Demonstration of short-term plasticity in the dorsolateral prefrontal cortex with theta burst stimulation: A TMS-EEG study

The Neurophysiological Effects of Theta Burst Stimulation as Measured by Electroencephalography: A Systematic Review

Theta burst stimulation (TBS) is a noninvasive brain stimulation technique that can modulate neural activity. The effect of TBS on regions beyond the motor cortex remains unclear. With increased interest in applying TBS to nonmotor regions for research and clinical purposes, these effects must be understood and characterized. We synthesized the electrophysiological effects of a single session of TBS, as indexed by electroencephalography (EEG) and concurrent transcranial magnetic stimulation and EEG, in nonclinical participants. We reviewed 79 studies that administered either continuous TBS or intermittent TBS protocols. Broadly, continuous TBS suppressed and intermittent TBS facilitated evoked response component amplitudes. Response to TBS as measured by spectral power and connectivity was much more variable. Variability increased in the presence of task stimuli. There was a large degree of heterogeneity in the research methodology across studies. Additionally, the effect of individual differences on TBS response has been insufficiently investigated. Future research investigating the effects of TBS as measured by EEG must consider methodological and individual factors that may affect TBS outcomes.

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.

Intermittent Theta Burst Stimulation Drives Bi-Directional Changes in Excitability in Prefrontal Cortex

Theta burst stimulation (TBS), an FDA-cleared treatment for depression, is hypothesized to modulate excitability in the prefrontal cortex, though this has not definitively been shown in vivo. We performed calcium imaging on glutamatergic neurons in awake rodents to understand the effects of theta burst stimulation at a cellular level. Our findings provide the first direct evidence that TBS bidirectionally modulates glutamatergic activity when delivered in vivo and directly links calcium activity changes during stimulation with post-stimulation plasticity.

Repetitive Transcranial Magnetic Stimulation-Induced Neuroplasticity and the Treatment of Psychiatric Disorders: State of the Evidence and Future Opportunities

Neuroplasticity, or activity-dependent neuronal change, is a crucial mechanism underlying the mechanisms of effect of many therapies for neuropsychiatric disorders, one of which is repetitive transcranial magnetic stimulation (rTMS). Understanding the neuroplastic effects of rTMS at different biological scales and on different timescales and how the effects at different scales interact with each other can help us understand the effects of rTMS in clinical populations and offers the potential to improve treatment outcomes. Several decades of research in the fields of neuroimaging and blood biomarkers is increasingly showing its clinical relevance, allowing measurement of the synaptic, functional, and structural changes involved in neuroplasticity in humans. In this narrative review, we describe the evidence for rTMS-induced neuroplasticity at multiple levels of the nervous system, with a focus on the treatment of psychiatric disorders. We also describe the relationship between neuroplasticity and clinical effects, discuss methods to optimize neuroplasticity, and identify future research opportunities in this area.

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).

A dose-response characterization of transcranial magnetic stimulation intensity and evoked potential amplitude in the dorsolateral prefrontal cortex

By combining transcranial magnetic stimulation (TMS) with electroencephalography, human cortical circuits can be directly interrogated. The resulting electrical trace contains TMS-evoked potential (TEP) components, and it is not known whether the amplitudes of these components are stimulus intensity dependent. We examined this in the left dorsolateral prefrontal cortex in nineteen healthy adult participants and extracted TEP amplitudes for the N40, P60, N120, and P200 components at 110%, 120%, and 130% of resting motor threshold (RMT). To probe plasticity of putative stimulus intensity dose-response relationships, this was repeated after participants received intermittent theta burst stimulation (iTBS; 600 pulses, 80% RMT). The amplitude of the N120 and P200 components exhibited a stimulus intensity dose-response relationship, however the N40 and P60 components did not. After iTBS, the N40 and P60 components continued to exhibit a lack of stimulus intensity dose-dependency, and the P200 dose-response was unchanged. In the N120 component, however, we saw evidence of change within the stimulus intensity dose-dependent relationship characterized by a decrease in absolute peak amplitudes at lower stimulus intensities. These data suggest that TEP components have heterogeneous dose-response relationships, with implications for standardizing and harmonizing methods across experiments. Moreover, the selective modification of the N120 dose-response relationship may provide a novel marker for iTBS plasticity in health and disease.

Increased theta-low gamma phase-amplitude coupling in resting electroencephalography after intermittent theta burst stimulation

OBJECTIVE

Intermittent theta burst stimulation (iTBS) is based on the phase-amplitude coupling (PAC) pattern. We aimed to investigate the effect of iTBS on PAC in resting electroencephalography (EEG), which may provide insight into the underlying mechanism.

METHODS

Twenty-one healthy volunteers were recruited and received both active and sham neuroimaging-guided iTBS on two separate days, which was precisely delivered to the right superior temporal gyrus. On each experimental day, resting EEG was recorded before and after stimulation for each participant. PACs across electrodes and frequency bands were calculated and compared to investigate the effect of iTBS.

RESULTS

Theta (4-6 Hz) -low gamma (45-55 Hz) PAC over the stimulation site had a significant interaction effect, which increased after the active iTBS but did not differ after the sham iTBS. No significant interaction effect occurred in other cross-frequency couplings such as delta-low gamma, alpha-low gamma, delta-high gamma, theta-high gamma, or alpha-high gamma PAC in the region of interest.

CONCLUSION

iTBS selectively modulated theta-low gamma PAC at the stimulation area, which exhibited both region- and frequency- specificity. This suggests that PAC may be a bridge connecting external neuromodulation to internal neuroplasticity.

Prefrontal stimulation as a tool to disrupt hippocampal and striatal reactivations underlying fast motor memory consolidation

BACKGROUND

Recent evidence suggests that hippocampal replay in humans support rapid motor memory consolidation during epochs of wakefulness interleaved with task practice.

