MEG response to median nerve stimulation correlates with recovery of sensory and motor function after stroke

Pathological changes of brain oscillations following ischemic stroke

Brain oscillations recorded in the extracellular space are among the most important aspects of neurophysiology data reflecting the activity and function of neurons in a population or a network. The signal strength and patterns of brain oscillations can be powerful biomarkers used for disease detection and prediction of the recovery of function. Electrophysiological signals can also serve as an index for many cutting-edge technologies aiming to interface between the nervous system and neuroprosthetic devices and to monitor the efficacy of boosting neural activity. In this review, we provided an overview of the basic knowledge regarding local field potential, electro- or magneto- encephalography signals, and their biological relevance, followed by a summary of the findings reported in various clinical and experimental stroke studies. We reviewed evidence of stroke-induced changes in hippocampal oscillations and disruption of communication between brain networks as potential mechanisms underlying post-stroke cognitive dysfunction. We also discussed the promise of brain stimulation in promoting post stroke functional recovery via restoring neural activity and enhancing brain plasticity.

Bilateral Transcutaneous Electrical Nerve Stimulation Improves Upper Limb Motor Recovery in Stroke: A Randomized Controlled Trial

BACKGROUND AND PURPOSE

Recent evidence has shown bilateral transcutaneous electrical nerve stimulation (Bi-TENS) combined with task-oriented training (TOT) to be superior to unilateral transcutaneous electrical nerve stimulation (Uni-TENS)+TOT in improving lower limb motor functioning following stroke. However, no research explored the effect of Bi-TENS+TOT in improving upper limb motor recovery. This study aimed to compare Bi-TENS+TOT with Uni-TENS+TOT, Placebo transcutaneous electrical nerve stimulation (Placebo-TENS)+TOT, and no treatment (Control) groups in upper limb motor recovery.

METHODS

This is a 4-group parallel design. One hundred and twenty subjects were given either Bi-TENS+TOT, Uni-TENS+TOT, Placebo-TENS+TOT, or Control without treatment in this randomized controlled trial. Twenty 60-minute sessions were administered 3× per week for 7 weeks. The outcome measure was the Fugl-Meyer Assessment of Upper Extremity, which was assessed at baseline, after 10 sessions (mid-intervention) and 20 sessions (post-intervention) of intervention, and at 1- and 3-month follow-up.

RESULTS

Patients in the Bi-TENS+TOT group showed greater improvement in the Fugl-Meyer Assessment of Upper Extremity scores than Uni-TENS+TOT (mean difference, 2.13; P=0.004), Placebo-TENS+TOT (mean difference, 2.63; P<0.001), and Control groups (mean difference, 3.11; P<0.001) at post-intervention. Both Bi-TENS+TOT (mean difference, 3.39; P<0.001) and Uni-TENS+TOT (mean difference, 1.26; P=0.018) showed significant within-group improvement in the Fugl-Meyer Assessment of Upper Extremity scores. Patients in the Bi-TENS+TOT group showed earlier within-group improvement in the Fugl-Meyer Assessment of Upper Extremity scores at mid-intervention than Uni-TENS+TOT. These improvements were maintained at the 3-month follow-up assessment.

CONCLUSIONS

Bi-TENS combined with TOT is an effective therapy for improving upper limb motor recovery following stroke. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03112473.

Magnetoencephalography: physics, techniques, and applications in the basic and clinical neurosciences

Magnetoencephalography (MEG) is a technique used to measure the magnetic fields generated from neuronal activity in the brain. MEG has a high temporal resolution on the order of milliseconds and provides a more direct measure of brain activity when compared with hemodynamic-based neuroimaging methods such as magnetic resonance imaging and positron emission tomography. The current review focuses on basic features of MEG such as the instrumentation and the physics that are integral to the signals that can be measured, and the principles of source localization techniques, particularly the physics of beamforming and the techniques that are used to localize the signal of interest. In addition, we review several metrics that can be used to assess functional coupling in MEG and describe the advantages and disadvantages of each approach. Lastly, we discuss the current and future applications of MEG.

