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TITLE
Cumulative Effects of Single TMS Pulses During Beta-tACS are Stimulation Intensity-Dependent

AUTHORS
Raco V; Bauer R; Norim S; Gharabaghi A.

SOURCE
Brain Stimulation. 10(6):1055-1060, 2017 Nov – Dec.

BACKGROUND
Single transcranial magnetic stimulation (TMS) pulses activate different components of the motor cortex neural circuitry in a stimulation intensity-dependent way and may lead to a cumulative increase of corticospinal excitability (CSE) during the same stimulation session. Furthermore, transcranial alternating current stimulation (tACS) has been shown to increase in a frequency-specific way the level of CSE probed by single-pulse TMS. The interaction of these two phenomena, i.e. cumulative increases and baseline shifts of CSE, and the involved neural circuitry has not been studied yet.

OBJECTIVE
The aim of this study was to investigate stimulation intensity-specific online effects of simultaneous TMS and tACS on CSE.

METHODS
Single-pulse TMS was applied concurrent to 20 Hz tACS over the left primary motor cortex of thirteen healthy subjects to probe CSE indexed by motor evoked potentials (MEPs) recorded from the contralateral extensor carpi radialis muscle of the right hand during rest. Six different TMS intensities (90%, 100%, 110%, 120%, 130%, and 140% of resting motor threshold, RMT) were studied in a randomized blocked design. In each block, 40 pulses were applied with an inter-stimulus interval of 5 s and a jitter of +/-0.5 s, i.e. at a stimulation frequency of 0.2-0.25 Hz.

RESULTS
Beta-tACS has a general facilitatory effect on CSE across the tested TMS intensities. The results of the block wise regression of the MEP amplitudes show a more specific effect. Combining tACS and TMS leads to a cumulative increase in CSE for the stimulation intensity of 120% RMT only (p = 0.0004).

CONCLUSION
CSE increases due to beta-tACS and cumulative TMS pulses may be mediated by different neuronal mechanisms.

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