Effect of conditioning and test stimulus intensity on cortical excitability using triad-conditioning transcranial magnetic stimulation

Shady Safwat Hassan,Carlos Trenado,Stefan Jun Groiss,Tarek Ali Rageh,Alfons Schnitzler

doi:10.1007/s00221-020-05812-z

Abstract

Cortical facilitation assessed with triad conditioning transcranial magnetic stimulation has been termed triad-conditioned facilitation (TCF). TCF has been supposed to reflect increased intracortical facilitation (ICF) at short interstimulus intervals (ISI) around 10 ms and an intrinsic rhythm of the motor cortex at longer ISI around 25 ms. To gain further insight into the pathophysiological mechanism of TCF, we systematically studied the effect of suprathreshold conditioning stimulus (CS) and test stimulus (TS) intensity on TCF. Various CS intensities and TS intensities were used in a triad-conditioning paradigm that was applied to 11 healthy subjects. ISI between pulses were studied between 5 and 200 ms. TCF at 10 ms ISI enhanced with increasing CS intensity but decreased with increasing TS intensity. The duration of facilitation was longer with higher CS intensity. However, TCF at 25 ms ISI could not be elicited with none of the CS and TS intensities addressed here. Our results are consistent with the notion of TCF at short ISI reflecting ICF. The enhanced and prolonged facilitation with increase of CS without additional isolated facilitation at longer ISI suggest a prolongation of ICF.

Highlights

Transcranial magnetic stimulation (TMS) provides a sensitive and non-invasive tool to modulate excitability of different components of neural tissues (Kobayashi and PascualLeone 2003)
The triad conditioning TMS induced Motor-evoked potentials (MEP) facilitation at 10 and 12.5 ms that was enhanced with increasing conditioning stimulus (CS) intensity but decreased with increasing test stimulus (TS) intensity, which supports the notion of triad conditioned facilitation (TCF) at short interstimulus intervals (ISI) sharing the same mechanisms as intracortical facilitation (ICF) (Kujirai 1993; Daskalakis 2004)
Reduction of TCF with higher intensity of TS might indicate less influence of conditioning stimuli. This will be in line with intensity dependent changes of ICF which have been described in paired-pulse TMS paradigms and, supports the hypothesis of TCF around 10 ms sharing the same mechanism as ICF (Kujirai 1993; Ziemann and Chapter 2003)

Summary

Introduction

Transcranial magnetic stimulation (TMS) provides a sensitive and non-invasive tool to modulate excitability of different components of neural tissues (Kobayashi and PascualLeone 2003). On the other hand, when preceded by a subthreshold CS at longer ISIs, such as 10–15 ms, MEPs are facilitated (IntraCortical Facilitation = ICF) (Kujirai 1993; Ziemann 1996; Ziemann and Chapter 2003; Hanajima 2002). Hanajima et al reported a new triad-conditioning method to study the modulation of motor cortical excitability in response to rhythmic stimulation of M1 (Hanajima (2009)). They used three monophasic TMS pulses over M1 applied at certain frequency in terms of rhythmic conditioning stimulation and found facilitation of the MEP to TS at an ISI around 10 ms and 25 ms, which was termed triad conditioned facilitation (TCF).

Objectives

Methods

Results

Discussion

Conclusion