Linearity Can Account for the Similarity Among Conventional, Frequency-Doubling, and Gabor-Based Perimetric Tests in the Glaucomatous Macula

Abstract

Purposes-The purposes of this study are to compare macular perimetric sensitivities for conventional size III, frequency-doubling, and Gabor stimuli in terms of Weber contrast and to provide a theoretical interpretation of the results.Methods-Twenty-two patients with glaucoma performed four perimetric tests: a conventional Swedish Interactive Threshold Algorithm (SITA) 10-2 test with Goldmann size III stimuli, two frequency-doubling tests (FDT 10-2, FDT Macula) with counterphase-modulated grating stimuli, and a laboratory-designed test with Gabor stimuli. Perimetric sensitivities were converted to the reciprocal of Weber contrast and sensitivities from different tests were compared using the BlandAltman method. Effects of ganglion cell loss on perimetric sensitivities were then simulated with a two-stage neural model.Results-The average perimetric loss was similar for all stimuli until advanced stages of ganglion cell loss, in which perimetric loss tended to be greater for size III stimuli than for frequency-doubling and Gabor stimuli. Comparison of the experimental data and model simulation suggests that, in the macula, linear relations between ganglion cell loss and perimetric sensitivity loss hold for all three stimuli.Conclusions-Linear relations between perimetric loss and ganglion cell loss for all three stimuli can account for the similarity in perimetric loss until advanced stages. The results do not support the hypothesis that redundancy for frequency-doubling stimuli is lower than redundancy for size III stimuli. Keywordsperimetry; glaucoma; macula; SITA 10-2; frequency-doubling; Gabor Conventional automated perimetry is widely used in diagnosis of glaucoma and in evaluating effects of glaucoma treatment. The most commonly performed conventional perimetric test uses a small luminance increment stimulus (Goldmann size III, 0.43° diameter) superimposed on a uniform white background. With this stimulus, test-retest variability is high in areas with decreased sensitivity, 1-3 which makes it difficult to detect progression of visual field loss.Correspondence to: HAO SUN. York, NY 10036 e-mail: hsun@sunyopt.edu. Hao Sun SUNY State College of Optometry 33 West 42nd Street New NIH Public Access Author ManuscriptOptom Vis Sci. Author manuscript; available in PMC 2007 July 1. Published in final edited form as:Optom Vis Sci. 2006 July ; 83(7): 455-465. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptTest-retest variability in these areas can be reduced 4,5 by using a larger stimulus (Goldmann size V, 1.72° diameter), but increasing stimulus size can decrease the depth of defect and the ability to detect visual field change. 5-7 An alternative is to use cathode ray tube (CRT)-based perimetric tests, which use large sinusoidal stimuli such as counterphase-modulated gratings used in frequency-doubling perimetry 8,9 or Gabor stimuli (sinusoidal gratings with Gaussian envelopes) used in contrast sensitivity perimetry. 10 A recent study has shown that test-retest variability is less...

Choice of Stimulus Range and Size Can Reduce Test-Retest Variability in Glaucomatous Visual Field Defects

“…22 This is in accord with a proposal that fixational instability in perimetry with SD ¼~0.58 can account for high variability for the size III stimulus in regions of the visual field with gradients in sensitivity 21,45 or with ''holes'' in ganglion cell arrays damaged by glaucoma. 20,26 Both of these explanations for the effect are consistent with our finding that test-retest variability may be decreased with only a modest increase in stimulus size when soft edges are used. The hard edge of the size III target means that a 0.58 displacement would stimulate responses from a completely different set of ganglion cells, so the soft edges of the blobs were designed to cause a smoother transition between responsive and not responsive ganglion cells.…”

“…These results provide guidance for design of perimetric stimuli and confirm predictions of neural modeling of the pathophysiology of glaucoma. 22,26 Imaging measures can be helpful in assessing progression in patients with early stages of disease, in large part because of lower between-subject variability than found in perimetry. 44 However, once a defect has reached an average of À5 dB, imaging measures reach a floor.…”

“…Contrast is a unitless measure defined as (peakÀmean)/mean; for luminance increments, this is equivalent to Weber contrast, and for sinusoids this is equivalent to Michelson contrast. 26 For size III perimetry, the device reports decibels as 25 þ 10 3 logCS. For frequency-doubling perimetry the device reports decibels as 20 3 logCS.…”

“…By comparison, frequency-doubling stimuli are sharp-edged square stimuli with widths of 108, 58, and 28. 12,13,26 It is possible that the size of sinusoidal stimuli, rather than their property of being sinusoidal, is the source of reduced variability. For instance, variability in glaucomatous defects can also be reduced 15 by using the size V stimulus, which is has a width of 1.78.…”

Development and Evaluation of a Contrast Sensitivity Perimetry Test for Patients with Glaucoma

“…7,8 However, normal between-subject and test-retest variability are high for FDP. A recent study 9 showed large, normal, between-subject variability on FDP: an MD of Ϫ5 dB was within 95% confidence limits of normal, as was a pattern standard deviation (PSD) of 5 dB (up to 8 dB in some cases).…”