Inter-individual variability in cortical excitability and motor network connectivity following multiple blocks of rTMS

Comparison between accelerated and standard or sham rTMS in the treatment of depression: A systematic review

Major depressive disorder (MDD) represents a major global health challenge, with a significant proportion of patients being resistant to drug treatment (TRD). Repetitive transcranial magnetic stimulation (rTMS) has shown promise in the treatment of MDD/TRD, with a single stimulation session per day for five days per week over several weeks (the "standard" protocol). The two main paradigms used are high-frequency rTMS and intermittent theta burst stimulation (iTBS) delivered to the left dorsolateral prefrontal cortex (DLPFC). Accelerated TMS (aTMS) protocols aim to make the treatment more effective, or at least more rapidly effective, by delivering more stimulations in a shorter time, which could also facilitate the implementation of the protocols for a larger number of patients. In this systematic literature review, articles comparing in the same study an aTMS protocol to a standard or sham rTMS protocol were retained for analysis. Thus, 23 articles were retained and the analysis focused on the efficacy of aTMS protocols used for the treatment of depression (MDD/TRD) as well as on the impact of various stimulation parameters, such as stimulation pattern, intersession interval, dosage, and methods of cortical targeting. Although some studies did not report significant differences between aTMS and standard or sham protocols, others suggested potential advantages of aTMS, such as twice-daily HF-rTMS of the left DLPFC or more intensive iTBS protocols with a long interval between two sessions and personalized cortical targeting. Our results highlight the influence of the number of sessions or pulses per session (dosage), the duration of the interval between sessions, and the precision of target localization (using image-guided neuronavigation) on therapeutic efficacy. However, limitations in sample size, few independent studies replicating the same methodology, and variability in the clinical profile of treated patients, given different definitions of treatment resistance or the presence of comorbidities, hamper definitive conclusions.

50 Hz-Repetitive transcranial magnetic stimulation modulates brain connectivity in Parkinson's disease

OBJECTIVES

High-frequency repetitive transcranial magnetic stimulation (rTMS) may modulate neuronal excitability and promote the presumed "pro-kinetic" gamma frequency, while attenuating the "anti-kinetic" beta frequency. This study explores whether 50 Hz-rTMS and intermittent Theta Burst Stimulation (iTBS), of the primary motor (M1) and dorsolateral prefrontal cortex (DLPFC) enhance the gamma activity and functional connectivity within the motor circuit in Parkinson's disease (PD).

METHODS

We investigated pre- and post-rTMS interventional EEG in 62 PD patients following 50 Hz-rTMS and iTBS. Power spectral analysis, along with coherence and mutual information embedded in metrics of graph theory, was applied to assess the functional connectivity across the whole brain.

RESULTS

We found changes in the cluster coefficient and local efficiency of gamma activity in the left M1 following iTBS, and wider-spread changes within the sensorimotor circuit following 50 Hz-rTMS. We found no changes in the power spectrum or entrainment of the gamma activity in the motor cortex or beyond.

CONCLUSION

The current 50 Hz-rTMS protocols modulate functional connectivity in PD patients, but not the power spectrum. These topological changes do not translate into clinical effects. These stimulation protocols may lack the specificity to be clinically effective.

SIGNIFICANCE

High frequency rTMS provides new insights in brain connectivity in the gamma bandwidth.

Effects of repetitive transcranial magnetic stimulation combined with cognitive training for improving response inhibition: A proof-of-concept, single-blind randomised controlled study

BACKGROUND

Impaired response inhibition is a common characteristic of various psychiatric disorders. Cognitive training (CT) can improve cognitive function, but the benefits may be limited. Repetitive transcranial magnetic stimulation (rTMS) is a promising tool to enhance neuroplasticity, and thereby augment the effects of CT. We aimed to investigate the augmentation effects of rTMS on CT for response inhibition in healthy participants.

METHODS

Sixty healthy participants were randomly assigned to two experimental groups: one with prolonged intermittent theta burst stimulation (iTBS) + CT and the other with sham iTBS + CT over four experimental sessions. Prolonged iTBS (1800 pulses) was used to stimulate the right inferior frontal cortex (rIFC) and pre-supplementary motor area (pre-SMA) in a counterbalanced order. Participants completed a Stop Signal training task following iTBS over one brain region, followed by the Go/No-Go training task after iTBS over the other brain region. The Stroop task with concomitant electroencephalography was conducted before and immediately after the intervention.

RESULTS

There were no significant differences between groups in behavioural outcomes on the Stop Signal task, Go/No-Go task, Stroop task or Behavior Rating Inventory of Executive Functioning for Adults. Similarly, analysis of event-related potentials (ERPs) from the Stroop task (N200 and N400) and exploratory cluster-based permutation analysis did not reveal any significant differences between groups. Subgroup analyses revealed that individuals with higher baseline impulsivity exhibited better learning effects in the active group.

CONCLUSIONS

This first proof of concept study did not find evidence that four sessions of active rTMS + CT could induce cognitive or neurophysiological effects on response inhibition in healthy participants. However, subgroup analyses suggests that rTMS combined with CT could be useful in improving response inhibition in individuals with high impulsivity. It is recommended that future proof of concept studies examine its potential in this clinical population.

A Novel Real-time Phase Prediction Network in EEG Rhythm

Closed-loop neuromodulation, especially using the phase of the electroencephalography (EEG) rhythm to assess the real-time brain state and optimize the brain stimulation process, is becoming a hot research topic. Because the EEG signal is non-stationary, the commonly used EEG phase-based prediction methods have large variances, which may reduce the accuracy of the phase prediction. In this study, we proposed a machine learning-based EEG phase prediction network, which we call EEG phase prediction network (EPN), to capture the overall rhythm distribution pattern of subjects and map the instantaneous phase directly from the narrow-band EEG data. We verified the performance of EPN on pre-recorded data, simulated EEG data, and a real-time experiment. Compared with widely used state-of-the-art models (optimized multi-layer filter architecture, auto-regress, and educated temporal prediction), EPN achieved the lowest variance and the greatest accuracy. Thus, the EPN model will provide broader applications for EEG phase-based closed-loop neuromodulation.

Neuromodulation and meditation: A review and synthesis toward promoting well-being and understanding consciousness and brain

The neuroscience of meditation is providing insight into meditation's beneficial effects on well-being and informing understanding of consciousness. However, further research is needed to explicate mechanisms linking brain activity and meditation. Non-invasive brain stimulation (NIBS) presents a promising approach for causally investigating neural mechanisms of meditation. Prior NIBS-meditation research has predominantly targeted frontal and parietal cortices suggesting that it might be possible to boost the behavioral and neural effects of meditation with NIBS. Moreover, NIBS has revealed distinct neural signatures in long-term meditators. Nonetheless, methodological variations in NIBS-meditation research contributes to challenges for definitive interpretation of previous results. Future NIBS studies should further investigate core substrates of meditation, including specific brain networks and oscillations, and causal neural mechanisms of advanced meditation. Overall, NIBS-meditation research holds promise for enhancing meditation-based interventions in support of well-being and resilience in both non-clinical and clinical populations, and for uncovering the brain-mind mechanisms of meditation and consciousness.

Randomised controlled trial comparing different intersession intervals of intermittent theta burst delivered to the dorsal medial prefrontal cortex

BACKGROUND

Intermittent theta burst stimulation (iTBS) is a form of repetitive transcranial magnetic stimulation (rTMS) that can be administered in a fraction of the time of standard rTMS. Applying multiple daily iTBS sessions (ie, accelerated iTBS) may enable patients to achieve remission more rapidly. However, questions remain regarding the optimal time interval between treatment sessions.

OBJECTIVE

The overall aim of this study was to compare the efficacy and tolerability of two accelerated bilateral iTBS protocols (ie, 30-min or 60-min intervals) and a once-daily bilateral iTBS protocol (ie, 0-min interval) while the number of pulses was held constant, in patients with treatment-resistant depression (TRD).

