Neuroplasticity and network connectivity of the motor cortex following stroke: A transcranial direct current stimulation study

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has potential for clinical utility in neurorehabilitation. However, recent evidence indicates that the responses to tDCS are highly variable. This study investigated whether electroencephalographic (EEG) measures of functional connectivity of the target network were associated with the response to ipsilesional anodal tDCS in stroke survivors. Ten chronic stroke patients attended two experimental sessions in a randomized cross‐over trial and received anodal or sham tDCS. Single‐pulse transcranial magnetic stimulation was used to quantify change in corticospinal excitability following tDCS. At the beginning of each session, functional connectivity was estimated using the debiased‐weighted phase lag index from EEG recordings at rest. Magnetic resonance imaging identified lesion location and lesion volume. Partial least squares regression identified models of connectivity which maximally accounted for variance in anodal tDCS responses. Stronger connectivity of a network with a seed approximating the stimulated ipsilesional motor cortex, and clusters of electrodes approximating the ipsilesional parietal cortex and contralesional frontotemporal cortex in the alpha band (8–13 Hz) was strongly associated with a greater increase of corticospinal excitability following anodal tDCS. This association was not observed following sham stimulation. Addition of a structural measure(s) of injury (lesion volume) provided an improved model fit for connectivity between the seed electrode and ipsilesional parietal cortex, but not the contralesional frontotemporal cortex. TDCS has potential to greatly assist stroke rehabilitation and functional connectivity appears a robust and specific biomarker of response which may assist clinical translation of this therapy.     

Focal enhancement of motor cortex excitability during motor imagery: a transcranial magnetic stimulation study

OBJECTIVES: In order to learn more about the physiology of the motor cortex during motor imagery, we evaluated the changes in excitability of two different hand muscle representations in the primary motor cortex (M1) of both hemispheres during two imagery conditions. MATERIALS AND METHODS: We applied focal transcranial magnetic stimulation (TMS) over each M1, recording motor evoked potentials (MEPs) from the contralateral abductor pollicis brevis (APB) and first dorsal interosseus (FDI) muscles during rest, imagery of contralateral thumb abduction (C-APB), and imagery of ipsilateral thumb abduction (I-APB). We obtained measures of motor threshold (MT), MEP recruitment curve (MEP-rc) and F waves. RESULTS: Motor imagery compared with rest significantly decreased the MT and increased MEPs amplitude at stimulation intensities clearly above MT in condition C-APB, but not in condition I-APB. These effects were not significantly different between right and left hemisphere. MEPs simultaneously recorded from the FDI, which was not involved in the task, did not show facilitatory effects. There were no significant changes in F wave amplitude during motor imagery compared with rest. CONCLUSIONS: Imagery of unilateral simple movements is associated with increased excitability only of a highly specific representation in the contralateral M1 and does not differ between hemispheres.     

Modulation of motor cortex excitability by paired peripheral and transcranial magnetic stimulation

Objective

Repetitive application of peripheral electrical stimuli paired with transcranial magnetic stimulation (rTMS) of M1 cortex at low frequency, known as paired associative stimulation (PAS), is an effective method to induce motor cortex plasticity in humans. Here we investigated the effects of repetitive peripheral magnetic stimulation (rPMS) combined with low frequency rTMS (‘magnetic-PAS’) on intracortical and corticospinal excitability and whether those changes were widespread or circumscribed to the cortical area controlling the stimulated muscle.

Anodal transcranial direct current stimulation of the motor cortex increases cortical voluntary activation and neural plasticity

Introduction: We examined the cumulative effect of 4 consecutive bouts of noninvasive brain stimulation on corticospinal plasticity and motor performance, and whether these responses were influenced by the brain‐derived neurotrophic factor (BDNF) polymorphism. Methods: In a randomized double‐blinded cross‐over design, changes in strength and indices of corticospinal plasticity were analyzed in 14 adults who were exposed to 4 consecutive sessions of anodal and sham transcranial direct current stimulation (tDCS). Participants also undertook a blood sample for BDNF genotyping (N = 13). Results: We observed a significant increase in isometric wrist flexor strength with transcranial magnetic stimulation revealing increased corticospinal excitability, decreased silent period duration, and increased cortical voluntary activation compared with sham tDCS. Conclusions: The results show that 4 consecutive sessions of anodal tDCS increased cortical voluntary activation manifested as an improvement in strength. Induction of corticospinal plasticity appears to be influenced by the BDNF polymorphism. Muscle Nerve 54 : 903–913, 2016     

Transient tinnitus suppression induced by repetitive transcranial magnetic stimulation and transcranial direct current stimulation

Modulation of activity in the left temporoparietal area (LTA) by 10 Hz repetitive transcranial magnetic stimulation (rTMS) results in a transient reduction of tinnitus. We aimed to replicate these results and test whether transcranial direct current stimulation (tDCS) of LTA could yield similar effect. Patients with tinnitus underwent six different types of stimulation in a random order: 10-Hz rTMS of LTA, 10-Hz rTMS of mesial parietal cortex, sham rTMS, anodal tDCS of LTA, cathodal tDCS of LTA and sham tDCS. A non-parametric analysis of variance showed a significant main effect of type of stimulation ( P  = 0.002) and post hoc tests showed that 10-Hz rTMS and anodal tDCS of LTA resulted in a significant reduction of tinnitus. These effects were short lasting. These results replicate the findings of the previous study and, in addition, show preliminary evidence that anodal tDCS of LTA induces a similar transient tinnitus reduction as high-frequency rTMS.     

