Reliability of transcranial magnetic stimulation for mapping the human motor cortex

Reliability of active robotic neuro-navigated transcranial magnetic stimulation motor maps

Transcranial magnetic stimulation (TMS) motor mapping is a safe, non-invasive method used to study corticomotor organization and intervention-induced plasticity. Reliability of resting maps is well established, but understudied for active maps and unestablished for active maps obtained using robotic TMS techniques. The objective of  this study was to determine the reliability of robotic neuro-navigated TMS motor map measures during active muscle contraction. We hypothesized that map area and volume would show excellent short- and medium-term reliability. Twenty healthy adults were tested on 3 days. Active maps of the first dorsal interosseous muscle were created using a 12 × 12 grid (7 mm spacing). Short- (24 h) and medium-term (3-5 weeks) relative (intra-class correlation coefficient) and absolute (minimal detectable change (MDC); standard error of measure) reliabilities were evaluated for map area, volume, center of gravity (CoG), and hotspot magnitude (peak-to-peak MEP amplitude at the hotspot), along with active motor threshold (AMT) and maximum voluntary contraction (MVC). This study found that AMT and MVC had good-to-excellent short- and medium-term reliability. Map CoG (x and y) were the most reliable map measures across sessions with excellent short- and medium-term reliability (p < 0.001). Map area, hotspot magnitude, and map volume followed with better reliability medium-term than short-term, with a change of 28%, 62%, and 78% needed to detect a true medium-term change, respectively. Therefore, robot-guided neuro-navigated TMS active mapping is relatively reliable but varies across measures. This, and MDC, should be considered in interventional study designs.

Active versus resting neuro-navigated robotic transcranial magnetic stimulation motor mapping

Transcranial magnetic stimulation (TMS) motor mapping is a safe, non-invasive method that can be used to study corticomotor organization. Motor maps are typically acquired at rest, and comparisons to maps obtained during muscle activation have been both limited and contradictory. Understanding the relationship between functional activation of the corticomotor system as recorded by motor mapping is crucial for their use clinically and in research. The present study utilized robotic TMS paired with personalized neuro-navigation to examine the relationship between resting and active motor map measures and their relationship with motor performance. Twenty healthy right-handed participants underwent resting and active robotic TMS motor mapping of the first dorsal interosseous to 10% maximum voluntary contraction. Motor map parameters including map area, volume, and measures of map centrality were compared between techniques using paired sample tests of difference and Bland-Altman plots and analysis. Map area, volume, and hotspot magnitude were larger in the active motor maps, while map center of gravity and hotspot locations remained consistent between both maps. No associations were observed between motor maps and motor performance as measured by the Purdue Pegboard Test. Our findings support previous suggestions that maps scale with muscle contraction. Differences in mapping outcomes suggest rest and active motor maps may reflect functionally different corticomotor representations. Advanced analysis methods may better characterize the underlying neurophysiology of both types of motor mapping.

Effects of Transcranial Direct Current Stimulation and High-Definition Transcranial Direct Current Stimulation Enhanced Motor Learning on Robotic Transcranial Magnetic Stimulation Motor Maps in Children

Introduction: Conventional transcranial direct current stimulation (tDCS) and high-definition tDCS (HD-tDCS) may improve motor learning in children. Mechanisms are not understood. Neuronavigated robotic transcranial magnetic stimulation (TMS) can produce individualised maps of primary motor cortex (M1) topography. We aimed to determine the effects of tDCS- and HD-tDCS-enhanced motor learning on motor maps. Methods: Typically developing children aged 12-18 years were randomised to right M1 anodal tDCS, HD-tDCS, or Sham during training of their left-hand on the Purdue Pegboard Task (PPT) over 5 days. Bilateral motor mapping was performed at baseline (pre), day 5 (post), and 6-weeks retention time (RT). Primary muscle was the first dorsal interosseous (FDI) with secondary muscles of abductor pollicis brevis (APB) and adductor digiti minimi (ADM). Primary mapping outcomes were volume (mm²/mV) and area (mm²). Secondary outcomes were centre of gravity (COG, mm) and hotspot magnitude (mV). Linear mixed-effects modelling was employed to investigate effects of time and stimulation type (tDCS, HD-tDCS, Sham) on motor map characteristics. Results: Twenty-four right-handed participants (median age 15.5 years, 52% female) completed the study with no serious adverse events or dropouts. Quality maps could not be obtained in two participants. No effect of time or group were observed on map area or volume. LFDI COG (mm) differed in the medial-lateral plane (x-axis) between tDCS and Sham (p = 0.038) from pre-to-post mapping sessions. Shifts in map COG were also observed for secondary left-hand muscles. Map metrics did not correlate with behavioural changes. Conclusion: Robotic TMS mapping can safely assess motor cortex neurophysiology in children undergoing motor learning and neuromodulation interventions. Large effects on map area and volume were not observed while changes in COG may occur. Larger controlled studies are required to understand the role of motor maps in interventional neuroplasticity in children.

Cortical Re-organization After Traumatic Brain Injury Elicited Using Functional Electrical Stimulation Therapy: A Case Report

Functional electrical stimulation therapy (FEST) can improve motor function after neurological injuries. However, little is known about cortical changes after FEST and weather it can improve motor function after traumatic brain injury (TBI). Our study examined cortical changes and motor improvements in one male participant with chronic TBI suffering from mild motor impairment affecting the right upper-limb during 3-months of FEST and during 3-months follow-up. In total, 36 sessions of FEST were applied to enable upper-limb grasping and reaching movements. Short-term assessments carried out using transcranial magnetic stimulation (TMS) showed reduced cortical silent period (CSP), indicating cortical and/or subcortical inhibition after each intervention. At the same time, no changes in motor evoked potentials (MEPs) were observed. Long-term assessments showed increased MEP corticospinal excitability after 12-weeks of FEST, which seemed to remain during both follow-ups, while no changes in CSP were observed. Similarly, long-term assessments using TMS mapping showed larger hand MEP area in the primary motor cortex (M1) after 12-weeks of FEST as well as during both follow-ups. Corroborating TMS results, functional magnetic resonance imaging (fMRI) data showed M1 activations increased during hand grip and finger pinch tasks after 12-weeks of FEST, while gradual reduction of activity compared to after the intervention was seen during follow-ups. Widespread changes were seen not only in the M1, but also sensory, parietal rostroventral, supplementary motor, and premotor areas in both contralateral and ipsilateral hemispheres, especially during the finger pinch task. Drawing test performance showed improvements after the intervention and during follow-ups. Our findings suggest that task-specific and repetitive FEST can effectively increase cortical activations by integrating voluntary motor commands and sensorimotor network through functional electrical stimulation (FES). Overall, our results demonstrated cortical re-organization in an individual with chronic TBI after FEST.