METHODS

182 patients with TRD were randomised to receive two sessions per day of bilateral iTBS of the dorsomedial prefrontal cortex (DMPFC) at 60-min, 30-min or 0-min intervals. Sham treatments were delivered using a shielded 'sham coil' which produced the auditory and tactile sensations of stimulation. The primary outcome measure was a change in depression scores on the 17-item Hamilton Rating Scale for Depression (HRSD-17) after 20 days of treatment.

RESULTS

HRSD-17 scores improved across all groups; however, these improvements did not significantly differ between the three groups after 20 days of treatment. Similarly, response and remission rates did not differ between the treatment groups.

CONCLUSIONS

These results suggest that contrary to our original hypothesis, implementing a 30-min or 60-min interval between two treatment sessions of DMPFC-iTBS does not lead to a more rapid improvement in symptoms, than once-daily iTBS administration.

TRIAL REGISTRATION NUMBER

NCT02778035.

Repeated spaced cortical paired associative stimulation promotes additive plasticity in the human parietal-motor circuit

OBJECTIVE

Repeated spaced sessions of repetitive transcranial magnetic stimulation (TMS) to the human primary motor cortex can lead to dose-dependent increases in motor cortical excitability. However, this has yet to be demonstrated in a defined cortical circuit. We aimed to examine the effects of repeated spaced cortical paired associative stimulation (cPAS) on excitability in the motor cortex.

METHODS

cPAS was delivered to the primary motor cortex (M1) and posterior parietal cortex (PPC) with two coils. In the multi-dose condition, three sessions of cPAS were delivered 50-min apart. The single-dose condition had one session of cPAS, followed by two sessions of a control cPAS protocol. Motor-evoked potentials were evaluated before and up to 40 min after each cPAS session as a measure of cortical excitability.

RESULTS

Compared to a single dose of cPAS, motor cortical excitability significantly increased after multi-dose cPAS. Increasing the number of cPAS sessions resulted in a cumulative, dose-dependent effect on excitability in the motor cortex, with each successive cPAS session leading to notable increases in potentiation.

CONCLUSION

Repeated spaced cPAS sessions summate to increase motor cortical excitability induced by single cPAS.

SIGNIFICANCE

Repeated spaced cPAS could potentially restore abilities lost due to disorders like stroke.

Immediate Effects of Preconditioning Intermittent Theta Burst Stimulation on Lower Extremity Motor Cortex Excitability in Healthy Participants

BACKGROUND

Previous studies have found that inhibitory priming with continuous theta burst stimulation (cTBS) can enhance the effect of subsequent excitatory conditioning stimuli with intermittent theta burst stimulation (iTBS) in the upper limbs. However, whether this combined stimulation approach elicits a comparable compensatory response in the lower extremities remains unclear. This study aimed to investigate how cTBS preconditioning modulated the effect of iTBS on motor cortex excitability related to the lower limb in healthy individuals.

METHODS

Using a randomised cross-over design, a total of 25 healthy participants (19 females, mean age = 24.80 yr) were recruited to undergo three different TBS protocols (cTBS + iTBS, sham cTBS + iTBS, sham cTBS + sham iTBS) in a random order. Each TBS intervention was administered with one-week intervals. cTBS and iTBS were administered at an intensity of 80% active motor threshold (AMT) delivering a total of 600 pulses. Before intervention (T0), immediately following intervention (T1), and 20 min after intervention (T2), the corticomotor excitability was measured for the tibialis anterior muscle of participants' non-dominant leg using a Magneuro100 stimulator and matched double-cone coil. The average amplitude of the motor-evoked potential (MEP) induced by applying 20 consecutive monopulse stimuli at an intensity of 130% resting motor threshold (RMT) was collected and analysed.

RESULTS

Compare with T0 time, the MEP amplitude (raw and normalised) at T1 and T2 showed a statistically significant increase following the cTBS + iTBS protocol (p < 0.01), but no significant differences were observed in amplitude changes following other protocols (sham cTBS + iTBS and sham cTBS + sham iTBS) (p > 0.05). Furthermore, no statistically significant difference was found among the three protocols at any given time point (p > 0.05).

CONCLUSIONS

Preconditioning the lower extremity motor cortex with cTBS prior to iTBS intervention can promptly enhance its excitability in healthy participants. This effect persists for a minimum duration of 20 min.

CLINICAL TRIAL REGISTRATION

No: ChiCTR2300069315. Registered 13 March, 2023, https://www.chictr.org.cn.

Theta-burst direct electrical stimulation remodels human brain networks

Theta-burst stimulation (TBS), a patterned brain stimulation technique that mimics rhythmic bursts of 3-8 Hz endogenous brain rhythms, has emerged as a promising therapeutic approach for treating a wide range of brain disorders, though the neural mechanism of TBS action remains poorly understood. We investigated the neural effects of TBS using intracranial EEG (iEEG) in 10 pre-surgical epilepsy participants undergoing intracranial monitoring. Here we show that individual bursts of direct electrical TBS at 29 frontal and temporal sites evoked strong neural responses spanning broad cortical regions. These responses exhibited dynamic local field potential voltage changes over the course of stimulation presentations, including either increasing or decreasing responses, suggestive of short-term plasticity. Stronger stimulation augmented the mean TBS response amplitude and spread with more recording sites demonstrating short-term plasticity. TBS responses were stimulation site-specific with stronger TBS responses observed in regions with strong baseline stimulation effective (cortico-cortical evoked potentials) and functional (low frequency phase locking) connectivity. Further, we could use these measures to predict stable and varying (e.g. short-term plasticity) TBS response locations. Future work may integrate pre-treatment connectivity alongside other biophysical factors to personalize stimulation parameters, thereby optimizing induction of neuroplasticity within disease-relevant brain networks.

Applications of Functional Near-Infrared Spectroscopy (fNIRS) Neuroimaging During Rehabilitation Following Stroke: A Review

Stroke is a cerebrovascular disease that impairs blood supply to localized brain tissue regions due to various causes. This leads to ischemic and hypoxic lesions, necrosis of the brain tissue, and a variety of functional disorders. Abnormal cortical activation and functional connectivity occur in the brain after a stroke, but the activation patterns and functional reorganization are not well understood. Rehabilitation interventions can enhance functional recovery in stroke patients. However, clinicians require objective measures to support their practice, as outcome measures for functional recovery are based on scale scores. Furthermore, the most effective rehabilitation measures for treating patients are yet to be investigated. Functional near-infrared spectroscopy (fNIRS) is a non-invasive neuroimaging method that detects changes in cerebral hemodynamics during task performance. It is widely used in neurological research and clinical practice due to its safety, portability, high motion tolerance, and low cost. This paper briefly introduces the imaging principle and the advantages and disadvantages of fNIRS to summarize the application of fNIRS in post-stroke rehabilitation.

Assessing the synergistic effectiveness of intermittent theta burst stimulation and the vestibular ocular reflex rehabilitation protocol in the treatment of Mal de Debarquement Syndrome: a randomised controlled trial

INTRODUCTION

Mal de Debarquement Syndrome (MdDS) is a rare central vestibular disorder characterised by a constant sensation of motion (rocking, swaying, bobbing), which typically arises after motion experiences (e.g. sea, air, and road travel), though can be triggered by non-motion events. The current standard of care is non-specific medications and interventions that only result in mild-to-moderate improvements. The vestibular ocular reflex (VOR) rehabilitation protocol, a specialised form of rehabilitation, has shown promising results in reducing symptoms amongst people with MdDS. Accumulating evidence suggests that it may be possible to augment the effects of VOR rehabilitation via non-invasive brain stimulation protocols, such as theta burst stimulation (TBS).

