Transcranial Alternating Current Stimulation at Beta Frequency: Lack of Immediate Effects on Excitation and Interhemispheric Inhibition of the Human Motor Cortex

Viola Rjosk, Elisabeth Kaminski, Maike Hoff, Christopher Gundlach, Arno Villringer, Bernhard Sehm, Patrick Ragert, Viola Rjosk, Elisabeth Kaminski, Maike Hoff, Christopher Gundlach, Arno Villringer, Bernhard Sehm, Patrick Ragert

Abstract

Transcranial alternating current stimulation (tACS) is a form of noninvasive brain stimulation and is capable of influencing brain oscillations and cortical networks. In humans, the endogenous oscillation frequency in sensorimotor areas peaks at 20 Hz. This beta-band typically occurs during maintenance of tonic motor output and seems to play a role in interhemispheric coordination of movements. Previous studies showed that tACS applied in specific frequency bands over primary motor cortex (M1) or the visual cortex modulates cortical excitability within the stimulated hemisphere. However, the particular impact remains controversial because effects of tACS were shown to be frequency, duration and location specific. Furthermore, the potential of tACS to modulate cortical interhemispheric processing, like interhemispheric inhibition (IHI), remains elusive. Transcranial magnetic stimulation (TMS) is a noninvasive and well-tolerated method of directly activating neurons in superficial areas of the human brain and thereby a useful tool for evaluating the functional state of motor pathways. The aim of the present study was to elucidate the immediate effect of 10 min tACS in the β-frequency band (20 Hz) over left M1 on IHI between M1s in 19 young, healthy, right-handed participants. A series of TMS measurements (motor evoked potential (MEP) size, resting motor threshold (RMT), IHI from left to right M1 and vice versa) was performed before and immediately after tACS or sham using a double-blinded, cross-over design. We did not find any significant tACS-induced modulations of intracortical excitation (as assessed by MEP size and RMT) and/or IHI. These results indicate that 10 min of 20 Hz tACS over left M1 seems incapable of modulating immediate brain activity or inhibition. Further studies are needed to elucidate potential aftereffects of 20 Hz tACS as well as frequency-specific effects of tACS on intracortical excitation and IHI.

Keywords: interhemispheric inhibition (IHI); motor cortical excitability; neuroplasticity; transcranial alternating current stimulation (tACS); transcranial magnetic stimulation (TMS).

Figures

Figure 1
Figure 1
Experimental procedures. Nineteen young and healthy participants took part in two experimental sessions in a cross-over design on 2 days separated by at least 48 h. At each experimental session, cortical excitability (motor evoked potential (MEP) size, resting motor threshold (RMT)) and interhemispheric inhibition (IHI) were measured by transcranial magnetic stimulation (TMS) single- and paired-pulse protocols over both primary motor cortex (M1) hand areas before and after transcranial alternating current stimulation (tACS; tACS session) or sham (sham session). IHI was investigated both from left M1 to right M1 (LIHI) and vice versa (RIHI) in a randomized order. MEPs of right and left first dorsal interosseus muscle (FDI) were recorded by electromyography (EMG). The experimental sessions only differed in the type of stimulation: 10 min of 20 Hz tACS vs. sham (30 s of 20 Hz tACS) stimulation with an intensity of 1 mA each. The active electrode was placed on the FDI hotspot of the left M1, the “reference” electrode was placed on Pz according to the International 10-20 EEG system. The order of stimulation was randomized between participants and the study was performed in a double-blinded manner. Throughout the experiment, participants were seated in a comfortable chair in a relaxed position and were asked to keep their eyes open. See also “Experimental Procedures” Section for a detailed description.
Figure 2
Figure 2
Interhemispheric inhibition. Depicted are the individual IHIs of each participant (slim gray lines) as well as the mean IHI (bold black lines) per session and direction of IHI measurement. IHI was assessed with paired-pulse TMS before (pre) and after (post) 10 min of 20 Hz tACS (tACS) or sham stimulation (sham). IHI from left to right M1 (LIHI) was obtained by delivering a conditioning stimulus (CS) to left M1 followed by a test stimulus (TS) delivered to right M1 with an interstimulus interval (ISI) of 10 ms. For RIHI, CS was applied to right M1 followed by TS applied to left M1. For LIHI_pre and RIHI_pre, the TS and CS intensity was adjusted to the minimum stimulus intensity to evoke 1 mV control MEPs. For LIHI_post and RIHI_post CS intensity was maintained, but TS intensity was adjusted, if necessary, to evoke 1 mV control MEPs after tACS or sham. (A,B) Depict LIHI and RIHI under sham stimulation and (C,D) depict LIHI and RIHI under 10 min of tACS. See “Interhemispheric Inhibition: Paired-Pulse TMS Protocols” and “IHI” Sections for details as well as Table 1.

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