TMS Motor Mapping Methodology and Reliability: A Structured Review

Motor cortical representation can be probed non-invasively using a transcranial magnetic stimulation (TMS) technique known as motor mapping. The mapping technique can influence features of the maps because of several controllable elements. Here we review the literature on six key motor mapping parameters, as well as their influence on outcome measures and discuss factors impacting their selection. 132 of 1,587 distinct records were examined in detail and synthesized to form the basis of our review. A summary of mapping parameters, their impact on outcome measures and feasibility considerations are reported to support the design and interpretation of TMS mapping studies.

Robotic transcranial magnetic stimulation motor maps and hand function in adolescents

Introduction

Transcranial magnetic stimulation (TMS) motor mapping can characterize the neurophysiology of the motor system. Limitations including human error and the challenges of pediatric populations may be overcome by emerging robotic systems. We aimed to show that neuronavigated robotic motor mapping in adolescents could efficiently produce discrete maps of individual upper extremity muscles, the characteristics of which would correlate with motor behavior.

Methods

Typically developing adolescents (TDA) underwent neuronavigated robotic TMS mapping of bilateral motor cortex. Representative maps of first dorsal interosseous (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles in each hand were created. Map features including area (primary), volume, and center of gravity were analyzed across different excitability regions (R100%, R75%, R50%, R25%). Correlations between map metrics and validated tests of hand motor function (Purdue Pegboard Test as primary) were explored.

Results

Twenty‐four right‐handed participants (range 12–18 years, median 15.5 years, 52% female) completed bilateral mapping and motor assessments with no serious adverse events or dropouts. Gender and age were associated with hand function and motor map characteristics. Full motor maps (R100%) for FDI did not correlate with motor function in either hand. Smaller excitability subset regions demonstrated reduced variance and dose‐dependent correlations between primary map variables and motor function in the dominant hemisphere.

Conclusions

Hand function in TDA correlates with smaller subset excitability regions of robotic TMS motor map outcomes. Refined motor maps may have less variance and greater potential to quantify interventional neuroplasticity. Robotic TMS mapping is safe and feasible in adolescents.

Understanding intracortical excitability in phantom limb pain: A multivariate analysis from a multicenter randomized clinical trial

OBJECTIVES

To explore associations of intracortical excitability with clinical characteristics in a large sample of subjects with phantom limb pain (PLP).

METHODS

Ancillary study using baseline and longitudinal data from a large multicenter randomized trial that investigated the effects of non-invasive brain stimulation combined with sensorimotor training on PLP. Multivariate regression modeling analyses were used to investigate the association of intracortical excitability, measured by percentages of intracortical inhibition (ICI) and facilitation (ICF) with clinical variables.

RESULTS

Ninety-eight subjects were included. Phantom sensation of itching was positively associated with ICI changes and at baseline in the affected hemisphere (contralateral to PLP). However, in the non-affected hemisphere (ipsilateral to PLP), the phantom sensation of warmth and PLP intensity were negatively associated with ICI (both models). For the ICF, PLP intensity (baseline model only) and age (longitudinal model) were negatively associated, while time since amputation and amputation level (both for longitudinal model only) were positively associated in the affected hemisphere. Additionally, use of antidepressants led to lower ICF in the non-affected hemisphere for the baseline model while higher amputation level also led to less changes in the ICF.

CONCLUSION

Results revealed clear associations of clinical variables and cortical excitability in a large chronic pain sample. ICI and ICF changes appear not to be mainly explained by PLP intensity. Instead, other variables associated with duration of neuroplasticity changes (such as age and duration of amputation) and compensatory mechanisms (such as itching and phantom limb sensation) seem to be more important in explaining these variables.

Reliability of robotic transcranial magnetic stimulation motor mapping

Robotic transcranial magnetic stimulation (TMS) is a noninvasive and safe tool that produces cortical motor maps using neuronavigational and neuroanatomical images. Motor maps are individualized representations of the primary motor cortex (M1) topography that may reflect developmental and interventional plasticity. Results of TMS motor map reliability testing have been variable, and robotic measures are undefined. We aimed to determine the short- and long-term reliability of robotic TMS motor maps. Twenty healthy participants underwent motor mapping at baseline, 24 h, and 4 wk. A 12 × 12 grid (7-mm spacing) was placed over the left M1, centered over the hand knob area. Four suprathreshold stimulations were delivered at each grid point. First dorsal interosseous (FDI) motor-evoked potentials (MEPs) were analyzed offline to generate map characteristics of area, volume, center of gravity (COG), and hotspot magnitude. Subsets of each outcome corresponding to 75%, 50%, and 25% of each map were determined. Reliability measures including intraclass correlation coefficient (ICC), minimal detectable change (MDC), and standard error of measure (SEM) were calculated. Map volume, COG, and hotspot magnitude were the most reliable measures (good-to-excellent) over both short- and long-term sessions. Map area reliability was poor-to-moderate for short- and long-term sessions. Smaller map percentile subsets showed decreased variability but only minimal improvements in reliability. MDC for most outcomes was >50%. Procedures were well tolerated with no serious adverse events. Robotic TMS motor mapping is relatively reliable over time, but careful consideration of specific outcomes is required for this method to interrogate plasticity in the human motor system.NEW & NOTEWORTHY Robotic transcranial magnetic stimulation (TMS) is a noninvasive and safe tool that produces cortical motor maps-individualized representations of the primary motor cortex (M1) topography-that may reflect developmental and interventional plasticity. This study is the first to evaluate short- and long-term relative and absolute reliability of TMS mapping outcomes at various M1 excitability levels using novel robotic neuronavigated TMS.

Structural and functional motor cortex asymmetry in unilateral lower limb amputation with phantom limb pain

OBJECTIVE

The role of motor cortex reorganization in the development and maintenance of phantom limb pain (PLP) is still unclear. This study aims to evaluate neurophysiological and structural motor cortex asymmetry in patients with PLP and its relationship with pain intensity.

METHODS

Cross-sectional analysis of an ongoing randomized-controlled trial. We evaluated the motor cortex asymmetry through two techniques: i) changes in cortical excitability indexed by transcranial magnetic stimulation (motor evoked potential, paired-pulse paradigms and cortical mapping), and ii) voxel-wise grey matter asymmetry analysis by brain magnetic resonance imaging.

RESULTS

We included 62 unilateral traumatic lower limb amputees with a mean PLP of 5.9 (SD = 1.79). We found, in the affected hemisphere, an anterior shift of the hand area center of gravity (23 mm, 95% CI 6 to 38, p = 0.005) and a disorganized and widespread representation. Regarding voxel-wise grey matter asymmetry analysis, data from 21 participants show a loss of grey matter volume in the motor area of the affected hemisphere. This asymmetry seems negatively associated with time since amputation. For TMS data, only the ICF ratio is negatively correlated with PLP intensity (r = -0.25, p = 0.04).

CONCLUSION

There is an asymmetrical reorganization of the motor cortex in patients with PLP, characterized by a disorganized, widespread, and shifted hand cortical representation and a loss in grey matter volume in the affected hemisphere. This reorganization seems to reduce across time since amputation. However, it is not associated with pain intensity.

SIGNIFICANCE

These findings are significant to understand the role of the motor cortex reorganization in patients with PLP, showing that the pain intensity may be related with other neurophysiological factors, not just cortical reorganization.

