Direct demonstration of the effects of repetitive paired-pulse transcranial magnetic stimulation at I-wave periodicity

Repetitive paired-pulse TMS increases motor cortex excitability and visuomotor skill acquisition in young and older adults

Transcranial magnetic stimulation (TMS) over primary motor cortex (M1) recruits indirect (I) waves that can be modulated by repetitive paired-pulse TMS (rppTMS). The purpose of this study was to examine the effect of rppTMS on M1 excitability and visuomotor skill acquisition in young and older adults. A total of 37 healthy adults (22 young, 18-32 yr; 15 older, 60-79 yr) participated in a study that involved rppTMS at early (1.4 ms) and late (4.5 ms) interstimulus intervals (ISIs), followed by the performance of a visuomotor training task. M1 excitability was examined with motor-evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. We found that rppTMS increased M1 excitability in young and old adults, with the greatest effects for PA TMS at the late ISI (4.5 ms). Motor skill acquisition was improved by rppTMS at an early (1.4 ms) but not late (4.5 ms) ISI in young and older adults. An additional study using a non-I-wave interval (3.5 ms) also showed increased M1 excitability and visuomotor skill acquisition. These findings show that rppTMS at both I-wave and non-I-wave intervals can alter M1 excitability and improve visuomotor skill acquisition in young and older adults.

Effect of current direction and muscle activation on motor cortex neuroplasticity induced by repetitive paired-pulse transcranial magnetic stimulation

Repetitive paired-pulse transcranial magnetic stimulation (TMS) at indirect (I)-wave periodicity (iTMS) can increase plasticity in primary motor cortex (M1). Both TMS coil orientation and muscle activation can influence I-wave activity, but it remains unclear how these factors influence M1 plasticity with iTMS. We therefore investigated the influence of TMS coil orientation and muscle activation on the response to iTMS. Thirty-two young adults (24.2 ± 4.8 years) participated in three experiments. Each experiment included two sessions using a modified iTMS intervention with either a posterior-anterior orientation (PA) or anterior-posterior (AP) coil orientation over M1. Stimulation was applied in resting (Experiments 1 and 3) or active muscle (Experiments 2 and 3). Effects of iTMS on M1 excitability were assessed by recording motor evoked potentials (MEPs) and short-interval intracortical facilitation (SICF) with PA and AP orientations in both resting (all experiments) and active (Experiment 2) muscle. For the resting intervention, MEPs were greater after AP iTMS (Experiment 1, P = .046), whereas SICF was comparable between interventions (all P > .10). For the active intervention, responses did not vary between PA and AP iTMS (Experiment 2, all P > .14), and muscle activation reduced the effect of AP iTMS during the intervention (Experiment 3, P = .002). Coil orientation influenced the MEP response after iTMS, and muscle activation reduced the response during iTMS. While this suggests that AP iTMS may be beneficial in producing a neuroplastic modulation of I-wave circuits in resting muscle, further exploration of factors such as dosing is required.

Different effects of I-wave periodicity repetitive TMS on motor cortex interhemispheric interaction

Background

Activity of the neural circuits in the human motor cortex can be probed using transcranial magnetic stimulation (TMS). Changing TMS-induced current direction recruits different cortical neural circuits. I-wave periodicity repetitive TMS (iTMS) substantially modulates motor cortex excitability through neural plasticity, yet its effect on interhemispheric interaction remains unclear.

Objective

To explore the modulation of interhemispheric interaction by iTMS applied in different current directions.

Materials and Methods

Twenty right-handed healthy young volunteers (aged 27.5 ± 5.0 years) participated in this study with three visits. On each visit, iTMS in posterior-anterior/anterior-posterior direction (PA-/AP-iTMS) or sham-iTMS was applied to the right hemisphere, with corticospinal excitability and intracortical facilitation of the non-stimulated left hemisphere evaluated at four timepoints. Ipsilateral silent period was also measured at each timepoint probing interhemispheric inhibition (IHI).

