Motor Evoked Potentials Recruitment Research Articles

Decoding firings of a large population of human motor units from high-density surface electromyogram in response to transcranial magnetic stimulation.

We describe a novel application of methodology for high-density surface electromyography (HDsEMG) decomposition to identify motor unit (MU) firings in response to transcranial magnetic stimulation (TMS). The method is based on the MU filter estimation from HDsEMG decomposition with convolution kernel compensation during voluntary isometric contractions and its application to contractions elicited by TMS. First, we simulated synthetic HDsEMG signals during voluntary contractions followed by simulated motor evoked potentials (MEPs) recruiting an increasing proportion of the motor pool. The estimation of MU filters from voluntary contractions and their application to elicited contractions resulted in high (>90%) precision and sensitivity of MU firings during MEPs. Subsequently, we conducted three experiments in humans. From HDsEMG recordings in first dorsal interosseous and tibialis anterior muscles, we demonstrated an increase in the number of identified MUs during MEPs evoked with increasing stimulation intensity, low variability in the MU firing latency and a proportion of MEP energy accounted for by decomposition similar to voluntary contractions. A negative relationship between the MU recruitment threshold and the number of identified MU firings was exhibited during the MEP recruitment curve, suggesting orderly MU recruitment. During isometric dorsiflexion we also showed a negative association between voluntary MU firing rate and the number of firings of the identified MUs during MEPs, suggesting a decrease in the probability of MU firing during MEPs with increased background MU firing rate. We demonstrate accurate identification of a large population of MU firings in a broad recruitment range in response to TMS via non-invasive HDsEMG recordings. KEY POINTS: Transcranial magnetic stimulation (TMS) of the scalp produces multiple descending volleys, exciting motor pools in a diffuse manner. The characteristics of a motor pool response to TMS have been previously investigated with intramuscular electromyography (EMG), but this is limited in its capacity to detect many motor units (MUs) that constitute a motor evoked potential (MEP) in response to TMS. By simulating synthetic signals with known MU firing patterns, and recording high-density EMG signals from two human muscles, we show the feasibility of identifying firings of many MUs that comprise a MEP. We demonstrate the identification of firings of a large population of MUs in the broad recruitment range, up to maximal MEP amplitude, with fewer required stimuli compared to intramuscular EMG recordings. The methodology demonstrates an emerging possibility to study responses to TMS on a level of individual MUs in a non-invasive manner.

Near-threshold recruitment characteristics of motor evoked potentials in transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) can induce motor evoked potentials (MEPs). In TMS applications, near-threshold stimulation intensities (SIs) are often used for characterizing corticospinal excitability using MEPs. We aimed to characterize the individual near-threshold recruitment of MEPs and to test the assumptions related to selection of the suprathreshold SI. We utilized MEP data from a right-hand muscle induced at variable SIs. The single-pulse TMS (spTMS) data from previous studies (27 healthy volunteers), as well as data from new measurements (10 healthy volunteers) that included also MEPs modulated by paired-pulse TMS (ppTMS), were included. The probability of MEP (pMEP) was represented with individually fitted cumulative distribution function (CDF) with two parameters: resting motor threshold (rMT) and spread relative to rMT. MEPs were recorded with 110% and 120% of rMT as well as with Mills-Nithi upper threshold (UT). The individual near-threshold characteristics varied with CDF parameters: the rMT and the relative spread (median: 0.052). The rMT was lower with ppTMS than with spTMS (p < 0.001), while the relative spread remained similar (p = 0.812). At suprathreshold SIs, the probability of MEP was similar between UT and 110% of rMT (pMEP > 0.88), and higher for 120% of rMT (pMEP > 0.98). The individual near-threshold characteristics determine how probably MEPs are produced at common suprathreshold SIs. At the population level, the used SIs UT and 110% of rMT produced MEPs at similar probability. The individual variability in the relative spread parameter was large; therefore, the method of determining the proper suprathreshold SI for TMS applications is of crucial importance.

The effect of acute intermittent hypoxia on human limb motoneurone output.

