Characterization of GABAB-receptor mediated neurotransmission in the human cortex by paired-pulse TMS-EEG

GABAB Receptor: Structure, Biological Functions, and Therapy for Diseases

Gamma-aminobutyric acid (GABA) B receptors (GABABRs) that acts slowly and maintains the inhibitory tone are versatile regulators in the complex nervous behaviors and their involvement in various neuropsychiatric disorders, such as anxiety, epilepsy, pain, drug addiction, and Alzheimer's disease. Additional study advances have implied the crucial roles of GABABRs in regulating feeding-related behaviors, yet their therapeutic potential in addressing the neuropsychiatric disorders, binge eating, and feeding-related disorders remains underutilized. This general review summarized the physiological structure and functions of GABABR, explored the regulation in various psychiatric disorders, feeding behaviors, binge eating, and metabolism disorders, and fully discussed the potential of targeting GABABRs and its regulator-binding sites for the treatment of different psychiatric disorders, binge eating and even obesity. While agonists that directly bind to GABABR1 have some negative side effects, positive allosteric modulators (PAMs) that bind to GABABR2 demonstrate excellent therapeutic efficacy and tolerability and have better safety and therapeutic indexes. Moreover, phosphorylation sites of downstream GABABRs regulators may be novel therapeutic targets for psychiatric disorders, binge eating, and obesity. Further studies, clinical trials in particular, will be essential for confirming the therapeutic value of PAMs and other agents targeting the GABABR pathways in a clinical setting.

Assessing Neuromodulation Effects of Theta Burst Stimulation to the Prefrontal Cortex Using Transcranial Magnetic Stimulation Electroencephalography (TMS-EEG)

Theta burst stimulation (TBS), a form of repetitive transcranial magnetic stimulation (TMS), is capable of non-invasively modulating cortical excitability. TBS is gaining popularity as a therapeutic tool for psychiatric disorders such as depression, in which the dorsolateral prefrontal cortex (DLPFC) is the main therapeutic target. However, the neuromodulatory effects of TBS on prefrontal regions remain unclear. Concurrent transcranial magnetic stimulation and electroencephalography (TMS-EEG) can assess neuromodulation in non-motor regions using TMS-evoked potentials (TEPs) and event-related synchronisation/desynchronisation (ERS/D). We assessed 24 healthy participants (13 males, mean age 25.2 ± 9.9 years) in a single-blinded crossover study design, following intermittent TBS, continuous TBS and sham applied to the left DLPFC. TEPs and ERS/D were obtained at baseline and 2-, 15- and 30-min post-stimulation. Four TEP components (N40, P60, N100 and P200) and two frequency bands (theta and gamma) were analysed using mixed effects repeated measures models (MRMM). Results indicated no significant effects for any assessed components or frequency bands. Relative to sham, the largest TEP effect size was obtained for the N100 component at 15 min post-iTBS (d = -0.50), and the largest frequency effect was obtained for gamma ERS at 15 min post-cTBS (d = 0.53). These results were in the same direction but smaller than found in previous studies, suggesting that effect sizes of the neuromodulatory effects of TBS may be lower than previously reported.

Stimulation Parameters Recruit Distinct Cortico-Cortical Pathways: Insights from Microstate Analysis on TMS-Evoked Potentials

Transcranial magnetic stimulation (TMS)-evoked potentials (TEPs) represent an innovative measure for examining brain connectivity and developing biomarkers of psychiatric conditions. Minimizing TEP variability across studies and participants, which may stem from methodological choices, is therefore vital. By combining classic peak analysis and microstate investigation, we tested how TMS pulse waveform and current direction may affect cortico-cortical circuit engagement when targeting the primary motor cortex (M1). We aim to disentangle whether changing these parameters affects the degree of activation of the same neural circuitry or may lead to changes in the pathways through which the induced activation spreads. Thirty-two healthy participants underwent a TMS-EEG experiment in which the pulse waveform (monophasic, biphasic) and current direction (posterior-anterior, anterior-posterior, latero-medial) were manipulated. We assessed the latency and amplitude of M1-TEP components and employed microstate analyses to test differences in topographies. Results revealed that TMS parameters strongly influenced M1-TEP components' amplitude but had a weaker role over their latencies. Microstate analysis showed that the current direction in monophasic stimulations changed the pattern of evoked microstates at the early TEP latencies, as well as their duration and global field power. This study shows that the current direction of monophasic pulses may modulate cortical sources contributing to TEP signals, activating neural populations and cortico-cortical paths more selectively. Biphasic stimulation reduces the variability associated with current direction and may be better suited when TMS targeting is blind to anatomical information.

Association of TMS-EEG interhemispheric imbalance with upper limb motor impairment in chronic stroke patients: An exploratory study

OBJECTIVE

We aimed to investigate the involvement of interhemispheric cortical dynamics as measured by combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) in recovery of upper limb (UL) motor functions in chronic stroke patients.

METHODS

Ten patients with a history of single ischemic chronic stroke were enrolled (4F, 63.8 ± 9.9 years). Each patient underwent TMS-EEG recordings to evaluate interhemispheric cortical dynamics as well as a reaching task recorded with inertial measurement units, and a series of clinical assessments. TMS-EEG neurophysiological data were analysed considering spatiotemporal, power response, and interhemispheric balance (IHB) dynamics.

RESULTS

We found that IHB index (IHBi) and low-frequency power (LFP) (4-13 Hz) in the affected hemisphere were associated with the degree of UL impairment.

CONCLUSION

Increased IHBi due to stroke is an unfavourable factor of UL' functions. Similarly, LFP of both hemispheres is strongly correlated with clinical and kinematic outcomes.

SIGNIFICANCE

TMS-EEG biomarkers of interhemispheric unbalance could be used to estimate functional recovery and drive tailored neuromodulation and neurorehabilitation approaches.

TMS and EEG Pharmacodynamic Effects of a Selective Sphingosine-1-Phosphate Subtype 1 Receptor Agonist on Cortical Excitability in Healthy Subjects

Current anti-epileptic drugs lack efficacy, cause many side effects and one third of all patients are treatment-resistant. Drugs targeting the sphingosine-1-phosphate receptor show potential anti-convulsant effects in animal models and decrease cortical excitability in patients with multiple sclerosis, but available compounds alter lymphocyte trafficking and cause immunosuppression, limiting their clinical anti-epileptic potential. TRV045 is a selective sphingosine-1-phosphate subtype 1 receptor agonist without effects on lymphocyte trafficking, demonstrating efficacy in animal models of epilepsy, with the potential to target abnormal cortical excitability. This randomized, double-blind, placebo-controlled, two-way cross-over, multiple-dose study evaluated the effects of TRV045 on cortical excitability in healthy male adults, measured by pharmaco-electroencephalography and transcranial magnetic stimulation (TMS). Subjects received TRV045 250 mg or placebo, once daily for 4 days, in randomized order. Endpoints were analyzed with a mixed effects model analysis of covariance. Twenty-five of the 27 subjects completed the study. There was a significant increase in alpha power with eyes open after treatment with TRV045 on Day 1, increasing after 4 days of dosing. Less pronounced significant effects in beta, gamma, and delta power were observed after 4 days. For TMS-Electromyography there was a non-significant decreased post-dose single-pulse peak-to-peak amplitude on Day 1 only, and there were no effects on paired-pulse parameters. Several significant TMS-Electroencephalography clusters were seen after 4 days of dosing. These findings show that TRV045 has central nervous system activity with evolving effects following repeated dosing. These data support further studies to elucidate the mechanism of action of TRV045 and its potential anti-epileptic effects.

Transcranial magnetic stimulation in attention-deficit/hyperactivity disorder: a systematic review and meta-analysis of cortical excitability and therapeutic efficacy

Background/Objectives

Attention-deficit/hyperactivity disorder (ADHD) currently lacks a universally accepted biomarker or diagnostic test, underscoring the need for objective and effective assessment methods. Transcranial magnetic stimulation (TMS) has emerged as a promising tool for both assessing cortical excitability and providing therapeutic interventions. This study conducted two independent meta-analyses to evaluate: 1) the potential of TMS in assessing cortical excitability, and 2) its therapeutic efficacy in managing ADHD symptoms.