Results

PA- and AP-iTMS potentiated cortical excitability concurrently in the stimulated right hemisphere. Corticospinal excitability of the non-stimulated left hemisphere increased 10 min after both PA- and AP-iTMS intervention, with a decrease in short-interval intracortical facilitation (SICF) observed in AP-iTMS only. Immediately after the intervention, PA-iTMS tilted the IHI balance toward inhibiting the non-stimulated hemisphere, while AP-iTMS shifted the balance toward the opposite direction.

Conclusions

Our findings provide systematic evidence on the plastic modulation of interhemispheric interaction by PA- and AP-iTMS. We show that iTMS induces an interhemispheric facilitatory effect, and that PA- and AP-iTMS differs in modulating interhemispheric inhibition.

Is it possible to compare inhibitory and excitatory intracortical circuits in face and hand primary motor cortex?

Abstract

Face muscles are important in a variety of different functions, such as feeding, speech and communication of non‐verbal affective states, which require quite different patterns of activity from those of a typical hand muscle. We ask whether there are differences in their neurophysiological control that might reflect this. Fifteen healthy individuals were studied. Standard single‐ and paired‐pulse transcranial magnetic stimulation (TMS) methods were used to compare intracortical inhibitory (short interval intracortical inhibition (SICI); cortical silent period (CSP)) and excitatory circuitries (short interval intracortical facilitation (SICF)) in two typical muscles, the depressor anguli oris (DAO), a face muscle, and the first dorsal interosseous (FDI), a hand muscle. TMS threshold was higher in DAO than in FDI. Over a range of intensities, resting SICF was not different between DAO and FDI, while during muscle activation SICF was stronger in FDI than in DAO (P = 0.012). At rest, SICI was stronger in FDI than in DAO (P = 0.038) but during muscle contraction, SICI was weaker in FDI than in DAO (P = 0.034). We argue that although many of the difference in response to the TMS protocols could result from the difference in thresholds, some, such as the reduction of resting SICI in DAO, may reflect fundamental differences in the physiology of the two muscle groups.

Key points

Transcranial magnetic stimulation (TMS) single‐ and paired‐pulse protocols were used to investigate and compare the activity of facilitatory and inhibitory intracortical circuits in a face (depressor anguli oris; DAO) and hand (first dorsal interosseous; FDI) muscles. Several TMS intensities and interstimulus intervals were tested with the target muscles at rest and when voluntarily activated.

At rest, intracortical inhibitory activity was stronger in FDI than in DAO. In contrast, during muscle contraction inhibitory activity was stronger in DAO than in FDI. As many previous reports have found, the motor evoked potential threshold was higher in DAO than in FDI.

Although many of the differences in response to the TMS protocols could result from the difference in thresholds, some, such as the reduction of resting short interval intracortical inhibition in DAO, may reflect fundamental differences in the physiology of the two muscle groups.

Cortical mechanisms underlying variability in intermittent theta-burst stimulation-induced plasticity: A TMS-EEG study

OBJECTIVE

To test the hypothesis that intermittent theta burst stimulation (iTBS) variability depends on the ability to engage specific neurons in the primary motor cortex (M1).

METHODS

In a sham-controlled interventional study on 31 healthy volunteers, we used concomitant transcranial magnetic stimulation (TMS) and electroencephalography (EEG). We compared baseline motor evoked potentials (MEPs), M1 iTBS-evoked EEG oscillations, and resting-state EEG (rsEEG) between subjects who did and did not show MEP facilitation following iTBS. We also investigated whether baseline MEP and iTBS-evoked EEG oscillations could explain inter and intraindividual variability in iTBS aftereffects.

RESULTS

The facilitation group had smaller baseline MEPs than the no-facilitation group and showed more iTBS-evoked EEG oscillation synchronization in the alpha and beta frequency bands. Resting-state EEG power was similar between groups and iTBS had a similar non-significant effect on rsEEG in both groups. Baseline MEP amplitude and beta iTBS-evoked EEG oscillation power explained both inter and intraindividual variability in MEP modulation following iTBS.

