Effects of subthalamic nucleus (STN) stimulation on motor cortex excitability

Deep Brain Stimulation restores information processing in parkinsonian cortical networks

Parkinson's disease (PD) is a neurodegenerative disorder associated with alterations of neural activity and information processing primarily in the basal ganglia and cerebral cortex. Deep brain stimulation (DBS) of the subthalamic nucleus (STN-DBS) is the most effective therapy when patients experience levodopa-induced motor complications. A growing body of evidence points towards a cortical effect of STN-DBS, restoring key electrophysiological markers, such as excessive beta band oscillations, commonly observed in PD. However, the mechanisms of STN-DBS remain elusive. Here, we aim to better characterize the cortical substrates underlying STN-DBS-induced improvement in motor symptoms. We recorded electroencephalograms (EEG) from PD patients and found that, although apparent EEG features were not different with or without therapy, EEG signals could more accurately predict limb movements under STN-DBS. To understand the origins of this enhanced information transmission under STN-DBS in the human EEG data, we investigated the information capacity and dynamics of a variety of computational models of cortical networks. The extent of improvement in decoding accuracy of complex naturalistic inputs under STN-DBS depended on the synaptic parameters of the network as well as its excitability and synchronization levels. Additionally, decoding accuracy could be optimized by adjusting STN-DBS parameters. Altogether, this work draws a comprehensive link between known alterations in cortical activity and the degradation of information processing capacity, as well as its restoration under DBS. These results also offer new perspectives for optimizing STN-DBS parameters based on clinically accessible measures of cortical information processing capacity.

Midbrain dopaminergic degeneration differentially modulates primary motor cortex activity and motor behavior in hemi-parkinsonian rats

Parkinson's disease (PD) is marked by degeneration in the nigrostriatal dopaminergic pathway, affecting motor control via complex changes in the cortico-basal ganglia-thalamic motor network, including the primary motor cortex (M1). The modulation of M1 neuronal activity by dopaminergic inputs, particularly from the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), plays a crucial role in PD pathophysiology. This study investigates how nigrostriatal dopaminergic degeneration influences M1 neuronal activity in rats using in vivo calcium imaging. Histological analysis confirmed dopaminergic lesion severity, with high lesion level rats showing significant motor deficits. Levodopa treatment improved fine motor abilities, particularly in high lesion level rats. Analysis of M1 calcium signals based on dopaminergic lesion severity revealed distinct M1 activity patterns. Animals with low dopaminergic lesion showed increased calcium events, while high lesion level rats exhibited decreased activity, partially restored by levodopa. These findings suggest that M1 activity is more sensitive to transient fluctuations in dopaminergic transmission, rather than to chronic high or low dopaminergic signaling. This study underscores the complex interplay between dopaminergic signaling and M1 neuronal activity in PD symptoms development. Further research integrating behavioral and calcium imaging data can elucidate mechanisms underlying motor deficits and therapeutic responses in PD.

Cortical Disinhibition Drives Freezing of Gait in Parkinson's Disease and an Exploratory Repetitive Transcranial Magnetic Stimulation Study

BACKGROUND

Dysfunction of the primary motor cortex, participating in regulation of posture and gait, is implicated in freezing of gait (FOG) in Parkinson's disease (PD).

OBJECTIVE

The aim was to reveal the mechanisms of "OFF-period" FOG (OFF-FOG) and "levodopa-unresponsive" FOG (ONOFF-FOG) in PD.

METHODS

We measured the transcranial magnetic stimulation (TMS) indicators and gait parameters in 21 healthy controls (HCs), 15 PD patients with ONOFF-FOG, 15 PD patients with OFF-FOG, and 15 PD patients without FOG (Non-FOG) in "ON" and "OFF" medication conditions. Difference of TMS indicators in the four groups and two conditions and its correlations with gait parameters were explored. Additionally, we explored the effect of 10 Hz repetitive TMS on gait and TMS indicators in ONOFF-FOG patients.

RESULTS

In "OFF" condition, short interval intracortical inhibition (SICI) exhibited remarkable attenuation in FOG patients (both ONOFF-FOG and OFF-FOG) compared to Non-FOG patients and HCs. The weakening of SICI correlated with impaired gait characteristics in FOG. However, in "ON" condition, SICI in ONOFF-FOG patients reduced compared to OFF-FOG patients. Pharmacological treatment significantly improved SICI and gait in OFF-FOG patients, and high-frequency repetitive TMS distinctly improved gait in ONOFF-FOG patients, accompanied by enhanced SICI.

CONCLUSIONS

Motor cortex disinhibition, represented by decreased SICI, is related to FOG in PD. Refractory freezing in ONOFF-FOG patients correlated with the their reduced SICI insensitive to dopaminergic medication. SICI can serve as an indicator of the severity of impaired gait characteristics in FOG and reflect treatments efficacy for FOG in PD patients. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Combining Transcranial Magnetic Stimulation and Deep Brain Stimulation: Current Knowledge, Relevance and Future Perspectives

Deep brain stimulation (DBS) has emerged as an invasive neuromodulation technique for the treatment of several neurological disorders, but the mechanisms underlying its effects remain partially elusive. In this context, the application of Transcranial Magnetic Stimulation (TMS) in patients treated with DBS represents an intriguing approach to investigate the neurophysiology of cortico-basal networks. Experimental studies combining TMS and DBS that have been performed so far have mainly aimed to evaluate the effects of DBS on the cerebral cortex and thus to provide insights into DBS's mechanisms of action. The modulation of cortical excitability and plasticity by DBS is emerging as a potential contributor to its therapeutic effects. Moreover, pairing DBS and TMS stimuli could represent a method to induce cortical synaptic plasticity, the therapeutic potential of which is still unexplored. Furthermore, the advent of new DBS technologies and novel treatment targets will present new research opportunities and prospects to investigate brain networks. However, the application of the combined TMS-DBS approach is currently limited by safety concerns. In this review, we sought to present an overview of studies performed by combining TMS and DBS in neurological disorders, as well as available evidence and recommendations on the safety of their combination. Additionally, we outline perspectives for future research by highlighting knowledge gaps and possible novel applications of this approach.

