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 non-linear 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.