CONCLUSIONS

The results show that variability in iTBS-associated plasticity depends on baseline corticospinal excitability and on the ability of iTBS to engage M1 beta oscillations.

SIGNIFICANCE

These observations can be used to optimize iTBS investigational and therapeutic applications.

A specific phase of transcranial alternating current stimulation at the β frequency boosts repetitive paired-pulse TMS-induced plasticity

Transcranial alternating current stimulation (tACS) at 20 Hz (β) has been shown to modulate motor evoked potentials (MEPs) when paired with transcranial magnetic stimulation (TMS) in a phase-dependent manner. Repetitive paired-pulse TMS (rPPS) with I-wave periodicity (1.5 ms) induced short-lived facilitation of MEPs. We hypothesized that tACS would modulate the facilitatory effects of rPPS in a frequency- and phase-dependent manner. To test our hypothesis, we investigated the effects of combined tACS and rPPS. We applied rPPS in combination with peak or trough phase tACS at 10 Hz (α) or β, or sham tACS (rPPS alone). The facilitatory effects of rPPS in the sham condition were temporary and variable among participants. In the β tACS peak condition, significant increases in single-pulse MEPs persisted for over 30 min after the stimulation, and this effect was stable across participants. In contrast, β tACS in the trough condition did not modulate MEPs. Further, α tACS parameters did not affect single-pulse MEPs after the intervention. These results suggest that a rPPS-induced increase in trans-synaptic efficacy could be strengthened depending on the β tACS phase, and that this technique could produce long-lasting plasticity with respect to cortical excitability.

Strength-Duration Relationship in Paired-pulse Transcranial Magnetic Stimulation (TMS) and Its Implications for Repetitive TMS

BACKGROUND

Paired-pulse protocols have played a pivotal role in neuroscience research using transcranial magnetic stimulation (TMS). Stimulus parameters have been optimized over the years. More recently, pulse width (PW) has been introduced to this field as a new parameter, which may further fine-tune paired-pulse protocols. The relationship between the PW and effectiveness of a stimulus is known as the "strength-duration relationship".

OBJECTIVE

To test the "strength-duration relationship", so as to improve paired-pulse TMS protocols, and to apply the results to develop new repetitive TMS (rTMS) methods.

METHODS

Four protocols were investigated separately: short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), short-interval intracortical facilitation (SICF) and long-interval intracortical inhibition (LICI). First, various stimulus parameters were tested to identify those yielding the largest facilitation or inhibition of the motor evoked potential (MEP) in each participant. Using these parameters, paired-pulse stimulations were repeated every five seconds for 30 minutes (repetitive paired-pulse stimulation, rPPS). The after-effects of rPPS were measured using MEP amplitude as an index of motor-cortical excitability.

RESULTS

Altogether, the effect of changing PW was similar to that of changing the stimulus intensity in the conventional settings. The best parameters were different for each participant. When these parameters were used, rPPS based on either SICF or ICF induced an increase in MEP amplitude.

CONCLUSIONS

PW was introduced as a new parameter in paired-pulse TMS. Modulation of PW influenced the results of paired-pulse protocols. rPPS using facilitatory protocols can be a good candidate to induce enhancement of motor-cortical excitability.

Two distinct interneuron circuits in human motor cortex are linked to different subsets of physiological and behavioral plasticity

How does a single brain region participate in multiple behaviors? Here we argue that two separate interneuron circuits in the primary motor cortex (M1) contribute differently to two varieties of physiological and behavioral plasticity. To test this in human brain noninvasively, we used transcranial magnetic stimulation (TMS) of M1 hand area to activate two independent sets of synaptic inputs to corticospinal neurons by changing the direction of current induced in the brain: posterior-to-anterior current (PA inputs) and anterior-to-posterior current (AP inputs). We demonstrate that excitability changes produced by repetitive activation of AP inputs depend on cerebellar activity and selectively alter model-based motor learning. In contrast, the changes observed with repetitive stimulation of PA inputs are independent of cerebellar activity and specifically modulate model-free motor learning. The findings are highly suggestive that separate circuits in M1 subserve different forms of motor learning.