Are there differences in cortical excitability between akinetic-rigid and tremor-dominant subtypes of Parkinson's disease?

OBJECTIVE

To assess by transcranial magnetic stimulation (TMS) the excitability of various cortical circuits in akinetic-rigid and tremor-dominant subtypes of Parkinson's disease (PD).

METHODS

The study included 92 patients with PD according to UK Brain Bank criteria, with akinetic-rigid (n = 64) or tremor-dominant (n = 28) subtype. Cortical excitability study, including resting and active motor thresholds (rMT and aMT), input-output curve of motor evoked potentials, contralateral and ipsilateral silent periods (cSP and iSP), short and long-interval intracortical inhibition (SICI and LICI), and intracortical facilitation (ICF) were measured. The results obtained were compared to a control group of 30 age- and sex-matched healthy subjects.

RESULTS

The patients in the tremor group had significantly lower rMT and aMT compared to controls and akinetic-rigid patients and significantly shorter iSP duration compared to akinetic-rigid patients, while iSP latency tended to be longer in akinetic-rigid patients compared to controls. There were no significant differences between the two PD subgroups regarding other cortical excitability parameters, including paired-pulse TMS parameters.

CONCLUSIONS

Only subtle differences of cortical excitability were found between patients with akinetic-rigid vs. tremor-dominant subtype of PD.

SIGNIFICANCE

The clinical heterogeneity of PD patients probably has an impact on cortical excitability measures, far beyond the akinetic-rigid versus tremor-dominant profile.

STN-DBS affects language processing differentially in Parkinson's disease: Multiple-case MEG study

OBJECTIVES

In this study, we investigated the effects of bilateral and unilateral deep brain stimulation of the subthalamic nucleus (STN-DBS) in PD patients on neural responses associated with two aspects of spoken language processing: semantics of action-related verbs and morphosyntactic processing.

MATERIALS AND METHODS

Using a passive unattended paradigm to present spoken linguistic stimuli, we recorded magnetoencephalographic (MEG) responses in three PD patients in four DBS conditions: left unilateral STN-DBS, right unilateral STN-DBS, bilateral STN-DBS, and no STN-DBS. To ensure that any observed effects of DBS on the neuromagnetic responses could be attributed to the linguistic context per se and were not merely induced by the electrical stimulation, we assessed the effects of STN-DBS on linguistic contrasts within each stimulation condition. Hence, we contrasted the processing of action vs. abstract verbs as well as the processing of correct vs. incorrect morphosyntactic inflections within each DBS condition.

RESULTS

The results revealed that, compared to the DBS-off state, both bilateral and right unilateral stimulation of the STN yielded significant dissociations in the processing of action and abstract verbs, with greater neuromagnetic responses for action verbs compared to abstract verbs. For morphosyntax processing, only left unilateral stimulation yielded significant dissociations (relative to the DBS-off state), with greater neuromagnetic responses to the incorrect inflections compared to the correct inflections.

CONCLUSION

The results reflect differential effects of unilateral and bilateral STN-DBS on neuromagnetic responses associated with the processing of spoken language. They suggest that different specific aspects of linguistic information processing in PD are affected differently by STN-DBS.

Impaired motor cortical facilitatory-inhibitory circuit interaction in Parkinson's disease

OBJECTIVE

Motor cortical (M1) inhibition and facilitation can be studied with short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF). These circuits are altered in Parkinson's disease (PD). The sensorimotor measure short latency afferent inhibition (SAI) is possibly altered in PD. The aim was to determine if the manner in which these circuits interact with each other is abnormal in PD.

METHODS

Fifteen PD patients were studied at rest in ON and OFF medication states, and were compared to 16 age-matched controls. A triple-stimulus transcranial magnetic stimulation paradigm was used to elicit a circuit of interest in the presence of another circuit.

RESULTS

SICF was increased in PD OFF and PD ON conditions compared to controls. SICI facilitated SICF in controls and PD ON, but not in PD OFF. SICF in the presence of SICI negatively correlated with UPDRS-III scores in OFF and ON medication conditions. SAI showed similar inhibition of SICI in controls, PD OFF and PD ON conditions.

CONCLUSIONS

The facilitatory effect of SICI on SICF is absent in PD OFF, but is restored with dopaminergic medication.

SIGNIFICANCE

Impaired interaction between M1 circuits is a pathophysiological feature of PD.

Effects of Deep Brain Stimulation and Dopaminergic Medication on Excitatory and Inhibitory Spinal Pathways in Parkinson Disease

PURPOSE

Abnormal activity within the corticospinal system is believed to contribute to the motor dysfunction associated with Parkinson disease. However, the effect of treatment for parkinsonian motor symptoms on dysfunctional descending input to the motor neuron pool remains unclear.

METHODS

We recruited nine patients with PD treated with deep brain stimulation and examined the time course of interaction between a conditioning pulse from transcranial magnetic stimulation and the soleus H-reflex. Patients with Parkinson disease were examined under four treatment conditions and compared with 10 age-matched control subjects.