OBJECTIVES/HYPOTHESES

The goal of this study was to test whether such reactivation patterns can be modulated with experimental interventions and in turn influence fast consolidation. We hypothesized that non-invasive brain stimulation targeting hippocampal and striatal networks via the prefrontal cortex would influence brain reactivation and the rapid form of motor memory consolidation.

METHODS

Theta-burst stimulation was applied to a prefrontal cluster functionally connected to both the hippocampus and striatum of young healthy participants before they learned a motor sequence task in a functional magnetic resonance imaging (fMRI) scanner. Neuroimaging data acquired during task practice and the interleaved rest epochs were analyzed to comprehensively characterize the effect of stimulation on the neural processes supporting fast motor memory consolidation.

RESULTS

Our results collectively show that active, as compared to control, theta-burst stimulation of the prefrontal cortex hindered fast motor memory consolidation. Converging evidence from both univariate and multivariate analyses of fMRI data indicate that active stimulation disrupted hippocampal and caudate responses during inter-practice rest, presumably altering the reactivation of learning-related patterns during the micro-offline consolidation episodes. Last, stimulation altered the link between the brain and the behavioral markers of the fast consolidation process.

CONCLUSION

These results suggest that stimulation targeting deep brain regions via the prefrontal cortex can be used to modulate hippocampal and striatal reactivations in the human brain and influence motor memory consolidation.

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.

Orbitofrontal cortex-hippocampus potentiation mediates relief for depression: A randomized double-blind trial and TMS-EEG study

It has been 15 years since repetitive transcranial magnetic stimulation (rTMS) targeting the dorsolateral prefrontal cortex (DLPFC) was approved by the FDA for clinical depression treatment. Yet, the underlying mechanisms for rTMS-induced depression relief are not fully elucidated. This study analyzes TMS-electroencephalogram (EEG) data from 64 healthy control (HC) subjects and 53 patients with major depressive disorder (MDD) before and after rTMS treatment. Prior to treatment, patients with MDD have lower activity in the DLPFC, the hippocampus (HPC), the orbitofrontal cortex (OFC), and DLPFC-OFC connectivity compared with HCs. Following active rTMS treatment, patients with MDD show a significant increase in the DLPFC, HPC, and OFC. Notably, the increase in HPC activity is specifically associated with amelioration of depressive symptoms but not anxiety or sleep quality. The orbitofrontal-hippocampal pathway plays a crucial role in mediating depression relief following rTMS treatment. These findings suggest potential alternative targets for brain stimulation therapy against depression (chictr.org.cn: ChiCTR2100052007).

The Impact of Sex Hormones on Transcranial Magnetic Stimulation Measures of Cortical Excitability: A Systematic Review and Considerations for Clinical Practice

ABSTRACT

Repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising alternative for the treatment of major depressive disorder (MDD), although its clinical effectiveness varies substantially. The effects of sex hormone fluctuations on cortical excitability have been identified as potential factors that can explain this variability. However, data on how sex hormone changes affect clinical response to rTMS is limited. To address this gap, we reviewed the literature examining the effects of sex hormones and hormonal treatments on transcranial magnetic stimulation (TMS) measures of cortical excitability. Results show that variations of endogenous estrogen, testosterone, and progesterone have modulatory effects on TMS-derived measures of cortical excitability. Specifically, higher levels of estrogen and testosterone were associated with greater cortical excitability, while higher progesterone was associated with lower cortical excitability. This highlights the importance of additional investigation into the effects of hormonal changes on rTMS outcomes and circuit-specific physiological variables. These results call for TMS clinicians to consider performing more frequent motor threshold (MT) assessments in patients receiving high doses of estrogen, testosterone, and progesterone in cases such as in vitro fertilization, hormone replacement therapy, and gender-affirming hormonal treatments. It may also be important to consider physiological hormonal fluctuations and their impact on depressive symptoms and the MT when treating female patients with rTMS.

TMS-evoked responses are driven by recurrent large-scale network dynamics

A compelling way to disentangle the complexity of the brain is to measure the effects of spatially and temporally synchronized systematic perturbations. In humans, this can be non-invasively achieved by combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG). Spatiotemporally complex and long-lasting TMS-EEG evoked potential (TEP) waveforms are believed to result from recurrent, re-entrant activity that propagates broadly across multiple cortical and subcortical regions, dispersing from and later re-converging on, the primary stimulation site. However, if we loosely understand the TEP of a TMS-stimulated region as the impulse response function of a noisy underdamped harmonic oscillator, then multiple later activity components (waveform peaks) should be expected even for an isolated network node in the complete absence of recurrent inputs. Thus emerges a critically important question for basic and clinical research on human brain dynamics: what parts of the TEP are due to purely local dynamics, what parts are due to reverberant, re-entrant network activity, and how can we distinguish between the two? To disentangle this, we used source-localized TMS-EEG analyses and whole-brain connectome-based computational modelling. Results indicated that recurrent network feedback begins to drive TEP responses from 100 ms post-stimulation, with earlier TEP components being attributable to local reverberatory activity within the stimulated region. Subject-specific estimation of neurophysiological parameters additionally indicated an important role for inhibitory GABAergic neural populations in scaling cortical excitability levels, as reflected in TEP waveform characteristics. The novel discoveries and new software technologies introduced here should be of broad utility in basic and clinical neuroscience research.

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.