Methods

A systematic search was conducted in EMBASE, MEDLINE, PsycINFO, ClinicalTrials, and PubMed following PRISMA guidelines. The "cortical excitability" meta-analysis included studies comparing TMS-EMG or TMS-EEG neurophysiological measures between ADHD patients and healthy controls. The "therapeutic" meta-analysis focused on randomized controlled trials (RCTs) evaluating repetitive TMS (rTMS) effects on ADHD symptoms. Standardized mean differences (SMDs) were calculated for pooled effect sizes.

Results

In the "cortical excitability" meta-analysis, 17 studies were included, demonstrating significantly reduced short-interval intracortical inhibition (SICI) in ADHD compared to healthy controls (pooled SMD = 0.65, 95% CI: 0.41-0.88, P < 0.00001). No significant differences were observed for motor evoked potentials (MEP), motor thresholds (aMT/rMT), cortical silent period (cSP), ipsilateral silent period (iSP), or intracortical facilitation (ICF). The "therapeutic" meta-analysis, encompassing 8 samples from 7 studies, demonstrated that rTMS significantly improved ADHD symptoms compared to control conditions (pooled SMD = 0.45, 95% CI: 0.19-0.70, P = 0.0006).

Conclusions

This study highlights the potential of TMS as both a diagnostic and therapeutic tool in ADHD. Reduced SICI appears to be a key neurophysiological marker of ADHD, reflecting cortical GABAergic dysfunction. Additionally, rTMS shows promise in alleviating ADHD symptoms, though further studies are needed to confirm long-term therapeutic benefits and optimize stimulation protocols.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42024507867.

Optimizing the identification of long-interval intracortical inhibition from the dorsolateral prefrontal cortex

OBJECTIVE

This study aimed to optimally evaluate the effect of the long-interval intracortical inhibition (LICI) in the dorsolateral prefrontal cortex (DLPFC) through transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) by eliminating the volume conductance with signal source estimation and using a realistic sham coil as a control.

METHODS

We compared the LICI effects from the DLPFC between the active and sham stimulation conditions in 27 healthy participants. Evoked responses between the two conditions were evaluated at the sensor and source levels.

RESULTS

At the sensor level, a significant LICI effect was confirmed in the active condition in the global mean field power analysis; however, in the local mean field power analysis focused on the DLPFC, no LICI effect was observed in the active condition. However, in the signal source estimation analysis for the DLPFC, we could reconfirm a significant LICI effect (p = 0.023) in the interval 30-250 ms post-stimulus, compared to the sham condition.

CONCLUSIONS

Our results demonstrate that application of realistic sham stimulation condition and source estimation method allows for a robust and optimal identification of the LICI effect in the DLPFC.

SIGNIFICANCE

The optimal DLPFC-LICI effect was identified by the use of the sophisticated sham coil.

GABALAGEN Alleviates Stress-Induced Sleep Disorders in Rats

(1) Background: Gamma-aminobutyric acid (GABA) is an amino acid and the primary inhibitory neurotransmitter in the brain. GABA has been shown to reduce stress and promote sleep. GABALAGEN (GBL) is the product of fermented fish collagen by Lactobacillus brevis BJ20 and Lactobacillus plantarum BJ21, naturally enriched with GABA through the fermentation process and characterized by low molecular weight. (2) Methods: The present study evaluated the GABAA affinity of GBL through receptor binding assay. The sedative effects of GBL were investigated through electroencephalography (EEG) analysis in an animal model of electro foot shock (EFS) stress-induced sleep disorder, and then we examined the expression of orexin and the GABAA receptor in the brain region using immunohistochemistry and an enzyme-linked immunosorbent assay (ELISA). (3) Results: We found that on the binding assay, GBL displayed high affinity to the GABAA receptor. Also, after treatment with GBL, the percentage of the total time in rapid eye movement (REM) and non-rapid eye movement (NREM) sleep was significantly and dose-dependently increased in EFS-induced rats. Consistent with behavioral results, the GBL-treated groups showed that the expression of GABAA receptor immune-positive cells in the VLPO was markedly and dose-dependently increased. Also, the GBL-treated groups showed that the expression of the orexin-A level in LH was significantly decreased. (4) Conclusions: GBL showed efficacy and potential to be used as an anti-stress therapy to treat sleep deprivation through the stimulation of GABAA receptors and the consequent inhibition of orexin activity.

Circuits and mechanisms for TMS-induced corticospinal waves: Connecting sensitivity analysis to the network graph

Transcranial magnetic stimulation (TMS) is a non-invasive, FDA-cleared treatment for neuropsychiatric disorders with broad potential for new applications, but the neural circuits that are engaged during TMS are still poorly understood. Recordings of neural activity from the corticospinal tract provide a direct readout of the response of motor cortex to TMS, and therefore a new opportunity to model neural circuit dynamics. The study goal was to use epidural recordings from the cervical spine of human subjects to develop a computational model of a motor cortical macrocolumn through which the mechanisms underlying the response to TMS, including direct and indirect waves, could be investigated. An in-depth sensitivity analysis was conducted to identify important pathways, and machine learning was used to identify common circuit features among these pathways. Sensitivity analysis identified neuron types that preferentially contributed to single corticospinal waves. Single wave preference could be predicted using the average connection probability of all possible paths between the activated neuron type and L5 pyramidal tract neurons (PTNs). For these activations, the total conduction delay of the shortest path to L5 PTNs determined the latency of the corticospinal wave. Finally, there were multiple neuron type activations that could preferentially modulate a particular corticospinal wave. The results support the hypothesis that different pathways of circuit activation contribute to different corticospinal waves with participation of both excitatory and inhibitory neurons. Moreover, activation of both afferents to the motor cortex as well as specific neuron types within the motor cortex initiated different I-waves, and the results were interpreted to propose the cortical origins of afferents that may give rise to certain I-waves. The methodology provides a workflow for performing computationally tractable sensitivity analyses on complex models and relating the results to the network structure to both identify and understand mechanisms underlying the response to acute stimulation.

The Cerebellum and the Motor Cortex: Multiple Networks Controlling Multiple Aspects of Behavior

The cerebellum and its thalamic projections to the primary motor cortex (M1) are well known to play an essential role in executing daily actions. Anatomic investigations in animals and postmortem humans have established the reciprocal connections between these regions; however, how these pathways can shape cortical activity in behavioral contexts and help promote recovery in neuropathological conditions remains not well understood. The present review aims to provide a comprehensive description of these pathways in animals and humans and discuss how novel noninvasive brain stimulation (NIBS) methods can be used to gain a deeper understanding of the cerebellar-M1 connections. In the first section, we focus on recent animal literature that details how information sent from the cerebellum and thalamus is integrated into an broad network of cortical motor neurons. We then discuss how NIBS approaches in humans can be used to reliably assess the connectivity between the cerebellum and M1. Moreover, we provide the latest perspectives on using advanced NIBS approaches to investigate and modulate multiple cerebellar-cortical networks involved in movement behavior and plasticity. Finally, we discuss how these emerging methods have been used in translation research to produce long-lasting modifications of cerebellar-thalamic-M1 to restore cortical activity and motor function in neurologic patients.

Utilization of Single-Pulse Transcranial-Evoked Potentials in Neurological and Psychiatric Clinical Practice: A Narrative Review

Background: The utility of single-pulse TMS (transcranial magnetic stimulation)-evoked EEG (electroencephalograph) potentials (TEPs) has been extensively studied in the past three decades. TEPs have been shown to provide insights into features of cortical excitability and connectivity, reflecting mechanisms of excitatory/inhibitory balance, in various neurological and psychiatric conditions. In the present study, we sought to review and summarize the most studied neurological and psychiatric clinical indications utilizing single-pulse TEP and describe its promise as an informative novel tool for the evaluation of brain physiology. Methods: A thorough search of PubMed, Embase, and Google Scholar for original research utilizing single-pulse TMS-EEG and the measurement of TEP was conducted. Our review focused on the indications and outcomes most clinically relevant, commonly studied, and well-supported scientifically. Results: We included a total of 55 publications and summarized them by clinical application. We categorized these publications into seven sub-sections: healthy aging, Alzheimer's disease (AD), disorders of consciousness (DOCs), stroke rehabilitation and recovery, major depressive disorder (MDD), Parkinson's disease (PD), as well as prediction and monitoring of treatment response. Conclusions: TEP is a useful measurement of mechanisms underlying neuronal networks. It may be utilized in several clinical applications. Its most prominent uses include monitoring of consciousness levels in DOCs, monitoring and prediction of treatment response in MDD, and diagnosis of AD. Additional applications including the monitoring of stroke rehabilitation and recovery, as well as a diagnostic aid for PD, have also shown encouraging results but require further evidence from randomized controlled trials (RCTs).

