Glaucoma Affects Steady State VEP Contrast Thresholds Before Psychophysics

Abstract

Frequency doubling technology (FDT) is a recent psychophysical test for glaucoma. It measures the contrast threshold to low spatial frequency, high temporal frequency sinusoidal luminance profile bars. We wanted to confirm, with stricter controls, Vaegan and Hollow's report that contrast thresholds of steady state visual evoked potentials (ssVEPs) to a stimulus resembling the central field of the FDT test was more sensitive to glaucoma than the subjective threshold to the same stimulus and to start to optimize the technique. A double masked trial using 57 eyes of 42 subjects. Both thresholds were estimated by modified binary search. In psychophysical testing, subjects were given a two alternative forced choice task. In ssVEP testing a significant signal in any one of eight channels was deemed to be a detection. In some subjects electrode positions were compared, both eyes were tested, tests were repeated to estimate reliability, stimulus frequencies were varied or full contrast functions were obtained. Thresholds and percent abnormal increased as a function of glaucoma severity for ssVEPs but not for psychophysics. Both threshold measures were reliable. Interocular correlations were low. SsVEP amplitude against contrast functions had similar thresholds to those found by modified binary search. The data was too irregular for individual thresholds to be estimated from a fitted exponential. Amplitudes were greatest at 7 to 10 Hz, < or =6 cm above inion and least laterally. Noise was similar across electrodes. SsVEP contrast thresholds to a stimulus like the central field of the FDT test detected glaucoma better than corresponding psychophysical thresholds at 18.29 Hz, when formal controls were used, as they had in a less controlled previous study at 7.14 Hz.