What we can know from bispectral analysis of EEG?

Bispectral analysis is an advanced signal processing technique that quantifies quadratic nonlinearities (phase-coupling) among the components of a signal. As bicoherence is the direct indicator of the degree of phase coupling, it is the most important element of bispectral analysis. Here we investigated the changes of EEG bicoherence induced by anesthesia or noxious stimuli. First, we investigated the changes of EEG bicoherence during isoflurane anesthesia. We found EEG bicoherence showed two peaks in fairly restricted regions of bi-frequency space and their heights were well correlated with isoflurane concentration. It is well known that EEG is influenced by noxious stimuli as well as anesthetic concentration. Then we investigated the influence of noxious stimuli on EEG bicoherence during isoflurane or sevoflurane anesthesia. Two peaks of EEG bicoherence were both diminished after skin incision, which re-emerged after fentanyl administration. On the other hand, EEG bicoherence was not changed after incision, when fentanyl was administered before incision. Their data suggested that EEG bicoherence was sensitive to noxious stimuli, which suggested that EEG bicoherence would become an indicator for adequacy of analgesia during anesthesia. In conclusion, we would be able to manage both hypnosis and analgesia during anesthesia by EEG bicoherence monitoring.