Accuracy of Estimating the Area of Cortical Muscle Representations from TMS Mapping Data Using Voronoi Diagrams

Motor evoked potentials (MEPs) caused by transcranial magnetic stimulation (TMS) provide a possibility of noninvasively mapping cortical muscle representations for clinical and research purposes. The interpretation of such results is complicated by the high variability in MEPs and the lack of a standard optimal mapping protocol. Comparing protocols requires the determination of the accuracy of estimated representation parameters (such as the area), which is problematic without ground truth data. We addressed this problem and obtained two main results: (1) the development of a bootstrapping-based approach for estimating the within-session variability and bias of representation parameters and (2) estimations of the area and amplitude-weighted area accuracies for motor representations using this approach. The method consists in the simulation of TMS mapping results by subsampling MEPs from a single map with a large number of stimuli. We studied the extensor digitorum communis (EDC) and flexor digitorum superficialis (FDS) muscle maps of 15 healthy subjects processed using Voronoi diagrams. We calculated the (decreasing) dependency of the errors in the area and weighted area on the number of stimuli. This result can be used to choose a number of stimuli sufficient for studying the effects of a given size (e.g., the protocol with 150 stimuli leads to relative errors of 7% for the area and 11% for the weighted area in 90% of the maps). The approach is applicable to other parameters (e.g., the center of gravity) and other map processing methods, such as spline interpolation.

Optimization of the Navigated TMS Mapping Algorithm for Accurate Estimation of Cortical Muscle Representation Characteristics

Navigated transcranial magnetic stimulation (nTMS) mapping of cortical muscle representations allows noninvasive assessment of the state of a healthy or diseased motor system, and monitoring changes over time. These applications are hampered by the heterogeneity of existing mapping algorithms and the lack of detailed information about their accuracy. We aimed to find an optimal motor evoked potential (MEP) sampling scheme in the grid-based mapping algorithm in terms of the accuracy of muscle representation parameters. The abductor pollicis brevis (APB) muscles of eight healthy subjects were mapped three times on consecutive days using a seven-by-seven grid with ten stimuli per cell. The effect of the MEP variability on the parameter accuracy was assessed using bootstrapping. The accuracy of representation parameters increased with the number of stimuli without saturation up to at least ten stimuli per cell. The detailed sampling showed that the between-session representation area changes in the absence of interventions were significantly larger than the within-session fluctuations and thus could not be explained solely by the trial-to-trial variability of MEPs. The results demonstrate that the number of stimuli has no universally optimal value and must be chosen by balancing the accuracy requirements with the mapping time constraints in a given problem.

Potential Corticomotor Plasticity in Those with and without Chronic Ankle Instability

INTRODUCTION

Quantifying corticomotor alterations is important to understand the neurophysiological mechanisms that likely contribute to the neuromuscular control deficits observed in patients with chronic ankle instability (CAI). Corticomotor output mapping provides further insight into the changes within the motor cortex and identifies potential changes in the area of the motor cortex associated with selected muscles. Therefore, this investigation compared the corticomotor map output for the fibularis longus (FL) muscle in patients with and without CAI.

METHODS

Eighteen CAI patients and 16 healthy controls (HC) volunteered. Transcranial magnetic stimulation was used to map the motor cortex's representation of the FL. The normalized average of three motor evoked potentials at 100% of active motor threshold intensity was recorded for each scalp site on a 6 × 6 cm grid. Corticomotor output map was compared between groups through 1) the size of the corticomotor map area, 2) the volume of the corticomotor map, and 3) the location of cortical representation. Independent t-tests were used to assess group differences in each mapping outcome variable. Cohen's d effect sizes along with 95% confidence intervals were calculated using the pooled SD values.

RESULTS

CAI patients exhibited less map volume (P = 0.018, CAI = 8.2 ± 3.2 cm mV vs HC = 11.3 ± 3.9 cm mV) and map area (P = 0.046, CAI = 12.8 ± 6.0 cm vs HC: 17.4 ± 6.9 cm) compared with HC.

CONCLUSIONS

The smaller map area and volume suggest a more concentrated area of neurons communicating with the FL muscle in patients with CAI. Consequently, motor cortical cells on the border of the FL excitation area are less committed to the proper function of the FL muscle and may be recruited by other surrounding areas. This may explain altered movement strategies that lead to ankle reinjury.

Neuromuscular Plasticity: Disentangling Stable and Variable Motor Maps in the Human Sensorimotor Cortex

Motor maps acquired with transcranial magnetic stimulation (TMS) are evolving as a biomarker for monitoring disease progression or the effects of therapeutic interventions. High test-retest reliability of this technique for long observation periods is therefore required to differentiate daily or weekly fluctuations from stable plastic reorganization of corticospinal connectivity. In this study, a novel projection, interpolation, and coregistration technique, which considers the individual gyral anatomy, was applied in healthy subjects for biweekly acquired TMS motor maps over a period of twelve weeks. The intraclass correlation coefficient revealed long-term reliability of motor maps with relevant interhemispheric differences. The sensorimotor cortex and nonprimary motor areas of the dominant hemisphere showed more extended and more stable corticospinal connectivity. Long-term correlations of the MEP amplitudes at each stimulation site revealed mosaic-like clusters of consistent corticospinal excitability. The resting motor threshold, centre of gravity, and mean MEPs across all TMS sites, as highly reliable cortical map parameters, could be disentangled from more variable parameters such as MEP area and volume. Cortical TMS motor maps provide high test-retest reliability for long-term monitoring when analyzed with refined techniques. They may guide restorative interventions which target dormant corticospinal connectivity for neurorehabilitation.

A Review of Transcranial Magnetic Stimulation and Multimodal Neuroimaging to Characterize Post-Stroke Neuroplasticity

Following stroke, the brain undergoes various stages of recovery where the central nervous system can reorganize neural circuitry (neuroplasticity) both spontaneously and with the aid of behavioral rehabilitation and non-invasive brain stimulation. Multiple neuroimaging techniques can characterize common structural and functional stroke-related deficits, and importantly, help predict recovery of function. Diffusion tensor imaging (DTI) typically reveals increased overall diffusivity throughout the brain following stroke, and is capable of indexing the extent of white matter damage. Magnetic resonance spectroscopy (MRS) provides an index of metabolic changes in surviving neural tissue after stroke, serving as a marker of brain function. The neural correlates of altered brain activity after stroke have been demonstrated by abnormal activation of sensorimotor cortices during task performance, and at rest, using functional magnetic resonance imaging (fMRI). Electroencephalography (EEG) has been used to characterize motor dysfunction in terms of increased cortical amplitude in the sensorimotor regions when performing upper limb movement, indicating abnormally increased cognitive effort and planning in individuals with stroke. Transcranial magnetic stimulation (TMS) work reveals changes in ipsilesional and contralesional cortical excitability in the sensorimotor cortices. The severity of motor deficits indexed using TMS has been linked to the magnitude of activity imbalance between the sensorimotor cortices. In this paper, we will provide a narrative review of data from studies utilizing DTI, MRS, fMRI, EEG, and brain stimulation techniques focusing on TMS and its combination with uni- and multimodal neuroimaging methods to assess recovery after stroke. Approaches that delineate the best measures with which to predict or positively alter outcomes will be highlighted.