Methodological Choices Matter: A Systematic Comparison of TMS-EEG Studies Targeting the Primary Motor Cortex

Transcranial magnetic stimulation (TMS) triggers time-locked cortical activity that can be recorded with electroencephalography (EEG). Transcranial evoked potentials (TEPs) are widely used to probe brain responses to TMS. Here, we systematically reviewed 137 published experiments that studied TEPs elicited from TMS to the human primary motor cortex (M1) in healthy individuals to investigate the impact of methodological choices. We scrutinized prevalent methodological choices and assessed how consistently they were reported in published papers. We extracted amplitudes and latencies from reported TEPs and compared specific TEP peaks and components between studies using distinct methods. Reporting of methodological details was overall sufficient, but some relevant information regarding the TMS settings and the recording and preprocessing of EEG data were missing in more than 25% of the included experiments. The published TEP latencies and amplitudes confirm the "prototypical" TEP waveform following stimulation of M1, comprising distinct N15, P30, N45, P60, N100, and P180 peaks. However, variations in amplitude were evident across studies. Higher stimulation intensities were associated with overall larger TEP amplitudes. Active noise masking during TMS generally resulted in lower TEP amplitudes compared to no or passive masking but did not specifically impact those TEP peaks linked to long-latency sensory processing. Studies implementing independent component analysis (ICA) for artifact removal generally reported lower TEP magnitudes. In summary, some aspects of reporting practices could be improved in future TEP studies to enable replication. Methodological choices, including TMS intensity and the use of noise masking or ICA, introduce systematic differences in reported TEP amplitudes. Further investigation into the significance of these and other methodological factors and their interactions is warranted.

Differential analgesic effects of high-frequency or accelerated intermittent theta burst stimulation of M1 on experimental tonic pain: Correlations with cortical activity changes assessed by TMS-EEG

Accelerated intermittent theta burst stimulation (AiTBS) has attracted much attention in the past few years as a new form of brain stimulation paradigm. However, it is unclear the relative efficacy of AiTBS on cortical excitability compared to conventional high-frequency rTMS. Using concurrent TMS and electroencephalogram (TMS-EEG), this study systematically compared the efficacy on cortical excitability and a typical clinical application (i.e. pain), between AiTBS with different intersession interval (ISIs) and 10-Hz rTMS. Participants received 10-Hz rTMS, AiTBS-15 (3 iTBS sessions with a 15-min ISI), AiTBS-50 (3 iTBS sessions with a 50-min ISI), or Sham stimulation over the primary motor cortex on four separate days. All four protocols included a total of 1800 pulses but with different session durations (10-Hz rTMS ​= ​18, AiTBS-15 ​= ​40, and AiTBS-50 ​= ​110 ​min). AiTBS-50 and 10-Hz rTMS were more effective in pain reduction compared to AiTBS-15. Using single-pulse TMS-induced oscillation, our data revealed low gamma oscillation as a shared cortical excitability change across all three active rTMS protocols but demonstrated completely opposite directions. Changes in low gamma oscillation were further associated with changes in pain perception across the three active conditions. In contrast, a distinct pattern of TMS-evoked potentials (TEPs) was revealed, with 10-Hz rTMS decreasing inhibitory N100 amplitude and AiTBS-15 reducing excitatory P60 amplitude. These changes in TEPs were also covarying with low gamma power changes. Sham stimulation indicated no significant effect on either cortical excitability or pain perception. These results are relevant only for provoked experimental pain, without being predictive for chronic pain, and revealed a change in low gamma oscillation, particularly around the very particular frequency of 40 ​Hz, shared between AiTBS and high-frequency rTMS. Conversely, cortical excitability (balance between excitation and inhibition) assessed by TEP recording was modulated differently by AiTBS and high-frequency rTMS paradigms.

A cogent technique to circumvent the use of (some) ratio measures in physiology

Physiologists often express the change in the value of a measurement made on two occasions as a ratio of the initial value. This is usually motivated by an assumption that the absolute change fails to capture the true extent of the alteration that has occurred in attaining the final value - if there is initial variation among individual cases. While it may appear reasonable to use ratios to standardize the magnitude of change in this way, the perils of doing so have been widely documented. Ratios frequently have intractable statistical properties, both when taken in isolation and when analysed using techniques such as regression. A new method of computing a standardized metric of change, based on principal components analysis (PCA), is described. It exploits the collinearity within sets of initial, absolute change and final values. When these sets define variables subjected to PCA, the standardized measure of change is obtained as the product of the loading of absolute change onto the first principal component (PC1) and the eigenvalue of PC1. It is demonstrated that a sample drawn from a population of these standardized measures: approximates a normal distribution (unlike the corresponding ratios); lies within the same range; and preserves the rank order of the ratios. It is also shown that this method can be used to express the magnitude of a physiological response in an experimental condition relative to that obtained in a control condition. KEY POINTS: The intractable statistical properties of ratios and the perils of using ratios to standardize the magnitude of change are well known. A new method of computing a standardized metric, based on principal components analysis (PCA), is described, which exploits the collinearity within sets of initial, absolute change and final values. A sample drawn from a population of these PCA-derived measures: approximates a normal distribution (unlike the corresponding ratios); lies within the same range as the ratios; and preserves the rank order of the ratios. The method can also be applied to express the magnitude of a physiological response in an experimental condition relative to a control condition.

Abnormal Global Cortical Responses in Drug-Naïve Patients With Schizophrenia Following Orbitofrontal Cortex Stimulation: A Concurrent Transcranial Magnetic Stimulation-Electroencephalography Study

BACKGROUND

Abnormalities in cortical excitability and plasticity have been considered to underlie the pathophysiology of schizophrenia. Transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) can provide a direct evaluation of cortical responses to TMS. Here, we employed TMS-EEG to investigate cortical responses to orbitofrontal cortex (OFC) stimulation in schizophrenia.

METHODS

In total, we recruited 92 drug-naïve patients with first-episode schizophrenia and 51 age- and sex-matched healthy individuals. For each participant, one session of 1-Hz repetitive TMS (rTMS) was delivered to the right OFC, and TMS-EEG data were obtained to explore the change in cortical-evoked activities before and immediately after rTMS during the eyes-closed state. The MATRICS Consensus Cognitive Battery was used to assess neurocognitive performance.

RESULTS

The cortical responses indexed by global mean field amplitudes (i.e., P30, N45, and P60) were larger in patients with schizophrenia than in healthy control participants at baseline. Furthermore, after one session of 1-Hz rTMS over the right OFC, the N100 amplitude was significantly reduced in the healthy control group but not in the schizophrenia group. In the healthy control participants, there was a significant correlation between modulation of P60 amplitude by rTMS and working memory; however, this correlation was absent in patients with schizophrenia.

CONCLUSIONS

Aberrant global cortical responses following right OFC stimulation were found in patients with drug-naïve first-episode schizophrenia, supporting its significance in the primary pathophysiology of schizophrenia.

Neurophysiological markers of disease severity and cognitive dysfunction in major depressive disorder: A TMS-EEG study

Background

Transcranial magnetic stimulation-electroencephalography (TMS-EEG) is a powerful technique to study the neuropathology and biomarkers of major depressive disorder (MDD). This study investigated cortical activity and its relationship with clinical symptoms and cognitive dysfunction in MDD patients by indexing TMS-EEG biomarkers in the dorsolateral prefrontal cortex (DLPFC).

