The effect of stimulus orientation on the visual evoked potential in multiple sclerosis.

Visual evoked potential (VEP) latency was measured in 89 patients with multiple sclerosis (MS) using both a checkerboard stimulus and an alternating vertical grating. Thirty-seven patients had abnormal VEP latencies to the check stimulus, but 63 were classified as abnormal when the grating stimulus was employed. In an additional 22 MS patients, each eye was tested with a checkerboard stimulus, a vertical grating, and a horizontal grating to determine if edge orientation of the stimuli was responsible for the difference between grating and check diagnostic yields. The resulting diagnostic differences could be explained by taking into account the sinusoidal components of these stimuli. After equating gratings and checks for the fundamental Fourier component, each eye of 15 of the MS patients was retested with gratings in three orientations: vertical, horizontal, and oblique. Nine of the 15 patients had an orientation-dependent abnormality of VEP latency. The results suggest that MS causes an orientational imbalance in the human visual system and that VEP testing with stimuli in more than one orientation can increase the diagnostic yield in MS patients.