The reliability of repeated TMS measures in older adults and in patients with subacute and chronic stroke

The reliability of transcranial magnetic stimulation (TMS) measures in healthy older adults and stroke patients has been insufficiently characterized. We determined whether common TMS measures could reliably evaluate change in individuals and in groups using the smallest detectable change (SDC), or could tell subjects apart using the intraclass correlation coefficient (ICC). We used a single-rater test-retest design in older healthy, subacute stroke, and chronic stroke subjects. At twice daily sessions on two consecutive days, we recorded resting motor threshold, test stimulus intensity, recruitment curves, short-interval intracortical inhibition, and facilitation, and long-interval intracortical inhibition. Using variances estimated from a random effects model, we calculated the SDC and ICC for each TMS measure. For all TMS measures in all groups, SDCs for single subjects were large; only with modest group sizes did the SDCs become low. Thus, while these TMS measures cannot be reliably used as a biomarker to detect individual change, they can reliably detect change exceeding measurement noise in moderate-sized groups. For several of the TMS measures, ICCs were universally high, suggesting that they can reliably discriminate between subjects. TMS measures should be used based on their reliability in particular contexts. More work establishing their validity, responsiveness, and clinical relevance is still needed.

Reproducibility of transcranial magnetic stimulation metrics in the study of proximal upper limb muscles

OBJECTIVE

Reproducibility of transcranial magnetic stimulation (TMS) metrics is essential in accurately tracking recovery and disease. However, majority of evidence pertains to reproducibility of metrics for distal upper limb muscles. We investigate for the first time, reliability of corticospinal physiology for a large proximal muscle - the biceps brachii and relate how varying statistical analyses can influence interpretations.

METHODS

14 young right-handed healthy participants completed two sessions assessing resting motor threshold (RMT), motor evoked potentials (MEPs), motor map and intra-cortical inhibition (ICI) from the left biceps brachii. Analyses included paired t-tests, Pearson's, intra-class (ICC) and concordance correlation coefficients (CCC) and Bland-Altman plots.

RESULTS

Unlike paired t-tests, ICC, CCC and Pearson's were >0.6 indicating good reliability for RMTs, MEP intensities and locations of map; however values were <0.3 for MEP responses and ICI.

CONCLUSIONS

Corticospinal physiology, defining excitability and output in terms of intensity of the TMS device, and spatial loci are the most reliable metrics for the biceps. MEPs and variables based on MEPs are less reliable since biceps receives fewer cortico-motor-neuronal projections. Statistical tests of agreement and associations are more powerful reliability indices than inferential tests.

SIGNIFICANCE

Reliable metrics of proximal muscles when translated to a larger number of participants would serve to sensitively track and prognosticate function in neurological disorders such as stroke where proximal recovery precedes distal.

Projecting Navigated TMS Sites on the Gyral Anatomy Decreases Inter-subject Variability of Cortical Motor Maps

BACKGROUND

Magnetic resonance images are being increasingly deployed in conjunction with navigated transcranial magnetic stimulation (nTMS) to account for inter-individual differences in brain anatomy as well as to reduce the variability of mapping findings.

OBJECTIVE

However, despite the fact that the individual gyral anatomy has a significant impact on the TMS-induced electrical field distributions, these approaches still project the TMS coil positions as a plane grid of target points on the brain surface and fail to account for differences in cortex morphology.

METHODS

In this study, we have introduced a technique for projecting nTMS sites onto the gyral anatomy to decrease the variability of cortical motor maps between subjects in normalized space. This involved interpolating the discrete map points in the normalized volume space and performing additional surface coregistration.

RESULTS

By applying this technique, we increased the spatial overlap between the cortical maps of the extensor digitorum communis muscle between subjects from 80% to 100%. We also managed to significantly reduce the mean Euclidean distance between the average center of gravity and the average hotspots to the respective individual spots from 8 mm to 6.5 mm.

CONCLUSION

Our approach facilitates the study of the functional topography of distinct behavioral properties with high spatial resolution, thereby constituting a valuable tool for precise group analysis of cortical TMS maps.

Use of transcranial magnetic stimulation in the treatment of selected movement disorders

Transcranial magnetic stimulation (TMS) is a valuable technique for assessing the underlying neurophysiology associated with various neuropathologies, and is a unique tool for establishing potential neural mechanisms responsible for disease progression. Recently, repetitive TMS (rTMS) has been advanced as a potential therapeutic technique to treat selected neurologic disorders. In healthy individuals, rTMS can induce changes in cortical excitability. Therefore, targeting specific cortical areas affected by movement disorders theoretically may alter symptomology. This review discusses the evidence for the efficacy of rTMS in Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and multiple sclerosis. It is hoped that gaining a more thorough understanding of the timing and parameters of rTMS in individuals with neurodegenerative disorders may advance both clinical care and research into the most effective uses of this technology.

TMS brain mapping in less than two minutes

BACKGROUND

Transcranial magnetic stimulation (TMS) corticospinal excitability maps are a valuable tool to study plasticity in the corticospinal tract. Traditionally, data acquisition for a single map is time consuming, limiting the method's applicability when excitability changes quickly, such as during motor learning, and in clinical investigations where assessment time is a limiting factor.

OBJECTIVE

To reduce the time needed to create a reliable map by 1) investigating the minimum interstimulus interval (ISI) at which stimuli may be delivered, and 2) investigating the minimum number of stimuli required to create a map.

METHOD

Frameless stereotaxy was used to monitor coil position as the coil was moved pseudorandomly within a 6 × 6 cm square. Maps were acquired using 1-4 s ISIs in 12 participants. The minimum number of stimuli was determined by randomly extracting data and comparing the resulting map to the original data set. To confirm validity, the pseudorandom walk method was compared against a traditional mapping method.

RESULTS

Reliable maps could be created with 63 stimuli recorded with a 1 s ISI. Maps created acquiring data using the pseudorandom walk method were not significantly different from maps acquired following the traditional method.

CONCLUSIONS

To account for inter-participant variability, outliers, coil positioning errors and, most importantly, participant comfort during data acquisition, we recommend creating a map with 80 stimuli and a 1.5 s ISI. This makes it possible to acquire TMS maps in 2 min, making mapping a more feasible tool to study short- and long-term changes in cortical organization.

Test-retest reliability of navigated transcranial magnetic stimulation of the motor cortex

BACKGROUND

Because navigated transcranial magnetic stimulation (nTMS) is increasingly used in neurosurgical research, interpretation of its results is of utmost importance.

OBJECTIVE

To evaluate the test-retest reliability of nTMS.

METHODS

Twelve healthy participants underwent nTMS at 2 different sessions separated by 10.3 ± 9.6 days. Investigated parameters included resting motor thresholds, hotspots, and centers of gravity calculated for the first dorsal interosseous, abductor pollicis brevis, extensor digitorum, tibial anterior, and abductor hallucis muscles.