METHODS

The aim of this randomised controlled trial was to evaluate the effectiveness of intermittent TBS (iTBS) over the dorsolateral prefrontal cortex in enhancing the effectiveness of a subsequently delivered VOR rehabilitation protocol in people with MdDS. Participants were allocated randomly to receive either Sham (n = 10) or Active (n = 10) iTBS, followed by the VOR rehabilitation protocol. Subjective outcome measures (symptom ratings and mental health scores) were collected 1 week pre-treatment and for 16 weeks post-treatment. Posturography (objective outcome) was recorded each day of the treatment week.

RESULTS

Significant improvements in subjective and objective outcomes were reported across both treatment groups over time, but no between-group differences were observed.

DISCUSSION

These findings support the effectiveness of the VOR rehabilitation protocol in reducing MdDS symptoms. Further research into iTBS is required to elucidate whether the treatment has a role in the management of MdDS. TRN: ACTRN12619001519145 (Date registered: 04 November 2019).

Driving innovation in addiction treatment: role of transcranial magnetic stimulation

Addictions comprises heterogenous psychiatric conditions caused by the complex interaction of genetic, neurobiological, psychological, and environmental factors with a chronic relapsing-remitting pattern. Despite the long-standing efforts of preclinical and clinical research studies, addiction field has seen relatively slow progress when it comes to the development of new therapeutic interventions, most of which failed to demonstrate a significant efficacy. This is likely due to the very complex interplay of many factors that contribute to both the development and expression of addictions. The imbalance between the salience and the reward brain network circuitry has been proposed as the neurobiological mechanisms explaining the pathognomonic symptoms of addictions.Non-invasive neuromodulation techniques have been proposed as a promising therapeutic intervention to restore these brain circuits dysfunctions. Here, we propose a multi-level strategy to innovate the diagnosis and the treatment of addictive disorders.

Explainable Deep-Learning Prediction for Brain-Computer Interfaces Supported Lower Extremity Motor Gains Based on Multistate Fusion

Predicting the potential for recovery of motor function in stroke patients who undergo specific rehabilitation treatments is an important and major challenge. Recently, electroencephalography (EEG) has shown potential in helping to determine the relationship between cortical neural activity and motor recovery. EEG recorded in different states could more accurately predict motor recovery than single-state recordings. Here, we design a multi-state (combining eyes closed, EC, and eyes open, EO) fusion neural network for predicting the motor recovery of patients with stroke after EEG-brain-computer-interface (BCI) rehabilitation training and use an explainable deep learning method to identify the most important features of EEG power spectral density and functional connectivity contributing to prediction. The prediction accuracy of the multi-states fusion network was 82%, significantly improved compared with a single-state model. The neural network explanation result demonstrated the important region and frequency oscillation bands. Specifically, in those two states, power spectral density and functional connectivity were shown as the regions and bands related to motor recovery in frontal, central, and occipital. Moreover, the motor recovery relation in bands, the power spectrum density shows the bands at delta and alpha bands. The functional connectivity shows the delta, theta, and alpha bands in the EC state; delta, theta, and beta mid at the EO state are related to motor recovery. Multi-state fusion neural networks, which combine multiple states of EEG signals into a single network, can increase the accuracy of predicting motor recovery after BCI training, and reveal the underlying mechanisms of motor recovery in brain activity.

Bayesian Optimization of Neurostimulation (BOONStim)

BACKGROUND

Transcranial magnetic stimulation (TMS) treatment response is influenced by individual variability in brain structure and function. Sophisticated, user-friendly approaches, incorporating both established functional magnetic resonance imaging (fMRI) and TMS simulation tools, to identify TMS targets are needed.

OBJECTIVE

The current study presents the development and validation of the Bayesian Optimization of Neuro-Stimulation (BOONStim) pipeline.

METHODS

BOONStim uses Bayesian optimization for individualized TMS targeting, automating interoperability between surface-based fMRI analytic tools and TMS electric field modeling. Bayesian optimization performance was evaluated in a sample dataset (N=10) using standard circular and functional connectivity-defined targets, and compared to grid optimization.

RESULTS

Bayesian optimization converged to similar levels of total electric field stimulation across targets in under 30 iterations, converging within a 5% error of the maxima detected by grid optimization, and requiring less time.

CONCLUSIONS

BOONStim is a scalable and configurable user-friendly pipeline for individualized TMS targeting with quick turnaround.

Intermittent theta burst stimulation and functional connectivity in people living with HIV/AIDS who smoke tobacco cigarettes: a preliminary pilot study

Background

People living with HIV (PLWHA) smoke at three times the rate of the general population and respond poorly to cessation strategies. Previous studies examined repetitive transcranial magnetic stimulation (rTMS) over left dorsolateral prefrontal cortex (L. dlPFC) to reduce craving, but no studies have explored rTMS among PLWHA who smoke. The current pilot study compared the effects of active and sham intermittent theta-burst stimulation (iTBS) on resting state functional connectivity (rsFC), cigarette cue attentional bias, and cigarette craving in PLWHA who smoke.

Methods

Eight PLWHA were recruited (single-blind, within-subject design) to receive one session of iTBS (n=8) over the L. dlPFC using neuronavigation and, four weeks later, sham iTBS (n=5). Cigarette craving and attentional bias assessments were completed before and after both iTBS and sham iTBS. rsFC was assessed before iTBS (baseline) and after iTBS and sham iTBS.

Results

Compared to sham iTBS, iTBS enhanced rsFC between the L. dlPFC and bilateral medial prefrontal cortex and pons. iTBS also enhanced rsFC between the right insula and right occipital cortex compared to sham iTBS. iTBS also decreased cigarette craving and cigarette cue attentional bias.

Conclusion

iTBS could potentially offer a therapeutic option for smoking cessation in PLWHA.

Predicting response to repetitive transcranial magnetic stimulation in patients with chronic insomnia disorder using electroencephalography: A pilot study

Predicting responsvienss to repetitive transcranial magnetic stimulation (rTMS) can facilitate personalized treatments with improved efficacy; however, predictive features related to this response are still lacking. We explored whether resting-state electroencephalography (rsEEG) functional connectivity measured at baseline or during treatment could predict the response to 10-day rTMS targeted to the right dorsolateral prefrontal cortex (DLPFC) in 36 patients with chronic insomnia disorder (CID). Pre- and post-treatment rsEEG scans and the Pittsburgh Sleep Quality Index (PSQI) were evaluated, with an additional rsEEG scan conducted after four rTMS sessions. Machine-learning approaches were employed to assess the ability of each connectivity measure to distinguish between responders (PSQI improvement > 25%) and non-responders (PSQI improvement ≤ 25%). Furthermore, we analyzed the connectivity trends of the two subgroups throughout the treatment. Our results revealed that the machine learning model based on baseline theta connectivity achieved the highest accuracy (AUC = 0.843) in predicting treatment response. Decreased baseline connectivity at the stimulated site was associated with higher responsiveness to TMS, emphasizing the significance of functional connectivity characteristics in rTMS treatment. These findings enhance the clinical application of EEG functional connectivity markers in predicting treatment outcomes.

Effects of prefrontal and parietal neuromodulation on magnitude processing and integration

Numerical cognition is an essential skill for survival, which includes the processing of discrete and continuous quantities, involving a mainly right fronto-parietal network. However, the neurocognitive systems underlying the processing and integration of discrete and continuous quantities are currently under debate. Noninvasive brain stimulation techniques have been used in the study of the neural basis of numerical cognition with a spatial, temporal and functional resolution superior to other neuroimaging techniques. The present randomized sham-controlled single-blinded trial addresses the involvement of the right dorsolateral prefrontal cortex and the right intraparietal sulcus in magnitude processing and integration. Multifocal anodal transcranial direct current stimulation was applied online during the execution of magnitude comparison tasks in three conditions: right prefrontal, right parietal and sham stimulation. The results show that prefrontal stimulation produced a moderated decrease in response times in all magnitude processing and integration tasks compared to sham condition. While parietal stimulation had no significant effect on any of the tasks. The effect found is interpreted as a generalized improvement in processing speed and magnitude integration due to right prefrontal neuromodulation, which may be attributable to domain-general or domain-specific factors.

