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Stuart Gardiner, Deborah Goren, Casie Goldman, William Swanson, Shaban Demirel; The Effect of Stimulus Size on the Relation between Sensitivity and Variability in Perimetry. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2636.
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© ARVO (1962-2015); The Authors (2016-present)
Variability increases as sensitivity declines in clinical automated perimetry, complicating analysis and interpretation of results. It has been suggested that using Size V stimuli reduces variability, steepening Frequency-of-Seeing (FOS) curves by summating responses from more retinal ganglion cells (RGCs). However, this has largely been assessed indirectly, using clinical perimetry to estimate sensitivity and test-retest variability. These are influenced by the algorithm used, not just by physiologic variability. In this study, we measure FOS curves in damaged areas, examining the effect of stimulus size.
FOS curves were recorded at four locations in 16 subjects with moderate to advanced glaucoma. The method of constant stimuli was used, with 35 presentations at each of 7 contrasts, on an Octopus perimeter controlled using the Open Perimetry Interface. The false positive rate was estimated based on 25 catch trials. The false negative rate was assumed to be 5%. A cumulative Gaussian curve was fit to the response probabilities at each location, with two free parameters: mean (50% detection, “sensitivity”) and standard deviation (SD, “variability”). This was repeated using Size III and Size V stimuli. SD was regressed against sensitivity using a generalized estimating equation (GEE) model to account for intra-subject correlations.
Sensitivities were higher for Size V stimuli, p<0.001. Plots of variability against sensitivity were well-described by a linear relation. With SD & sensitivity (Sens) expressed in dB, for Size III stimuli SD=10.02-0.32*Sens (R2=0.73), and for Size V stimuli SD=10.84-0.33*Sens (R2=0.67). Combining data, variability for a given sensitivity did not differ significantly between stimulus sizes III and V (p=0.105, GEE regression).
Testing the same location in a subject’s field with Size V stimuli instead of Size III increases sensitivity and hence decreases variability. However for a given sensitivity, variability appears constant between these two stimulus sizes. This is consistent with both sensitivity and variability being determined solely by the number of remaining RGCs at that location. Modifying the stimulus size while keeping contrast fixed, instead of modifying contrast for a fixed stimulus size, could result in variability being more constant across disease stages in glaucoma.
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