RESULTS

Excellent reliability of resting motor thresholds was observed. Hotspots and centers of gravity showed moderate to excellent repeatability along the anteroposterior axis (intraclass correlation coefficient, 0.54-0.89), whereas the x coordinate presented mainly poor to moderate stability (intraclass correlation coefficient, 0.11-0.89). Movement of centers of gravity over sessions was 0.57 ± 0.32 cm, and hotspots laid 0.79 ± 0.47 cm apart. Calculation of coefficient of variation revealed high reliability of investigated parameters in upper extremities; in lower extremity muscles, high variation across sessions was observed.

CONCLUSION

nTMS can be considered a reliable tool, thus opening new fields of noninvasive investigations in neurosurgery. The results presented here should be considered in the interpretation of individual nTMS results.

Corticospinal excitability modulation during action observation

This study used the transcranial magnetic stimulation/motor evoked potential (TMS/MEP) technique to pinpoint when the automatic tendency to mirror someone else's action becomes anticipatory simulation of a complementary act. TMS was delivered to the left primary motor cortex corresponding to the hand to induce the highest level of MEP activity from the abductor digiti minimi (ADM; the muscle serving little finger abduction) as well as the first dorsal interosseus (FDI; the muscle serving index finger flexion/extension) muscles. A neuronavigation system was used to maintain the position of the TMS coil, and electromyographic (EMG) activity was recorded from the right ADM and FDI muscles. Producing original data with regard to motor resonance, the combined TMS/MEP technique has taken research on the perception-action coupling mechanism a step further. Specifically, it has answered the questions of how and when observing another person's actions produces motor facilitation in an onlooker's corresponding muscles and in what way corticospinal excitability is modulated in social contexts.

Inter- and intraobserver variability in motor mapping of the hotspot for the abductor policis brevis muscle

Background

For accuracy in navigated transcranial magnetic stimulation (nTMS), determination of the hotspot location of small hand muscles is crucial because it is the basis for the resting motor threshold (RMT) and, therefore, its spatial resolution. We investigated intra- and interobserver differences of hotspot mapping to provide evidence for the reproducibility of this method.

Ten subjects underwent nTMS motor mapping of the hotspot for the abductor pollicis brevis muscle (APB) three times. The first two sessions were performed by the same examiner; the third mapping was performed by a different examiner. Distances between the first and second mappings (intraobserver variability) and between the second and third mappings (interobserver variability) were measured.

Results

Intraobserver variability had a mean of 8.1 ± 3.3 mm (limits of agreement (LOA) 1.7 to 14.6 mm), whereas mean interobserver variability was 10.3 ± 3.3 mm (LOA 3.8 to 16.7 mm). Concerning RMT, CCC was 0.725 (95% CI: 0.276; 0.914). The mean variability in the same cortical depth was measured as 5.7 ± 3.3 mm (LOA −0.7 to 12.2 mm) for intraobserver and 9.2 ± 3.3 mm (LOA 2.7 to 15.8 mm) for interobserver examinations. When evaluating the RMT, CCC was 0.709 (95% CI: 0.244; 0.909).

Conclusions

Overall, intraobserver variability showed higher reliability than interobserver variability. Our findings show that we can achieve good reliability in hotspot determination, ranging within the calculated precision of the system.

The reliability of topographic measurements from navigated transcranial magnetic stimulation in healthy volunteers and tumor patients

BACKGROUND

Navigated transcranial magnetic stimulation (nTMS) is increasingly being used for preoperative mapping of the motor cortex. Any new technology should undergo rigorous validation before being widely adopted in routine clinical practice. The aim of this experimental study was to assess the intraexaminer and interexaminer reliability of topographic mapping with nTMS.

METHODS

nTMS mapping of the motor cortex for the first dorsal interosseous (FDI) muscle was performed by an expert and a novice examiner, twice in ten healthy volunteers and once in ten tumor patients. The distances between the centers-of-gravity and hotspots were calculated, as were coefficients of variation. This study also compared orthogonal versus variable orientation of the stimulation coil.

RESULTS

The mean (range) distance between centers-of-gravity for the expert examiner in the test-retest protocol with healthy volunteers was 4.40 (1.86-7.68) mm. The mean (range) distance between centers-of-gravity for the expert vs. novice examiner was 4.89 (2.39-9.22) mm. There were no significant differences in this result between healthy volunteers and tumor patients.

CONCLUSIONS

nTMS is sufficiently reliable for clinical use, but examiners should make efforts to minimize sources of error. The reliability of nTMS in tumor patients appears comparable to healthy subjects.

Assessing brain plasticity across the lifespan with transcranial magnetic stimulation: why, how, and what is the ultimate goal?

Sustaining brain and cognitive function across the lifespan must be one of the main biomedical goals of the twenty-first century. We need to aim to prevent neuropsychiatric diseases and, thus, to identify and remediate brain and cognitive dysfunction before clinical symptoms manifest and disability develops. The brain undergoes a complex array of changes from developmental years into old age, putatively the underpinnings of changes in cognition and behavior throughout life. A functionally “normal” brain is a changing brain, a brain whose capacity and mechanisms of change are shifting appropriately from one time-point to another in a given individual's life. Therefore, assessing the mechanisms of brain plasticity across the lifespan is critical to gain insight into an individual's brain health. Indexing brain plasticity in humans is possible with transcranial magnetic stimulation (TMS), which, in combination with neuroimaging, provides a powerful tool for exploring local cortical and brain network plasticity. Here, we review investigations to date, summarize findings, and discuss some of the challenges that need to be solved to enhance the use of TMS measures of brain plasticity across all ages. Ultimately, TMS measures of plasticity can become the foundation for a brain health index (BHI) to enable objective correlates of an individual's brain health over time, assessment across diseases and disorders, and reliable evaluation of indicators of efficacy of future preventive and therapeutic interventions.

Variability of motor cortical excitability using a novel mapping procedure

The purpose of this study was to assess the reliability of a novel TMS motor cortex mapping procedure. The procedure was designed to take less time and be more clinically useful by delivering fewer MEPS over fewer skull locations. Resting motor evoked potentials (MEPs) were recorded from the first dorsal interosseus muscle of 6 individuals over a fixed 15-point grid. Mean MEP amplitudes, map center of gravity (CoG), and stimulus-response characteristics were assessed before and after a 30-min rest session. As a novel feature, subregions of the map were analyzed for regions of highest test-retest reliability for use as a global measure of cortical excitability. Mean MEP amplitudes between sessions were highly reliable (ICC=0.90-0.92). Reproducibility of MEPs was highest along an axis approximately 45° to the nasion-inion. Stimulus-response MEP amplitudes showed moderate to high reliability (ICC 0.54-0.95). Mean CoG shift between sessions was 2.79±1.2mm. This mapping procedure is reliable and allows efficient assessment of motor cortex excitability.

