Intermittent theta-burst stimulation induces correlated changes in cortical and corticospinal excitability in healthy older subjects

No effect of whole-hand water flow stimulation on skill acquisition and retention during sensorimotor adaptation

Introduction

Repetitive somatosensory stimulation (RSS) is a conventional approach to modulate the neural states of both the primary somatosensory cortex (S1) and the primary motor cortex (M1). However, the impact of RSS on skill acquisition and retention in sensorimotor adaptation remains debated. This study aimed to investigate whether whole-hand water flow (WF), a unique RSS-induced M1 disinhibition, influences sensorimotor adaptation by examining the hypothesis that whole-hand WF leads to M1 disinhibition; thereby, enhancing motor memory retention.

Methods

Sixty-eight young healthy participants were randomly allocated to three groups based on the preconditioning received before motor learning: control, whole-hand water immersion (WI), and whole-hand WF. The experimental protocol for all the participants spanned two consecutive days. On the initial day (day 1), baseline transcranial magnetic stimulation (TMS) assessments (T0) were executed before any preconditioning. Subsequently, each group underwent their respective 30 min preconditioning protocol. To ascertain the influence of each preconditioning on the excitability of the M1, subsequent TMS assessments were conducted (T1). Following this, all participants engaged in the motor learning (ML) of a visuomotor tracking task, wherein they were instructed to align a cursor with a target trajectory by modulating the pinch force. Upon completion of the ML session, final TMS assessments (T2) were conducted. All participants were required to perform the same motor learning 24 h later on day 2.

Results

The results revealed that whole-hand WF did not significantly influence skill acquisition during sensorimotor adaptation, although it did reduce intracortical inhibition. This phenomenon is consistent with the idea that S1, rather than M1, is involved in skill acquisition during the early stages of sensorimotor adaptation. Moreover, memory retention 24 h after skill acquisition did not differ significantly across the three groups, challenging our initial hypothesis that whole-hand WF enhances memory retention throughout sensorimotor adaptation. This could be due to the inability of whole-hand WF to alter sensorimotor connectivity and integration, as well as the nature of the plastic response elicited by the preconditioning.

Discussion

In conclusion, these findings suggest that although whole-hand WF attenuates intracortical inhibition, it does not modulate skill acquisition or motor memory retention during sensorimotor adaptation.

Age-related changes in responsiveness to non-invasive brain stimulation neuroplasticity paradigms: A systematic review with meta-analysis

OBJECTIVES

We aimed to summarise and critically appraise the available evidence for the effect of age on responsiveness to non-invasive brain stimulation (NBS) paradigms delivered to the primary motor cortex.

METHODS

Four databases (Medline, Embase, PsycINFO and Scopus) were searched from inception to February 7, 2023. Studies investigating age group comparisons and associations between age and neuroplasticity induction from NBS paradigms were included. Only studies delivering neuroplasticity paradigms to the primary motor cortex and responses measured via motor-evoked potentials (MEPs) in healthy adults were considered.

RESULTS

39 studies, encompassing 40 experiments and eight NBS paradigms were included: paired associative stimulation (PAS; n = 12), repetitive transcranial magnetic stimulation (rTMS; n = 2), intermittent theta burst stimulation (iTBS; n = 8), continuous theta burst stimulation (cTBS; n = 7), transcranial direct and alternating current stimulation ((tDCS; n = 7; tACS; n = 2)), quadripulse stimulation (QPS; n = 1) and i-wave periodic transcranial magnetic stimulation (iTMS; n = 1). Pooled findings from PAS paradigms suggested older adults have reduced post-paradigm responses, although there was considerable heterogeneity. Mixed results were observed across all other NBS paradigms and post-paradigm timepoints.

CONCLUSIONS/SIGNIFICANCE

Whilst age-dependent reduction in corticospinal excitability is possible, there is extensive inter- and intra-individual variability both within and between studies, making it difficult to draw meaningful conclusions from pooled analyses.