A pilot study to examine the association between COX-2 rs5275 polymorphism and the response to repetitive transcranial stimulation in schizophrenia

High frequency (HF)-rTMS has been shown to improve cognitive functions in patients with schizophrenia (SCZ). This study aimed to investigate whether COX-2 rs5275 variants were associated with cognitive improvements following rTMS treatment in patients with SCZ. Forty-eight hospitalized patients with SCZ were assigned to the neuronavigation HF-rTMS group and 28 patients to the sham group over left DLPFC for 1 month. Cognitive function was evaluated using the repeatable battery for the assessment of neuropsychological status (RBANS) at weeks 0 and 4. COX-2 rs5275 polymorphism was genotyped by a technician. At baseline, C allele carriers showed better cognitive performance relative to patients with TT homozygote. Additionally, C allele carriers had greater improvement in memory from the follow-up to baseline following rTMS stimulation, while patients with the TT genotype showed no significant improvement in memory index. More importantly, we found that COX-2 rs5275 was correlated with the response to rTMS after controlling for the covariates. This study data indicate that COX-2 rs5275 was associated with improvements in immediate memory after HF-rTMS treatment in patients with SCZ. rTMS shows an effect on memory only in C allele carriers, but not in those with the TT genotype.

TMS over the pre-SMA enhances semantic cognition via remote network effects on task-based activity and connectivity

BACKGROUND

The continuous decline of executive abilities with age is mirrored by increased neural activity of domain-general networks during task processing. So far, it remains unclear how much domain-general networks contribute to domain-specific processes such as language when cognitive demands increase. The current neuroimaging study explored the potential of intermittent theta-burst stimulation (iTBS) over a domain-general hub to enhance executive and semantic processing in healthy middle-aged to older adults.

METHODS

We implemented a cross-over within-subject study design with three task-based neuroimaging sessions per participant. Using an individualized stimulation approach, each participant received once effective and once sham iTBS over the pre-supplementary motor area (pre-SMA), a region of domain-general control. Subsequently, task-specific stimulation effects were assessed in functional MRI using a semantic and a non-verbal executive task with varying cognitive demand.

RESULTS

Effective stimulation increased activity only during semantic processing in visual and dorsal attention networks. Further, iTBS induced increased seed-based connectivity in task-specific networks for semantic and executive conditions with high cognitive load but overall reduced whole-brain coupling between domain-general networks. Notably, stimulation-induced changes in activity and connectivity related differently to behavior: While stronger activity of the parietal dorsal attention network was linked to poorer semantic performance, its enhanced coupling with the pre-SMA was associated with more efficient semantic processing.

CONCLUSIONS

iTBS modulates networks in a task-dependent manner and generates effects at regions remote to the stimulation site. These neural changes are linked to more efficient semantic processing, which underlines the general potential of network stimulation approaches in cognitive aging.

Train duration and inter-train interval determine the direction and intensity of high-frequency rTMS after-effects

Background and objective

It has been proved that repetitive transcranial magnetic stimulation (rTMS) triggers the modulation of homeostatic metaplasticity, which causes the effect of rTMS to disappear or even reverse, and a certain length of interval between rTMS trains might break the modulation of homeostatic metaplasticity. However, it remains unknown whether the effects of high-frequency rTMS can be modulated by homeostatic metaplasticity by lengthening the train duration and whether homeostatic metaplasticity can be broken by prolonging the inter-train interval.

Methods

In this study, 15 subjects participated in two experiments including different rTMS protocols targeting the motor cortex. In the first experiment, high-frequency rTMS protocols with different train durations (2 s and 5 s) and an inter-train interval of 25 s were adopted. In the second experiment, high-frequency rTMS protocols with a train duration of 5 s and different inter-train intervals (50 s and 100 s) were adopted. A sham protocol was also included. Changes of motor evoked potential amplitude acquired from electromyography, power spectral density, and intra-region and inter-region functional connectivity acquired from electroencephalography in the resting state before and after each rTMS protocol were evaluated.

Results

High-frequency rTMS with 2 s train duration and 25 s inter-train interval increased cortex excitability and the power spectral density of bilateral central regions in the alpha frequency band and enhanced the functional connectivity between central regions and other brain regions. When the train duration was prolonged to 5 s, the after-effects of high-frequency rTMS disappeared. The after-effects of rTMS with 5 s train duration and 100 s inter-train interval were the same as those of rTMS with 2 s train duration and 25 s inter-train interval.

Conclusion

Our results indicated that train duration and inter-train interval could induce the homeostatic metaplasticiy and determine the direction of intensity of rTMS after-effects, and should certainly be taken into account when performing rTMS in both research and clinical practice.

Neuromodulatory effects and reproducibility of the most widely used repetitive transcranial magnetic stimulation protocols

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is widely used in both research and clinical settings to modulate human brain function and behavior through the engagement of the mechanisms of plasticity. Based upon experiments using single-pulse TMS as a probe, the physiologic mechanism of these effects is often assumed to be via changes in cortical excitability, with 10 Hz rTMS increasing and 1 Hz rTMS decreasing the excitability of the stimulated region. However, the reliability and reproducibility of these rTMS protocols on cortical excitability across and within individual subjects, particularly in comparison to robust sham stimulation, have not been systematically examined.

OBJECTIVES

In a cohort of 28 subjects (39 ± 16 years), we report the first comprehensive study to (1) assess the neuromodulatory effects of traditional 1 Hz and 10 Hz rTMS on corticospinal excitability against both a robust sham control, and two other widely used patterned rTMS protocols (intermittent theta burst stimulation, iTBS; and continuous theta burst stimulation, cTBS), and (2) determine the reproducibility of all rTMS protocols across identical repeat sessions.

RESULTS

At the group level, neither 1 Hz nor 10 Hz rTMS significantly modulated corticospinal excitability. 1 Hz and 10 Hz rTMS were also not significantly different from sham and both TBS protocols. Reproducibility was poor for all rTMS protocols except for sham. Importantly, none of the real rTMS and TBS protocols demonstrated greater neuromodulatory effects or reproducibility after controlling for potential experimental factors including baseline corticospinal excitability, TMS coil deviation and the number of individual MEP trials.

CONCLUSIONS

These results call into question the effectiveness and reproducibility of widely used rTMS techniques for modulating corticospinal excitability, and suggest the need for a fundamental rethinking regarding the potential mechanisms by which rTMS affects brain function and behavior in humans.

Accelerated Repetitive Transcranial Magnetic Stimulation to Treat Major Depression: The Past, Present, and Future

Repetitive transcranial magnetic stimulation (rTMS) is an effective and evidence-based therapy for treatment-resistant major depressive disorder. A conventional course of rTMS applies 20-30 daily sessions over 4-6 weeks. The schedule of rTMS delivery can be accelerated by applying multiple stimulation sessions per day, which reduces the duration of a treatment course with a predefined number of sessions. Accelerated rTMS reduces time demands, improves clinical efficiency, and potentially induces faster onset of antidepressant effects. However, considerable heterogeneity exists across study designs. Stimulation protocols vary in parameters such as the stimulation target, frequency, intensity, number of pulses applied per session or over a course of treatment, and duration of intersession intervals. In this article, clinician-researchers and neuroscientists who have extensive research experience in accelerated rTMS synthesize a consensus based on two decades of investigation and development, from early studies ("Past") to contemporaneous theta burst stimulation, a time-efficient form of rTMS gaining acceptance in clinical settings ("Present"). We propose descriptive nomenclature for accelerated rTMS, recommend avenues to optimize therapeutic and efficiency potential, and suggest using neuroimaging and electrophysiological biomarkers to individualize treatment protocols ("Future"). Overall, empirical studies show that accelerated rTMS protocols are well tolerated and not associated with serious adverse effects. Importantly, the antidepressant efficacy of accelerated rTMS appears comparable to conventional, once daily rTMS protocols. Whether accelerated rTMS induces antidepressant effects more quickly remains uncertain. On present evidence, treatment protocols incorporating high pulse dose and multiple treatments per day show promise and improved efficacy.

