STIMULUS-INDUCED GAMMA-OSCILLATIONS - HARMONICS OF ALPHA-ACTIVITY

EVENT-RELATED changes of spectral power of the EEG were studied for ea ch integer frequency between 5 and 100 Hz in three subjects during mem ory storage and retrieval. Spectra were calculated for successive, ove rlapping time epochs in seven channels. In one subject a stimulus-lock ed increase of power was observed at 12 Hz, while in the other two alp ha 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 a frequencies. An analysis of the cross covariance of the alpha, beta and gamma activity revealed high coefficients for harmonic frequencie s only while all other covariances were negligibly small. It is argued that event-related gamma activity may be an epiphenomenon of event-re lated changes within the alpha band.