Monitoring Changes in TMS-Evoked EEG and EMG Activity During 1 Hz rTMS of the Healthy Motor Cortex

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive brain stimulation technique capable of inducing neuroplasticity as measured by changes in peripheral muscle electromyography (EMG) or electroencephalography (EEG) from pre-to-post stimulation. However, temporal courses of neuromodulation during ongoing rTMS are unclear. Monitoring cortical dynamics via TMS-evoked responses using EMG (motor-evoked potentials; MEPs) and EEG (transcranial-evoked potentials; TEPs) during rTMS might provide further essential insights into its mode of action - temporal course of potential modulations. The objective of this study was to first evaluate the validity of online rTMS-EEG and rTMS-EMG analyses, and second to scrutinize the temporal changes of TEPs and MEPs during rTMS. As rTMS is subject to high inter-individual effect variability, we aimed for single-subject analyses of EEG changes during rTMS. Ten healthy human participants were stimulated with 1,000 pulses of 1 Hz rTMS over the motor cortex, while EEG and EMG were recorded continuously. Validity of MEPs and TEPs measured during rTMS was assessed in sensor and source space. Electrophysiological changes during rTMS were evaluated with model fitting approaches on a group- and single-subject level. TEPs and MEPs appearance during rTMS was consistent with past findings of single pulse experiments. Heterogeneous temporal progressions, fluctuations or saturation effects of brain activity were observed during rTMS depending on the TEP component. Overall, global brain activity increased over the course of stimulation. Single-subject analysis revealed inter-individual temporal courses of global brain activity. The present findings are in favor of dose-response considerations and attempts in personalization of rTMS protocols.

cTBS over primary motor cortex increased contralateral corticomuscular coupling and interhemispheric functional connection

Objective.Transcranial magnetic stimulation is a non-invasive brain stimulation technique that changes the activity of the cerebral cortex. Contralesional continuous theta burst stimulation (cTBS) has been proposed and verified beneficial to stroke motor recovery. However, the underlying mechanism is still unclear.Approach.20 healthy right-handed subjects were recruited in this study, receiving real-cTBS over their left primary motor cortex or sham-cTBS. We designed the finger tapping task (FTT) before and after stimulation and recorded the accuracy and reaction time (RT) of the task. The electroencephalogram and surface electromyogram signals were recorded during the left finger pinching task (FPT) before and after stimulation. We calculated cortico-muscular coherence (CMC) in the contralateral hemisphere and cortico-cortical coherence (CCC) in the bilateral hemisphere. The two-way repeated measures analysis of variance was used to analyze the effect of cTBS.Main results.In the FTT, there was a significant main effect of 'time' on RT (F(1, 38) = 24.739,p< 0.001). In the FPT, the results showed that there was a significant interaction effect on the CMC peak and area in the beta band (peak:F(1, 38) = 8.562,p= 0.006; area:F(1, 38) = 5.273,p= 0.027), on the CCC peak in the alpha band (F(1, 38) = 4.815,p= 0.034) and area in the beta band (F(1, 38) = 4.822,p= 0.034). The post hoc tests showed that the CMC peak (W= 20,p= 0.002), the CMC area (W= 13,p= 0.003) and the CCC peak (t= -2.696,p= 0.014) increased significantly after real-cTBS. However, there was no significant decrease or increase after sham-cTBS.Significance.Our study found that cTBS can improve CMC of contralateral hemisphere and CCC of bilateral hemisphere, indicating that cTBS can strengthen cortico-muscular and cortico-cortical coupling.

Pharmacological adjuncts and transcranial magnetic stimulation-induced synaptic plasticity: a systematic review

BACKGROUND

Transcranial magnetic stimulation (TMS) is a noninvasive neurostimulation modality that has been used to study human synaptic plasticity. Leveraging work in ex vivo preparations, mechanistically informed pharmacological adjuncts to TMS have been used to improve our fundamental understanding of TMS-induced synaptic plasticity.

METHODS

We systematically reviewed the literature pairing pharmacological adjuncts with TMS plasticity-induction protocols in humans. We searched MEDLINE, PsycINFO, and Embase from 2013 to Mar. 10, 2023. Studies published before 2013 were extracted from a previous systematic review. We included studies using repetitive TMS, theta-burst stimulation, paired associative stimulation, and quadripulse stimulation paradigms in healthy and clinical populations.

