slow temporal fluctuations (envelope) of the orthogonalized MEG signals (for a review see O'Neill et al., 2015). Moreover, MEG is able to track neuronal activity at its characteristic time scale, i.e., milliseconds, and it is thus ideally suited to assess the faster dynamics of the different brain areas as well as of their functional connectivity. Indeed, this characteristic makes MEG also able to investigate, with high spectral resolution, neural oscillations which are known to subserve brain connectivity and to play important roles in cognitive processes (Varela et al., 2001; Engel et al., 2013). In more detail, it has been hypothesized that only coherently oscillating neuronal groups can interact effectively. This hypothesis, namely Communication Through Coherence (CTC; Fries, 2005), is grounded in the fact that neural oscillations are associated to neuronal excitability fluctuations: two neuronal groups can communicate only when they share the same excitability state, with a possible time lag between the two that takes into account the speed of traveling signals (Bastos et al., 2015; Fries, 2015). Thus, long range connectivity can be assessed through the characterization of phase-phase coupling (hereinafter simply referred to as phase coupling) between MEG signals which has also been related to the concept of