What is the central question of this study? Does a single session of repeated bouts of acute intermittent hypoxic breathing enhance the motoneuronal output of the limb muscles of healthy able-bodied participants? What is the main finding and its importance? Compared to breathing room air, there were some increases in motoneuronal output following acute intermittent hypoxia, but the increases were variable across participants and in time after the intervention and depended on which neurophysiological measure was checked. Acute intermittent hypoxia (AIH) induces persistent increases in output from rat phrenic motoneurones. Studies in people with spinal cord injury (SCI) suggest that AIH improves limb performance, perhaps via postsynaptic changes at cortico-motoneuronal synapses. We assessed whether limb motoneurone output in response to reflex and descending synaptic activation is facilitated after one session of AIH in healthy able-bodied volunteers. Fourteen participants completed two experimental days, with either AIH or a sham intervention (randomised crossover design). We measured H-reflex recruitment curves and homosynaptic post-activation depression (HPAD) of the H-reflex in soleus, and motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) and their recruitment curves in first dorsal interosseous. All measurements were performed at rest and occurred at baseline, 0, 20, 40 and 60min post-intervention. The intervention was 30min of either normoxia (sham, ≈0.21) or AIH (alternate 1-min hypoxia [ ≈0.09], 1-min normoxia). After AIH, the H-reflex recruitment curve shifted leftward. Lower stimulation intensities were needed to evoke 5%, 50% and 99% of the maximal H-reflex at 40 and 60min after AIH (P<0.04). The maximal H-reflex, recruitment slope and HPAD were unchanged after AIH. MEPs evoked by constant intensity TMS were larger 40min after AIH (P=0.027). There was no change in MEP recruitment or the maximal MEP. In conclusion, some measures of the evoked responses from limb motoneurones increased after a single AIH session, but only at discrete time points. It is unclear to what extent these changes alter functional performance.

Transcutaneous Electrical Neuromodulation of the Cervical Spinal Cord Depends Both on the Stimulation Intensity and the Degree of Voluntary Activity for Training. A Pilot Study

Electrical enabling motor control (eEmc) through transcutaneous spinal cord stimulation offers promise in improving hand function. However, it is still unknown which stimulus intensity or which muscle force level could be better for this improvement. Nine healthy individuals received the following interventions: (i) eEmc intensities at 80%, 90% and 110% of abductor pollicis brevis motor threshold combined with hand training consisting in 100% handgrip strength; (ii) hand training consisting in 100% and 50% of maximal handgrip strength combined with 90% eEmc intensity. The evaluations included box and blocks test (BBT), maximal voluntary contraction (MVC), F wave persistency, F/M ratio, spinal and cortical motor evoked potentials (MEP), recruitment curves of spinal MEP and cortical MEP and short-interval intracortical inhibition. The results showed that: (i) 90% eEmc intensity increased BBT, MVC, F wave persistency, F/M ratio and cortical MEP recruitment curve; 110% eEmc intensity increased BBT, F wave persistency and cortical MEP and recruitment curve of cortical MEP; (ii) 100% handgrip strength training significantly modulated MVC, F wave persistency, F/M wave and cortical MEP recruitment curve in comparison to 50% handgrip strength. In conclusion, eEmc intensity and muscle strength during training both influence the results for neuromodulation at the cervical level.

Contraction Phase and Force Differentially Change Motor Evoked Potential Recruitment Slope and Interhemispheric Inhibition in Young Versus Old.

