Influence of Waveform and Current Direction on Short-Interval Intracortical Facilitation: A Paired-Pulse TMS Study

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.

Cortical inhibition and facilitation are mediated by distinct physiological processes

A complex interaction of inhibitory and facilitatory interneuronal processes may underlie development of cortical excitability in the human motor cortex. To determine whether distinct interneuronal processes mediated cortical excitability, threshold tracking transcranial magnetic stimulation was utilised to assess cortical excitability, with figure-of-eight coil oriented in posterior-anterior (PA), anterior-posterior (AP) and latero-medial (LM) directions. Motor evoked potential (MEP) responses were recorded over the contralateral abductor pollicis brevis. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF) and intracortical facilitation were recorded. Significant effects of coil orientation were evident on SICI (F = 8.560, P = 0.002) and SICF (F = 7.132, P = 0.003). SICI was greater when recorded with PA (9.7 ± 10.9%, P = 0.029) and AP (13.1 ± 7.0%, P = 0.003) compared to LM (5.2 ± 7.3%) directed currents. SICF was significantly greater with PA (-14.7 ± 8.1%, P = 0.016) and LM (-14.7 ± 8.8%, P = 0.005) compared to AP (-9.1 ± 7.2%) coil orientations. SICI recorded with LM and PA coil orientations were correlated (R = 0.7, P = 0.002), as was SICF recorded with AP vs LM (R = 0.60, P = 0.019) and LM vs PA (R = 0.69, P = 0.002) coil orientations. RMT was significantly smaller with PA compared to AP (P < 0.001) and LM (P = 0.018) stimulation. Recruitment of distinct interneuronal processes with variable cortical orientation and thresholds underlies short interval intracortical inhibition and facilitation.

Classical conditioning of faciliatory paired-pulse TMS

In this proof-of-concept study, we questioned whether the influence of TMS on cortical excitability can be applied to classical conditioning. More specifically, we investigated whether the faciliatory influence of paired-pulse TMS on the excitability of the human motor cortex can be transferred to a simultaneously presented auditory stimulus through conditioning. During the conditioning phase, 75 healthy young participants received 170 faciliatory paired TMS pulses (1st pulse at 95% resting motor threshold, 2nd at 130%, interstimulus interval 12 ms), always presented simultaneously with one out of two acoustic stimuli. In the test phase, 20 min later, we pseudorandomly applied 100 single TMS pulses (at 130% MT), 50 paired with the conditioned tone-50 paired with a control tone. Using the Wilcoxon-Signed Rank test, we found significantly enhanced median amplitudes of motor evoked potentials (MEPs) paired with the conditioned tone as compared to the control tone, suggesting successful conditioning (p = 0.031, responder rate 55%, small effect size of r = - 0.248). The same comparison in only those participants with a paired-pulse amplitude < 2 mV in the conditioning phase, increased the responder rate to 61% (n = 38) and effect size to moderate (r = - 0.389). If we considered only those participants with a median paired-pulse amplitude < 1 mV, responder rate increased further to 79% (n = 14) and effect size to r = - 0.727 (i.e., large effect). These findings suggest increasingly stronger conditioning effects for smaller MEP amplitudes during paired-pulse TMS conditioning. These proof-of-concept findings extend the scope of classical conditioning to faciliatory paired-pulse TMS.

TMS combined with EEG: Recommendations and open issues for data collection and analysis

Transcranial magnetic stimulation (TMS) evokes neuronal activity in the targeted cortex and connected brain regions. The evoked brain response can be measured with electroencephalography (EEG). TMS combined with simultaneous EEG (TMS-EEG) is widely used for studying cortical reactivity and connectivity at high spatiotemporal resolution. Methodologically, the combination of TMS with EEG is challenging, and there are many open questions in the field. Different TMS-EEG equipment and approaches for data collection and analysis are used. The lack of standardization may affect reproducibility and limit the comparability of results produced in different research laboratories. In addition, there is controversy about the extent to which auditory and somatosensory inputs contribute to transcranially evoked EEG. This review provides a guide for researchers who wish to use TMS-EEG to study the reactivity of the human cortex. A worldwide panel of experts working on TMS-EEG covered all aspects that should be considered in TMS-EEG experiments, providing methodological recommendations (when possible) for effective TMS-EEG recordings and analysis. The panel identified and discussed the challenges of the technique, particularly regarding recording procedures, artifact correction, analysis, and interpretation of the transcranial evoked potentials (TEPs). Therefore, this work offers an extensive overview of TMS-EEG methodology and thus may promote standardization of experimental and computational procedures across groups.

Cerebellar transcranial direct current stimulation disrupts neuroplasticity of intracortical motor circuits

While previous research using transcranial magnetic stimulation (TMS) suggest that cerebellum (CB) influences the neuroplastic response of primary motor cortex (M1), the role of different indirect (I) wave inputs in M1 mediating this interaction remains unclear. The aim of this study was therefore to assess how CB influences neuroplasticity of early and late I-wave circuits. 22 young adults (22 ± 2.7 years) participated in 3 sessions in which I-wave periodicity repetitive transcranial magnetic stimulation (iTMS) was applied over M1 during concurrent application of cathodal transcranial direct current stimulation over CB (tDCS_CB). In each session, iTMS either targeted early I-waves (1.5 ms interval; iTMS_1.5), late I-waves (4.5 ms interval; iTMS_4.5), or had no effect (variable interval; iTMS_Sham). Changes due to the intervention were examined with motor evoked potential (MEP) amplitude using TMS protocols measuring corticospinal excitability (MEP_1mV) and the strength of CB-M1 connections (CBI). In addition, we indexed I-wave activity using short-interval intracortical facilitation (SICF) and low-intensity single-pulse TMS applied with posterior-anterior (MEP_PA) and anterior-posterior (MEP_AP) current directions. Following both active iTMS sessions, there was no change in MEP_1mV, CBI or SICF (all P > 0.05), suggesting that tDCS_CB broadly disrupted the excitatory response that is normally seen following iTMS. However, although MEP_AP also failed to facilitate after the intervention (P > 0.05), MEP_PA potentiated following both active iTMS sessions (both P < 0.05). This differential response between current directions could indicate a selective effect of CB on AP-sensitive circuits.