RESULTS

In healthy controls, transcranial magnetic stimulation conditioning led to early inhibition of the H-reflex (76.2% ± 6.3%) at a condition-test interval of -2 ms. This early inhibition was absent when patients were OFF medication/OFF stimulation (132.5% ± 20.4%; P > 0.05) but was maximally restored toward control levels ON medication/ON stimulation (80.3% ± 7.0%). Of note, early inhibition ON medication/ON stimulation tended to be stronger than when medication (85.4% ± 5.9%) or deep brain stimulation (95.7% ± 9.4%) were applied separately. Late facilitation was observed in controls at condition-test intervals ≥5 ms but was significantly reduced (by 50% to 80% of controls) in Parkinson disease OFF stimulation at condition-test intervals ≥15 ms. The late facilitation was akin to control subjects when patients were ON stimulation.

CONCLUSIONS

The present pilot study demonstrates that the recruitment of early inhibition and late facilitation is disrupted in untreated Parkinson disease and that medication and deep brain stimulation may act together to normalize supraspinal drive to the motor neuron pool.

On the Use of TMS to Investigate the Pathophysiology of Neurodegenerative Diseases

Neurodegenerative diseases are a collection of disorders that result in the progressive degeneration and death of neurons. They are clinically heterogenous and can present as deficits in movement, cognition, executive function, memory, visuospatial awareness and language. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation tool that allows for the assessment of cortical function in vivo. We review how TMS has been used for the investigation of three neurodegenerative diseases that differ in their neuroanatomical axes: (1) Motor cortex-corticospinal tract (motor neuron diseases), (2) Non-motor cortical areas (dementias), and (3) Subcortical structures (parkinsonisms). We also make four recommendations that we hope will benefit the use of TMS in neurodegenerative diseases. Firstly, TMS has traditionally been limited by the lack of an objective output and so has been confined to stimulation of the motor cortex; this limitation can be overcome by the use of concurrent neuroimaging methods such as EEG. Given that neurodegenerative diseases progress over time, TMS measures should aim to track longitudinal changes, especially when the aim of the study is to look at disease progression and symptomatology. The lack of gold-standard diagnostic confirmation undermines the validity of findings in clinical populations. Consequently, diagnostic certainty should be maximized through a variety of methods including multiple, independent clinical assessments, imaging and fluids biomarkers, and post-mortem pathological confirmation where possible. There is great interest in understanding the mechanisms by which symptoms arise in neurodegenerative disorders. However, TMS assessments in patients are usually carried out during resting conditions, when the brain network engaged during these symptoms is not expressed. Rather, a context-appropriate form of TMS would be more suitable in probing the physiology driving clinical symptoms. In all, we hope that the recommendations made here will help to further understand the pathophysiology of neurodegenerative diseases.

Motor cortical circuits in Parkinson disease and dystonia

We review the motor cortical and basal ganglia involvement in two important movement disorders: Parkinson's disease (PD) and dystonia. Single and paired pulse transcranial magnetic stimulation studies showed altered excitability and cortical circuits in PD with decreased silent period, short interval intracortical inhibition, intracortical facilitation, long afferent inhibition, interhemispheric inhibition, and cerebellar inhibition, and increased long interval intracortical inhibition and short interval intracortical facilitation. In dystonia, there is decreased silent period, short interval intracortical inhibition, long afferent inhibition, interhemispheric inhibition, and increased intracortical facilitation. Plasticity induction protocols revealed deficient plasticity in PD and normal and exaggerated plasticity in dystonia. In the basal ganglia, there is increased β (14-30Hz) rhythm in PD and characteristic 5-18Hz band synchronization in dystonia. These motor cortical circuits, cortical plasticity, and oscillation profiles of the basal ganglia are altered with medications and deep brain stimulation treatment. There is considerable variability in these measures related to interindividual variations, different disease characteristics, and methodological considerations. Nevertheless, these pathophysiologic studies have expanded our knowledge of cortical excitability, plasticity, and oscillations in PD and dystonia, improved our understanding of disease pathophysiology, and helped to develop new treatments for these conditions.

Deep brain stimulation for stroke: Current uses and future directions

BACKGROUND

Survivors of stroke often experience significant disability and impaired quality of life related to ongoing maladaptive responses and persistent neurologic deficits. Novel therapeutic options are urgently needed to augment current approaches. One way to promote recovery and ameliorate symptoms may be to electrically stimulate the surviving brain. Various forms of brain stimulation have been investigated for use in stroke, including deep brain stimulation (DBS).

OBJECTIVE/METHODS

We conducted a comprehensive literature review in order to 1) review the use of DBS to treat post-stroke maladaptive responses including pain, dystonia, dyskinesias, and tremor and 2) assess the use and potential utility of DBS for enhancing plasticity and recovery from post-stroke neurologic deficits.

RESULTS/CONCLUSIONS

A large variety of brain structures have been targeted in post-stroke patients, including motor thalamus, sensory thalamus, basal ganglia nuclei, internal capsule, and periventricular/periaqueductal grey. Overall, the reviewed clinical literature suggests a role for DBS in the management of several post-stroke maladaptive responses. More limited evidence was identified regarding DBS for post-stroke motor deficits, although existing work tentatively suggests DBS-particularly DBS targeting the posterior limb of the internal capsule-may improve paresis in certain circumstances. Substantial future work is required both to establish optimal targets and parameters for treatment of maladapative responses and to further investigate the effectiveness of DBS for post-stroke paresis.

