Investigating human motor control by transcranial magnetic stimulation

Exploring the Influence of Inter-Trial Interval on the Assessment of Short-Interval Intracortical Inhibition

Short-interval intracortical inhibition (SICI) is a common paired-pulse transcranial magnetic stimulation (TMS) measure used to assess primary motor cortex (M1) interneuron activity in healthy populations and in neurological disorders. Many of the parameters of TMS stimulation to most accurately measure SICI have been determined. However, one TMS parameter that has not been investigated is the time between SICI trials (termed inter-trial interval; ITI). This is despite a series of single-pulse TMS studies which have reported that motor evoked potential (MEP) amplitude were suppressed for short, but not long ITIs in approximately the initial ten trials of a TMS block of 20-30 trials. The primary purpose was to examine the effects of ITI on the quantification of SICI at rest. A total of 23 healthy adults completed an experimental session that included four SICI trial blocks. Each block utilized a different ITI (4, 6, 8, and 10 s) and was comprised of a total of 26 SICI trials divided into three epochs. ANOVA revealed that the main effects for ITI and epoch as well as their interaction were all non-statistically significant for SICI. We conclude that the shorter (4-6 s) ITIs used in studies investigating SICI should not alter the interpretation of M1 activity, while having the advantages of being more comfortable to participants and reducing the experimental time needed to evaluate perform single and paired-pulse TMS experiments.

Evaluation of Orexin-A Salivary Levels and its Correlation with Attention After Non-invasive Brain Stimulation in Female Volleyball Players

BACKGROUND

The capacity to change attention from one area to another depending on the many environmental circumstances present is a crucial aspect of selective attention and is strictly correlated to reaction time. The cholinergic system of the basal forebrain is crucial for attentive abilities. Several inputs, particularly orexin neurons, whose cell bodies are found in the postero-lateral hypothalamus, can activate the cholinergic system. The aim of this study was to investigate if high frequencies rTMS at dorsolateral prefrontal cortex (DLPFC) in highly trained volleyball players can change Orexin-A levels, attention and reaction time. This study was a double-blinded (participant and evaluator) matched-pair experimental design. Twenty right-handed female volleyball players were recruited for the study (age 24.6 ± 2.7 years; height 177.0 ± 5.5 cm; body mass 67.5 ± 6.5 kg; BMI 21.5 ± 1.2).

RESULTS

The main finding of this study was that 10 Hz rTMS to the DLPFC seems to increase Orexin-A salivary levels and the percentage of correct answers, while decreasing RT. After rTMS, the athletes show an increase in the percentage of correct answers immediately after the end of stimulation, and also after 15 and 30 min. Moreover, the athletes show decreases in reaction time after the end of stimulation and after 15 and 30 min to the end of stimulation, while no differences were found at the end of stimulation. Finally, the athletes show significant increases in Orexin-A salivary levels after stimulation with a peak after 30' of the end.

CONCLUSION

The results of our study seem to indicate that there is a relationship between salivary Orexin-A levels and RT. These results could provide useful tools for modulating sports training; in fact, if confirmed, they could lead coaches to offer their athletes rTMS sessions appropriately integrated with training. In fact, alternating attention is a mental flexibility that enables people to change their point of focus and switch between tasks requiring various levels of cognition.

Horizontal foot orientation affects the distribution of neural drive between gastrocnemii during plantarflexion, without changing neural excitability

The distribution of activation among muscles from the same anatomical group can be affected by the mechanical constraints of the task, such as limb orientation. For example, the distribution of activation between the gastrocnemius medialis (GM) and lateralis (GL) muscles during submaximal plantarflexion depends on the orientation of the foot in the horizontal plane. The neural mechanisms behind these modulations are not known. The overall aim of this study was to determine whether the excitability of the two gastrocnemius muscles is differentially affected by changes in foot orientation. Nineteen males performed isometric plantarflexions with their foot internally (toes-in) or externally (toes-out) rotated. GM and GL motor unit discharge characteristics were estimated from high-density surface electromyography to estimate neural drive. GM and GL corticospinal excitability and intracortical activity were assessed using transcranial magnetic stimulation through motor-evoked potentials. The efficacy of synaptic transmission between Ia-afferent fibers and α-motoneurons of the GM and GL was evaluated through the Hoffmann reflex. We observed a differential change in neural drive between GM (toes-out > toes-in) and GL (toes-out < toes-in). However, there was no foot orientation-related modulation in corticospinal excitability of the GM or GL, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. These results demonstrate that change in the motor pathway excitability is not the mechanism controlling the different distribution of neural drive between GM and GL with foot orientation.NEW & NOTEWORTHY Horizontal foot orientation affects the distribution of neural drive between the gastrocnemii during plantarflexion. There is no foot orientation-related modulation in the corticospinal excitability of the gastrocnemii, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. Change in motor pathway excitability is not the mechanism controlling the different distribution of neural drive between gastrocnemius medialis and lateralis with foot orientation.

Corticospinal and spinal responses following a single session of lower limb motor skill and resistance training

Prior studies suggest resistance exercise as a potential form of motor learning due to task-specific corticospinal responses observed in single sessions of motor skill and resistance training. While existing literature primarily focuses on upper limb muscles, revealing a task-dependent nature in eliciting corticospinal responses, our aim was to investigate such responses after a single session of lower limb motor skill and resistance training. Twelve participants engaged in a visuomotor force tracking task, self-paced knee extensions, and a control task. Corticospinal, spinal, and neuromuscular responses were measured using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS). Assessments occurred at baseline, immediately post, and at 30-min intervals over two hours. Force steadiness significantly improved in the visuomotor task (P < 0.001). Significant fixed-effects emerged between conditions for corticospinal excitability, corticospinal inhibition, and spinal excitability (all P < 0.001). Lower limb motor skill training resulted in a greater corticospinal excitability compared to resistance training (mean difference [MD] = 35%, P < 0.001) and control (MD; 37%, P < 0.001). Motor skill training resulted in a lower corticospinal inhibition compared to control (MD; - 10%, P < 0.001) and resistance training (MD; - 9%, P < 0.001). Spinal excitability was lower following motor skill training compared to control (MD; - 28%, P < 0.001). No significant fixed effect of Time or Time*Condition interactions were observed. Our findings highlight task-dependent corticospinal responses in lower limb motor skill training, offering insights for neurorehabilitation program design.

The Influence of Different Inter-Trial Intervals on the Quantification of Intracortical Facilitation in the Primary Motor Cortex

Intracortical facilitation (ICF) is a paired-pulse transcranial magnetic stimulation (TMS) measurement used to quantify interneuron activity in the primary motor cortex (M1) in healthy populations and motor disorders. Due to the prevalence of the technique, most of the stimulation parameters to optimize ICF quantification have been established. However, the underappreciated methodological issue of the time between ICF trials (inter-trial interval; ITI) has been unstandardized, and different ITIs have never been compared in a paired-pulse TMS study. This is important because single-pulse TMS studies have found motor evoked potential (MEP) amplitude reductions over time during TMS trial blocks for short, but not long ITIs. The primary purpose was to determine the influence of different ITIs on the measurement of ICF. Twenty adults completed one experimental session that involved 4 separate ICF trial blocks with each utilizing a different ITI (4, 6, 8, and 10 s). Two-way ANOVAs indicated no significant ITI main effects for test MEP amplitudes, condition-test MEP amplitudes, and therefore ICF. Accordingly, all ITIs studied provided nearly identical ICF values when averaged over entire trial blocks. Therefore, it is recommended that ITIs of 4-6 s be utilized for ICF quantification to optimize participant comfort and experiment time efficiency.

Acute non invasive brain stimulation improves performances in volleyball players

OBJECTIVES

The ability to redirect one's attention in response to various environmental situations is a crucial aspect of selective attention in team sports. Thus, the aim of this study was to investigate whether repetitive transcranial magnetic stimulation (rTMS) in volleyball players can improve Posner test response and cortical excitability. This study had a double-blinded (participant and evaluator) matched-pair experimental design.

METHODS

Twenty right-handed female volleyball players were recruited for the study and randomly assigned to either the active rTMS group (n = 10) or the sham stimulation group (n = 10). The stimulation was performed in one session with 10 Hz, 80% of the resting motor threshold (RMT), 5 s of stimulation, and 15 s of rest, for a total of 1,500 pulses. Before and after stimulation, the Posner test and cortical excitability were evaluated.

