Decreased sensory cortical excitability after 1 Hz rTMS over the ipsilateral primary motor cortex

Neural correlates of approach-avoidance behavior in healthy subjects: Effects of low-frequency repetitive transcranial magnetic stimulation (rTMS) over the right dorsolateral prefrontal cortex

The dorsolateral prefrontal cortex (dlPFC) is implicated in top-down regulation of emotion, but the detailed network mechanisms require further elucidation. To investigate network-level functions of the dlPFC in emotion regulation, this study measured changes in task-based activation, resting-state and task-based functional connectivity (FC) patterns following suppression of dlPFC excitability by 1-Hz repetitive transcranial magnetic stimulation (rTMS). In a sham-controlled within-subject design, 1-Hz active or sham rTMS was applied to the right dlPFC of 19 healthy volunteers during two separate counterbalanced sessions. Following active and sham rTMS, functional magnetic resonance imaging (fMRI) was conducted in the resting state (rs-fMRI) and during approach-avoidance task responses to pictures with positive and negative emotional content (task-based fMRI). Activation and generalized psychophysiological interaction analyses were performed on task-based fMRI, and seed-based FC analysis was applied to rs-fMRI data. Task-based fMRI revealed greater and more lateralized activation in the right hemisphere during negative picture responses compared to positive picture responses. After active rTMS, greater activation was observed in the left middle prefrontal cortex compared to sham rTMS. Further, rTMS reduced response times and error rates in approach to positive pictures compared to negative pictures. Significant FC changes due to rTMS were observed predominantly in the frontoparietal network (FPN) and visual network (VN) during the task, and in the default mode network (DMN) and VN at rest. Suppression of right dlPFC activity by 1-Hz rTMS alters large-scale neural networks and modulates emotion, supporting potential applications for the treatment of mood disorders.

Facilitation of Early and Middle Latency SEP after tDCS of M1: No Evidence of Primary Somatosensory Homeostatic Plasticity

Homeostatic plasticity is a mechanism that stabilizes cortical excitability within a physiological range. Most homeostatic plasticity protocols have primed and tested the homeostatic response of the primary motor cortex (M1). This study investigated if a homeostatic response could be recorded from the primary sensory cortex (S1) after inducing homeostatic plasticity in M1. In 31 healthy participants, homeostatic plasticity was induced over M1 with a priming and testing block of transcranial direct current stimulation (tDCS) in two different sessions (anodal and cathodal). S1 excitability was assessed by early (N20, P25) and middle-latency (N33-P45) somatosensory evoked potentials (SEP) extracted from 4 electrodes (CP5, CP3, P5, P3). Baseline and post-measures (post-priming, 0-min, 10-min, and 20-min after homeostatic induction) were taken. Anodal M1 homeostatic plasticity induction significantly facilitated the N20-P25, P45 peak, and N33-P45 early SEP components up to 20-min post-induction, without any indication of a homeostatic response (i.e., reduced SEP). Cathodal homeostatic induction did not induce any significant effect on early or middle latency SEPs. M1 homeostatic plasticity induction by anodal stimulation protocol to the primary motor cortex did not induce a homeostatic response in SEPs.

Somatosensory evoked potentials amplitude is enhanced after non-invasive brain stimulation in chronic ischemic stroke: Preliminary results from a randomised control trial

OBJECTIVE

To investigate the effects of transcranial electrical and magnetic non-invasive brain stimulation (NIBS) protocols on somatosensory evoked potential (SEP) in chronic ischemic stroke.

METHODS

33 patients were randomly assigned to one of the four treatment groups of the transcranial direct current stimulation (tDCS) and/or repetitive transcranial magnetic stimulation (rTMS) protocol. SEP parameters were recorded before and after ten days of the treatment session. All the statistical analyses were carried out using SPSS version 19.

RESULTS

It was found that there is a statistically significant improvement in the N20-P22 mean amplitude after treatment sessions in all groups except the group where tDCS and rTMS groups were sham. On paired t-tests, the difference betweeen post and pre-stimulation SEP amplitudes for the real tDCS and real rTMS coupled group was 1.045 ± 0.732 (p value = 0.005). For sham tDCS+real rTMS group, 1.05 ± 0.96 (P = 0.04); for real tDCS+sham rTMS 0.543 ± 0.332 (P = 0.01) and for double sham stimulation, 0.204 ± 0.648 (P =  0.4) respectively CONCLUSION: In ischemic stroke patients, either or coupled true transcranial tDCS and rTMS was found to be safe and significantly enhanced the amplitude of cortical somatosensory potentials when combined with standard physiotherapy, in the interim analysis of an ongoing randomised controlled trial.

CLINICAL TRIAL REGISTRY OF INDIA

CTRI/2019/11/022009 SIGNIFICANCE: The results of this research indicates the importance of RCTs in developing robust improved NIBS protocols coupled to physiotherapy to enhance the sensory-motor functional recovery following ischemic stroke.

The 'Postural Rhythm' of the Ground Reaction Force during Upright Stance and Its Conversion to Body Sway-The Effect of Vision, Support Surface and Adaptation to Repeated Trials

The ground reaction force (GRF) recorded by a platform when a person stands upright lies at the interface between the neural networks controlling stance and the body sway deduced from centre of pressure (CoP) displacement. It can be decomposed into vertical (VGRF) and horizontal (HGRF) vectors. Few studies have addressed the modulation of the GRFs by the sensory conditions and their relationship with body sway. We reconsidered the features of the GRFs oscillations in healthy young subjects (n = 24) standing for 90 s, with the aim of characterising the possible effects of vision, support surface and adaptation to repeated trials, and the correspondence between HGRF and CoP time-series. We compared the frequency spectra of these variables with eyes open or closed on solid support surface (EOS, ECS) and on foam (EOF, ECF). All stance trials were repeated in a sequence of eight. Conditions were randomised across different days. The oscillations of the VGRF, HGRF and CoP differed between each other, as per the dominant frequency of their spectra (around 4 Hz, 0.8 Hz and <0.4 Hz, respectively) featuring a low-pass filter effect from VGRF to HGRF to CoP. GRF frequencies hardly changed as a function of the experimental conditions, including adaptation. CoP frequencies diminished to <0.2 Hz when vision was available on hard support surface. Amplitudes of both GRFs and CoP oscillations decreased in the order ECF > EOF > ECS ≈ EOS. Adaptation had no effect except in ECF condition. Specific rhythms of the GRFs do not transfer to the CoP frequency, whereas the magnitude of the forces acting on the ground ultimately determines body sway. The discrepancies in the time-series of the HGRF and CoP oscillations confirm that the body's oscillation mode cannot be dictated by the inverted pendulum model in any experimental conditions. The findings emphasise the robustness of the VGRF "postural rhythm" and its correspondence with the cortical theta rhythm, shed new insight on current principles of balance control and on understanding of upright stance in healthy and elderly people as well as on injury prevention and rehabilitation.

