The Effect of Stimulus Size on the Reliable Stimulus Range of Perimetry

Abstract

Purpose: Automated perimetry uses a 3.5 log unit (35dB) range of stimulus contrasts to assess function within the visual field. Using 'Size III' stimuli (0.43°), presenting stimuli within the highest 15dB of available contrast may not increase the response probability at locations damaged by glaucoma, due to retinal ganglion cell response saturation. This experiment examines the effect of instead using 'Size V' (1.72°) stimuli.

Methods: Luminance increment thresholds for circular spot stimuli of each stimulus size were measured in 35 participants (mean deviation -20.9 to -3.4 dB, ages 52-87) using the method of constant stimuli, at four locations per participant. Frequency-of-seeing curves were fit at each size and location, with three free parameters: mean, standard deviation, and asymptotic maximum response probability. These were used to estimate the contrasts to which each participant would respond on 25% of presentations (c25).

Results: Using segmented orthogonal regression, the maximum observed response probabilities for size III stimuli began to decline at c25 = 25.2 dB (95% confidence interval 23.3-29.0 dB from bootstrap resampling). This decline started at similar contrast for the size V stimulus: c25 = 25.0dB (22.0-26.8 dB). Among locations at which the sensitivity was above these split-points for both stimulus sizes, c25 averaged 5.6 dB higher for size V than size III stimuli.

Conclusions: The lower limit of the reliable stimulus range did not differ significantly between stimulus sizes. However, more locations remained within the reliable stimulus range when using the size V stimulus.

Translational relevance: Size V stimuli enable reliable clinical testing later into the glaucomatous disease process.

Keywords: glaucoma; perimetry; psychophysics; retinal ganglion cells.

Figure 1.

Examples of FOS curves obtained at four locations. In each case, the black symbols represent the response probability at each stimulus contrast for a Size III target; the solid black line shows the best fit curve through those data; and the horizontal dashed black lines show the 90% confidence interval for the fitted asymptotic maximum response probability, obtained empirically through bootstrapping. The red points and lines show the equivalent for a Size V target.

Figure 2.

The observed response probability for a maximal 3.7-dB stimulus at each location, out of 35 trials, plotted against the contrast (dB) that would result in 50% (left) or 25% (right) response probability according to the fitted frequency-of-seeing curve. Black points are from testing with a size III stimulus, and red points are from testing with a size V stimulus.

Figure 3.

The contrasts (dB) that would give 50% (left) or 25% (right) response probability for each stimulus size, based on the fitted frequency-of-seeing curves, at each location tested. The solid lines give the difference between the stimulus sizes, based only on those locations with sensitivity above the split-points derived from Figure 2, indicating that a maximal contrast stimulus would be reliably detected. Therefore the line is based on locations with c50 greater than or equal to 23.1 dB and c25 greater than or equal to 25.2 dB, respectively, for size III stimuli. Locations for which the asymptotic maximum response probability was below 50% (left) or 25% (right) for either stimulus size are omitted.