Coherence and Phase Coupling in the Scalp EEG and between LORETA Model Sources, and Microstates as Putative Mechanisms of Temporo-Spatial Functional Organization

What are the brain mechanisms that incorporate the seemingly homogeneous experience of a percept or a thought? Higher brain functions assumedly are generated by sets of neural sub-systems that are distributed in various brain regions and that are temporally coordinated. EEG and ERP measures of coherence or phase coupling aim at such functional temporal connections between regions. However, results obtained from scalp recorded electric data (including recomputations to CSD) are ambiguous concerning source localization because electric sources are oriented, and are not necessarily perpendicular to the head surface. Thus, there is no meaningful functional interpretation of scalp results. On the other hand, time series computed from intracerebral model sources (LORETA cortical areas) permit coherence and phase computations whose results are non-ambiguous, and which may be very different from scalp-obtained results. Whether functional connectivity is executed exclusively by transmission through fibers or to what extent electric field effects directly influence brain cell activity still needs clarification.-An alternative conceptualization assumes that brain activity contributes to a homogeneous experience as long as it occurs within a short, elementary time window. Temporal parsing of global brain electric activity into microstates leads to the identification of such sub-second building blocks in the range of <100 ms. The functional significance of various classes of spontaneous and event-related microstates has been established. The results support the hypothesis that brain work for conscious homogeneous experience might be implemented by these brief temporal packages of a quasi-stable state.