Altered somatosensory cortex neuronal activity in a rat model of Parkinson's disease and levodopa-induced dyskinesias

Several findings support the concept that sensorimotor integration is disturbed in Parkinson's disease (PD) and in levodopa-induced dyskinesias. In this study, we explored the neuronal firing activity of excitatory pyramidal cells and inhibitory interneurons in the forelimb region of the primary somatosensory cortex (S1FL-Ctx), along with its interaction with oscillatory activity of the primary motor cortex (MCtx) in 6-hydroxydopamine lesioned hemiparkinsonian (HP) and levodopa-primed dyskinetic (HP-LID) rats as compared to controls under urethane (1.4g/kg, i.p.) anesthesia. Further, gene expression patterns of distinct markers for inhibitory GABAergic neurons were analyzed in both cortical regions. While firing frequency and burst activity of S1FL-Ctx inhibitory interneurons were reduced in HP and HP-LID rats, measures of irregularity were enhanced in pyramidal cells. Further, enhanced coherence of distinct frequency bands of the theta/alpha, high-beta, and gamma frequency, together with enhanced synchronization of putative pyramidal cells and interneurons with MCtx oscillatory activity were observed. While GABA level was similar, gene expression levels of interneuron and GABAergic markers in S1FL-Ctx and MCtx of HP-LID rats differed to some extent. Our study shows that in a rat model of PD with dyskinesias, neuronal activity in putative interneurons was reduced, which was accompanied by high beta and gamma coherence between S1FL-Ctx and MCtx, together with changes in gene expression, indicating maladaptive neuroplasticity after long term levodopa treatment.

Subthalamic stimulation and levodopa modulate cortical reactivity in Parkinson's patients

BACKGROUND

The effects of deep brain stimulation of the subthalamic nucleus (DBS-STN) and L-dopa (LD) on cortical activity in Parkinson's disease (PD) are poorly understood.

OBJECTIVES

By combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG) we explored the effects of STN-DBS, either alone or in combination with L-Dopa (LD), on TMS-evoked cortical activity in a sample of implanted PD patients.

METHODS

PD patients were tested in three clinical conditions: i) LD therapy with STN-DBS turned on (ON/ON condition); ii) without LD therapy with STN-DBS turned on (OFF/ON condition); iii) without LD therapy with STN-DBS turned off (OFF/OFF condition). TMS pulses were delivered over left M1 while simultaneously acquiring EEG. Eight age-matched healthy volunteers (HC) were tested as a control group.

RESULTS

STN-DBS enhanced early global TMS-evoked activity (∼45-80ms) and high-alpha TMS-evoked oscillations (11-13 Hz) as compared to OFF/OFF condition, independently from concomitant LD therapy. LD intake (ON/ON condition) produced a further increase of late TMS-evoked activity (∼80-130ms) and beta TMS-evoked oscillations (13-30 Hz), as compared to OFF/OFF and OFF/ON conditions, that normalized reactivity as compared to HC range of values.

CONCLUSIONS

Our data reveal that bilateral STN-DBS and LD therapy induce a modulation of specific cortical components and specific ranges of frequency. These findings demonstrate that STN-DBS and LD therapy may have synergistic effects on motor cortical activity.

Effects of movement imitation training in Parkinson's disease: A virtual reality pilot study

BACKGROUND

Hypometria is a clinical motor sign in Parkinson's disease. Its origin likely emerges from basal ganglia dysfunction, leading to an impaired control of inhibitory intracortical motor circuits. Some neurorehabilitation approaches include movement imitation training; besides the effects of motor practice, there might be a benefit due to observation and imitation of un-altered movement patterns. In this sense, virtual reality facilitates the process by customizing motor-patterns to be observed and imitated.

OBJECTIVE

To evaluate the effect of a motor-imitation therapy focused on hypometria in Parkinson's disease using virtual reality.

METHODS

We carried out a randomized controlled pilot-study. Sixteen patients were randomly assigned in experimental and control groups. Groups underwent 4-weeks of training based on finger-tapping with the dominant hand, in which imitation was the differential factor (only the experimental group imitated). We evaluated self-paced movement features and cortico-spinal excitability (recruitment curves and silent periods in both hemispheres) before, immediately after, and two weeks after the training period.

RESULTS

Movement amplitude increased significantly after the therapy in the experimental group for the trained and un-trained hands. Motor thresholds and silent periods evaluated with transcranial magnetic stimulation were differently modified by training in the two groups; although the changes in the input-output recruitment were similar.

CONCLUSIONS

This pilot study suggests that movement imitation therapy enhances the effect of motor practice in patients with Parkinson's disease; imitation-training might be helpful for reducing hypometria in these patients. These results must be clarified in future larger trials.

Motor and Nonmotor Circuitry Activation Induced by Subthalamic Nucleus Deep Brain Stimulation in Patients With Parkinson Disease: Intraoperative Functional Magnetic Resonance Imaging for Deep Brain Stimulation

OBJECTIVE

To test the hypothesis suggested by previous studies that subthalamic nucleus (STN) deep brain stimulation (DBS) in patients with Parkinson disease would affect the activity of motor and nonmotor networks, we applied intraoperative functional magnetic resonance imaging (fMRI) to patients receiving DBS.

PATIENTS AND METHODS

Ten patients receiving STN DBS for Parkinson disease underwent intraoperative 1.5-T fMRI during high-frequency stimulation delivered via an external pulse generator. The study was conducted between January 1, 2013, and September 30, 2014.

