Transcranial magnetic stimulation with a half-sine wave pulse elicits direction-specific effects in human motor cortex

Nikolai H Jung,Volker Mall,Igor Delvendahl,Norbert Gattinger,Astrid Pechmann,Hartwig R Siebner,Bernhard Gleich

doi:10.1186/1471-2202-13-139

Abstract

BackgroundTranscranial magnetic stimulation (TMS) commonly uses so-called monophasic pulses where the initial rapidly changing current flow is followed by a critically dampened return current. It has been shown that a monophasic TMS pulse preferentially excites different cortical circuits in the human motor hand area (M1-HAND), if the induced tissue current has a posterior-to-anterior (PA) or anterior-to-posterior (AP) direction. Here we tested whether similar direction-specific effects could be elicited in M1-HAND using TMS pulses with a half-sine wave configuration.ResultsIn 10 young participants, we applied half-sine pulses to the right M1-HAND which elicited PA or AP currents with respect to the orientation of the central sulcus.Measurements of the motor evoked potential (MEP) revealed that PA half-sine stimulation resulted in lower resting motor threshold (RMT) than AP stimulation. When stimulus intensity (SI) was gradually increased as percentage of maximal stimulator output, the stimulus–response curve (SRC) of MEP amplitude showed a leftward shift for PA as opposed to AP half-sine stimulation. Further, MEP latencies were approximately 1 ms shorter for PA relative to AP half-sine stimulation across the entire SI range tested. When adjusting SI to the respective RMT of PA and AP stimulation, the direction-specific differences in MEP latencies persisted, while the gain function of MEP amplitudes was comparable for PA and AP stimulation.ConclusionsUsing half-sine pulse configuration, single-pulse TMS elicits consistent direction-specific effects in M1-HAND that are similar to TMS with monophasic pulses. The longer MEP latency for AP half-sine stimulation suggests that PA and AP half-sine stimulation preferentially activates different sets of cortical neurons that are involved in the generation of different corticospinal descending volleys.

Highlights

Transcranial magnetic stimulation (TMS) commonly uses so-called monophasic pulses where the initial rapidly changing current flow is followed by a critically dampened return current
Experiment 1: Differences in resting motor threshold (RMT), motor evoked potential (MEP) amplitude and MEP latency We first assessed in this pilot experiment differences in RMT as well as differences in mean MEP amplitude and latencies for MEPs evoked at moderate suprathreshold intensity (Figure 1)
Experiment 2: SRCMSO measurements for MEP amplitude and latency We examined whether the change in mean MEP amplitude and latency with increasing stimulus intensity depended on the direction of the induced current in right M1-HAND using stimulus intensities that were adjusted to maximal stimulator output (MSO)

Summary

Introduction

Transcranial magnetic stimulation (TMS) commonly uses so-called monophasic pulses where the initial rapidly changing current flow is followed by a critically dampened return current. Transcranial magnetic stimulation (TMS) of the primary motor hand area (M1-HAND) can induce multiple descending corticospinal volleys which can be recorded from the upper spinal cord using invasive techniques [1,2]. These descending volleys are caused by either direct or indirect activation of fastconducting pyramidal tract neurons that connect monosynaptically to spinal motoneurons [3]. Since early and late I-waves are thought to be generated by different intracortical circuits, it has been concluded that different sets of intracortical neurons are excited in the motor cortex when inverting the current direction of a monophasic stimulus from PA to AP or vice versa [5,7]

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Discussion

Conclusion