Sequential bilateral accelerated theta burst stimulation in adolescents with suicidal ideation associated with major depressive disorder: Protocol for a randomized controlled trial

BACKGROUND

Suicide is a leading cause of death in adolescents worldwide. Previous research findings suggest that suicidal adolescents with depression have pathophysiological dorsolateral prefrontal cortex (DLPFC) deficits in γ-aminobutyric acid neurotransmission. Interventions with transcranial magnetic stimulation (TMS) directly address these underlying pathophysiological deficits in the prefrontal cortex. Theta burst stimulation (TBS) is newer dosing approach for TMS. Accelerated TBS (aTBS) involves administering multiple sessions of TMS daily as this dosing may be more efficient, tolerable, and rapid acting than standard TMS.

MATERIALS AND METHODS

This is a randomized, double-blind, sham-controlled trial of sequential bilateral aTBS in adolescents with major depressive disorder (MDD) and suicidal ideation. Three sessions are administered daily for 10 days. During each session, continuous TBS is administered first to the right DPFC, in which 1,800 pulses are delivered continuously over 120 seconds. Then intermittent TBS is applied to the left DPFC, in which 1,800 pulses are delivered in 2-second bursts and repeated every 10 seconds for 570 seconds. The TBS parameters were adopted from prior research, with 3-pulse, 50-Hz bursts given every 200 ms (at 5 Hz) with an intensity of 80% active motor threshold. The comparison group will receive 3 daily sessions of bilateral sham TBS treatment for 10 days. All participants will receive the standard of care for patients with depression and suicidal ideation including daily psychotherapeutic skill sessions. Long-interval intracortical inhibition (LICI) biomarkers will be measured before and after treatment. Exploratory measures will be collected with TMS and electroencephalography for biomarker development.

DISCUSSION

This is the first known randomized controlled trial to examine the efficacy of sequential bilateral aTBS for treating suicidal ideation in adolescents with MDD. Results from this study will also provide opportunities to further understand the neurophysiological and molecular mechanisms of suicidal ideation in adolescents.

TRIAL REGISTRATION

Investigational device exemption (IDE) Number: G200220, ClinicalTrials.gov (ID: NCT04701840). Registered August 6, 2020. https://clinicaltrials.gov/ct2/show/NCT04502758?term=NCT04701840&draw=2&rank=1.

Investigation of neurobiological responses to theta burst stimulation during recovery from mild traumatic brain injury (mTBI)

OBJECTIVE

The ability of the brain to recover following neurological insult is of interest for mild traumatic brain injury (mTBI) populations. Investigating whether non-invasive brain stimulation (NIBS) can modulate neurophysiology and cognition may lead to the development of therapeutic interventions post injury. The purpose of this study was to investigate neurobiological effects of one session of intermittent theta burst stimulation (iTBS) to the dorsolateral prefrontal cortex (DLPFC) in participants recovering from mTBI.

METHOD

Changes to neurophysiology were assessed with electroencephalography (EEG) and transcranial magnetic stimulation combined with EEG (TMS-EEG). Digit span working memory accuracy assessed cognitive performance. 30 patients were assessed within one-month of sustaining a mTBI and 26 demographically matched controls were assessed. Participants were also assessed at 3-months (mTBI: N = 21, control: N = 26) and 6-months (mTBI: N = 15, control: N = 24).

RESULTS

Analyses demonstrated iTBS did not reliably modulate neurophysiological activity, and no differences in cognitive performance were produced by iTBS at any assessment time-point.

CONCLUSIONS

Factors responsible for our null results are unclear. Possible limitations to our experimental design are discussed.

SIGNIFICANCE

Our findings suggest additional research is required to establish the effects of iTBS on plasticity following mTBI, prior to therapeutic application.

DATA AND CODE AVAILABILITY STATEMENT

We do not have ethical approval to make this data publicly available, as our approval predated our inclusion of such approvals (which we now do routinely).

Analgesic efficacy of theta-burst stimulation for postoperative pain

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) may be a relevant method to assist postoperative pain. However, studies to date have only used conventional 10 Hz rTMS and targeted the DLPFC for postoperative pain. A more recent form of rTMS, termed intermittent Theta Burst Stimulation (iTBS), enables to increase cortical excitability in a short period of time. This preliminary double-blind, randomised, sham controlled study was designed to evaluate the efficacy of iTBS in postoperative care across two distinct stimulation targets.

METHODS

A group of 45 patients post laparoscopic surgery were randomised to receive a single session of iTBS over either the dorsolateral prefrontal cortex (DLPFC), primary motor cortex (M1), or Sham stimulation (1:1:1 ratio). Outcome measurements were number of pump attempts, total anaesthetic volume used, and self-rated pain experience, assessed at 1 hour, 6 hours, 24 hours, and 48 hours post stimulation. All randomised patients were analysed (n = 15 in each group).

RESULTS

Compared to Sham stimulation, DLPFC-iTBS reduced pump attempts at 6 (DLPFC = 0.73 ± 0.88, Sham = 2.36 ± 1.65, P = 0.031), 24 (DLPFC = 1.40 ± 1.24, Sham = 5.03 ± 3.87, P = 0.008), and 48 (DLPFC = 1.47 ± 1.41, Sham = 5.87 ± 4.34, P = 0.014) hours post-surgery, whereby M1 stimulation had no effect. No group effect was observed on total anaesthetics, which was mainly provided through the continuous administration of opioids at a set speed for each group. There was also no group or interaction effect on pain ratings. Pump attempts were positively associated with pain ratings in the DLPFC (r = 0.59, P = 0.02) and M1 (r = 0.56, P = 0.03) stimulation.

CONCLUSIONS

Our findings show that iTBS to the DLPFC reduces pump attempts for additional anaesthetics following a laparoscopic surgery. However, reduced pump attempts by DLPFC stimulation did not translate into a significantly smaller volume of total anaesthetic, due to the continuous administration of opioids at a set speed for each group.

SIGNIFICANCE

Our findings therefore provide preliminary evidence for iTBS targeting the DLPFC to be used to improve postoperative pain management.