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

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

TMS with fast and accurate electronic control: Measuring the orientation sensitivity of corticomotor pathways

BACKGROUND

Transcranial magnetic stimulation (TMS) coils allow only a slow, mechanical adjustment of the stimulating electric field (E-field) orientation in the cerebral tissue. Fast E-field control is needed to synchronize the stimulation with the ongoing brain activity. Also, empirical models that fully describe the relationship between evoked responses and the stimulus orientation and intensity are still missing.

OBJECTIVE

We aimed to (1) develop a TMS transducer for manipulating the E-field orientation electronically with high accuracy at the neuronally meaningful millisecond-level time scale and (2) devise and validate a physiologically based model describing the orientation selectivity of neuronal excitability.

METHODS

We designed and manufactured a two-coil TMS transducer. The coil windings were computed with a minimum-energy optimization procedure, and the transducer was controlled with our custom-made electronics. The electronic E-field control was verified with a TMS characterizer. The motor evoked potential amplitude and latency of a hand muscle were mapped in 3° steps of the stimulus orientation in 16 healthy subjects for three stimulation intensities. We fitted a logistic model to the motor response amplitude.

RESULTS

The two-coil TMS transducer allows one to manipulate the pulse orientation accurately without manual coil movement. The motor response amplitude followed a logistic function of the stimulus orientation; this dependency was strongly affected by the stimulus intensity.

CONCLUSION

The developed electronic control of the E-field orientation allows exploring new stimulation paradigms and probing neuronal mechanisms. The presented model helps to disentangle the neuronal mechanisms of brain function and guide future non-invasive stimulation protocols.

The Interactions Between Primary Somatosensory and Motor Cortex during Human Grasping Behaviors

Afferent inputs to the primary somatosensory cortex (S1) are differentially processed during precision and power grip in humans. However, it remains unclear how S1 interacts with the primary motor cortex (M1) during these two grasping behaviors. To address this question, we measured short-latency afferent inhibition (SAI), reflecting S1-M1 interactions via thalamo-cortical pathways, using paired-pulse transcranial magnetic stimulation (TMS) during precision and power grip. The TMS coil over the hand representation of M1 was oriented in the posterior-anterior (PA) and anterior-posterior (AP) direction to activate distinct sets of corticospinal neurons. We found that SAI increased during precision compared with power grip when AP, but not PA, currents were applied. Notably, SAI tested in the AP direction were similar during two-digit than five-digit precision grip. The M1 receives movement information from S1 through direct cortico-cortical pathways, so intra-hemispheric S1-M1 interactions using dual-site TMS were also evaluated. Stimulation of S1 attenuated M1 excitability (S1-M1 inhibition) during precision and power grip, while the S1-M1 inhibition ratio remained similar across tasks. Taken together,our findings suggest that distinct neural mechanisms for S1-M1 interactions mediate precision and power grip, presumably by modulating neural activity along thalamo-cortical pathways.

Effect of stimulus orientation and intensity on short-interval intracortical inhibition (SICI) and facilitation (SICF): A multi-channel transcranial magnetic stimulation study

Besides stimulus intensities and interstimulus intervals (ISI), the electric field (E-field) orientation is known to affect both short-interval intracortical inhibition (SICI) and facilitation (SICF) in paired-pulse transcranial magnetic stimulation (TMS). However, it has yet to be established how distinct orientations of the conditioning (CS) and test stimuli (TS) affect the SICI and SICF generation. With the use of a multi-channel TMS transducer that provides electronic control of the stimulus orientation and intensity, we aimed to investigate how changes in the CS and TS orientation affect the strength of SICI and SICF. We hypothesized that the CS orientation would play a major role for SICF than for SICI, whereas the CS intensity would be more critical for SICI than for SICF. In eight healthy subjects, we tested two ISIs (1.5 and 2.7 ms), two CS and TS orientations (anteromedial (AM) and posteromedial (PM)), and four CS intensities (50, 70, 90, and 110% of the resting motor threshold (RMT)). The TS intensity was fixed at 110% RMT. The intensities were adjusted to the corresponding RMT in the AM and PM orientations. SICI and SICF were observed in all tested CS and TS orientations. SICI depended on the CS intensity in a U-shaped manner in any combination of the CS and TS orientations. With 70% and 90% RMT CS intensities, stronger PM-oriented CS induced stronger inhibition than weaker AM-oriented CS. Similar SICF was observed for any CS orientation. Neither SICI nor SICF depended on the TS orientation. We demonstrated that SICI and SICF could be elicited by the CS perpendicular to the TS, which indicates that these stimuli affected either overlapping or strongly connected neuronal populations. We concluded that SICI is primarily sensitive to the CS intensity and that CS intensity adjustment resulted in similar SICF for different CS orientations.

Two forms of short-interval intracortical inhibition in human motor cortex

Background

Pulses of transcranial magnetic stimulation (TMS) with a predominantly anterior-posterior (AP) or posterior-anterior (PA) current direction over the primary motor cortex appear to activate distinct excitatory inputs to corticospinal neurons. In contrast, very few reports have examined whether the inhibitory neurons responsible for short-interval intracortical inhibition (SICI) are sensitive to TMS current direction.

Objectives

To investigate whether SICI evaluated with AP and PA conditioning stimuli (CS_PA and CS_AP) activate different inhibitory pathways. SICI was always assessed using a PA-oriented test stimulus (TS_PA).

