Functional Connectivity States of Alpha Rhythm Sources in the Human Cortex at Rest: Implications for Real-Time Brain State Dependent EEG-TMS.

Davide Tabarelli,Paolo Belardinelli,Christoph Zrenner,Arianna Brancaccio

doi:10.3390/brainsci12030348

Abstract

Alpha is the predominant rhythm of the human electroencephalogram, but its function, multiple generators and functional coupling patterns are still relatively unknown. In this regard, alpha connectivity patterns can change between different cortical generators depending on the status of the brain. Therefore, in the light of the communication through coherence framework, an alpha functional network depends on the functional coupling patterns in a determined state. This notion has a relevance for brain-state dependent EEG-TMS because, beyond the local state, a network connectivity overview at rest could provide further and more comprehensive information for the definition of ‘instantaneous state’ at the stimulation moment, rather than just the local state around the stimulation site. For this reason, we studied functional coupling at rest in 203 healthy subjects with MEG data. Sensor signals were source localized and connectivity was studied at the Individual Alpha Frequency (IAF) between three different cortical areas (occipital, parietal and prefrontal). Two different and complementary phase-coherence metrices were used. Our results show a consistent connectivity between parietal and prefrontal regions whereas occipito-prefrontal connectivity is less marked and occipito-parietal connectivity is extremely low, despite physical closeness. We consider our results a relevant add-on for informed, individualized real-time brain state dependent stimulation, with possible contributions to novel, personalized non-invasive therapeutic approaches.

Highlights

The origin of alpha waves and the function they subserve constitute long-lasting scientific issues in neuroscience
When considering the same values but extracted from Weighted Phase Lag Index (WPLI), the pattern is less evident (Γ(O, P) = 0.012; Γ(O, F) = 0.015 and Γ(P, F) = 0.013), since the mean connectivity between sectors as detected by the imaginary part of the coherence is almost the same. This has to be interpreted in the light of the different sensitivity of the two metrics with respect to the absolute value of phase shift at Individual Alpha Frequency (IAF) between the source activity in the two areas, which is related to the propagation time of the nervous signal on the pathway connecting the ROIs under consideration
Alpha phase-dependent cortical excitability has been studied both in healthy controls and patients via brain-state dependent stimulation [8,13–15]

Summary

Introduction

The origin of alpha waves and the function they subserve constitute long-lasting scientific issues in neuroscience. The “pulsed inhibition hypothesis”, for instance, proposes that alpha oscillations actively inhibit neuronal firing in a phasic manner, opening and closing interleaved periods of “high” and “low” excitability of the cortex by cyclically producing bouts of inhibition [5,6]. This hypothesis is in line with the idea behind brain-state dependent stimulation, that the outcome of an electro/magnetic perturbation depends on the instantaneous phase state of a specific brain rhythm in a given area. Studies investigating alpha in the occipital cortex demonstrate that the alpha phase at the instant of a visual stimulus predicts high or low probability of detection [7]

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