Investigating the Effect of Visual Field Eccentricity on Spatial Summation in Mesopic Microperimetry

Abstract

Purpose : To measure the upper limit of complete spatial summation (Ricco’s area) across the central visual field in healthy observers under the conditions of mesopic microperimetry.

Methods : Achromatic contrast thresholds were measured for six differently-sized incremental spot stimuli (area: -2.07 – 0.32 log deg²) of fixed duration (200 ms), at twelve visual field locations (four each at 2.5°, 5°, and 10° eccentricity) in five healthy observers (mean age: 61.4 years, range: 60-63). All stimuli were generated using MATLAB (R2016b, The MathWorks Inc., USA) with Psychtoolbox (v3.0) and presented on a gamma-corrected CRT display (SONY 420-GSM; refresh rate 75Hz, pixel resolution 1280x1024) with a uniform background luminance of 1.52 cd/m². Iterative two-phase regression analysis was used to estimate both Ricco’s area and the associated luminance contrast at threshold for a Ricco’s area scaled stimulus at each test location. Changes in these measures as a function of visual field eccentricity were investigated.

Results : At all locations, median Ricco’s area (log deg²) was smaller than the standard Goldmann III stimulus (-0.84) used in microperimetry (median [IQR]: -1.29 [-1.58 to -1.14], -1.04 [-1.16 to -0.98] and -1.09 [-1.38 to -0.94] at 2.5°, 5°, and 10° respectively, Fig. 1). Log contrast thresholds (DI, cd/m²) for Ricco’s area-sized stimuli (median [IQR]) were -0.23 [-0.27 to -0.13], -0.24 [-0.31 to -0.14] and 0.20 [-0.074 to 0.449]. No statistically significant change in either Ricco’s area or contrast thresholds at Ricco’s area were found with increasing visual field eccentricity (Friedman test, all P>0.05 after Holm-Bonferroni correction).

Conclusions : Under mesopic conditions Ricco’s area does not vary significantly across the central visual field. In contrast to photopic conditions where Ricco’s area enlarges as a function of visual field eccentricity, a flattened hill-of-vision under mesopic testing conditions may account for this finding. Further work is required to determine if compensatory enlargements in Ricco’s area occur in age-related macular degeneration to inform the selection of optimal stimulus parameters for the detection and monitoring of functional changes in the condition using microperimetry.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.

 

Figure 1: Ricco’s area measurements with varying visual field eccentricity across all test locations.