Methods

Using two superimposed TMS coils, CS_PA and CS_AP were applied at interstimulus intervals (ISI) of 1–5 ms before a TS_PA, and at a range of different intensities. Using a triple stimulation design, we then tested whether SICI at ISI of 3 ms using opposite directions of CS (SICI_CSPA3 and SICI_CSAP3) interacted differently with three other forms of inhibition, including SICI at ISI of 2 ms (SICI_CSPA2), cerebellum-motor cortex inhibition (CBI 5 ms) and short-latency afferent inhibition (SAI 22 ms). Finally, we compared the effect of tonic and phasic voluntary contraction on SICI_CSPA3 and SICI_CSAP3.

Results

CS_AP produced little SICI at ISIs = 1 and 2 ms. However, at ISI = 3 ms, both CS_AP and CS_PA were equally effective at the same percent of maximum stimulator output. Despite this apparent similarity, combining SICI_CSPA3 or SICI_CSAP3 with other forms of inhibition led to quite different results: SICI_CSPA3 interacted in complex ways with CBI, SAI and SICI_CSPA2, whereas the effect of SICI_CSAP3 appeared to be quite independent of them. Although SICI_CSPA and SICI_CSAP were both reduced by the same amount during voluntary tonic contraction compared with rest, in a simple reaction time task SICI_CSAP was disinhibited much earlier following the imperative signal than SICI_CSPA.

Conclusions

SICI_CSPA appears to activate a different inhibitory pathway to that activated by SICI_CSAP. The difference is behaviourally relevant since the pathways are controlled differently during volitional contraction. The results may explain some previous pathological data and open the possibility of testing whether these pathways are differentially recruited in a range of tasks.

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Opposite directions of conditioning stimulus (CS) used to suppress MEPs evoked by a conventional test stimulus.

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Different directions of CS have different time courses of short-interval intracortical inhibition (SICI).

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They also interact differently with short-latency afferent inhibition and with cerebellar inhibition.

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They are differently affected in a reaction time task.

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We suggest there are two forms of SICI in motor cortex.

Modulation of Motor Cortex Plasticity by Repetitive Paired-Pulse TMS at Late I-Wave Intervals Is Influenced by Intracortical Excitability

The late indirect (I)-waves recruited by transcranial magnetic stimulation (TMS) over primary motor cortex (M1) can be modulated using I-wave periodicity repetitive TMS (iTMS). The purpose of this study was to determine if the response to iTMS is influenced by different interstimulus intervals (ISIs) targeting late I-waves, and whether these responses were associated with individual variations in intracortical excitability. Seventeen young (27.2 ± 6.4 years, 12 females) healthy adults received iTMS at late I-wave intervals (4.0, 4.5, and 5.0 ms) in three separate sessions. Changes due to each intervention were examined with motor evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using both posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. Changes in MEP amplitude and SICF were influenced by iTMS ISI, with the greatest facilitation for ISIs at 4 and 5 ms with PA TMS, and 4 ms with AP TMS. Maximum SICF at baseline (irrespective of ISI) was associated with increased iTMS response, but only for PA stimulation. These results suggest that modifying iTMS parameters targeting late I-waves can influence M1 plasticity. They also suggest that maximum SICF may be a means by which responders to iTMS targeting the late I-waves could be identified.

Preferential Activation of Unique Motor Cortical Networks With Transcranial Magnetic Stimulation: A Review of the Physiological, Functional, and Clinical Evidence

OBJECTIVES

The corticospinal volley produced by application of transcranial magnetic stimulation (TMS) over primary motor cortex consists of a number of waves generated by trans-synaptic input from interneuronal circuits. These indirect (I)-waves mediate the sensitivity of TMS to cortical plasticity and intracortical excitability and can be assessed by altering the direction of cortical current induced by TMS. While this methodological approach has been conventionally viewed as preferentially recruiting early or late I-wave inputs from a given populations of neurons, growing evidence suggests recruitment of different neuronal populations, and this would strongly influence interpretation and application of these measures. The aim of this review is therefore to consider the physiological, functional, and clinical evidence for the independence of the neuronal circuits activated by different current directions.

MATERIALS AND METHODS

To provide the relevant context, we begin with an overview of TMS methodology, focusing on the different techniques used to quantify I-waves. We then comprehensively review the literature that has used variations in coil orientation to investigate the I-wave circuits, grouping studies based on the neurophysiological, functional, and clinical relevance of their outcomes.

RESULTS

Review of the existing literature reveals significant evidence supporting the idea that varying current direction can recruit different neuronal populations having unique functionally and clinically relevant characteristics.

CONCLUSIONS

Further research providing greater characterization of the I-wave circuits activated with different current directions is required. This will facilitate the development of interventions that are able to modulate specific intracortical circuits, which will be an important application of TMS.

Direct comparison of efficacy of the motor cortical plasticity induction and the interindividual variability between TBS and QPS

BACKGROUND

Theta burst stimulation (TBS) and quadripulse stimulation (QPS) are known to induce synaptic plasticity in humans. There have been no head-to-head comparisons of the efficacy and variability between TBS and QPS.

OBJECTIVE

To compare the efficacy and interindividual variability between the original TBS and QPS protocols. We hypothesized that QPS would be more effective and less variable than TBS.

METHODS

Forty-six healthy subjects participated in this study. Thirty subjects participated in the main comparison experiment, and the other sixteen subjects participated in the experiment to obtain natural variation in motor-evoked potentials. The facilitatory effects were compared between intermittent TBS (iTBS) and QPS5, and the inhibitory effects were compared between continuous TBS (cTBS) and QPS50. The motor-evoked potential amplitudes elicited by transcranial magnetic stimulation over the primary motor cortex were measured before the intervention and every 5 min after the intervention for 1 h. To investigate the interindividual variability, the responder/nonresponder/opposite-responder rates were also analyzed.