Opposite modulations of corticospinal excitability by intermittent and continuous peripheral electrical stimulation in healthy subjects

Peripheral electrical stimulation (PES) modulates the excitability of the corticospinal tract (CST). This modulation of CST excitability depends on the PES intensity, defined by the amplitude and the width of each pulse, the total pulse number, the stimulation frequency, and the intervention duration. Another key PES parameter is the stimulation pattern; little is known about how PES pattern affects CST excitability, as previous studies did not control other PES parameters. Here, we investigated the effect of the net difference in PES pattern on CST excitability. We use three controlled PESs, intermittent PES (30 Hz) (stimulation trains at 30 Hz with pauses), continuous PES (12 Hz) (constant stimulation at 12 Hz without pauses), and continuous PES (30 Hz) with the same stimulation frequency as the intermittent PES (30 Hz), to compare the effect of the stimulation frequency. The motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs) of healthy subjects were recorded before and after these three types of PESs in separate sessions. We found that intermittent PES (30 Hz) increased MEP amplitudes, whereas continuous PES (12 and 30 Hz) decreased amplitudes. A significant change in subcortical SEP component occurred during continuous PES (12 and 30 Hz), but not intermittent PES (30 Hz), whereas cortical SEP components showed similar behavior in three types of PESs. We conclude that (1) opposing modulations of CST excitability were induced by the differences in the PES pattern, and (2) these modulations appear to be mediated through different processes in the sensorimotor system. Our findings suggest the possibility that it may be preferable to select the PES pattern in therapeutic interventions based on the putative desired effect and the neural structure being targeted.

Biomarkers of Stroke Recovery: Consensus-Based Core Recommendations from the Stroke Recovery and Rehabilitation Roundtable

The most difficult clinical questions in stroke rehabilitation are "What is this patient's potential for recovery?" and "What is the best rehabilitation strategy for this person, given her/his clinical profile?" Without answers to these questions, clinicians struggle to make decisions regarding the content and focus of therapy, and researchers design studies that inadvertently mix participants who have a high likelihood of responding with those who do not. Developing and implementing biomarkers that distinguish patient subgroups will help address these issues and unravel the factors important to the recovery process. The goal of the present paper is to provide a consensus statement regarding the current state of the evidence for stroke recovery biomarkers. Biomarkers of motor, somatosensory, cognitive and language domains across the recovery timeline post-stroke are considered; with focus on brain structure and function, and exclusion of blood markers and genetics. We provide evidence for biomarkers that are considered ready to be included in clinical trials, as well as others that are promising but not ready and so represent a developmental priority. We conclude with an example that illustrates the utility of biomarkers in recovery and rehabilitation research, demonstrating how the inclusion of a biomarker may enhance future clinical trials. In this way, we propose a way forward for when and where we can include biomarkers to advance the efficacy of the practice of, and research into, rehabilitation and recovery after stroke.

Biomarkers of stroke recovery: Consensus-based core recommendations from the Stroke Recovery and Rehabilitation Roundtable

The most difficult clinical questions in stroke rehabilitation are "What is this patient's potential for recovery?" and "What is the best rehabilitation strategy for this person, given her/his clinical profile?" Without answers to these questions, clinicians struggle to make decisions regarding the content and focus of therapy, and researchers design studies that inadvertently mix participants who have a high likelihood of responding with those who do not. Developing and implementing biomarkers that distinguish patient subgroups will help address these issues and unravel the factors important to the recovery process. The goal of the present paper is to provide a consensus statement regarding the current state of the evidence for stroke recovery biomarkers. Biomarkers of motor, somatosensory, cognitive and language domains across the recovery timeline post-stroke are considered; with focus on brain structure and function, and exclusion of blood markers and genetics. We provide evidence for biomarkers that are considered ready to be included in clinical trials, as well as others that are promising but not ready and so represent a developmental priority. We conclude with an example that illustrates the utility of biomarkers in recovery and rehabilitation research, demonstrating how the inclusion of a biomarker may enhance future clinical trials. In this way, we propose a way forward for when and where we can include biomarkers to advance the efficacy of the practice of, and research into, rehabilitation and recovery after stroke.