Corticospinal activity evoked and modulated by non-invasive stimulation of the intact human motor cortex

A number of methods have been developed recently that stimulate the human brain non-invasively through the intact scalp. The most common are transcranial magnetic stimulation (TMS), transcranial electric stimulation (TES) and transcranial direct current stimulation (TDCS). They are widely used to probe function and connectivity of brain areas as well as therapeutically in a variety of conditions such as depression or stroke. They are much less focal than conventional invasive methods which use small electrodes placed on or in the brain and are often thought to activate all classes of neurones in the stimulated area. However, this is not true. A large body of evidence from experiments on the motor cortex shows that non-invasive methods of brain stimulation can be surprisingly selective and that adjusting the intensity and direction of stimulation can activate different classes of inhibitory and excitatory inputs to the corticospinal output cells. Here we review data that have elucidated the action of TMS and TES, concentrating mainly on the most direct evidence available from spinal epidural recordings of the descending corticospinal volleys. The results show that it is potentially possible to test and condition specific neural circuits in motor cortex that could be affected differentially by disease, or be used in different forms of natural behaviour. However, there is substantial interindividual variability in the specificity of these protocols. Perhaps in the future it will be possible, with the advances currently being made to model the electrical fields induced in individual brains, to develop forms of stimulation that can reliably target more specific populations of neurones, and open up the internal circuitry of the motor cortex for study in behaving humans.

Evidence for high-fidelity timing-dependent synaptic plasticity of human motor cortex

A single transcranial magnetic stimulation (TMS) pulse typically evokes a short series of spikes in corticospinal neurons [known as indirect (I)-waves] which are thought to arise from transynaptic input. Delivering a second pulse at inter-pulse intervals (IPIs) corresponding to the timing of these I-waves leads to a facilitation of the response, and if stimulus pairs are delivered repeatedly, a persistent LTP-like increase in excitability can occur. This has been demonstrated at an IPI of 1.5 ms, which corresponds to the first I-wave interval, in an intervention referred to as ITMS (I-wave TMS), and it has been argued that this may have similarities with timing-dependent plasticity models. Consequently, we hypothesized that if the second stimulus is delivered so as not to coincide with I-wave timing, it should lead to LTD. We performed a crossover study in 10 subjects in which TMS doublets were timed to coincide (1.5-ms IPI, ITMS1.5) or not coincide (2-ms IPI, ITMS2) with I-wave firing. Single pulse motor-evoked potential (MEP) amplitude, resting motor threshold (RMT), and short-interval cortical inhibition (SICI) were measured from the first dorsal interosseous (FDI) muscle. After ITMS1.5 corticomotor excitability was increased by ∼60% for 15 min ( P < 0.05) and returned to baseline by 20 min. Increasing the IPI by just 500 μs to 2 ms reversed the aftereffect, and MEP amplitude was significantly reduced (∼35%, P < 0.05) for 15 min before returning to baseline. This reduction was not associated with an increase in SICI, suggesting a reduction in excitatory transmission rather than an increase in inhibitory efficacy. RMT also remained unchanged, suggesting that these changes were not due to changes in membrane excitability. Amplitude-matching ITMS2 did not modulate excitability. The results are consistent with timing-dependent synaptic LTP/D-like effects and suggest that there are plasticity mechanisms operating in the human motor cortex with a temporal resolution of the order of a few hundreds of microseconds.

A new approach for corticospinal tract reconstruction based on navigated transcranial stimulation and standardized fractional anisotropy values

PURPOSE

To establish a novel approach for fiber tracking based on navigated transcranial magnetic stimulation (nTMS) mapping of the primary motor cortex and to propose a new algorithm for determination of an individualized fractional anisotropy value for reliable and objective fiber tracking.