RESULTS

Thirty-six studies met our inclusion criteria (28 in healthy and 8 in clinical populations). Most pharmacological agents have targeted the glutamatergic N-methyl-d-aspartate (NMDA; 15 studies) or dopamine receptors (13 studies). The NMDA receptor is necessary for TMS-induced plasticity; however, sufficiency has not been shown across protocols. Dopaminergic modulation of TMS-induced plasticity appears to be dose-dependent. The GABAergic, cholinergic, noradrenergic, and serotonergic neurotransmitter systems have small evidence bases supporting modulation of TMS-induced plasticity, as do voltage-gated calcium and sodium channels. Studies in clinical populations suggest that pharmacological adjuncts to TMS may rescue motor cortex plasticity, with implications for therapeutic applications of TMS and a promising clinical trial in depression.

LIMITATIONS

This review is limited by the predominance in the literature of studies with small sample sizes and crossover designs.

CONCLUSION

Pharmacologically enhanced TMS largely parallels findings from ex vivo preparations. As this area expands and novel targets are tested, adequately powered samples in healthy and clinical populations will inform the mechanisms of TMS-induced plasticity in health and disease.

Using TMS-EEG to assess the effects of neuromodulation techniques: a narrative review

Over the past decades, among all the non-invasive brain stimulation (NIBS) techniques, those aiming for neuromodulatory protocols have gained special attention. The traditional neurophysiological outcome to estimate the neuromodulatory effect is the motor evoked potential (MEP), the impact of NIBS techniques is commonly estimated as the change in MEP amplitude. This approach has several limitations: first, the use of MEP limits the evaluation of stimulation to the motor cortex excluding all the other brain areas. Second, MEP is an indirect measure of brain activity and is influenced by several factors. To overcome these limitations several studies have used new outcomes to measure brain changes after neuromodulation techniques with the concurrent use of transcranial magnetic stimulation (TMS) and electroencephalogram (EEG). In the present review, we examine studies that use TMS-EEG before and after a single session of neuromodulatory TMS. Then, we focused our literature research on the description of the different metrics derived from TMS-EEG to measure the effect of neuromodulation.

Effect of regional intrinsic activity following two kinds of theta burst stimulation on precuneus

Theta burst stimulation (TBS) has been widely used in the treatment of mental disorders, but the cerebral functional difference between intermittent TBS (iTBS) and continuous TBS (cTBS) after one single session of stimulation is not clear. Here we applied resting-state functional magnetic resonance imaging (RS-FMRI) to evaluate the alterations in intrinsic brain activity after iTBS and cTBS in the precuneus. We recruited 32 healthy young adults and performed a single session each of iTBS and cTBS at a 1-week interval. RS-fMRI was collected at baseline before and immediately after the stimulation. Parameters for regional brain activity (ALFF/fALFF/ReHo) and functional connectivity (FC) with the stimulated site of the precuneus after iTBS and cTBS were calculated and compared between each stimulation using a paired t-test. Correlation analysis among those parameters was calculated to explore whether changes in functional connectivity were associated with local spontaneous activity. After iTBS stimulation, fALFF increased in the bilateral precuneus, while fALFF decreased in the bilateral middle temporal gyrus. Reductions in precuneus FC were found in the bilateral cuneus, superior occipital gyrus, superior temporal gyrus, precentral gyrus, and postcentral gyrus, which correlated with regional activity. After cTBS, fALFF decreased in the bilateral insula, and precuneus FC was decreased in the bilateral inferior occipital gyrus and increased in the thalamus. In the current study, we observed that one session of iTBS or cTBS could cause inhibitory effects in remote brain regions, but only iTBS caused significant local activation in the target region.

Corticomotor plasticity as a predictor of response to high frequency transcranial magnetic stimulation treatment for major depressive disorder

BACKGROUND

Many patients with treatment-resistant depression (TRD) respond to repetitive transcranial magnetic stimulation (rTMS) treatment. This study aimed to investigate whether modulation of corticomotor excitability by rTMS predicts response to rTMS treatment for TRD in 10 Hz and intermittent theta-burst stimulation (iTBS) protocols.