Modulation of somatosensation by transcranial magnetic stimulation over somatosensory cortex: a systematic review

Over the last three decades, transcranial magnetic stimulation (TMS) has gained popularity as a tool to modulate human somatosensation. However, the effects of different stimulation types on the multiple distinct subdomains of somatosensation (e.g., tactile perception, proprioception and pain) have not been systematically compared. This is especially notable in the case of newer theta-burst stimulation protocols now in widespread use. Here, we aimed to systematically and critically review the existing TMS literature and provide a complete picture of current knowledge regarding the role of TMS in modulating human somatosensation across stimulation protocols and somatosensory domains. Following the PRISMA guidelines, fifty-four studies were included in the current review and were compared based on their methodologies and results. Overall, findings from these studies provide evidence that different types of somatosensation can be both disrupted and enhanced by targeted stimulation of specific somatosensory areas. Some mixed results, however, were reported in the literature. We discussed possible reasons for these mixed results, methodological limitations of existing investigations, and potential avenues for future research.

The relationships between motor behavior and sensory gating in the ball rotation task

During voluntary muscle contraction, sensory information induced by electrostimulation of the nerves supplying the contracting muscle is inhibited and the amplitude of the corresponding somatosensory evoked potential (SEP) decreases. This phenomenon is called "gating." The reduction of the SEP amplitude is reportedly significantly larger when task performance is high. However, the relationship between dexterous movement skills and gating remains unclear. In this study, we investigated through a ball rotation (BR) task how dexterous movement skills affect the SEP amplitudes. Thirty healthy subjects performed the BR task comprising the rotation of two wooden balls as quickly as possible. We estimated the median number of ball rotations for each participant and classified the participants into two (fast and slow) groups based on the results. Moreover, we recorded SEPs, while the subjects performed BR tasks or rested. SEP amplitude reduction (P45) was significantly larger in the fast than in the slow group. We also observed that the P45 amplitude during the BR task was attenuated even more so in the case of the participants with better dexterous movement skills. Our results suggest that the participants with better dexterous movement skills might display stronger somatosensory information suppression because of increasing the motor cortex activity and the afferent input during the BR task.

Therapeutic benefits of noninvasive somatosensory cortex stimulation on cortical plasticity and somatosensory function: a systematic review

Optimal limb coordination requires efficient transmission of somatosensory information to the sensorimotor cortex. The primary somatosensory cortex (S1) is frequently damaged by stroke, resulting in both somatosensory and motor impairments. Noninvasive brain stimulation (NIBS) to the primary motor cortex is thought to induce neural plasticity that facilitates neurorehabilitation. Several studies have also examined if NIBS to the S1 can enhance somatosensory processing as assessed by somatosensory-evoked potentials (SEPs) and improve behavioral task performance, but it remains uncertain if NIBS can reliably modulate S1 plasticity or even whether SEPs can reflect this plasticity. This systematic review revealed that NIBS has relatively minor effects on SEPs or somatosensory task performance, but larger early SEP changes after NIBS can still predict improved performance. Similarly, decreased paired-pulse inhibition in S1 post-NIBS is associated with improved somatosensory performance. However, several studies still debate the role of inhibitory function in somatosensory performance after NIBS in terms of the direction of the change (that, disinhibition or inhibition). Altogether, early SEP and paired-pulse inhibition (particularly inter-stimulus intervals of 30-100 ms) may become useful biomarkers for somatosensory deficits, but improved NIBS protocols are required for therapeutic applications.

Functional Near-Infrared Spectroscopy as a Target Navigator for rTMS Modulation in Patients with Hemiplegia: A Randomized Control Study

Introduction

Although repetitive transcranial magnetic stimulation (rTMS) is efficacious for motor neuromodulation in stroke survivors, high interindividual variability for responsiveness remains a concern. Target probing on the skull using a proper brain-mapping technique may help overcome this challenge. This study assessed the feasibility of functional near-infrared spectroscopy (fNIRS) as a target navigator in rTMS treatment for motor facilitation in patients with stroke.

Methods

Fifty-one patients with stroke were enrolled in this randomized controlled study. The patients were assigned to three groups: fNIRS-guided rTMS treatment (fNIRS group, n = 20), motor evoked potential (MEP)-guided rTMS treatment (MEP group, n = 16), and sham (n = 15) group. Motor assessments, including Fugl–Meyer Assessment (FMA), Wolf Motor Function Test (WMFT), and muscle strength, were conducted at baseline and after the 10-session rTMS treatment.

Results

The fNIRS-guided hotspot (fNIRS-HS) was obtained for each patient, even those for whom the MEP-guided hotspot was undetectable. Both intervention groups exhibited significant improvements in muscle strength, FMA, and WMFT scores (P < 0.001) compared with the sham group. The fNIRS group achieved significantly greater improvement in elbow function (P = 0.001) than the MEP group.

Conclusion

fNIRS can be a reliable tool for hotspot navigation for motor neuromodulation in patients with stroke. With high sensitivity to cortical oxygenation changes, this navigation system achieved a superior outcome to the traditional MEP-based method in patients with stroke. fNIRS-based systems may also facilitate the integration of machine learning, thus enabling precision medicine for neuromodulation.

