doi:10.1016/j.visres.2006.12.012 
Copyright © 2007 Elsevier Ltd All rights reserved.
Variability of visual field measurements is correlated with the gradient of visual sensitivity
Harry J. Wyatta, c, 
, 
, Mitchell W. Dulb, c and William H. Swansonb, c
aDepartment of Biological Sciences, State University of New York, State College of Optometry, 33 West 42nd Street, New York, NY 10036, USA
bDepartment of Clinical Sciences, State University of New York, State College of Optometry, 33 West 42nd Street, New York, NY 10036, USA
cGlaucoma Institute of SUNY, State University of New York, State College of Optometry, 33 West 42nd Street, New York, NY 10036, USA
Received 6 January 2006;
revised 29 November 2006;
accepted 1 December 2006.
Available online 23 February 2007.
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Abstract
Conventional static automated perimetry provides important clinical information, but its utility is limited by considerable test–retest variability. Fixational eye movements during testing could contribute to variability. To assess this possibility, it is important to know how much sensitivity change would be caused by a given eye movement. To investigate this, we have evaluated the gradient, the rate at which sensitivity changes with location. We tested one eye each, twice within 3 weeks, of 29 patients with glaucoma, 17 young normal subjects and 13 older normal subjects. The 10-2 test pattern with the SITA Standard algorithm was used to assess sensitivity at locations with 2° spacing. Variability and gradient were calculated at individual test locations. Matrix correlations were determined between variability and gradient, and were substantial for the patients with glaucoma. The results were consistent with a substantial contribution to test–retest variability from small fixational eye movements interacting with visual field gradient. Successful characterization of the gradient of sensitivity appears to require sampling at relatively close spacing, as in the 10-2 test pattern.
Keywords: Visual field; Perimetry; Variability; Eye-movement; Glaucoma; Gradient
Fig. 1. (a) Visual field data (10-2 SITA Standard) from one eye of an 88-year-old patient with glaucoma. Left to right: sensitivity data from a single visual field, averaged sensitivity data from 2 fields, the SD values for each test point, and the magnitude of the gradient determined from the average sensitivity data. (Decimal places are omitted, to improve readability.) The heavy outline indicates the 44 “core” test locations discussed in the text. (b) 24-2 SITA Standard data from the same subject, showing the arrangement of test locations.
Fig. 2. Contour plots of average sensitivity, variability (i.e., the SD) and |gradient|. Average values are superimposed on the plots. Top row: patient of Fig. 1. Correlation (variability, |gradient|) = 0.651. Bottom row: visual field data from a 68-year-old patient with glaucoma. Correlation (variability, |gradient|) = 0.593.
Fig. 3. 10-2 data from a 50-year-old patient with glaucoma that gave the lowest observed correlation between variability and gradient (−0.260).
Fig. 4. Histogram of values of Matrix correlation between variability and gradient for patients with glaucoma. Open bars: 10-2 data; filled bars: 24/30-2 data.
Fig. 5. Scatterplots for individual test locations in visual fields of patients glaucoma. (a) Variability vs. sensitivity. (b–d) Variability vs. |Gradient|. (b) 10-2 data. (c) 24-2 data for eccentricities <10 deg. (d) 24-2 data for eccentricities >10 deg. Note the shorter gradient axis in (c and d) compared to (b); gradients from 24-2 data were all <3.0. Linear regressions are included in (b–d); the dashed line in (b) is the regression after exclusion of points with gradients less than 1.
Fig. 6. Matrix correlations based on 10-2 data plotted against Mean Deviation for the same eye. Filled circles represent correlations of Variability with −1* sensitivity; open circles represent correlations of Variability with |gradient|.
Fig. 7. Distributions of gradient values determined for 10-2 data (top), 24-2 data for eccentricities <10 deg (middle), and 24-2 data for eccentricities >10 deg (bottom). Diamonds indicate mean values.
Fig. 8. Average sensitivity, variability, and gradient values for young normal subjects (left column) and older normal subjects (middle column). Values have been averaged into one half-quadrant. Values in open cells are for core locations (compare to Fig. 1a). Differences between the two groups are shown in the right column.
Fig. 9. Results from a 67-year-old patient with glaucoma. This eye had six core test locations with sensitivity in the low-sensitivity/low-gradient class (see text).
Fig. 10. Comparison of variability data from individual test locations in 10-2 fields, grouped by sensitivity, together with magnitude of gradient for the same data grouped by sensitivity. Also shown for comparison are the variability vs. sensitivity data from Artes et al. (2002) for 24-2 data, based on their Fig. 6.
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