Abstract
Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that allows interaction with endogenous cortical oscillatory rhythms by means of external sinusoidal potentials. The physiological mechanisms underlying tACS effects are still under debate. Whereas online (e.g., ongoing) tACS over the motor cortex induces robust state-, phase- and frequency-dependent effects on cortical excitability, the offline effects (i.e. after-effects) of tACS are less clear. Here, we explored online and offline effects of tACS in two single-blind, sham-controlled experiments. In both experiments we used neuronavigated transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) as a probe to index changes of cortical excitability and delivered M1 tACS at 10 Hz (alpha), 20 Hz (beta) and sham (30 s of low-frequency transcranial random noise stimulation; tRNS). Corticospinal excitability was measured by single pulse TMS-induced motor evoked potentials (MEPs). tACS was delivered online in Experiment 1 and offline in Experiment 2. In Experiment 1, the increase of MEPs size was maximal with the 20 Hz stimulation, however in Experiment 2 neither the 10 Hz nor the 20 Hz stimulation induced tACS offline effects. These findings support the idea that tACS affects cortical excitability only during online application, at least when delivered on the scalp overlying M1, thereby contributing to the development of effective protocols that can be applied to clinical populations.
Highlights
Transcranial alternating current stimulation is a non-invasive brain stimulation technique that allows interaction with endogenous cortical oscillatory rhythms by means of external sinusoidal potentials
Post-hoc comparisons showed that motor evoked potentials (MEPs) change during 20 Hz Transcranial alternating current stimulation (tACS) was higher compared to 10 Hz (p = 0.005, average raw MEPs size: 916 μV ± 696 μV) and sham (p = 0.026, average raw MEPs size: 908 μV ± 546), which did not differ from one another (p = 0.570)
Methods of Experiment 2 were identical to the Methods of Experiment 1, with some adjustments to allow the measurement of tACS offline effects: (1) MEPs were recorded after the termination of the stimulation, which lasted 15 min
Summary
Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique that allows interaction with endogenous cortical oscillatory rhythms by means of external sinusoidal potentials. In Experiment 1, the increase of MEPs size was maximal with the 20 Hz stimulation, in Experiment 2 neither the 10 Hz nor the 20 Hz stimulation induced tACS offline effects These findings support the idea that tACS affects cortical excitability only during online application, at least when delivered on the scalp overlying M1, thereby contributing to the development of effective protocols that can be applied to clinical populations. One of the main controversial issues is whether transcranial electrical stimulation (tES) (and tACS in particular) exerts its effects only when delivered online, that is, during ongoing stimulation, only offline (i.e. after-effects), after the stimulation has ceased, or b oth[17,21,22,23] This question can be addressed with different methods, including functional magnetic resonance imaging (fMRI), electro- and magneto-encephalography (EEG and MEG, respectively) and transcranial magnetic stimulation (TMS). Amplitude of TMS-induced MEPs gives the possibility to measure both online[7] and o ffline[30] effects of tACS
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