Trial Registration

https://clinicaltrials.gov; Unique identifier: NCT02006615.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40120-021-00300-0.

Inhibition of the occipital face area modulates the electrophysiological signals of face familiarity: A combined cTBS-EEG study

The occipital face area (OFA) is hierarchically one of the first stages of the face processing network. It has originally been thought to be involved in early, structural processing steps, but currently more and more studies challenge this view and propose that it also takes part in higher level face processing, such as identification and recognition. Here we tested whether the OFA is involved in the initial steps of recognition memory and plays a causal role in the differential processing of familiar and unfamiliar faces. We used an offline, inhibitory continuous theta-burst stimulation (cTBS) protocol over the right OFA and the vertex as control site. Electroencephalographic (EEG) recording of event-related potentials (ERPs), elicited by visually presented familiar (famous) and unfamiliar faces was performed before and after stimulation. We observed a difference in ERPs for famous and unfamiliar faces in a time-window corresponding to the N250 component. Importantly, this difference was significantly increased by cTBS of the right OFA, suggesting its causal role in the differential processing of familiar and unfamiliar faces. The enhancement occurred focally, at electrodes close to the right hemispheric cTBS site, as well as over similar occipito-temporal sites of the contralateral hemisphere. To the best of our knowledge, this is the first study showing the causal role of the rOFA in the differential processing of familiar and unfamiliar faces, using combined cTBS and EEG recording methods. These results are discussed with respect to the nature of familiar face representations, supported by an extensive, bilateral network.

Changes in beta and high-gamma power in resting-state electrocorticogram induced by repetitive transcranial magnetic stimulation of primary motor cortex in unanesthetized macaque monkeys

Repetitive transcranial magnetic stimulation (rTMS) is now widely used as a means of neuromodulation, but the details of the mechanisms by which rTMS works remain unclarified. As a step forward to unveiling the neural phenomena occurring underneath the TMS coil, we conducted an electrophysiological study using awake and unanesthetized monkeys with subdural electrocorticogram (ECoG) electrodes implanted over the primary motor cortex (MI). We evaluated the effects of low-frequency (1 Hz) and high-frequency (10 Hz) rTMS on the resting-state ECoG signals in the stimulated MI, as well as the motor evoked potentials (MEPs) in the contralateral hand. Following the 1-Hz rTMS application, the ECoG beta band power and the MEP amplitude were significantly decreased. Following the 10-Hz rTMS application, the ECoG high-gamma power and the MEP amplitude significantly increased. Given that beta and high-gamma activities in the ECoG reflect the synchronous firing and the firing frequency of cell assemblies, respectively, in local neural circuits, these results suggest that low-frequency rTMS inhibits neural activity by desynchronizing the firing activity of local circuits, whereas high-frequency rTMS facilitates neural activity by increasing the firing rate of cell assemblies in the local circuits.

The Suppressive Effect of the Motor System on the Sensory System in Patients With Tourette Syndrome

Objective: Tourette syndrome (TS) is a complicated sensorimotor disorder. Some patients with TS relieve their involuntary premonitory urges via tics. However, the effect of the motor system on the sensory system has not yet been elucidated. The purpose of the present study was to investigate changes in the excitability of the sensory cortex following repetitive transcranial magnetic stimulation (rTMS) of the motor cortex in patients with TS.

Methods: Twenty-nine patients with TS and 20 healthy, age-matched controls were enrolled in this study. All subjects were divided into four groups: patients with rTMS, patients with sham-rTMS, controls with rTMS, and controls with sham-rTMS. The clinical severity of tics was evaluated using the Yale Global Tic Severity Scale. Single somatosensory evoked potentials (SEPs) and paired SEPs were recorded by stimulating the median nerve at the wrist of all subjects. The resting motor threshold (RMT) was tested in each subject in the rTMS group. Afterwards, all four groups were administered rTMS (1 Hz, 90% RMT) or sham-rTMS for 200 s, followed by a 15-min rest. Finally, single SEPs and paired SEPs were repeated for each subject.

Results: No significant differences in RMT, the amplitudes of single SEPs, or the suppression of paired SEPs were observed between patients with TS and controls at baseline. After rTMS, a significant suppression of the peak-to-peak amplitude of the N20–P25 responses of single SEPs was observed in both controls (p = 0.049) and patients (p < 0.0001). The suppression of the N20–P25 peak-to-peak amplitude was more significant in patients than in controls (p = 0.039). A significant difference in the suppression of paired SEPs after rTMS was not observed between groups.

Conclusions: The more significant suppression of N20–P25 components of single SEPs with normal suppressed paired SEPs in patients with TS after 1-Hz rTMS of the motor cortex suggests that the suppressive effect of the motor system on the sensory system might originate from the motor-sensory cortical circuits rather than the sensory system itself.

New Developments in Non-invasive Brain Stimulation in Chronic Pain

Purpose of Review

The goal of this review is to present a summary of the recent literature of a non-invasive brain stimulation (NIBS) to alleviate pain in people with chronic pain syndromes. This article reviews the current evidence for the use of transcranial direct current (tDCS) and repetitive transcranial magnetic stimulation (rTMS) to improve outcomes in chronic pain. Finally, we introduce the reader to novel stimulation methods that may improve therapeutic outcomes in chronic pain.

Recent Findings

While tDCS is approved for treatment of fibromyalgia in Canada and the European Union, no NIBS method is currently approved for chronic pain in the United States. Increasing sample sizes in randomized clinical trials (RCTs) seems the most efficient way to increase confidence in initial promising results. Trends at funding agencies reveal increased interest and support for NIBS such as recent Requests for Application from the National Institutes of Health. NIBS in conjunction with cognitive behavioral therapy and physical therapy may enhance outcomes in chronic pain. Novel stimulation methods, such as transcranial ultrasound stimulation, await rigorous study in chronic pain.

