Differences in Static and Kinetic Perimetry Results are Eliminated in Retinal Disease when Psychophysical Procedures are Equated

Abstract

Purpose: We tested the hypothesis that clinical statokinetic dissociation (SKD, defined as the difference in sensitivity to static and kinetic stimuli) at the scotoma border in retinal disease is due to individual criterion bias and that SKD can be eliminated by equating the psychophysical procedures for testing static and kinetic stimulus detection.

Methods: Six subjects with glaucoma and six with retinitis pigmentosa (RP) were tested. Clinical procedures (standard automated perimetry [SAP] and manual kinetic perimetry [MKP]) were used to determine clinical SKD and the region of interest for laboratory-based testing. Two-way Method of Limits (MoL) was used to establish the isocontrast region at the scotoma border in glaucoma and RP subjects. Method of Constant Stimuli (MoCS) and a two-interval forced choice (2IFC) procedure then were used to present static or kinetic (inward or outward) stimuli at different eccentricities within the isocontrast region. The results were fitted with psychometric functions to determine threshold eccentricities.

Results: Clinical SKD was found in glaucoma and RP subjects, with variable magnitude among subjects, but significantly exceeding expected typical measurement variability. The resultant psychometric functions when using MoCS and 2IFC showed equal sensitivity to static and kinetic targets, thus eliminating SKD.

Conclusions: Clinical SKD found using clinical techniques is due to methodologic differences and criterion bias, and is eliminated by using an equated and more objective psychophysical task, similar to normal subjects.

Translational relevance: Eliminating SKD using a psychophysical approach minimizing criterion bias suggests that it is not useful to distinguish between normal and diseased fields.

Keywords: Goldmann perimetry; glaucoma; retinitis pigmentosa; standard automated perimetry; statokinetic dissociation.

Figure 1

Examples of static and kinetic perimetry results from one of the RP subjects in our study: Humphrey Field Analyzer 30-2 test grid (A), 10-2 test grid (B) and one-way MoL kinetic perimetry using a III4e target (C). In (A-C), all results have been magnified, but show different extents of the VF and different levels of detail, depending on the resolution of the test grid. These differences are made more apparent in (D), when scaled down to their respective grid sizes and superimposed (kinetic results at 100% transparency and static results at 30% transparency). An example region of interest tested in our study is highlighted by the green box.

Figure 2

(A) MoL for determining the “inner” isopter. The black square is the fixation mark for the subject. A stimulus moves in an inward direction (straight path along the meridian) at a constant speed and the subject responds when they first see the target. (B) MoL for determining the “outer” isopter. The stimulus moves outward along the meridian and the subject responds when the target disappears. (C) MoCS and 2-interval forced choice procedure. The fixation mark is shown. After 200 ms, there is a tone signaling the first interval, followed by a 200 ms pause, then another tone signaling the second interval. After both intervals are shown, the fixation mark is shown again as the program waits for a response from the first subject. During one of the two intervals, a stimulus is shown. The stimulus was randomly presented up to 4° inward or up to 4° outward in 1° steps around the midpoint of the isopters found in (A) and (B) (black dashed inset). The stimulus conditions included a static stimulus, inward moving stimulus or outward moving stimulus. All stimuli were shown for 200 ms.

Figure 3

A comparison of the difference between the midpoint of inner and outer isopters generated using the two-way MoL and the defect position found on SAP, typically regarded as statokinetic dissociation found using clinical instruments (SKD) for glaucoma (A) and RP (B) subjects. The relative difference (sign dependent, where a positive y-axis value indicates a more outward position obtained using the MoL) between SAP and MoL for each subject is shown by the black filled circle. Error bars: 95% CI. The standard deviations (SD) of the differences are also plotted for each subject (open circle). Similar figures are shown when the differences between one-way MoL (inward isopter, more conventionally used in practice) and SAP border (red filled circle; Error bars: 95% CI) and SD of the differences (red open circle) are considered (C, D). In all figures, the black dotted horizontal line indicates no disparity between SAP and MoL techniques, that is no clinical SKD found.

Figure 4

Psychometric functions (proportion seen as a function of eccentricity, in degrees) for the individual RP subjects (n = 6) for static (black), inward moving (blue), and outward moving (red) stimuli. Note that for consistency across all subjects the x-axis has been flipped, such that the lower eccentricity indicates a location closer to fixation. The cardinal visual field direction of testing has been noted for each patient (0° indicates the nasal direction, increasing in a counterclockwise direction). Each datum point represents the average of at least 20 responses. Error bars: 1 SEM. The horizontal dashed line indicates the threshold value (0.75). The vertical dashed lines indicate the inner and outer isopter positions measured using the laboratory-based two-way MoL, with the gray zone representing the isocontrast zone.

Figure 5

Psychometric functions (proportion seen as a function of eccentricity, in degrees) for the individual glaucoma subjects (n = 6) for static (black), inward moving (blue), and outward moving (red) stimuli. Functions are plotted as per Figure 4.

Figure 6

Slope values calculated by dividing the IQR of the psychometric function by 0.5 for each diagnostic category. Each datum point represents the result from one observer, and the horizontal bars indicate the median and IQR of slope values.