Chapter 16 - Neural synchronization in fMRI and EEG/MEG—a methodological guide

Abstract

Neural synchronization is a fundamental feature of the brain at both infraslow (0.01–0.1 Hz)/fMRI and faster (1–60 Hz)/EEG/MEG frequencies. Synchrony is mediated by the phases and how they synchronize across different regions and frequencies. The present chapter focuses on the distinct kinds of phase-related processes in the brain's neural activity and how we can measure them in brain imaging (fMRI, EEG). With a strong, detailed methodological component, the more general reader may want to just glance over it. One measure of neural synchronization is functional connectivity (FC), as used in fMRI. We compare time-varying and phase-based approaches to FC; this marks FC as an instance of neural synchronization. That is followed by shedding some light on the brain's synchronization through its global activity and how its global synchronization can be measured and is viewed in current neuroscience. Turning to EEG/MEG, we show how coherence between electrodes in EEG/MEG can be calculated by the phase locking value. We also demonstrate how phase coherence or variance over different trials of the same input/stimulus can be calculated and simulated. Finally, we introduce peak frequency, which measures the power at the phase angles at each time point. Together, we conclude that there are a variety of different phase-based measures in both fMRI and EEG. This makes it clear that neural synchronization is far from homogenous and may, by itself, include and operate across different timescales. Future work is necessary to shed more light on the heterogeneity of the brain's neural synchronization in its timescales, the relationship between the different measures, and their potentially different shaping of perception and cognition.