Frequency-specific effects of low-intensity rTMS can persist for up to 2 weeks post-stimulation: A longitudinal rs-fMRI/MRS study in rats

BACKGROUND

Evidence suggests that repetitive transcranial magnetic stimulation (rTMS), a non-invasive neuromodulation technique, alters resting brain activity. Despite anecdotal evidence that rTMS effects wear off, there are no reports of longitudinal studies, even in humans, mapping the therapeutic duration of rTMS effects.

OBJECTIVE

Here, we investigated the longitudinal effects of repeated low-intensity rTMS (LI-rTMS) on healthy rodent resting-state networks (RSNs) using resting-state functional MRI (rs-fMRI) and on sensorimotor cortical neurometabolite levels using proton magnetic resonance spectroscopy (MRS).

METHODS

Sprague-Dawley rats received 10 min LI-rTMS daily for 15 days (10 Hz or 1 Hz stimulation, n = 9 per group). MRI data were acquired at baseline, after seven days and after 14 days of daily stimulation and at two more timepoints up to three weeks post-cessation of daily stimulation.

RESULTS

10 Hz stimulation increased RSN connectivity and GABA, glutamine, and glutamate levels. 1 Hz stimulation had opposite but subtler effects, resulting in decreased RSN connectivity and glutamine levels. The induced changes decreased to baseline levels within seven days following stimulation cessation in the 10 Hz group but were sustained for at least 14 days in the 1 Hz group.

CONCLUSION

Overall, our study provides evidence of long-term frequency-specific effects of LI-rTMS. Additionally, the transient connectivity changes following 10 Hz stimulation suggest that current treatment protocols involving this frequency may require ongoing "top-up" stimulation sessions to maintain therapeutic effects.

Modified ischaemic nerve block of the forearm: use for the induction of cortical plasticity in distal hand muscles

KEY POINTS

Ischaemic nerve block (INB) of the forearm rapidly reduces somatosensory input to a part of the body, which leads to the functional reorganization of the temporarily deafferented primary motor cortex (M1). We applied a novel modified INB (mINB) to the forearm, maintaining mean blood pressure, to assess cortical plasticity in the primary somatosensory cortex (S1) and the M1 regions associated with small hand muscles. S1 excitability was measured by median nerve somatosensory-evoked potentials (SEPs), while M1 excitability was evaluated by motor-evoked potentials (MEPS), using transcranial magnetic stimulation. The finding that S1 excitability increased and M1 excitability decreased after the mINB was removed reflects the differential short-term cortical plasticity of the S1 and M1 regions. These opposite effects observed for the S1 and M1 regions following the mINB may indicate a possible intra-hemispheric interaction between the S1 and M1 regions.

ABSTRACT

Ischaemic nerve block (INB) causes short-term sensory deprivation, leading to functional reorganization in the deafferented motor cortex (M1). We used a modified INB (mINB) to evaluate cortical plasticity in the somatosensory cortex (S1) and M1 region associated with small hand muscles, because INB strongly inhibits muscles distal to the tourniquet. Thirty-three healthy adults participated in different combinations of four experiments. A pneumatic tourniquet was placed just below the right elbow and inflated to induce a mINB. We recorded the median nerve somatosensory- and motor-evoked potentials (SEPs and MEPs) before, during and after mINB placement and assessed spinal cord excitability using F-wave measurements. SEPs at Erb's point (N9) were abolished during the mINB; those at cortical N20 were suppressed. After removing the mINB, N20 amplitudes increased significantly, while those at N9 did not fully recover. P14 amplitudes after tourniquet deflation immediately recovered to baseline levels. M1-MEP amplitudes decreased during the mINB, and Erb-MEPs were suppressed. After the mINB was removed, M1-MEPs remained suppressed, while Erb-MEPs fully recovered. F-waves were not affected by the intervention. Therefore, sensory, but not motor, nerve function was affected by the mINB. S1 excitability was enhanced after the mINB was removed, indicating that S1 and M1 excitability were modulated in opposing directions after deflation. These after-effects may reflect isolated effects or interactions between the S1 and M1 regions. Our findings may facilitate improved understanding of the sensorimotor modulations that occur distal to the tourniquet due to temporal deafferentation and lead to development of novel neuromodulation protocols.

Visuomotor task acquisition is reduced by priming paired associative stimulation in older adults

Transcranial magnetic stimulation may represent an effective means for improving motor function in the elderly. The aim of this study was therefore to investigate the effects of paired associative stimulation (PAS; a plasticity-inducing transcranial magnetic stimulation paradigm) on acquisition of a novel visuomotor task in young and older adults. Fourteen young (20.4 ± 0.6 years) and 13 older (69.0 ± 1.6 years) adults participated in 3 experimental sessions during which training was preceded (primed) by PAS. Within each session, the interstimulus interval used for PAS was set at either the N20 latency plus 5 ms (PAS_LTP), the N20 latency minus 10 ms (PAS_LTD), or a constant 100 ms (PAS_Control). After training, the level of motor skill was not different between PAS conditions in young subjects (all p-values > 0.2), but was reduced by both PAS_LTP (p = 0.02) and PAS_LTD (p = 0.0001) in older subjects. Consequently, priming PAS was detrimental to skill acquisition in older adults, possibly suggesting a need for interventions that are optimized for use in elderly populations.

Modulation of the rubber hand illusion by transcranial direct current stimulation over the contralateral somatosensory cortex

In the rubber hand illusion (RHI), illusory bodily ownership is induced by synchronous touch of a participant's hidden hand and a visible surrogate. This paradigm allows investigating how the brain resolves conflicting multisensory evidence during perceptual inference. Previous studies suggest that the conflict between visual and proprioceptive information preceding the RHI is solved by attenuation of the somatosensory input. To investigate whether excitability-decreasing transcranial direct current stimulation (cathodal tDCS) over the primary somatosensory cortex may enhance the RHI, thirty healthy subjects underwent RHI without (baseline) and during tDCS. Each subject received cathodal, anodal, and sham stimulation at independent sessions on three separate days. The RHI paradigm was applied at six interval distances between the real and artificial hand. Occurrence of the RHI was evaluated by a questionnaire (illusion score) and the perceived hand misplacement (relative drift). Compared to sham, neither cathodal, nor anodal tDCS induced significant changes of the illusion score. However, cathodal tDCS was associated with significantly higher illusion scores compared to anodal stimulation. The relative drift was comparable between stimulation modes. Our findings point to a differential impact of cathodal vs. anodal tDCS over the somatosensory region on RHI perception. This may indicate that an attenuation - in contrast to an enhancement - of somatosensory precision might pave the way for the integration of an artificial limb into one's body schema.

