Nonstationary nature of the brain activity as revealed by EEG/MEG: Methodological, practical and conceptual challenges

Revealing the functional meaning of EEG and MEG signals’ nonstationarity and metastability is one of the major topics in current brain research. Indeed, the explicit quasi-stationary phenomena in the activity of large neuronal populations are still largely unknown. However, the fast dynamics of quasi-stationary episodes in EEG/MEG signal, together with rapid transitive periods between them, fit to the time scale of our conscious experience on the one hand, and to the theory of coupled nonlinear dynamical subsystems on the other hand. The global integrity of local quasi-stationary states of EEG/MEG signal is the other side of metastable brain dynamics. In the current review paper we present methodologies for studying the quasi-stationary composition of both local EEGs/MEGs and the inherent synchrony between quasi-stationary structures in pairs of EEG/MEG channels. To obtain quantitative characteristics of segmental organization and structural synchrony of multichannel EEG/MEG signal, the original algorithms and program tools have been used. Convincing results obtained for the experimental models and simulated data are presented and discussed in detail. A novel framework for the analysis of EEG/MEG time series that alternate between different operating modes is suggested.