METHODS

50 patients (22 females, 28 males, median age 58 years (20-80)) with brain tumors compromising the primary motor cortex and the corticospinal tract underwent preoperative MR imaging and nTMS mapping. Stimulation spots evoking muscle potentials (MEP) closest to the tumor were imported into the fiber tracking software and set as seed points for tractography. Next the individual FA threshold, i.e. the highest FA value leading to visualization of tracts at a predefined minimum fiber length of 110 mm, was determined. Fiber tracking was then performed at a fractional anisotropy value of 75% and 50% of the individual FA threshold. In addition, fiber tracking according to the conventional knowledge-based approach was performed. Results of tractography of either method were presented to the surgeon for preoperative planning and integrated into the navigation system and its impact was rated using a questionnaire.

RESULTS

Mapping of the motor cortex was successful in all patients. A fractional anisotropy threshold for corticospinal tract reconstruction could be obtained in every case. TMS-based results changed or modified surgical strategy in 23 of 50 patients (46%), whereas knowledge-based results would have changed surgical strategy in 11 of 50 patients (22%). Tractography results facilitated intraoperative orientation and electrical stimulation in 28 of 50 (56%) patients. Tracking at 75% of the individual FA thresholds was considered most beneficial by the respective surgeons.

CONCLUSIONS

Fiber tracking based on nTMS by the proposed standardized algorithm represents an objective visualization method based on functional data and provides a valuable instrument for preoperative planning and intraoperative orientation and monitoring.

Transcranial magnetic stimulation in cognitive rehabilitation

Repetitive transcranial magnetic stimulation (rTMS) can generate an increase or a decrease of neuronal excitability, which can modulate cognition and behaviour. Transcranial magnetic stimulation-induced cortical changes have been shown to result in neural plasticity. Thus, TMS provides an important opportunity to gain more insight into the mechanisms responsible for the remarkable flexibility of the central nervous system. The aim of this review was to cover the topics that could be useful when using TMS in the cognitive rehabilitation field after brain damage. The basic TMS principles are introduced, together with the clinical application for diagnosis and prognosis, the biological aspects, and the use in cognitive neuroscience studies. Finally, several hypotheses are discussed to explain the likely mechanisms induced by TMS that favour the recovery of a function after brain damage and cause the adult brain to undergo plasticity. The possibility of non-invasively interacting with the functioning of the brain and its plasticity mechanisms - a possibility that may eventually lead to cognitive and behavioural modifications - opens new and exciting scenarios in the cognitive neurorehabilitation field.

Enhancing the effect of repetitive I-wave paired-pulse TMS (iTMS) by adjusting for the individual I-wave periodicity

BACKGROUND

Repeated application of paired-pulse TMS over the primary motor cortex (M1) in human subjects with an inter-pulse interval (IPI) of 1.5 ms (iTMS(1.5 ms)) has been shown to significantly increase paired-pulse MEP (ppMEP) amplitudes during the stimulation period and increased single-pulse MEP amplitudes for up to 10 minutes after termination of iTMS.

RESULTS

Here we show in a cross-over design that a modified version of the iTMS(1.5 ms) protocol with an I-wave periodicity adjusted to the individual I1-peak wave latency (iTMS(adj)) resulted in a stronger effect on ppMEPs relative to iTMS(1.5 ms).

CONCLUSIONS

Based on these findings, our results indicate that the efficiency of iTMS strongly depends on the individual choice of the IPI and that parameter optimization of the conventional iTMS(1.5 ms) protocol might improve the outcome of this novel non-invasive brain stimulation technique.