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.

Effects of working memory span training on top-down attentional asymmetry at both neural and behavioral levels

The leftward asymmetry of the visual field and posterior brain regions, a feature of the normal attention process, can be strengthened by brain stimulation, e.g. administering alpha frequency stimulation to the left posterior cortex. However, whether it can be strengthened by cognitive training, especially with nonlateralized tasks, is unknown. We used a dataset from a 2-month-long randomized controlled trial and compared the control group with 2 training groups trained with backward or forward memory span tasks. A lateralized change detection task with varied memory loads was administered as the pre-, mid-, and post-tests with simultaneous electroencephalographic recording. Intrasubject response variability (IRV) and the alpha modulation index (MI) were calculated. Analysis of IRV showed more enhanced leftward attentional bias in the backward group than in the other groups. Consistently, analysis of MI found that its enhancements in the left hemisphere (but not the right hemisphere) of the backward group were significantly higher than those of the other groups. Further analysis revealed that left MI changes predicted left IRV improvement. All of these results indicated that backward memory span training enhanced leftward attentional asymmetry at both the behavioral and neural levels.

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.

Neuromodulatory effects of theta burst stimulation to the prefrontal cortex

Theta burst stimulation (TBS) is a new form of repetitive transcranial magnetic stimulation (TMS) capable of non-invasively modulating cortical excitability. In recent years TBS has been increasingly used as a neuroscientific investigative tool and therapeutic intervention for psychiatric disorders, in which the dorsolateral prefrontal cortex (DLPFC) is often the primary target. However, the neuromodulatory effects of TBS on prefrontal regions remain unclear. Here we share EEG and ECG recordings and structural MRI scans, including high-resolution DTI, from twenty-four healthy participants who received intermittent TBS (two sessions), continuous TBS (two sessions), and sham stimulation (one session) applied to the left DLPFC using a single-blinded crossover design. Each session includes eyes-open resting-state EEG and single-pulse TMS-EEG obtained before TBS and 2-, 15-, and 30-minutes post-stimulation. This dataset enables foundational basic science investigations into the neuromodulatory effects of TBS on the DLPFC.

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

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

Changes in the TMS-evoked potential N100 in the dorsolateral prefrontal cortex as a function of depression severity in adolescents

Studies using transcranial magnetic stimulation with simultaneous electroencephalography (TMS-EEG) revealed an imbalance between cortical excitation and inhibition (E/I) in the dorsolateral prefrontal cortex (DLPFC) in depression. As adolescence is a developmental period with an increase in depression prevalence and profound neural changes, it is crucial to study the relationship between depression and cortical excitability in adolescence. We aimed to investigate the cortical excitability of the DLPFC in adolescents with depression and a dependency of the TMS-evoked potential N100 on the depression severity. 36 clinical patients (12–18 years of age; 21 females) with a major depressive episode were assessed twice in a longitudinal design: shortly after admission (T0) and after six weeks of intervention (T1). GABA-B-mediated cortical inhibition in the left and right DLPFC, as assessed by the N100, was recorded with EEG. Significantly higher depression scores were reported at T0 compared to T1 (p < 0.001). N100 amplitudes were significantly increased (i.e., more negative) at T0 compared to T1 (p = 0.03). No significant hemispheric difference was found in the N100 component. The correlation between the difference in depression severity and the difference in N100 amplitudes (T0–T1) obtained during stimulation of the left DLPFC did not remain significant after correction for testing in both hemispheres. Higher N100 amplitudes during a state of greater depression severity are suggestive of an E/I imbalance in the DLPFC in adolescents with an acute depressive episode. The N100 reduction potentially reflects a normalization of DLPFC over inhibition in association with decreased depressive symptomatology, indicating severity dependency.

Paired associative stimulation applied to the cortex can increase resting-state functional connectivity: A proof of principle study

INTRODUCTION

There is emerging evidence that high Beta coherence (hBc) between prefrontal and motor corticies, measured with resting-state electroencephalography (rs-EEG), can be an accurate predictor of motor skill learning and stroke recovery. However, it remains unknown whether and how intracortical connectivity may be influenced using neuromodulation. Therefore, a cortico-cortico PAS (ccPAS) paradigm may be used to increase resting-state intracortical connectivity (rs-IC) within a targeted neural circuit.

PURPOSE

Our purpose is to demonstrate proof of principle that ccPAS can be used to increase rs-IC between a prefrontal and motor cortical region.

METHODS

Eleven non-disabled adults were recruited (mean age 26.4, sd 5.6, 5 female). Each participant underwent a double baseline measurement, followed by a real and control ccPAS condition, counter-balanced for order. Control and ccPAS conditions were performed over electrodes of the right prefrontal and motor cortex. Both ccPAS conditions were identical apart from the inter-stimulus interval (i.e ISI 5 ms: real ccPAS and 500 ms: control ccPAS). Whole brain rs-EEG of high Beta coherence (hBc) was acquired before and after each ccPAS condition and then analyzed for changes in rs-IC along the targeted circuit.

RESULTS

Compared to ccPAS500 and baseline, ccPAS5 induced a significant increase in rs-IC, measured as coherence between electrodes over right prefrontal and motor cortex, (p <.05).

CONCLUSION

These findings demonstrate proof of principle that ccPAS with an STDP derived ISI, can effectively increase hBc along a targeted circuit.