Disentangling Somatosensory Evoked Potentials of the Fingers: Limitations and Clinical Potential

In searching for clinical biomarkers of the somatosensory function, we studied reproducibility of somatosensory potentials (SEP) evoked by finger stimulation in healthy subjects. SEPs induced by electrical stimulation and especially after median nerve stimulation is a method widely used in the literature. It is unclear, however, if the EEG recordings after finger stimulation are reproducible within the same subject. We tested in five healthy subjects the consistency and reproducibility of responses through bootstrapping as well as test–retest recordings. We further evaluated the possibility to discriminate activity of different fingers both at electrode and at source level. The lack of consistency and reproducibility suggest responses to finger stimulation to be unreliable, even with reasonably high signal-to-noise ratio and adequate number of trials. At sources level, somatotopic arrangement of the fingers representation was only found in one of the subjects. Although finding distinct locations of the different fingers activation was possible, our protocol did not allow for non-overlapping dipole representations of the fingers. We conclude that despite its theoretical advantages, we cannot recommend the use of somatosensory potentials evoked by finger stimulation to extract clinical biomarkers.

The sensory side of post-stroke motor rehabilitation

Contemporary strategies to promote motor recovery following stroke focus on repetitive voluntary movements. Although successful movement relies on efficient sensorimotor integration, functional outcomes often bias motor therapy toward motor-related impairments such as weakness, spasticity and synergies; sensory therapy and reintegration is implied, but seldom targeted. However, the planning and execution of voluntary movement requires that the brain extracts sensory information regarding body position and predicts future positions, by integrating a variety of sensory inputs with ongoing and planned motor activity. Neurological patients who have lost one or more of their senses may show profoundly affected motor functions, even if muscle strength remains unaffected. Following stroke, motor recovery can be dictated by the degree of sensory disruption. Consequently, a thorough account of sensory function might be both prognostic and prescriptive in neurorehabilitation. This review outlines the key sensory components of human voluntary movement, describes how sensory disruption can influence prognosis and expected outcomes in stroke patients, reports on current sensory-based approaches in post-stroke motor rehabilitation, and makes recommendations for optimizing rehabilitation programs based on sensory stimulation.

Quantification of task-dependent cortical activation evoked by robotic continuous wrist joint manipulation in chronic hemiparetic stroke

Background

Cortical damage after stroke can drastically impair sensory and motor function of the upper limb, affecting the execution of activities of daily living and quality of life. Motor impairment after stroke has been thoroughly studied, however sensory impairment and its relation to movement control has received less attention. Integrity of the somatosensory system is essential for feedback control of human movement, and compromised integrity due to stroke has been linked to sensory impairment.

Methods

The goal of this study is to assess the integrity of the somatosensory system in individuals with chronic hemiparetic stroke with different levels of sensory impairment, through a combination of robotic joint manipulation and high-density electroencephalogram (EEG). A robotic wrist manipulator applied continuous periodic disturbances to the affected limb, providing somatosensory (proprioceptive and tactile) stimulation while challenging task execution. The integrity of the somatosensory system was evaluated during passive and active tasks, defined as ‘relaxed wrist’ and ‘maintaining 20% maximum wrist flexion’, respectively. The evoked cortical responses in the EEG were quantified using the power in the averaged responses and their signal-to-noise ratio.

Results

Thirty individuals with chronic hemiparetic stroke and ten unimpaired individuals without stroke participated in this study. Participants with stroke were classified as having severe, mild, or no sensory impairment, based on the Erasmus modification of the Nottingham Sensory Assessment. Under passive conditions, wrist manipulation resulted in contralateral cortical responses in unimpaired and chronic stroke participants with mild and no sensory impairment. In participants with severe sensory impairment the cortical responses were strongly reduced in amplitude, which related to anatomical damage. Under active conditions, participants with mild sensory impairment showed reduced responses compared to the passive condition, whereas unimpaired and chronic stroke participants without sensory impairment did not show this reduction.

Conclusions

Robotic continuous joint manipulation allows studying somatosensory cortical evoked responses during the execution of meaningful upper limb control tasks. Using such an approach it is possible to quantitatively assess the integrity of sensory pathways; in the context of movement control this provides additional information required to develop more effective neurorehabilitation therapies.

