M- versus P-function: Relationship to Visual Field Loss in Patients with Open Angle Glaucoma

Abstract

Abstract: : Purpose: To compare the severity of glaucomatous visual field loss to the relative losses in magnocellular (M)- and parvocellular (P)-pathway functions using swept-parameter visual evoked potentials (VEPs) to luminance and chromatic contrast. Methods: Ten patients with open angle glaucoma (OAG) and 10 age-similar control subjects participated. Visual field thresholds were obtained using the Humphrey Visual Field Analyzer 24-2 program. VEPs were elicited by modulation (appearance/disappearance) of an array of 32 x 32 isolated checks presented on a steady yellow background. Stimuli were viewed monocularly. The patterns were either modulated in chromatic contrast (red checks) at 6 Hz (isoluminant) or in luminance contrast at 6 Hz or 12 Hz. Isoluminance was determined using a VEP technique (Zemon, Siegfried, & Gordon, 1991). VEP frequency components were quantified using Fourier methods. Results: For control subjects, the amplitude of the fundamental component increased steeply with increasing luminance contrast (M) and saturated at approximately 30% contrast. For the chromatic contrast condition (P), the amplitude rises out of the noise at approximately 16% and does not saturate. For the majority of the patients, there was little or no response at any chromatic contrast (P). For these patients, a prominent deficit was noted as well for the ON subdivision of the M-pathway when low luminance contrast stimuli (8% peak) were presented at 12 Hz. There was no relationship between the severity of visual field loss and the M- or P-pathway VEP losses. Conclusions: In this group of patients with OAG, responses to stimulation of the P-pathway and the ON subdivision of the M-pathway were severely disrupted even though visual field tests showed no involvement of central visual function.

Keywords: electrophysiology: clinical