Evidence of Neuroplastic Changes after Transcranial Magnetic, Electric, and Deep Brain Stimulation

Electric and magnetic stimulation of the human brain can be used to excite or inhibit neurons. Numerous methods have been designed over the years for this purpose with various advantages and disadvantages that are the topic of this review. Deep brain stimulation (DBS) is the most direct and focal application of electric impulses to brain tissue. Electrodes are placed in the brain in order to modulate neural activity and to correct parameters of pathological oscillation in brain circuits such as their amplitude or frequency. Transcranial magnetic stimulation (TMS) is a non-invasive alternative with the stimulator generating a magnetic field in a coil over the scalp that induces an electric field in the brain which, in turn, interacts with ongoing brain activity. Depending upon stimulation parameters, excitation and inhibition can be achieved. Transcranial electric stimulation (tES) applies electric fields to the scalp that spread along the skull in order to reach the brain, thus, limiting current strength to avoid skin sensations and cranial muscle pain. Therefore, tES can only modulate brain activity and is considered subthreshold, i.e., it does not directly elicit neuronal action potentials. In this review, we collect hints for neuroplastic changes such as modulation of behavior, the electric activity of the brain, or the evolution of clinical signs and symptoms in response to stimulation. Possible mechanisms are discussed, and future paradigms are suggested.

Cortical excitability and plasticity in Alzheimer's disease and mild cognitive impairment: A systematic review and meta-analysis of transcranial magnetic stimulation studies

BACKGROUND

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique. When stimulation is applied over the primary motor cortex and coupled with electromyography measures, TMS can probe functions of cortical excitability and plasticity in vivo. The purpose of this meta-analysis is to evaluate the utility of TMS-derived measures for differentiating patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI) from cognitively normal older adults (CN).

METHODS

Databases searched included PubMed, Embase, APA PsycInfo, Medline, and CINAHL Plus from inception to July 2021.

RESULTS

Sixty-one studies with a total of 2728 participants (1454 patients with AD, 163 patients with MCI, and 1111 CN) were included. Patients with AD showed significantly higher cortical excitability, lower cortical inhibition, and impaired cortical plasticity compared to the CN cohorts. Patients with MCI exhibited increased cortical excitability and reduced plasticity compared to the CN cohort. Additionally, lower cognitive performance was significantly associated with higher cortical excitability and lower inhibition. No seizure events due to TMS were reported, and the mild adverse response rate is approximately 3/1000 (i.e., 9/2728).

CONCLUSIONS

Findings of our meta-analysis demonstrate the potential of using TMS-derived cortical excitability and plasticity measures as diagnostic biomarkers and therapeutic targets for AD and MCI.

Identifying novel biomarkers with TMS-EEG - Methodological possibilities and challenges

Biomarkers are essential for understanding the underlying pathologies in brain disorders and for developing effective treatments. Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) is an emerging neurophysiological tool that can be used for biomarker development. This method can identify biomarkers associated with the function and dynamics of the inhibitory and excitatory neurotransmitter systems and effective connectivity between brain areas. In this review, we outline the current state of the TMS-EEG biomarker field by summarizing the existing protocols and the possibilities and challenges associated with this methodology.

Enhancing Visuospatial Working Memory Performance Using Intermittent Theta-Burst Stimulation Over the Right Dorsolateral Prefrontal Cortex

Noninvasive brain stimulation provides a promising approach for the treatment of neuropsychiatric conditions. Despite the increasing research on the facilitatory effects of this kind of stimulation on the cognitive processes, the majority of the studies have used the standard stimulation approaches such as the transcranial direct current stimulation and the conventional repetitive transcranial magnetic stimulation (rTMS) which seem to be limited in robustness and the duration of the transient effects. However, a recent specialized type of rTMS, theta-burst stimulation (TBS), patterned to mimic the natural cross-frequency coupling of the human brain, may induce robust and longer-lasting effects on cortical activity. Here, we aimed to investigate the effects of the intermittent TBS (iTBS), a facilitatory form of TBS, over the right DLPFC (rDLPFC), a brain area implicated in higher-order cognitive processes, on visuospatial working memory (VSWM) performance. Therefore, iTBS was applied over either the rDLPFC or the vertex of 24 healthy participants, in two separate sessions. We assessed VSWM performance using 2-back and 4-back visuospatial tasks before iTBS (at the baseline (BL), and after the iTBS. Our results indicate that the iTBS over the rDLPFC significantly enhanced VSWM performance in the 2-back task, as measured by the discriminability index and the reaction time. However, the 4-back task performance was not significantly modulated by iTBS. These findings demonstrate that the rDLPFC plays a critical role in VSWM and that iTBS is a safe and effective approach for investigating the causal role of the specific brain areas.

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.

Designing and comparing cleaning pipelines for TMS-EEG data: a theoretical overview and practical example

Combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) is growing in popularity as a method for probing the reactivity and connectivity of neural circuits in basic and clinical research. However, using EEG to measure the neural responses to TMS is challenging due to the unique artifacts introduced by combining the two techniques. In this paper, we overview the artifacts present in TMS-EEG data and the offline cleaning methods used to suppress these unwanted signals. We then describe how open science practices, including the development of open-source toolboxes designed for TMS-EEG analysis (e.g., TESA - the TMS-EEG signal analyser), have improved the availability and reproducibility of TMS-EEG cleaning methods. We provide theoretical and practical considerations for designing TMS-EEG cleaning pipelines and then give an example of how to compare different pipelines using TESA. We show that changing even a single step in a pipeline designed to suppress decay artifacts results in TMS-evoked potentials (TEPs) with small differences in amplitude and spatial topography. The variability in TEPs resulting from the choice of cleaning pipeline has important implications for comparing TMS-EEG findings between research groups which use different online and offline approaches. Finally, we discuss the challenges of validating cleaning pipelines and recommend that researchers compare outcomes from TMS-EEG experiments using multiple pipelines to ensure findings are not related to the choice of cleaning methods. We conclude that the continued improvement, availability, and validation of cleaning pipelines is essential to ensure TMS-EEG reaches its full potential as a method for studying human neurophysiology.

