Measuring Contralateral Silent Period Induced by Single-Pulse Transcranial Magnetic Stimulation to Investigate M1 Corticospinal Inhibition

Summary

Contralateral silent period (cSP) assessment is a promising biomarker to index cortical excitability and treatment response. We demonstrate a protocol to assess cSP intended for studying M1 corticospinal inhibition of upper and lower limbs.

Abstract

Contralateral silent period (cSP) is a period of suppression in the background electrical muscle activity captured by electromyography (EMG) after a motor evoked potential (MEP). To obtain this, an MEP is elicited by a suprathreshold transcranial magnetic stimulation (TMS) pulse delivered to the primary motor cortex (M1) of the target muscle selected, while the participant provides a standardized voluntary target muscle contraction. The cSP is a result of inhibitory mechanisms that occur after the MEP; it provides a broad temporal assessment of spinal inhibition in its initial ~50 ms, and cortical inhibition after. Researchers have tried to better understand the neurobiological mechanism behind the cSP to validate it as a potential diagnostic, surrogate, and predictive biomarker for different neuropsychiatric diseases. Therefore, this article describes a method to measure M1 cSP of lower and upper limbs, including a selection of target muscle, electrode placement, coil positioning, method of measuring voluntary contraction stimulation, intensity setup, and data analysis to obtain a representative result. It has the educational objective of giving a visual guideline in performing a feasible, reliable, and reproducible cSP protocol for lower and upper limbs and discussing practical challenges of this technique.

Introduction

The silent period (SP) is a period of electromyographic (EMG) silence that follows a motor-evoked potential (MEP) induced by transcranial magnetic stimulation (TMS) applied during sustained muscle contraction. The suprathreshold TMS pulse can either be applied to the contralateral or ipsilateral primary motor cortex (M1) of the target muscle from which the EMG activity is being recorded yielding two phenomena: contralateral silent period (cSP) and ipsilateral silent period (iSP).

Even though iSP and cSP share similar features, they may reflect slightly different components. The first is thought to reflect transcallosal inhibition and thus b....

Protocol

This protocol involves research on human subjects and is in alliance with institutional and ethical guidelines of local ethical committees and the Declaration of Helsinki. Informed consent was obtained from subjects for using their data in the study.

1. Pre-experimental procedures

1. Screening of the subject. Screen the subject for intracranial implants, epilepsy, history of seizures, and pregnancy. Use questionnaire guidelines to ensure compliance with up-to-date safety precautions³².
   1. The delivery of electromagnetic pulses with TMS is contraindicated for individuals with intracr....

Results

After following the step-by-step procedure, the delivery of a suprathreshold TMS pulse (120% of the RMT) will elicit an observable MEP in the EMG recording of the target muscle, and a subsequent period of background EMG activity suppression of approximately 150 ms to 300 ms (Figure 2). From that EMG pattern, it's possible to calculate the cSP metrics. The most reported outcomes are the duration (in the range of ms) of the relative and absolute SP. The relative SP is measured from the MEP.......

Discussion

The default SI to elicit MEP and SPs can vary according to the population. Intensities as low as 80% RMT have been shown to elicit cSP in healthy individuals³⁹, still studies on both healthy and diseased populations have used intensities as high as 150% RMT^49,50,51. Although this source of heterogeneity can be inherent to the nature of the target population, it should not be neglected as different SIs hav.......

Materials

Name	Company	Catalog Number	Comments
Alcohol pads	Medline		Preparation with 70% isopropyl alcohol
Conductive gel	Weaver and Company		Used on the electrode
Echo Pinch	JTECH medical	0902A302	Digital dynamometer.
Mega-EMG	Soterix Medical	NS006201	Digital multiple channel EMG with built in software.
MEGA-TMS coil	Soterix Medical	NS063201	8 shaped TMS coil
Mega-TMS stimulator	Soterix Medical	6990061	Single Pulse TMS
Neuro-MEP.NET	Soterix Medical		EMG software used to analyse the muscles eletrical activity.
Swim cap	Kiefer