Abstract
Corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) consist of high-frequency bursts (≈667 and ≈333 Hz). However, intracortical circuits producing such corticospinal high-frequency bursts are unknown. We here investigated whether neurons activated by single TMS pulses over M1 are resonant to high-frequency oscillations, using a combined transcranial alternating current stimulation (tACS)-TMS approach. We applied 667, 333 Hz or sham-tACS and, concurrently, we delivered six single-pulse TMS protocols using monophasic or biphasic pulses, different stimulation intensities, muscular states, types and orientations of coils. We recorded motor evoked potentials (MEPs) before, during and after tACS. 333 Hz tACS facilitated MEPs evoked by biphasic TMS through a figure-of-eight coil at active motor threshold (AMT), and by monophasic TMS with anterior-to-posterior-induced current in the brain. 333 Hz tACS also facilitated MEPs evoked by monophasic TMS through a circular coil at AMT, an effect that weakly persisted after the stimulation. 667 Hz tACS had no effects. 333 Hz, but not 667 Hz, tACS may have reinforced the synchronization of specific neurons to high-frequency oscillations enhancing this activity, and facilitating MEPs. Our findings suggest that different bursting modes of corticospinal neurons are produced by separate circuits with different oscillatory properties.
Highlights
Corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) consist of high-frequency bursts (≈667 and ≈333 Hz)
The rmANOVA conducted on motor evoked potentials (MEPs) amplitude in Experiment 1 disclosed a significant ‘session’ × ‘protocol’ × ‘time-point’ interaction (F16,224 = 1.97, p = 0.016), suggesting that this measure was modified by transcranial alternating current stimulation (tACS) in one or more TMS protocols, during and/or after the stimulation
Since the modulation we found with 333 Hz tACS was present in protocols implying voluntary muscular contraction during TMS, we verified that the amount of EMG activity preceding TMS was not significantly different in all the time-points and protocols tested
Summary
Corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) consist of high-frequency bursts (≈667 and ≈333 Hz). At higher stimulus intensities the lower frequency bursting is no longer evident, and it is replaced by the 667 Hz activity To date, it is unknown whether the increase of the synchronization of motor cortex circuits at 667 Hz and/ or at the lower frequencies corresponding to the bursts evoked by AP or biphasic stimulation, would eventually result in an increase of the corticospinal output. It is unknown whether the increase of the synchronization of motor cortex circuits at 667 Hz and/ or at the lower frequencies corresponding to the bursts evoked by AP or biphasic stimulation, would eventually result in an increase of the corticospinal output It is unclear whether corticospinal bursts at higher and lower frequencies are produced by independent oscillatory intracortical circuits with different characteristics, as suggested by modeling studies[8], or rather reflect the activity of a single neuronal generator able to discharge at harmonically-related frequencies (667 and 333 Hz). In order to verify whether such putative effects only occur during tACS or, outlast the stimulation, we repeated the same recordings after tACS
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