Electrical activity of the human brain, termed electroencephalogram (EEG), was first recorded by the German physiologist and psychiatrist Hans Berger (1873–1941) in 1924 (cf. [1]). The EEG activity recorded during specific cognitive events has been studied either in the time domain as event-related potentials (ERPs) comprising trial-averaged waveforms, or in the frequency domain as event-related oscillations (EROs) with time-frequency characteristics. According to Basar [2], specific frequencies of EROs underlie different cognitive functions, and selectively distributed delta, theta, alpha and gamma oscillatory systems act as resonant communication networks through large populations of neurons during cognitive processing. ERO signals with phase-alignment across the trials of cognitive events are termed ‘evoked’ or ‘phase-locked’ oscillations, while the signals that are ‘out-of-phase’ are ‘induced’ or ‘non-phase-locked’ oscillations [3]. The ‘total’ ERO power consists of both evoked and induced signals [4].