Interhemispheric interactions are important for arm coordination and hemispheric specialization. Unilateral voluntary static contraction is known to increase bilateral corticospinal motor evoked potential (MEP) amplitude. It is unknown how increasing and decreasing contraction affect the opposite limb. Since dynamic muscle contraction is more ecologically relevant to daily activities, we studied MEP recruitment using a novel method and short interval interhemispheric inhibition (IHI) from active to resting hemisphere at 4 phases of contralateral ECR contraction: Rest, Ramp Up [increasing at 25% of maximum voluntary contraction (MVC)], Execution (tonic at 50% MVC), and Ramp Down (relaxation at 25% MVC) in 42 healthy adults. We analyzed the linear portion of resting extensor carpi radialis (ECR) MEP recruitment by stimulating at multiple intensities and comparing slopes, expressed as mV per TMS stimulation level, via linear mixed modeling. In younger participants (age ≤ 30), resting ECR MEP recruitment slopes were significantly and equally larger both at Ramp Up (slope increase = 0.047, p < 0.001) and Ramp Down (slope increase = 0.031, p < 0.001) compared to rest, despite opposite directions of force change. In contrast, Active ECR MEP recruitment slopes were larger in Ramp Down than all other phases (Rest:0.184, p < 0.001; Ramp Up:0.128, p = 0.001; Execution: p = 0.003). Older (age ≥ 60) participants’ resting MEP recruitment slope was higher than younger participants across all phases. IHI did not reduce MEP recruitment slope equally in old compared to young. In conclusion, our data indicate that MEP recruitment slope in the resting limb is affected by the homologous active limb contraction force, irrespective of the direction of force change. The active arm MEP recruitment slope, in contrast, remains relatively unaffected. Older participants had steeper MEP recruitment slopes and less interhemispheric inhibition compared to younger participants.

Priming Effects of Water Immersion on Paired Associative Stimulation-Induced Neural Plasticity in the Primary Motor Cortex.

We aimed to verify whether indirect-wave (I-wave) recruitment and cortical inhibition can regulate or predict the plastic response to paired associative stimulation with an inter-stimulus interval of 25 ms (PAS25), and also whether water immersion (WI) can facilitate the subsequent PAS25-induced plasticity. To address the first question, we applied transcranial magnetic stimulation (TMS) to the M1 hand area, while alternating the direction of the induced current between posterior-to-anterior and anterior-to-posterior to activate two independent synaptic inputs to the corticospinal neurons. Moreover, we used a paired stimulation paradigm to evaluate the short-latency afferent inhibition (SAI) and short-interval intracortical inhibition (SICI). To address the second question, we examined the motor evoked potential (MEP) amplitudes before and after PAS25, with and without WI, and used the SAI, SICI, and MEP recruitment curves to determine the mechanism underlying priming by WI on PAS25. We demonstrated that SAI, with an inter-stimulus interval of 25 ms, might serve as a predictor of the response to PAS25, whereas I-wave recruitment evaluated by the MEP latency difference was not predictive of the PAS25 response, and found that 15 min WI prior to PAS25 facilitated long-term potentiation (LTP)-like plasticity due to a homeostatic increase in cholinergic activity.

The Effects of Biological Sex and Ovarian Hormones on Exercise-Induced Neuroplasticity

Acute aerobic exercise induces short-term neuroplasticity, although it remains unknown whether biological sex and ovarian hormones influence this response. The present study investigated the effects of biological sex and ovarian hormones on short-term neuroplasticity induced by acute aerobic exercise. Young active adults (n = 17 males and n = 17 females; 21 ± 2 years) participated in two sessions in which transcranial magnetic stimulation (TMS) measures were acquired immediately before and after a 20-min bout of moderate-intensity cycling at 65–70% of maximal heart rate. Females were tested in the follicular (~day 7) and luteal (~day 21) phases of the menstrual cycle. Males were tested on two sessions separated by ~14 days. Measures of motor-evoked potential (MEP) recruitment curves and short-interval intracortical inhibition (SICI) were obtained using TMS. Estradiol, progesterone, testosterone, brain-derived neurotrophic factor (BDNF), and insulin-like growth factor 1 (IGF-1) were measured in venous blood samples obtained prior to exercise. MEP recruitment curves increased and SICI decreased after exercise in both sexes, regardless of menstrual cycle phase. BDNF and IGF-1 were not different between sexes or across the menstrual cycle. Females had a greater estradiol to progesterone ratio (E:P) in the follicular phase compared to the luteal phase, while males had similar testosterone levels on both occasions. We conclude that biological sex and ovarian hormones do not impact short-term neuroplasticity induced by acute exercise. Significance StatementAcute exercise induces short-term changes indicative of neuroplasticity within the primary motor cortex and corticospinal pathway. This research reveals that increases in corticospinal excitability and decreases in intracortical inhibition occur similarly in males and females, and that female hormones do not influence these changes. These findings may be used to assist with developing exercise interventions aimed at promoting neuroplasticity in both sexes.