RESULTS

The significant finding of this paper was that 10 Hz rTMS to the DLPFC seemed to improve Posner test response, and also resulted in a significantly decreased RMT and MEP latency of the ipsilateral motor cortex. After stimulation, the active group showed a significant decrease in the percentage of errors in the Posner test. Moreover, active group showed faster RT after rTMS, suggesting that HF stimulation could enhance performance. Additionally, significant differences in RMT emerged in the active rTMS group after stimulation, while no differences were observed in MEP latency and MEP amplitude.

CONCLUSION

In conclusion, we believe that these results may be of great interest to the scientific community and could have practical implications in the future.

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.

Restoring Axonal Organelle Motility and Regeneration in Cultured FUS-ALS Motoneurons through Magnetic Field Stimulation Suggests an Alternative Therapeutic Approach

Amyotrophic lateral sclerosis (ALS) is a devastating motoneuron disease characterized by sustained loss of neuromuscular junctions, degenerating corticospinal motoneurons and rapidly progressing muscle paralysis. Motoneurons have unique features, essentially a highly polarized, lengthy architecture of axons, posing a considerable challenge for maintaining long-range trafficking routes for organelles, cargo, mRNA and secretion with a high energy effort to serve crucial neuronal functions. Impaired intracellular pathways implicated in ALS pathology comprise RNA metabolism, cytoplasmic protein aggregation, cytoskeletal integrity for organelle trafficking and maintenance of mitochondrial morphology and function, cumulatively leading to neurodegeneration. Current drug treatments only have marginal effects on survival, thereby calling for alternative ALS therapies. Exposure to magnetic fields, e.g., transcranial magnetic stimulations (TMS) on the central nervous system (CNS), has been broadly explored over the past 20 years to investigate and improve physical and mental activities through stimulated excitability as well as neuronal plasticity. However, studies of magnetic treatments on the peripheral nervous system are still scarce. Thus, we investigated the therapeutic potential of low frequency alternating current magnetic fields on cultured spinal motoneurons derived from induced pluripotent stem cells of FUS-ALS patients and healthy persons. We report a remarkable restoration induced by magnetic stimulation on axonal trafficking of mitochondria and lysosomes and axonal regenerative sprouting after axotomy in FUS-ALS in vitro without obvious harmful effects on diseased and healthy neurons. These beneficial effects seem to derive from improved microtubule integrity. Thus, our study suggests the therapeutic potential of magnetic stimulations in ALS, which awaits further exploration and validation in future long-term in vivo studies.

High frequencies (HF) repetitive transcranial magnetic stimulation (rTMS) increase motor coordination performances in volleyball players

INTRODUCTION

It is widely demonstrated that high frequency (HF) repetitive transcranial magnetic stimulation (rTMS) has facilitative effects and is therefore capable to inducing changes in motor responses. One of the most investigated areas is the dorsolateral prefrontal cortex (DLPFC) as it plays a special executive attention role in actively preserving access to stimulus representations and objectives in environments with plenty of distraction such as those of team sports. Volleyball is a team sport in which the attention and coordination components are essential for achieving performance. Thus, the aim of this study was to investigate if HF rTMS at DLPFC in volleyball players can improve homolateral motor coordination and cortical excitability.

RESULTS

This study was a double-blinded (participant and evaluator) matched-pair experimental design. Twenty right-handed female volleyball players were recruited for the study and were randomly assigned either the active rTMS (n = 10) or the sham stimulation group (n = 10). The stimulation was performed in one session with 10 Hz, 80% of the resting motor threshold (RMT) of the right first dorsal interosseous muscle, 5 s of stimulation, and 15 s of rest, for a total of 1500 pulses. Before and after stimulation, the coordination and the cortical excitability were evaluated. The significant finding of this paper was that HF-rTMS of the DLPFC improved performance in terms of the homolateral interlimb coordination, with a significantly decreased in resting motor threshold and MEP latency of the ipsilateral motor cortex. It seem that HF-rTMS could increase coordination performances when the velocity of the execution is higher (120 bpm and 180 bpm).

CONCLUSION

Moreover, in active rTMS group significant differences emerged after stimulation in RMT and in MEP latency, while no differences emerged after stimulation in MEP amplitude. In conclusion we believe that these results may be of great interest to the scientific community and may also have practical implications in the future.

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.

Tactile perception of right middle fingertip suppresses excitability of motor cortex supplying right first dorsal interosseous muscle

The present study examined whether tactile perception of the fingertip modulates excitability of the motor cortex supplying the intrinsic hand muscle and whether this modulation is specific to the fingertip stimulated and the muscle and hand tested. Tactile stimulation was given to one of the five fingertips in the left or right hand, and transcranial magnetic stimulation eliciting motor evoked potential in the first dorsal interosseous muscle (FDI) or abductor digiti minimi was given 200 ms after the onset of tactile stimulation. The corticospinal excitability of the FDI at rest was suppressed by the tactile stimulation of the right middle fingertip, but such suppression was absent for the other fingers stimulated and for the other muscle or hand tested. The persistence and amplitude of the F-wave was not significantly influenced by tactile stimulation of the fingertip in the right hand. These findings indicate that tactile perception of the right middle fingertip suppresses excitability of the motor cortex supplying the right FDI at rest. The suppression of corticospinal excitability was absent during tonic contraction of the right FDI, indicating that the motor execution process interrupts the tactile perception-induced suppression of motor cortical excitability supplying the right FDI. These findings are in line with a view that the tactile perception of the right middle finger induces surround inhibition of the motor cortex supplying the prime mover of the finger neighboring the stimulated finger.

The effect of cathodal transspinal direct current stimulation on tibialis anterior stretch reflex components in humans

Spinal DC stimulation (tsDCS) shows promise as a technique for the facilitation of functional recovery of motor function following central nervous system (CNS) lesion. However, the network mechanisms that are responsible for the effects of tsDCS are still uncertain. Here, in a series of experiments, we tested the hypothesis that tsDCS increases the excitability of the long-latency stretch reflex, leading to increased excitability of corticospinal neurons in the primary motor cortex. Experiments were performed in 33 adult human subjects (mean age 28 ± 7 years/14 females). Subjects were seated in a reclining armchair with the right leg attached to a footplate, which could be quickly plantarflexed (100 deg/s; 6 deg amplitude) to induce stretch reflexes in the tibialis anterior (TA) muscle at short (45 ms) and longer latencies (90-95 ms). This setup also enabled measuring motor evoked potentials (MEPs) and cervicomedullary evoked potentials (cMEPs) from TA evoked by transcranial magnetic stimulation (TMS) and electrical stimulation at the cervical junction, respectively. Cathodal tsDCS at 2.5 and 4 mA was found to increase the long-latency reflex without any significant effect on the short-latency reflex. Furthermore, TA MEPs, but not cMEPs, were increased following tsDCS. We conclude that cathodal tsDCS over lumbar segments may facilitate proprioceptive transcortical reflexes in the TA muscle, and we suggest that the most likely explanation of this facilitation is an effect on ascending fibers in the dorsal columns.

Neuromuscular Electric Stimulation in Addition to Exercise Therapy in Patients with Lower Extremity Paresis Due to Acute Ischemic Stroke. A proof-of-concept randomised controlled trial

INTRODUCTION

Exercise therapy and neuromuscular electrical stimulation (NMES) during the initial 14 days after stroke may benefit recovery of gait. We aimed to determine whether poststroke NMES of vastus medial and tibial muscles during exercise therapy is more effective than exercise therapy alone.

MATERIALS AND METHODS

In this proof-of-concept randomised trial patients with first-ever acute ischemic stroke and a leg paresis (40-85 years of age) were randomised (1:1) to 10 min of daily NMES + exercise therapy or exercise therapy alone. Primary outcome was the between-group difference in change in 6 min Walk Test (6MWT) at 90 days post stroke estimated with a mixed regression model. Secondary outcomes included 10 m Walk Test, Fugl-Meyer Motor Assessment, Guralnik Timed Standing Balance, Sit to Stand, Timed Up and Go, EQ-5D-5L, Montreal Cognitive Assessment and Becks Depression Inventory.

RESULTS

50 stroke survivors (25 in each group) with a mean age of 67 years (range 43-83) were included. An insignificant between-group difference in change of 28.3 m (95%CI -16.0 to 72.6, p = 0.23, adjusted for baseline) in 6MWT at 90-days follow-up was found, in favour of the NMES group. All secondary outcomes showed no statistically significant between-group difference. The conclusion was that adding NMES to exercise therapy had no effect on poststroke walking distance measured by the 6 MWT or any of the secondary outcomes.

