The effect of current flow direction on motor hot spot allocation by transcranial magnetic stimulation

Caspar Stephani,Martin Sommer,Walter Paulus

doi:10.14814/phy2.12666

Abstract

The objective of this study was to investigate the significance of pulse configurations and current direction for corticospinal activation using transcranial magnetic stimulation (TMS). In 11 healthy subjects (8 female), a motor map for the motor evoked potentials (MEPs) recorded from the first dorsal interosseus (FDI), abductor digiti minimi (ADM), extensor carpi radialis, and biceps brachii (BB) muscles of the dominant side was established. Starting from a manually determined hot spot of the FDI representation, we measured MEPs at equal oriented points on an hexagonal grid, with 7 MEPs recorded at each point, using the following pulse configurations: posteriorly directed monophasic (Mo-P), anteriorly directed monophasic (Mo-A), biphasic with the more relevant second cycle oriented posteriorly (Bi-P) as well as a reversed biphasic condition (Bi-A). For each pulse configuration, a hot spot was determined and a center of gravity (CoG) was calculated. We found that the factor current direction had an effect on location of the CoG-adjusted hot spot in the cranio-caudal axis but not in the latero-medial direction with anteriorly directed pulses locating the CoG more anteriorly and vice versa. In addition, the CoG for the FDI was more laterally than the cortical representations for the abductor digiti minimi (ADM) and extensor carpi radialis (ECR) which were registered as well. The results indicate that direction of the current pulse should be taken into account for determination of the motor representation of a muscle by TMS.

Highlights

Three main types of pulse configuration – monophasic, half-sine and biphasic pulses – are used in studies applying transcranial magnetic stimulation (TMS) (Sommer et al 2006)
The percentage of maximum stimulator output (MSO) at motor evoked potentials (MEPs) thresholds and when MEPs were 1 mV amplitude at the hot spot of the first dorsal interosseus (FDI) were significantly different across current direction (F(1,7) = 12.704, P = 0.009 and F(1,7) = 40.048, P < 0.001) and pulse configurations (F(1,7) = 150.306, P < 0.001 and F(1,7) = 132.713, P < 0.001), while an interaction between them was found (F(1,7) = 39.436, P < 0.001 and F(1,7) = 43.043, P < 0.001)
Since regularly there were few or no MEPs evoked at the biceps brachii (BB)-EMG, this muscle was excluded from topographic statistical analysis

Summary

Introduction

Three main types of pulse configuration – monophasic, half-sine and biphasic pulses – are used in studies applying TMS (Sommer et al 2006). Triggering pulse configurations and current direction of single transcranial magnetic pulses influence resting and active motor threshold, latency of motor evoked potentials (MEP), cortical silent period and aftereffects of repetitive TMS (Kammer et al 2001; Orth and Rothwell 2004; Sommer et al 2006, 2013). Regarding pulse configuration, resting, and active motor threshold are both lower upon biphasic pulses compared to monophasic pulses, with half-sine pulses being of intermediate effectiveness (Kammer et al 2001; Sommer et al 2006). Biphasic pulses, being directed first posteriorly and anteriorly in the second cycle demand the overall lowest current intensity to induce an MEP.

Objectives

Methods

Results

Conclusion