Methods

133 patients with MDD and 76 healthy individuals participated in this study. Single-pulse TMS was performed on the left DLPFC to obtain TMS-evoked potential (TEP) indices. TMS-EEG waveforms and components were determined by global mean field amplitude. We used the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) to measure participants' cognitive function.

Results

Patients with MDD had a lower excitatory P180 index compared to healthy controls, and P180 amplitude was negatively correlated with the severity of depressive and anxiety symptoms in patients with MDD. In the MDD group, P30 amplitude was negatively associated with RBANS Visuospatial/ Constructional index and total score.

Conclusions

TMS-EEG findings suggest that abnormal cortical excitation and inhibition induced by TMS on the DLPFC are associated with the severity of clinical symptoms and cognitive dysfunction in patients with MDD. P180 and P30 have the potential to serve as neurophysiological biomarkers of clinical symptoms and cognitive dysfunction in MDD patients, respectively.

Investigating cortical excitability and inhibition in patients with schizophrenia: A TMS-EEG study

BACKGROUND

Transcranial magnetic stimulation (TMS) combined with electromyography (EMG) has widely been used as a non-invasive brain stimulation tool to assess excitation/inhibition (E/I) balance. E/I imbalance is a putative mechanism underlying symptoms in patients with schizophrenia. Combined TMS-electroencephalography (TMS-EEG) provides a detailed examination of cortical excitability to assess the pathophysiology of schizophrenia. This study aimed to investigate differences in TMS-evoked potentials (TEPs), TMS-related spectral perturbations (TRSP) and intertrial coherence (ITC) between patients with schizophrenia and healthy controls.

MATERIALS AND METHODS

TMS was applied over the motor cortex during EEG recording. Differences in TEPs, TRSP and ITC between the patient and healthy subjects were analysed for all electrodes at each time point, by applying multiple independent sample t-tests with a cluster-based permutation analysis to correct for multiple comparisons.

RESULTS

Patients demonstrated significantly reduced amplitudes of early and late TEP components compared to healthy controls. Patients also showed a significant reduction of early delta (50-160 ms) and theta TRSP (30-250ms),followed by a reduction in alpha and beta suppression (220-560 ms; 190-420 ms). Patients showed a reduction of both early (50-110 ms) gamma increase and later (180-230 ms) gamma suppression. Finally, the ITC was significantly lower in patients in the alpha band, from 30 to 260 ms.

CONCLUSION

Our findings support the putative role of impaired GABA-receptor mediated inhibition in schizophrenia impacting excitatory neurotransmission. Further studies can usefully elucidate mechanisms underlying specific symptoms clusters using TMS-EEG biometrics.

Decreased short-latency afferent inhibition in individuals with mild cognitive impairment: A TMS-EEG study

TMS combined with EEG (TMS-EEG) is a tool to characterize the neurophysiological dynamics of the cortex. Among the TMS paradigms, short-latency afferent inhibition (SAI) allows the investigation of inhibitory effects mediated by the cholinergic system. The aim of this study was to compare cholinergic function in the DLPFC between individuals with mild cognitive impairment (MCI) and healthy controls (HC) using TMS-EEG with the SAI paradigm. In this study, 30 MCI and 30 HC subjects were included. The SAI paradigm consisted of 80 single pulse TMS and 80 SAI stimulations applied to the left DLPFC. N100 components, global mean field power (GMFP) and total power were calculated. As a result, individuals with MCI showed reduced inhibitory effects on N100 components and GMFP at approximately 100 ms post-stimulation and on β-band activity at 200 ms post-stimulation compared to HC. Individuals with MCI showed reduced SAI, suggesting impaired cholinergic function in the DLPFC compared to the HC group. We conclude that these findings underscore the clinical applicability of the TMS-EEG method as a powerful tool for assessing cholinergic function in individuals with MCI.

Young Adults with Anxiety Disorders Show Reduced Inhibition in the Dorsolateral Prefrontal Cortex at Higher Trait Anxiety Levels: A TMS-EEG Study

Background

The neuropathology of anxiety disorders, including specific phobias, social phobias, and generalized anxiety disorders (GAD), has been believed to be rooted in a reduced inhibition of limbic areas by the dorsolateral prefrontal cortex (DLPFC). Trait anxiety has been linked to insufficient recruitment of DLPFC mechanisms for attentional control. Despite limited research on individuals with anxiety disorders, our study utilized transcranial magnetic stimulation to assess DLPFC cortical activity and emotional states using the N100 as an indicator of GABA-B-mediated cortical inhibition. Additionally, we aimed to correlate trait anxiety scores with cortical activity.

Methods

A total of 20 subjects with social phobia and GAD and 21 subjects with specific phobia were compared to 24 control subjects regarding their inhibitory N100 in the DLPFC. Therefore, TMS was applied on the left and right DLPFC during an emotional task with fearful, angry, and neutral faces and a rest condition.

Results

Smaller N100 amplitudes after DLPFC stimulation were found in subjects with social phobia, GAD, and social phobias compared to the control group. Furthermore, a correlation between trait anxiety scores and smaller N100 amplitudes, independent of group effects, was found.

Conclusion

There appears to be a decrease in GABA-B-mediated cortical inhibition in the DLPFC in subjects with anxiety disorders. The correlation between trait anxiety and N100 amplitudes suggests a trait-related modulation of cortical inhibition.

Real-time cortical dynamics during motor inhibition

The inhibition of action is a fundamental executive mechanism of human behaviour that involve a complex neural network. In spite of the progresses made so far, many questions regarding the brain dynamics occurring during action inhibition are still unsolved. Here, we used a novel approach optimized to investigate real-time effective brain dynamics, which combines transcranial magnetic stimulation (TMS) with simultaneous electroencephalographic (EEG) recordings. 22 healthy volunteers performed a motor Go/NoGo task during TMS of the hand-hotspot of the primary motor cortex (M1) and whole-scalp EEG recordings. We reconstructed source-based real-time spatiotemporal dynamics of cortical activity and cortico-cortical connectivity throughout the task. Our results showed a task-dependent bi-directional change in theta/gamma supplementary motor cortex (SMA) and M1 connectivity that, when participants were instructed to inhibit their response, resulted in an increase of a specific TMS-evoked EEG potential (N100), likely due to a GABA-mediated inhibition. Interestingly, these changes were linearly related to reaction times, when participants were asked to produce a motor response. In addition, TMS perturbation revealed a task-dependent long-lasting modulation of SMA-M1 natural frequencies, i.e. alpha/beta activity. Some of these results are shared by animal models and shed new light on the physiological mechanisms of motor inhibition in humans.

Changes in Cortical Activation by Transcranial Magnetic Stimulation Due to Coil Rotation Are Not Attributable to Cranial Muscle Activation

Transcranial magnetic stimulation coupled with electroencephalography (TMS-EEG) allows for the study of brain dynamics in health and disease. Cranial muscle activation can decrease the interpretability of TMS-EEG signals by masking genuine EEG responses and increasing the reliance on preprocessing methods but can be at least partly prevented by coil rotation coupled with the online monitoring of signals; however, the extent to which changing coil rotation may affect TMS-EEG signals is not fully understood. Our objective was to compare TMS-EEG data obtained with an optimal coil rotation to induce motor evoked potentials (M1standard) while rotating the coil to minimize cranial muscle activation (M1emg). TMS-evoked potentials (TEPs), TMS-related spectral perturbation (TRSP), and intertrial phase clustering (ITPC) were calculated in both conditions using two different preprocessing pipelines based on independent component analysis (ICA) or signal-space projection with source-informed reconstruction (SSP-SIR). Comparisons were performed with cluster-based correction. The concordance correlation coefficient was computed to measure the similarity between M1standard and M1emg TMS-EEG signals. TEPs, TRSP, and ITPC were significantly larger in M1standard than in M1emg conditions; a lower CCC than expected was also found. These results were similar across the preprocessing pipelines. While rotating the coil may be advantageous to reduce cranial muscle activation, it may result in changes in TMS-EEG signals; therefore, this solution should be tailored to the specific experimental context.