White Matter Biomarkers Associated with Motor Change in Individuals with Stroke: A Continuous Theta Burst Stimulation Study

Continuous theta burst stimulation (cTBS) is a form of noninvasive repetitive brain stimulation that, when delivered over the contralesional hemisphere, can influence the excitability of the ipsilesional hemisphere in individuals with stroke. cTBS applied prior to skilled motor practice interventions may augment motor learning; however, there is a high degree of variability in individual response to this intervention. The main objective of the present study was to assess white matter biomarkers of response to cTBS paired with skilled motor practice in individuals with chronic stroke. We tested the effects of stimulation of the contralesional hemisphere at the site of the primary motor cortex (M1c) or primary somatosensory cortex (S1c) and a third group who received sham stimulation. Within each stimulation group, individuals were categorized into responders or nonresponders based on their capacity for motor skill change. Baseline diffusion tensor imaging (DTI) indexed the underlying white matter microstructure of a previously known motor learning network, named the constrained motor connectome (CMC), as well as the corticospinal tract (CST) of lesioned and nonlesioned hemispheres. Across practice, there were no differential group effects. However, when categorized as responders vs. nonresponders using change in motor behaviour, we demonstrated a significant difference in CMC microstructural properties (as measured by fractional anisotropy (FA)) for individuals in M1c and S1c groups. There were no significant differences between responders and nonresponders in clinical baseline measures or microstructural properties (FA) in the CST. The present study identifies a white matter biomarker, which extends beyond the CST, advancing our understanding of the importance of white matter networks for motor after stroke.

Locating primary somatosensory cortex in human brain stimulation studies: systematic review and meta-analytic evidence

Transcranial magnetic stimulation (TMS) over human primary somatosensory cortex (S1), unlike over primary motor cortex (M1), does not produce an immediate, objective output. Researchers must therefore rely on one or more indirect methods to position the TMS coil over S1. The “gold standard” method of TMS coil positioning is to use individual functional and structural magnetic resonance imaging (f/sMRI) alongside a stereotactic navigation system. In the absence of these facilities, however, one common method used to locate S1 is to find the scalp location that produces twitches in a hand muscle (e.g., the first dorsal interosseus, M1-FDI) and then move the coil posteriorly to target S1. There has been no systematic assessment of whether this commonly reported method of finding the hand area of S1 is optimal. To do this, we systematically reviewed 124 TMS studies targeting the S1 hand area and 95 fMRI studies involving passive finger and hand stimulation. Ninety-six TMS studies reported the scalp location assumed to correspond to S1-hand, which was on average 1.5–2 cm posterior to the functionally defined M1-hand area. Using our own scalp measurements combined with similar data from MRI and TMS studies of M1-hand, we provide the estimated scalp locations targeted in these TMS studies of the S1-hand. We also provide a summary of reported S1 coordinates for passive finger and hand stimulation in fMRI studies. We conclude that S1-hand is more lateral to M1-hand than assumed by the majority of TMS studies.

Locating primary somatosensory cortex in human brain stimulation studies: experimental evidence

Transcranial magnetic stimulation (TMS) over human primary somatosensory cortex (S1) does not produce immediate outputs. Researchers must therefore rely on indirect methods for TMS coil positioning. The "gold standard" is to use individual functional and structural magnetic resonance imaging (MRI) data, but the majority of studies don't do this. The most common method to locate the hand area of S1 (S1-hand) is to move the coil posteriorly from the hand area of primary motor cortex (M1-hand). Yet, S1-hand is not directly posterior to M1-hand. We localized the index finger area of S1-hand (S1-index) experimentally in four ways. First, we reanalyzed functional MRI data from 20 participants who received vibrotactile stimulation to their 10 digits. Second, to assist the localization of S1-hand without MRI data, we constructed a probabilistic atlas of the central sulcus from 100 healthy adult MRIs and measured the likely scalp location of S1-index. Third, we conducted two experiments mapping the effects of TMS across the scalp on tactile discrimination performance. Fourth, we examined all available neuronavigation data from our laboratory on the scalp location of S1-index. Contrary to the prevailing method, and consistent with systematic review evidence, S1-index is close to the C3/C4 electroencephalography (EEG) electrode locations on the scalp, ~7-8 cm lateral to the vertex, and ~2 cm lateral and 0.5 cm posterior to the M1-hand scalp location. These results suggest that an immediate revision to the most commonly used heuristic to locate S1-hand is required. The results of many TMS studies of S1-hand need reassessment. NEW & NOTEWORTHY Noninvasive human brain stimulation requires indirect methods to target particular brain areas. Magnetic stimulation studies of human primary somatosensory cortex have used scalp-based heuristics to find the target, typically locating it 2 cm posterior to the motor cortex. We measured the scalp location of the hand area of primary somatosensory cortex and found that it is ~2 cm lateral to motor cortex. Our results suggest an immediate revision of the prevailing method is required.

Resting-state fMRI study of brain activation using low-intensity repetitive transcranial magnetic stimulation in rats

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive neuromodulation technique used to treat many neuropsychiatric conditions. However, the mechanisms underlying its mode of action are still unclear. This is the first rodent study using resting-state functional MRI (rs-fMRI) to examine low-intensity (LI) rTMS effects, in an effort to provide a direct means of comparison between rodent and human studies. Using anaesthetised Sprague-Dawley rats, rs-fMRI data were acquired before and after control or LI-rTMS at 1 Hz, 10 Hz, continuous theta burst stimulation (cTBS) or biomimetic high-frequency stimulation (BHFS). Independent component analysis revealed LI-rTMS-induced changes in the resting-state networks (RSN): (i) in the somatosensory cortex, the synchrony of resting activity decreased ipsilaterally following 10 Hz and bilaterally following 1 Hz stimulation and BHFS, and increased ipsilaterally following cTBS; (ii) the motor cortex showed bilateral changes following 1 Hz and 10 Hz stimulation, a contralateral decrease in synchrony following BHFS, and an ipsilateral increase following cTBS; and (iii) hippocampal synchrony decreased ipsilaterally following 10 Hz, and bilaterally following 1 Hz stimulation and BHFS. The present findings demonstrate that LI-rTMS modulates functional links within the rat RSN with frequency-specific outcomes, and the observed changes are similar to those described in humans following rTMS.