RESULTS

The facilitatory effects of QPS5 were greater than those of iTBS, and the inhibitory effects of QPS50 were much stronger than those of cTBS. The responder rate of QPS was significantly higher than that of TBS. QPS had a smaller number of opposite responders than TBS.

CONCLUSION

QPS is more effective and stable for synaptic plasticity induction than TBS.

I-waves in motor cortex revisited

I-waves represent high-frequency (~ 600 Hz) repetitive discharge of corticospinal fibers elicited by single-pulse stimulation of motor cortex. First detected and examined in animal preparations, this multiple discharge can also be recorded in humans from the corticospinal tract with epidural spinal electrodes. The exact underpinning neurophysiology of I-waves is still unclear, but there is converging evidence that they originate at the cortical level through synaptic input from specific excitatory interneuronal circuitries onto corticomotoneuronal cells, controlled by GABAAergic interneurons. In contrast, there is at present no supportive evidence for the alternative hypothesis that I-waves are generated by high-frequency oscillations of the membrane potential of corticomotoneuronal cells upon initial strong depolarization. Understanding I-wave physiology is essential for understanding how TMS activates the motor cortex.

TMS coil orientation and muscle activation influence lower limb intracortical excitability

INTRODUCTION

Previous research with transcranial magnetic stimulation (TMS) indicates that coil orientation (TMS current direction) and muscle activation state (rest or active) modify corticospinal and intracortical excitability of upper limb muscles. However, the extent to which these factors influence corticospinal and intracortical excitability of lower limb muscles is unknown. This study aimed to examine how variations in coil orientation and muscle activation affect corticospinal and intracortical excitability of tibialis anterior (TA), a lower leg muscle.

METHODS

In 21 young (21.6 ± 3.3 years, 11 female) adults, TMS was administered to the motor cortical representation of TA in posterior-anterior (PA) and mediolateral (ML) orientations at rest and during muscle activation. Single-pulse TMS measures of motor evoked potential amplitude, in addition to resting and active motor thresholds, were used to index corticospinal excitability, whereas paired-pulse TMS measures of short-interval intracortical inhibition (SICI) and facilitation (SICF), and long-interval intracortical inhibition (LICI), were used to assess excitability of intracortical circuits.

RESULTS

For single-pulse TMS, motor thresholds and test TMS intensity were lower for ML stimulation (all P < 0.05). In a resting muscle, ML TMS produced greater SICI (P < 0.001) and less SICF (both P < 0.05) when compared with PA TMS. In contrast, ML TMS in an active muscle resulted in reduced SICI but increased SICF (both P ≤ 0.001) when compared with PA TMS.

CONCLUSION

TMS coil orientation and muscle activation influence measurements of intracortical excitability recorded in the tibialis anterior, and are therefore important considerations in TMS studies of lower limb muscles.

Characterising the influence of cerebellum on the neuroplastic modulation of intracortical motor circuits

The cerebellum (CB) has extensive connections with both cortical and subcortical areas of the brain, and is known to strongly influence function in areas it projects to. In particular, research using non-invasive brain stimulation (NIBS) has shown that CB projections to primary motor cortex (M1) are likely important for facilitating the learning of new motor skills, and that this process may involve modulation of late indirect (I) wave inputs in M1. However, the nature of this relationship remains unclear, particularly in regards to how CB influences the contribution of the I-wave circuits to neuroplastic changes in M1. Within the proposed research, we will therefore investigate how CB effects neuroplasticity of the I-wave generating circuits. This will be achieved by downregulating CB excitability while concurrently applying a neuroplastic intervention that specifically targets the I-wave circuitry. The outcomes of this study will provide valuable neurophysiological insight into key aspects of the motor network, and may inform the development of optimized interventions for modifying motor learning in a targeted way.

Cerebellar-Motor Cortex Connectivity: One or Two Different Networks?

Anterior-posterior (AP) and posterior-anterior (PA) pulses of transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) appear to activate distinct interneuron networks that contribute differently to two varieties of physiological plasticity and motor behaviors (Hamada et al., 2014). The AP network is thought to be more sensitive to online manipulation of cerebellar (CB) activity using transcranial direct current stimulation. Here we probed CB-M1 interactions using cerebellar brain inhibition (CBI) in young healthy female and male individuals. TMS over the cerebellum produced maximal CBI of PA-evoked EMG responses at an interstimulus interval of 5 ms (PA-CBI), whereas the maximum effect on AP responses was at 7 ms (AP-CBI), suggesting that CB-M1 pathways with different conduction times interact with AP and PA networks. In addition, paired associative stimulation using ulnar nerve stimulation and PA TMS pulses over M1, a protocol used in human studies to induce cortical plasticity, reduced PA-CBI but not AP-CBI, indicating that cortical networks process cerebellar inputs in distinct ways. Finally, PA-CBI and AP-CBI were differentially modulated after performing two different types of motor learning tasks that are known to process cerebellar input in different ways. The data presented here are compatible with the idea that applying different TMS currents to the cerebral cortex may reveal cerebellar inputs to both the premotor cortex and M1. Overall, these results suggest that there are two independent CB-M1 networks that contribute uniquely to different motor behaviors.SIGNIFICANCE STATEMENT Connections between the cerebellum and primary motor cortex (M1) are essential for performing daily life activities, as damage to these pathways can result in faulty movements. Therefore, developing and understanding novel approaches to probe this pathway are critical to advancing our understanding of the pathophysiology of diseases involving the cerebellum. Here, we show evidence for two distinct cerebellar-cerebral interactions using cerebellar stimulation in combination with directional transcranial magnetic stimulation (TMS) over M1. These distinct cerebellar-cerebral interactions respond differently to physiological plasticity and to distinct motor learning tasks, which suggests they represent separate cerebellar inputs to the premotor cortex and M1. Overall, we show that directional TMS can probe two distinct cerebellar-cerebral pathways that likely contribute to independent processes of learning.

