Examining the effect of test stimulus size and movement on the detection of contrast in visual field testing

Abstract

Backgrounds and Aims: Visual field testing is a common procedure used in clinical assessments to provide a functional measure of vision. It is usually conducted by measuring the detectability of a spot of light presented to discrete locations across the visual field (static perimetry) or by moving a spot of light across the visual field until it is visible (kinetic perimetry). Despite the popularity of visual field testing in both research and clinical assessment, a number of limitations have been noted with both methods. In particular, it has been noted that contrast sensitivity measured by automated visual field does not hold good concordance with structural loss (resulting from eye disease) and that static and kinetic perimetry yield different results, despite that they assess the hill of vision. In this thesis, visual field testing was examined, and the effect of spatial summation (Chapter 3), and the relationship between static and kinetic perimetry (Chapter 4) were investigated in separate studies. Hypothesis This thesis is built upon the hypothesis that testing the retina with a stimulus smaller than critical area will provide a better sensitivity to central retinal defects. In addition, static and kinetic testing is highly correlated if the testing method of Kinetic testing is modified. Experiments In the first study, a review of previous studies examined how the stimulus size and spatial summation (the degree to which the visual system spatially integrates information to detect contrast) affected contrast sensitivity. Following this, spatial summation was measured empirically, and the critical area (Ac) at different retinal locations was quantified. The Ac, which is the point at which contrast threshold no longer improves as the stimulus size is increased, (derived from data in previous studies and empirically) was observed to increase with retinal eccentricity, and this was uniform across the visual field. In particular, a Goldmann size III (the gold standard target size) is larger than the Ac within the central 25-30 degrees of arc; this means that size III can be less sensitive to detect the threshold, but is more sensitive at higher eccentricities because it is less than the Ac. This explains why size III can capture glaucoma at the periphery at early stages of the disease. Accordingly, current visual field testing (static, 30-2 screening test), which uses a fixed-size stimulus for central and peripheral retina, does not test the visual field equally (as spatial summation is dependent on retinal eccentricity, and thus contrast sensitivity is dependent on stimulus size) and is influenced by the different spatial summation characteristic of different retinal locations. In study 2, kinetic perimetry was performed to identify isopters along different angular meridians, but in two conditions in which the target was moved from the center of the stimulus to the periphery or in the opposite conditions. This two-way method of limits corrected for the directional bias associated with testing in one direction. This experiment identified a specific region on the retina, where the contrast of the kinetic stimulus was the same as the contrast threshold measured using static perimetry; we called it isocontrast region. This finding suggests that static and kinetic perimetry are highly correlated and previous reports of outcome differences most likely stem from methodological differences.

Conclusion The findings of the present study indicate that considerations of the test stimulus size and the method used to measure visual field are most important in visual field testing because they greatly contribute to the accuracy in measuring contrast sensitivity.