Intracortical inhibition of lower limb motor-evoked potentials after paired transcranial magnetic stimulation

Corticospinal excitability across lower limb muscles in humans

Electrophysiological studies in nonhuman primates reported the existence of strong corticospinal output from the primary motor cortex to distal compared with proximal hindlimb muscles. The extent to which corticospinal output differs across muscles in the leg in humans remains poorly understood. Using transcranial magnetic stimulation over the leg representation of the primary motor cortex, we constructed motor evoked potential (MEP) recruitment curves to measure the resting motor threshold (RMT), maximum MEP amplitude (MEP-max), and slope in the biceps femoris, rectus femoris, tibialis anterior, soleus, and a foot muscle (i.e., abductor hallucis) in intact humans. We found that the RMT was lower and the MEP-max and slope were larger in the abductor hallucis compared with most other muscles tested. In contrast, the RMT was higher and the MEP-max and slope were lower in the biceps femoris compared to all other muscles tested. Corticospinal responses in the rectus femoris, tibialis anterior, and soleus were in between those obtained from other leg muscles, with the soleus having a higher RMT and lower MEP-max and slope than the rectus femoris and tibialis anterior. To examine the origin of increases in corticospinal excitability in the abductor hallucis, we compared short-interval intracortical inhibition (SICI) and F-waves between the abductor hallucis and tibialis anterior. SICI was similar across muscles while the F-wave amplitude was larger in the abductor hallucis compared with the tibialis anterior. These results support a nonuniform distribution of corticospinal output to leg muscles, highlighting that increases in corticospinal excitability in a foot muscle could be related to a spinal origin.NEW & NOTEWORTHY We provide evidence on how corticospinal output differs across muscles in the leg in intact humans. We found that corticospinal responses were larger in a distal intrinsic foot muscle and were smaller in the biceps femoris compared to all other muscles in the leg. Increases in corticospinal excitability to an intrinsic foot muscle could have a spinal origin.

Using transcranial magnetic stimulation to map the cortical representation of lower-limb muscles

Objective

To evaluate the extent to which transcranial magnetic stimulation (TMS) can identify discrete cortical representation of lower-limb muscles in healthy individuals.

Methods

Motor evoked potentials were recorded from resting vastus medialis, rectus femoris, vastus lateralis, medial and lateral hamstring, and medial and lateral gastrocnemius muscles on the right leg of 16 young healthy adults using bipolar surface electrodes. TMS was delivered through a 110-mm double-cone coil at 63 sites over the left hemisphere. Location and size of cortical representation and number of discrete peaks were quantified.

Results

Within the quadriceps group there was a main effect of muscle on anterior-posterior centre of gravity (p = 0.010), but the magnitude of the difference was small. There was also a main effect of muscle on medial-lateral hotspot (p = 0.027) and map volume (p = 0.047), but no post-hoc tests were significant. The topography of each lower-limb muscle was complex and variable across individuals.

Conclusions

TMS delivered with a 110-mm double-cone coil could not reliably identify discrete cortical representations of resting lower-limb muscles when responses were measured using bipolar surface electromyography.

Significance

The characteristics of the cortical representation provide a basis against which to evaluate cortical reorganisation in clinical populations.

Effect of paired-pulse stimulus parameters on the two phases of short interval intracortical inhibition in the quadriceps muscle group

BACKGROUND

Short interval intracortical inhibition (SICI) is commonly used to assess inhibition in the motor cortex and is known to be affected by the paired-pulse stimulus parameters (i.e., interstimulus interval [ISI], conditioning stimulus intensity [CSI] and test stimulus intensity [TSI]) used during testing. While the effects of stimulus parameters are well-studied in the upper-extremity, evidence in the lower-extremity is lacking.

OBJECTIVE

To comprehensively examine the effects of alterations in paired-pulse stimulus parameters on the two phases of SICI in the quadriceps muscle group.

METHODS

Seventeen adults (8 males, 9 females) volunteered to participate in this study. SICI was examined over a range of CSIs (70-90% active motor threshold [AMT]), TSIs (100-140% AMT), and ISIs (1.0-3.0 ms) using both EMG and torque responses elicited by transcranial magnetic stimulation (TMS).

RESULTS

The results indicated that SICI at 1.0 ms ISI was best revealed with a CSI of 70% and TSI ≥110% AMT, whereas SICI at 2.5 ms ISI was best revealed with a CSI of 80-90% and a TSI of ≥130% AMT. Unlike upper-extremity muscles, evaluating SICI with a CSI of 70% AMT and an ISI of 1.0 ms produced the greatest inhibition for all TSIs. In general, inhibitory effects were contaminated by facilitatory effects when using a TSI of 100% AMT.

CONCLUSIONS

The amount of inhibition was dependent on the stimulation parameters used during testing. A CSI of 70% AMT, ISI of 1.0 ms, and TSI of ≥110% AMT appear to be optimal for measuring SICI in the quadriceps muscle; however, other parameters can be used if careful consideration is given to the described interaction between the parameters.

Deep continuous theta burst stimulation of the operculo-insular cortex selectively affects Aδ-fibre heat pain

KEY POINTS

Deep continuous theta burst stimulation (cTBS) of the right operculo-insular cortex delivered with a double cone coil selectively impairs the ability to perceive thermonociceptive input conveyed by Aδ-fibre thermonociceptors without concomitantly affecting the ability to perceive innocuous warm, cold or vibrotactile sensations. Unlike deep cTBS, superficial cTBS of the right operculum delivered with a figure-of-eight coil does not affect the ability to perceive thermonociceptive input conveyed by Aδ-fibre thermonociceptors. The effect of deep operculo-insular cTBS on the perception of Aδ-fibre input was present at both the contralateral and the ipsilateral hand. The magnitude of the increase in Aδ-heat detection threshold induced by the deep cTBS was significantly correlated with the intensity of the cTBS pulses. Deep cTBS delivered over the operculo-insular cortex is associated with a risk of transcranial magnetic stimulation-induced seizure.