Transcranial direct current stimulation effects on I-wave activity in humans

Transcranial direct current stimulation (tDCS) of the human cerebral cortex modulates cortical excitability noninvasively in a polarity-specific manner: anodal tDCS leads to lasting facilitation and cathodal tDCS to inhibition of motor cortex excitability. To further elucidate the underlying physiological mechanisms, we recorded corticospinal volleys evoked by single-pulse transcranial magnetic stimulation of the primary motor cortex before and after a 5-min period of anodal or cathodal tDCS in eight conscious patients who had electrodes implanted in the cervical epidural space for the control of pain. The effects of anodal tDCS were evaluated in six subjects and the effects of cathodal tDCS in five subjects. Three subjects were studied with both polarities. Anodal tDCS increased the excitability of cortical circuits generating I waves in the corticospinal system, including the earliest wave (I1 wave), whereas cathodal tDCS suppressed later I waves. The motor evoked potential (MEP) amplitude changes immediately following tDCS periods were in agreement with the effects produced on intracortical circuitry. The results deliver additional evidence that tDCS changes the excitability of cortical neurons.

Interventional repetitive I-wave transcranial magnetic stimulation (TMS): the dimension of stimulation duration

BACKGROUND

A range of transcranial magnetic stimulation (TMS) techniques are now available to modulate human corticomotor excitability and plasticity. One presumably critical aspect of these interventions is their duration of application.

OBJECTIVE

In the current study, we investigated whether doubling the duration of an intervention would offer any additional benefit, or invoke self-limiting mechanisms controlling corticomotor excitability or synaptic plasticity.

METHODS

We compared (in a cross-over design) corticomotor excitability (to the first dorsal interosseous muscle) during and after a 15-minute (I15) and 30-minute (I30) TMS intervention targeting indirect (I-) wave interaction (iTMS). The interventions consisted of equi-intensity paired stimuli with an interpulse interval (IPI) of 1.5 milliseconds, corresponding to I-wave periodicity, delivered at a frequency of 0.2 Hz.

RESULTS

During both the I15 and I30 interventions, paired-pulse (I-wave) motor evoked potential (iMEP) amplitude significantly increased (by 98.3% and 120.6%, respectively, last versus first minute, P = .001). The increase for I30 occurred in the first 15 minutes, and there was no further change during the remainder of the intervention. Both interventions were equally effective overall. Postintervention, single-pulse MEP amplitude increased by a mean of 91% and 106% (I15 and I30, respectively, P < .01) with no significant difference between interventions.

CONCLUSIONS

We conclude that repetitive iTMS can increase corticomotor excitability after a relatively short intervention period of stimulation, and that a longer stimulation period has no additional benefit or detriment, perhaps as a result of the action of regulatory mechanisms.

Modulation of corticomotor excitability by an I-wave intervention delivered during low-level voluntary contraction

Transcranial magnetic stimulation (TMS) interventions that modulate cortical plasticity may achieve a more functional benefit if combined with neuro-rehabilitation therapies. With a TMS protocol targeting I-wave dynamics, it is possible to deliver stimuli while a subject performs a motor task, and this may more effectively target functional networks related to the task. However, the efficacy of this intervention during a simple task such as a low-level voluntary contraction is not known. We delivered paired-pulse TMS at an inter-pulse interval (IPI) of 1.5 ms for 15 min while subjects performed a 10 ± 2.5% voluntary contraction of the first dorsal interosseous (FDI) muscle and made motor evoked potential (MEP) amplitude and short-interval intracortical facilitation (SICF) curve measurements. Pre-intervention SICF curves showed only a single peak at 1.3-1.5 ms IPI. During the intervention, MEP amplitude steadily increased (P < 0.001) to 137 ± 13% of its initial value. After the intervention, SICF curves were increased in amplitude (P < 0.001) and later peaks emerged at 2.8 and 4.3 ms IPIs. A control experiment, replacing paired-pulse stimulation with single-pulse stimulation showed no effect on MEP amplitude (P = 0.951). We conclude that the I-wave intervention can be administered concurrently with a simple motor task and that it acts by increasing trans-synaptic efficacy across a number of I-waves. The ability to perform a motor task simultaneously with a TMS intervention could confer a degree of specificity to the induced excitability changes and may be beneficial for functional neuro-rehabilitation programs built around motor learning and retraining.