Development of advanced signal processing and source imaging methods for superparamagnetic relaxometry

Superparamagnetic relaxometry (SPMR) is a highly sensitive technique for the in vivo detection of tumor cells and may improve early stage detection of cancers. SPMR employs superparamagnetic iron oxide nanoparticles (SPION). After a brief magnetizing pulse is used to align the SPION, SPMR measures the time decay of SPION using super-conducting quantum interference device (SQUID) sensors. Substantial research has been carried out in developing the SQUID hardware and in improving the properties of the SPION. However, little research has been done in the pre-processing of sensor signals and post-processing source modeling in SPMR. In the present study, we illustrate new pre-processing tools that were developed to: (1) remove trials contaminated with artifacts, (2) evaluate and ensure that a single decay process associated with bounded SPION exists in the data, (3) automatically detect and correct flux jumps, and (4) accurately fit the sensor signals with different decay models. Furthermore, we developed an automated approach based on multi-start dipole imaging technique to obtain the locations and magnitudes of multiple magnetic sources, without initial guesses from the users. A regularization process was implemented to solve the ambiguity issue related to the SPMR source variables. A procedure based on reduced chi-square cost-function was introduced to objectively obtain the adequate number of dipoles that describe the data. The new pre-processing tools and multi-start source imaging approach have been successfully evaluated using phantom data. In conclusion, these tools and multi-start source modeling approach substantially enhance the accuracy and sensitivity in detecting and localizing sources from the SPMR signals. Furthermore, multi-start approach with regularization provided robust and accurate solutions for a poor SNR condition similar to the SPMR detection sensitivity in the order of 1000 cells. We believe such algorithms will help establishing the industrial standards for SPMR when applying the technique in pre-clinical and clinical settings.

Investigating the Effects of Peripheral Electrical Stimulation on Corticomuscular Functional Connectivity Stroke Survivors

BACKGROUND

Electrical stimulation (ES) in the periphery can induce brain plasticity and has been used clinically to promote motor recovery in patients with central nervous system lesion. Electroencephalogram (EEG) and electromyogram (EMG) are readily applicable in clinical settings and can detect real-time functional connectivity between motor cortex and muscles with EEG-EMG (corticomuscular) coherence.

OBJECTIVE

The purpose of this study was to determine whether EEG-EMG coherence can detect changes in corticomuscular control induced by peripheral ES.

METHODS

Fifteen healthy young adults and 15 stroke survivors received 40-min electrical stimulation session on median nerve. The stimulation (1-ms rectangular pulse, 100 Hz) was delivered with a 20-s on-20-s off cycle, and the intensity was set at the subjects' highest tolerable level without muscle contraction or pain. Both before and after the stimulation session, subjects performed a 20-s steady-hold thumb flexion at 50% maximal voluntary contraction (MVC) while EEG and EMG were collected.

RESULTS

Our results demonstrated that after ES, EEG-EMG coherence in gamma band increased significantly for 22.1 and 48.6% in healthy adults and stroke survivors, respectively. In addition, after ES, force steadiness was also improved in both groups, as indicated by the decrease in force fluctuation during steady-hold contraction (-1.7% MVC and -3.9%MVC for healthy and stroke individuals, respectively).

CONCLUSIONS

Our results demonstrated that EEG-EMG coherence can detect ES-induced changes in the neuromuscular system. Also, because gamma coherence is linked to afferent inputs encoding, improvement in motor performance is likely related to ES-elicited strong sensory input and enhanced sensorimotor integration.

Neuromagnetic beta and gamma oscillations in the somatosensory cortex after music training in healthy older adults and a chronic stroke patient

OBJECTIVE

Extensive rehabilitation training can lead to functional improvement even years after a stroke. Although neuronal plasticity is considered as a main origin of such ameliorations, specific subtending mechanisms need further investigation. Our aim was to obtain objective neuromagnetic measures sensitive to brain reorganizations induced by a music-supported training.

METHODS

We applied 20-Hz vibrotactile stimuli to the index finger and the ring finger, recorded somatosensory steady-state responses with magnetoencephalography, and analyzed the cortical sources displaying oscillations synchronized with the external stimuli in two groups of healthy older adults before and after musical training or without training. In addition, we applied the same analysis for an anecdotic report of a single chronic stroke patient with hemiparetic arm and hand problems, who received music-supported therapy (MST).

RESULTS

Healthy older adults showed significant finger separation within the primary somatotopic map. Beta dipole sources were more anterior located compared to gamma sources. An anterior shift of sources and increases in synchrony between the stimuli and beta and gamma oscillations were observed selectively after music training. In the stroke patient a normalization of somatotopic organization was observed after MST, with digit separation recovered after training and stimulus induced gamma synchrony increased.

CONCLUSIONS

The proposed stimulation paradigm captures the integrity of primary somatosensory hand representation. Source position and synchronization between the stimuli and gamma activity are indices, sensitive to music-supported training. Responsiveness was also observed in a chronic stroke patient, encouraging for the music-supported therapy. Notably, changes in somatosensory responses were observed, even though the therapy did not involve specific sensory discrimination training.