Transcranial Static Magnetic Field Stimulation over the Primary Motor Cortex Induces Plastic Changes in Cortical Nociceptive Processing

Transcranial static magnetic field stimulation (tSMS) is a novel and inexpensive, non-invasive brain stimulation (NIBS) technique. Here, we performed non-invasive modulation of intra-epidermal electrical stimulation-evoked potentials (IES-EPs) by applying tSMS or sham stimulation over the primary motor (M1) and somatosensory (S1) cortices in 18 healthy volunteers for 15 min. We recorded EPs after IES before, right after, and 10 min after tSMS. The IES-EP amplitude was significantly reduced immediately after tSMS over M1, whereas tSMS over S1 and sham stimulation did not affect the IES-EP amplitude. Thus, tSMS may affect cortical nociceptive processing. Although the results of intervention for experimental acute pain in healthy subjects cannot be directly translated into the clinical situation, tSMS may be a potentially useful NIBS method for managing chronic pain, in addition to standard of care treatments.

Probing the timing network: A continuous theta burst stimulation study of temporal categorization

Time perception in the millisecond and second ranges is thought to be processed by different neural mechanisms. However, whether there is a sharp boundary between these ranges and whether they are implemented in the same, overlapped or separate brain areas is still not certain. To probe the role of the right dorsolateral prefrontal cortex (dlPFC), the right supplementary motor area (SMA), and the cerebellum on time perception, we temporarily altered their activity on healthy volunteers on separate sessions using transcranial magnetic stimulation with the continuous Theta Burst Stimulation (cTBS) protocol. A control session was reserved for the stimulation of the primary somatosensory cortex (S1). Before and after stimulation, participants were tested on a temporal categorization task using intervals in the hundreds and thousands of milliseconds ranges, as well as on a pitch categorization task which was used as a further control. We then looked for changes in the Relative Threshold and the Constant Error, which, respectively, reflect participants' sensitivity to interval duration and their accuracy at setting an interval that acts as a boundary between categories. We found that after cTBS in all of the studied regions, the Relative Threshold, but not the Constant Error, was affected and only when hundreds of milliseconds intervals were being categorized. Categorization of thousands of milliseconds intervals and of pitch was not affected. These results suggest that the fronto-cerebellar circuit is particularly involved in the estimation of intervals in the hundreds of milliseconds range.

What makes you tic? Translational approaches to study the role of stress and contextual triggers in Tourette syndrome

Tourette syndrome (TS) is a neurodevelopmental condition characterized by multiple, recurring motor and phonic tics. Rich empirical evidence shows that the severity of tics and associated manifestations is increased by several stressors and contextual triggers; however, the neurobiological mechanisms responsible for symptom exacerbation in TS remain poorly understood. This conceptual gap partially reflects the high phenotypic variability in tics, as well as the existing difficulties in operationalizing and standardizing stress and its effects in a clinical setting. Animal models of TS may be highly informative tools to overcome some of these limitations; these experimental preparations have already provided critical insights on key aspects of TS pathophysiology, and may prove useful to identify the neurochemical alterations induced by different stressful contingencies. In particular, emerging knowledge on the role of contextual triggers in animal models of TS may inform the development of novel pharmacological interventions to reduce tic fluctuations in this disorder.

Non-invasive modulation of somatosensory evoked potentials by the application of static magnetic fields over the primary and supplementary motor cortices

This study was performed to investigate the possibility of non-invasive modulation of SEPs by the application of transcranial static magnetic field stimulation (tSMS) over the primary motor cortex (M1) and supplementary motor cortex (SMA), and to measure the strength of the NdFeB magnetic field by using a gaussmeter. An NdFeB magnet or a non-magnetic stainless steel cylinder (for sham stimulation) was settled on the scalp over M1 and SMA of 14 subjects for periods of 15 min. SEPs following right median nerve stimulation were recorded before and immediately after, 5 min after, and 10 min after tSMS from sites C3′ and F3. Amplitudes of the N33 component of SEPs at C3′ significantly decreased immediately after tSMS over M1 by up to 20%. However, tSMS over the SMA did not affect the amplitude of any of the SEP components. At a distance of 2–3 cm (rough depth of the cortex), magnetic field strength was in the range of 110–190 mT. Our results that tSMS over M1 can reduce the amplitude of SEPs are consistent with those of low-frequency repeated TMS and cathodal tDCS studies. Therefore, tSMS could be a useful tool for modulating cortical somatosensory processing.

Understanding the role of the primary somatosensory cortex: Opportunities for rehabilitation

Emerging evidence indicates impairments in somatosensory function may be a major contributor to motor dysfunction associated with neurologic injury or disorders. However, the neuroanatomical substrates underlying the connection between aberrant sensory input and ineffective motor output are still under investigation. The primary somatosensory cortex (S1) plays a critical role in processing afferent somatosensory input and contributes to the integration of sensory and motor signals necessary for skilled movement. Neuroimaging and neurostimulation approaches provide unique opportunities to non-invasively study S1 structure and function including connectivity with other cortical regions. These research techniques have begun to illuminate casual contributions of abnormal S1 activity and connectivity to motor dysfunction and poorer recovery of motor function in neurologic patient populations. This review synthesizes recent evidence illustrating the role of S1 in motor control, motor learning and functional recovery with an emphasis on how information from these investigations may be exploited to inform stroke rehabilitation to reduce motor dysfunction and improve therapeutic outcomes.