ABSTRACT

Previous studies have suggested a pivotal role of the insular cortex in nociception and pain perception. Using a double-cone coil designed for deep transcranial magnetic stimulation, our objective was to assess (1) whether continuous theta burst stimulation (cTBS) of the operculo-insular cortex affects differentially the perception of different types of thermal and mechanical somatosensory inputs, (2) whether the induced after-effects are lateralized relative to the stimulated hemisphere, and (3) whether the after-effects are due to neuromodulation of the insula or neuromodulation of the more superficial opercular cortex. Seventeen participants took part in two experiments. In Experiment 1, thresholds and perceived intensity of Aδ- and C-fibre heat pain elicited by laser stimulation, non-painful cool sensations elicited by contact cold stimulation and mechanical vibrotactile sensations were assessed at the left hand before, immediately after and 20 min after deep cTBS delivered over the right operculo-insular cortex. In Experiment 2, Aδ-fibre heat pain and vibrotactile sensations elicited by stimulating the contralateral and ipsilateral hands were evaluated before and after deep cTBS or superficial cTBS delivered using a flat figure-of-eight coil. Only the threshold to detect Aδ-fibre heat pain was significantly increased 20 min after deep cTBS. This effect was present at both hands. No effect was observed after superficial cTBS. Neuromodulation of the operculo-insular cortex using deep cTBS induces a bilateral reduction of the ability to perceive Aδ-fibre heat pain, without concomitantly affecting the ability to perceive innocuous warm, cold or vibrotactile sensations.

Differences in motor cortical control of the Soleus and Tibialis

The tibialis anterior (TA) and the soleus (SOL) are both ankle joint muscles with functionally very different tasks. Thus, differences in motor cortical control between the TA and the SOL have been debated. This study compared the activity of the primary motor cortex during dynamic plantar- and dorsiflexions and compared this with measures obtained during rest. Single- and paired-pulse transcranial magnetic stimulations known as short-interval intracortical inhibition (SICI) were applied to the cortical representation of either the soleus or the tibialis muscle. The results show that the range of SICI from rest to activity is significantly greater in the TA compared with the SOL. Furthermore, when the TA acts as the agonist muscle during dorsiflexions of the ankle, SICI is almost absent (2.9%). When acting as the antagonist during plantarflexions, intracortical inhibition is significantly increased (28.7%). This task-specific modulation is far less pronounced in the SOL, which displayed higher levels of SICI when acting as agonist (10.9%) during plantarflexion, but there was no significant inhibition (6.5%) as antagonist during dorsiflexion. Furthermore, the cortical silent period (CSP) during plantarflexions was significantly longer in the SOL compared with the TA during dorsiflexions, accompanied by a greater corticospinal excitability in the TA. Thus, cortical control considerably differs between the SOL and the TA in a way that inhibitory cortical control (SICI and CSP) of the TA is task-specifically adapted in a broader range of movements, whereas inhibition in the SOL muscle is less specific and more limited in its magnitude of modulation.

High-frequency rTMS on leg motor area in the early phase of stroke

Although repetitive transcranial magnetic stimulation (rTMS) for upper limb motor area in stroke patients is beneficial, it has been poorly investigated in rTMS for leg motor area. Furthermore, no study has examined the usefulness of rTMS for leg motor area in patients in the early phase of stroke. Twenty-one patients with a hemispheric stroke lesion in the early phase were randomly assigned into two groups: the high-frequency (HF)-rTMS group [N = 11] and the sham stimulation group [N = 10]. Patients received rTMS for 5 consecutive days, beginning 10.9 ± 6.6 days on average after the onset. Brunnstrom Recovery Stages (BRS) for the lower limbs and the Ability for Basic Movement Scale Revised (ABMS II) were assessed before and after the intervention. The improvement in BRS for the lower limbs was significant after the intervention in the HF-rTMS group. Although both the HF-rTMS and sham stimulation groups had significant improvements in ABMS II scores, the extent of improvement in the AMBS II was significantly greater in the HF-rTMS group than in the sham stimulation group. Application of HF-rTMS over the bilateral leg motor areas has potential to be a new rehabilitation therapy for patients in the acute phase of stroke.

Variability, frequency composition, and temporal regularity of submaximal isometric elbow flexion force in subacute stroke

We compared variability, frequency composition, and temporal regularity of submaximal isometric elbow flexion force at 10, 20, 35, and 50 % of peak torque between 34 stroke subjects (5-48 days post-onset, both arms) and 24 age-matched controls (dominant arm), and related the findings in the paretic arm to motor impairment. Force variability was quantified by the coefficient of variation (CV), frequency composition by the median frequency and relative power in 0-3-, 4-6-, and 8-12-Hz bands, and regularity by the sample entropy (SampEn). The paretic elbow flexors showed significantly increased CV and relative power in 0-3-Hz band, decreased power in 4-6- and 8-12-Hz bands, and decreased SampEn compared to both the non-paretic and control elbow flexors (P ≤ 0.0002), with no differences between the latter two (P ≥ 0.012). With increasing contraction intensity, the relative power in different frequency bands was insufficiently modulated and SampEn excessively decreased in the paretic elbow flexors. Also, CV in the paretic elbow flexors was non-linearly related to the relative power in different frequency bands and SampEn across contraction intensities (rectangular hyperbolic fit, 0.21 ≤ R ² ≤ 0.55, P ≤ 0.006), whereas no force parameter correlated with arm motor impairment. These results largely extend our previous findings in the paretic knee extensors to the elbow flexors in subacute stroke, except that here force variability was increased only in the paretic elbow flexors and modulation of force regularity with increasing contraction intensity showed the opposite, decreasing pattern, which was considerably exaggerated in the paretic muscles.