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.

Investigating Neurophysiological Markers of Symptom Severity in Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is characterized by a progressive decline in cognitive functioning for which there is a stark lack of effective treatments. Investigating the neurophysiological markers of symptom severity in AD may aid in the identification of alternative treatment targets.

OBJECTIVE

In the current study, we used a multimodal approach to investigate the association between functional connectivity (specifically between scalp electrodes placed over frontal and parietal regions) and symptom severity in AD, and to explore the relationship between connectivity and cortical excitability.

METHODS

40 people with AD (25 mild severity, 15 moderate severity) underwent neurobiological assessment (resting state electroencephalography (EEG) and prefrontal transcranial magnetic stimulation (TMS) with EEG) and cognitive assessment. Neurobiological outcomes were resting state functional connectivity and TMS-evoked potentials. Cognitive outcomes were scores on the Alzheimer's Disease Assessment Scale-Cognitive Subscale, Mini-Mental Status Examination, and a measure of episodic verbal learning.

RESULTS

Greater contralateral functional theta connectivity between frontal scalp electrodes and parietal scalp electrodes was associated with poorer cognitive performance. In addition, significant correlations were seen between the contralateral theta connectivity and the N100 and P60 TMS-evoked potentials measured from electrodes over the left dorsolateral prefrontal cortex.

CONCLUSION

Together these findings provide initial support for the use of a multimodal neurophysiological approaches to investigate potential therapeutic targets in AD. Suggestions for future research are discussed.

Intermittent theta burst stimulation over the parietal cortex has a significant neural effect on working memory

The crucial role of the parietal cortex in working memory (WM) storage has been identified by fMRI studies. However, it remains unknown whether repeated parietal intermittent theta‐burst stimulation (iTBS) can improve WM. In this within‐subject randomized controlled study, under the guidance of fMRI‐identified parietal activation in the left hemisphere, 22 healthy adults received real and sham iTBS sessions (five consecutive days, 600 pulses per day for each session) with an interval of 9 months between the two sessions. Electroencephalography signals of each subject before and after both iTBS sessions were collected during a change detection task. Changes in contralateral delay activity (CDA) and K‐score were then calculated to reflect neural and behavioral WM improvement. Repeated‐measures ANOVA suggested that real iTBS increased CDA more than the sham one (p = .011 for iTBS effect). Further analysis showed that this effect was more significant in the left hemisphere than in the right hemisphere (p = .029 for the hemisphere‐by‐iTBS interaction effect). Pearson correlation analyses showed significant correlations for two conditions between CDA changes in the left hemisphere and K score changes (ps <.05). In terms of the behavioral results, significant K score changes after real iTBS were observed for two conditions, but a repeated‐measures ANOVA showed a nonsignificant main effect of iTBS (p = .826). These results indicate that the current iTBS protocol is a promising way to improve WM capability based on the neural indicator (CDA) but further optimization is needed to produce a behavioral effect.

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.

Hippocampal and striatal responses during motor learning are modulated by prefrontal cortex stimulation

While it is widely accepted that motor sequence learning (MSL) is supported by a prefrontal-mediated interaction between hippocampal and striatal networks, it remains unknown whether the functional responses of these networks can be modulated in humans with targeted experimental interventions. The present proof-of-concept study employed a multimodal neuroimaging approach, including functional magnetic resonance (MR) imaging and MR spectroscopy, to investigate whether individually-tailored theta-burst stimulation of the dorsolateral prefrontal cortex can modulate responses in the hippocampus and the basal ganglia during motor learning. Our results indicate that while stimulation did not modulate motor performance nor task-related brain activity, it influenced connectivity patterns within hippocampo-frontal and striatal networks. Stimulation also altered the relationship between the levels of gamma-aminobutyric acid (GABA) in the stimulated prefrontal cortex and learning-related changes in both activity and connectivity in fronto-striato-hippocampal networks. This study provides the first experimental evidence, to the best of our knowledge, that brain stimulation can alter motor learning-related functional responses in the striatum and hippocampus.

Accelerated theta burst stimulation for the treatment of depression: A randomised controlled trial

INTRODUCTION

Theta burst pattern repetitive transcranial magnetic stimulation (TBS) is increasingly applied to treat depression. TBS's brevity is well-suited to application in accelerated schedules. Sizeable trials of accelerated TBS are lacking; and optimal TBS parameters such as stimulation intensity are not established.

METHODS

We conducted a three arm, single blind, randomised, controlled, multi-site trial comparing accelerated bilateral TBS applied at 80 % or 120 % of the resting motor threshold and left unilateral 10 Hz rTMS. 300 patients with treatment-resistant depression (TRD) were recruited. TBS arms applied 20 bilateral prefrontal TBS sessions over 10 days, while the rTMS arm applied 20 daily sessions of 10 Hz rTMS to the left prefrontal cortex over 4 weeks. Primary outcome was depression treatment response at week 4.

RESULTS

The overall treatment response rate was 43.7 % and the remission rate was 28.2 %. There were no significant differences for response (p = 0.180) or remission (p = 0.316) across the three groups. Response rates between accelerated bilateral TBS applied at sub- and supra-threshold intensities were not significantly different (p = 0.319). Linear mixed model analysis showed a significant effect of time (p < 0.01), but not rTMS type (p = 0.680).

CONCLUSION

This is the largest accelerated bilateral TBS study to date and provides evidence that it is effective and safe in treating TRD. The accelerated application of TBS was not associated with more rapid antidepressant effects. Bilateral sequential TBS did not have superior antidepressant effect to unilateral 10 Hz rTMS. There was no significant difference in antidepressant efficacy between sub- and supra-threshold accelerated bilateral TBS.