SIGNIFICANCE

The proposed protocol can be used for monitoring changes in neuronal organization during training and will improve the understanding of the brain mechanisms underlying rehabilitation.

No somatotopy of sensorimotor alpha-oscillation responses to differential finger stimulation

The somatotopic layout of the primary somatosensory cortex is known for its fine spatial structure as delineated in single cell recordings and macroscopic EEG evoked responses. While a gross somatotopic layout has been revealed also for neuronal oscillations responding to sensorimotor stimulation of distant body parts (e.g. hand vs. foot), it is still unclear whether these oscillatory dynamics exhibit fine spatial layout comparable to those found in evoked responses. In twelve healthy subjects we applied electric stimuli to the first (D1) and fifth finger (D5) of the same hand while performing high-density electroencephalography. We used Common Spatial Pattern analysis to optimally extract components showing the strongest Event-Related Desynchronization (ERD) in neuronal alpha oscillations. In agreement with the previous studies, dipole locations of Somatosensory Evoked Potentials (SEPs) confirmed the existence of spatially distinct representations of each finger. In contrast, dipole locations of alpha-ERD patterns did not yield spatially different source locations, indicating that the stimulation of different fingers most likely resulted in oscillatory activity of overlapping neuronal populations. When both fingers were stimulated simultaneously the SEP dipole strength was found increased in comparison to a stimulation of either finger alone, in agreement with spatially distinct SEP to finger stimulation. The strength of ERD, on the other hand, was the same regardless of whether either one or both fingers were stimulated. Our findings might reflect anatomical constraints on the sequential temporal activation of fingers' skin where almost simultaneous activation of many fingers usually occurs in everyday activities, such as grasping or holding objects. Such simultaneity is unlikely to benefit from slow amplitude modulation of alpha oscillations, which would rather be beneficial for contrasting somatosensory processing of distinct body parts.

Changes in neuromagnetic beta-band oscillation after music-supported stroke rehabilitation

Precise timing of sound is crucial in music for both performing and listening. Indeed, listening to rhythmic sound sequences activates not only the auditory system but also the sensorimotor system. Previously, we showed the significance of neural beta-band oscillations (15-30 Hz) for the timing processing that involves such auditory-motor coordination. Thus, we hypothesized that motor rehabilitation training incorporating music playing will stimulate and enhance auditory-motor interaction in stroke patients. We examined three chronic patients who received Music-Supported Therapy following the protocols practiced by Schneider. Neuromagnetic beta-band activity was remarkably alike during passive listening to a metronome and during finger tapping, with or without the metronome, for either the paretic or nonparetic hand, suggesting a shared mechanism of the beta modulation. In the listening task, the magnitude of the beta decrease after the tone onset was more pronounced at the posttraining time point and was accompanied by improved arm and hand skills. The present case data give insight into the neural underpinnings of rehabilitation with music making and rhythmic auditory stimulation.

Somatosensory system deficits in schizophrenia revealed by MEG during a median-nerve oddball task

Although impairments related to somatosensory perception are common in schizophrenia, they have rarely been examined in functional imaging studies. In the present study, magnetoencephalography (MEG) was used to identify neural networks that support attention to somatosensory stimuli in healthy adults and abnormalities in these networks in patient with schizophrenia. A median-nerve oddball task was used to probe attention to somatosensory stimuli, and an advanced, high-resolution MEG source-imaging method was applied to assess activity throughout the brain. In nineteen healthy subjects, attention-related activation was seen in a sensorimotor network involving primary somatosensory (S1), secondary somatosensory (S2), primary motor (M1), pre-motor (PMA), and paracentral lobule (PCL) areas. A frontal-parietal-temporal "attention network", containing dorsal- and ventral-lateral prefrontal cortex (DLPFC and VLPFC), orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), superior parietal lobule (SPL), inferior parietal lobule (IPL)/supramarginal gyrus (SMG), and temporal lobe areas, was also activated. Seventeen individuals with schizophrenia showed early attention-related hyperactivations in S1 and M1 but hypo-activation in S1, S2, M1, and PMA at later latency in the sensorimotor network. Within this attention network, hypoactivation was found in SPL, DLPFC, orbitofrontal cortex, and the dorsal aspect of ACC. Hyperactivation was seen in SMG/IPL, frontal pole, and the ventral aspect of ACC in patients. These findings link attention-related somatosensory deficits to dysfunction in both sensorimotor and frontal-parietal-temporal networks in schizophrenia.