CONCLUSIONS

In this proof-of-concept RCT, we demonstrated that NMES in addition to exercise therapy during the first 14 days after onset of ischemic stroke did not improve walking distance or any of the secondary outcomes. Future studies with a longer trial period, stratifying patients into subgroups with comparable patterns of expected spontaneous recovery - if possible within 48 h post stroke, and greater sample size, than in this study are suggestions of how rehabilitation research could go on exploring the potential for NMES as an amplifier in stroke recovery.

Dancing in Your Head: An Interdisciplinary Review

The aim of this review is to highlight the most relevant contributions on dance in neuroscientific research. Neuroscience has analyzed the mirror system through neuroimaging techniques, testing its role in imitative learning, in the recognition of other people's emotions and especially in the understanding of the motor behavior of others. This review analyses the literature related to five general areas: (I) breakthrough studies on the mirror system, and subsequent studies on its involvement in the prediction, the execution, the control of movement, and in the process of "embodied simulation" within the intersubjective relationship; (II) research focused on investigating the neural networks in action observation, and the neural correlates of motor expertise highlighted by comparative studies on different dance styles; (III) studies dealing with the viewer's experience of dance according to specific dance repertoires, which revealed the relevance of choreographic choices for aesthetic appreciation; (IV) studies focused on dance as an aesthetic experience, where both the emotional and the cultural dimension play a significant role, and whose investigation paves the way to further progress both in empirical and in phenomenological research methodologies; (V) collaboration-based experiments, in which neuroscientists and choreographers developed expertise-related questions, especially focusing on the multiple phenomena that underlie motor imagery.

Relationship between lower limb function and functional connectivity assessed by EEG among motor-related areas after stroke

Background: Neural connectivity in brain has been known as indicators for neural function and recovery of brain. Although previous studies reported that neural connectivity predicted the recovery of upper limb function after stroke, the relationship between neural connectivity and lower limb function has not been clear.Objectives: To clarify whether functional connectivity (FC) assessed by electroencephalographiy (EEG) with five electrodes placed on motor-related areas could be related to the functional motor recovery of the lower limbs in patients after stroke.Methods: Twenty-four patients with stroke during the recovery phase were recruited. Motor function of the lower limbs was assessed using Fugl-Meyer Assessment lower limb section (FMAL). EEG signals were recorded by five electrodes (C3, C4, FC3, FC4, and FCz) at rest and during ankle movement. Amplitude envelope correlations, as values for FC, were calculated in α (8-12 Hz), β (13-30 Hz), low-β (13-19 Hz), and high-β (20-30 Hz) frequency bands. The predictive regression equation of the FMAL score in the eighth week after stroke (8 W) was created by FCs in the fourth week (4 W).Results: The higher intra-hemispheric FC in both hemispheres in the resting state and during the ankle movement at 4 W was related to a higher lower limb function at 8 W. Additionally, the higher inter-hemispheric FC between M1 on both sides during the ankle movement was related to a higher function recovery.Conclusions: The intra- and inter-hemispheric FC among motor-related areas at 4 W after stroke might be related to the functional recovery of the lower limbs at 8 W.

Integration of Convergent Sensorimotor Inputs Within Spinal Reflex Circuits in Healthy Adults

The output from motor neuron pools is influenced by the integration of synaptic inputs originating from descending corticomotor and spinal reflex pathways. In this study, using paired non-invasive brain and peripheral nerve stimulation, we investigated how descending corticomotor pathways influence the physiologic recruitment order of the soleus Hoffmann (H-) reflex. Eleven neurologically unimpaired adults (9 females; mean age 25 ± 3 years) completed an assessment of transcranial magnetic stimulation (TMS)-conditioning of the soleus H-reflex over a range of peripheral nerve stimulation (PNS) intensities. Unconditioned H-reflex recruitment curves were obtained by delivering PNS pulses to the posterior tibial nerve. Subsequently, TMS-conditioned H-reflex recruitment curves were obtained by pairing PNS with subthreshold TMS at short (−1.5 ms) and long (+10 ms) intervals. We evaluated unconditioned and TMS-conditioned H-reflex amplitudes along the ascending limb, peak, and descending limb of the H-reflex recruitment curve. Our results revealed that, for long-interval facilitation, TMS-conditioned H-reflex amplitudes were significantly larger than unconditioned H-reflex amplitudes along the ascending limb and peak of the H-reflex recruitment curve. Additionally, significantly lower PNS intensities were needed to elicit peak H-reflex amplitude (Hmax) for long-interval facilitation compared to unconditioned. These findings suggest that the influence of descending corticomotor pathways, particularly those mediating long-interval facilitation, contribute to changing the recruitment gain of the motor neuron pool, and can inform future methodological protocols for TMS-conditioning of H-reflexes. By characterizing and inducing short-term plasticity in circuitry mediating short- and long-interval TMS-conditioning of H-reflex amplitudes, future studies can investigate supraspinal and spinal circuit contributions to abnormal motor control, as well as develop novel therapeutic targets for neuromodulation.

Crossed Corticospinal Facilitation Between Arm and Trunk Muscles Correlates With Trunk Control After Spinal Cord Injury

Objective: To investigate whether crossed corticospinal facilitation between arm and trunk muscles is preserved following spinal cord injury (SCI) and to elucidate these neural interactions for postural control during functional arm movements.

Methods: Using transcranial magnetic stimulation (TMS) in 22 subjects with incomplete SCI motor evoked potentials (MEPs) in the erector spinae (ES) muscle were examined when the contralateral arm was at rest or performed 20% of maximal voluntary contraction (MVC) of biceps brachii (BB) or triceps brachii (TB). Trunk function was assessed with rapid shoulder flexion and forward-reaching tasks.

Results: MEP amplitudes in ES were increased during elbow flexion in some subjects and this facilitatory effect was more prominent in subjects with thoracic SCI than in the subjects with cervical SCI. Those who showed the increased MEPs during elbow flexion had faster reaction times and quicker anticipatory postural adjustments of the trunk in the rapid shoulder flexion task. The onset of EMG activity in ES during the rapid shoulder flexion task correlated with the trunk excursion in forward-reaching.

Conclusions: Our findings demonstrate that crossed corticospinal facilitation in the trunk muscles can be preserved after SCI and is reflected in trunk control during functional arm movements.

Distinct Corticospinal and Reticulospinal Contributions to Voluntary Control of Elbow Flexor and Extensor Muscles in Humans with Tetraplegia

Humans with cervical spinal cord injury (SCI) often recover voluntary control of elbow flexors and, to a much lesser extent, elbow extensor muscles. The neural mechanisms underlying this asymmetrical recovery remain unknown. Anatomical and physiological evidence in animals and humans indicates that corticospinal and reticulospinal pathways differentially control elbow flexor and extensor motoneurons; therefore, it is possible that reorganization in these pathways contributes to the asymmetrical recovery of elbow muscles after SCI. To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the arm representation of the primary motor cortex, maximal voluntary contractions, the StartReact response (a shortening in reaction time evoked by a startling stimulus), and the effect of an acoustic startle cue on MEPs elicited by cervicomedullary stimulation (CMEPs) on biceps and triceps brachii in males and females with and without chronic cervical incomplete SCI. We found that SCI participants showed similar MEPs and maximal voluntary contractions in biceps but smaller responses in triceps compared with controls, suggesting reduced corticospinal inputs to elbow extensors. The StartReact and CMEP facilitation was larger in biceps but similar to controls in triceps, suggesting enhanced reticulospinal inputs to elbow flexors. These findings support the hypothesis that the recovery of biceps after cervical SCI results, at least in part, from increased reticulospinal inputs and that the lack of these extra inputs combined with the loss of corticospinal drive contribute to the pronounced weakness found in triceps.SIGNIFICANCE STATEMENT Although a number of individuals with cervical incomplete spinal cord injury show limited functional recovery of elbow extensors compared with elbow flexor muscles, to date, the neural mechanisms underlying this asymmetrical recovery remain unknown. Here, we provide for the first time evidence for increased reticulospinal inputs to biceps but not triceps brachii and loss of corticospinal drive to triceps brachii in humans with tetraplegia. We propose that this reorganization in descending control contributes to the asymmetrical recovery between elbow flexor and extensor muscles after cervical spinal cord injury.

