Steady-state visual evoked response amplitudes and concurrent electroencephalographic activity.

Among the possible causes of steady-state visual evoked response (VER) amplitude variability are: concurrent electroencephalographic (EEG) activity occurring at frequencies other than that of the VER (alpha rhythms, etc.), and EEG activity occurring at the same frequency as the VER (Noise). To evaluate these two factors, 10 steady-state VER's (7.8 Hz pattern reversal rate) and 10 samples of resting EEG activity were obtained from each of 20 normal subjects. The correlations between VER and concurrent EEG activity at other frequencies were calculated from simultaneous determinations of Fourier-derived VER and EEG amplitudes. No significant correlations were found. Because EEG Noise occurring at the same frequency and time as the VER cannot easily be separated from the VER by Fourier transformation, two indirect techniques for assessing the amplitude of this Noise were evaluated: measurement of simultaneously recorded activity at frequencies directly adjacent to the 15.6 Hz VER frequency and measurement of EEG activity at 15.6 Hz obtained just before the VER measurements. Neither of these procedures could predict trial-by-trial variations in the amplitude of the Noise at 15.6 Hz. However, across subjects both procedures provided good estimates of the mean EEG Noise underlying the VER.