Impaired trial-by-trial adjustment of cognitive control in obsessive compulsive disorder improves after deep repetitive transcranial magnetic stimulation

BACKGROUND

Adaptive decision making requires the adjustment of behaviour following an error. Some theories suggest that repetitive thoughts and behaviours in obsessive compulsive disorder (OCD) are driven by malfunctioning error monitoring. This malfunction may relate to demonstrated hyperactivity in the medial prefrontal cortex (mPFC), including the dorsal anterior cingulate cortex. In this study, we measured aspects of error monitoring in individuals with OCD and administered deep low frequency repetitive transcranial magnetic stimulation (rTMS) in an attempt to modulate error monitoring capacity.

METHODS

For this pilot study, ten OCD patients and 10 aged-matched healthy controls completed modified versions of the Eriksen Flanker task before and after one session of deep 1 Hz rTMS (1200 pulses) over the mPFC (Brodmann areas 24 and 32). OCD patients received nine additional sessions of daily rTMS to assess their clinical response. Flanker tasks were repeated with patients post-treatment.

RESULTS

Overall error rates were higher for patients compared to controls. When subjects were asked to report their errors, OCD patients were able to report fewer of their errors than the control group. In contrast to controls, patients did not demonstrate a normal post-error slowing (PES) phenomenon. This abnormal PES was mainly driven by abnormally slow response times (RTs) following correct responses rather than a failure to slow down after errors. Patients' symptoms and slowed RTs following correct responses improved after ten sessions of rTMS.

CONCLUSIONS

Certain aspects of error monitoring, namely conscious error report and post error slowing, are impaired in OCD. These impairments can at least be partly corrected by 1 Hz deep rTMS over the mPFC. Simultaneous improvement of OCD symptoms by this method might suggest a correlation between error monitoring impairment and OCD pathophysiology.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02541812; 09/02/2015.

Convergence of flexor reflex and corticospinal inputs on tibialis anterior network in humans

OBJECTIVE

Integration between descending and ascending inputs at supraspinal and spinal levels is a key characteristic of neural control of movement. In this study, we characterized convergence of the flexor reflex and corticospinal inputs on the tibialis anterior (TA) network in healthy human subjects. Specifically, we characterized the modulation profiles of the spinal TA flexor reflex following subthreshold and suprathreshold transcranial magnetic stimulation (TMS). We also characterized the modulation profiles of the TA motor evoked potentials (MEPs) following medial arch foot stimulation at sensory and above reflex threshold.

METHODS

TA flexor reflexes were evoked following stimulation of the medial arch of the foot with a 30 ms pulse train at innocuous intensities. TA MEPs were evoked following TMS of the leg motor cortex area.

RESULTS

TMS at 0.7 and at 1.2 MEP resting threshold increased the TA flexor reflex when TMS was delivered 40-100 ms after foot stimulation, and decreased the TA flexor reflex when TMS was delivered 25-110 ms before foot stimulation. Foot stimulation at sensory and above flexor reflex threshold induced a similar time-dependent modulation in resting TA MEPs, that were facilitated when foot stimulation was delivered 40-100 ms before TMS. The flexor reflex and MEPs recorded from the medial hamstring muscle were modulated in a similar manner to that observed for the TA flexor reflex and MEP.

CONCLUSION

Cutaneomuscular afferents from the distal foot can increase the output of the leg motor cortex area. Descending motor volleys that directly or indirectly depolarize flexor motoneurons increase the output of the spinal FRA interneuronal network. The parallel facilitation of flexor MEPs and flexor reflexes is likely cortical in origin.

SIGNIFICANCE

Afferent mediated facilitation of corticospinal excitability can be utilized to strengthen motor cortex output in neurological disorders.

The efficacy of deep repetitive transcranial magnetic stimulation over the medial prefrontal cortex in obsessive compulsive disorder: results from an open-label study

BACKGROUND

Significant numbers of patients with obsessive compulsive disorder (OCD) respond minimally to currently available treatments. Furthermore, the application of both high- and low-frequency repetitive transcranial magnetic stimulation (rTMS) to dorsolateral prefrontal cortex has shown to be ineffective in the reduction of OCD symptoms. In this study, we instead targeted the medial prefrontal cortex (mPFC) and applied low-frequency rTMS to patients with OCD and assessed their clinical response.

METHODS

In an open-label design, 10 OCD patients with no other current axis I psychiatric disorders were recruited. Twelve hundred pulses of 1-Hz frequency were applied over the mPFC (Brodmann areas 24 and 32) using a double-cone coil for 10 days. Regions of interest were located on participants' MRIs using neuronavigation software. Patients' symptoms were rated using the Yale-Brown Obsessive Compulsive Scale (Y-BOCS).

RESULTS

All patients demonstrated improvement in their OCD symptoms after 10 sessions of rTMS as shown by a mean improvement in Y-BOCS score of 39% (SD = 15%; P < .001, F = 62.95). This improvement persisted 1 month following the last session of rTMS.

CONCLUSIONS

Our results suggest the use of low frequency deep rTMS as a promising and robust intervention in OCD symptom reduction. However, this study is limited by its open-label nature and its lack of a control group, so further randomized clinical trials are needed to confirm these results.