METHODS

Thirteen TRD patients underwent two evaluations of corticomotor plasticity-assessed as the post-rTMS (10 Hz, iTBS) percent change (%∆) in motor evoked potential (MEP) amplitude elicited by single-pulse TMS. Following corticomotor plasticity evaluations, patients subsequently underwent a standard 6-week course of 10 Hz rTMS (4 s train, 26 s inter-train interval, 3000 total pulses, 120% of motor threshold) to the left dorsolateral prefrontal cortex. Treatment efficacy was assessed by the Beck Depression Inventory II (BDI-II) and Hamilton Depression Rating Scale (HAM-D). The change in MEPs was compared between 10 Hz and iTBS conditions and related to the change in BDI-II and HAM-D scores.

RESULTS

Analyses of variance revealed that across all time-points, higher post-10 Hz MEP change was a significant predictor of greater improvement on the BDI-II (p < 0.001) and HAM-D (p = 0.022). This relationship was not observed with iTBS (p-values≥0.100). Post-hoc tests revealed the MEP change 20 min post-10 Hz was the strongest predictor of BDI-II improvement.

LIMITATIONS

Cortical excitability was measured from the motor cortex, rather than the dorsolateral prefrontal cortex, where treatment is applied. The 10 Hz and iTBS protocols were performed at different intensities consistent with common practice.

CONCLUSIONS

Modulation of corticomotor excitability by 10 Hz can predict response to rTMS treatment with 10 Hz rTMS.

Prefrontal theta phase-dependent rTMS-induced plasticity of cortical and behavioral responses in human cortex

BACKGROUND

Prefrontal theta oscillations are involved in neuronal information transfer and retention. Phases along the theta cycle represent varied excitability states, whereby high-excitability states correspond to high-frequency neuronal activity and heightened capacity for plasticity induction, as demonstrated in animal studies. Human studies corroborate this model and suggest a core role of prefrontal theta activity in working memory (WM).

OBJECTIVE/HYPOTHESIS

We aimed at modulating prefrontal neuronal excitability and WM performance in healthy humans, using real-time EEG analysis for triggering repetitive transcranial magnetic stimulation (rTMS) theta-phase synchronized to the left dorsomedial prefrontal cortex.

METHODS

16 subjects underwent 3 different rTMS interventions on separate days, with pulses triggered according to the individual's real-time EEG activity: 400 rTMS gamma-frequency (100 Hz) triplet bursts applied during either the negative peak of the prefrontal theta oscillation, the positive peak, or at random phase. Changes in cortical excitability were assessed with EEG responses following single-pulse TMS, and behavioral effects by using a WM task.

RESULTS

Negative-peak rTMS increased single-pulse TMS-induced prefrontal theta power and theta-gamma phase-amplitude coupling, and decreased WM response time. In contrast, positive-peak rTMS decreased prefrontal theta power, while no changes were observed after random-phase rTMS.

CONCLUSION

Findings point to the feasibility of EEG-TMS technology in a theta-gamma phase-amplitude coupling mode for effectively modifying WM networks in human prefrontal cortex, with potential for therapeutic applications.

Intermittent Theta Burst Stimulation Increases Natural Oscillatory Frequency in Ipsilesional Motor Cortex Post-Stroke: A Transcranial Magnetic Stimulation and Electroencephalography Study

Objective

Intermittent theta burst stimulation (iTBS) has been widely used as a neural modulation approach in stroke rehabilitation. Concurrent use of transcranial magnetic stimulation and electroencephalography (TMS-EEG) offers a chance to directly measure cortical reactivity and oscillatory dynamics and allows for investigating neural effects induced by iTBS in all stroke survivors including individuals without recordable MEPs. Here, we used TMS-EEG to investigate aftereffects of iTBS following stroke.

Methods

We studied 22 stroke survivors (age: 65.2 ± 11.4 years; chronicity: 4.1 ± 3.5 months) with upper limb motor deficits. Upper-extremity component of Fugl-Meyer motor function assessment and action research arm test were used to measure motor function of stroke survivors. Stroke survivors were randomly divided into two groups receiving either Active or Sham iTBS applied over the ipsilesional primary motor cortex. TMS-EEG recordings were performed at baseline and immediately after Active or Sham iTBS. Time and time-frequency domain analyses were performed for quantifying TMS-evoked EEG responses.

