Measuring large-scale synchronization with human MEG and EEG: Challenges and solutions

Specific kinds of neuronal interactions, such as phase coupling of neuronal oscillations, are likely to be essential systems-level mechanisms for coordinating neuronal communication, integration, and segregation. The functional roles of these interactions during cognitive tasks in healthy humans can be investigated with magneto- and electroencephalography (MEG/EEG), the only means for noninvasive electrophysiological recordings of human cortical activity. While advances in source modeling have opened new avenues for assessing inter-areal interactions with MEG/EEG, several factors limit the accuracy and inferential value of such analyses. In this chapter, we provide an overview of common source analysis strategies for mapping inter-areal interactions with MEG/EEG. Linear mixing between sources, as caused by volume conduction and signal mixing, is the principal confounder in connectivity analysis and always leads to false positive observations. We discuss the sensitivity of different interaction metrics to directly and indirectly caused false positives and conclude with approaches to mitigate these problems. In conclusion, MEG and EEG are becoming increasingly useful for assessing inter-areal neuronal interaction in humans.