Visual evoked potentials in multiple sclerosis: frequency response shows reduced alpha amplitude.

Visual evoked potentials were measured in a group of 16 multiple sclerosis (MS) patients and in a control group of 20 subjects. With respect to vertex and occipital recordings, latencies of main peaks were prolonged and response amplitudes were reduced in the MS group. As a result of frequency domain analysis we found that the amplitude reduction was not uniform in all frequency ranges: alpha (7-12 (Hz) components of EPs were markedly reduced whereas theta (4-7 Hz) responses were not altered. It is remarkable that the frequency components were altered to a different degree--this may shed some light on the physiological roles of the frequency components: As MS is frequently associated with optic neuritis, our interpretation of this frequency-dependent pattern is based on regarding MS as a model of impaired sensory input to the brain: the fact that in this situation alpha responses are markedly reduced hints at a link between alpha responses and primary sensory processing. Theta responses turned out to be unaltered--i.e., less dependent on sensory inputs--and might thus reflect associative sensory processing. This conclusion for functional roles of EP frequency components has also been drawn from previous investigations of topographic differences of EP frequency components.