Cortical and Subcortical Contributions to Neuroplasticity after Repetitive Transspinal Stimulation in Humans

The objectives of this study were to establish cortical and subcortical contributions to neuroplasticity induced by noninvasive repetitive transspinal stimulation in human subjects free of any neurological disorder. To meet our objectives, before and after 40 minutes of transspinal stimulation we established changes in tibialis anterior (TA) motor-evoked potentials (MEPs) in response to paired transcranial magnetic stimulation (TMS) pulses at interstimulus intervals (ISIs) consistent with I-wave periodicity. In order to establish to what extent similar actions are exerted at the spinal cord and motor axons, changes in soleus H-reflex and transspinal evoked potential (TEP) amplitude following transspinal and group Ia afferent conditioning stimulation, respectively, were established. After 40 min of transspinal stimulation, the TA MEP consecutive peaks of facilitation produced by paired TMS pulses were significantly decreased supporting for depression of I-waves. Additionally, the soleus H-reflex and ankle TEP depression following transspinal and group Ia afferent conditioning stimulation was potentiated at intervals when both responses interacted at the spinal cord and nerve axons. These findings support the notion that repetitive transspinal stimulation decreases corticocortical inputs onto corticospinal neurons and promotes a surround inhibition in the spinal cord and nerve axons. This novel method may be a suitable neuromodulation tool to alter excitability at cortical and subcortical levels in neurological disorders.

Effects of bilateral priming on motor cortex function in healthy adults

Bilateral priming is a rehabilitation adjuvant that can improve upper limb motor recovery poststroke. It uses a table-top device to couple the upper limbs together such that active flexion and extension of one wrist leads to passive movement of the opposite wrist in a mirror symmetric pattern. Bilateral priming increases corticomotor excitability (CME) in the primary motor cortex (M1) of the passively driven wrist; however, the neurophysiological mechanisms underlying this increase remain unclear. This study explored these mechanisms by using transcranial magnetic stimulation over the right M1 and recording motor-evoked potentials from the passively driven left extensor carpi radialis of healthy adults. Intracortical measures were recorded before and 5 and 35 min after a single 15-min session of priming. One-millisecond short-interval intracortical inhibition, long-interval intracortical inhibition, late cortical disinhibition (LCD), and intracortical facilitation were recorded with a posterior-anterior (PA) intracortical current, whereas CME and short-interval intracortical facilitation (SICF) were recorded with both PA and anterior-posterior (AP) currents. CME with PA stimulation was also recorded ~1 h postpriming. PA CME was elevated 35 min postpriming and remained elevated ~1 h postpriming. LCD decreased, and AP SICF increased at both 5 and 35 min postpriming. However, these changes in LCD and AP SICF are unlikely to be the cause of the increased PA CME because of the differing timelines of their effects and AP and PA currents activating separate interneuron circuits. These results suggest that bilateral priming does not increase CME through alterations of the intracortical circuits investigated here. NEW & NOTEWORTHY This is the first study to measure how bilateral priming modulates corticomotor excitability with posterior-anterior and anterior-posterior intracortical currents, 1-ms short-interval intracortical inhibition, late cortical disinhibition, intracortical facilitation, and short-interval intracortical facilitation. We found corticomotor excitability with a posterior-anterior current increased by 35 min until ~1 h postpriming. Short-interval intracortical facilitation with an anterior-posterior current was greater for at least 35 min postpriming. This provides further insight into the neurophysiological mechanisms underlying bilateral priming.

The Effects of Waveform and Current Direction on the Efficacy and Test-Retest Reliability of Transcranial Magnetic Stimulation

The pulse waveform and current direction of transcranial magnetic stimulation (TMS) influence its interactions with the neural substrate; however, their role in the efficacy and reliability of single- and paired-pulse TMS measures is not fully understood. We investigated how pulse waveform and current direction affect the efficacy and test-retest reliability of navigated, single- and paired-pulse TMS measures. 23 healthy adults (aged 18-35 years) completed two identical TMS sessions, assessing resting motor threshold (RMT), motor-evoked potentials (MEPs), cortical silent period (cSP), short- and long-interval intra-cortical inhibition (SICI and LICI), and intracortical facilitation (ICF) using either monophasic posterior-anterior (monoPA; n = 9), monophasic anterior-posterior (monoAP; n = 7), or biphasic (biAP-PA; n = 7) pulses. Averages of each TMS measure were compared across the three groups and intraclass correlation coefficients were calculated to assess test-retest reliability. RMT was the lowest and cSP was the longest with biAP-PA pulses, whereas MEP latency was the shortest with monoPA pulses. SICI and LICI had the largest effect with monoPA pulses, whereas only monoAP and biAP-PA pulses resulted in significant ICF. MEP amplitude was more reliable with either monoPA or monoAP than with biAP-PA pulses. LICI was the most reliable with monoAP pulses, whereas ICF was the most reliable with biAP-PA pulses. Waveform/current direction influenced RMT, MEP latency, cSP, SICI, LICI, and ICF, as well as the reliability of MEP amplitude, LICI, and ICF. These results show the importance of considering TMS pulse parameters for optimizing the efficacy and reliability of TMS neurophysiologic measures.

