Effect of stimulus orientation and intensity on short-interval intracortical inhibition (SICI) and facilitation (SICF): A multi-channel transcranial magnetic stimulation study.

Pantelis Lioumis,Ulf Ziemann,Risto J Ilmoniemi,Vadim V Nikulin,Victor H Souza,Sergei Tugin,Maria A Nazarova,Pavel A Novikov,Aino E Tervo,Giuseppe Lanza,Jaakko O Nieminen

doi:10.1371/journal.pone.0257554

Abstract

Besides stimulus intensities and interstimulus intervals (ISI), the electric field (E-field) orientation is known to affect both short-interval intracortical inhibition (SICI) and facilitation (SICF) in paired-pulse transcranial magnetic stimulation (TMS). However, it has yet to be established how distinct orientations of the conditioning (CS) and test stimuli (TS) affect the SICI and SICF generation. With the use of a multi-channel TMS transducer that provides electronic control of the stimulus orientation and intensity, we aimed to investigate how changes in the CS and TS orientation affect the strength of SICI and SICF. We hypothesized that the CS orientation would play a major role for SICF than for SICI, whereas the CS intensity would be more critical for SICI than for SICF. In eight healthy subjects, we tested two ISIs (1.5 and 2.7 ms), two CS and TS orientations (anteromedial (AM) and posteromedial (PM)), and four CS intensities (50, 70, 90, and 110% of the resting motor threshold (RMT)). The TS intensity was fixed at 110% RMT. The intensities were adjusted to the corresponding RMT in the AM and PM orientations. SICI and SICF were observed in all tested CS and TS orientations. SICI depended on the CS intensity in a U-shaped manner in any combination of the CS and TS orientations. With 70% and 90% RMT CS intensities, stronger PM-oriented CS induced stronger inhibition than weaker AM-oriented CS. Similar SICF was observed for any CS orientation. Neither SICI nor SICF depended on the TS orientation. We demonstrated that SICI and SICF could be elicited by the CS perpendicular to the TS, which indicates that these stimuli affected either overlapping or strongly connected neuronal populations. We concluded that SICI is primarily sensitive to the CS intensity and that CS intensity adjustment resulted in similar SICF for different CS orientations.

Highlights

Manual dexterity, along with motor learning, requires fine-tuning of neuronal activity in the motor cortex, which is mediated through discrete interactions of excitatory and inhibitory interneuronal circuits [1]
At 1.5-ms interstimulus intervals (ISI), when the test stimulus (TS) was delivered in the AM orientation, with 50% resting motor threshold (RMT) conditioning stimulus (CS) intensity, the AM-oriented CS led to 1.5 times stronger motor evoked potential (MEP) inhibition than in the PM-oriented CS
Using paired-pulse transcranial magnetic stimulation (TMS) with the intensities normalized to the RMT in different orientations, we demonstrated that the orientation-specific CS intensity, but not the electric field (E-field) orientation affects the level of short-interval intracortical inhibition (SICI)

Summary

Introduction

Along with motor learning, requires fine-tuning of neuronal activity in the motor cortex, which is mediated through discrete interactions of excitatory and inhibitory interneuronal circuits [1]. Paired-pulse transcranial magnetic stimulation (TMS) is widely used to assess the intracortical inhibitory and excitatory processes, offering a possibility to probe non-invasively the interactions of the cortical neuronal circuits in healthy and pathological conditions [4,5]. A supra-threshold CS 1.3−1.7 or 2.3−3.0 ms before the TS enhances the MEP amplitudes [10], causing short-interval ICF (SICF). These three phenomena seem to have different neuronal origins. ICF at longer ISIs is mediated by cortical neuronal circuits distinct from the circuits involved in the generation of SICI and SICF [8,14,15]

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