Transcutaneous spinal direct current stimulation increases corticospinal transmission and enhances voluntary motor output in humans

Optimization of motor performance is of importance in daily life, in relation to recovery following injury as well as for elite sports performance. The present study investigated whether transcutaneous spinal direct current stimulation (tsDCS) may enhance voluntary ballistic activation of ankle muscles and descending activation of spinal motor neurons in able-bodied adults. Forty-one adults (21 men; 24.0 ± 3.2 years) participated in the study. The effect of tsDCS on ballistic motor performance and plantar flexor muscle activation was assessed in a double-blinded sham-controlled cross-over experiment. In separate experiments, the underlying changes in excitability of corticospinal and spinal pathways were probed by evaluating soleus (SOL) motor evoked potentials (MEPs) following single-pulse transcranial magnetic stimulation (TMS) over the primary motor cortex, SOL H-reflexes elicited by tibial nerve stimulation and TMS-conditioning of SOL H-reflexes. Measures were obtained before and after cathodal tsDCS over the thoracic spine (T11-T12) for 10 min at 2.5 mA. We found that cathodal tsDCS transiently facilitated peak acceleration in the ballistic motor task compared to sham tsDCS. Following tsDCS, SOL MEPs were increased without changes in H-reflex amplitudes. The short-latency facilitation of the H-reflex by subthreshold TMS, which is assumed to be mediated by the fast conducting monosynaptic corticomotoneuronal pathway, was also enhanced by tsDCS. We argue that tsDCS briefly facilitates voluntary motor output by increasing descending drive from corticospinal neurones to spinal plantar flexor motor neurons. tsDCS can thus transiently promote within-session CNS function and voluntary motor output and holds potential as a technique in the rehabilitation of motor function following central nervous lesions.

Freely Chosen Cadence During Cycling Attenuates Intracortical Inhibition and Increases Intracortical Facilitation Compared to a Similar Fixed Cadence

In contrast to other rhythmic tasks such as running, the preferred movement rate in cycling does not minimize energy consumption. It is possible that neurophysiological mechanisms contribute to the choice of cadence, however this phenomenon is not well understood. Eleven participants cycled at a fixed workload of 125 W and different cadences including a freely chosen cadence (FCC, ∼72), and fixed cadences of 70, 80, 90 and 100 revolutions per minute (rpm) during which transcranial magnetic stimulation (TMS) was used to measure short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). There was a significant increase in SICI at 70 (P = 0.004), 80 (P = 0.008) and 100 rpm (P = 0.041) compared to FCC. ICF was significantly reduced at 70 rpm compared to FCC (P = 0.04). Inhibition-excitation ratio (SICI divided by ICF) declined (P = 0.014) with an increase in cadence. The results demonstrate that SICI is attenuated during FCC compared to fixed cadences. The outcomes suggest that the attenuation of intracortical inhibition and augmentation of ICF may be a contributing factor for FCC.

Effects of twelve weeks' aerobic training on motor cortex excitability

BACKGROUND

Regular physical activity or aerobic exercise is well known to increase brain plasticity. Recent studies have reported that aerobic exercise enhances neuroplasticity and motor learning. The aim of this study was to investigate if 12 weeks' aerobic training can modify cortical excitability and motor evoked potential (MEP) responses.

METHODS

Fifteen untrained males were recruited. Cortical excitability was investigated using TMS. VO2<inf>max</inf> was estimated using Cooper's test. Aerobic intervention lasted 12 weeks. The subjects performed a 6-week supervised aerobic workout, 3 times a week, at 60-75% of their maximum heart rate (HR<inf>max</inf>). Over the following 6 weeks, they performed a supervised aerobic workout 3 times a week at 70-75% of FC<inf>max</inf>.

RESULTS

After 8 weeks of aerobic training there was a significant increase of distance covered during Cooper's test (P<0.001) and a significant increase of VO2<inf>max</inf> (P<0.001); there was also an improvement in resting motor threshold (rMT decreased from 60.5±6.6% [T0] to 55.8±5.9% [T2]; P<0.001), motor evoked potential latency decreased (from 25.3±0.8 ms [T0] to 24.1±0.8 ms [T2]; P<0.001), and motor evoked potential amplitude increased (from 0.58±0.09 mV [T0] to 0.65±0.08 mV [T2]; P<0.001). Furthermore, after 12 weeks' aerobic training there were improvements in all parameters.

CONCLUSIONS

This study shows that aerobic activity seems to induce changes in cortical excitability if performed for a period longer than 4 weeks, in addition to typical cardiorespiratory benefits in previously untrained males.

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.

Obstructive Sleep Apnea Syndrome: A Preliminary Navigated Transcranial Magnetic Stimulation Study

PURPOSE

An increase in resting motor threshold (RMT), prolonged cortical silent period duration (CSP), and reduced short-latency afferent inhibition (SAI), confirmed with previous transcranial magnetic stimulation (TMS), suggest decreased cortical excitability in obstructive sleep apnea syndrome (OSAS). The present study included MRI of OSAS patients for navigated TMS assessment of the RMT, as an index of the threshold for corticospinal activation at rest, and SAI as an index of cholinergic neurotransmission. We hypothesize to confirm findings on SAI and RMT with adding precision in the targeting of motor cortex in OSAS.

SUBJECTS AND METHODS

After acquiring head MRIs for 17 severe right-handed OSAS and 12 healthy subjects, the motor cortex was mapped with nTMS to assess the RMT and SAI, with motor evoked potentials (MEPs) recorded from the abductor-pollicis brevis (APB) muscle. The 120%RMT intensity was used for the SAI by a paired-pulse paradigm in which the electrical stimulation to the median nerve is followed by magnetic stimulation of the motor cortex at inter-stimulus intervals (ISIs) of 18-28 ms (ISIs_18-28). The SAI control condition included a recording of MEPs without peripheral stimulation. Latency and amplitude of MEP at RMT at 120%RMT for eleven different at ISIs_18-28 were analyzed.

RESULTS

The study showed a significantly lower percentage deviation of MEP amplitude at ISIs_(18-28ms) from the control condition between OSAS and healthy subjects (U=44.0, p=0.01). The intensity of stimulation at RMT was significantly higher in OSAS subjects (U=55.0, p=0.04*). Correlation analysis showed that BMI significantly negatively correlated (ρ=-0.47) with MEP amplitude percentage deviation in OSAS patients.

CONCLUSION

The nTMS study results in increased RMT, and reduced cortical afferent inhibition in OSAS patients for SAI at ISIs_18-28, confirming previous findings of impaired cortical afferent inhibition in OSAS. Future nTMS studies are desirable to elucidate the role of RMT and SAI in diagnostics and treatment of OSAS, and to elucidate the usefulness of nTMS in OSAS research.

Skillful Cycling Training Induces Cortical Plasticity in the Lower Extremity Motor Cortex Area in Healthy Persons

Cycling exercise is commonly used in rehabilitation to improve lower extremity (LE) motor function and gait performance after stroke. Motor learning is important for regaining motor skills, suggesting that training of motor skills influences cortical plasticity. However, the effects of motor skill learning in dynamic alternating movements of both legs on cortical plasticity remain unclear. Here, we examined the effects of skillful cycling training on cortical plasticity of the LE motor area in healthy adults. Eleven healthy volunteers participated in the following three sessions on different days: skillful cycling training, constant-speed cycling training, and rest condition. Skillful cycling training required the navigation of a marker up and down curves by controlling the rotation speed of the pedals. Participants were instructed to fit the marker to the target curves as accurately as possible. Amplitudes of motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) evoked using transcranial magnetic stimulation (TMS) were assessed at baseline, after every 10 min of the task (a total of 30 min), and 30 min after the third and final trial. A decrease in tracking errors was representative of the formation of motor learning following skillful cycling training. Compared to baseline, SICI was significantly decreased after skillful cycling training in the tibialis anterior (TA) muscle. The task-induced alterations of SICI were more prominent and lasted longer with skillful cycling training than with the other conditions. The changes in SICI were negatively correlated with a change in tracking error ratio at 20 min the task. MEP amplitudes were not significantly altered with any condition. In conclusion, skillful cycling training induced long-lasting plastic changes of intracortical inhibition, which corresponded to the learning process in the LE motor cortex. These findings suggest that skillful cycling training would be an effective LE rehabilitation method after stroke.

Examining changes in corticospinal excitability and balance performance in response to social-comparative feedback

BACKGROUND

Social-comparative feedback informs an individual that their performance was better or worse than the group. Previous studies have found that compared to knowledge of results alone, social-comparative feedback produces a valence response that results in larger improvements in balance performance. However, the neural processes contributing to these motor improvements have not yet been examined.