Noninvasive Neuromodulation in Poststroke Gait Disorders

Walking rehabilitation is one of the primary goals in stroke survivors because of its great potential for recovery and its functional relevance in daily living activities. Although 70% to 80% of people in the chronic poststroke phases are able to walk, impairment of gait often persists, involving speed, endurance, and stability. Walking involves several brain regions, such as the sensorimotor cortex, supplementary motor area, cerebellum, and brainstem, which are approachable by the application of noninvasive brain stimulation (NIBS). NIBS techniques, such as repetitive transcranial magnetic stimulation and transcranial direct current stimulation, have been reported to modulate neural activity beyond the period of stimulation, facilitating neuroplasticity. NIBS methods have been largely applied for improving paretic hand motor function and stroke-associated cognitive deficits. Recent studies suggest a possible effectiveness of these techniques also in the recovery of poststroke gait disturbance. This article is a selective review about functional investigations addressing the mechanisms of lower-limb motor system reorganization after stroke and the application of NIBS for neurorehabilitation.

Posture-related modulation of cortical excitability in the tibialis anterior muscle in humans

Corticospinal excitability in the lower leg muscles is enhanced during standing as compared to other postures. In the present study, we investigated how the excitability of intracortical circuits that control the tibialis anterior muscle (TA) is modulated during standing. Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed by the paired-pulse transcranial magnetic stimulation technique during standing (STD) and sitting (SIT) with a comparable background activity level in both the soleus and the TA muscle. The results demonstrated that SICI was less effective during STD than during SIT, whereas ICF was more effective during STD than during SIT. These findings suggest that the excitabilities of these cortical neural circuits are modulated depending on posture. A decrease in SICI and an increase in ICF may reflect subliminal enhancement of the cortical excitability in the TA muscle during standing as compared with that during sitting.

Bilateral motor tasks involve more brain regions and higher neural activation than unilateral tasks: an fMRI study

Movements that involve simultaneous coordination of muscles of the right and left lower limbs form a large part of our daily activities (e.g., standing, rising from a chair). This study used functional magnetic resonance imaging to determine which brain areas are used to control coordinated lower-limb movements, specifically comparing regions that are activated during bilateral exertions to those performed unilaterally. Plantarflexor exertions were produced at a target force level of 15% of the participants' maximum voluntary contraction, in three conditions, with their right (dominant) foot, with their left foot, and with both feet simultaneously. A voxel-wise analysis determined which regions were active in the bilateral, but not in the unilateral conditions. In addition, a region of interest (ROI) approach was used to determine differences in the percent signal change (PSC) between the conditions within motor areas. The voxel-wise analysis showed a large number of regions (cortical, subcortical, and cerebellar) that were active during the bilateral condition, but not during either unilateral condition. The ROI analysis showed several motor regions with higher activation in the bilateral condition than unilateral conditions; further, the magnitude of bilateral PSC was more than the sum of the two unilateral conditions in several of these regions. We postulate that the greater levels of activation during bilateral exertions may arise from interhemispheric inhibition, as well as from the greater need for motor coordination (e.g., synchronizing the two limbs to activate together) and visual processing (e.g., monitoring of two visual stimuli).

High-frequency rTMS using a double cone coil for gait disturbance

OBJECTIVE

It is difficult to stimulate leg motor areas with magnetic current using a figure-of-eight coil due to the deep anatomical location of the areas. However, a double cone coil is useful for stimulating deep brain regions. We postulated that the use of the same coil may allow repetitive transcranial magnetic stimulation (rTMS) to modulate the neural activity of the same areas. The purpose of this study is to investigate the effect of high-frequency rTMS applied over bilateral leg motor areas with a double cone coil on walking function after stroke.

MATERIALS AND METHODS

Eighteen post-stroke hemiparetic patients with gait disturbances attended two experimental sessions with more than 24 h apart, in a cross-over, double-blind paradigm. In one session, high-frequency rTMS of 10 Hz was applied over the leg motor area bilaterally in a 10-s train using a double cone coil for 20 min (total 2,000 pulses). In the other session, sham stimulation was applied for 20 min at the same site. To assess walking function, walking velocity, and Physiological Cost Index (PCI) were evaluated serially before, immediately after, and 10 and 20 min after each stimulation.

RESULTS

The walking velocity was significantly higher for 20 min after stimulation in the high-frequency rTMS group than the sham group. PCI was lower in the high-frequency rTMS group than the sham group, but this was significant only immediately after stimulation.

CONCLUSIONS

High-frequency rTMS of bilateral leg motor areas using a double cone coil can potentially improve walking function in post-stroke hemiparetic patients.

Short-interval intracortical inhibition in knee extensors during locomotor cycling

AIM

Short-interval intracortical inhibition (SICI) can provide information on changes in cortical responsiveness during voluntary contractions. It is, however, unknown whether the magnitude of SICI changes throughout the cycle of rhythmic movements such as leg cycling.

METHODS

The effects of four conditioning stimulus (CS) intensities, 70, 80, 90 and 95% of active motor threshold (AMT), on the magnitude of SICI were tested during three conditions: (1) activation phase of the electromyography (EMG) burst, (2) deactivation phase of the EMG burst and (3) static contractions. The three conditions were matched for EMG amplitude and test motor-evoked potential (MEP) size with reference to the vastus lateralis muscle. Responses were also recorded from rectus femoris and vastus medialis.

RESULTS

short-interval cortical inhibition was weak during static knee contractions (15% reduction in control MEP) relative to previous reports during contractions in other muscle groups. SICI was abolished during the activation phase of the knee extensor EMG burst (P > 0.05), but present (approx. 90% of control MEP size) during the deactivation phase of EMG (P < 0.05). Furthermore, inhibition was elicited at a lower CS intensity during the deactivation phase of EMG during cycling than during static contractions (70 AMT vs. 90% AMT).

