Phase Dependency of the Human Primary Motor Cortex and Cholinergic Inhibition Cancelation During Beta tACS.

Andrea Guerra,Vincenzo Di Lazzaro,Florinda Ferreri,Alek Pogosyan,Peter Brown,Magdalena Nowak,Huiling Tan

doi:10.1093/cercor/bhw245

Abstract

The human motor cortex has a tendency to resonant activity at about 20 Hz so stimulation should more readily entrain neuronal populations at this frequency. We investigated whether and how different interneuronal circuits contribute to such resonance by using transcranial magnetic stimulation (TMS) during transcranial alternating current stimulation (tACS) at motor (20 Hz) and a nonmotor resonance frequency (7 Hz). We tested different TMS interneuronal protocols and triggered TMS pulses at different tACS phases. The effect of cholinergic short-latency afferent inhibition (SAI) was abolished by 20 Hz tACS, linking cortical beta activity to sensorimotor integration. However, this effect occurred regardless of the tACS phase. In contrast, 20 Hz tACS selectively modulated MEP size according to the phase of tACS during single pulse, GABAAergic short-interval intracortical inhibition (SICI) and glutamatergic intracortical facilitation (ICF). For SICI this phase effect was more marked during 20 Hz stimulation. Phase modulation of SICI also depended on whether or not spontaneous beta activity occurred at ~20 Hz, supporting an interaction effect between tACS and underlying circuit resonances. The present study provides in vivo evidence linking cortical beta activity to sensorimotor integration, and for beta oscillations in motor cortex being promoted by resonance in GABAAergic interneuronal circuits.

Highlights

Transcranial alternating current stimulation is a novel, noninvasive neurophysiological technique able to induce or entrain brain oscillations by causing coherent changes in the firing rate and timing of neuronal populations (Antal and Paulus 2013; Reato et al 2013)
We aimed to investigate the variations of the Motor evoked potentials (MEPs) size that were caused by phase-dependent effects within each transcranial magnetic stimulation (TMS) protocol and at each transcranial alternating current stimulation (tACS) frequency
Post hoc analysis with paired t-tests showed that intracortical facilitation (ICF) significantly increased the MEP (1917 ± 245 μV, P < 0.005 compared with the MEP with single-pulse TMS)

Summary

Introduction

Transcranial alternating current stimulation (tACS) is a novel, noninvasive neurophysiological technique able to induce or entrain brain oscillations by causing coherent changes in the firing rate and timing of neuronal populations (Antal and Paulus 2013; Reato et al 2013). It is capable of modulating cognitive functions TACS effects tend to be frequency and area selective (Kanai et al 2008; Feurra et al 2011a; Riecke et al 2015)

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