Transcranial Magnetic Stimulation-Electroencephalography for Biomarker Discovery in Psychiatry

Current diagnosis and treatment of psychiatric illnesses are still based on behavioral observations and self-reports, commonly leading to prolonged untreated illness. Biological markers (biomarkers) may offer an opportunity to revolutionize clinical psychiatry practice by helping provide faster and potentially more effective therapies. Transcranial magnetic stimulation concurrent with electroencephalography (TMS-EEG) is a noninvasive brain mapping methodology that can assess the functions and dynamics of specific brain circuitries in awake humans and aid in biomarker discovery. This article provides an overview of TMS-EEG-based biomarkers that may hold potential in psychiatry. The methodological readiness of the TMS-EEG approach and steps in the validation of TMS-EEG biomarkers for clinical utility are discussed. Biomarker discovery with TMS-EEG is in the early stages, and several validation steps are still required before clinical implementations are realized. Thus far, TMS-EEG predictors of response to magnetic brain stimulation treatments in particular have shown promise for translation to clinical practice. Larger-scale studies can confirm validation followed by biomarker-informed trials to assess added value compared to existing practice.

Corticospinal excitability reductions during action preparation and action stopping in humans: Different sides of the same inhibitory coin?

Motor functions and cognitive processes are closely associated with each other. In humans, this linkage is reflected in motor system state changes both when an action must be prepared and stopped. Single-pulse transcranial magnetic stimulation showed that both action preparation and action stopping are accompanied by a reduction of corticospinal excitability, referred to as preparatory and response inhibition, respectively. While previous efforts have been made to describe both phenomena extensively, an updated and comprehensive comparison of the two phenomena is lacking. To ameliorate such deficit, this review focuses on the role and interpretation of single-coil (single-pulse and paired-pulse) and dual-coil TMS outcome measures during action preparation and action stopping in humans. To that effect, it aims to identify commonalities and differences, detailing how TMS-based outcome measures are affected by states, traits, and psychopathologies in both processes. Eventually, findings will be compared, and open questions will be addressed to aid future research.

Baseline markers of cortical excitation and inhibition predict response to theta burst stimulation treatment for youth depression

Theta burst stimulation (TBS), a specific form of repetitive transcranial magnetic stimulation (TMS), is a promising treatment for youth with Major Depressive Disorder (MDD) who do not respond to conventional therapies. However, given the variable response to TBS, a greater understanding of how baseline features relate to clinical response is needed to identify which patients are most likely to benefit from this treatment. In the current study, we sought to determine if baseline neurophysiology, specifically cortical excitation and/or inhibition, is associated with antidepressant response to TBS. In two independent open-label clinical trials, youth (aged 16-24 years old) with MDD underwent bilateral dorsolateral prefrontal cortex (DLPFC) TBS treatment. Clinical trial one and two consisted of 10 and 20 daily sessions of bilateral DLPFC TBS, respectively. At baseline, single-pulse TMS combined with electroencephalography was used to assess the neurophysiology of 4 cortical sites: bilateral DLPFC and inferior parietal lobule. Measures of cortical excitation and inhibition were indexed by TMS-evoked potentials (i.e., P30, N45, P60, N100, and P200). Depression severity was measured before, during and after treatment completion using the Hamilton Rating Scale for Depression-17. In both clinical trials, the baseline left DLPFC N45 and P60, which are believed to reflect inhibitory and excitatory mechanisms respectively, were predictors of clinical response. Specifically, greater (i.e., more negative) N45 and smaller P60 baseline values were associated with greater treatment response to TBS. Accordingly, cortical excitation and inhibition circuitry of the left DLPFC may have value as a TBS treatment response biomarker for youth with MDD.Clinical trial 1 registration number: NCT02472470 (June 15, 2015).Clinical trial 2 registration number: NCT03708172 (October 17, 2018).

The external evocation and movement-related modulation of motor cortex inhibition in children and adolescents with Tourette syndrome - a TMS/EEG study

Objective

This study tested the reactivity of motor cortex inhibition to different intensities of external stimulation by transcranial magnetic stimulation (TMS) and its internal modulation during different motor states in children and adolescents with Tourette syndrome.

Methods

TMS-evoked N100 served as an indirect measure of GABAB receptor function which is related to cortical inhibition. Combined TMS/EEG was used to analyze the TMS-evoked N100 component evoked by different stimulation intensities as well as during resting condition, movement preparation (contingent negative variation task) and movement execution. The study included 18 early adolescents with Tourette syndrome and 15 typically developing control subjects.

Results

TMS-evoked N100 showed a less steep increase with increasing TMS intensity in Tourette syndrome together with less modulation (disinhibition) over the primary motor cortex during the motor states movement preparation and movement execution. Children with Tourette syndrome showed equally high N100 amplitudes at 110% resting motor threshold (RMT) intensity during resting condition and a parallel decline of RMT and N100 amplitude with increasing age as control subjects.

Conclusion

Our study yields preliminary evidence that modulation of motor cortical inhibitory circuits, during external direct stimulation by different TMS intensities and during volitional movement preparation and execution is different in children and adolescents with Tourette syndrome compared to controls. These results suggest that a reduced resting motor cortical inhibitory "reserve" could contribute to the production of unwanted movements. Our findings are compatible with increased regulation of motor cortex excitability by perception-action binding in Tourette syndrome instead of top-down / motor regulation and need to be replicated in further studies.

TMS-evoked EEG potentials demonstrate altered cortical excitability in migraine with aura

Migraine is associated with altered sensory processing, that may be evident as changes in cortical responsivity due to altered excitability, especially in migraine with aura. Cortical excitability can be directly assessed by combining transcranial magnetic stimulation with electroencephalography (TMS-EEG). We measured TMS evoked potential (TEP) amplitude and response consistency as these measures have been linked to cortical excitability but were not yet reported in migraine.We recorded 64-channel EEG during single-pulse TMS on the vertex interictally in 10 people with migraine with aura and 10 healthy controls matched for age, sex and resting motor threshold. On average 160 pulses around resting motor threshold were delivered through a circular coil in clockwise and counterclockwise direction. Trial-averaged TEP responses, frequency spectra and phase clustering (over the entire scalp as well as in frontal, central and occipital midline electrode clusters) were compared between groups, including comparison to sham-stimulation evoked responses.Migraine and control groups had a similar distribution of TEP waveforms over the scalp. In migraine with aura, TEP responses showed reduced amplitude around the frontal and occipital N100 peaks. For the migraine and control groups, responses over the scalp were affected by current direction for the primary motor cortex, somatosensory cortex and sensory association areas, but not for frontal, central or occipital midline clusters.This study provides evidence of altered TEP responses in-between attacks in migraine with aura. Decreased TEP responses around the N100 peak may be indicative of reduced cortical GABA-mediated inhibition and expand observations on enhanced cortical excitability from earlier migraine studies using more indirect measurements.

Transferability of cathodal tDCS effects from the primary motor to the prefrontal cortex: A multimodal TMS-EEG study

Neurophysiological effects of transcranial direct current stimulation (tDCS) have been extensively studied over the primary motor cortex (M1). Much less is however known about its effects over non-motor areas, such as the prefrontal cortex (PFC), which is the neuronal foundation for many high-level cognitive functions and involved in neuropsychiatric disorders. In this study, we, therefore, explored the transferability of cathodal tDCS effects over M1 to the PFC. Eighteen healthy human participants (11 males and 8 females) were involved in eight randomized sessions per participant, in which four cathodal tDCS dosages, low, medium, and high, as well as sham stimulation, were applied over the left M1 and left PFC. After-effects of tDCS were evaluated via transcranial magnetic stimulation (TMS)-electroencephalography (EEG), and TMS-elicited motor evoked potentials (MEP), for the outcome parameters TMS-evoked potentials (TEP), TMS-evoked oscillations, and MEP amplitude alterations. TEPs were studied both at the regional and global scalp levels. The results indicate a regional dosage-dependent nonlinear neurophysiological effect of M1 tDCS, which is not one-to-one transferable to PFC tDCS. Low and high dosages of M1 tDCS reduced early positive TEP peaks (P30, P60), and MEP amplitudes, while an enhancement was observed for medium dosage M1 tDCS (P30). In contrast, prefrontal low, medium and high dosage tDCS uniformly reduced the early positive TEP peak amplitudes. Furthermore, for both cortical areas, regional tDCS-induced modulatory effects were not observed for late TEP peaks, nor TMS-evoked oscillations. However, at the global scalp level, widespread effects of tDCS were observed for both, TMS-evoked potentials and oscillations. This study provides the first direct physiological comparison of tDCS effects applied over different brain areas and therefore delivers crucial information for future tDCS applications.