Post-stroke tactile allodynia and its modulation by vestibular stimulation: a MEG case study

BACKGROUND

There is behavioural evidence that caloric vestibular stimulation (CVS) can alleviate central pain. Several such patients have also noted that it reduces tactile allodynia, an especially ill-understood phenomenon in these patients.

AIMS OF THE STUDY

The first aim is to use magnetoencephalography (MEG) to study neural activity associated with tactile allodynia in central post-stroke pain (CPSP). The second is to assess how this would be affected, if at all, by CVS. The third is to assess the ability of the VESTAL solution for MEG to detect anterior cingulate activation.

METHODS

A 58-year-old woman with CPSP, and marked unilateral tactile allodynia, participated in a MEG study with imaging pre- and post-CVS.

RESULTS

Tactile simulation within the patient's allodynic area resulted in contralateral activation of the primary motor and anterior cingulate cortices, which had normalized 24 h post-CVS.

CONCLUSIONS

We suggest that the unexpected primary motor cortex activation in response to light touch in the allodynic area arises from inappropriate activation of a normal mechanism, which may occur when a threat to homeostasis is present, to lower motor thresholds and allow for more rapid performance of corrective actions. We propose this may be mediated by the interoceptive cortex in the dorsal posterior insula.

Interhemispheric asymmetry of primary hand representation and recovery after stroke: A MEG study

In patients affected by monohemispheric stroke in the middle cerebral artery territory, who do not regain a normal neurological function, a positive contribution to the clinical recovery seems to be made by the involvement of primary hand representation areas in the affected hemisphere (AH), excessively asymmetric to its homologous in the unaffected hemisphere (UH). We investigated primary sensory hand areas in 41 chronic patients who had improved their clinical status without reaching complete recovery. The location and strength of the first cerebral sources activated by a contralateral galvanic median nerve stimulation (M20 and M30) were evaluated in both hemispheres, together with their interhemispheric differences. The source displacement in the AH with respect to the UH was positively correlated with clinical recovery (Spearman's rho=0.584, p=0.003). The excessive interhemispheric asymmetry - as defined on the basis of reference ranges in the healthy population - could be interpreted as the involvement of neuronal pools in the AH outside the hand 'omega zone' of the Rolandic sulcus, revealing the presence of plasticity phenomena. The present data provide support to a positive role of cerebral plasticity phenomena, partially contributing to post-stroke recovery in patients unable to achieve normal neurological function.

High-resolution magnetoencephalographic functional mapping of the cortical network mediating intentional movement

Magnetoencephalography (MEG) is a sensitive technique that can detect and map cortical electrophysiologic activations with high spatial (mm) and temporal (msecs) resolutions. We used 148-channel whole-head MEG to record the activation sequence for the somatosensory and motor cortical network during cued hand movements in a healthy 39-yr-old subject. The complex sequence and topography of cortical activations were superimposed onto the subject's brain magnetic resonance images. Frontal premotor and supplementary motor and cingulate areas activated well before the primary motor area and again repetitively from 200 msecs onward with activations alternating repeatedly between frontal and parietal areas. The network's very close functional integration of supplementary motor areas suggests how brain injury that is localized to these regions, but not to the primary motor area itself, can disrupt integrity of movement, and why preservation of functional integrity of some areas traditionally viewed as extramotor may be necessary for recovery from neurologic disability.

Recovery from ischemic stroke: a translational research perspective for neurology

Ischemic stroke is the most frequent neurological disease, characterized by an age-related incidence and chronic disability in the majority of patients. A great challenge in acute stroke is to predict the degree to which a patient will eventually recover. Magnetic resonance imaging has revealed that treatment-induced reperfusion limits the extent of ischemic brain damage, thereby enabling rapid and profound recovery. Nevertheless, patients may retain deficits in motor, sensory or cognitive functions due to the residual lesion. Functional neuroimaging and transcranial magnetic stimulation have shown that recovery is associated with abnormal activation in the perilesional vicinity and in brain areas remote from the lesion. This is likely related to altered functional properties or morphological changes in both cerebral hemispheres. Recent neurorehabilitative strategies, including forced use, mental imagery and peripheral nerve or cortex stimulation, aim at modulating these functional networks. Accordingly, translational research has provided new vistas on the neurobiological mechanisms of recovery and opened future avenues for science-based pharmacological and neurophysiological training strategies in stroke.