CONCLUSION

The results suggest that the efficacy of intracortical inhibitory projections to knee extensor corticomotoneurons is particularly weak during muscle activation. A lower threshold of activation for inhibitory cells during deactivation phase of cycling EMG was evident, and there was a phasic modulation of intracortical inhibition affecting corticospinal projections to the working muscles.

Transcranial magnetic stimulation of the ventromedial prefrontal cortex impairs theory of mind learning

Imaging and lesion studies indicate that the prefrontal cortex plays a prominent role in mediating theory of mind (ToM) functioning. Particularly, the ventromedial prefrontal cortex (VMPFC) appears to be involved in mediating ToM functioning. This study utilized slow repetitive transcranial magnetic stimulation (rTMS) over the VMPFC in 13 healthy subjects in order to test whether normal functioning of the VMPFC is necessary for ToM functioning. We found that rTMS to the VMPFC, but not sham-rTMS, significantly disrupted ToM learning. Performance on a control task, not involving affective ToM functioning, was not significantly altered after applying rTMS to the VMPFC or sham-rTMS. In an additional experiment, rTMS to the vertex did not significantly affect ToM learning, confirming specificity of the VMPFC region. These findings indicate that the VMPFC is critical for intact ToM learning and shed further light on the concept and localization of ToM in particular and empathic functioning in general.

Cortical and spinal excitability changes after robotic gait training in healthy participants

BACKGROUND

Recent studies have proposed a role for robotic gait training in participants with acquired brain injury, but the effects on the excitability of cortical and spinal neurons even in healthy participants are uncertain.

OBJECTIVE

To investigate changes in corticospinal excitability in healthy participants after active and passive robotic gait training in a driven gait orthosis (DGO), the Lokomat.

METHODS

Thirteen healthy participants took part in 2 experiments. Each participant performed 20 minutes of active and passive gait training in a DGO. Motor evoked potentials (MEP), short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), F-wave frequency, and Mmax were measured in the right tibialis anterior muscle before and after training.

RESULTS

Active training led to a decline in MEP amplitude and F-wave frequency. The MEP decline was associated with subjective muscle fatigue. Passive training induced a decrease in SICI lasting for 20 minutes after training.

CONCLUSIONS

The decline in MEP after active training is most likely because of central fatigue, whereas the decreased F-wave frequency might represent short-term plastic changes in the spinal cord. The decrease in SICI after passive training probably reflects a decrease in intracortical GABA activity, which could benefit the acquisition of new motor skills.

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.

Interaction of paired cortical and peripheral nerve stimulation on human motor neurons

This paper contrasts responses in the soleus muscle of normal human subjects to two major inputs: the tibial nerve (TN) and the corticospinal tract. Paired transcranial magnetic stimulation (TMS) of the motor cortex at intervals of 10-25 ms strongly facilitated the motor evoked potential (MEP) produced by the second stimulus. In contrast, paired TN stimulation produced a depression of the reflex response to the second stimulus. Direct activation of the pyramidal tract did not facilitate a second response, suggesting that the MEP facilitation observed using paired TMS occurred in the cortex. A TN stimulus also depressed a subsequent MEP. Since the TN stimulus depressed both inputs, the mechanism is probably post-synaptic, such as afterhyperpolarization of motor neurons. Presynaptic mechanisms, such as homosynaptic depression, would only affect the pathway used as a conditioning stimulus. When TN and TMS pulses were paired, the largest facilitation occurred when TMS preceded TN by about 5 ms, which is optimal for summation of the two pathways at the level of the spinal motor neurons. A later, smaller facilitation occurred when a single TN stimulus preceded TMS by 50-60 ms, an interval that allows enough time for the sensory afferent input to reach the sensory cortex and be relayed to the motor cortex. Other work indicates that repetitively pairing nerve stimuli and TMS at these intervals, known as paired associative stimulation, produces long-term increases in the MEP and may be useful in strengthening residual pathways after damage to the central nervous system.

Role of Sustained Excitability of the Leg Motor Cortex After Transcranial Magnetic Stimulation in Associative Plasticity

Changes in the strength of corticospinal projections to muscles in the upper and lower limbs are induced in conscious humans after paired associative stimulation (PAS) to the motor cortex. We tested whether an intervention of PAS consisting of 90 low-frequency (0.1-Hz) stimuli to the common peroneal nerve combined with suprathreshold transcranial magnetic stimulation (TMS) produces specific changes to the motor-evoked potentials (MEPs) in lower leg muscles if the afferent volley from peripheral stimulation is timed to arrive at the motor cortex after TMS-induced firing of corticospinal neurons. Unlike PAS in the hand, MEP facilitation in the leg was produced when sensory inputs were estimated to arrive at the motor cortex over a range of 15 to 90 ms after cortical stimulation. We examined whether this broad range of facilitation occurred as a result of prolonged subthreshold excitability of the motor cortex after a single pulse of suprathreshold TMS so that coincident excitation from sensory inputs arriving many milliseconds after TMS can occur. We found that significant facilitation of MEP responses (>200%) occurred when the motor cortex was conditioned with suprathreshold TMS tens of milliseconds earlier. Likewise, it was possible to induce strong MEP facilitation (85% at 60 min) when afferent inputs were directly paired with subthreshold TMS. We argue that in the leg motor cortex, facilitation of MEP responses from PAS occurred over a large range of interstimulus intervals as a result of the paired activation of sensory inputs with sustained, subthreshold activity of cortical neurons that follow a pulse of suprathreshold TMS.