Regional Precuneus Cortical Hyperexcitability in Alzheimer's Disease Patients

OBJECTIVE

Neuronal excitation/inhibition (E/I) imbalance is a potential cause of neuronal network malfunctioning in Alzheimer's disease (AD), contributing to cognitive dysfunction. Here, we used a novel approach combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to probe cortical excitability in different brain areas known to be directly involved in AD pathology.

METHODS

We performed TMS-EEG recordings targeting the left dorsolateral prefrontal cortex (l-DLPFC), the left posterior parietal cortex (l-PPC), and the precuneus (PC) in a large sample of patients with mild-to-moderate AD (n = 65) that were compared with a group of age-matched healthy controls (n = 21).

RESULTS

We found that patients with AD are characterized by a regional cortical hyperexcitability in the PC and, to some extent, in the frontal lobe, as measured by TMS-evoked potentials. Notably, cortical excitability assessed over the l-PPC was comparable between the 2 groups. Furthermore, we found that the individual level of PC excitability was associated with the level of cognitive impairment, as measured with Mini-Mental State Examination, and with corticospinal fluid levels of Aβ₄₂ .

INTERPRETATION

Our data provide novel evidence that precuneus cortical hyperexcitability is a key feature of synaptic dysfunction in patients with AD. The current results point to the combined approach of TMS and EEG as a novel promising technique to measure hyperexcitability in patients with AD. This index could represent a useful biomarker to stage disease severity and evaluate response to novel therapies. ANN NEUROL 2023;93:371-383.

Co-ultramicronized palmitoylethanolamide/luteolin normalizes GABAB-ergic activity and cortical plasticity in long COVID-19 syndrome

OBJECTIVE

Transcranial magnetic stimulation (TMS) studies showed that patients with cognitive dysfunction and fatigue after COVID-19 exhibit impaired cortical GABA_B-ergic activity, as revealed by reduced long-interval intracortical inhibition (LICI). Aim of this study was to test the effects of co-ultramicronized palmitoylethanolamide/luteolin (PEA-LUT), an endocannabinoid-like mediator able to enhance GABA-ergic transmission and to reduce neuroinflammation, on LICI.

METHODS

Thirty-nine patients (26 females, mean age 49.9 ± 11.4 years, mean time from infection 296.7 ± 112.3 days) suffering from persistent cognitive difficulties and fatigue after mild COVID-19 were randomly assigned to receive either PEA-LUT 700 mg + 70 mg or PLACEBO, administered orally bid for eight weeks. The day before (PRE) and at the end of the treatment (POST), they underwent TMS protocols to assess LICI. We further evaluate short-latency afferent inhibition (SAI) and long-term potentiation (LTP)-like cortical plasticity.

RESULTS

Patients treated with PEA-LUT but not with PLACEBO showed a significant increase of LICI and LTP-like cortical plasticity. SAI remained unaffected.

CONCLUSIONS

Eight weeks of treatment with PEA-LUT restore GABA_B activity and cortical plasticity in long Covid patients.

SIGNIFICANCE

This study confirms altered physiology of the motor cortex in long COVID-19 syndrome and indicates PEA-LUT as a candidate for the treatment of this post-viral condition.

Concurrent TMS-EEG and EEG reveal neuroplastic and oscillatory changes associated with self-compassion and negative emotions

Background/Objective

Self-compassion has a consensual relevance for overall mental health, but its mechanisms remain unknown. Using intermittent theta burst stimulation (iTBS) and concurrent transcranial magnetic stimulation-electroencephalography (TMS-EEG), this study investigated the causal relationship of the dorsolateral prefrontal cortex (DLPFC) with self-compassion and explored the changes in neuroplasticity and neural dynamics.

Method

Thirty-two healthy participants received iTBS or sham stimulation over the DLPFC, before and after which they were instructed to either use self-compassionate strategies or to be rejected in the context of social rejection and to report the level of self-compassion or negative affect. TMS-evoked potentials were evaluated as novel neuroplastic techniques with N45, P60, N100, and P180.

Results

iTBS uniquely decreased P180 amplitude measured with TMS-EEG whereby sham stimulation had no effect on neuroplasticity. In line with neuroplasticity changes, iTBS enhanced a widespread gamma band power and coherence, which correlated consistently with increased engagement in self-compassion. Meanwhile, iTBS demonstrated opposite effects on theta activity dependent on the social contexts whereby self-compassion decreased and social rejection enhanced it respectively. This unique effect of iTBS on theta activity was also supplemented by the enhancement of theta band coherence following iTBS.

Conclusions

We found a causal relationship between DLPFC and self-compassion. We also provide evidence to indicate widespread gamma activity and connectivity to correlate with self-compassion as well as the critical role of the DLPFC in modulating theta activity and negative emotions.

Identification and verification of a 'true' TMS evoked potential in TMS-EEG

The concurrent combination of transcranial magnetic stimulation and electroencephalography (TMS-EEG) can unveil functional neural mechanisms with applications in basic and clinical research. In particular, TMS-evoked potentials (TEPs) potentially allow studying excitability and connectivity of the cortex in a causal manner that is not easily or non-invasively attainable with other neuroimaging techniques. The TEP waveform is obtained by isolating the EEG responses phase-locked to the time of TMS application. The intended component in a TEP waveform is the cortical activation by the TMS-induced electric current, free of instrumental and physiological artifact sources. This artifact-free cortical activation can be referred to as 'true' TEP. However, due to many unwanted auxiliary effects of TMS, the interpretation of 'true' TEPs has not been free of controversy. This paper reviews the most recent understandings of 'true' TEPs and their application. In the first part of the paper, TEP components are defined according to recommended methodologies. In the second part, the verification of 'true' TEP is discussed along with its sensitivity to brain-state, age, and disease. The various proposed origins of TEP components are then presented in the context of existing literature. Throughout the paper, lessons learned from the past TMS-EEG studies are highlighted to guide the identification and interpretation of 'true' TEPs in future studies.

Ultrasound stimulation of the motor cortex during tonic muscle contraction

Transcranial ultrasound stimulation (tUS) shows potential as a noninvasive brain stimulation (NIBS) technique, offering increased spatial precision compared to other NIBS techniques. However, its reported effects on primary motor cortex (M1) are limited. We aimed to better understand tUS effects in human M1 by performing tUS of the hand area of M1 (M1hand) during tonic muscle contraction of the index finger. Stimulation during muscle contraction was chosen because of the transcranial magnetic stimulation-induced phenomenon known as cortical silent period (cSP), in which transcranial magnetic stimulation (TMS) of M1hand involuntarily suppresses voluntary motor activity. Since cSP is widely considered an inhibitory phenomenon, it presents an ideal parallel for tUS, which has often been proposed to preferentially influence inhibitory interneurons. Recording electromyography (EMG) of the first dorsal interosseous (FDI) muscle, we investigated effects on muscle activity both during and after tUS. We found no change in FDI EMG activity concurrent with tUS stimulation. Using single-pulse TMS, we found no difference in M1 excitability before versus after sparsely repetitive tUS exposure. Using acoustic simulations in models made from structural MRI of the participants that matched the experimental setups, we estimated in-brain pressures and generated an estimate of cumulative tUS exposure experienced by M1hand for each subject. We were unable to find any correlation between cumulative M1hand exposure and M1 excitability change. We also present data that suggest a TMS-induced MEP always preceded a near-threshold cSP.