Network-specific metabolic and haemodynamic effects elicited by non-invasive brain stimulation

Repetitive transcranial magnetic stimulation (TMS), when applied to the dorsolateral prefrontal cortex (dlPFC), treats depression. Therapeutic effects are hypothesized to arise from propagation of local dlPFC stimulation effects across distributed networks; however, the mechanisms of this remain unresolved. dlPFC contains representations of different networks. As such, dlPFC TMS may exert different effects depending on the network being stimulated. Here, to test this, we applied high-frequency TMS to two nearby dlPFC targets functionally embedded in distinct anti-correlated networks-the default and salience networks- in the same individuals in separate sessions. Local and distributed TMS effects were measured with combined 18fluorodeoxyglucose positron emission tomography and functional magnetic resonance imaging. Identical TMS patterns caused opposing effects on local glucose metabolism: metabolism increased at the salience target following salience TMS but decreased at the default target following default TMS. At the distributed level, both conditions increased functional connectivity between the default and salience networks, with this effect being dramatically larger following default TMS. Metabolic and haemodynamic effects were also linked: across subjects, the magnitude of local metabolic changes correlated with the degree of functional connectivity changes. These results suggest that TMS effects upon dlPFC are network specific. They also invoke putative antidepressant mechanisms of TMS: network de-coupling.

A systematic review of the neurobiological effects of theta-burst stimulation (TBS) as measured using functional magnetic resonance imaging (fMRI)

Theta burst stimulation (TBS) is associated with the modulation of a range of clinical, cognitive, and behavioural outcomes, but specific neurobiological effects remain somewhat unclear. This systematic literature review investigated resting-state and task-based functional magnetic resonance imaging (fMRI) outcomes post-TBS in healthy human adults. Fifty studies that applied either continuous-or intermittent-(c/i) TBS, and adopted a pretest-posttest or sham-controlled design, were included. For resting-state outcomes following stimulation applied to motor, temporal, parietal, occipital, or cerebellar regions, functional connectivity generally decreased in response to cTBS and increased in response to iTBS, though there were some exceptions to this pattern of response. These findings are mostly consistent with the assumed long-term depression (LTD)/long-term potentiation (LTP)-like plasticity effects of cTBS and iTBS, respectively. Task-related outcomes following TBS were more variable. TBS applied to the prefrontal cortex, irrespective of task or state, also produced more variable responses, with no consistent patterns emerging. Individual participant and methodological factors are likely to contribute to the variability in responses to TBS. Future studies assessing the effects of TBS via fMRI must account for factors known to affect the TBS outcomes, both at the level of individual participants and of research methodology.

Association Between Responsiveness to Transcranial Magnetic Stimulation and Interhemispheric Functional Connectivity of Sensorimotor Cortex in Older Adults

Introduction: Repetitive transcranial magnetic stimulation (rTMS) is a promising therapeutic technique, and is believed to accomplish its effect by influencing the stimulated and remotely connected areas. However, responsiveness to rTMS shows high interindividual variability, and this intersubject variability is particularly high in older adults. It remains unclear whether baseline resting-state functional connectivity (rsFC) contributes to this variability in older adults. The aims of this study are to (1) examine rTMS effects over the primary motor cortex (M1) in older adults, and (2) identify baseline network properties that may contribute to the interindividual variability. Methods: We tested response to intermittent theta burst stimulation (iTBS), an effective rTMS protocol, over M1 by using both electromyography and resting-state functional magnetic resonance imaging in older adults. Outcome measures included motor-evoked potential (MEP) elicited by single-pulse transcranial magnetic stimulation and rsFC before and after an iTBS session. Results: iTBS significantly increased MEP amplitudes and rsFC between the stimulation site, sensorimotor cortex, and supplementary motor area (SMA) in older adults. iTBS-induced changes in MEP amplitude were positively correlated with increases in interhemispheric rsFC after iTBS. Furthermore, older adults with lower baseline interhemispheric rsFC between sensorimotor cortex and SMA exhibited stronger MEP response after iTBS. Discussion: Findings of the study suggest that different levels of interhemispheric communication during resting state might contribute to the response heterogeneity to iTBS in older adults. Interhemispheric rsFC may have great potential serving as a useful marker for predicting iTBS responsiveness in older adults. ClinicalTrials.gov ID: 1707654427 Impact statement Factors contributing to interindividual variability of the responsive to repetitive transcranial magnetic stimulation (rTMS) in older adults remain poorly understood. In this study, we examined the effects of rTMS over the primary motor cortex in older adults, and found that response to rTMS is associated with prestimulation interhemispheric connectivity in the sensorimotor and premotor areas. Findings of the study have great potential to be translated into a connectivity-based strategy for identification of responders for rTMS in older adults.

Revisiting the Rotational Field TMS Method for Neurostimulation

Transcranial magnetic stimulation (TMS) is a non-invasive technique that has shown high efficacy in the treatment of major depressive disorder (MDD) and is increasingly utilized for various neuropsychiatric disorders. However, conventional TMS is limited to activating only a small fraction of neurons that have components parallel to the induced electric field. This likely contributes to the significant variability observed in clinical outcomes. A novel method termed rotational field TMS (rfTMS or TMS 360°) enables the activation of a greater number of neurons by reducing the sensitivity to orientation. Recruitment of a larger number of neurons offers the potential to enhance efficacy and reduce variability in the treatment of clinical indications for which neuronal recruitment and organization may play a significant role, such as MDD and stroke. The potential of the method remains to be validated in clinical trials. Here, we revisit and describe in detail the rfTMS method, its principles, mode of operation, effects on the brain, and potential benefits for clinical TMS.

Neuronavigated Repetitive Transcranial Stimulation Improves Neurocognitive Functioning in Veterans with Schizophrenia: A Possible Role of BDNF Polymorphism

It has been reported in the previous literatures that high-frequency (HF) neuronavigated repetitive transcranial magnetic stimulation (rTMS) may improve neurocognitive functioning in patients with schizophrenia. Nonetheless, the heterogeneity of the research findings with regards to the effectiveness of HF-rTMS on the neurocognitive functioning in patients with schizophrenia greatly hinders its clinical application. The current study was designed to determine the predictive role of BDNF variants for neurocognitive improvements after rTMS administration in veterans with schizophrenia. 109 hospitalized veterans with schizophrenia were randomly allocated to active HF-rTMS (n=63) or sham stimulation (n=46) over left DLPFC for 4 consecutive weeks. Neurocognitive functions were assessed by using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) at baseline and at the end of week 4. BDNF polymorphism was genotyped by the technicians. Compared with sham stimulation sessions, the immediate memory performance was significantly increased in active sessions after neuronavigated HF-rTMS administration. In addition, patients with the CC homozygotes demonstrated greater improvement of immediate memory after rTMS treatment, while T allele carriers showed no significant improvement in immediate memory domain relative to baseline performance of immediate memory. Our findings suggest that add-on neuronavigated HF-rTMS is beneficial on immediate memory only in patients with CC homozygotes, but not in T allele carriers. This pilot study provides further evidence for BDNF as a promise biomarker in predicting the clinical response to rTMS stimulation.

Assessing the mechanisms of brain plasticity by transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a non-invasive technique for focal brain stimulation based on electromagnetic induction where a fluctuating magnetic field induces a small intracranial electric current in the brain. For more than 35 years, TMS has shown promise in the diagnosis and treatment of neurological and psychiatric disorders in adults. In this review, we provide a brief introduction to the TMS technique with a focus on repetitive TMS (rTMS) protocols, particularly theta-burst stimulation (TBS), and relevant rTMS-derived metrics of brain plasticity. We then discuss the TMS-EEG technique, the use of neuronavigation in TMS, the neural substrate of TBS measures of plasticity, the inter- and intraindividual variability of those measures, effects of age and genetic factors on TBS aftereffects, and then summarize alterations of TMS-TBS measures of plasticity in major neurological and psychiatric disorders including autism spectrum disorder, schizophrenia, depression, traumatic brain injury, Alzheimer's disease, and diabetes. Finally, we discuss the translational studies of TMS-TBS measures of plasticity and their therapeutic implications.

