Practical issues in bispectral analysis of electroencephalographic signals.

B ispectral analysis is an advanced signal processing technique that quantifies quadratic nonlinearities (phase-coupling) among the components of a signal. There are only a few reports concerning the bispectrum of electroencephalogram (EEG). Barnett et al. (1) first reported the Bispectral analysis of EEG in 1971. Sigl and Chamoun (2) introduced the detailed principle and concept of bispectral analysis in 1994. Ning and Bronzino (3) reported the changes of bispectrum of the rat EEG during various vigilance states, and Muthuswamy et al. (4) reported the bispectral analysis of burst patterns in EEG. This analytic technique is also known as a core technology of the Bispectral Index System (BIS) monitor (Aspect Medical Systems, Natick, MA). Although bispectral analysis involves complicated mathematics, today’s computers are powerful enough for real-time Bispectral analysis of EEG data. Nevertheless, at the time of this writing, there were no reports accurately showing the relationship between the “depth of anesthesia” and bispectrum and bicoherence, normalized variable of bispectrum, of EEG. To investigate such relationship, we developed a software application that runs under Microsoft Windows 95/98 (Microsoft Corp., Redmond, WA). While developing the software, we discovered several theoretical and practical problems with the bispectral analysis of EEG. The aim of this report was to confirm the methodology of bispectral analysis of EEG.