RESEARCH QUESTION

Does social-comparative feedback alter corticospinal excitability and consequently, balance performance?

METHODS

Thirty-six healthy young adults stood and maintained their balance on a stabiliometer for eight trials. After three of the trials, the neutral (i.e., only knowledge of results) group received their performance feedback (i.e., time on balance) while the other two groups also received positive (i.e., performed better than the group) or negative (i.e., performed worse than the group) social-comparative feedback. To measure corticospinal excitability, soleus motor-evoked potentials were elicited using transcranial magnetic stimulation at the beginning of the experiment, after the presentation of feedback, and at the end of the experiment. Pre- and post- ratings of confidence, perceived skill, motivation, and anxiety were also collected.

RESULTS

The negative feedback group reported decreases in perceived skill (43 ± 29%) and balance confidence (26 ± 28%), while the positive group reported a 13 ± 17% increase in perceived skill. Despite these group differences in feedback perception, all three groups improved their balance performance by ≈35% (p < 0.001) by the eighth trial. However, this improvement in balance performance was not matched by any changes in corticospinal excitability over time (19.2 ± 55.9% change; p = 0.340) or between groups (p = 0.734).

SIGNIFICANCE

Our findings suggest that social-comparative feedback, as presented in this study, does not affect corticospinal excitability and balance performance differently than knowledge of results (neutral feedback) alone. More arousing and more frequent forms of social-comparative feedback may be necessary for observing larger changes in the functional or neural control of balance.

Neuromuscular effects of dorsiflexor training with and without blood flow restriction

Blood flow restriction training (BFRT) has been proposed for elderly and clinical populations with weakness. Before being used in these populations it is important to understand the neurological effects of, and subject perceptions to, BFRT. Seventeen healthy subjects were recruited and performed 2 experimental sessions, BFRT and training without blood flow restriction (TR-only), on separate days. Four sets of concentric/eccentric dorsiflexion contractions against theraband resistance were performed. Surface electromyography of the tibialis anterior was recorded during exercise and for the electrophysiological measures. At baseline, immediately-post, 10-min-post and 20-min-post exercise, motor evoked potentials (MEPs) from single pulse transcranial magnetic stimulation (TMS), paired-pulse TMS with interstimulus intervals of 2-ms (SICI) and 15-ms (ICF), and the M-max amplitude were recorded in the resting TA. Following training, subjects provided a numerical rating of the levels of pain, discomfort, fatigue, focus and difficulty during training. Muscle activation was higher in the last 20 contractions during BFRT compared to TR. There was no difference (time × condition interaction) between BFRT and TR for single-pulse MEP, SICI, ICF or M-max amplitude. There was a significant main effect of timepoint for single-pulse MEP and M-max amplitudes with both significantly reduced for 20-min-post exercise. No reductions were observed for SICI and ICF amplitudes. Taken together, BFRT and TR-only were only different during exercise and both regimes induced similar significant reductions in M-Max and MEP-amplitude post-training. Due to the lack of changes in SICI and ICF, it is unlikely that changes occurred in cortical sites related to these pathways. The increased surface electromyography activity in the last 20 contractions, indicate that the training regimes are different and that BFRT possibly induces more fatigue than TR. As such, BFRT could be used as an adjunct to conventional training. However, as subjects perceived BFRT as more painful, difficult and uncomfortable than TR-only, people should be selected carefully to undertake BFRT.

Imbalanced Corticospinal and Reticulospinal Contributions to Spasticity in Humans with Spinal Cord Injury

Damage to the corticospinal and reticulospinal tract has been associated with spasticity in humans with upper motor neuron lesions. We hypothesized that these descending motor pathways distinctly contribute to the control of a spastic muscle in humans with incomplete spinal cord injury (SCI). To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the leg representation of the primary motor cortex, maximal voluntary contractions (MVCs), and the StartReact response (shortening in reaction time evoked by a startling stimulus) in the quadriceps femoris muscle in male and females with and without incomplete SCI. A total of 66.7% of the SCI participants showed symptoms of spasticity, whereas the other 33.3% showed no or low levels of spasticity. We found that participants with spasticity had smaller MEPs and MVCs and larger StartReact compared with participants with no or low spasticity and control subjects. These results were consistently present in spastic subjects but not in the other populations. Clinical scores of spasticity were negatively correlated with MEP-max and MVC values and positively correlated with shortening in reaction time. These findings provide evidence for lesser corticospinal and larger reticulospinal influences to spastic muscles in humans with SCI and suggest that these imbalanced contributions are important for motor recovery.SIGNIFICANCE STATEMENT Although spasticity is one of the most common symptoms manifested in humans with spinal cord injury (SCI) to date, its mechanisms of action remain poorly understood. We provide evidence, for the first time, of imbalanced contributions of the corticospinal and reticulospinal tract to control a spastic muscle in humans with chronic incomplete SCI. We found that participants with SCI with spasticity showed small corticospinal responses and maximal voluntary contractions and larger reticulospinal gain compared with participants with no or low spasticity and control subjects. These results were consistently present in spastic subjects but not in the other populations. We showed that imbalanced corticospinal and reticulospinal tract contributions are more pronounced in participants with chronic incomplete SCI with lesser recovery.

Changes in Spinal and Corticospinal Excitability in Patients with Chronic Ankle Instability: A Systematic Review with Meta-Analysis

The objective of this systematic review with meta-analysis was to determine alterations in spinal and corticospinal excitability of ankle muscles in patients with chronic ankle instability (CAI) compared to uninjured controls. Independent researchers performed comprehensive literature searches of electronic databases and included studies that compared groups with and without CAI and investigated neural excitability with Hoffmann reflex (H-reflex) and/or transcranial magnetic stimulation (TMS). A fixed-effect meta-analysis was conducted to determine group differences for (1) soleus and fibularis maximal H-reflex (Hmax)/maximal M-wave (Mmax)-ratios, and (2) soleus and fibularis longus cortical motor thresholds (CMTs). Seventeen studies were included in the current meta-analysis. They showed that the Hmax/Mmax-ratios of the soleus and the fibularis longus in the CAI group were significantly lower than those in the uninjured control group (soleus: d = −0.41, p < 0.001; fibularis longus: d = −0.27, p = 0.04). There was no evidence for changes in the CMT. This systematic review is the first to demonstrate evidence that patients with CAI present decreased spinal reflex excitability in the soleus and fibularis longus. However, there is no evidence of changes in supraspinal excitability when considering only the CMT. The latter result needs to be interpreted with caution as all except one study demonstrate some changes at the supraspinal level with CAI.

Corticospinal excitability of tibialis anterior and soleus differs during passive ankle movement

The purpose of this study was to assess corticospinal excitability of soleus (SOL) and tibialis anterior (TA) at a segmental level during passive ankle movement. Four experimental components were performed to assess the effects of passive ankle movement and muscle length on corticospinal excitability (MEP/M_max) at different muscle lengths, subcortical excitability at the level of lumbar spinal segments (LEP/M_max), intracortical inhibition (SICI) and facilitation (ICF), and H-reflex in SOL and TA. In addition, the degree of fascicle length changes between SOL and TA was assessed in a subpopulation during passive ankle movement. Fascicles shortened and lengthened with joint movement during passive shortening and lengthening of SOL and TA to a similar degree (p < 0.001). Resting motor threshold was greater in SOL compared to TA (p ≤ 0.014). MEP/M_max was facilitated in TA during passive shortening relative to the static position (p ≤ 0.023) and passive lengthening (p ≤ 0.001), but remained similar during passive ankle movement in SOL (p ≥ 0.497), regardless of muscle length at the point of stimulus (p = 0.922). LEP/M_max (SOL: p = 0.075, TA: p = 0.071), SICI (SOL: p = 0.427, TA: p = 0.540), and ICF (SOL: p = 0.177, TA: p = 0.777) remained similar during passive ankle movement. H-reflex was not different across conditions in TA (p = 0.258), but was reduced during passive lengthening compared to shortening in SOL (p = 0.048). These results suggest a differential modulation of corticospinal excitability between plantar and dorsiflexors during passive movement. The corticospinal behaviour observed might be mediated by an increase in corticospinal drive as a result of reduced afferent input during muscle shortening and appears to be flexor-biased.

