The Role of Pulse Shape in Motor Cortex Transcranial Magnetic Stimulation Using Full-Sine Stimuli

Abstract

A full-sine (biphasic) pulse waveform is most commonly used for repetitive transcranial magnetic stimulation (TMS), but little is known about how variations in duration or amplitude of distinct pulse segments influence the effectiveness of a single TMS pulse to elicit a corticomotor response. Using a novel TMS device, we systematically varied the configuration of full-sine pulses to assess the impact of configuration changes on resting motor threshold (RMT) as measure of stimulation effectiveness with single-pulse TMS of the non-dominant motor hand area (M1). In young healthy volunteers, we (i) compared monophasic, half-sine, and full-sine pulses, (ii) applied two-segment pulses consisting of two identical half-sines, and (iii) manipulated amplitude, duration, and current direction of the first or second full-sine pulse half-segments. RMT was significantly higher using half-sine or monophasic pulses compared with full-sine. Pulses combining two half-sines of identical polarity and duration were also characterized by higher RMT than full-sine stimuli resulting. For full-sine stimuli, decreasing the amplitude of the half-segment inducing posterior-anterior oriented current in M1 resulted in considerably higher RMT, whereas varying the amplitude of the half-segment inducing anterior-posterior current had a smaller effect. These findings provide direct experimental evidence that the pulse segment inducing a posterior-anterior directed current in M1 contributes most to corticospinal pathway excitation. Preferential excitation of neuronal target cells in the posterior-anterior segment or targeting of different neuronal structures by the two half-segments can explain this result. Thus, our findings help understanding the mechanisms of neural stimulation by full-sine TMS.

Short-Interval Intracortical Facilitation Improves Efficacy in nTMS Motor Mapping of Lower Extremity Muscle Representations in Patients with Supra-Tentorial Brain Tumors

“…However, brain lesions influence the patients' CSE given the presence of peritumoral edema, lesion spread into the motor cortex or CST, and frequent antiepileptic drug intake in such patients, having considerable effect on nTMS parameters and motor map location and extent [32,38]. Higher effectiveness in these patient cases may closely relate to facilitatory effects of paired-pulse stimulation combined with biphasic wave forms, which seems more energy efficient and powerful in eliciting MEPs when compared to sp-nTMS [19,[25][26][27]. Previously, application of preoperative nTMS motor mapping has shown to improve clinical outcome in patients with motor-eloquent brain lesions [12][13][14][15]; hence, the opportunity of pp-nTMS to provide reliable motor maps particularly in patients with motor function at high risk is of high clinical merit.…”

“…Paired-pulse stimulation is fundamentally different from sp-nTMS in that it makes advantage of two consecutive stimuli that exert inhibitory or facilitatory effects as determined by their respective inter-stimulus interval, order, and intensity [20][21][22][23][24]. The combination with biphasic pulse character to induce SICF drives energy efficiency and requires lower power to produce a similar effect of stimulation as with other alternatives [25][26][27]. Thus, particularly motor mapping of lE muscle representations and assessments in patients with reduced corticospinal excitability (CSE)-as frequently found related to brain tumors, edema, or treatment with certain antiepileptic drugs-may benefit from this novel alternative to conventionally used sp-nTMS.…”

Cooperation Not Competition: Bihemispheric tDCS and fMRI Show Role for Ipsilateral Hemisphere in Motor Learning

“…This was implemented using a Magstim BiStim2 with a 5 cm figure-of-eight coil positioned tangentially to the medial-sagittal plane skull at a 45° angle and with the handle pointing posteriorly and a monophasic pulse was delivered. This angle was chosen because it elicits the strongest the strongest perpendicular fields ( Janssen et al, 2015 ), which is optimal for stimulating corticospinal neurons transsynaptically via horizontal corticocortical connections ( Di Lazzaro et al, 2008 ; Delvendahl et al, 2014 ). tDCS was administered over 4 consecutive training days for the first 25 min of an ∼60 min session of sequence training.…”

TMS of primary motor cortex with a biphasic pulse activates two independent sets of excitable neurones

“…Less common are devices which produce a half sine pulse, consisting of two opposing quarter-cycle phases ( Fig. 1 ), which more closely approximates a traditional biphasic pulse in that it is symmetrical with regard to duration and amplitude of the first and second phase [ 8 , 9 ] The present experiments used a new cTMS3 device [ 10 ], which can produce quasi-rectangular pulses and allows independent control of the duration and amplitude of each phase of a traditional, symmetrical biphasic pulse ( Fig. 1 ).…”