Vector-based spatial–temporal minimum L1-norm solution for MEG

Minimum L1-norm solutions have been used by many investigators to analyze MEG responses because they provide high spatial resolution images. However, conventional minimum L1-norm approaches suffer from instability in spatial construction, and poor smoothness of the reconstructed source time-courses. Activity commonly "jumps" from one grid point to (usually) the neighboring grid points. Equivalently, the time-course of one specific grid point can show substantial "spiky-looking" discontinuity. In the present study, we present a new vector-based spatial-temporal analysis using a L1-minimum-norm (VESTAL). This approach is based on a principle of MEG physics: the magnetic waveforms in sensor-space are linear functions of the source time-courses in the imaging-space. Our computer simulations showed that VESTAL provides good reconstruction of the source amplitude and orientation, with high stability and resolution in both the spatial and temporal domains. "Spiky-looking" discontinuity was not observed in the source time-courses. Importantly, the simulations also showed that VESTAL can resolve sources that are 100% correlated. We then examined the performance of VESTAL in the analysis of human median-nerve MEG responses. The results demonstrated that this method easily distinguishes sources very spatially close to each other, including individual primary somatosensory areas (BA 1, 2, 3b), primary motor area (BA 4), and other regions in the somatosensory system (e.g., BA 5, 7, SII, SMA, and temporal-parietal junction) with high temporal stability and resolution. VESTAL's potential for obtaining information on source extent was also examined.

Brain plasticity in recovery from stroke: an MEG assessment

The aim of this paper was to deepen understanding about the role played by brain plasticity in obtaining clinical recovery. Eighteen patients, who had recovered partially or totally from dysfunctions due to a monohemispheric infarction within the middle cerebral artery territory, underwent magnetoencephalographic (MEG) recordings of rolandic areas cerebral activity both in rest state (spectral power properties) and in response to the electrical stimulation of the contralateral median nerve (M20 and M30 cortical sources). MEG evaluation was performed in acute (T0: mean 5 days from ischemic attach) and post-acute phase (T1: median 6 months). At T1, all the inter-hemispheric asymmetries were reduced for both spontaneous and evoked activity parameters with respect to T0. In post-acute phase, lower cortical excitability, higher delta and theta power and lower spectral entropy were associated to a worse clinical state. An unusual recruitment-as revealed by an excessive inter-hemispheric asymmetry of M20 cortical source position-correlated with higher level of clinical amelioration in the patients who showed a partial recovery. In addition to confirmative evidence that "normalization" of neural activity in both the affected and unaffected hemispheres subtends best clinical recovery, present data provide support to the positive role of cerebral plasticity phenomena--i.e. unusual neural recruitments--to regain lost functions in those patients unable to achieve total recovery.

Prediction on affected upper extremity function in hemiplegic patients after thalamic hemorrhage using somatosensory evoked magnetic fields

The aim of the present study was to investigate the prognostic value of somatosensory evoked magnetic fields (SEFs) at an acute stage on recovery of an affected upper extremity (UE) function as practicality in hemiplegic patients after thalamic hemorrhage. Nine hemiplegic patients after thalamic hemorrhage were enrolled in this study. Median nerve SEFs, evoked by electrical stimulation at the wrist of the affected UE, were measured using a 204 channel whole-head magnetoencephalography system within 72 hours after the onset of thalamic hemorrhage (acute stage). Assessments on the affected UE, which included the motor palsies of the UE and fingers (Brunnstrom's motor recovery stage: BS), sensory disturbance (the thumb localizing test) and UE function (the UE ability test), were performed at both the acute stage and 3 months after the onset of thalamic hemorrhage (chronic stage). Almost all the patients showing any median nerve SEF components that originated from the somatosensory cortex in the affected hemisphere and occurred between about 20 ms and 100 ms post-stimulus at the acute stage demonstrated good outcomes in the motor palsies (BSV), sensory disturbance (normal) and affected UE function (practical hand) at the chronic stage. In contrast, majority of patients not showing them at all demonstrated poor outcomes in the motor palsies (BSIII or less), sensory disturbance (severely impaired) and affected UE function (disabled hand) at the chronic stage. These results suggest that the findings of the median nerve SEFs at the acute stage would contribute to the early outcome prediction on the affected UE function in hemiplegic patients after thalamic hemorrhage.