Efficient Mapping of the Motor Cortex with Navigated Biphasic Paired-Pulse Transcranial Magnetic Stimulation

Navigated transcranial magnetic stimulation (nTMS) can be applied to locate cortical muscle representations. Usually, single TMS pulses are targeted to the motor cortex with the help of neuronavigation and by measuring motor evoked potential (MEP) amplitudes from the peripheral muscles. The efficacy of single-pulse TMS to induce MEPs has been shown to increase by applying facilitatory paired-pulse TMS (ppTMS). Therefore, the aim was to study whether the facilitatory ppTMS could enable more efficient motor mapping. Biphasic single-pulse TMS and ppTMS with inter-stimulus intervals (ISIs) of 1.4 and 2.8 ms were applied to measure resting motor thresholds (rMTs) as a percentage of the maximal stimulator output and to determine the cortical representation areas of the right first dorsal interosseous muscle in healthy volunteers. The areas, shapes, hotspots, and center of gravities (CoGs) of the representations were calculated. Biphasic ppTMS with ISI of 1.4 ms resulted in lower rMTs than those obtained with the other protocols (p = 0.001). With ISI of 2.8 ms, rMT was lower than with single-pulse TMS (p = 0.032). The ppTMS mapping was thus performed with lower intensity than when using single-pulse TMS. The areas, shapes, hotspots, and CoGs of the muscle representations were in agreement. Hence, biphasic ppTMS has potential in the mapping of cortical hand representations in healthy individuals as an alternative for single-pulses, but with lower stimulation intensity by utilizing cortical facilitatory mechanism. This could improve application of nTMS in subjects with low motor tract excitability.

Distinct Corticocortical Contributions to Human Precision and Power Grip

The corticospinal tract contributes to the control of finger muscles during precision and power grip. The involvement of different sets of cortical interneuronal circuits during these distinct grasping behaviors remains unknown. To examine this question in humans we used noninvasive transcranial magnetic stimulation (TMS) over the hand representation of the primary motor cortex to elicit motor evoked potentials (MEPs) in an intrinsic finger muscle during index finger abduction (control task), precision grip, and power grip. The TMS coil was oriented to induce currents in the brain in the latero-medial (LM), posterior-anterior (PA), and anterior-posterior (AP) direction to preferentially activate corticospinal axons directly and early and late synaptic inputs to corticospinal neurons, respectively. We found that AP-LM MEP latency differences were consistently longer during power grip compared with index finger abduction and precision grip, while PA-LM differences remained similar across tasks. Short-interval intracortical facilitation, targeting AP but not PA inputs, increased during power grip compared with other tasks. Our novel findings suggest that cortical structures activated by PA and AP stimuli are differentially active during precision and power grip. We propose that a preferential recruitment of late synaptic inputs to corticospinal neurons may be achieved when humans perform a power grip.

Age-related changes in late I-waves influence motor cortex plasticity induction in older adults

KEY POINTS

The response to neuroplasticity interventions using transcranial magnetic stimulation (TMS) is reduced in older adults, which may be due, in part, to age-related alterations in interneuronal (I-wave) circuitry. The current study investigated age-related changes in interneuronal characteristics and whether they influence motor cortical plasticity in older adults. While I-wave recruitment was unaffected by age, there was a shift in the temporal characteristics of the late, but not the early I-waves. Using I-wave periodicity repetitive TMS (iTMS), we showed that these differences in I-wave characteristics influence the induction of cortical plasticity in older adults.

ABSTRACT

Previous research shows that neuroplasticity assessed using transcranial magnetic stimulation (TMS) is reduced in older adults. While this deficit is often assumed to represent altered synaptic modification processes, age-related changes in the interneuronal circuits activated by TMS may also contribute. Here we assessed age-related differences in the characteristics of the corticospinal indirect (I) waves and how they influence plasticity induction in primary motor cortex. Twenty young (23.7 ± 3.4 years) and 19 older adults (70.6 ± 6.0 years) participated in these studies. I-wave recruitment was assessed by changing the direction of the current used to activate the motor cortex, whereas short-interval intracortical facilitation (SICF) was recorded to assess facilitatory I-wave interactions. In a separate study, I-wave periodicity TMS (iTMS) was used to examine the effect of I-wave latency on motor cortex plasticity. Data from the motor-evoked potential (MEP) onset latency produced using different coil orientations suggested that there were no age-related differences in preferential I-wave recruitment (P = 0.6). However, older adults demonstrated significant reductions in MEP facilitation at all 3 SICF peaks (all P values < 0.05) and a delayed latency of the second and third SICF peaks (all P values < 0.05). Using I-wave intervals that were optimal for young and older adults, these changes in the late I-waves were shown to influence the plasticity response in older adults after iTMS. These findings suggest that temporal characteristics are delayed for the late I-waves in older adults, and that optimising TMS interventions based on I-wave characteristics may improve the plasticity response in older adults.

Noninvasive Stimulation of the Human Brain: Activation of Multiple Cortical Circuits

Noninvasive brain stimulation methods, such as transcranial electric stimulation and transcranial magnetic stimulation are widely used tools for both basic research and clinical applications. However, the cortical circuits underlying their effects are poorly defined. Here we review the current knowledge based on data mostly coming from experiments performed on human subjects, and also to a lesser extent on rodent or primate models. The data suggest that multiple mechanisms are likely to be involved, such as the direct activation of layer V pyramidal neurons, but also of different types of GABAergic interneurons. In this regard, we propose a key role for a specific type of interneuron known as neurogliaform cell.

Quadri-Pulse Theta Burst Stimulation using Ultra-High Frequency Bursts - A New Protocol to Induce Changes in Cortico-Spinal Excitability in Human Motor Cortex