Exploring the effect of adaptogenic Rhodiola Rosea extract on neuroplasticity in humans

ObjectivesRhodiola rosea extract is one of the most common herbal treatment for stress. Its mechanism of action in humans still need to be determined. We investigated the effect of a single dose intake of Rhodiola rosea extract on the plastic after-effects induced by anodal and cathodal motor cortex transcranial Direct Current Stimulation in humans. MethodsTwenty-eight healthy volunteers were required to intake 500 mg of either RRE or placebo. Transcranial Magnetic Stimulation was used to investigate cortical excitability. Motor threshold, recruitment of motor-evoked potentials, cortical silent period, short-interval intracortical inhibition and intracortical facilitation were assessed at different time points. Spinal excitability and peripheral nerve conduction were measured by F-response and M-wave. Furthermore, we assessed the modulation of cortical plasticity using transcranial direct current stimulation after-effects on Motor Evoked Potentials amplitudes. ResultsRhodiola rosea extract acute intake prevented cathodal transcranial Direct Current Stimulation-induced Long-term depression-like plasticity. The extract intake did not affect cortical excitability. ConclusionsOur findings suggest that a single oral dose of Rhodiola rosea extract intake modulates cortical plasticity in humans preventing the activity-dependent reduction in the efficacy of neuronal synapses. These results suggest that the adaptogenic and antidepressant effects of Rhodiola rosea extract might be based on its modulation of brain plasticity.

Transcranial focused ultrasound neuromodulation of the human primary motor cortex

Transcranial focused ultrasound is an emerging form of non-invasive neuromodulation that uses acoustic energy to affect neuronal excitability. The effect of ultrasound on human motor cortical excitability and behavior is currently unknown. We apply ultrasound to the primary motor cortex in humans using a novel simultaneous transcranial ultrasound and magnetic stimulation paradigm that allows for concurrent and concentric ultrasound stimulation with transcranial magnetic stimulation (TMS). This allows for non-invasive inspection of the effect of ultrasound on motor neuronal excitability using the motor evoked potential (MEP). We test the effect of ultrasound on single pulse MEP recruitment curves and paired pulse protocols including short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). In addition, we test the effect of ultrasound to motor cortex on a stimulus response reaction time task. Results show ultrasound inhibits the amplitude of single-pulse MEPs and attenuates intracortical facilitation but does not affect intracortical inhibition. Ultrasound also reduces reaction time on a simple stimulus response task. This is the first report of the effect of ultrasound on human motor cortical excitability and motor behavior and confirms previous results in the somatosensory cortex that ultrasound results in effective neuronal inhibition that confers a performance advantage.

Hypericum perforatum extract modulates cortical plasticity in humans.

Hypericum perforatum (HYP) extract is one of the most commonly used complementary alternative medicines (CAMs) for the treatment of mild-to-moderate depression. Non-invasive brain stimulation protocols can be used to investigate the effect of psychoactive substances on the human brain. In this study, we explored the effect of a single dose of HYP extract (WS 5570) intake on corticospinal excitability and plasticity in humans. Twenty-eight healthy subjects were required to intake 900mg of either HYP extract or placebo. Cortical excitability was assessed using single and paired transcranial magnetic stimulation (TMS). The electrophysiological parameters of motor threshold, recruitment of motor-evoked potentials (MEPs), cortical silent period (CSP), short interval intracortical inhibition (SICI), and intracortical facilitation (ICF) were tested before and 2 and 5h after the oral intake. Spinal and neuromuscular excitability and peripheral nerve excitability were measured by F response and M-wave. Cortical plasticity was induced using transcranial direct current stimulation (tDCS). Subjects received either HYP extract or placebo before anodal and cathodal tDCS of the primary motor cortex. Plasticity was assessed by MEP amplitudes. HYP extract reversed cathodal tDCS-induced long-term depression (LTD)-like plasticity into facilitation, as compared to placebo. HYP extract did not have a significant effect on anodal tDCS-induced plasticity and TMS measures of motor cortex and spinal/neuromuscular excitability. Our findings suggest that a single oral dose of HYP extract modulates cortical plasticity in healthy subjects and provide new insight into its possible mechanism of action in humans.