Transcranial Magnetic Stimulation and Neocortical Neurons: The Micro-Macro Connection

Understanding the operation of cortical circuits is an important and necessary task in both neuroscience and neurorehabilitation. The functioning of the neocortex results from integrative neuronal activity, which can be probed non-invasively by transcranial magnetic stimulation (TMS). Despite a clear indication of the direct involvement of cortical neurons in TMS, no explicit connection model has been made between the microscopic neuronal landscape and the macroscopic TMS outcome. Here we have performed an integrative review of multidisciplinary evidence regarding motor cortex neurocytology and TMS-related neurophysiology with the aim of elucidating the micro–macro connections underlying TMS. Neurocytological evidence from animal and human studies has been reviewed to describe the landscape of the cortical neurons covering the taxonomy, morphology, circuit wiring, and excitatory–inhibitory balance. Evidence from TMS studies in healthy humans is discussed, with emphasis on the TMS pulse and paradigm selectivity that reflect the underlying neural circuitry constitution. As a result, we propose a preliminary neuronal model of the human motor cortex and then link the TMS mechanisms with the neuronal model by stimulus intensity, direction of induced current, and paired-pulse timing. As TMS bears great developmental potential for both a probe and modulator of neural network activity and neurotransmission, the connection model will act as a foundation for future combined studies of neurocytology and neurophysiology, as well as the technical advances and application of TMS.

Cerebellar noninvasive neuromodulation influences the reactivity of the contralateral primary motor cortex and surrounding areas: a TMS-EMG-EEG study

Understanding cerebellar-cortical physiological interactions is of fundamental importance to advance the efficacy of neurorehabilitation strategies for patients with cerebellar damage. Previous works have aimed to modulate this pathway by applying transcranial electrical or magnetic stimulation (TMS) over the cerebellum and probing the resulting changes in the primary motor cortex (M1) excitability with motor-evoked potentials (MEPs). While these protocols produce changes in cerebellar excitability, their ability to modulate MEPs has produced inconsistent results, mainly due to the MEP being a highly variable outcome measure that is susceptible to fluctuations in the excitability of M1 neurons and spinal interneurons. To overcome this limitation, we combined TMS with electroencephalography (EEG) to directly record TMS-evoked potentials (TEPs) and oscillations from the scalp. In three sessions, we applied intermittent theta-burst stimulation (iTBS), cathodal direct current stimulation (c-DC) or sham stimulation to modulate cerebellar activity. To assess the effects on M1 and nearby cortex, we recorded TMS-EEG and MEPs before, immediately after (T1) and 15 min (T2) following cerebellar neuromodulation. We found that cerebellar iTBS immediately increased TMS-induced alpha oscillations and produced lasting facilitatory effects on TEPs, whereas c-DC immediately decreased TMS-induced alpha oscillations and reduced TEPs. We also found increased MEP following iTBS but not after c-DC. All of the TMS-EEG measures showed high test-retest repeatability. Overall, this work importantly shows that cerebellar neuromodulation influences both cortical and corticospinal physiological measures; however, they are more pronounced and detailed when utilizing TMS-EEG outcome measures. These findings highlight the advantage of using TMS-EEG over MEPs when assessing the effects of neuromodulation.

EEG Evoked Potentials to Repetitive Transcranial Magnetic Stimulation in Normal Volunteers: Inhibitory TMS EEG Evoked Potentials

The impact of repetitive magnetic stimulation (rTMS) on cortex varies with stimulation parameters, so it would be useful to develop a biomarker to rapidly judge effects on cortical activity, including regions other than motor cortex. This study evaluated rTMS-evoked EEG potentials (TEP) after 1 Hz of motor cortex stimulation. New features are controls for baseline amplitude and comparison to control groups of sham stimulation. We delivered 200 test pulses at 0.20 Hz before and after 1500 treatment pulses at 1 Hz. Sequences comprised AAA = active stimulation with the same coil for test–treat–test phases (n = 22); PPP = realistic placebo coil stimulation for all three phases (n = 10); and APA = active coil stimulation for tests and placebo coil stimulation for treatment (n = 15). Signal processing displayed the evoked EEG waveforms, and peaks were measured by software. ANCOVA was used to measure differences in TEP peak amplitudes in post-rTMS trials while controlling for pre-rTMS TEP peak amplitude. Post hoc analysis showed reduced P60 amplitude in the active (AAA) rTMS group versus the placebo (APA) group. The N100 peak showed a treatment effect compared to the placebo groups, but no pairwise post hoc differences. N40 showed a trend toward increase. Changes were seen in widespread EEG leads, mostly ipsilaterally. TMS-evoked EEG potentials showed reduction of the P60 peak and increase of the N100 peak, both possibly reflecting increased slow inhibition after 1 Hz of rTMS. TMS-EEG may be a useful biomarker to assay brain excitability at a seizure focus and elsewhere, but individual responses are highly variable, and the difficulty of distinguishing merged peaks complicates interpretation.

Concurrent TMS-EEG to Reveal the Neuroplastic Changes in the Prefrontal and Insular Cortices in the Analgesic Effects of DLPFC-rTMS

The dorsolateral prefrontal cortex (DLPFC) is an important target for repetitive transcranial magnetic stimulation (rTMS) to reduce pain. However, the analgesic efficacy of DLPFC-rTMS needs to be optimized, in which the mechanisms of action remain unclear. Concurrent TMS and electroencephalogram (TMS-EEG) is able to evaluate neuroplastic changes beyond the motor cortex. Using TMS-EEG, this study was designed to investigate the local and distributed neuroplastic changes associated with DLPFC analgesia. Thirty-four healthy adults received DLPFC or sham stimulation in a randomized, crossover design. In each session, participants underwent cold pain and TMS-EEG assessment both before and after 10-Hz rTMS. We provide novel findings that DLPFC analgesia is associated with a smaller N120 amplitude in the contralateral prefrontal cortex as well as with a larger N120 peak in the ipsilateral insular cortex. Furthermore, there was a strong negative correlation between N120 changes of these two regions whereby the amplitude changes of this dyad were associated with increased pain threshold. In addition, DLPFC stimulation enhanced coherence between the prefrontal and somatosensory cortices oscillating in the gamma frequency. Overall, our data present novel evidence on local and distributed neuroplastic changes associated with DLPFC analgesia.

Bridging the gap: TMS-EEG from Lab to Clinic

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) has reached technological maturity and has been an object of significant scientific interest for over two decades. Ιn parallel, accumulating evidence highlights the potential of TMS-EEG as a useful tool in the field of clinical neurosciences. Nevertheless, its clinical utility has not yet been established, partly because technical and methodological limitations have created a gap between an evolving scientific tool and standard clinical practice. Here we review some of the identified gaps that still prevent TMS-EEG moving from science laboratories to clinical practice. The principal and partly overlapping gaps include: 1) complex and laborious application, 2) difficulty in obtaining high-quality signals, 3) suboptimal accuracy and reliability, and 4) insufficient understanding of the neurobiological substrate of the responses. All these four aspects need to be satisfactorily addressed for the method to become clinically applicable and enter the diagnostic and therapeutic arena. In the current review, we identify steps that might be taken to address these issues and discuss promising recent studies providing tools to aid bridging the gaps.

Fearful facial expressions reduce inhibition levels in the dorsolateral prefrontal cortex in subjects with specific phobia

BACKGROUND

Specific phobias have the highest prevalence among anxiety disorders. Cognitive control involving the dorsolateral prefrontal cortex (DLPFC) is crucial for coping abilities in anxiety disorders. However, there is little research on the DLPFC in specific phobia.

METHODS

Using transcranial magnetic stimulation (TMS), we investigated the TMS-evoked potential component N100 in the DLPFC at rest and while watching emotional expressions. The TMS-evoked N100 provides a parameter for gamma-aminobutyric acid (GABA)-B-mediated cortical inhibition. Twenty-two drug-free subjects with specific phobia (21 females and 1 male) were compared with 26 control subjects (23 females and 3 males) regarding N100 in the DLPFC at rest and during an emotional 1-back task with fearful, angry, and neutral facial expressions.

RESULTS

At rest, we found reduced N100 amplitudes in the specific phobia compared with the control group. Furthermore, the specific phobia group showed a further reduction in N100 amplitude when memorizing fearful compared with neutral facial expressions.

CONCLUSION

There appears to be a decrease in GABA-B-mediated inhibition in the DLPFC in subjects with a specific phobia at rest. This decrease was more pronounced under emotional activation by exposure to fearful facial expressions, pointing towards additional state effects of emotional processing on inhibitory function in the DLPFC.