Age, Height, and Sex on Motor Evoked Potentials: Translational Data From a Large Italian Cohort in a Clinical Environment

Introduction

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics in relatively small sample sizes have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots in a large cohort.

Methods

A total of 587 subjects clinically and neuroradiologically intact were included. MEPs were recorded during mild tonic contraction through a circular coil applied over the "hot spot" of the first dorsal interosseous and tibialis anterior muscles (TAs), bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured.

Results

After Bonferroni correction, general linear (multiple) regression analysis showed that both MEP cortical latency and PMCT at four limbs positively correlated with age and height. At lower limbs, an independent effect of sex on the same measures was also observed (with females showing smaller values than males). CMCT correlated with both age (negatively) and height (positively) when analyzed by a single regression; however, with a multiple regression analysis this significance disappeared, due to the correction for the multicollinearity within the dataset.

Conclusion

Physical individual features need to be considered for a more accurate and meaningful MEPs interpretation. Both in clinical practice and in research setting, patients and controls should be matched for age, height, and sex.

Age-related changes in leg proprioception: implications for postural control

In addition to being a prerequisite for many activities of daily living, the ability to maintain steady upright standing is a relevant model to study sensorimotor integrative function. Upright standing requires managing multimodal sensory inputs to produce finely tuned motor output that can be adjusted to accommodate changes in standing conditions and environment. The sensory information used for postural control mainly arises from the vestibular system of the inner ear, vision, and proprioception. Proprioception (sense of body position and movement) encompasses signals from mechanoreceptors (proprioceptors) located in muscles, tendons, and joint capsules. There is general agreement that proprioception signals from leg muscles provide the primary source of information for postural control. This is because of their exquisite sensitivity to detect body sway during unperturbed upright standing that mainly results from variations in leg muscle length induced by rotations around the ankle joint. However, aging is associated with alterations of muscle spindles and their neural pathways, which induce a decrease in the sensitivity, acuity, and integration of the proprioceptive signal. These alterations promote changes in postural control that reduce its efficiency and thereby may have deleterious consequences for the functional independence of an individual. This narrative review provides an overview of how aging alters the proprioceptive signal from the legs and presents compelling evidence that these changes modify the neural control of upright standing.

Redundancy Among Parameters Describing the Input-Output Relation of Motor Evoked Potentials in Healthy Subjects and Stroke Patients

Background: Transcranial magnetic stimulation (TMS) is widely used to probe corticospinal excitability through Motor Evoked Potential (MEP) amplitude measurements. The input-output (I/O) curve is a sigmoid-shaped relation between the MEP amplitude at incremented TMS intensities. The aim of this study was to examine the relationships between seven parameters derived from the sigmoid function.

Methods: Principal Component Analysis and Spearman's rank correlation matrices were used to determine if the seven I/O curve parameters capture similar or, conversely, different aspects of the corticospinal excitability in 24 healthy subjects and 40 stroke survivors with a hand motor impairment.

Results: Maximum amplitude (MEP_max), peak slope, area under the I/O curve (AUC), and MEP amplitude recorded at 140% of the resting motor threshold showed strong linear relationships with each other (ρ > 0.72, p < 0.001). Results were found to be similar in healthy subjects and in both hemispheres of stroke patients. Our results did not support an added benefit of sampling entire I/O curves in both healthy subjects and stroke patients, with the exception of S₅₀, the stimulus intensity needed to obtain half of MEP_max amplitude.

Conclusions: This demonstrates that MEP elicited at a single stimulus intensity allows to capture the same characteristics of the corticospinal excitability as measured by the AUC, MEP_max and the peak slope, which may be of interest in both clinical and research settings. However, it is still necessary to plot I/O curves if an effect or a difference is expected at S₅₀.

Walking and finger tapping can be done with independent rhythms

Rhythmic movements occur in many aspects of daily life. Examples include clapping the hands and walking. The production of two independent rhythms with multiple limbs is considered to be extremely difficult. In the present study we evaluated whether two different, independent rhythms that involved finger tapping and walking could be produced. In Experiment I, twenty subjects that had no experience of musical instrument training performed rhythmic finger tapping with the right index finger and one of four different lower limb movements; (1) self-paced walking, (2) given-paced walking, (3) alternative bilateral heel tapping from a sitting position, and (4) unilateral heel tapping with the leg ipsilateral to the tapping finger from a sitting position. The target intervals of finger tapping and heel strikes for walking step/heel tapping were set at 375 ms and 600 ms, respectively. The even distribution of relative phases between instantaneous finger tapping and heel strike was taken as the criteria of independency for the two rhythms. In the self-paced walking and given-paced walking tasks, 16 out of 20 subjects successfully performed finger tapping and walking with independent rhythms without any special practice. On the other hand, in the bipedal heels striking and unipedal heel striking tasks 19 subjects failed to perform the two movements independently, falling into interrelated rhythms with the ratio mostly being 2:1. In Experiment II, a similar independency of finger tapping and walking at a given pace was observed for heel strike intervals of 400, 600, and 800 ms, as well as at the constant 375 ms for finger tapping. These results suggest that finger tapping and walking are controlled by separate neural control mechanisms, presumably with a supra-spinal locus for finger tapping, and a spinal location for walking.

Contribution of corticospinal drive to ankle plantar flexor muscle activation during gait in adults with cerebral palsy

Impaired plantar flexor muscle activation during push-off in late stance contributes importantly to reduced gait ability in adults with cerebral palsy (CP). Here we used low-intensity transcranial magnetic stimulation (TMS) to suppress soleus EMG activity during push-off as an estimate of corticospinal drive in CP adults and neurologically intact (NI) adults. Ten CP adults (age 34 years, SD 14.6, GMFCS I-II) and ten NI adults (age 33 years, SD 9.8) walked on a treadmill at their preferred walking speed. TMS of the leg motor cortex was elicited just prior to push-off during gait at intensities below threshold for motor-evoked potentials. Soleus EMG from steps with and without TMS were averaged and compared. Control experiments were performed while standing and in NI adults during gait at slow speed. TMS induced a suppression at a latency of about 40 ms. This suppression was similar in the two populations when differences in control EMG and gait speed were taken into account (CP 18%, NI 16%). The threshold of the suppression was higher in CP adults. The findings suggest that corticospinal drive to ankle plantar flexors at push-off is comparable in CP and NI adults. The higher threshold of the suppression in CP adults may reflect downregulation of cortical inhibition to facilitate corticospinal drive. Interventions aiming to facilitate excitability in cortical networks may contribute to maintain or even improve efficient gait in CP adults.

Age, Height, and Sex on Motor Evoked Potentials: Translational Data From a Large Italian Cohort in a Clinical Environment

INTRODUCTION

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics in relatively small sample sizes have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots in a large cohort.

METHODS

A total of 587 subjects clinically and neuroradiologically intact were included. MEPs were recorded during mild tonic contraction through a circular coil applied over the "hot spot" of the first dorsal interosseous and tibialis anterior muscles (TAs), bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured.

RESULTS

After Bonferroni correction, general linear (multiple) regression analysis showed that both MEP cortical latency and PMCT at four limbs positively correlated with age and height. At lower limbs, an independent effect of sex on the same measures was also observed (with females showing smaller values than males). CMCT correlated with both age (negatively) and height (positively) when analyzed by a single regression; however, with a multiple regression analysis this significance disappeared, due to the correction for the multicollinearity within the dataset.

CONCLUSION

Physical individual features need to be considered for a more accurate and meaningful MEPs interpretation. Both in clinical practice and in research setting, patients and controls should be matched for age, height, and sex.

Does Resting Motor Threshold Predict Motor Hand Recovery After Stroke?