Effect of group-based vs individualized stimulation site selection on reliability of network-targeted TMS

BACKGROUND

Transcranial magnetic stimulation (TMS) is a widely used technique for the noninvasive assessment and manipulation of brain activity and behavior. Although extensively used for research and clinical purposes, recent studies have questioned the reliability of TMS findings because of the high inter-individual variability that has been observed.

OBJECTIVE

In this study, we compared the efficacy and reliability of different targeting scenarios on the TMS-evoked response.

METHODS

24 subjects underwent a single pulse stimulation protocol over two parietal nodes belonging to the Dorsal Attention (DAN) and Default Mode (DMN) Networks respectively. Across visits, the stimulated target for both networks was chosen either based on group-derived networks' maps or personalized network topography based on individual anatomy and functional profile. All stimulation visits were conducted twice, one month apart, during concomitant electroencephalography recording.

RESULTS

At the network level, we did not observe significant differences in the TMS-evoked response between targeting conditions. However, reliable patterns of activity were observed- for both networks tested- following the individualized targeting approach. When the same analyses were carried out at the electrode space level, evidence of reliable patterns was observed following the individualized stimulation of the DAN, but not of the DMN.

CONCLUSIONS

Our findings suggest that individualization of stimulation sites might ensure reliability of the evoked TMS-response across visits. Furthermore, individualized stimulation sites appear to be of foremost importance in highly variable, high order task-positive networks, such as the DAN.

Simultaneous transcranial electrical and magnetic stimulation boost gamma oscillations in the dorsolateral prefrontal cortex

Neural oscillations in the gamma frequency band have been identified as a fundament for synaptic plasticity dynamics and their alterations are central in various psychiatric and neurological conditions. Transcranial magnetic stimulation (TMS) and alternating electrical stimulation (tACS) may have a strong therapeutic potential by promoting gamma oscillations expression and plasticity. Here we applied intermittent theta-burst stimulation (iTBS), an established TMS protocol known to induce LTP-like cortical plasticity, simultaneously with transcranial alternating current stimulation (tACS) at either theta (θtACS) or gamma (γtACS) frequency on the dorsolateral prefrontal cortex (DLPFC). We used TMS in combination with electroencephalography (EEG) to evaluate changes in cortical activity on both left/right DLPFC and over the vertex. We found that simultaneous iTBS with γtACS but not with θtACS resulted in an enhancement of spectral gamma power, a trend in shift of individual peak frequency towards faster oscillations and an increase of local connectivity in the gamma band. Furthermore, the response to the neuromodulatory protocol, in terms of gamma oscillations and connectivity, were directly correlated with the initial level of cortical excitability. These results were specific to the DLPFC and confined locally to the site of stimulation, not being detectable in the contralateral DLPFC. We argue that the results described here could promote a new and effective method able to induce long-lasting changes in brain plasticity useful to be clinically applied to several psychiatric and neurological conditions.

From mechanisms to markers: novel noninvasive EEG proxy markers of the neural excitation and inhibition system in humans

Brain function is a product of the balance between excitatory and inhibitory (E/I) brain activity. Variation in the regulation of this activity is thought to give rise to normal variation in human traits, and disruptions are thought to potentially underlie a spectrum of neuropsychiatric conditions (e.g., Autism, Schizophrenia, Downs' Syndrome, intellectual disability). Hypotheses related to E/I dysfunction have the potential to provide cross-diagnostic explanations and to combine genetic and neurological evidence that exists within and between psychiatric conditions. However, the hypothesis has been difficult to test because: (1) it lacks specificity-an E/I dysfunction could pertain to any level in the neural system- neurotransmitters, single neurons/receptors, local networks of neurons, or global brain balance - most researchers do not define the level at which they are examining E/I function; (2) We lack validated methods for assessing E/I function at any of these neural levels in humans. As a result, it has not been possible to reliably or robustly test the E/I hypothesis of psychiatric disorders in a large cohort or longitudinal patient studies. Currently available, in vivo markers of E/I in humans either carry significant risks (e.g., deep brain electrode recordings or using Positron Emission Tomography (PET) with radioactive tracers) and/or are highly restrictive (e.g., limited spatial extent for Transcranial Magnetic Stimulation (TMS) and Magnetic Resonance Spectroscopy (MRS). More recently, a range of novel Electroencephalography (EEG) features has been described, which could serve as proxy markers for E/I at a given level of inference. Thus, in this perspective review, we survey the theories and experimental evidence underlying 6 novel EEG markers and their biological underpinnings at a specific neural level. These cheap-to-record and scalable proxy markers may offer clinical utility for identifying subgroups within and between diagnostic categories, thus directing more tailored sub-grouping and, therefore, treatment strategies. However, we argue that studies in clinical populations are premature. To maximize the potential of prospective EEG markers, we first need to understand the link between underlying E/I mechanisms and measurement techniques.

Neurochemical profiles of the anterior temporal lobe predict response of repetitive transcranial magnetic stimulation on semantic processing

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique used to modulate cortical excitability in the human brain. However, one major challenge with rTMS is that the responses to stimulation are highly variable across individuals. The underlying reasons why responses to rTMS are highly variable between individuals still remain unclear. Here, we investigated whether the response to continuous theta-burst stimulation (cTBS) - an effective rTMS protocol for decreasing cortical excitability - is related to individual differences in glutamate and GABA neurotransmission. We acquired resting-state magnetic resonance spectroscopy (MRS) and functional magnetic resonance imaging (fMRI) during semantic processing. Then, we applied cTBS over the anterior temporal lobe (ATL), a hub for semantic representation, to explore the relationship between the baseline neurochemical profiles in this region and the response to cTBS. We found that the baseline excitation-inhibition balance (glutamate + glutamine/GABA ratio) in the ATL was associated with individual cTBS responsiveness during semantic processing. Specifically, individuals with lower excitation-inhibition balance showed stronger inhibitory effect - poorer semantic performance. Our results revealed that non-responders (subjects who did not show an inhibitory effect of cTBS on subsequent semantic performance) had higher excitatory-inhibitory balance in the ATL, which led to up-regulated task-induced regional activity as well as increased ATL-connectivity with other semantic regions compared to responders. These results disclose that the baseline neurochemical state of a cortical region can be a significant factor in predicting responses to cTBS.

Adding a Second iTBS Block in 15 or 60 Min Time Interval Does Not Increase iTBS Effects on Motor Cortex Excitability and the Responder Rates

The use of metaplasticity-based intermittent theta-burst stimulation (iTBS) protocols including several stimulation blocks could be a possible approach to increasing stimulation effectiveness. Our aim was to investigate the neurophysiological effects of two protocols with a short and a long interval between blocks. Seventeen healthy volunteers received four protocols in a pseudorandomized order: iTBS 0-15 (two blocks of active iTBS of primary motor cortex (M1) separated by 15 min and a control stimulation block of the vertex in 60 min from the first block); iTBS 0-60 (active iTBS, a control block in 15 min, and an active block in 60 min); iTBS 0 (active iTBS and two control blocks with the same intervals); and Control (three control blocks). The motor evoked potentials (MEPs) were measured before the first and after the second and third blocks. We have shown no significant differences between the effects of the protocols on both the motor cortex excitability and the responder rates. No significant changes of MEPs were observed after all the protocols. The reliability for the responsiveness to a single block between two sessions was insignificant. Our data confirm low reproducibility of the response to iTBS and suggest that the use of repeated protocols does not increase the responder rates or neurophysiological effects of iTBS.