RESULTS

We observed blood oxygen level-dependent (BOLD) signal changes (false discovery rate <0.001) in the motor circuitry (including the primary motor, premotor, and supplementary motor cortices; thalamus; pedunculopontine nucleus; and cerebellum) and in the limbic circuitry (including the cingulate and insular cortices). Activation of the motor network was observed also after applying a Bonferroni correction (P<.001) to the data set, suggesting that across patients, BOLD changes in the motor circuitry are more consistent compared with those occurring in the nonmotor network.

CONCLUSION

These findings support the modulatory role of STN DBS on the activity of motor and nonmotor networks and suggest complex mechanisms as the basis of the efficacy of this treatment modality. Furthermore, these results suggest that across patients, BOLD changes in the motor circuitry are more consistent than those in the nonmotor network. With further studies combining the use of real-time intraoperative fMRI with clinical outcomes in patients treated with DBS, functional imaging techniques have the potential not only to elucidate the mechanisms of DBS functioning but also to guide and assist in the surgical treatment of patients affected by movement and neuropsychiatric disorders.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01809613.

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.

Effect of subthalamic nucleus stimulation on penicillin induced focal motor seizures in primate

BACKGROUND

Drug-resistant motor epilepsies are particularly incapacitating for the patients. In a primate model of focal motor seizures induced by intracortical injection of penicillin, we recently showed that seizures propagated from the motor cortex towards the basal ganglia.

OBJECTIVE

Using the same animal model here, we hypothesized that disruption of subthalamic nucleus (STN) activity by chronic high frequency stimulation (HFS) could modify pathological excessive cortical synchronisation occurring during focal motor seizures, and therefore could reduce seizure activity.

METHODS

Two monkeys were chronically implanted with one electrode positioned into the STN. In each experiment, seizures were induced during 6 hours by injecting penicillin into the motor cortex. During stimulation sessions, HFS-STN was applied at the beginning of penicillin injection.

RESULTS

Our results indicate that HFS-STN improved focal motor seizures by delaying the occurrence of the first seizure, by decreasing the number of seizures by 47% and therefore the total time spent seizing by 53% compared to control. These results argue for a therapeutic use of HFS-STN in motor seizures because they were obtained in a very severe primate model of motor status similar to that seen in human. Furthermore, HFS-STN was much more efficient than direct cortical HFS of the epileptic focus, which we already tested in the same primate model.

CONCLUSIONS

The present study suggests that HFS-STN could be used as an experimental therapy when other therapeutic strategies are not possible or have failed in humans suffering from motor epilepsy but the present study still warrants controlled studies in humans.

Music in disorders of consciousness

This review presents an overview of the use of music therapy in neurological early rehabilitation of patients with coma and other disorders of consciousness (DOC) such as unresponsive wakefulness syndrome (UWS) or minimally conscious state (MCS). There is evidence that patients suffering from UWS show emotional processing of auditory information, such as listening to speech. Thus, it seems reasonable to believe that music listening-as part of an enriched environment setting-may be of therapeutic value in these patients. There is, however, a considerable lack of evidence. The authors strongly encourage further studies to evaluate the efficacy of music listening in patients with DOC in neurological early rehabilitation. These studies should consider a precise clinical definition and homogeneity of the patient cohort with respect to the quality (coma vs. UWS vs. MCS), duration (rather weeks to months than days) and cause (traumatic vs. non-traumatic) of DOC, a standardized intervention protocol, valid clinical outcome parameters over a longer observation period (weeks to months), monitoring of neurophysiological and vegetative parameters and, if available, neuroimaging to confirm diagnosis and to demonstrate responses and functional changes in the patients' brains.

Treatment and physiology in Parkinson's disease and dystonia: using transcranial magnetic stimulation to uncover the mechanisms of action

Transcranial magnetic stimulation (TMS) has served as an important technological breakthrough in the field of the physiology of movement disorders over the last three decades. TMS has grown popular owing to the ease of application as well as its painless and noninvasive character. The technique has provide important insights into understanding the pathophysiology of movement disorders, particularly Parkinson's disease and dystonia. The basic applications have included the study of motor cortex excitability, functioning of excitatory and inhibitory circuits, study of interactions between sensory and motor systems, and the plasticity response of the brain. TMS has also made important contributions to understanding the response to treatments such as dopaminergic medications, botulinum toxin injections, and deep brain stimulation surgery. This review summarizes the knowledge gained to date with TMS in Parkinson's disease and dystonia, and highlights the current challenges in the use of TMS technology.

Closed-loop cortical neuromodulation in Parkinson's disease: An alternative to deep brain stimulation?

Deep brain stimulation (DBS) is usually performed to treat advanced Parkinson's disease (PD) patients with electrodes permanently implanted in basal ganglia while the stimulator delivers electrical impulses continuously and independently of any feedback (open-loop stimulation). Conversely, in closed-loop stimulation, electrical stimulation is delivered as a function of neuronal activities recorded and analyzed online. There is an emerging development of closed-loop DBS in the treatment of PD and a growing discussion about proposing cortical stimulation rather than DBS for this purpose. Why does it make sense to "close the loop" to treat parkinsonian symptoms? Could closed-loop stimulation applied to the cortex become a valuable therapeutic strategy for PD? Can mathematical modeling contribute to the development of this technique? We review the various evidences in favor of the use of closed-loop cortical stimulation for the treatment of advanced PD, as an emerging technique which might offer substantial clinical benefits for PD patients.

