Early magnocellular loss in glaucoma demonstrated using the pseudorandomly stimulated flash visual evoked potential.

Components of the pseudorandomly stimulated flash visual evoked potential (VEP) have now been identified that appear to arise predominantly from each of the magnocellular (M-cell) and parvocellular (P-cell) systems. In this study, the relative damage to magnocellular and parvocellular systems at different stages of glaucoma using pseudorandomly stimulated flash VEP was investigated. Pseudorandomly stimulated flash VEP was recorded in 15 normal eyes and 28 eyes with different stages of glaucoma using the VERIS-3 recording system (Electro-Diagnostic Imaging, San Francisco, CA). Two levels of luminance contrast of the stimulus (32% and 99%) were tested. The first slice of the second-order kernel from only the central (8 degrees) stimulated area was extracted for analysis. Data recorded from normal eyes demonstrated early saturation of the response/contrast function of the first slice of the second-order kernel. The ratio of the VEP amplitude recorded at 32% and 99% of the luminance contrast was close to unity. In eyes with early glaucoma, although the amplitude of the responses to both low- and high-contrast stimulation decreased, the relative reduction of the low-contrast VEP (M-cell) was more prominent. However, the amplitude of the high-contrast response (P-cell) declined more rapidly later in the disease. These results are consistent with relatively earlier damage of the magnocellular pathway in glaucoma.