A Brief History and the Significance of the GABAB Receptor

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. GABA type B (GABA_B) receptors (GABA_BRs) are the only metabotropic G protein-coupled receptors for GABA and can be found distributed not only in the central nervous system, but also in the periphery. This chapter introduces important, fundamental knowledge related to GABA_BR function and the various potential therapeutic applications of the development of novel GABA_BR-active compounds, as documented through extensive studies presented in subsequent chapters of this Current Topic in Behavioral Neurosciences volume on the role of the neurobiology of GABA_BR function. The compounds that have received increased attention in the last few years compared to GABA_BR agonists and antagonists - the positive allosteric modulators - exhibit better pharmacological profiles and fewer side effects. As we continue to unveil the mystery of GABA_BRs at the molecular and cellular levels, we further understand the significance of these receptors. Future directions should aim for developing highly selective GABA_BR compounds for treating neuropsychiatric disorders and their symptomatology.

Modulation of Methamphetamine-Related Attention Bias by Intermittent Theta-Burst Stimulation on Left Dorsolateral Prefrontal Cortex

Background

Previous studies have identified the treatment effect of repetitive transcranial magnetic stimulation (rTMS) on cravings of patients with methamphetamine use disorder (MUD). However, the mechanism underlying the treatment effect remains largely unknown. A potential candidate mechanism could be that rTMS over the dorsolateral prefrontal cortex (DLPFC) modulates the attention bias to methamphetamine-related cues. The purpose of this study is therefore to determine the modulation of rTMS on methamphetamine-related attention bias and the corresponding electrophysiological changes.

Methods

Forty-nine patients with severe MUD were included for analysis. The subjects were randomized to receive the active intermittent theta-burst stimulation (iTBS) or sham iTBS targeting DLPFC for 20 sessions. Participants performed the Addiction Stroop Task before and after the treatment while being recorded by a 64-channel electroencephalogram. Baseline characteristics were collected through the Addiction Severity Index.

Results

Post-treatment evaluations showed a reduced error rate in discriminating the color of methamphetamine words in the active iTBS group compared with the sham iTBS group. Following rTMS treatment, we found the significant time-by-group effect for the N1 amplitude (methamphetamine words > neutral words) and P3 latency (methamphetamine words > neutral words). The change of N1 amplitude was positively correlated with cravings in the active group. Moreover, reduced power of neural oscillation in the beta band, manifesting at frontal central areas, was also found in the active group.

Conclusion

This study suggests that attention bias and the beta oscillation during the attentional processing of methamphetamine words in patients with MUD could be modulated by iTBS applied to left DLPFC.

Investigating the Effect of Transcutaneous Auricular Vagus Nerve Stimulation on Cortical Excitability in Healthy Males

OBJECTIVES

As a potential treatment for epilepsy, transcutaneous auricular vagus nerve stimulation (taVNS) has yielded inconsistent results. Combining transcranial magnetic stimulation with electromyography (TMS-EMG) and electroencephalography (TMS-EEG) can be used to investigate the effect of interventions on cortical excitability by evaluating changes in motor evoked potentials (MEPs) and TMS-evoked potentials (TEPs). The goal of this study is to objectively evaluate the effect of taVNS on cortical excitability with TMS-EMG and TMS-EEG. These findings are expected to provide insight in the mechanism of action and help identify more optimal stimulation paradigms.

MATERIALS AND METHODS

In this prospective single-blind cross-over study, 15 healthy male subjects underwent active and sham taVNS for 60 min, using a maximum tolerated stimulation current. Single and paired pulse TMS was delivered over the right-sided motor hotspot to evaluate MEPs and TEPs before and after the intervention. MEP statistical analysis was conducted with a two-way repeated measures ANOVA. TEPs were analyzed with a cluster-based permutation analysis. Linear regression analysis was implemented to investigate an association with stimulation current.

RESULTS

MEP and TEP measurements were not affected by taVNS in this study. An association was found between taVNS stimulation current and MEP outcome measures indicating a decrease in cortical excitability in participants who tolerated higher taVNS currents. A subanalysis of participants (n = 8) who tolerated a taVNS current ≥2.5 mA showed a significant increase in the resting motor threshold, decrease in MEP amplitude and modulation of the P60 and P180 TEP components.

CONCLUSIONS

taVNS did not affect cortical excitability measurements in the overall population in this study. However, taVNS has the potential to modulate specific markers of cortical excitability in participants who tolerate higher stimulation levels. These findings indicate the need for adequate stimulation protocols based on the recording of objective outcome parameters.

Examining and Modulating Neural Circuits in Psychiatric Disorders With Transcranial Magnetic Stimulation and Electroencephalography: Present Practices and Future Developments

Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation technique uniquely equipped to both examine and modulate neural systems and related cognitive and behavioral functions in humans. As an examination tool, TMS can be used in combination with EEG (TMS-EEG) to elucidate directly, objectively, and noninvasively the intrinsic properties of a specific cortical region, including excitation, inhibition, reactivity, and oscillatory activity, irrespective of the individual's conscious effort. Additionally, when applied in repetitive patterns, TMS has been shown to modulate brain networks in healthy individuals, as well as ameliorate symptoms in individuals with psychiatric disorders. The key role of TMS in assessing and modulating neural dysfunctions and associated clinical and cognitive deficits in psychiatric populations is therefore becoming increasingly evident. In this article, the authors review TMS-EEG studies in schizophrenia and mood disorders, as most TMS-EEG studies to date have focused on individuals with these disorders. The authors present the evidence on the efficacy of repetitive TMS (rTMS) and theta burst stimulation (TBS), when targeting specific cortical areas, in modulating neural circuits and ameliorating symptoms and abnormal behaviors in individuals with psychiatric disorders, especially when informed by resting-state and task-related neuroimaging measures. Examples of how the combination of TMS-EEG assessments and rTMS and TBS paradigms can be utilized to both characterize and modulate neural circuit alterations in individuals with psychiatric disorders are also provided. This approach, along with the evaluation of the behavioral effects of TMS-related neuromodulation, has the potential to lead to the development of more effective and personalized interventions for individuals with psychiatric disorders.

TMS-EEG: An emerging tool to study the neurophysiologic biomarkers of psychiatric disorders

The etiology of psychiatric disorders remains largely unknown. The exploration of the neurobiological mechanisms of mental illness helps improve diagnostic efficacy and develop new therapies. This review focuses on the application of concurrent transcranial magnetic stimulation and electroencephalography (TMS-EEG) in various mental diseases, including major depressive disorder, bipolar disorder, schizophrenia, autism spectrum disorder, attention-deficit/hyperactivity disorder, substance use disorder, and insomnia. First, we summarize the commonly used protocols and output measures of TMS-EEG; then, we review the literature exploring the alterations in neural patterns, particularly cortical excitability, plasticity, and connectivity alterations, and studies that predict treatment responses and clinical states in mental disorders using TMS-EEG. Finally, we discuss the potential mechanisms underlying TMS-EEG in establishing biomarkers for psychiatric disorders and future research directions.

Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

In the human brain, aging is characterized by progressive neuronal loss, leading to disruption of synapses and to a degree of failure in neurotransmission. However, there is increasing evidence to support the notion that the aged brain has a remarkable ability to reorganize itself, with the aim of preserving its physiological activity. It is important to develop objective markers able to characterize the biological processes underlying brain aging in the intact human, and to distinguish them from brain degeneration associated with many neurological diseases. Transcranial magnetic stimulation (TMS), coupled with electromyography or electroencephalography (EEG), is particularly suited to this aim, due to the functional nature of the information provided, and thanks to the ease with which it can be integrated with behavioral manipulation. In this review, we aimed to provide up to date information about the role of TMS and TMS-EEG in the investigation of brain aging. In particular, we focused on data about cortical excitability, connectivity and plasticity, obtained by using readouts such as motor evoked potentials and transcranial evoked potentials. Overall, findings in the literature support an important potential contribution of TMS to the understanding of the mechanisms underlying normal brain aging. Further studies are needed to expand the current body of information and to assess the applicability of TMS findings in the clinical setting.