Potentiation of Short-Latency Cortical Responses by High-Frequency Repetitive Transcranial Magnetic Stimulation

It is generally accepted that low- and high-frequency repetitive transcranial magnetic stimulation (rTMS) induces changes in cortical excitability, but there is only indirect evidence of its effects despite a large number of studies employing different stimulation parameters. Typically the cortical modulations are inferred through indirect measurements, such as recording the change in electromyographic responses. Recently it has become possible to directly evaluate rTMS-induced changes at the cortical level using electronencephalography (EEG). The present study investigates the modulation induced by high-frequency rTMS via EEG by evaluating changes in the latency and amplitude of TMS-evoked responses. In this study, rTMS was applied to the left primary motor cortex (MI) in 16 participants while an EEG was simultaneously acquired from 29 scalp electrodes. The rTMS consisted of 40 trains at 20 Hz with 10 stimuli each (a total of 400 stimuli) that were delivered at the individual resting motor threshold. The on-line modulation induced by the high-frequency TMS was characterized by a sequence of EEG responses. Two of the rTMS-induced responses, P5 and N8, were specifically modulated according to the protocol. Their latency decreased from the first to the last TMS stimuli, while the amplitude values increased. These results provide the first direct, on-line evaluation of the effects of high-frequency TMS on EEG activity. In addition, the results provide a direct demonstration of cortical potentiation induced by rTMS in humans.

Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research

This article is based on a consensus conference, which took place in Certosa di Pontignano, Siena (Italy) on March 7-9, 2008, intended to update the previous safety guidelines for the application of transcranial magnetic stimulation (TMS) in research and clinical settings. Over the past decade the scientific and medical community has had the opportunity to evaluate the safety record of research studies and clinical applications of TMS and repetitive TMS (rTMS). In these years the number of applications of conventional TMS has grown impressively, new paradigms of stimulation have been developed (e.g., patterned repetitive TMS) and technical advances have led to new device designs and to the real-time integration of TMS with electroencephalography (EEG), positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Thousands of healthy subjects and patients with various neurological and psychiatric diseases have undergone TMS allowing a better assessment of relative risks. The occurrence of seizures (i.e., the most serious TMS-related acute adverse effect) has been extremely rare, with most of the few new cases receiving rTMS exceeding previous guidelines, often in patients under treatment with drugs which potentially lower the seizure threshold. The present updated guidelines review issues of risk and safety of conventional TMS protocols, address the undesired effects and risks of emerging TMS interventions, the applications of TMS in patients with implanted electrodes in the central nervous system, and safety aspects of TMS in neuroimaging environments. We cover recommended limits of stimulation parameters and other important precautions, monitoring of subjects, expertise of the rTMS team, and ethical issues. While all the recommendations here are expert based, they utilize published data to the extent possible.

Physiology of repetitive transcranial magnetic stimulation of the human brain

During the last two decades, transcranial magnetic stimulation (TMS) has rapidly become a valuable method to investigate noninvasively the human brain. In addition, repetitive TMS (rTMS) is able to induce changes in brain activity that last after stimulation. Therefore, rTMS has therapeutic potential in patients with neurologic and psychiatric disorders. It is, however, unclear by which mechanism rTMS induces these lasting effects on the brain. The effects of rTMS are often described as LTD- or LTP-like, because the duration of these alterations seems to implicate changes in synaptic plasticity. In this review we therefore discuss, based on rTMS experiments and knowledge about synaptic plasticity, whether the physiologic basis of rTMS-effects relates to changes in synaptic plasticity. We present seven lines of evidence that strongly suggest a link between the aftereffects induced by rTMS and the induction of synaptic plasticity. It is, nevertheless, important to realize that at present it is impossible to demonstrate a direct link between rTMS on the one hand and synaptic plasticity on the other. Therefore, we provide suggestions for future, innovating research, aiming to investigate both the local effects of rTMS on the synapse and the effects of rTMS on other, more global levels of brain organization. Only in that way can the aftereffects of rTMS on the brain be completely understood.