A parietal–frontal network studied by somatosensory oddball MEG responses, and its cross-modal consistency

Previous studies using functional magnetic resonance imaging (fMRI) and event-related potentials (ERPs) of the brain have found that a distributed parietal-frontal neuronal network is activated in normals during both auditory and visual oddball tasks. The common cortical regions in this network are inferior parietal lobule (IPL)/supramarginal gyrus (SMG), anterior cingulate cortex (ACC), and dorsolateral prefrontal cortex (DLPFC). It is not clear whether the same network is activated by oddball tasks during somatosensory stimulation. The present study addressed this question by testing healthy adults as they performed a novel median-nerve oddball paradigm while undergoing magnetoencephalography (MEG). An automated multiple dipole analysis technique, the Multi-Start Spatio-Temporal (MSST) algorithm, localized multiple neuronal generators, and identified their time-courses. IPL/SMG, ACC, and DLPFC were reliably localized in the MEG median-nerve oddball responses, with IPL/SMG activation significantly preceding ACC and DLPFC activation. Thus, the same parietal-frontal neuronal network that shows activation during auditory and visual oddball tests is activated in a median-nerve oddball paradigm. Regions uniquely related to somatosensory oddball responses (e.g., primary and secondary somatosensory, dorsal premotor, primary motor, and supplementary motor areas) were also localized. Since the parietal-frontal network supports attentional allocation during performance of the task, this study may provide a novel method, as well as normative baseline data, for examining attention-related deficits in the somatosensory system of patients with neurological or psychiatric disorders.

Neural plasticity and recovery of function

Recovery of the function after stroke is a consequence of many factors including resolution of oedema and survival of the ischaemic penumbra. In addition there is a growing interest in the role of central nervous system (CNS) reorganization. Much of the evidence supporting this comes from animal models of focal brain injury, but non-invasive techniques such as functional magnetic resonance imaging, transcranial magnetic stimulation, electroencephalography and magnetoencephalography now allow the study of the working human brain. Using these techniques it is apparent that the motor system of the brain adapts to damage in a way that attempts to preserve motor function. This has been demonstrated after stroke, as part of the ageing process, and even after disruption of normal motor cortex with repetitive transcranial magnetic stimulation. The result of this reorganization is a new functional architecture, one which will vary from patient to patient depending on the anatomy of the damage, the biological age of the patient and lastly the chronicity of the lesion. The success of any given therapeutic intervention will depend on how well it interacts with this new functional architecture. Thus it is crucial that the study of novel therapeutic strategies for treating motor impairment after stroke take account of this. This review maps out the attempts to describe functionally relevant adaptive changes in the human brain following focal damage. A greater understanding of how these changes are related to the recovery process will allow not only the development of novel therapeutic techniques that are based on neurobiological principles and designed to minimize impairment in patients suffering from stroke, but also to target these therapies at the appropriate patients.

Functional reorganization of the cerebral motor system after stroke

PURPOSE OF REVIEW

Recovery of function after stroke is now widely considered to be a consequence of central nervous system reorganization. Non-invasive techniques such as functional magnetic resonance imaging, transcranial magnetic stimulation, electroencephalography and magnetoencephalography now allow the study of the working human brain. Studies in stroke patients can now address how cerebral networks in the human brain respond to focal injury and whether these changes are related to functional recovery. This understanding may in turn lead to the development of techniques that will drive cerebral reorganization in a way that promotes functional improvement.

RECENT FINDINGS

The relationship between cerebral reorganization and functional recovery has been examined in both cross-sectional and longitudinal studies. It appears that the motor system reacts to damage in a way that attempts to generate motor output through surviving brain regions and networks. There are changes in cortical excitability after stroke that may provide the substrate whereby the effects of motor practice or experience can be more effective in driving long lasting changes in motor networks. This will be particularly important in intact portions of neural networks subserving motor skills learning.

SUMMARY

Functionally relevant adaptive changes occur in the human brain following focal damage. A greater understanding of how these changes are related to the recovery process will allow the development of novel therapeutic techniques that are based on neurobiological principles and which are designed to minimize impairment in appropriately targeted patients suffering from stroke.