Quadratic phase coupling of electroencephalogram (EEG) and evoked potentials (EP)

The electroencephalogram (EEG) and evoked potentials (EP) are widely studied within the framework of linear systems theory. However, recent studies on EEG and brain dynamics imply that some nonlinear mechanisms may be partly responsible for the generation of these signals. The bispectrum can be used to detect quadratic nonlinearities in a time-series. After assuring the gaussianity of the signals by Kolmogorov-Smirnov test, the bispectral analysis is applied to the EEG and auditory evoked potentials (AEP). The bispectra and bicoherence indices showed prominent peaks at low frequency components of the EEG, which should be attributed to the nonlinear (quadratic) phase couplings in the waveform.