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Andrew J Anderson, Allison M McKendrick, Andrew Turpin; Behavioural evidence that retinal ganglion cells do not saturate with 15dB perimetric stimuli. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3921.
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© ARVO (1962-2015); The Authors (2016-present)
A recent proposal for why glaucomatous perimetric sensitivities of approximately 15 to 19dB or less are unreliable involves the idea that normal retinal ganglion cells saturate for intense stimuli less than approximately 15dB (Ophthalmology 2014;121:1359-1369). We therefore measured contrast discrimination performance for such stimuli, which should be impaired if saturation occurs.
We used a two-interval forced-choice method of constant stimuli (7 steps, 40 presentations/step) to measure contrast discrimination performance for Size III perimetric stimuli at 0, 9 & 21 degrees eccentricity in 3 observers aged <50 years. Baseline intensities from which contrast increments were judged were 27, 23, 19, 15 and 11dB (Humphrey Field Analyzer equivalents). Each observer performed a total of 4,200 judgments. Frequency of seeing probabilities versus log change in Weber contrast were fit using cumulative normal distributions. Intraocular scatter was measured using an Oculus C-Quant.
For centrally fixated targets, linear regression found no significant change (p=0.62) in discrimination thresholds (= mean of cumulative normal distribution) with baseline intensity. Slope of the frequency of seeing curve (=standard deviation of cumulative normal distribution) also did not significantly change as a function of baseline intensity (p=0.14), being 0.32 for 27dB stimuli and 0.43 for 11dB stimuli. Frequency of seeing curves reached an average upper asymptote >0.98 detection probability, indicating that participants could reliably discriminate an increase in stimulus intensity even when stimuli were already very intense (11dB). Similar results were found at both 9 & 21 degrees eccentricity. Discrimination thresholds increased significantly with eccentricity (RM ANOVA, p=0.02), although there was no significant alteration in frequency of seeing slopes (RM ANOVA, p=0.12). All observers had intra-ocular scatter lower than age-matched average values.
Contrast discrimination performance changes little as the intensity of perimetric stimuli increases. Our results are not readily consistent with a simple model of retinal ganglion cell saturation for intense perimetric stimuli of 15dB or less.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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