Investigating neurophysiological markers of impaired cognition in schizophrenia

Cognitive impairment is highly prevalent in schizophrenia and treatment options are severely limited. A greater understanding of the pathophysiology of impaired cognition would have broad implications, including for the development of effective treatments. In the current study we used a multimodal approach to identify neurophysiological markers of cognitive impairment in schizophrenia. Fifty-seven participants (30 schizophrenia, 27 controls) underwent neurobiological assessment (electroencephalography [EEG] and Transcranial Magnetic Stimulation combined with EEG [TMS-EEG]) and assessment of cognitive functioning using an n-back task and the MATRICS Consensus Cognitive Battery. Neurobiological outcome measures included oscillatory power during a 2-back task, TMS-related oscillations and TMS-evoked potentials (TEPs). Cognitive outcome measures were d prime and accurate reaction time on the 2-back and MATRICS domain scores. Compared to healthy controls, participants with schizophrenia showed significantly reduced theta oscillations in response to TMS, and trend level decreases in task-related theta and cortical reactivity (i.e. reduced N100 and N40 TEPs). Participants with schizophrenia also showed significantly impaired cognitive performance across all measures. Correlational analysis identified significant associations between cortical reactivity and TMS-related oscillations in both groups; and trend level associations between task-related oscillations and impaired cognition in schizophrenia. The current study provides experimental support for possible neurophysiological markers of cognitive impairment in schizophrenia. The potential implications of these findings, including for treatment development, are discussed.

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.

Characterizing Cortical Oscillatory Responses in Major Depressive Disorder Before and After Convulsive Therapy: A TMS-EEG Study

BACKGROUND

Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) is emerging as a powerful technique for interrogating neural circuit dysfunction in psychiatric disorders. Here, we utilized time-frequency analyses to characterize differences in neural oscillatory dynamics between subjects with major depressive disorder (MDD) and healthy controls (HC). We further examined changes in TMS-related oscillatory power following convulsive therapy.

METHODS

Oscillatory power was examined following TMS over the dorsolateral prefrontal and motor cortices (DLPFC and M1) in 38 MDD subjects, and 22 HCs. We further investigated how these responses changed in the MDD group following an acute course of convulsive therapy (either magnetic seizure therapy [MST, n = 24] or electroconvulsive therapy [ECT, n = 14]).

RESULTS

Prior to treatment, MDD subjects exhibited increased oscillatory power within delta, theta, and alpha frequency bands with TMS-EEG over the DLPFC, but showed no differences to HCs with stimulation over M1. Following MST, DLPFC stimulation revealed attenuated baseline-normalized power in the delta and theta bands, with reductions in the delta, theta, and alpha power following ECT. TMS over M1 revealed reduced delta and theta power following ECT, with no changes observed following MST. An association was also observed between the treatment- induced change in alpha power and depression severity score.

LIMITATIONS

Limitations include the modest sample size, open-label MST and ECT treatment designs, and lack of a placebo condition.

CONCLUSIONS

These results provide evidence of alterations in TMS-related oscillatory activity in MDD, and further suggest modulation of oscillatory power following ECT and MST.

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.

Repetitive Transcranial Magnetic Stimulation for Adolescent Major Depressive Disorder: A Focus on Neurodevelopment

Adolescent depression is a potentially lethal condition and a leading cause of disability for this age group. There is an urgent need for novel efficacious treatments since half of adolescents with depression fail to respond to current therapies and up to 70% of those who respond will relapse within 5 years. Repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising treatment for major depressive disorder (MDD) in adults who do not respond to pharmacological or behavioral interventions. In contrast, rTMS has not demonstrated the same degree of efficacy in adolescent MDD. We argue that this is due, in part, to conceptual and methodological shortcomings in the existing literature. In our review, we first provide a neurodevelopmentally focused overview of adolescent depression. We then summarize the rTMS literature in adult and adolescent MDD focusing on both the putative mechanisms of action and neurodevelopmental factors that may influence efficacy in adolescents. We then identify limitations in the existing adolescent MDD rTMS literature and propose specific parameters and approaches that may be used to optimize efficacy in this uniquely vulnerable age group. Specifically, we suggest ways in which future studies reduce clinical and neural heterogeneity, optimize neuronavigation by drawing from functional brain imaging, apply current knowledge of rTMS parameters and neurodevelopment, and employ an experimental therapeutics platform to identify neural targets and biomarkers for response. We conclude that rTMS is worthy of further investigation. Furthermore, we suggest that following these recommendations in future studies will offer a more rigorous test of rTMS as an effective treatment for adolescent depression.

The Effects of Bilateral Theta-burst Stimulation on Executive Functions and Affective Symptoms in Major Depressive Disorder

Major depressive disorder (MDD) is characterized by severe affective as well as cognitive symptoms. Moreover, cognitive impairment in MDD can persist after the remission of affective symptoms. Theta-burst stimulation (TBS) is a promising tool to manage the affective symptoms of major depressive disorder (MDD); however, its cognition-enhancing effects are sparsely investigated. Here, we aimed to examine whether the administration of bilateral TBS has pro-cognitive effects in MDD. Ten daily sessions of neuronavigated active or sham TBS were delivered bilaterally over the dorsolateral prefrontal cortex to patients with MDD. The n-back task and the attention network task were administered to assess working memory and attention, respectively. Affective symptoms were measured using the 21-item Hamilton Depression Rating Scale. We observed moderate evidence that the depressive symptoms of patients receiving active TBS improved compared to participants in the sham stimulation. No effects of TBS on attention and working memory were detected, supported by a moderate-to-strong level of evidence. The effects of TBS on psychomotor processing speed should be further investigated. Bilateral TBS has a substantial antidepressive effect with no immediate adverse effects on executive functions.