Results

At baseline, natural frequency was slower in the ipsilesional compared with the contralesional hemisphere (P = 0.006). Baseline natural frequency in the ipsilesional hemisphere was positively correlated with upper limb motor function following stroke (P = 0.007). After iTBS, natural frequency in the ipsilesional hemisphere was significantly increased (P < 0.001).

Conclusions

This is the first study to investigate the acute neural adaptations after iTBS in stroke survivors using TMS-EEG. Our results revealed that natural frequency is altered following stroke which is related to motor impairments. iTBS increases natural frequency in the ipsilesional motor cortex in stroke survivors. Our findings implicate that iTBS holds the potential to normalize natural frequency in stroke survivors, which can be utilized in stroke rehabilitation.

Improving speech perception in noise in young and older adults using transcranial magnetic stimulation

Normal aging is associated with speech perception in noise (SPiN) difficulties. The objective of this study was to determine if SPiN performance can be enhanced by intermittent theta-burst stimulation (iTBS) in young and older adults.

METHOD

We developed a sub-lexical SPiN test to evaluate the contribution of age, hearing, and cognition to SPiN performance in young and older adults. iTBS was applied to the left posterior superior temporal sulcus (pSTS) and the left ventral premotor cortex (PMv) to examine its impact on SPiN performance.

RESULTS

Aging was associated with reduced SPiN accuracy. TMS-induced performance gain was greater after stimulation of the PMv compared to the pSTS. Participants with lower scores in the baseline condition improved the most.

DISCUSSION

SPiN difficulties can be reduced by enhancing activity within the left speech-processing network in adults. This study paves the way for the development of TMS-based interventions to reduce SPiN difficulties in adults.

Intermittent Theta Burst Stimulation to the Primary Motor Cortex Reduces Cortical Inhibition: A TMS-EEG Study

Introduction: The aim of this study was to reveal the effects of intermittent theta burst stimulation (iTBS) in modulating cortical networks using transcranial magnetic stimulation and electroencephalography (TMS-EEG) recording. Methods: Eighteen young adults participated in our study and received iTBS to the primary motor cortex (M1), supplementary motor area, and the primary visual cortex in three separate sessions. A finger tapping task and ipsilateral single-pulse TMS-EEG recording for the M1 were administrated before and after iTBS in each session. The effects of iTBS in motor performance and TMS-evoked potentials (TEPs) were investigated. Results: The results showed that iTBS to the M1, but not supplementary motor area or the primary visual cortex, significantly reduced the N100 amplitude of M1 TEPs in bilateral hemispheres (p = 0.019), with a more prominent effect in the contralateral hemisphere than in the stimulated hemisphere. Moreover, only iTBS to the M1 decreased global mean field power (corrected ps < 0.05), interhemispheric signal propagation (t = 2.53, p = 0.030), and TMS-induced early α-band synchronization (p = 0.020). Conclusion: Our study confirmed the local and remote after-effects of iTBS in reducing cortical inhibition in the M1. TMS-induced oscillations after iTBS for changed cortical excitability in patients with various neurological and psychiatric conditions are worth further exploration.

Reproducibility of cortical response modulation induced by intermittent and continuous theta-burst stimulation of the human motor cortex

Background:

Over the past decade, the number of experimental and clinical studies using theta-burststimulation (TBS) protocols of transcranial magnetic stimulation (TMS) to modulate brain activity has risen substantially. The use of TBS is motivated by the assumption that these protocols can reliably and lastingly modulate cortical excitability despite their short duration and low number of stimuli. However, this assumption, and thus the experimental validity of studies using TBS, is challenged by recent work showing large inter- and intra-subject variability in response to TBS protocols.

Objectives:

To date, the reproducibility of TBS effects in humans has been exclusively assessed with motor evoked potentials (MEPs), which provide an indirect and limited measure of cortical excitability. Here we combined TMS with electroencephalography (TMS-EEG) and report the first comprehensive investigation of (1) direct TMS-evoked cortical responses to intermittent (iTBS) and continuous TBS (cTBS) of the human motor cortex, and (2) reproducibility of both iTBS- and cTBS-induced cortical response modulation against a robust sham control across repeat visits with commonly used cortical responsivity metrics.

