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H. J. Wyatt, J. Lam, J. Chan, W. H. Swanson, M. W. Dul; Retest Variability Is Correlated With Gradient of the Visual Field Near the Normal Physiological Blind Spot. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1621.
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© ARVO (1962-2015); The Authors (2016-present)
Last year, we reported that variability in visual fields of patients with glaucoma is correlated with the gradient of sensitivity. (The gradient is a measure of the local steepness of the hill of vision.) We proposed that small fixational eye movements could cause much of the variability in areas with steep gradients. Alternatively, cells at scotoma edges might be particularly variable in their behavior; therefore, we wanted to study variability near a physiological scotoma and compare it with our previous findings near pathological scotomas.We also wanted to measure eye position during the testing.
Seven normal subjects were tested three times, within a 6-week period, on a customized visual field test employing stimuli displayed on a CRT monitor. 28 test locations were spaced 2 deg apart in an array (4 wide x 7 high) centered at 14 deg temporal, 2 deg inferior to fixation. 12 other test locations were located in the nasal, superior, and inferior field to keep attention global. A full-threshold protocol was used to measure sensitivity, employing a 2 dB/1 dB staircase at each location. Background luminance was 5 cd/m2. During the tests, eye movements were measured with an infrared eyetracker. Gradients were calculated from average sensitivity data.
For the rectangular array of test locations, the matrix correlation of variability with the gradient of sensitivity was 0.60 ± 0.08. This is similar to the correlation with gradient that we previously reported, based on 10-2 fields of patients with glaucoma who had field loss within 10 degrees of fixation. Variability ranged from 0 to 7 dB (ave 1.6 dB). Horizontal and vertical eye position were approx normally distributed, with SD's of 0.2-0.5 deg.
Near the normal physiological blind spot, high variability was strongly correlated with "steep slopes" (large gradients) of visual sensitivity. This agrees with the suggestion that a significant component of perimetric variability is related to eye movements. For the gradients calculated, the observed variability of eye position was of sufficient magnitude to account for most of the observed variability. A comparison of the edge of the blind spot with the edge of scotomas suggests that gradients can be steeper at scotoma edges, causing greater variability. This could explain the results of Haefliger and Flammer (1991), which indicated greater variability at the edges of scotomas due to glaucoma than at the edge of the blind spot.
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