Cumulative effects of single TMS pulses during beta-tACS are stimulation intensity-dependent

Abstract

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.

Keywords: Beta-band; Non-invasive stimulation; Sensorimotor rhythm; Transcranial alternating current stimulation; Transcranial magnetic stimulation.