Repeated motor cortex theta-burst stimulation produces persistent strengthening of corticospinal motor output and durable spinal cord structural changes in the rat

BACKGROUND

The strength of connections between motor cortex (MCX) and muscle can be augmented with a variety of stimulation protocols. Augmenting MCX-to-muscle connection strength by neuromodulation may be a way to enhance the intact motor system's capacity for acquiring motor skills and promote function after injury to strengthen spared connections. But this enhancement must be maintained for functional improvements.

OBJECTIVE

We determined if brief MCX muscle evoked potential (MEP) enhancement produced by intermittent theta burst stimulation (iTBS) can be converted into a longer and structurally durable form of response enhancement with repeated daily and longer-term application.

METHODS

Electrical iTBS was delivered through an implanted MCX epidural electrode and MEPs were recorded using implanted EMG electrodes in awake naïve rats. MCX activity was modulated further using chemogenetic (DREADDs) excitation and inhibition. Corticospinal tract (CST) axons were traced and immunochemistry used to measure CST synapses.

RESULTS

A single MCX iTBS block (600 pulses) produced MEP LTP lasting ∼30-45 min. Concatenating five iTBS blocks within a 30-min session produced MEP LTP lasting 24-48 h, which could be strengthened or weakened by bidirectional MCX activity modulation. Effect duration was not changed. Finally, daily induction of this persistent MEP LTP with daily iTBS for 10-days produced MEP enhancement outlasting the stimulation period by at least 10 days, and accompanied by CST axonal outgrowth and structural changes at the CST-spinal interneuron synapse.

CONCLUSION

Our findings inform the mechanisms of iTBS and provide a framework for designing neuromodulatory strategies to promote durable enhancement of cortical motor actions.

Effect of neuroanatomy on corticomotor excitability during and after transcranial magnetic stimulation and intermittent theta burst stimulation

Individual neuroanatomy can influence motor responses to transcranial magnetic stimulation (TMS) and corticomotor excitability after intermittent theta burst stimulation (iTBS). The purpose of this study was to examine the relationship between individual neuroanatomy and both TMS response measured using resting motor threshold (RMT) and iTBS measured using motor evoked potentials (MEPs) targeting the biceps brachii and first dorsal interosseus (FDI). Ten nonimpaired individuals completed sham‐controlled iTBS sessions and underwent MRI, from which anatomically accurate head models were generated. Neuroanatomical parameters established through fiber tractography were fiber tract surface area (FTSA), tract fiber count (TFC), and brain scalp distance (BSD) at the point of stimulation. Cortical magnetic field induced electric field strength (EFS) was obtained using finite element simulations. A linear mixed effects model was used to assess effects of these parameters on RMT and iTBS (post‐iTBS MEPs). FDI RMT was dependent on interactions between EFS and both FTSA and TFC. Biceps RMT was dependent on interactions between EFS and and both FTSA and BSD. There was no groupwide effect of iTBS on the FDI but individual changes in corticomotor excitability scaled with RMT, EFS, BSD, and FTSA. iTBS targeting the biceps was facilitatory, and dependent on FTSA and TFC. MRI‐based measures of neuroanatomy highlight how individual anatomy affects motor system responses to different TMS paradigms and may be useful for selecting appropriate motor targets when designing TMS based therapies.

Brain-Derived Neurotrophic Factor Gene Polymorphism Predicts Response to Continuous Theta Burst Stimulation in Chronic Stroke Patients

OBJECTIVES

The efficacy of repetitive transcranial magnetic stimulation (rTMS) in clinically relevant neuroplasticity research depends on the degree to which stimulation induces robust, reliable effects. The high degree of interindividual and intraindividual variability observed in response to rTMS protocols, such as continuous theta burst stimulation (cTBS), therefore represents an obstacle to its utilization as treatment for neurological disorders. Brain-derived neurotrophic factor (BDNF) is a protein involved in human synaptic and neural plasticity, and a common polymorphism in the BDNF gene (Val66Met) may influence the capacity for neuroplastic changes that underlie the effects of cTBS and other rTMS protocols. While evidence from healthy individuals suggests that Val66Met polymorphism carriers may show diminished or facilitative effects of rTMS compared to their homozygous Val66Val counterparts, this has yet to be demonstrated in the patient populations where neuromodulatory therapies are most relevant.

MATERIALS AND METHODS

We examined the effects of BDNF Val66Met polymorphism on cTBS aftereffects in stroke patients. We compared approximately 30 log-transformed motor-evoked potentials (LnMEPs) obtained per time point: at baseline and at 0, 10, 20, and 30 min after cTBS-600, from 18 patients with chronic stroke using single TMS pulses. We used linear mixed-effects regression with trial-level data nested by subject for higher statistical power.

RESULTS

We found a significant interaction between BDNF genotype and pre-/post-cTBS LnMEPs. Val66Val carriers showed decrease in cortical excitability, whereas Val66Met carriers exhibited a modest increase in cortical excitability for 20 min poststimulation, followed by inhibition 30 min after cTBS-600.

CONCLUSIONS

Our findings strongly suggest that BDNF genotype differentially affects neuroplastic responses to TMS in individuals with chronic stroke. This provides novel insight into potential sources of variability in cTBS response in patients, which has important implications for optimizing the utility of this neuromodulation approach. Incorporating BDNF polymorphism genetic screening to stratify patients prior to use of cTBS as a neuromodulatory technique in therapy or research may optimize response rates.

Concurrent frontal and parietal network TMS for modulating attention

Transcranial magnetic stimulation (TMS) has been applied to frontal eye field (FEF) and intraparietal sulcus (IPS) in isolation, to study their role in attention. However, these nodes closely interact in a "dorsal attention network". Here, we compared effects of inhibitory TMS applied to individually fMRI-localized FEF or IPS (single-node TMS), to effects of simultaneously inhibiting both regions ("network TMS"), and sham. We assessed attention performance using the lateralized attention network test, which captures multiple facets of attention: spatial orienting, alerting, and executive control. TMS showed no effects on alerting and executive control. For spatial orienting, only network TMS showed a reduction of the orienting effect in the right hemifield compared to the left hemifield, irrespective of the order of TMS application (IPS→FEF or FEF→IPS). Network TMS might prevent compensatory mechanisms within a brain network, which is promising for both research and clinical applications to achieve superior neuromodulation effects.

Continuous and intermittent theta burst stimulation to the visual cortex do not alter GABA and glutamate concentrations measured by magnetic resonance spectroscopy

BACKGROUND

Theta burst stimulation (TBS), a form of repetitive transcranial magnetic stimulation (rTMS), uses repeated high-frequency bursts to non-invasively modulate neural processes in the brain. An intermittent TBS (iTBS) protocol is generally considered "excitatory," while continuous TBS (cTBS) is considered "inhibitory." However, the majority of work that has led to these effects being associated with the respective protocols has been done in the motor cortex, and it is well established that TMS can have variable effects across the brain.

OBJECTIVES AND METHOD

We investigated the effects of iTBS and cTBS to the primary visual cortex (V1) on composite levels of gamma-aminobutyric acid + co-edited macromolecules (GABA+) and glutamate + glutamine (Glx) since these are key inhibitory and excitatory neurotransmitters, respectively. Participants received a single session of cTBS, iTBS, or sham TBS to V1. GABA+ and Glx were quantified in vivo at the stimulation site using spectral-edited proton magnetic resonance spectroscopy (¹ H-MRS) at 3T. Baseline pre-TBS GABA+ and Glx levels were compared to immediate post-TBS and 1 h post-TBS levels.

RESULTS

There were no significant changes in GABA+ or Glx following either of the TBS conditions. Visual cortical excitability, measured using phosphene thresholds, remained unchanged following both cTBS and iTBS conditions. There was no relationship between excitability thresholds and GABA+ or Glx levels. However, TBS did alter the relationship between GABA+ and Glx for up to 1 h following stimulation.

CONCLUSIONS

These findings demonstrate that a single session of TBS to the visual cortex can be used without significant effects on the tonic levels of these key neurotransmitters; and add to our understanding that TBS has differential effects at visual, motor, and frontal cortices.