Background: Resting Motor threshold (rMT) is one of the measurement obtained by Transcranial Magnetic Stimulation (TMS) that reflects corticospinal excitability. As a functional marker of the corticospinal pathway, the question arises whether rMT is a suitable biomarker for predicting post-stroke upper limb function. To that aim, we conducted a systematic review of relevant studies that investigated the clinical significance of rMT in stroke survivors by using correlations between upper limb motor scores and rMT. Methods: Studies that reported correlations between upper limb motor function and rMT as a measure of corticospinal excitability in distal arm muscle were identified via a literature search in stroke patients. Two authors extracted the data using a home-made specific form. Subgroup analyses were carried out with patients classified with respect to time post-stroke onset (early vs. chronic stage) and stroke location (cortical, subcortical, or cortico-subcortical). Methodological quality of the study was also evaluated by a published checklist. Results: Eighteen studies with 22 groups (n = 508 stroke patients) were included in this systematic review. Mean methodological quality score was 14.75/24. rMT was often correlated with motor function or hand dexterity (n = 15/22, 68%), explaining on average 31% of the variance of the motor score. Moreover, the results did not seem impacted if patients were examined at the early or chronic stages of stroke. Two findings could not be properly interpreted: (i) the fact that the rMT is an independent predictor of motor function as several confounding factors are well-established, and, (ii) whether the stroke location impacts this prediction. Conclusion: Most of the studies found a correlation between rMT and upper limb motor function after stroke. However, it is still unclear if rMT is an independent predictor of upper limb motor function when taking into account for age, time post stroke onset and level of corticospinal tract damage as confounding factors. Clear-cut conclusions could not be drawn at that time but our results suggest that rMT could be a suitable candidate although future investigations are needed. Systematic Review Registration Number: (https://www.crd.york.ac.uk/prospero/): ID 114317.

Blood oxygenation changes resulting from subthreshold high frequency repetitive transcranial magnetic stimulation

Effects of high frequency repetitive transcranial magnetic stimulation (rTMS) with a subthreshold intensity on hemodynamic response in brain cortices (both motor and prefrontal cortices) was investigated using the functional near infrared spectroscopy (fNIRS) technique. FNIRS signals of the motor and prefrontal cortices were acquired in healthy volunteers (n=7) at rest and during rTMS intervention. A significant reduction in oxygenated hemoglobin (HbO) concentration during the entire stimulation process was observed from both motor and prefrontal cortices. Results showed that the fNIRS technique can provide a reliable measure of regional cortical brain activation that could be valuable in studying cortical excitability connectivity in combination with rTMS.

Cortical excitability varies across different muscles

The aim of the present study was to determine whether significant differences in cortical excitability were evident across different body regions in healthy humans. Threshold tracking transcranial magnetic stimulation (TMS) was undertaken in 28 healthy controls. Short-interval intracortical inhibition [SICI between interstimulus intervals (ISI) 1-7 ms], intracortical facilitation (ICF, between ISI 10-30 ms), resting motor threshold (RMT), cortical silent period (CSP) duration (generated at stimulus intensity 150% RMT), and motor evoked potential amplitude were recorded from the abductor pollicis brevis (APB), tibialis anterior (TA), and trapezius muscles. These muscles were selected as they are frequently affected in neurodegenerative diseases, such as amyotrophic lateral sclerosis. SICI and ICF are measured as a percentage difference between conditioned and an unconditioned test response. SICI was significantly greater when recorded over the APB (9.9 ± 1.5%) and TA (8.6 ± 1.4%) muscles compared with the trapezius (4.5 ± 1.9%, P < 0.05). The CSP duration was significantly shorter (CSP_trapezius, 131.0 ± 6.3 ms; CSP_TA, 175.7 ± 9.9 ms; CSP_APB, 188.3 ± 4.0 ms; P < 0.001) and ICF greater ( P < 0.01) in the trapezius muscle. There were no significant correlations between inhibitory and facilitatory processes recorded across the three muscles. The present study established significant differences in cortical excitability across three body regions, with evidence of more prominent inhibition and less facilitation in the limb muscles. NEW & NOTEWORTHY Cortical excitability of muscles with differing motor functions was assessed using threshold tracking transcranial magnetic stimulation. Significantly greater intracortical inhibition and less facilitation were evident over the limb muscles. These findings could relate to differences in the functional organization of the corticomotoneuronal system innervating different muscle regions.

Motivational goal-priming with or without awareness produces faster and stronger force exertion

Previous research has demonstrated that barely visible (subliminal) goal-priming with motivational reward can alter the state of the motor system and enhance motor output. Research shows that these affective-motivational effects result from associations between goal representations and positive affect without conscious awareness. Here, we tested whether motivational priming can increase motor output even if the priming is fully visible (supraliminal), and whether the priming effect occurs through increased cortical excitability. Groups of participants were primed with either barely visible or fully visible words related to effort and control sequences of random letters that were each followed by fully visible positively reinforcing words. The priming effect was measured behaviourally by handgrip force and reaction time to the grip cue after the priming was complete. Physiologically, the effects were measured by pupil dilation and motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation during the priming task. Analysis showed that for both the supraliminal and subliminal conditions, reaction time decreased and total force, MEP magnitude, and pupil dilation increased. None of the priming-induced changes in behaviour or physiology differed significantly between the supraliminal and the subliminal groups, indicating that implicit motivation towards motor goals might not require conscious perception of the goals.

Feedforward neural control of toe walking in humans

KEY POINTS

Activation of ankle muscles at ground contact during toe walking is unaltered when sensory feedback is blocked or the ground is suddenly dropped. Responses in the soleus muscle to transcranial magnetic stimulation, but not peripheral nerve stimulation, are facilitated at ground contact during toe walking. We argue that toe walking is supported by feedforward control at ground contact.

ABSTRACT

Toe walking requires careful control of the ankle muscles in order to absorb the impact of ground contact and maintain a stable position of the joint. The present study aimed to clarify the peripheral and central neural mechanisms involved. Fifteen healthy adults walked on a treadmill (3.0 km h^-1 ). Tibialis anterior (TA) and soleus (Sol) EMG, knee and ankle joint angles, and gastrocnemius-soleus muscle fascicle lengths were recorded. Peripheral and central contributions to the EMG activity were assessed by afferent blockade, H-reflex testing, transcranial magnetic brain stimulation (TMS) and sudden unloading of the planter flexor muscle-tendon complex. Sol EMG activity started prior to ground contact and remained high throughout stance. TA EMG activity, which is normally seen around ground contact during heel strike walking, was absent. Although stretch of the Achilles tendon-muscle complex was observed after ground contact, this was not associated with lengthening of the ankle plantar flexor muscle fascicles. Sol EMG around ground contact was not affected by ischaemic blockade of large-diameter sensory afferents, or the sudden removal of ground support shortly after toe contact. Soleus motor-evoked potentials elicited by TMS were facilitated immediately after ground contact, whereas Sol H-reflexes were not. These findings indicate that at the crucial time of ankle stabilization following ground contact, toe walking is governed by centrally mediated motor drive rather than sensory driven reflex mechanisms. These findings have implications for our understanding of the control of human gait during voluntary toe walking.

Characterization of corticospinal activation of finger motor neurons during precision and power grip in humans

Direct and indirect corticospinal pathways to finger muscles may play a different role in control of the upper extremity. We used transcranial magnetic stimulation (TMS) and coherence analysis to characterize the corticospinal drive to the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) when active during a precision and power grip task. In experiment 1, single motor units were recorded during precision grip and power grip in 20 adults (25.2 ± 7.1 years). Post-stimulus time histograms (PSTH) were obtained following TMS. In experiment 2, coherence and cross-correlation analysis of the FDI and APB surface EMG were used to investigate the temporal organization of corticospinal drive during precision grip and power grip in 15 adults (27.4 ± 8.1 years). We found no significant differences in PSTH peak onset (26.6 ± 1.9 vs. 26.7 ± 2.0 ms, p = 0.75), maximal peak (27.4 ± 1.9 vs. 27.4 ± 1.9 ms, p = 1.0) or peak duration (2.3 ± 1.1 vs. 2.3 ± 1.0 ms, p = 0.75) for the 11 recovered motor units during precision grip and power grip. Also, no significant difference in coherence or the width of the synchronization peaks during precision grip (7.2 ± 3.7 ms) and power grip (7.9 ± 3.1 ms) could be observed (p = 0.59). The short duration of peaks elicited in the PSTH of single motor units following TMS and central synchronization peaks of voluntarily activated motor units during precision and power grip suggests that the direct corticospinal pathway (the corticomotoneuronal system) is equally involved in the control of both tasks. The data do not support that indirect pathways would make a larger contribution to power grip.

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.

EMG-triggered stimulation post spinal cord injury: A case report

BACKGROUND

Mechanical injury in patients with spinal cord injury (SCI) rarely transects the cord completely, even when the injury is classified as complete. These patients can show sub-clinical evidence of spared motor connections, which might be amenable to targeted rehabilitation. Neurophysiological evaluations can complement the clinical evaluation by providing objective data about conduction across the SCI site.