Motor cortex evaluation by nTMS after surgery of central region tumors: a feasibility study

BACKGROUND

Largely discussed during the past decade, motor cortex reorganization in brain tumor surgery has been investigated only by few studies. We therefore aimed to investigate cortical motor representation after resection of perirolandic WHO grade II and III gliomas using navigated transcranial magnetic stimulation (nTMS).

METHODS

Five patients were examined before neurosurgery and after a follow-up period of 17.7 ± 6.8 months. As a control, five healthy age-matched subjects were equally studied by nTMS in two sessions spaced 12.6 (range 2-35) days apart. Resting motor thresholds (RMT), hotspots and centers of gravity (CoG) were identified for the first dorsal interosseous (FDI), abductor pollicis brevis (APB), extensor digitorum (EXT), tibialis anterior (TA) and abductor hallucis (AH) muscles. Euclidian distances, coefficients of variance and intraclass correlation coefficients (ICC) were calculated.

RESULTS

Healthy subjects showed moderate to excellent reliability measurement of RMT (ICC = 0.69-0.94). Average displacement of CoGs across sessions was 0.68 ± 0.34 cm in the dominant and 0.76 ± 0.38 cm in the non-dominant hemisphere; hotspots moved 0.87 ± 0.51 cm and 0.83 ± 0.45 cm, respectively. In one patient these parameters differed significantly from the control group (p < 0.05 for both CoGs and hotspots). Overall, all patients' CoGs moved 1.12 ± 0.93 cm, and hotspots were 1.06 ± 0.7 cm apart. In both patients and healthy subjects, movement of assessed parameters was more important along the X- than the Y-axis.

CONCLUSIONS

nTMS allows evaluating cortical reorganization after brain tumor surgery. It may contribute to the understanding of neurofunctional dynamics, thus influencing therapeutic strategy.

Motor map reliability and aging: a TMS/fMRI study

This study compared the reliability of motor maps over 3 sessions from both neuronavigated transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) data between younger and older adults. Seven younger (ages 19-31) and seven older (ages 64-76) adults participated in three joint TMS/fMRI assessment sessions separated by 7 or 14 days. Sessions involved mapping of the right first dorsal interosseous muscle using single-pulse TMS immediately followed by block-design fMRI scanning involving volitional right-hand index finger to thumb oppositional squeeze. Intersession reliability of map volume, evaluated by intraclass correlation and Jaccard Coefficient between testing sessions, was more consistent for younger adults in both fMRI and TMS. A positive correlation was evidenced between fMRI and TMS map volumes and Jaccard Coefficients indicating spatial consistency across sessions between the two measures. Comparisons of map reliability between age groups showed that younger adults have more stable motor maps in both fMRI and TMS. fMRI and TMS maps show consistency across modalities. Future interpretation of motor maps should attempt to account for potential increased variability of such mapping in older age groups. Despite these age group differences in reliability, fMRI and TMS appear to offer consistent and complementary information about cortical representation of the first dorsal interosseous muscle.

Brain mapping using transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a novel brain stimulation technique that has advanced the understanding of brain physiology, and has diagnostic value as well as therapeutic potential for several neuropsychiatric disorders. The stimulation involves restricted cortical and subcortical regions, and, when used in combination with a visually guided technique, results in improved accuracy to target specific areas, which may also influence the outcome desired. This article reviews the principles underlying the mechanism of action of TMS, and discusses its use to obtain functional maps of the motor and visual cortex, including technical considerations for accuracy and reproducibility of mapping procedures.

Reliability of TMS-related measures of tibialis anterior muscle in patients with chronic stroke and healthy subjects

A lack of normative data for transcranial magnetic stimulation (TMS)-related measures of the lower limb muscles in patients with stroke prevents us from understanding whether changes in TMS-related measures are induced by treatment or are due to their variability and/or the natural evolution of the disease. The purpose of this study was to determine the reliability of three TMS-related measures: motor threshold (MT), motor evoked potential latency (MEP Lat) and MEP amplitude (MEP Amp), linked to the corticospinal control of the tibialis anterior (TA) muscle in sixteen patients with chronic stroke and in sixteen aged-matched healthy subjects. Test-retest reliability was estimated using the intraclass correlation coefficient (ICC) with its 95% confidence interval (95% CI) and standard error of measurement (SEM). In healthy subjects the reliability of all the TMS-related measures yielded an ICC≥0.75. Similar reproducibility levels were found in patients with chronic stroke, with the exception of MEP Amp on the paretic side (ICC=0.38). These results suggest that the TMS-related measures investigated are reliable both in healthy subjects and, with the exception of MEP Amp on the paretic side, in patients with chronic stroke.

Plasticity of motor cortex induced by coordination and training

OBJECTIVE

To study the modifications induced by training of a coordinated movement on the primary motor cortex (M1) maps of one proximal muscle and one distal muscle activated alone and during their co-contraction.

METHODS

Six healthy female sport students performed a 6-week training program during which they were trained in darts 3-4 times a week. At the end each subject had made more than 1200 throws. Transcranial magnetic stimulation (TMS) was used to map the proximal medial deltoid (MD) and the distal brachio-radialis (BR) muscle representations on M1. Motor evoked potentials (MEPs) amplitude and excitability curves were used to test corticomotor excitability.

RESULTS

The cortical representation areas of each muscle separately increased after training. The cortical representation and the excitability curve of the BR muscle increased during co-activation with the MD. Combining co-contraction and training produced a further enlargement of the M1 representation of the BR muscle.

CONCLUSIONS

The enlargement of the BR representation in M1 suggests the development of overlapping zones specifying functional synergies between distal and proximal muscles.

SIGNIFICANCE

Our findings support the idea that training of a coordinated movement involving several muscles and joints requires an activity-dependent coupling of cortical networks.

Re-emergence of hand-muscle representations in human motor cortex after hand allograft

The human primary motor cortex (M1) undergoes considerable reorganization in response to traumatic upper limb amputation. The representations of the preserved arm muscles expand, invading portions of M1 previously dedicated to the hand, suggesting that former hand neurons are reassigned to the control of remaining proximal upper limb muscles. Hand allograft offers a unique opportunity to study the reversibility of such long-term cortical changes. We used transcranial magnetic stimulation in patient LB, who underwent bilateral hand transplantation 3 years after a traumatic amputation, to longitudinally track both the emergence of intrinsic (from the donor) hand muscles in M1 as well as changes in the representation of stump (upper arm and forearm) muscles. The same muscles were also mapped in patient CD, the first bilateral hand allograft recipient. Newly transplanted intrinsic muscles acquired a cortical representation in LB's M1 at 10 months postgraft for the left hand and at 26 months for the right hand. The appearance of a cortical representation of transplanted hand muscles in M1 coincided with the shrinkage of stump muscle representations for the left but not for the right side. In patient CD, transcranial magnetic stimulation performed at 51 months postgraft revealed a complete set of intrinsic hand-muscle representations for the left but not the right hand. Our findings show that newly transplanted muscles can be recognized and integrated into the patient's motor cortex.

