Stimulus-induced gamma oscillations: harmonics of alpha activity?

Event-related changes of spectral power of the EEG were studied for each integer frequency between 5 and 100 Hz in three subjects during memory storage and retrieval. Spectra were calculated for successive, overlapping time epochs in seven channels. In one subject a stimulus-locked increase of power was observed at 12 Hz, while in the other two alpha power decreased at the individual peak frequencies of 9 and 11 Hz, respectively. In all subjects corresponding changes of power appeared at frequencies which were integer multiples of the individual dominant alpha frequencies. An analysis of the cross covariance of the alpha, beta and gamma activity revealed high coefficients for harmonic frequencies only while all other covariances were negligibly small. It is argued that event-related gamma activity may be an epiphenomenon of event-related changes within the alpha band.