Prediction of Motor Recovery Using Quantitative Parameters of Motor Evoked Potential in Patients With Stroke

ObjectiveTo investigate the clinical significance of quantitative parameters in transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEP) which can be adopted to predict functional recovery of the upper limb in stroke patients in the early subacute phase.MethodsOne hundred thirteen patients (61 men, 52 women; mean age 57.8±12.2 years) who suffered faiarst-ever stroke were included in this study. All participants underwent TMS-induced MEP session to assess the corticospinal excitability of both hand motor cortices within 3 weeks after stroke onset. After the resting motor threshold (rMT) was assessed, five sweeps of MEP were performed, and the mean amplitude of the MEP was measured. Latency of MEP, volume of the MEP output curve, recruitment ratios, and intracortical inhibition and facilitation were also measured. Motor function was assessed using the Fugl-Meyer Assessment scale (FMA) within 3 weeks and at 3 months after stroke onset. Correlation analysis was performed between TMS-induced MEP derived measures and FMA scores.ResultsIn the MEP response group, rMT and rMT ratio measures within 3 weeks after stroke onset showed a significant negative correlation with the total and upper limb FMA scores at 3 months after stroke (p<0.001). Multiple regression analysis revealed that FMA score and rMT ratio, but not rMT within 3 weeks were independent prognostic factors for FMA scores at 3 months after stroke.ConclusionThese results indicated that the quantitative parameter of TMS-induced MEP, especially rMT ratio in the early subacute phase, could be used as a parameter to predict motor function in patients with stroke.

Modulation of sensorimotor cortex by repetitive peripheral magnetic stimulation.

This study examines with transcranial magnetic stimulation (TMS) and with functional magnetic resonance imaging (fMRI) whether 20 min of repetitive peripheral magnetic stimulation (rPMS) has a facilitating effect on associated motor controlling regions. Trains of rPMS with a stimulus intensity of 150% of the motor threshold (MT) were applied over right hand flexor muscles of healthy volunteers. First, with TMS, 10 vs. 25 Hz rPMS was examined and compared to a control group. Single and paired pulse motor evoked potentials (MEPs) from flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles were recorded at baseline (T0), post rPMS (T1), 30 min post (T2), 1 h post (T3) and 2 h post rPMS (T4). Then, with fMRI, 25 Hz rPMS was compared to sham stimulation by utilizing a finger tapping activation paradigm. Changes in bloodoxygen level dependent (BOLD) contrast were examined at baseline (PRE), post rPMS (POST1) and 1 h post rPMS (POST2). With TMS facilitation was observed in the target muscle (FCR) following 25 Hz rPMS: MEP recruitment curves (RCs) were increased at T1, T2 and T3, and intracortical facilitation (ICF) was increased at T1 and T2. No effects were observed following 10 Hz rPMS. With fMRI the BOLD contrast at the left sensorimotor area was increased at POST1. Compared to inductions protocols based on transcutaneous electrical stimulation and mechanical stimulation, the rPMS induced effects appeared shorter lasting.

Distinct interneuronal networks influence excitability of the surround during movement initiation.

Surround inhibition (SI) is a feature of motor control in which activation of task-related muscles is associated with inhibition of neighboring, nonprotagonist muscles, allowing selective motor control. The physiological basis for SI still remains unknown. In all previous studies, SI in the motor system was measured during movement initiation by using transcranial magnetic stimulation (TMS) to deliver a posteroanterior current at a single suprathreshold intensity. To expand our understanding of SI, we explored this phenomenon at a wide range of intensities and by stimulating motor cortex with currents along anteroposterior and lateromedial directions. Fifteen healthy volunteers performed a brief isometric index finger flexion on hearing a tone. Electromyography was recorded from the synergist and surround finger muscles. Single-pulse TMS was applied to stimulate the surround muscle at different intensities at rest or movement initiation. The motor evoked potential (MEP) amplitudes were then plotted against stimulation intensities to obtain the MEP recruitment curves for the rest and movement initiation conditions and for the three current directions for every subject. From the recruitment curves, we found that surround inhibition could be elicited only by the posteroanterior current. Hence, we postulate that surround inhibition is mediated by intracortical circuits in the motor cortex. Also, for the first time, we observed surround facilitation when the motor cortex was stimulated with anteroposterior current. Further studies are needed to investigate the mechanisms underlying both these phenomena individually in healthy subjects and patients with dystonia and other movement disorders.

