Spatial frequency evoked visuograms in multiple sclerosis.

We obtained steady-state visual evoked potentials (VEPs) to sinusoidal gratings alternating at 4 Hz with spatial frequencies varying from 0.5 to 8 cpd in 21 normal controls and 21 patients with multiple sclerosis (MS), and analyzed responses by fast Fourier transform. Amplitude- and phase-spatial frequency functions were obtained and referred to as amplitude and phase "visuograms." We observed two types of abnormalities in the phase visuograms of MS patients: (1) abnormal responses at all spatial frequencies tested (37%), and (2) abnormal responses only at selective spatial frequencies (52%). Some patients had phase lag limited to low, middle, or high spatial frequencies. Steady-state and transient VEPs to 2 and 4 cpd showed a similar percent of abnormalities. The use of more than one spatial frequency stimulus increased the diagnostic yield by 17%. Our data confirm that MS may selectively affect specific neuronal channels within the visual pathways.