Modulation of sensorimotor cortex by repetitive peripheral magnetic stimulation

Abstract

This study examines with transcranial magnetic stimulation (TMS) and with functional magnetic resonance imaging (fMRI) whether 20 min of repetitive peripheral magnetic stimulation (rPMS) has a facilitating effect on associated motor controlling regions. Trains of rPMS with a stimulus intensity of 150% of the motor threshold (MT) were applied over right hand flexor muscles of healthy volunteers. First, with TMS, 10 vs. 25 Hz rPMS was examined and compared to a control group. Single and paired pulse motor evoked potentials (MEPs) from flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles were recorded at baseline (T0), post rPMS (T1), 30 min post (T2), 1 h post (T3) and 2 h post rPMS (T4). Then, with fMRI, 25 Hz rPMS was compared to sham stimulation by utilizing a finger tapping activation paradigm. Changes in bloodoxygen level dependent (BOLD) contrast were examined at baseline (PRE), post rPMS (POST1) and 1 h post rPMS (POST2). With TMS facilitation was observed in the target muscle (FCR) following 25 Hz rPMS: MEP recruitment curves (RCs) were increased at T1, T2 and T3, and intracortical facilitation (ICF) was increased at T1 and T2. No effects were observed following 10 Hz rPMS. With fMRI the BOLD contrast at the left sensorimotor area was increased at POST1. Compared to inductions protocols based on transcutaneous electrical stimulation and mechanical stimulation, the rPMS induced effects appeared shorter lasting.

Keywords: TMS; afferent-induced facilitation; cortical plasticity; fMRI; motor cortex; peripheral magnetic stimulation.

Figure 1

Experimental procedures for the transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) assessments. TMS assessments were carried out before (T0), immediately after (T1), 0.5 h after (T2), 1 h after (T3) and 2 h after (T4) the repetitive peripheral magnetic stimulation (rPMS) treatment. fMRI assessments were carried out before (PRE), immediately after (POST1) and 1 h after (POST2) the rPMS or sham treatment.

Figure 2

MEP recruitment curves at baseline (T0), immediately after (T1), 0.5 h (T2), 1 h (T3) and 2 h (T4) after 20 min rPMS. Upper plots represent the results from the flexor carpi radialis (FCR), lower plots represent the results from the extensor carpi radialis (ECR). MEPs are normalized to the maximum mean MEP at T0 from each subject individually and then pooled for all subjects for each rPMS separately. For each stimulus intensity mean and standard error of mean (SEM) of the normalized MEP amplitude is plotted. The significant differences between T0 and the corresponding post stimulation assessment are presented (*p < 0.05; ⁺p < 0.01).

Figure 3

Normalized paired-pulse responses at baseline (T0), immediately after (T1), 0.5 h (T2), 1 h (T3) and 2 h (T4) after 20 min rPMS. Upper plots represent the results of both muscles for interstimulus intervals (ISI) of 3 ms (SICI), lower plots represent the results for ISI 13 ms (ICF). Values are normalized for each subject to their corresponding values in single pulse stimulation and plotted as mean (SEM). The significant differences between T0 and the corresponding post stimulation assessment are presented (*p < 0.05; ⁺p < 0.01).

Figure 4

Random effects between-groups analysis. STIM vs. NOSTIM group after 25 Hz rPMS at lime level POST1. Significant changes in activation pattern within the left precentral/postcentral gyrus (p = 0.01, uncorrected, p < 0.05, corrected on cluster level).