Continuous theta burst stimulation of the supplementary motor area: effect upon perception and somatosensory and motor evoked potentials

BACKGROUND

The supplementary motor area (SMA) has been implicated in many aspects of movement preparation and execution. In addition to motor roles, the SMA is responsive to somesthetic stimuli though it is unclear exactly what role the SMA plays in a somatosensory network.

OBJECTIVE/HYPOTHESIS

It is the purpose of this study to assess how continuous theta burst stimulation (cTBS) of the SMA affects both somatosensory (SEPs) and motor evoked potentials (MEPs) and if cTBS leads to alterations in tactile perception thresholds of the index fingertip.

METHODS

In experiment 1, cTBS was delivered over scalp sites FCZ (SMA stimulation) (n = 10) and CZ (control stimulation) (n = 10) in separate groups for 40 s (600 pulses) at 90% of participants' resting motor threshold. For both groups, median nerve SEPs were elicited from the right wrist at rest via electrical stimulation (0.5 ms pulse) before and at 10 min intervals post-cTBS out to 30 min (t = pre, 10, 20, and 30 min). Subjects' perceptual thresholds were assessed at similar time intervals as the SEP data using a biothesiometer (120 Hz vibration). In experiment 2 (n = 10) the effect of cTBS to SMA upon single and paired-pulse MEP amplitudes from the right first dorsal interosseous (FDI) was assessed.

RESULTS

cTBS to scalp site FCZ (SMA stimulation) reduced the frontal N30 SEP and increased tactile perceptual thresholds 30 min post-stimulation. However, parietal SEPs and MEP amplitudes from both single and paired-pulse stimulation were unaffected at all time points post-stimulation. cTBS to stimulation site CZ (control) did not result in any physiological or behavioral changes.

CONCLUSION(S)

These data demonstrate cTBS to the SMA reduces the amplitude of the N30 coincident with an increase in vibration sensation threshold but does not affect primary somatosensory or motor cortex excitability. The SMA may play a significant role in a somatosensory tactile attention network.

Cortical hemoglobin concentration changes underneath the coil after single-pulse transcranial magnetic stimulation: a near-infrared spectroscopy study

Using near-infrared spectroscopy (NIRS) and multichannel probes, we studied hemoglobin (Hb) concentration changes when single-pulse transcranial magnetic stimulation (TMS) was applied over the left hemisphere primary motor cortex (M1). Seventeen measurement probes were centered over left M1. Subjects were studied in both active and relaxed conditions, with TMS intensity set at 100%, 120%, and 140% of the active motor threshold. The magnetic coils were placed so as to induce anteromedially directed currents in the brain. Hb concentration changes were more prominent at channels over M1 and posterior to it. Importantly, Hb concentration changes at M1 after TMS differed depending on whether the target muscle was in an active or relaxed condition. In the relaxed condition, Hb concentration increased up to 3–6 s after TMS, peaking at ∼6 s, and returned to the baseline. In the active condition, a smaller increase in Hb concentrations continued up to 3–6 s after TMS (early activation), followed by a decrease in Hb concentration from 9 to 12 s after TMS (delayed deactivation). Hb concentration changes in the active condition at higher stimulus intensities were more pronounced at locations posterior to M1 than at M1. We conclude that early activation occurs when M1 is activated transsynaptically. The relatively late deactivation may result from the prolonged inhibition of the cerebral cortex after activation. The posterior-dominant activation at higher intensities in the active condition may result from an additional activation of the sensory cortex due to afferent inputs from muscle contraction evoked by the TMS.

Modifying somatosensory processing with non-invasive brain stimulation

Purposeful manipulation of cortical plasticity and excitability within somatosensory regions may have therapeutic potential. Non-invasive brain stimulation (NBS) techniques such as transcranial magnetic stimulation (TMS) or transcranial direct current stimulation (tDCS) have shown promise towards this end with certain NBS protocols augmenting somatosensory processing and others down-regulating it. Here, we review NBS protocols which, when applied to primary somatosensory cortex, facilitate cortical excitability and tactile acuity (i.e., high-frequency repetitive TMS (rTMS), intermittent theta burst stimulation (TBS), paired associative stimulation (PAS) N20-5 to 0, anodal tDCS), and protocols that inhibit the same (i.e., low-frequency rTMS, continuous TBS, PAS N20-20, cathodal tDCS). Other studies have targeted multisensory regions of the brain to modulate somatosensory processing. These studies in full present a wide array of strategies in which NBS can be utilized to influence somatosensory processing in a behaviorally and clinically relevant capacity.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.

[French guidelines on the use of repetitive transcranial magnetic stimulation (rTMS): safety and therapeutic indications]

During the past decade, a large amount of work on transcranial magnetic stimulation (TMS) has been performed, including the development of new paradigms of stimulation, the integration of imaging data, and the coupling of TMS techniques with electroencephalography or neuroimaging. These accumulating data being difficult to synthesize, several French scientific societies commissioned a group of experts to conduct a comprehensive review of the literature on TMS. This text contains all the consensual findings of the expert group on the mechanisms of action, safety rules and indications of TMS, including repetitive TMS (rTMS). TMS sessions have been conducted in thousands of healthy subjects or patients with various neurological or psychiatric diseases, allowing a better assessment of risks associated with this technique. The number of reported side effects is extremely low, the most serious complication being the occurrence of seizures. In most reported seizures, the stimulation parameters did not follow the previously published recommendations (Wassermann, 1998) [430] and rTMS was associated to medication that could lower the seizure threshold. Recommendations on the safe use of TMS / rTMS were recently updated (Rossi et al., 2009) [348], establishing new limits for stimulation parameters and fixing the contraindications. The recommendations we propose regarding safety are largely based on this previous report with some modifications. By contrast, the issue of therapeutic indications of rTMS has never been addressed before, the present work being the first attempt of a synthesis and expert consensus on this topic. The use of TMS/rTMS is discussed in the context of chronic pain, movement disorders, stroke, epilepsy, tinnitus and psychiatric disorders. There is already a sufficient level of evidence of published data to retain a therapeutic indication of rTMS in clinical practice (grade A) in chronic neuropathic pain, major depressive episodes, and auditory hallucinations. The number of therapeutic indications of rTMS is expected to increase in coming years, in parallel with the optimisation of stimulation parameters.