Assessment of corticospinal excitability after traumatic spinal cord injury using MEP recruitment curves: a preliminary TMS study.

Transcranial magnetic stimulation study. To further investigate the corticospinal excitability changes after spinal cord injury (SCI), as assessed by means of transcranial magnetic stimulation (TMS). Merano (Italy) and Salzburg (Austria). We studied resting motor threshold (RMT), motor evoked potential (MEP) amplitude and recruitment curve in five subjects with good recovery after traumatic incomplete cervical SCI. RMT did not differ significantly between patients and controls, whereas the slope of MEP recruitment curve was significantly increased in the patients. This abnormal finding may represent an adaptive response after SCI. The impaired ability of the motor cortex to generate proper voluntary movement may be compensated by increasing spinal excitability. The easily performed measurement of MEP recruitment curve may provide a useful additional tool to improve the assessment and monitoring of motor cortical function in subjects with SCI. Increasing our knowledge of the corticospinal excitability changes in the functional recovery after SCI may also support the development of effective therapeutic strategies.

Short-latency afferent inhibition in chronic spinal cord injury.

BackgroundShort-latency afferent inhibition (SAI) results when somatosensory afferent input inhibits the corticospinal output from primary motor cortex (M1). The present study examined SAI in the flexor carpi radialis (FCR) muscle in individuals with spinal cord injury (SCI) and uninjured controls.MethodsShort-latency afferent inhibition (SAI) was evoked by stimulating the median nerve at the elbow at intervals of 15, 20 and 25 ms in advance of a transcranial magnetic stimulation (TMS) pulse over M1. SAI was tested with the FCR at rest and also during ~20% of maximum voluntary contraction. Corticospinal output was assessed through measuring both motor thresholds and motor evoked potential (MEP) recruitment curves. The afferent volley was assessed via the N20–P25 amplitude of the somatosensory evoked potential (SEP) and the amplitude of sensory nerve action potentials (SNAP) recorded over the median nerve at the elbow.ResultsSAI is reduced in SCI in both the contracted and non-contracted FCR muscle. MEP recruitment curves and thresholds were decreased in SCI only in the active state and not the resting state. N20–P25 amplitude was similar between groups in both the resting and active states although SNAP was significantly reduced in SCI at rest.ConclusionsWe conclude that reduced SAI in SCI is likely attributed to neuroplasticity altering the intrinsic M1 circuitry mediating SAI and/or reduced afferent input traversing a direct thalamocortical route to M1. These data provide a new avenue of research aimed at identifying therapeutic approaches to alter SAI to improve upper limb function in individuals with SCI.

Enhanced motor cortex excitability after spinal cord injury.

Enhanced motor cortex excitability after spinal cord injury.

119. Cortical excitability in patients affected by brain tumors

In this study we investigated cortical excitability, inhibitory and excitatory intracortical circuitries, by using transcranial magnetic stimulation, in both hemispheres of patients affected by brain tumors. Cortical excitability was probed with transcranial magnetic stimulation in 10 right-handed patients (3 women), aged 49–70 years (mean age: 54.8 years) and 10 age-matched healthy controls. We tested motor thresholds (MT), motor evoked potential (MEP) recruitment curve, cortical silent period (CSP), short and long intracortical inhibition (SICI and LICI), intracortical facilitation (ICF), short and long afferent inhibition (SAI and LAI) and inter-hemispheric inhibition (IHI). RMT and AMT were higher in both hemispheres of patients with brain tumors than healthy controls. The slope of MEP recruitment curve was deeper in both hemispheres of patients with brain tumors. SICI, LICI, SAI and LAI are reduced in patients with brain tumors compared with aged matched healthy controls. Cortical excitability, specially inhibitory intracortical circuitries, are impaired in both hemispheres of patients affected by brain tumors.

