ABSTRACT
In the overwhelming majority of studies of brain electrical activity (EA) the frequency band of 1-30 Hz (from delta up to beta-3) has been studied. However, recently a number of reports have been published concerning the involvement of frequencies higher than 30 Hz (the gamma-range of the human EEG) in processes of cognitive activity (Bressler, 1990; De France and Sheer, 1988; Pulvermuller et al., 1995; Ray and Cole, 1985; Sokolov, 1990, 1996). Moreover, ideas about a functional role for this range encompass not only processes of sensory integration and perception, but have even considered it as a basic condition for the formation of consciousness (Barinaga, 1990; Crick and Koch, 1990). It is necessary to emphasize that gamma-frequency activity is characterized by its low amplitude (up to 10 µV), several times below that of the dominant alpha-waves (40-50 µV). High-
frequency (HF) fluctuations are commonly attributed to the combined gamma-band, regarded by many authors as a summation of noise, which masks any useful signal. The difficulties of recording the HFcomponents in humans through the scalp are due to the occurrence of artefacts from muscle potentials in the head. In addition, the averaging properties of the scalp make any evaluation of the localization of gamma-waves essentially more complicated.