Exploring the physiological effects of double-cone coil TMS over the medial frontal cortex on the anterior cingulate cortex: an H2(15)O PET study

Transcranial magnetic stimulation (TMS) using a double-cone coil over the medial frontal cortex has the potential to clarify the function of the anterior cingulate cortex (ACC) in cognition, emotion and mood disorders. Following demonstration of disruption of performance on psychological tasks closely linked to cingulate function using this TMS technique, the current study aimed to directly measure the regional distribution of physiological effects of stimulation in the brain with H2(15)O PET. Experiment 1 assessed the effect of increasing numbers of pulse trains of TMS on regional cerebral blood flow (rCBF). Experiment 2 assessed the capacity of medial frontal TMS to modulate brain activity associated with the Stroop task using medial parietal TMS as a control site of stimulation. SPM99 analyses, using the ACC as a region of interest, revealed clusters of increased rCBF during medial frontal TMS in Brodmann area 24 and reduced rCBF in more ventral ACC, the latter occurring in both experiments. In a whole-brain analysis, striking changes in rCBF were observed distal to the ACC following medial frontal TMS. Although TMS reliably affected Stroop task performance in early trials, there was no interaction between TMS and Stroop condition in rCBF. Our results suggest that medial frontal TMS using the double-cone coil can affect ACC activity. However, a number of more distal cortical areas were also affected in these experiments. These additional changes may reflect either 'downstream' effects of altered cingulate cortex activity or direct effects of the coil.

Reduction of intracortical inhibition in soleus muscle during postural activity

Short-interval intracortical inhibition (SICI) decreases during voluntary contraction of the target muscle. It is unknown whether this effect also occurs with postural contractions. We have compared the effects of voluntary and postural contractions on SICI in the soleus (SOL) muscle. We applied transcranial magnetic stimuli (TMS) in subjects under three tasks: sitting at rest (Rest), sitting while activating the SOL muscle (Voluntary), or standing quietly (Postural). In control trials, we applied suprathreshold TMS to obtain unconditioned motor-evoked potentials (MEPs). In test trials, the same TMS was preceded by a subthreshold TMS at different interstimulus intervals (ISIs), to obtain a conditioned MEP. SICI and intracortical facilitation (ICF) were expressed as the decrease or increase in MEP size relative to unconditioned MEPs. There was significant effect of task in mean SICI or mean ICF in SOL. Mean SICI in SOL was 52% in Rest and decreased to 21% in Voluntary and 15% in Postural. Mean ICF in SOL was 132% and decreased to 113% in Voluntary and to 108% in Postural. Mean SICI in SOL was not different in Voluntary and Postural tasks. There was no effect of task in mean SICI or mean ICF in TA. Our results indicate that decrease of SICI with muscle contraction occurs to a similar extent with tonic voluntary and postural activation, suggesting that those contractions require a similar type of cortical involvement. However, it cannot be excluded that some part of the SICI reduction with muscle contraction depends on changes in segmental excitability.

Neurophysiological examination of the corticospinal system and voluntary motor control in motor-incomplete human spinal cord injury

This study employed neurophysiological methods to relate the condition of the corticospinal system with the voluntary control of lower-limb muscles in persons with motor-incomplete spinal cord injury. It consisted of two phases. In a group of ten healthy subjects, single and paired transcranial magnetic stimulation (TMS) of the motor cortex was used to study the behavior of the resulting motor evoked potentials (MEP) in lower-limb muscles. Interstimulus intervals (ISIs) of 15-100 ms were examined for augmentation of test MEPs by threshold or subthreshold conditioning stimuli. The second phase of this study examined eight incomplete spinal cord injured (iSCI) subjects, American Spinal Injury Association Impairment Scale C (n = 5) and D (n = 3) in whom voluntary motor control was quantified using the surface EMG (sEMG) based Voluntary Response Index (VRI). The VRI is calculated to characterize relative output patterns across ten lower-limb muscles recorded during a standard protocol of elementary voluntary motor tasks. VRI components were calculated by comparing the distribution of sEMG in iSCI subjects with prototype patterns collected from 15 healthy subjects using the same rigidly administered protocol, The resulting similarity index (SI) and magnitude values provided the measure of voluntary motor control. Corticospinal system connections were characterized by the thresholds for MEPs in key muscles. Key muscles were those that function as the prime-movers, or agonists for the voluntary movements from which the VRI data were calculated. Results include healthy-subject data that showed significant increases in conditioned MEP responses with paired stimuli of 15-50 ms ISI. Stimulus pairs of 75 and 100 ms showed no increase in MEP peak amplitude over that of the single-pulse conditioning stimulus alone, usually no response. For the iSCI subjects, 42% of the agonists responded to single-pulse TMS and 25% required paired-pulse TMS to produce an MEP. American Spinal Injury Association Impairment Scale component motor scores for agonist muscles, Quadriceps, Tibialis Anterior, and Triceps Surae, were significantly lower where MEPs could not be obtained (p < 0.05). VRI values were also significantly lower for motor tasks with agonists that had no resting MEP (p < 0.01). Therefore, the presence of a demonstrable connection between the motor cortex and spinal motor neurons in persons with SCI was related to the quality of post-injury voluntary motor control as assessed by the VRI.