Subthalamic nucleus high-frequency stimulation restores altered electrophysiological properties of cortical neurons in parkinsonian rat

Electrophysiological recordings performed in parkinsonian patients and animal models have confirmed the occurrence of alterations in firing rate and pattern of basal ganglia neurons, but the outcome of these changes in thalamo-cortical networks remains unclear. Using rats rendered parkinsonian, we investigated, at a cellular level in vivo, the electrophysiological changes induced in the pyramidal cells of the motor cortex by the dopaminergic transmission interruption and further characterized the impact of high-frequency electrical stimulation of the subthalamic nucleus, a procedure alleviating parkinsonian symptoms. We provided evidence that a lesion restricted to the substantia nigra pars compacta resulted in a marked increase in the mean firing rate and bursting pattern of pyramidal neurons of the motor cortex. These alterations were underlain by changes of the electrical membranes properties of pyramidal cells including depolarized resting membrane potential and increased input resistance. The modifications induced by the dopaminergic loss were more pronounced in cortico-striatal than in cortico-subthalamic neurons. Furthermore, subthalamic nucleus high-frequency stimulation applied at parameters alleviating parkinsonian signs regularized the firing pattern of pyramidal cells and restored their electrical membrane properties.

Long-term subthalamic nucleus stimulation improves sensorimotor integration and proprioception

OBJECTIVE

Sensorimotor integration is impaired in patients with Parkinson's disease (PD). Short latency afferent inhibition (SAI) and long latency afferent inhibition (LAI) measured with transcranial magnetic stimulation (TMS) can be used to measure sensorimotor integration. Subthalamic nucleus (STN) deep brain stimulation (DBS) has been found to restore these abnormalities, but the time course of these changes is not known. We prospectively evaluated the short-term and long-term effects of STN DBS on SAI, LAI and proprioception. We hypothesised plasticity changes induced by chronic stimulation are necessary to normalise sensorimotor integration and proprioception.

METHODS

Patients with PD were studied preoperatively, at 1 month and more than 6 months postoperatively. SAI was tested with median nerve stimulation to the wrist preceding TMS pulse to motor cortex by ~20 ms and LAI by 200 ms. Proprioception (distance and spatial errors) in the arm was quantitatively assessed. For postoperative assessments, patients were studied in the medication-off/stimulator-off, medication-off/stimulator-on, medication-on/stimulator-off and medication-on/stimulator-on conditions.

RESULTS

11 patients with PD and 10 controls were enrolled. Preoperatively, SAI and proprioception was abnormal during the medication-on conditions and LAI was reduced regardless of the medication status. STN DBS had no significant effect on SAI, LAI and proprioception at 1 month. However, at 6 months SAI, LAI and distance errors were normalised in the medication-on/stimulator-on condition. Spatial error was normalised with DBS on and off.

CONCLUSIONS

Chronic STN DBS in PD normalises sensorimotor integration and proprioception, likely through long-term plastic changes in the basal ganglia thalamocortical circuit.

Increased motor cortical facilitation and decreased inhibition in Parkinson disease

OBJECTIVE

To identify the changes in motor cortical facilitatory and inhibitory circuits in Parkinson disease (PD) by detailed studies of their time courses and interactions.

METHODS

Short-interval intracortical facilitation (SICF) and short-interval intracortical inhibition (SICI) were measured with a paired-pulse paradigm using transcranial magnetic stimulation. Twelve patients with PD in both ON and OFF medication states and 12 age-matched healthy controls were tested. The first experiment tested the time course of SICF in PD and controls. The second experiment tested SICI at different times corresponding to SICF peaks and troughs to investigate whether SICI was affected by SICF.

RESULTS

SICF was increased in PD OFF state and was reduced by dopaminergic medications. The reduction in SICF from the OFF to ON state correlated with the improvement in PD motor signs. SICI was reduced in PD OFF state and was only partially normalized by dopaminergic medications. At SICF peaks, improvement in SICI with medication correlated with improvement in PD motor sign. Principal component analysis showed that variations of SICF and SICI were explained by the same principal component only in the PD OFF group, suggesting that decreased SICI in the OFF state is related to increased SICF.

CONCLUSIONS

Motor cortical facilitation is increased and inhibition is decreased in PD. Increased cortical facilitation partly accounts for the decreased inhibition, but there is also impairment in synaptic inhibition in PD. Increased cortical facilitation may be a compensatory mechanism in PD.

Safety of transcranial magnetic stimulation in Parkinson's disease: a review of the literature

BACKGROUND

Transcranial magnetic stimulation (TMS) has been used in both physiological studies and, more recently, the therapy of Parkinson's disease (PD). Prior TMS studies in healthy subjects and other patient populations demonstrate a slight risk of seizures and other adverse events. Our goal was to estimate these risks and document other safety concerns specific to PD patients.

METHODS

We performed an English-Language literature search through PudMed to review all TMS studies involving PD patients. We documented any seizures or other adverse events associated with these studies. Crude risks were calculated per subject and per session of TMS.

RESULTS

We identified 84 single pulse (spTMS) and/or paired-pulse (ppTMS) TMS studies involving 1091 patients and 77 repetitive TMS (rTMS) studies involving 1137 patients. Risk of adverse events was low in all protocols. spTMS and ppTMS risk per patient for any adverse event was 0.0018 (95% CI: 0.0002-0.0066) per patient and no seizures were encountered. Risk of an adverse event from rTMS was 0.040 (95% CI: 0.029-0.053) per patient and no seizures were reported. Other adverse events included transient headaches, scalp pain, tinnitus, nausea, increase in pre-existing pain, and muscle jerks. Transient worsening of Parkinsonian symptoms was noted in one study involving rTMS of the supplementary motor area (SMA).

