Stereo-VEPs in normals and glaucomas
A. Bergua, F. K. Horn, A. Jünemann, M. Dzialach, M. Korth
Background: Since the introduction of dynamic random dot stereograms (dRDS) by Julesz (1971) it is possible to record specific visual evoked potentials (VEP) using stereoscopic patterns. Aim of this study was to optimize recording and stimulus conditions for VEP measurements in normals and in glaucoma patients.
Methods: dRDS were generated with a stimulation unity consisting of a TV-Monitor (framerate: 120 Hz, 20" Eizo F77) and a pattern generator (Vision Works, VRG). A checkerboard pattern (checksize: 2.7° x 2.7°) was presented to the subject at a distance of 75 cm. Three-dimensional perception of the checkerboard pattern onset/offset (500 ms each) could be realized with wireless "stereo-shutter-glasses" (NuVision). Five disparities have been tested (9, 13, 22, 35 and 40 min-arc). 4 recording electrodes have been positioned on the scalp at different localizations in the midline (1, 3, 5, 7 cm above inion; reference: forehead and right ear, grounded to left ear) to find the best test position. Subjects: 10 normal controls (42 ±9 years) and 26 glaucoma patients (51 ±11 years).
Results: A small positive onset response (p120), followed by a larger negativity and a positive offset response could be observed in all subjects studied. The onset peak time was significantly prolonged (148 ms vs 124 ms, p=0.001) and the amplitude reduced (4,4m V vs 7,8 m V, p=0.03) in our glaucoma cohort using a 40 min of arc disparity and the 5 cm above inion derivation.
Conclusion: Reduced stereo-VEP responses confirm the result of earlier psychophysical studies which found decrease of stereo acuity in glaucoma patients.
Supported by: DFG, grant SFB 539
Univ. Augenklinik, Schwabachanlage 6, D-91054 Erlangen