Effects of rTMS on the brain: is there value in variability?

The ability of repetitive transcranial magnetic stimulation (rTMS) to non-invasively induce neuroplasticity in the human cortex has opened exciting possibilities for its application in both basic and clinical research. Changes in the amplitude of motor evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation has so far provided a convenient model for exploring the neurophysiology of rTMS effects on the brain, influencing the ways in which these stimulation protocols have been applied therapeutically. However, a growing number of studies have reported large inter-individual variability in the mean MEP response to rTMS, raising legitimate questions about the usefulness of this model for guiding therapy. Although the increasing application of different neuroimaging approaches has made it possible to probe rTMS-induced neuroplasticity outside the motor cortex to measure changes in neural activity that impact other aspects of human behaviour, the high variability of rTMS effects on these measurements remains an important issue for the field to address. In this review, we seek to move away from the conventional facilitation/inhibition dichotomy that permeates much of the rTMS literature, presenting a non-standard approach for measuring rTMS-induced neuroplasticity. We consider the evidence that rTMS is able to modulate an individual's moment-to-moment variability of neural activity, and whether this could have implications for guiding the therapeutic application of rTMS.

Neurophysiological markers of response to theta burst stimulation in youth depression

BACKGROUND

Theta burst stimulation (TBS) has recently been proposed as a novel treatment for youth depression. However, the impact of TBS on the youth brain and neurophysiological predictors of response to TBS in this population have not been investigated.

METHODS

Cortical reactivity was assessed at baseline and following 2 weeks of bilateral dorsolateral prefrontal cortex (DLPFC) TBS treatment in 16 youth with depression (aged 16-24 years old). In 16 age-matched health youths, cortical reactivity was assessed twice, 2 weeks apart. Transcranial magnetic stimulation (TMS) combined with electroencephalography was used to assess TMS-evoked potentials in bilateral DLPFC, motor cortices, and intraparietal lobules (IPL). Resting-state functional magnetic resonance imaging (fMRI) data was also collected at baseline.

RESULTS

Left DLPFC pretreatment cortical reactivity, specifically the negativity at 45 ms (i.e., N45), which is related to GABAA neurotransmission, was associated with changes in depressive symptoms. Furthermore, TBS treatment was found to alter the N45 in the right IPL, a site distal to the treatment sites. The magnitude of the right IPL N45 modulation was correlated with the baseline fMRI connectivity between the right IPL and right DLPFC.

CONCLUSIONS

TMS-probed cortical inhibition at the site of TBS application may have potential as a predictor of treatment response in youth depression. Furthermore, pre-treatment functional connectivity may predict the impact of TBS on the neurophysiology of regions distal to the stimulation site. Collectively, these results offer novel neurophysiological insights into the application of TBS for youth depression, which may facilitate its wider use in the youth population.

Influence of theta-burst transcranial magnetic stimulation over the dorsolateral prefrontal cortex on emotion processing in healthy volunteers

Repetitive transcranial magnetic stimulation is a potential treatment option for depression, with the newer intermittent theta-burst stimulation (iTBS) protocols providing brief intervention. However, their mechanism of action remains unclear. We investigated the hypothesis that iTBS influences brain circuits involved in emotion processing that are also affected by antidepressants. We predicted that iTBS would lead to changes in performance on emotion-processing tasks. We investigated the effects of intermittent TBS (iTBS) over the left dorsolateral prefrontal cortex (DLPFC) on the processing of emotional information (word recall and categorization, facial emotion recognition, and decision-making) in 28 healthy volunteers by contrasting these effects with those of sham stimulation. Each volunteer received iTBS and sham stimulation in a blinded crossover design and completed the emotion-processing tasks before and after stimulation. Compared to sham stimulation, iTBS increased positive affective processing for word recall, yet had an unexpected effect on facial emotion recognition for happy and sad faces. There was no evidence of an effect on decision-making or word categorization. We found support for our hypothesis that iTBS influences emotion processing, though some changes were not in the expected direction. These findings suggest a possible common mechanism of action between iTBS and antidepressants, and a complex neural circuitry involved in emotion processing that could potentially be tapped into via brain stimulation. Future research should investigate the neural correlates of emotion processing more closely to inform future iTBS protocols.

Intermittent theta burst stimulation facilitates functional connectivity from the dorsal premotor cortex to primary motor cortex

Background

Motor information in the brain is transmitted from the dorsal premotor cortex (PMd) to the primary motor cortex (M1), where it is further processed and relayed to the spinal cord to eventually generate muscle movement. However, how information from the PMd affects M1 processing and the final output is unclear. Here, we applied intermittent theta burst stimulation (iTBS) to the PMd to alter cortical excitability not only at the application site but also at the PMd projection site of M1. We aimed to determine how PMd iTBS–altered information changed M1 processing and the corticospinal output.

Methods

In total, 16 young, healthy participants underwent PMd iTBS with 600 pulses (iTBS600) or sham-iTBS600. Corticospinal excitability, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were measured using transcranial magnetic stimulation before and up to 60 min after stimulation.

Results

Corticospinal excitability in M1 was significantly greater 15 min after PMd iTBS600 than that after sham-iTBS600 (p = 0.012). Compared with that after sham-iTBS600, at 0 (p = 0.014) and 15 (p = 0.037) min after iTBS600, SICI in M1 was significantly decreased, whereas 15 min after iTBS600, ICF in M1 was significantly increased (p = 0.033).

Conclusion

Our results suggested that projections from the PMd to M1 facilitated M1 corticospinal output and that this facilitation may be attributable in part to decreased intracortical inhibition and increased intracortical facilitation in M1. Such a facilitatory network may inform future understanding of the allocation of resources to achieve optimal motion output.