Reliability of transcranial magnetic stimulation for mapping swallowing musculature in the human motor cortex

OBJECTIVE

Although transcranial magnetic stimulation (TMS) has been widely used to study motor cortex organization and excitability, the reliability of this technique has not been thoroughly investigated. Furthermore, previous reports of TMS reliability have been restricted to upper limb musculature. We sought to determine the test-retest reliability for TMS mapping of motor representations for swallowing musculature.

METHODS

Twenty healthy volunteers were tested twice over two weeks using TMS to determine motor threshold, map area, map volume, maximal MEP site location and maximal MEP site size for the suprahyoid complex and pharyngeal musculature.

RESULTS

Good test-retest reliability was found in both swallowing muscle sites for the following test parameters: motor map area, maximal MEP site location: lateral coordinate, maximal MEP site size and motor threshold (ICC=0.76-0.98). Moderate reliability was observed for motor map volume and maximal MEP site location: anterior-posterior coordinate (ICC=0.68-0.74).

CONCLUSIONS

TMS assessments of motor representation size, location and excitability appear to be highly reproducible, although the reliability of these measures may vary according to the specific muscle under investigation.

SIGNIFICANCE

These works provide much needed psychometric data to validate the use of TMS to assess the cortical representation of swallowing musculature.

Transcranial magnetic stimulation mapping: a model based on spline interpolation

Transcranial magnetic stimulation (TMS) is a non-invasive method to investigate motor pathways and to create a map of the somatotopical organization of the motor cortex: ordinary mapping procedures requires a focal brain stimulation over different spots of the scalp and electromyographic (EMG) recording from a muscle. Finding an appropriate and a valid visual representation of collected data is a crucial step in research and clinical field to allow a relatively fast, intra- and inter-patient comparison of motor cortex mapping. Aim of this study was to develop and to validate a method to map cortical representation of an intrinsic hand muscle (abductor digiti minimi, ADM) using a two-dimensional spline interpolation of EMG peak amplitudes obtained with TMS. The interpolated model will result in a graphical colour-scaled representation of the motor cortex for the investigated muscle; fitted model was finally validated by comparing derived parameters with those directly measured to ensure the strength and reliability of the model. Ten healthy volunteers (mean age+/-S.D.: 35.3+/-4.7 years, 4 males and 6 females) were enrolled in the study. Transcranial stimulation was performed by placing a figure-of-eight coil over a predefined grid on the scalp of the subject. EMG responses were recorded from the right abductor digiti minimi (ADM): averaged EMG peak amplitudes obtained at each node were then used to perform spline interpolation and to derive other parameters like center of gravity (CoG). Arithmetical mean of all resting motor threshold at the hotspot was 50.6+/-3.4% of the maximal stimulator output. Average amplitude at the hotspot was 1.72+/-0.80 mV and its coordinates, expressed as median, were x=4.5 cm and y=0.0 cm. Mean CoG was located at x=4.86+/-0.57 cm and y=0.35+/-0.10 cm. Mean interpolated peak coordinates for ADM were xf=4.86+/-0.58 cm and yf=0.36+/-0.12 cm, while mean fitted peak amplitude was 0.87+/-0.47 mV. Results suggest how it is possible to map the primary motor cortex using two-dimensional spline interpolation of peak-to-peak amplitudes obtained by single pulse TMS delivered on several scalp positions, which will result in a smooth, easy to read, colour-scaled map. However, like other visual representation modalities, the interpolation should become complementary to traditional methods and not a substitute of a precise and accurate cortical motor mapping.

Putting the brain on the map: use of transcranial magnetic stimulation to assess and induce cortical plasticity of upper-extremity movement

The plasticity of the brain is an increasingly important topic for physical therapists interested in childhood development, learning, and repair following injury. The study of plasticity directly in the human nervous system presents numerous challenges, such as the ability to assess neuronal function in vivo because of physical impediments, such as the skull, skin, and dura. Transcranial magnetic stimulation (TMS), however, has become a suitable, noninvasive, and painless technique that can be applied to detect changes in cortical excitability as an indicator of neurological changes. Furthermore, repetitive trains of TMS themselves can induce plasticity. This article discusses the use of TMS to investigate and manipulate plasticity in the human nervous system.

Reliability of intracortical and corticomotor excitability estimates obtained from the upper extremities in chronic stroke

We estimated the trial-to-trial variability and the test-retest reliability of several intracortical and corticomotor excitability parameters for the upper extremity in chronic stroke patients. Nine patients with hemiparesis of the upper extremity were enrolled 8-17 months after a unilateral stroke. Transcranial magnetic stimulation was used to obtain repeated measures over a two week interval of motor evoked potential (MEP) recruitment curves and cortical silent periods in the first dorsal interosseus muscle of each hand. Five trials would have provided accurate estimates of the MEP amplitude and silent period duration for the unlesioned side in all patients, but 25% of the datasets from the lesioned side provided poor estimates of MEP amplitude even with 10 trials. Intraclass correlations were >0.70 for all parameters obtained from the lesioned side and for the MEP amplitude, slope of the recruitment curve, silent period, and silent period slope from the unlesioned side. MEP amplitude varied across sessions within subject by 20% on both sides, whereas other parameters showed less variability on the unlesioned side relative to the lesioned side. The Fugl-Meyer upper extremity motor score and the time to complete the 6 fine-motor items from the Wolf Motor Function Test (WMFT) were also found to be highly reliable over this interval. We conclude that the functional and most of the excitability parameters are reliable across time in patients with variable lesions due to stroke. Due to high intrasubject variability, the use of some excitability parameters as indicators of functional neuroplasticity in response to treatment may be limited to interventions with large effect sizes.

Cortical plasticity and motor activity studied with transcranial magnetic stimulation

For decades cortical representations of the parts of the body have been considered to be unchangeable. This view has changed radically during the past 20 years using new tools designed to study plasticity in the adult human brain. Transcranial magnetic stimulation (TMS) is a valuable non-invasive technique for exploring the ability of the motor cortex to change during motor skill acquisition. Results obtained with TMS in neurological patients as well as in normal subjects demonstrate that cortical plasticity is a necessity for correct adaptation to the continuously changing environment. Topographical reorganization of the motor cortex depends on the types of movements performed by the subjects. During simple training, the cortical representation is enlarged, and it returns to its initial size when the task is overlearned. These transient modifications characterize simple motor training. Motor skills in which coordination of distal and proximal muscles, precision of the task and spatio-temporal constraints are associated, has a different impact on cortical reorganization. We propose that years of practice of a complex motor skill induces a new cortical topography that must be interpreted as structural plasticity which provides the capacity to execute a plastic behaviour instead of a stereotypical movement. We review the neuronal mechanisms underlying plasticity in different types of movement. We stress new emerging notions, such as overlap of cortical maps, and system dynamics at single neuron and network levels, to explain the reorganization of movement representations that encode motor skill. Dendritic arborizations as functional computing elements, newly generated neurons in adult brain, and plastic architectures of cortical networks operating as distributed functional modules are new hypotheses for structural plasticity.