Patterned transcranial magnetic stimulation (TMS) such as theta burst stimulation (TBS) or quadri-pulse stimulation (QPS) can induce changes in cortico-spinal excitability, commonly referred to as long-term potentiation (LTP)-like and long-term depression (LTD)-like effects in human motor cortex (M1). Here, we aimed to test the plasticity-inducing capabilities of a novel protocol that merged TBS and QPS. 360 bursts of quadri-pulse TBS (qTBS) were continuously given to M1 at 90% of active motor threshold (1440 full-sine pulses). In a first experiment, stimulation frequency of each burst was set to 666 Hz to mimic the rhythmicity of the descending cortico-spinal volleys that are elicited by TMS (i.e., I-wave periodicity). In a second experiment, burst frequency was set to 200 Hz to maximize postsynaptic Ca²⁺ influx using a temporal pattern unrelated to I-wave periodicity. The second phase of sinusoidal TMS pulses elicited either a posterior-anterior (PA) or anterior-posterior (AP) directed current in M1. Motor evoked potentials (MEPs) were recorded before and after qTBS to probe changes in cortico-spinal excitability. PA-qTBS at 666 Hz caused a decrease in PA-MEP amplitudes, whereas AP-qTBS at 666 Hz induced an increase in mean AP-MEP amplitudes. At a burst frequency of 200 Hz, PA-qTBS and AP-qTBS produced an increase in cortico-spinal excitability outlasting for at least 60 minutes in PA- and AP-MEP amplitudes, respectively. Continuous qTBS at 666 Hz or 200 Hz can induce lasting changes in cortico-spinal excitability. Induced current direction in the brain appears to be relevant when qTBS targets I-wave periodicity, corroborating that high-fidelity spike timing mechanisms are critical for inducing bi-directional plasticity in human M1.

Pulse Duration as Well as Current Direction Determines the Specificity of Transcranial Magnetic Stimulation of Motor Cortex during Contraction

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Selective stimulation of inputs to corticospinal neurons may be achieved by manipulating current direction and pulse duration.

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Neural populations recruited by brief (30 μs) anterior–posterior currents exhibited the greatest sensitivity to somatosensory input.

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Pulse duration is an important determinant of what is activated with TMS in human motor cortex.

Background

Previous research suggested that anterior–posterior (AP) directed currents induced by TMS in motor cortex (M1) activate interneuron circuits different from those activated by posterior–anterior currents (PA). The present experiments provide evidence that pulse duration also determines the activation of specific interneuron circuits.

Objective

To use single motor unit (SMU) recordings to confirm the difference in onset latencies of motor-evoked potentials (MEPs) evoked by different current directions and pulse durations: AP₃₀, AP₁₂₀, PA₃₀ and PA₁₂₀. To test whether the amplitude of the MEPs is differentially influenced by somatosensory inputs from the hand (short-latency afferent inhibition, SAI), and examine the sensitivity of SAI to changes in cerebellar excitability produced by direct current stimulation (tDCS_Cb).

Methods

Surface electromyograms and SMUs were recorded from the first dorsal interosseous muscle. SAI was tested with an electrical stimulus to median or digital nerves ~20–25 ms prior to TMS delivered over the M1 hand area via a controllable pulse parameter TMS (cTMS) device. SAI was also tested during the application of anodal or sham tDCS_Cb. Because TMS pulse specificity is greatest at low stimulus intensities, most experiments were conducted with weak voluntary contraction to reduce stimulus threshold.

Results

AP₃₀ currents recruited the longest latency SMU and surface MEP responses. During contraction SAI was greater for AP₃₀ responses versus all other pulses. Online anodal tDCS_Cb reduced SAI for the AP₃₀ currents only.

Conclusions

AP₃₀ currents activate an interneuron circuit with functional properties different from those activated by other pulse types. Pulse duration and current direction determine what is activated in M1 with TMS.

Facilitatory effect of paired-pulse stimulation by transcranial magnetic stimulation with biphasic wave-form

Transcranial magnetic stimulation (TMS) is used to probe corticospinal excitability by stimulating the motor cortex. Our aim was to enhance the effects of biphasic TMS by coupling a suprathreshold test pulse and a following subthreshold priming pulse to induce short-interval intracortical facilitation (SICF), which is conventionally produced with monophasic TMS. Biphasic TMS could potentially induce the SICF effect with better energy-efficiency and with lower stimulus intensities. This would make the biphasic paired-pulses better applicable in patients with reduced cortical excitability. A prototype stimulator was built to produce biphasic paired-pulses. Resting motor thresholds (rMTs) from the right and left hand abductor pollicis brevis muscles, and the right tibialis anterior muscle of eight healthy volunteers were determined using single-pulse paradigm with neuronavigated TMS. The rMTs and MEPs were measured using single-pulses and three paired-pulse setups (interstimulus interval, ISI of 3, 7 or 15ms). The rMTs were lower and MEPs were higher with biphasic paired-pulses compared to single-pulses. The SICF effect was greatest at 3ms ISI. This suggests that the application of biphasic paired-pulses to enhance stimulation effects is possible.

Technical aspects of neurostimulation: Focus on equipment, electric field modeling, and stimulation protocols

Neuromodulation is a field of science, medicine, and bioengineering that encompasses implantable and non-implantable technologies for the purpose of improving quality of life and functioning of humans. Brain neuromodulation involves different neurostimulation techniques: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS), which are being used both to study their effects on cognitive brain functions and to treat neuropsychiatric disorders. The mechanisms of action of neurostimulation remain incompletely understood. Insight into the technical basis of neurostimulation might be a first step towards a more profound understanding of these mechanisms, which might lead to improved clinical outcome and therapeutic potential. This review provides an overview of the technical basis of neurostimulation focusing on the equipment, the present understanding of induced electric fields, and the stimulation protocols. The review is written from a technical perspective aimed at supporting the use of neurostimulation in clinical practice.

Effect of coil orientation on strength-duration time constant and I-wave activation with controllable pulse parameter transcranial magnetic stimulation

OBJECTIVE

To compare the strength-duration (S-D) time constants of motor cortex structures activated by current pulses oriented posterior-anterior (PA) or anterior-posterior (AP) across the central sulcus.

METHODS

Motor threshold and input-output curve, along with motor evoked potential (MEP) latencies, of first dorsal interosseus were determined at pulse widths of 30, 60, and 120 μs using a controllable pulse parameter (cTMS) device, with the coil oriented PA or AP. These were used to estimate the S-D time constant and we compared with data for responses evoked by cTMS of the ulnar nerve at the elbow.