Differences between Han Chinese and Caucasians in transcranial magnetic stimulation parameters

The study was conducted to investigate the difference between Han Chinese and Caucasians on various parameters measured from responses to transcranial magnetic brain stimulation (TMS). Sixteen subjects were studied in each group. A circular coil at the vertex was used for stimulation, whilst recording surface electromyograms from right first dorsal interosseous. In the passive state, motor-evoked potential (MEP) threshold, MEP recruitment, short-interval intracortical inhibition (SICI) and intracortical facilitation were measured. The MEP threshold, recruitment and silent period were also measured in the active state. Chinese subjects showed significantly higher passive thresholds (P < 0.005), less inhibition of the motor response (SICI, P < 0.0005) and a shorter silent period (P < 0.05). Differences in SICI appeared to be a consequence of the differences in passive threshold and were not seen when active threshold was used to determine the conditioning stimulus intensity. Differences in silent period may also reflect differences in cortical excitability rather than inhibitory processes, as they were not seen when the silent-period duration was expressed as a function of MEP size, rather than TMS intensity. There appears to be a significant difference in some TMS parameters between Han Chinese and Caucasian subjects. This may reflect an underlying difference in cortical excitability.

Outlasting corticomotor excitability changes induced by 25 Hz whole-hand mechanical stimulation

The objective was to investigate if whole-hand mechanical stimulation (MSTIM) in the tapping-flutter frequency range induces outlasting post-stimulus changes in the hand region of the primary motor cortex. MSTIM was delivered to 12 healthy subjects for 20 min using a therapeutic stimulation device (Swisswing BMR 2000). Frequencies of 10 and 25 Hz were tested in separate sessions, and for control additionally the foot sole was stimulated at 25 Hz. Motor evoked potentials (MEPs) after single (recruitment curves) and paired-pulse transcranial magnetic stimulation (TMS) were recorded from FDI and APB muscles of the right hand. TMS assessments were carried out at baseline (T0), immediately after (T1), 30 min (T2), 1 h (T3) and 2 h (T4) after end of MSTIM. After MSTIM with 25 Hz, MEP recruitment curves were increased at all post-stimulation assessments in both muscles. The most significant effect was achieved at T3 (1 h). Intracortical inhibition was decreased within the first hour, while intracortical facilitation was increased at all post-stimulation assessments. No significant effects were found following MSTIM with 10 Hz and following foot vibration. We conclude that 20 min MSTIM with a frequency of 25 Hz induces outlasting plastic changes in the primary motor cortex. Paired-pulse stimulation further confirms that intrinsic intracortical mechanisms are involved in these changes. Spinal adaptation could be excluded (F-wave assessments). These results could be of relevance for hemiplegic patients with motor deficits, to improve the rehabilitation outcome with vibration exercise in combination with motor training.

Increased motor cortical excitability after whole-hand electrical stimulation: A TMS study

To examine the neuromodulatory effect of whole-hand mesh-glove (MG) stimulation on motor cortical pathways, we explored motor cortical excitability before and after suprathreshold whole-hand MG stimulation using transcranial magnetic stimulation (TMS). Twenty-eight healthy volunteers (14 controls) were studied at baseline, immediately post and 1h post-MG stimulation for 30 min. Motor thresholds (MTs), motor evoked potentials (MEPs) recruitment curve, short intracortical inhibition (SICI) and intracortical facilitation (ICF) after paired magnetic stimuli were evaluated. After MG stimulation the MTs were significantly reduced and slope of MEP recruitment curve significantly increased; furthermore, the stimulation led to a sustained decrease of SICI and increase of ICF in the contralateral motor cortex. These effects lasted for at least 60 min and were stronger 1h post-stimulation compared with testing immediately after stimulation. A sham group did not show any differences before and after MG stimulation. We provide a first demonstration that MG whole-hand stimulation induces increases in motor cortical excitability lasting at least 1h. Both the strength of the corticospinal projections and the inhibitory and facilitatory intracortical mechanisms are involved. Synaptic modifications such as long-term potentiation mechanisms may underlie this stimulation-induced cortical plasticity changes. Present results prove the MG stimulation to be a promising tool in neurorehabilitation.