We performed a detailed analysis of the variability of a steady-state human evoked potential (EP) and the spectral properties of the simultaneously-recorded electroencephalogram (EEG). This allowed us to determine whether the background EEG was influenced by the evoked-potential stimulus, and to what extent variability of evoked-potential estimates are simply due to the addition of the background EEG.
Steady-state visual evoked potentials (VEPs) were elicited by a checkerboard undergoing contrast-reversal modulation at one of three fundamental frequencies f: 5.0 Hz, 7.5 Hz, and 10.0 Hz. To a first approximation, the evoked potential (at frequency 2f) and the undriven components of the EEG combined linearly. However, two kinds of interactions were present. (i) Patterned visual stimulation decreased the power in the undriven EEG in the 5 Hz to 17 Hz range by as much as a factor of 2. (ii) Superimposed on this overall effect of pattern stimulation, there were changes in the EEG power at specific harmonics of the input frequency. Power increased by as much as 6-fold at the stimulus reversal rate (2f) and its second harmonic (4f). These findings imply a complex nonlinear interaction between the visual input and the EEG.