RESULTS

The S-D time constant with PA was shorter than for AP stimulation (230.9 ± 97.2 vs. 294.2 ± 90.9 μs; p<0.001). These values were similar to those calculated after stimulation of ulnar nerve (197 ± 47 μs). MEP latencies to AP, but not PA stimulation were affected by pulse width, showing longer latencies following short duration stimuli.

CONCLUSION

PA and AP stimuli appear to activate the axons of neurons with different time constants. Short duration AP pulses are more selective than longer pulses in recruiting longer latency corticospinal output.

SIGNIFICANCE

More selective stimulation of neural elements may be achieved by manipulating pulse width and orientation.

Subcortical contribution to late TMS-induced I-waves in intact humans

Paired-pulse transcranial magnetic stimulation (TMS) of the human motor cortex results in consecutive facilitatory motor evoked potential (MEP) peaks in surface electromyography. It has been proposed that early and late MEP peaks involve different mechanisms of action; however, little is known about the characteristics of the later peaks. Using paired-pulse TMS over the hand motor cortex at different test (S1) and conditioning (S2) interstimulus intervals and intensities we examined early (first) and late (second and third) MEP peaks in a resting finger muscle. We demonstrate that the third peak had reduced amplitude and duration compared with the second, regardless of the S1 intensity. Higher S2 intensity increased the amplitude of the third but not the second peak, suggesting that the third peak had a higher threshold. The interval between the second and third peak was longer than between the first and second peak in all conditions even though all peaks had a similar latency dispersion. No differences were found in the amplitude, duration, and threshold of the first and second peaks. A threshold electrical S2 over the cervicomedullary junction facilitated the second and third but not the first peak similarly to TMS. Our results indicate that the third MEP peak is smaller and has higher threshold than the second peak and the similarities between the first and second peak suggest that this is less likely explained by a reduced effectiveness in recruitment. We argue that subcortical pathways might contribute to differences found between late TMS-induced peaks in intact humans.

The influence of sensory afferent input on local motor cortical excitatory circuitry in humans

In human, sensorimotor integration can be investigated by combining sensory input and transcranial magnetic stimulation (TMS). Short latency afferent inhibition (SAI) refers to motor cortical inhibition 20-25 ms after median nerve stimulation. We investigated the interaction between SAI and short-interval intracortical facilitation (SICF), an excitatory motor cortical circuit. Seven experiments were performed. Contrary to expectations, SICF was facilitated in the presence of SAI (SICF(SAI)). This effect is specific to SICF since there was no effect at SICF trough 1 when SICF was absent. Furthermore, the facilitatory SICF(SAI) interaction increased with stronger SICF or SAI. SAI and SICF correlated between individuals, and this relationship was maintained when SICF was delivered in the presence of SAI, suggesting an intrinsic relationship between SAI and SICF in sensorimotor integration. The interaction was present at rest and during muscle contraction, had a broad degree of somatotopic influence and was present in different interneuronal SICF circuits induced by posterior-anterior and anterior-posterior current directions. Our results are compatible with the finding that projections from sensory to motor cortex terminate in both superficial layers where late indirect (I-) waves are thought to originate, as well as deeper layers with more direct effect on pyramidal output. This interaction is likely to be relevant to sensorimotor integration and motor control.

Impaired Organization of Paired-Pulse TMS-Induced I-Waves After Human Spinal Cord Injury

Paired-pulse transcranial magnetic stimulation (TMS) of the human motor cortex results in consecutive facilitatory motor-evoked potential (MEP) peaks in surface electromyography in intact humans. Here, we tested the effect of an incomplete cervical spinal cord injury (SCI) on early (first) and late (second and third) MEP peaks in a resting intrinsic finger muscle. We found that all peaks had decreased amplitude in SCI subjects compared with controls. The second and third peaks were delayed with the third peak also showing an increased duration. The delay of the third peak was smaller than that seen in controls at lower stimulation intensity, suggesting lesser influence of decreased corticospinal inputs. A mathematical model showed that after SCI the third peak aberrantly contributed to spinal motoneurone recruitment, regardless on the motor unit threshold tested. Temporal and spatial aspects of the late peaks correlated with MEP size and hand motor output. Thus, early and late TMS-induced MEP peaks undergo distinct modulation after SCI, with the third peak likely reflecting a decreased ability to summate descending volleys at the spinal level. We argue that the later corticospinal inputs on the spinal cord might be crucial for recruitment of motoneurones after human SCI.

The role of pulse shape in motor cortex transcranial magnetic stimulation using full-sine stimuli

A full-sine (biphasic) pulse waveform is most commonly used for repetitive transcranial magnetic stimulation (TMS), but little is known about how variations in duration or amplitude of distinct pulse segments influence the effectiveness of a single TMS pulse to elicit a corticomotor response. Using a novel TMS device, we systematically varied the configuration of full-sine pulses to assess the impact of configuration changes on resting motor threshold (RMT) as measure of stimulation effectiveness with single-pulse TMS of the non-dominant motor hand area (M1). In young healthy volunteers, we (i) compared monophasic, half-sine, and full-sine pulses, (ii) applied two-segment pulses consisting of two identical half-sines, and (iii) manipulated amplitude, duration, and current direction of the first or second full-sine pulse half-segments. RMT was significantly higher using half-sine or monophasic pulses compared with full-sine. Pulses combining two half-sines of identical polarity and duration were also characterized by higher RMT than full-sine stimuli resulting. For full-sine stimuli, decreasing the amplitude of the half-segment inducing posterior-anterior oriented current in M1 resulted in considerably higher RMT, whereas varying the amplitude of the half-segment inducing anterior-posterior current had a smaller effect. These findings provide direct experimental evidence that the pulse segment inducing a posterior-anterior directed current in M1 contributes most to corticospinal pathway excitation. Preferential excitation of neuronal target cells in the posterior-anterior segment or targeting of different neuronal structures by the two half-segments can explain this result. Thus, our findings help understanding the mechanisms of neural stimulation by full-sine TMS.