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.
Highlights
Transcranial alternating current stimulation is a form of noninvasive brain stimulation
The aim of the present study was to investigate the immediate effect of 10 min of 20 Hz Transcranial alternating current stimulation (tACS) over left M1 on bidirectional IHI in young, healthy participants in a sham-controlled, cross-over design
These results indicate that 10 min of 20 Hz tACS over left M1 does not seem capable of modulating immediate brain activity/or inhibition and is unlikely to induce immediate neuroplastic effects in M1
Summary
Transcranial alternating current stimulation (tACS) is a form of noninvasive brain stimulation. It is known that this relatively weak sinusoidal current can influence brain oscillations and can modulate cortical networks (Fröhlich and McCormick, 2010; Ozen et al, 2010; Thut et al, 2012; Herrmann et al, 2013). The endogenous oscillation frequency in sensorimotor areas is an idling beta activity (13–30 Hz) peaking at 20 Hz, which typically occurs during maintenance of tonic motor output and declines during sensory information processing and active movements (Niedermeyer, 1999; Baker, 2007). TACS applied in specific frequency bands (e.g., α, β or θ band) over primary motor cortex (M1) or the visual cortex modulates cortical excitability within the stimulated hemisphere by modulating natural brain rhythms (Zaghi et al, 2010a,b)
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