Results:

We show that although single pulse TMS generates stable and reproducible cortical responses across visits, the modulatory effects of TBS vary substantially both between and within individuals. Overall, at the group level, most measures of the iTBS and cTBS-induced effects were not significantly different from sham-TBS. Most importantly, none of the significant TBS-induced effects observed in visit1 were reproduced in visit-2.

Conclusions:

Our findings suggest that the generally accepted mechanisms of TBS-induced neuromodulation, i.e. through changes in cortical excitability, may not be accurate. Future research is needed to determine the mechanisms underlying the established therapeutic effects of TBS in neuropsychiatry and examine reproducibility of TBS-induced neuromodulation through oscillatory response dynamics.

Exploring and optimizing the neuroplastic effects of anodal transcranial direct current stimulation over the primary motor cortex of older humans

BACKGROUND

tDCS modulates cortical plasticity and has shown potential to improve cognitive/motor functions in healthy young humans. However, age-related alterations of brain structure and functions might require an adaptation of tDCS-parameters to achieve a targeted plasticity effect in older humans and conclusions obtained from young adults might not be directly transferable to older adults. Thus, our study aimed to systematically explore the association between tDCS-parameters and induced aftereffects on motor cortical excitability to determine optimal stimulation protocols for older individuals, as well as to investigate age-related differences of motor cortex plasticity in two different age groups of older adults.

METHODS

32 healthy, volunteers from two different age groups of Young-Old (50-65 years, n = 16) and Old-Old (66-80 years, n = 16) participated in this study. Anodal tDCS was applied over the primary motor cortex, with respective combinations of three intensities (1, 2, and 3 mA) and durations (15, 20, and 30 min), in a sham-controlled cross-over design. Cortical excitability alterations were monitored by single-pulse TMS-induced MEPs until the next day morning after stimulation.

RESULTS

All active stimulation conditions resulted in a significant enhancement of motor cortical excitability in both age groups. The facilitatory aftereffects of anodal tDCS did not significantly differ between age groups. We observed prolonged plasticity in the late-phase range for two protocols with the highest stimulation intensity (i.e., 3 mA-20 min, 3 mA-30 min).

CONCLUSIONS

Our study highlights the role of stimulation dosage in tDCS-induced neuroplastic aftereffects in the motor cortex of healthy older adults and delivers crucial information about optimized tDCS protocols in the domain of the primary motor cortex. Our findings might set the grounds for the development of optimal stimulation protocols to reinstate neuroplasticity in different cortical areas and induce long-lasting, functionally relevant plasticity in normal aging and in pathological conditions, which would require however systematic tDCS titration studies over respective target areas.

Human Theta Burst Stimulation Combined with Subsequent Electroacupuncture Increases Corticospinal Excitability

Objective

Intermittent theta burst stimulation (iTBS) is a widely used noninvasive brain stimulation for the facilitation of corticospinal excitability (CSE). Previous studies have shown that acupuncture applied to acupoints associated with motor function in healthy people can reduce the amplitude of the motor-evoked potentials (MEPs), which reflects the inhibition of CSE. In our work, we wanted to test whether the combination of iTBS and electroacupuncture (EA) would have different effects on CSE in humans.

Methods

A single-blind sham-controlled crossover design study was conducted on 20 healthy subjects. Subjects received 20 minutes' sham or real EA stimulation immediately after sham or real iTBS. MEPs, short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), cortical silent period (CSP), and central motor conduction time (CMCT) were recorded before each trial, and immediately, 20 minutes, and 40 minutes after the end of stimulation.

Results

In the sham iTBS group, EA produced a reduction in MEPs amplitude, lasting approximately 40 minutes, while in the real iTBS group, EA significantly increased MEPs amplitude beyond 40 minutes after the end of stimulation. In sham EA group, the recorded MEPs amplitude showed no significant trend over time compared to baseline. Among all experiments, there were no significant changes in SICI, ICF, CSP, CMCT, etc.

Conclusion

These data indicate that immediate application of EA after iTBS significantly increased corticospinal excitability. This trial was registered in the Chinese Clinical Trial Registry (registration no. ChiCTR1900025348).

