Visual evoked spectrum array and interhemispheric variations.

An EEG spectrum analysis during visual stimulation was computed in 19 normal subjects. Visual stimulation consisted of trains of flashes at frequencies ranging between 2.5 and 20 flashes per second. Recordings were carried out simultaneously from the right and left occipital regions with bipolar and referential montages. Compressed spectral arrays were computed for eight-second epochs at each recording site using a fast Fourier transform. The ratio of the spectral energy from homologus regions of right and left hemispheres at each stimulation frequency was determined. The ratios were graphically displayed in a visual evoked spectrum array (VESA) ratio plot (VESA-GRAM); the mean of the ratio plot was designated the VESA coefficient. The range of variation for these measurements was determined for normal subjects. An application of the technique to patients with hemianopia showed abnormal VESA (characterized by smaller spectral amplitudes over the appropriate hemisphere), abnormal VESA ratio plots, and high VESA coefficients. These preliminary findings suggest that VESA may be a promising method to detect retrochiasmatic visual defects.