]
  49. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet. 1986;1:307–310.[CrossRef][ISI][Medline][Order article via Infotrieve]
  50. Cohen J. Statistical Power Analysis for the Behavioural Sciences. 1988; 2nd ed. Erlbaum Associates Hillsdale, NJ.
  51. Middel B. Statistical significant change versus relevant or important change in (quasi) experimental design: Some conceptual and methodological problems in estimating magnitude of intervention-related change in health services research. Int J Integr Care. 2002;2:1–22.[Medline][Order article via Infotrieve]
  52. Owsley C, Sekuler R, Siemsen D. Contrast sensitivity throughout adulthood. Vision Res. 1983;23:689–699.[CrossRef][ISI][Medline][Order article via Infotrieve]
  53. Crassini B, Brown B, Bowman K. Age-related changes in contrast sensitivity in central and peripheral retina. Perception. 1988;17:315–332.[CrossRef][ISI][Medline][Order article via Infotrieve]
  54. Elliott DB, Sanderson K, Conkey A. The reliability of the Pelli-Robson contrast sensitivity chart. Ophthalmic Physiol Opt. 1990;10:21–24.[CrossRef][ISI][Medline][Order article via Infotrieve]
  55. Wood JM, Bullimore MA. Changes in the lower displacement limit for motion with age. Ophthalmic Physiol Opt. 1995;15:31–36.[CrossRef][ISI][Medline][Order article via Infotrieve]
  56. Beck RW, Diehl L, Cleary PA, Optic Neuritis Study Group. The Pelli-Robson Letter Chart: normative data for young adults. Clin Vision Sci. 1993;8:207–210.
  57. Elliott DB, Bullimore MA. Assessing the reliability, discriminative ability, and validity of disability glare tests. Invest Ophthalmol Vis Sci. 1993;34:108–119.[Abstract/Free Full Text]
  58. Reeves BC, Wood JM, Hill AR. Reliability of high- and low-contrast letter charts. Ophthalmic Physiol Opt. 1993;13:17–26.[ISI][Medline][Order article via Infotrieve]
  59. Mantyjarvi M, Laitinen T. Normal values for the Pelli-Robson contrast sensitivity test. J Cataract Refract Surg. 2001;27:261–266.[CrossRef][ISI][Medline][Order article via Infotrieve]
  60. Puell MC, Palomo C, Sanchez-Ramos C, Villena C. Normal values for photopic and mesopic letter contrast sensitivity. J Refract Surg. 2004;20:484–488.[Medline][Order article via Infotrieve]
  61. Simpson TL, Regan D. Test-retest variability and correlations between tests of texture processing, motion processing, visual acuity, and contrast sensitivity. Optom Vis Sci. 1995;72:11–16.[CrossRef][ISI][Medline][Order article via Infotrieve]
  62. Haymes SA, Chen J. Reliability and validity of the Melbourne Edge Test and High/Low Contrast Visual Acuity chart. Optom Vis Sci. 2004;81:308–316.[Medline][Order article via Infotrieve]
  63. Kiser AK, Mladenovich D, Eshraghi F, Bourdeau D, Dagnelie G. Reliability and consistency of visual acuity and contrast sensitivity measures in advanced eye disease. Optom Vis Sci. 2005;82:946–954.[Medline][Order article via Infotrieve]
  64. Elliott DB, Sheridan M. The use of accurate visual acuity measurements in clinical anti-cataract formulation trials. Ophthalmic Physiol Opt. 1988;8:397–401.[ISI][Medline][Order article via Infotrieve]
  65. Blackhurst DW, Maguire MG. Reproducibility of refraction and visual acuity measurement under a standard protocol. The Macular Photocoagulation Study Group. Retina. 1989;9:163–169.[ISI][Medline][Order article via Infotrieve]
  66. Rosser DA, Murdoch IE, Cousens SN. The effect of optical defocus on the test-retest variability of visual acuity measurements. Invest Ophthalmol Vis Sci. 2004;45:1076–1079.[Abstract/Free Full Text]
  67. Verbaken JH, Jacobs RJ. The technical problems of producing photographic prints for the measurement of human contrast thresholds. Ophthalmic Physiol Opt. 1985;7:459–465.[CrossRef]



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Regression to the Mean Could Account for This Discrepancy
Michael D. Crossland, et al.
IOVS Online, 8 Feb 2007 [Full text]
Author Response: Regression to the Mean Could Account for This Discrepancy
Sharon A. Haymes
IOVS Online, 8 Feb 2007 [Full text]
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