Influence of repetitive hand movements on intracortical inhibition

Repetitive movements have been reported to induce task-specific changes of intracortical inhibition and facilitation, but the mechanism operating shortly after hand movement is unclear. Transcranial magnetic single and paired stimuli (2 ms) were applied to 15 healthy subjects at rest and 1 s after repetitive (every 6 s) active and passive hand extensions. Motor evoked potentials (MEPs) were recorded from hand extensors (agonists) and flexors (antagonists). A strong overall inhibitory effect was observed after applying paired stimuli. In agonists only, active movements produced significantly larger MEPs. Inhibition, however, did not differ between active or passive movements and rest. This suggests that MEP increases produced by active movements in agonists are not caused by disinhibition, but are rather due to excitation (facilitation). This finding may also have implications for future studies evaluating the preferential activation of target muscles in physiotherapy.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

A coil design for transcranial magnetic stimulation of deep brain regions

Noninvasive magnetic stimulation of the human central nervous system has been used in research and the clinic for several years. However, the coils used previously stimulated mainly the cortical brain regions but could not stimulate deeper brain regions directly. The purpose of the current study was to develop a coil to stimulate deep brain regions. Stimulation of the nucleus accumbens and the nerve fibers connecting the prefrontal cortex with the nucleus accumbens was one major target of the authors' coil design. Numeric simulations of the electrical field induced by several types of coils were performed and accordingly an optimized coil for deep brain stimulation was designed. The electrical field induced by the new coil design was measured in a phantom brain and compared with the double-cone coil. The numeric simulations show that the electrical fields induced by various types of coils are always greater in cortical regions (closer to the coil placement); however, the decrease in electrical field within the brain (as a function of the distance from the coil) is markedly slower for the new coil design. The phantom brain measurements basically confirmed the numeric simulations. The suggested coil is likely to have the ability of deep brain stimulation without the need to increase the intensity to levels that stimulate cortical regions to a much higher extent and possibly cause undesirable side effects.

Motor-evoked potential facilitation during progressive cortical suppression by propofol

We characterized the effects of various stimulation patterns on motor-evoked potentials (MEPs) elicited by repetitive transcranial magnetoelectric stimulation at different levels of cortical suppression by propofol. In 20 patients undergoing lumbar disk surgery, propofol target plasma concentrations (PTPCs) were increased incrementally by target plasma-level controlled infusion during the induction of anesthesia. MEPs were recorded from the muscles of the upper extremities after single, double, and quadruple magnetoelectric stimulation at 500, 200, and 100 Hz. The mean PTPC during loss of responsiveness to verbal instructions (CP50) was 3 microg/mL (CP(95), 5 microg/mL). At PTPCs <3 microg/mL, maximal MEP amplitudes were elicited by quadruple stimulation at 100 Hz. At PTPCs > or =3 microg/mL, four pulses at 200 Hz yielded peak MEP amplitudes. Therefore, quadruple magnetoelectric stimulation at 100 Hz yields peak myogenic responses in awake patients. With progressive cortical suppression resulting from PTPCs beyond 3 microg/mL, the most effective stimulation frequency shifts to 200 Hz. This may be explained by a differential dose-dependent action of propofol on GABAergic cortical interneurons, corresponding to the clinically observed state of vigilance. Recording of spinal cord evoked potentials will aid in further elucidation of the modulatory effects of general anesthesia on intracortical facilitation.

IMPLICATIONS

We investigated the effect of different transcranial magnetoelectric stimulation paradigms on motor-evoked potentials during different levels of cortical suppression by propofol. The most effective stimulation frequency seems to change from 100 to 200 Hz during progressive propofol dose escalation, possibly because of specific interaction with cortical interneurons.

Analogous corticocortical inhibition and facilitation in ipsilateral and contralateral human motor cortex representations of the tongue

How the human brain controls activation of the ipsilateral part of midline muscles is unknown. We studied corticospinal and corticocortical network excitability of both ipsilateral and contralateral motor representations of the tongue to determine whether they are under analogous or disparate inhibitory and facilitatory corticocortical control. Motor evoked potentials (MEPs) to unilateral focal transcranial magnetic stimulation (TMS) of the tongue primary motor cortex were recorded simultaneously from the ipsilateral and contralateral lingual muscles. Single-pulse TMS was used to assess motor threshold (MT) and MEP recruitment. Paired-pulse TMS was used to study intracortical inhibition (ICI) and intracortical facilitation (ICF) at various interstimulus intervals (ISIs) between the conditioning stimulus (CS) and the test stimulus (TS), and at different CS and TS intensities, respectively. Focal TMS invariably produced MEPs in both ipsilateral and contralateral lingual muscles. MT was lower and MEP recruitment was steeper when recorded from the contralateral muscle group. ICI and ICF were identical in the ipsilateral and contralateral representations, with inhibition occurring at short ISIs (2 and 3 ms) and facilitation occurring at longer ISIs (10 and 15 ms). Moreover, changing one stimulus parameter regularly produced analogous changes in MEP size bilaterally, revealing strong linear correlations between ipsilateral and contralateral ICI and ICF (P < 0.0001). These findings indicate that the ipsilateral and contralateral representations of the tongue are under analogous inhibitory and facilitatory control, possibly by a common intracortical network.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Predominant activation of I1-waves from the leg motor area by transcranial magnetic stimulation

We performed transcranial magnetic stimulation (TMS) to elucidate the D- and I-wave components comprising the motor evoked potentials (MEPs) elicited from the leg motor area, especially at near-threshold intensity. Recordings were made from the tibialis anterior muscle using needle electrodes. A figure-of-eight coil was placed so as to induce current in the brain in eight different directions, starting from the posterior-to-anterior direction and rotating it in 45 degrees steps. The latencies were compared with those evoked by transcranial electrical stimulation (TES) and TMS using a double cone coil. Although the latencies of MEPs ranged from D to I3 waves, the most prominent component evoked by TMS at near-threshold intensity represented the I1 wave. With the double cone coil, the elicited peaks always represented I1 waves, and D waves were evoked only at very high stimulus intensities, suggesting a high effectiveness of this coil in inducing I1 waves. Using the figure-of-eight coil, current flowing anteriorly or toward the hemisphere contralateral to the recorded muscle was more effective in eliciting large responses than current flowing posteriorly or toward the ipsilateral hemisphere. The effective directions induced I1 waves with the lowest threshold, whereas the less effective directions elicited I1 and I2 waves with a similar frequency. Higher stimulus intensities resulted in concomitant activation of D through I3 waves with increasing amount of D waves, but still the predominance of I1 waves was apparent. The amount of I waves, especially of I1 waves, was greater than predicted by the hypothesis that TMS over the leg motor area activates the output cells directly, but rather suggests predominant transsynaptic activation. The results accord with those of recent human epidural recordings.