Parietal transcranial direct current stimulation modulates primary motor cortex excitability

The posterior parietal cortex is part of the cortical network involved in motor learning and is structurally and functionally connected with the primary motor cortex (M1). Neuroplastic alterations of neuronal connectivity might be an important basis for learning processes. These have however not been explored for parieto‐motor connections in humans by transcranial direct current stimulation (tDCS). Exploring tDCS effects on parieto‐motor cortical connectivity might be functionally relevant, because tDCS has been shown to improve motor learning. We aimed to explore plastic alterations of parieto‐motor cortical connections by tDCS in healthy humans. We measured neuroplastic changes of corticospinal excitability via motor evoked potentials (MEP) elicited by single‐pulse transcranial magnetic stimulation (TMS) before and after tDCS over the left posterior parietal cortex (P3), and 3 cm posterior or lateral to P3, to explore the spatial specificity of the effects. Furthermore, short‐interval intracortical inhibition/intracortical facilitation (SICI/ICF) over M1, and parieto‐motor cortical connectivity were obtained before and after P3 tDCS. The results show polarity‐dependent M1 excitability alterations primarily after P3 tDCS. Single‐pulse TMS‐elicited MEPs, M1 SICI/ICF at 5 and 7 ms and 10 and 15 ms interstimulus intervals (ISIs), and parieto‐motor connectivity at 10 and 15 ms ISIs were all enhanced by anodal stimulation. Single pulse‐TMS‐elicited MEPs, and parieto‐motor connectivity at 10 and 15 ms ISIs were reduced by cathodal tDCS. The respective corticospinal excitability alterations lasted for at least 120 min after stimulation. These results show an effect of remote stimulation of parietal areas on M1 excitability. The spatial specificity of the effects and the impact on parietal cortex–motor cortex connections suggest a relevant connectivity‐driven effect.     

Psychogenic paralysis and recovery after motor cortex transcranial magnetic stimulation

Psychogenic paralysis presents a real treatment challenge. Despite psychotherapy, physiotherapy, antidepressants, acupuncture, or hypnosis, the outcome is not always satisfactory with persistent symptoms after long‐term follow‐up. We conducted a retrospective study to assess clinical features and to propose an alternative treatment based on repetitive transcranial magnetic stimulation (rTMS). Seventy patients (44 F/26 M, mean age: 24.7 ± 16.6 years) experienced paraparesis (57%), monoparesis (37%), tetraparesis (3%), or hemiparesis (3%). A precipitating event was observed in 42 patients, primarily as a psychosocial event or a physical injury. An average of 30 stimuli over the motor cortex contralateral to the corresponding paralysis was delivered at low frequency with a circular coil. The rTMS was effective in 89% of cases, with a significantly better outcome for acute rather than chronic symptoms. In conclusion, motor cortex rTMS seem to be very effective in patients with psychogenic paralysis and could be considered a useful therapeutic option. © 2010 Movement Disorder Society     

Systematic assessment of duration and intensity of anodal transcranial direct current stimulation on primary motor cortex excitability

Since the initial demonstration of linear effects of stimulation duration and intensity on the strength of after‐effects associated with transcranial direct current stimulation (tDCS), few studies have systematically assessed how varying these parameters modulates corticospinal excitability. Therefore, the objective of this study was to systematically evaluate the effects of anodal tDCS on corticospinal excitability at two stimulation intensities (1 mA, 2 mA) and durations (10 min, 20 min), and determine the value of several variables in predicting response. Two groups of 20 individuals received, in two separate sessions, 1 and 2 mA anodal tDCS (left primary motor cortex (M1)‐right supra‐orbital montage) for either 10‐ or 20‐min. Transcranial magnetic stimulation was delivered over left M1 and motor evoked potentials (MEPs) of the contralateral hand were recorded prior to tDCS and every 5 min for 20‐min post‐tDCS. The following predictive variables were evaluated: I‐wave recruitment, stimulation intensity, baseline M1 excitability and inter‐trial MEP variability. Results show that anodal tDCS failed to significantly modulate corticospinal excitability in all conditions. Furthermore, low response rates were identified across all parameter combinations. No baseline measure was significantly correlated with increases in MEP amplitude. However, a decrease in inter‐trial MEP variability was linked to response to anodal tDCS. In conclusion, the present findings are consistent with recent reports showing high levels of inter‐subject variability in the neurophysiological response to tDCS, which may partly explain inconsistent group results. Furthermore, the level of variability in the neurophysiological outcome measure, i.e. MEPs, appears to be related to response.