CASE DESCRIPTION

A twenty-four year old patient with SCI was admitted to a rehabilitation centre 49 days post traumatic SCI. His injury was categorized as motor and sensory complete (AIS A) with a neurological level of C4. The strength of his triceps bilaterally was recorded 0/5 repeatedly by his therapists during the five-month period post-injury. As a result, no training was provided for these muscles during the rehabilitation program. Neurophysiological Assessment: Motor evoked potentials (MEPs) were recorded from his left triceps with transcranial magnetic stimulation (TMS) which confirmed the existence of spared corticospinal connections to this muscle post-injury.

INTERVENTION

He completed a series of active-assisted exercises with an EMG-triggered neuromuscular stimulation (NMS) device for his left triceps comprising 20-minutes elbow extension (15 trials), three times per day for 4 weeks.

OUTCOME

The strength of his left triceps gradually improved to 2/5.

DISCUSSION

Neurophysiological evaluation can be useful in identifying residual function below the level of injury, which can, in turn, be enhanced through appropriate rehabilitation strategies.

The TWIST Algorithm Predicts Time to Walking Independently After Stroke

BACKGROUND AND OBJECTIVE

The likelihood of regaining independent walking after stroke is of concern to patients and their families and influences hospital discharge planning. The objective of this study was to explore factors that could be combined in an algorithm for predicting whether and when a patient will walk independently after stroke.

METHODS

Adults with new lower limb weakness were recruited within 3 days of having a stroke. Clinical assessment, transcranial magnetic stimulation, and magnetic resonance imaging were completed 1 to 2 weeks poststroke. Classification and regression tree (CART) analysis was used to identify factors that predicted whether a patient achieved independent walking by 6 or 12 weeks, or remained dependent at 12 weeks.

RESULTS

We recruited 41 patients (24 women; median age 72 years, range 43-96 years). The CART analysis results were used to create the Time to Walking Independently after STroke (TWIST) algorithm, which made accurate predictions for 95% of patients. Patients with a trunk control test score >40 at 1 week walked independently within 6 weeks. Patients with a trunk control test score <40 only achieved independent walking by 12 weeks if they also had hip extension strength of Medical Research Council grade 3 or more. Neurophysiological and neuroimaging measures did not predict independent walking after stroke.

CONCLUSIONS

In this exploratory study, the TWIST algorithm accurately predicted whether and when an individual patient walked independently after stroke using simple bedside measures 1 week poststroke. Further work is required to develop and validate this algorithm in a larger study.

Immediate effect of vibratory stimuli on quadriceps function in healthy adults

INTRODUCTION

The purpose of this study was to compare the effect of whole body vibration (WBV) and local muscle vibration (LMV) on quadriceps function.

METHODS

Sixty adults were randomized to WBV, LMV, or control groups. Quadriceps function [Hoffmann (H)-reflex, active motor threshold (AMT), motor evoked potential (MEP) and electromyographic amplitude, peak torque (PT), rate of torque development (RTD), and central activation ratio (CAR)] was assessed before and immediately after and 10 and 20 minutes after interventions.

RESULTS

WBV improved PT, CAR, AMT, EMG, and MEP amplitude, and EMG amplitude and CAR were greater than control after application. LMV improved EMG amplitude and AMT, and EMG amplitude was greater than control after application. AMT remained lower 10 and 20 minutes after WBV and LMV. No differences were noted between LMV and WBV. Vibration did not influence H-reflex or RTD.

CONCLUSIONS

WBV and LMV increased quadriceps function and may be used to enhance the efficacy of strengthening protocols. Muscle Nerve 54: 469-478, 2016.

How much detail is needed in modeling a transcranial magnetic stimulation figure-8 coil: Measurements and brain simulations

Background

Despite TMS wide adoption, its spatial and temporal patterns of neuronal effects are not well understood. Although progress has been made in predicting induced currents in the brain using realistic finite element models (FEM), there is little consensus on how a magnetic field of a typical TMS coil should be modeled. Empirical validation of such models is limited and subject to several limitations.

Methods

We evaluate and empirically validate models of a figure-of-eight TMS coil that are commonly used in published modeling studies, of increasing complexity: simple circular coil model; coil with in-plane spiral winding turns; and finally one with stacked spiral winding turns. We will assess the electric fields induced by all 3 coil models in the motor cortex using a computer FEM model. Biot-Savart models of discretized wires were used to approximate the 3 coil models of increasing complexity. We use a tailored MR based phase mapping technique to get a full 3D validation of the incident magnetic field induced in a cylindrical phantom by our TMS coil. FEM based simulations on a meshed 3D brain model consisting of five tissues types were performed, using two orthogonal coil orientations.

Results

Substantial differences in the induced currents are observed, both theoretically and empirically, between highly idealized coils and coils with correctly modeled spiral winding turns. Thickness of the coil winding turns affect minimally the induced electric field, and it does not influence the predicted activation.

Conclusion

TMS coil models used in FEM simulations should include in-plane coil geometry in order to make reliable predictions of the incident field. Modeling the in-plane coil geometry is important to correctly simulate the induced electric field and to correctly make reliable predictions of neuronal activation

Corticospinal excitability as a biomarker of myofascial pain syndrome

INTRODUCTION

Myofascial pain syndrome (MPS) is a common chronic pain disorder that lacks effective diagnostic criteria. To better understand neurophysiological changes in chronic pain, several trials exploring corticospinal excitability in different populations of patients with chronic pain have been performed.

OBJECTIVES

In this systematic review, we aimed to investigate the current literature on MPS and intracortical disinhibition, by means of increased intracortical facilitation and decreased intracortical inhibition (ICI).

METHODS

We performed a search on PubMed to identify clinical trials on MPS and transcranial magnetic stimulation measurements. We then applied the Harford Hill criteria to the identified studies to assess the possible causal relationship between intracortical disinhibition measurements and MPS. Finally, we compared our findings on MPS with other chronic pain conditions.

RESULTS

Four studies assessing corticospinal excitability in patients with MPS were found. Although the amount of trials available is limited, all the reported studies indicated an increased intracortical disinhibition in patients with MPS. Importantly, these measurements were also correlated with psychological factors, such as pain catastrophism, or anxiety. However, based on the Harford Hill criteria, we could not assert a strong causal relationship between these markers and MPS. Although intracortical disinhibition has been consistently found in patients having MPS, this lack of cortical inhibition was not only observed in this specific chronic pain syndrome but also in fibromyalgia and neuropathic pain conditions.

CONCLUSION

Intracortical disinhibition seems to be a marker that has been consistently observed in MPS. Future prospective cohort studies could provide new insights in the development of neoplastic and maladaptive changes occurring in chronic pain syndromes and help the development of new therapeutic options.

A critical period of corticomuscular and EMG-EMG coherence detection in healthy infants aged 9-25 weeks

KEY POINTS

The early postnatal development of functional corticospinal connections in human infants is not fully clarified. Corticospinal drive to upper and lower limb muscle shows developmental changes with an increased functional coupling in infants between 9 and 25 weeks in the beta frequency band. The changes in functional coupling coincide with the developmental period where fidgety movements are present in healthy infants. Data support a possible sensitive period where functional connections between corticospinal tract fibres and spinal motoneurones undergo activity-dependent reorganization.

ABSTRACT

The early postnatal development of functional corticospinal connections in human infants is not fully clarified. We used EEG and EMG to investigate the development of corticomuscular and intramuscular coherence as indicators of functional corticospinal connectivity in healthy infants aged 1-66 weeks. EEG was recorded over leg and hand area of motor cortex. EMG recordings were made from right ankle dorsiflexor and right wrist extensor muscles. Quantification of the amount of corticomuscular coherence in the 20-40 Hz frequency band showed a significantly larger coherence for infants aged 9-25 weeks compared to younger and older infants. Coherence between paired EMG recordings from tibialis anterior muscle in the 20-40 Hz frequency band was also significantly larger for the 9-25 week age group. A low-amplitude, broad-duration (40-50 ms) central peak of EMG-EMG synchronization was observed for infants younger than 9 weeks, whereas a short-lasting (10-20 ms) central peak was observed for EMG-EMG synchronization in older infants. This peak was largest for infants aged 9-25 weeks. These data suggest that the corticospinal drive to lower and upper limb muscles shows significant developmental changes with an increase in functional coupling in infants aged 9-25 weeks, a period which coincides partly with the developmental period of normal fidgety movements. We propose that these neurophysiological findings may reflect the existence of a sensitive period where the functional connections between corticospinal tract fibres and spinal motoneurones undergo activity-dependent reorganization. This may be relevant for the timing of early therapy interventions in infants with pre- and perinatal brain injury.