Brain-derived neurotrophic factor in Alzheimer's disease and its pharmaceutical potential

Synaptic abnormalities are a cardinal feature of Alzheimer's disease (AD) that are known to arise as the disease progresses. A growing body of evidence suggests that pathological alterations to neuronal circuits and synapses may provide a mechanistic link between amyloid β (Aβ) and tau pathology and thus may serve as an obligatory relay of the cognitive impairment in AD. Brain-derived neurotrophic factors (BDNFs) play an important role in maintaining synaptic plasticity in learning and memory. Considering AD as a synaptic disorder, BDNF has attracted increasing attention as a potential diagnostic biomarker and a therapeutical molecule for AD. Although depletion of BDNF has been linked with Aβ accumulation, tau phosphorylation, neuroinflammation and neuronal apoptosis, the exact mechanisms underlying the effect of impaired BDNF signaling on AD are still unknown. Here, we present an overview of how BDNF genomic structure is connected to factors that regulate BDNF signaling. We then discuss the role of BDNF in AD and the potential of BDNF-targeting therapeutics for AD.

Transcranial magnetic stimulation as a tool to induce and explore plasticity in humans

Activity-dependent synaptic plasticity is the main theoretical framework to explain mechanisms of learning and memory. Synaptic plasticity can be explored experimentally in animals through various standardized protocols for eliciting long-term potentiation and long-term depression in hippocampal and cortical slices. In humans, several non-invasive protocols of repetitive transcranial magnetic stimulation and transcranial direct current stimulation have been designed and applied to probe synaptic plasticity in the primary motor cortex, as reflected by long-term changes in motor evoked potential amplitudes. These protocols mimic those normally used in animal studies for assessing long-term potentiation and long-term depression. In this chapter, we first discuss the physiologic basis of theta-burst stimulation, paired associative stimulation, and transcranial direct current stimulation. We describe the current biophysical and theoretical models underlying the molecular mechanisms of synaptic plasticity and metaplasticity, defined as activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), in the human motor cortex including calcium-dependent plasticity, spike-timing-dependent plasticity, the role of N-methyl-d-aspartate-related transmission and gamma-aminobutyric-acid interneuronal activity. We also review the putative microcircuits responsible for synaptic plasticity in the human motor cortex. We critically readdress the issue of variability in studies investigating synaptic plasticity and propose available solutions. Finally, we speculate about the utility of future studies with more advanced experimental approaches.

Excitatory-Inhibitory Homeostasis and Diaschisis: Tying the Local and Global Scales in the Post-stroke Cortex

Maintaining a balance between excitatory and inhibitory activity is an essential feature of neural networks of the neocortex. In the face of perturbations in the levels of excitation to cortical neurons, synapses adjust to maintain excitatory-inhibitory (EI) balance. In this review, we summarize research on this EI homeostasis in the neocortex, using stroke as our case study, and in particular the loss of excitation to distant cortical regions after focal lesions. Widespread changes following a localized lesion, a phenomenon known as diaschisis, are not only related to excitability, but also observed with respect to functional connectivity. Here, we highlight the main findings regarding the evolution of excitability and functional cortical networks during the process of post-stroke recovery, and how both are related to functional recovery. We show that cortical reorganization at a global scale can be explained from the perspective of EI homeostasis. Indeed, recovery of functional networks is paralleled by increases in excitability across the cortex. These adaptive changes likely result from plasticity mechanisms such as synaptic scaling and are linked to EI homeostasis, providing a possible target for future therapeutic strategies in the process of rehabilitation. In addition, we address the difficulty of simultaneously studying these multiscale processes by presenting recent advances in large-scale modeling of the human cortex in the contexts of stroke and EI homeostasis, suggesting computational modeling as a powerful tool to tie the meso- and macro-scale processes of recovery in stroke patients.

BDNF Val66Met gene polymorphism modulates brain activity following rTMS-induced memory impairment

The BDNF Val66Met gene polymorphism is a relevant factor explaining inter-individual differences to TMS responses in studies of the motor system. However, whether this variant also contributes to TMS-induced memory effects, as well as their underlying brain mechanisms, remains unexplored. In this investigation, we applied rTMS during encoding of a visual memory task either over the left frontal cortex (LFC; experimental condition) or the cranial vertex (control condition). Subsequently, individuals underwent a recognition memory phase during a functional MRI acquisition. We included 43 young volunteers and classified them as 19 Met allele carriers and 24 as Val/Val individuals. The results revealed that rTMS delivered over LFC compared to vertex stimulation resulted in reduced memory performance only amongst Val/Val allele carriers. This genetic group also exhibited greater fMRI brain activity during memory recognition, mainly over frontal regions, which was positively associated with cognitive performance. We concluded that BDNF Val66Met gene polymorphism, known to exert a significant effect on neuroplasticity, modulates the impact of rTMS both at the cognitive as well as at the associated brain networks expression levels. This data provides new insights on the brain mechanisms explaining cognitive inter-individual differences to TMS, and may inform future, more individually-tailored rTMS interventions.

Microstructural Properties of Human Brain Revealed by Fractional Anisotropy Can Predict the After-Effect of Intermittent Theta Burst Stimulation

Intermittent theta burst stimulation (iTBS) delivered by transcranial magnetic stimulation (TMS) produces a long-term potentiation-like after-effect useful for investigations of cortical function and of potential therapeutic value. However, the iTBS after-effect over the primary motor cortex (M1) as measured by changes in motor evoked potential (MEP) amplitude exhibits a largely unexplained variability across individuals. Here, we present evidence that individual differences in white matter (WM) and gray matter (GM) microstructural properties revealed by fractional anisotropy (FA) predict the magnitude of the iTBS-induced after-effect over M1. The MEP amplitude change in the early phase (5–10 min post-iTBS) was associated with FA values in WM tracts such as right superior longitudinal fasciculus and corpus callosum. By contrast, the MEP amplitude change in the late phase (15–30 min post-iTBS) was associated with FA in GM, primarily in right frontal cortex. These results suggest that the microstructural properties of regions connected directly or indirectly to the target region (M1) are crucial determinants of the iTBS after-effect. FA values indicative of these microstructural differences can predict the potential effectiveness of repetitive TMS for both investigational use and clinical application.

Spontaneous Fluctuations in Oscillatory Brain State Cause Differences in Transcranial Magnetic Stimulation Effects Within and Between Individuals

Transcranial magnetic stimulation (TMS) can cause measurable effects on neural activity and behavioral performance in healthy volunteers. In addition, TMS is increasingly used in clinical practice for treating various neuropsychiatric disorders. Unfortunately, TMS-induced effects show large intra- and inter-subject variability, hindering its reliability, and efficacy. One possible source of this variability may be the spontaneous fluctuations of neuronal oscillations. We present recent studies using multimodal TMS including TMS-EMG (electromyography), TMS-tACS (transcranial alternating current stimulation), and concurrent TMS-EEG-fMRI (electroencephalography, functional magnetic resonance imaging), to evaluate how individual oscillatory brain state affects TMS signal propagation within targeted networks. We demonstrate how the spontaneous oscillatory state at the time of TMS influences both immediate and longer-lasting TMS effects. These findings indicate that at least part of the variability in TMS efficacy may be attributable to the current practice of ignoring (spontaneous) oscillatory fluctuations during TMS. Ignoring this state-dependent spread of activity may cause great individual variability which so far is poorly understood and has proven impossible to control. We therefore also compare two technical solutions to directly account for oscillatory state during TMS, namely, to use (a) tACS to externally control these oscillatory states and then apply TMS at the optimal (controlled) brain state, or (b) oscillatory state-triggered TMS (closed-loop TMS). The described multimodal TMS approaches are paramount for establishing more robust TMS effects, and to allow enhanced control over the individual outcome of TMS interventions aimed at modulating information flow in the brain to achieve desirable changes in cognition, mood, and behavior.

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.