Intra- and intersubject reliability of abductor pollicis brevis muscle motor map characteristics with transcranial magnetic stimulation

OBJECTIVE

To determine consistency in motor maps of the abductor pollicis brevis (APB) muscle using transcranial magnetic stimulation (TMS) with conventional and close-spaced surface electrode recording placements.

DESIGN

Nonrandom convenience sample.

SETTING

TMS laboratory.

PARTICIPANTS

Eleven participants without any known neurologic disorders.

INTERVENTIONS

The left hemisphere of right-hand-dominant subjects was mapped using TMS at each of 2 sessions, separated by 1 to 8 weeks.

MAIN OUTCOME MEASURES

Measurements at each session included the resting motor threshold, active site locations and average motor response amplitude for active sites, normalized map volume, and center of gravity (COG) for both electrode array configurations.

RESULTS

The normalized map volume was larger for the conventional electrode placement than for the close electrode placement (4.23 cm2 vs 3.44 cm2, P=.010). Resting motor threshold was higher for the close-spaced than for the conventional-spaced electrodes (48.36% vs 46.59%, P=.028). No statistical difference was found between sessions and within electrode placements for the normalized map volume, for resting motor threshold, or for the mean motor amplitude of the most active site for each map. No statistical difference was observed for the COG distance between electrode placements.

CONCLUSIONS

This study revealed differences between close-spaced and conventional-spaced surface electrode placement for TMS motor map volumes and for resting motor thresholds. The motor maps for the APB muscle were reliably reproducible between sessions for each electrode array.

Contemporary linkages between EMG, kinetics and stroke rehabilitation

EMG and kinetic measures have been primary tools in the study of movement and have provided the foundation for much of the work presented in this journal. Recently, novel ways of combining these tools have provided opportunities to examine elements of motor learning and brain plasticity. This presentation reviews the quantification of EMG within the context of transcranial magnetic stimulation. This vehicle permits acquisition of measures that are fundamental to examining prospects for cortical reorganization among patients with stroke and employs a therapeutic approach called "constraint induced therapy" as a model to demonstrate the interpretation of changes in EMG measures among patients with stroke. Moreover, interfacing novel uses of kinetic measurements during functional task performances is highlighted to illustrate how EMG and kinetics can provide further insight into mechanisms related to reacquisition of movement and concomitant changes in plasticity. Clinicians and researchers interested in expanding their use of these measurement tools are encouraged to learn more about application possibilities.

Finger extensor variability in TMS parameters among chronic stroke patients

BACKGROUND

This study determined the reliability of topographic motor cortical maps and MEP characteristics in the extensor digitorum communis (EDC) evoked by single-pulse TMS among patients with chronic stroke.

METHODS

Each of ten patients was studied on three occasions. Measures included location of the EDC hotspot and center of gravity (COG), threshold of activation and average amplitude of the hotspot, number of active sites, map volume, and recruitment curve (RC) slope.

RESULTS

Consistent intrahemispheric measurements were obtained for the three TMS mapping sessions for all measured variables. No statistically significant difference was observed between hemispheres for the number of active sites, COG distance or the RC slope. The magnitude and range of COG movement between sessions were similar to those reported previously with this muscle in able-bodied individuals. The average COG movement over three sessions in both hemispheres was 0.90 cm. The average COG movement in the affected hemisphere was 1.13 (+/- 0.08) cm, and 0.68 (+/- 0.04) cm) for the less affected hemisphere. However, significant interhemispheric variability was seen for the average MEP amplitude, normalized map volume, and resting motor threshold.

CONCLUSION

The physiologic variability in some TMS measurements of EDC suggest that interpretation of TMS mapping data derived from hemiparetic patients in the chronic stage following stroke should be undertaken cautiously. Irrespective of the muscle, potential causes of variability should be resolved to accurately assess the impact of pharmacological or physical interventions on cortical organization as measured by TMS among patients with stroke.

The NMDA antagonist memantine affects training induced motor cortex plasticity--a study using transcranial magnetic stimulation

BACKGROUND

Training of a repetitive synchronised movement of two limb muscles leads to short-term plastic changes in the primary motor cortex, which can be assessed by transcranial magnetic stimulation (TMS) mapping. We used this paradigm to study the effect of memantine, a NDMA antagonist, on short-term motor cortex plasticity in 20 healthy human subjects, and we were especially interested in possible differential effects of different treatment regimens. In a randomised double-blinded cross over study design we therefore administered placebo or memantine either as a single dosage or as an ascending dosage over 8 days. Before and after one hour of motor training, which consisted of a repetitive co-contraction of the abductor pollicis brevis (APB) and the deltoid muscle, we assessed the motor output map of the APB muscle by TMS under the different conditions.

RESULTS

We found a significant medial shift of the APB motor output map after training in the placebo condition, indicating training-induced short-term plastic changes in the motor cortex. A single dosage of memantine had no significant effect on this training-induced plasticity, whereas memantine administered in an ascending dosage over 8 days was able to block the cortical effect of the motor training. The memantine serum levels after 8 days were markedly higher than the serum levels after a single dosage of memantine, but there was no individual correlation between the shift of the motor output map and the memantine serum level. Besides, repeated administration of a low memantine dosage also led to an effective blockade of training-induced cortical plasticity in spite of serum levels comparable to those reached after single dose administration, suggesting that the repeated administration was more important for the blocking effect than the memantine serum levels.

CONCLUSION

We conclude that the NMDA-antagonist memantine is able to block training-induced motor cortex plasticity when administered over 8 days, but not after administration of a single dose. This differential effect might be mainly due to the prolonged action of memantine at the NMDA receptor. These findings must be considered if clinical studies are designed, which aim at evaluating the potency of memantine to prevent "maladaptive" plasticity, e.g. after limb amputation.

Intra-subject reliability of parameters contributing to maps generated by transcranial magnetic stimulation in able-bodied adults

OBJECTIVE

This study evaluated the reliability of several parameters contributing to topographic motor cortical maps of the extensor digitorum communis (EDC) within able-bodied participants, across 3 sessions and from both hemispheres with greater precision than previously reported.

METHODS

Nine healthy right-handed males aged 44-75 years were studied at 3 separate sessions, spaced 7-14 days apart. Transcranial magnetic stimulation (TMS) was used to elicit motor evoked potentials (MEP) in the contralateral EDC. Closely spaced surface electrodes were used to record the MEPs.

RESULTS

TMS-related parameters did not demonstrate a significant difference within participants across sessions and between hemispheres, with the exception of the hotspot distance, center of gravity distance, and normalized map volume. Hotspot and COG distances were determined from the Euclidean equation to calculate the distance in x,y coordinates traveled over sessions: one to two (distance A) and two to three (distance B). The hotspot distance, center of gravity distance and normalized map volume demonstrated a significant difference between right and left hemispheres, within participants. Adjusting for time and examining mean changes for hemispheres across sessions revealed that there was a 9-fold greater movement over sessions in the left hemisphere among these variables.

CONCLUSIONS

TMS-related parameters are reliable within participants across 3 sessions. These data should be useful for planning and interpreting TMS studies using a healthy or patient population before and after an intervention.