On-line effects of quadripulse transcranial magnetic stimulation (QPS) on the contralateral hemisphere studied with somatosensory evoked potentials and near infrared spectroscopy

To evaluate on-line effects of quadripulse stimulation (QPS) over the primary motor cortex (M1) on cortical areas in the contralateral hemisphere. QPS consisted of 24 bursts of transcranial magnetic stimulation (TMS) pulses with an inter-burst interval of 5 s for 2 min (for on-line effect study) or 360 bursts for 30 min (for after-effect study). Each burst consisted of four TMS pulses (i.e. QPS) separated by an interstimulus interval of 5 or 50 ms (QPS-5 or QPS-50). QPSs were delivered over the left M1. Experiment 1 [on-line effect on somatosensory evoked potential (SEP)]: Left median nerve SEPs were recorded before, during and after QPS. Experiment 2 (after effect on SEP): After-effects of QPS were evaluated by following up SEPs after the QPS sessions. Experiment 3 (on-line effect on NIRS): Near infrared spectroscopy (NIRS) was also recorded at the right hemisphere during all QPS paradigms. Both QPS-5 and QPS-50 enlarged a cortical component of the contralateral SEP during stimulation. On the other hand, concerning the after effects, QPS-5 over M1 potentiated the contralateral SEP and QPS-50 tended to depress it. In NIRS study, both QPS-5 and QPS-50 induced a significant oxy-Hb decrease (deactivation pattern) at the right hemisphere during stimulation whereas sham stimulations unaffected them. We have shown the unidirectional on-line effects evoked by QPS-5 and QPS-50 on both SEP and NIRS, and bidirectional after effects on SEP at the contralateral hemisphere. The discrepancy between on-line effect and after effect may be explained by the differences in the underlying mechanisms between them. The former may be mainly explained by pure electrophysiological property changes in the membrane or synapses. The latter may be explained by synaptic efficacy changes which need some protein syntheses at least partly. Another discrepancy shown here is the direction of on-line effects. Electrophysiological (SEP) function was potentiated by both QPSs whereas hemodynamic (NIRS) function was depressed. This may be explained by which sensory areas contribute to NIRS or SEP generation.

Transcranial direct current stimulation over the motor association cortex induces plastic changes in ipsilateral primary motor and somatosensory cortices

OBJECTIVE

This study was performed to elucidate whether transcranial direct current stimulation (tDCS) over the motor association cortex modifies the excitability of primary motor (M1) and somatosensory (S1) cortices via neuronal connectivity.

METHODS

Anodal, cathodal, and sham tDCS (1 mA) over the left motor association cortex was applied to 10 subjects for 15 min using electrodes of two sizes (9 and 18 cm(2)). Both motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs) were recorded before, immediately after, and 15 min after tDCS. Electrode positions were confirmed by overlaying them on MRI anatomical surface images of two individuals.

RESULTS

After applying anodal tDCS using the large electrode, amplitudes of MEP components significantly decreased, whereas those of early SEP components (N20 and P25) increase. Opposite effects were observed on MEPs and SEPs after cathodal tDCS. However, a small electrode did not significantly influence either MEPs or SEPs, irrespective of polarity. The small electrode covered mainly the dorsal premotor cortex (PMd) while the large electrode involved the supplementary motor area (SMA) in addition to PMd.

CONCLUSIONS

These results suggest that anodal tDCS over PMd together with SMA enhanced the inhibitory input to M1 and excitatory input to S1, and that cathodal tDCS might lead to an opposite effect.

SIGNIFICANCE

The finding that only the large electrode modulated M1 and S1 implies that activation of PMd together with SMA by tDCS can induce plastic changes in primary sensorimotor areas.

Bilateral somatosensory evoked potentials following intermittent theta-burst repetitive transcranial magnetic stimulation

Background

Intermittent theta-burst stimulation (iTBS) is a form of repetitive transcranial magnetic stimulation that may alter cortical excitability in the primary somatosensory cortex (SI). The present study investigated the effects of iTBS on subcortical and early cortical somatosensory evoked potentials (SEPs) recorded over left, iTBS stimulated SI and the right-hemisphere non-stimulated SI. SEPs were recorded before and at 5, 15, and 25 minutes following iTBS.

Results

Compared to pre-iTBS, the amplitude of cortical potential N20/P25 was significantly increased for 5 minutes from non-stimulated SI and for 15 to 25 minutes from stimulated SI. Subcortical potentials recorded bilaterally remained unaltered following iTBS.

Conclusion

We conclude that iTBS increases the cortical excitability of SI bilaterally and does not alter thalamocortical afferent input to SI. ITBS may provide one avenue to induce cortical plasticity in the somatosensory cortex.

Repetitive transcranial magnetic stimulation of dorsolateral prefrontal cortex affects performance of the wisconsin card sorting task during provision of feedback

Early functional neuroimaging studies of tasks evaluating executive processes, such as the Wisconsin card sorting task (WCST), only assessed trials in blocks that may contain a large amount of different cognitive processes. More recently, we showed using event-related fMRI that the dorsolateral prefrontal cortex (DL-PFC) significantly increased activity during feedback but not matching periods of the WCST, consistent with its proposed role in the monitoring of information in working memory. Repetitive transcranial magnetic stimulation (rTMS) is a method that allows to disrupt processing within a given cortical region and to affect task performance for which this region is significantly solicited. Here we applied rTMS to test the hypothesis that the DL-PFC stimulation influences monitoring of working memory without interfering with other executive functions. We applied rTMS to the right DL-PFC and the vertex (control site) in different time points of the WCST. When rTMS was applied to the DL-PFC specifically during the period when subjects were receiving feedback regarding their previous response, WCST performance deteriorated, while rTMS did not affect performance during matching either when maintaining set or during set-shifting. This selective impairment of the DL-PFC is consistent with its proposed role in monitoring of events in working memory.