CONCLUSION

We conclude that current TMS and rTMS protocols do not pose significant risks to PD patients. We would recommend that TMS users in this population follow the most recent safety guidelines but do not warrant additional precautions.

Therapeutic deep brain stimulation in Parkinsonian rats directly influences motor cortex

Much recent discussion about the origin of Parkinsonian symptoms has centered around the idea that they arise with the increase of beta frequency waves in the EEG. This activity may be closely related to an oscillation between subthalamic nucleus (STN) and globus pallidus. Since STN is the target of deep brain stimulation, it had been assumed that its action is on the nucleus itself. By means of simultaneous recordings of the firing activities from populations of neurons and the local field potentials in the motor cortex of freely moving Parkinsonian rats, this study casts doubt on this assumption. Instead, we found evidence that the corrective action is upon the cortex, where stochastic antidromic spikes originating from the STN directly modify the firing probability of the corticofugal projection neurons, destroy the dominance of beta rhythm, and thus restore motor control to the subjects, be they patients or rodents.

Parkinson's disease

In advanced Parkinson's disease (PD), the emergence of symptoms refractory to conventional therapy poses therapeutic challenges. The success of deep brain stimulation (DBS) and advances in the understanding of the pathophysiology of PD have raised interest in noninvasive brain stimulation as an alternative therapeutic tool. The rationale for its use draws from the concept that reversing abnormalities in brain activity and physiology thought to cause the clinical deficits may restore normal functioning. Currently the best evidence in support of this concept comes from DBS, which improves motor deficits, and modulates brain activity and motor cortex physiology, although whether a causal interaction exists remains largely undetermined. Most trials of noninvasive brain stimulation in PD have applied repetitive transcranial magnetic stimulation (rTMS), targeting the motor cortex. Current studies suggest a possible therapeutic potential for rTMS and transcranial direct current stimulation (tDCS), but clinical effects so far have been small and negligible with regard to functional independence and quality of life. Approaches to potentiate the efficacy of rTMS include increasing stimulation intensity and novel stimulation parameters that derive their rationale from studies on brain physiology. These novel parameters are intended to simulate normal firing patterns or to act on the hypothesized role of oscillatory activity in the motor cortex and basal ganglia with regard to motor control and its contribution to the pathogenesis of motor disorders. Noninvasive brain stimulation studies will enhance our understanding of PD pathophysiology and might provide further evidence for potential therapeutic applications.

Parkinson's disease

There have been a large number of basic research studies of noninvasive brain stimulation in Parkinson's disease. Initial work focused on measuring: (1) the excitability of corticospinal output with threshold and input-output measures, and (2) the effectiveness of intracortical γ-aminobutyric acid (GABA)ergic inhibitory systems using short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and silent period measures. Early suggestions of increased excitability and reduced inhibition have been progressively modified. There are conflicting reports on changes in excitability, silent period, and LICI, and the more consistent reduction in SICI is now viewed as a superimposed excitation rather than a primary deficit in a GABAergic mechanism. A small number of studies have suggested that premovement increases in corticospinal excitability may be prolonged in Parkinson's disease, consistent with the suggestion of slower buildup of the motor command to move; there are also modifications of interhemispheric connections in patients with mirror movements. Transcranial magnetic stimulation (TMS) has also been used to explore the involvement of motor cortex and cerebellum in resting and postural tremors by examining how readily they can be reset by single TMS pulses over each area. It can also probe the effects of deep brain stimulation of motor cortex excitability. Finally, new TMS techniques that examine synaptic plasticity in motor cortex have shown reduced excitability in patients off therapy which is restored when on therapy. Data are also emerging about the possible role of cortical plasticity in compensating for gradual loss of dopaminergic function prior to onset of clinical symptoms.

Non-homogeneous effect of levodopa on inhibitory circuits in Parkinson's disease and dyskinesia

INTRODUCTION

Levodopa-induced dyskinesia in patients with Parkinson's disease (PD) has been shown to be associated with an abnormal plasticity in the motor cortex. We investigated whether changes in the excitability of inhibitory and excitatory motor circuits could underlie maladaptive mechanisms associated with dyskinesia.

METHODS

Using single and paired transcranial magnetic stimulation (TMS), we studied motor threshold, silent period (SP) duration, intracortical facilitation (ICF), short intracortical inhibition (SICI) and low- and high-intensity long intracortical inhibition (LICI) in 10 dyskinetic and 10 non-dyskinetic patients, matched for disease and treatment duration, before (OFF state) and after (ON state) levodopa, and in 10 healthy controls.

RESULTS

In the OFF state, the two groups of patients showed similar motor cortex excitability with a reduced SICI compared to controls. LICI was weaker and increasing stimulation intensity had a lower effect on SP duration in dyskinetic patients than in controls. In dyskinetic patients, in contrast to non-dyskinetic patients, levodopa failed to increase SICI and SP duration, and potentiated to a lesser extent the effect of increasing the stimulation intensity on LICI. Although levodopa improved motor symptoms to a similar extent in both dyskinetic and non-dyskinetic patients, it failed to activate effectively the excitability of the inhibitory systems in dyskinetic patients.

DISCUSSION

These findings suggest that dyskinesia is associated with an abnormal effect of levodopa on cortical motor inhibitory circuits.