Repetitive transcranial magnetic stimulation-associated neurobehavioral gains during coma recovery

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive method to induce changes in cortical neural excitability. This report presents findings from the first participant of a safety and efficacy study that examined a therapeutic rTMS protocol for persons with severe traumatic brain injury (TBI).

OBJECTIVE

The primary hypothesis was that there will be no adverse events related to the provision of a 6-week rTMS protocol for persons with severe TBI who remain, at best, in a minimally conscious state for longer than 3 months. The secondary hypothesis was that the rTMS protocol would induce significant neurobehavioral gains during treatment and that these gains would persist at 6-week follow-up.

METHODS

A 6-week rTMS protocol (30 sessions) was delivered to a 26-year-old man who remained in a vegetative state 287 days after severe TBI. Stimulation was directed over the right dorsolateral prefrontal cortex. Repeated safety measures, neurobehavioral assessments, clinical examinations, and evoked potentials (EP) were obtained at baseline, every fifth rTMS session (weekly), and at a 6-week follow-up.

RESULTS

There were no adverse events related to the provision of rTMS treatment. A trend toward significant (P = .066) neurobehavioral gains was temporally related to provision of rTMS. Left-sided brain stem auditory EP wave V latencies and waves I to V interpeak latencies improved along with neurobehavioral gains during provision of rTMS, suggesting that improved neural conduction in the pathway mediated the neurobehavioral improvements.

CONCLUSIONS

Repetitive TMS merits further investigation as a safe therapeutic intervention to alter neural activity, to modulate neural activity, and/or to facilitate recovery in persons with disordered consciousness subsequent to severe TBI.

Consensus: New methodologies for brain stimulation

We briefly summarized several new stimulation techniques. There are many new methods of human brain stimulation, including modification of already known methods and brand-new methods. In this article, we focused on theta burst stimulation (TBS), repetitive monophasic pulse stimulation, paired- and quadri-pulse stimulation, transcranial alternating current stimulation (tACS), paired associative stimulation, controllable pulse shape TMS (cTMS), and deep-brain TMS. For every method, we summarized the state of the art and discussed issues that remain to be addressed.

Consensus: Motor cortex plasticity protocols

Noninvasive transcranial stimulation is being increasingly used by clinicians and neuroscientists to alter deliberately the status of the human brain. Important applications are the induction of virtual lesions (for example, transient dysfunction) to identify the importance of the stimulated brain network for a certain sensorimotor or cognitive task, and the induction of changes in neuronal excitability, synaptic plasticity or behavioral function outlasting the stimulation, for example, for therapeutic purposes. The aim of this article is to review critically the properties of the different currently used stimulation protocols, including a focus on their particular strengths and weaknesses, to facilitate their appropriate and conscientious application.

Transcranial magnetic stimulation and synaptic plasticity: experimental framework and human models

Interest in the therapeutic potential of non-invasive human brain stimulation has been boosted by an improved understanding of the mechanisms of synaptic plasticity and the stimulus protocols that can induce plasticity in experimental preparations. A range of transcranial magnetic stimulation (TMS) protocols are available that have the potential to mimic these experimental protocols in the human. Repetitive TMS emulates aspects of activity-dependent plasticity, and theta-burst refinements may be able to take into account excitatory and inhibitory networks, paired associative stimulation can extend network considerations to incorporate sensorimotor integration, inhibitory networks may be targeted with short-interval paired stimulation and finally even the precision of spike-timing dependent plasticity may be accessible through I-(indirect)wave dynamics. This review will provide a synthesis of current concepts of activity- and time-dependent plasticity and their homeostatic regulation based on experimental studies, and relate these concepts to the promising range of TMS interventions that are available to target human brain plasticity.