Facilitatory I wave interaction in proximal arm and lower limb muscle representations of the human motor cortex

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits direct and indirect (I) waves in the corticospinal tract. Facilitatory I wave interaction has been demonstrated with a suprathreshold first stimulus (S1) followed by a subthreshold to threshold second stimulus (S2). Intracortical inhibition (ICI) and intracortical facilitation (ICF) can be studied by another paired TMS paradigm with a subthreshold conditioning stimulus (CS) followed by a suprathreshold test stimulus. Facilitatory I wave interaction in motor representations other than the hand area and its relationship to ICI and ICF has not been studied. We studied I wave interaction, ICI and ICF in an intrinsic hand muscle (abductor pollicis brevis, APB), in a proximal arm muscle (biceps brachii, BB) and in a lower limb muscle (tibialis anterior, TA) in 11 normal subjects. I wave facilitation was studied by paired TMS at 24 interstimulus intervals (ISIs) from 0.5 to 5.1 ms. For APB and TA, facilitation occurred in three distinct peaks at ISIs of 0.9-1.7, 2. 5-3.5, and 4.1-5.1 ms. For BB, facilitation was significant for the first two peaks. The latencies of the peaks were similar among different muscles, but the magnitude of facilitation was much greater for APB and TA compared with BB. For all three muscles, changing the S2 to transcranial electrical stimulation (TES) resulted in much less facilitation of the first peak. For APB, there was significant I wave facilitation with S2 at 72% motor threshold (MT). The same stimulus used as the CS did not elicit ICF at ISI of 15 ms, suggesting that the threshold for eliciting I wave facilitation is lower than that for ICF. For BB and TA, there was no I wave facilitation with S2 at 90% of APB MT, and the same stimulus used as CS led to ICI. Thus in BB and TA the threshold for eliciting ICI is lower than that for I wave facilitation. We conclude that the circuits that mediate I wave interactions are present in the proximal arm and lower limb representations of the motor cortex. I wave facilitation occurs predominately in the cortex and may be primarily related to the monosynaptic corticomotoneuronal (CM) system. The reduced I wave facilitation for BB compared with APB and TA may be related to less extensive CM projection and involvement of other polysynaptic descending pathways. I wave facilitation, ICI, and ICF appears to be mediated by different neuronal circuits.

Mind, muscles and motoneurones

This review considers some of the adaptations which take place in the central nervous system to allow optimal performance of the musculoskeletal system for the smallest to the largest "efforts". Mental imagery of exercise helps performance but the way in which it works is multifactional: it evokes muscle contraction sufficient to activate muscle receptors. Furthermore, it is possible for subjects to focus specifically on control of particular muscles even without feedback from them. On the other hand maximal voluntary efforts, at least in isometric and in concentric contractions, can drive the motoneurones sufficiently to ensure full force production by the muscle. Many neural factors contribute to maintain force output during repetitive activity, including a feedback loop whereby increased central command during fatigue acts to enhance muscle perfusion. As peripheral muscle fatigue develops, changes occur in the excitability of the motor cortex. Recent evidence suggests that "central" factors leading to reduced drive to muscles in isometric contractions act "upstream" of motor cortical output.

Intracortical inhibition and facilitation in different representations of the human motor cortex

Intracortical inhibition and facilitation in different representations of the human motor cortex. J. Neurophysiol. 80: 2870-2881, 1998. Intracortical inhibition (ICI) and intracortical facilitation (ICF) of the human motor cortex can be studied with paired transcranial magnetic stimulation (TMS). Plastic changes and some neurological disorders in humans are associated with changes in ICI and ICF. Although well characterized in the hand representation, it is not known if ICI and ICF vary across different body part representations. Therefore we studied ICI and ICF in different motor representations of the human motor cortex. The target muscles were rectus abdominus (RA), biceps brachii (BB), abductor pollicis brevis (APB), quadriceps femoris (QF), and abductor hallucis (AH). For each muscle, we measured the rest and active motor thresholds (MTs), the motor-evoked potential (MEP) stimulus-response curve (MEP recruitment), ICI, and ICF. The effects of different interstimulus intervals (ISIs) were studied with a conditioning stimulus (CS) intensity of 80% active MT. The effects of different CS intensities were studied at ISI of 2 ms for ICI and ISI of 15 ms for ICF. MT was lowest for APB, followed by BB, AH, and QF, and was highest for RA. Except for BB, MEP recruitment was generally steeper for muscles with lower MT. ICI and ICF were present in all the motor representations tested. The stimulus intensity necessary to elicit ICI was consistently lower than that required to elicit ICF, suggesting that they are mediated by separate mechanisms. Despite wide differences in MT and MEP recruitment, the absolute CS intensities (expressed as percentage of the stimulator's output) required to elicit ICI and ICF appear unrelated to MT and MEP recruitment in the different muscles tested. These findings suggest that the intracortical mechanisms for inhibition and facilitation in different motor representations are not related to the strength of corticospinal projections.