Visual evoked potential assessment of the effects of glaucoma on visual subsystems

The purpose of this study is to test the hypothesis that glaucoma leads to selective deficits in parallel pathways or channels. Sweep VEPs were obtained to isolated-check stimuli that were modulated sinusoidally in either isoluminant chromatic contrast or in positive and negative luminance contrast. Response functions were obtained from 14 control subjects, 15 patients with open-angle glaucoma, and seven glaucoma suspects. For all three groups of subjects we found characteristic differences between the VEP response functions to isoluminant chromatic contrast stimuli and to luminance contrast stimuli. The isoluminant chromatic stimulus conditions appeared to favor activity of the P-pathway, whereas the luminance contrast stimuli at low depths of modulation favored M-pathway activity. VEP responses for patients with OAG were significantly reduced for chromatic contrast and luminance contrast conditions, whereas VEP responses for glaucoma suspects were significantly reduced only for the 15-Hz positive luminance contrast condition. Our results suggest that both M- and P-pathways are affected by glaucoma.