A Checklist to Reduce Response Variability in Studies Using Transcranial Magnetic Stimulation for Assessment of Corticospinal Excitability: A Systematic Review of the Literature

Response variability between individuals (interindividual variability) and within individuals (intraindividual variability) is an important issue in the transcranial magnetic stimulation (TMS) literature. This has raised questions of the validity of TMS to assess changes in corticospinal excitability (CSE) in a predictable and reliable manner. Several participant-specific factors contribute to this observed response variability with a current lack of consensus on the degree each factor contributes. This highlights a need for consistency and structure in reporting study designs and methodologies. Currently, there is no summarized review of the participant-specific factors that can be controlled and may contribute to response variability. This systematic review aimed to develop a checklist of methodological measures taken by previously published research to increase the homogeneity of participant selection criteria, preparation of participants before experimental testing, participant scheduling, and the instructions given to participants throughout experimental testing to minimize their effect on response variability. Seven databases were searched in full. Studies were included if CSE was measured via TMS and included methodological measures to increase the homogeneity of the participants. Eighty-four studies were included. Twenty-three included measures to increase participant selection homogeneity, 21 included measures to increase participant preparation homogeneity, while 61 included measures to increase participant scheduling and instructions during experimental testing homogeneity. These methodological measures were summarized into a user-friendly checklist with considerations, suggestions, and rationale/justification for their inclusion. This may provide the framework for further insights into ways to reduce response variability in TMS research.

Nil effects of μ-rhythm phase-dependent burst-rTMS on cortical excitability in humans: A resting-state EEG and TMS-EEG study

Repetitive transcranial magnetic stimulation (rTMS) can induce excitability changes of a stimulated brain area through synaptic plasticity mechanisms. High-frequency (100 Hz) triplets of rTMS synchronized to the negative but not the positive peak of the ongoing sensorimotor μ-rhythm isolated with the concurrently acquired electroencephalography (EEG) resulted in a reproducible long-term potentiation like increase of motor evoked potential (MEP) amplitude, an index of corticospinal excitability (Zrenner et al. 2018, Brain Stimul 11:374–389). Here, we analyzed the EEG and TMS-EEG data from (Zrenner et al., 2018) to investigate the effects of μ-rhythm-phase-dependent burst-rTMS on EEG-based measures of cortical excitability. We used resting-state EEG to assess μ- and β-power in the motor cortex ipsi- and contralateral to the stimulation, and single-pulse TMS-evoked and induced EEG responses in the stimulated motor cortex. We found that μ-rhythm-phase-dependent burst-rTMS did not significantly change any of these EEG measures, despite the presence of a significant differential and reproducible effect on MEP amplitude. We conclude that EEG measures of cortical excitability do not reflect corticospinal excitability as measured by MEP amplitude. Most likely this is explained by the fact that rTMS induces complex changes at the molecular and synaptic level towards both excitation and inhibition that cannot be differentiated at the macroscopic level by EEG.

Non-invasive Brain Stimulation: Probing Intracortical Circuits and Improving Cognition in the Aging Brain

The impact of cognitive aging on brain function and structure is complex, and the relationship between aging-related structural changes and cognitive function are not fully understood. Physiological and pathological changes to the aging brain are highly variable, making it difficult to estimate a cognitive trajectory with which to monitor the conversion to cognitive decline. Beyond the information on the structural and functional consequences of cognitive aging gained from brain imaging and neuropsychological studies, non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) can enable stimulation of the human brain in vivo, offering useful insights into the functional integrity of intracortical circuits using electrophysiology and neuromodulation. TMS measurements can be used to identify and monitor changes in cortical reactivity, the integrity of inhibitory and excitatory intracortical circuits, the mechanisms of long-term potentiation (LTP)/depression-like plasticity and central cholinergic function. Repetitive TMS and tDCS can be used to modulate neuronal excitability and enhance cortical function, and thus offer a potential means to slow or reverse cognitive decline. This review will summarize and critically appraise relevant literature regarding the use of TMS and tDCS to probe cortical areas affected by the aging brain, and as potential therapeutic tools to improve cognitive function in the aging population. Challenges arising from intra-individual differences, limited reproducibility, and methodological differences will be discussed.