Intra- and Inter-Regional Priming of Ipsilateral Human Primary Motor Cortex With Continuous Theta Burst Stimulation Does Not Induce Consistent Neuroplastic Effects.

Michael Do,Linda K Byrne,Soukayna Bekkali,Melissa Kirkovski,Peter G Enticott,Charlotte B Davies

doi:10.3389/fnhum.2018.00123

Abstract

Human responses to non-invasive brain stimulation (NIBS) techniques can be highly variable. Recently, priming protocols involving a conditioning round of NIBS applied to a target brain region prior to the application of a test protocol have shown promise in inducing more reliable effects. We investigated whether intra- or inter-regional priming of the left primary motor cortex (M1) using continuous theta burst stimulation (cTBS) can induce consistent, and reliable modulation of corticospinal excitability. Twenty healthy adults (six males) underwent four cTBS protocols. For intra-regional priming, cTBS was applied twice to the left M1 (M1-M1). For inter-regional M1 priming, cTBS was applied to the ipsilateral (left) dorsal premotor cortex (dPMC-M1), and ipsilateral (left) dorsolateral prefrontal cortex (DLPFC-M1). In the control condition, sham stimulation was applied to left M1, followed by active cTBS also applied to the left M1 (sham-M1). Each round of cTBS was separated by 10 min. Neuroplastic responses were indexed using motor evoked potentials (MEPs) elicited from the left M1 hand region, and measured from the contralateral first dorsal interosseous (right hand). MEP measurements were taken before the first round of cTBS priming, then immediately, 10, 20 and 30 min after the second test round of cTBS. The primary two-way repeated measures ANOVA revealed no significant differences in MEP responses across each condition (no main effects or interaction). Intra- and inter-regional priming of the left M1 using cTBS does not induce consistent neuroplastic effects. Further work is required to identify factors which contribute to such variability in human responses to NIBS.

Highlights

Neuroplasticity refers to the brain’s capacity for neural restructuring, and underlies our capacity to acquire knowledge, retain memories, and to recover from injury and trauma (Hebb, 1949; Pascual-Leone et al, 2005)
Goldsworthy et al (2012, 2015) have shown that two rounds of continuous theta burst stimulation applied intra-regionally to the human primary motor cortex (M1) can induce strong, and persistent suppression of corticospinal excitability lasting up to 2 h
For the main analysis comparing the effect of all four protocols on motor evoked potentials (MEPs) responses, the two-way repeated measures

Summary

Introduction

Neuroplasticity refers to the brain’s capacity for neural restructuring, and underlies our capacity to acquire knowledge, retain memories, and to recover from injury and trauma (Hebb, 1949; Pascual-Leone et al, 2005). Inter- and Intra-Regional Priming of M1 human neocortex, similar LTD- and LTP-like effects can be induced using non-invasive brain stimulation (NIBS) techniques (Fitzgerald et al, 2006; Ridding and Ziemann, 2010; Chung et al, 2016). NIBS techniques can modulate corticospinal excitability, the neuroplastic effects reported in humans are often highly variable, limiting their potential utility (Ridding and Ziemann, 2010; Pell et al, 2011; Hamada et al, 2012; Goldsworthy et al, 2014). Goldsworthy et al (2012, 2015) have shown that two rounds of continuous theta burst stimulation (cTBS; a NIBS technique involving the delivery of very high frequency electromagnetic pulse bursts in the theta range) applied intra-regionally to the human primary motor cortex (M1) can induce strong, and persistent suppression of corticospinal excitability lasting up to 2 h

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