Evoked and induced gamma-band activity of the human cortex

The evoked gamma-band activity is an event related rhythmic response which persists within the first 100 ms after the stimulus onset. It shows spectral peaks between 30 and 40 Hz in the auditory, between 45 and 55 Hz in the somatosensory and between 100 and 110 Hz in the visual system. After separation of the wide-band activity in slow and gamma-band activity, a moving single equivalent current dipole model accounts for each activity almost completely. The induced gamma-band activity is not phase-locked to the stimulus or it is strongly gittering and thus it cannot be extracted in time domain. In this case we are using signal analysis methods in frequency domain. The evaluation of the induced brain gamma-band activity around 30 Hz shows differences to word and nonword stimuli. It was supposed that the induced gamma-band activity represents the synchronized activity of Hebbian cell assemblies correlated to words.