Speed effects of deep brain stimulation for Parkinson's disease

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) accelerates reaction time (RT) in patients with Parkinson's disease (PD), particularly in tasks in which decisions on the response side have to be made. This might indicate that DBS speeds up both motor and nonmotor operations. Therefore, we studied the extent to which modifications of different processing streams could explain changes of RT under subthalamic DBS. Ten PD patients on-DBS and off-DBS and 10 healthy subjects performed a choice-response task (CRT), requiring either right or left finger button presses. At the same time, EEG recordings were performed, so that RTs could be assessed together with lateralized readiness potentials (LRP), indicative of movement preparation. Additionally, an oddball task (OT) was run, in which right finger responses to target stimuli were recorded along with cognitive P300 responses. Generally, PD patients off-DBS had longer RTs than controls. Subthalamic DBS accelerated RT only in CRT. This could largely be explained by analog shortenings of LRP. No DBS-dependent changes were identified in OT, neither on the level of RT nor on the level of P300 latencies. It follows that RT accelerations under DBS of the STN are predominantly due to effects on the timing of motor instead of nonmotor processes. This starting point explains why DBS gains of response speed are low in tasks in which reactions are initiated from an advanced level of movement preparation (as in OT), and high whenever motor responses have to be raised from scratch (as in CRT).

Slow (1 Hz) repetitive transcranial magnetic stimulation (rTMS) induces a sustained change in cortical excitability in patients with Parkinson's disease

OBJECTIVE

Low-frequency (< or =1 Hz) rTMS (LF-rTMS) can reduce excitability in the underlying cortex and/or promote inhibition. In patients with Parkinson's disease (PD) several TMS elicited features of motor corticospinal physiology suggest presence of impaired inhibitory mechanisms. These include shortened silent period (SP) and slightly steeper input-output (I-O) curve of motor evoked potential (MEP) size than in normal controls. However, studies of LF-rTMS effects on inhibitory mechanisms in PD are scarce. In this companion paper to the clinical paper describing effects of four consecutive days of LF-rTMS on dyskinesia in PD (Filipović et al., 2009), we evaluate the delayed (24h) effects of the LF-rTMS treatment on physiological measures of excitability of the motor cortex in the same patients. There are very few studies of physiological follow up of daily rTMS treatments.

METHODS

Nine patients with PD in Hoehn and Yahr stages 2 or 3 and prominent medication-induced dyskinesia were studied. This was a placebo-controlled, crossover study, with two treatment arms, "real" rTMS and "sham" rTMS (placebo). In each of the treatment arms, rTMS (1800 pulses; 1 Hz rate; intensity of the real stimuli just-below the active motor threshold) was delivered over the motor cortex for four consecutive days. Motor cortex excitability was evaluated at the beginning of the study and the next day following each of the four-day rTMS series (real and sham) with patients first in the practically defined "off" state, following 12h withdrawal of medication, and subsequently in a typical "on" state following usual morning medication dose.

RESULTS

The SP was significantly longer following real rTMS in comparison to both baseline and sham rTMS. The effect was independent from the effects of dopaminergic treatment. There was no difference in MEP size, rest and active motor threshold. The I-O curve, recorded from the relaxed muscle, showed a trend towards diminished slope in comparison to baseline, but the difference was not significant. There was no consistent correlation between prolongation of SP and concomitant reduction in dyskinesia following real rTMS.

CONCLUSIONS

Low-frequency rTMS delivered over several consecutive days changes the excitability of motor cortex by increasing the excitability of inhibitory circuits. The effects persist for at least a day after rTMS.

SIGNIFICANCE

The results confirm the existence of a residual after-effect of consecutive daily applications of rTMS that might be relevant to the clinical effect that was observed in this group of patients and could be further exploited for potential therapeutic uses.

Functional imaging and related techniques: an introduction for rehabilitation researchers

Functional neuroimaging and related neuroimaging techniques are becoming important tools for rehabilitation research. Functional neuroimaging techniques can be used to determine the effects of brain injury or disease on brain systems related to cognition and behavior and to determine how rehabilitation changes brain systems. These techniques include: functional magnetic resonance imaging (fMRI), positron emission tomography (PET), electroencephalography (EEG), magnetoencephalography (MEG), near infrared spectroscopy (NIRS), and transcranial magnetic stimulation (TMS). Related diffusion weighted magnetic resonance imaging techniques (DWI), including diffusion tensor imaging (DTI) and high angular resolution diffusion imaging (HARDI), can quantify white matter integrity. With the proliferation of these imaging techniques in rehabilitation research, it is critical that rehabilitation researchers, as well as consumers of rehabilitation research, become familiar with neuroimaging techniques, what they can offer, and their strengths and weaknesses The purpose to this review is to provide such an introduction to these neuroimaging techniques.

Role of D1 and D2 receptors in the regulation of voluntary movements

The effect of dopamine receptor agonist cabergoline on muscle tone and contractility was studied in healthy volunteers. Variations in muscle tone were evaluated by means of transcranial magnetic stimulation under resting conditions. Muscle contractility was estimated from kinematic parameters of voluntary movements. Oral administration of cabergoline in a dose of 2 mg was followed by a decrease in muscle tone and increase in muscle contractility. Our findings indicate that the brain dopaminergic system regulates voluntary movements by decreasing the tone and increasing contractility of skeletal muscles. Under resting conditions, prolonged exposure of D1 receptors to dopamine in a low concentration decreases excitability threshold of the motor cortex and reduces muscle tone. During voluntary movements, short-term stimulation of D2 receptors with dopamine in a high concentration increases excitability of the motor cortex and induces muscle contraction. The movement occurs when D2 receptor-mediated excitation of the cortex and induced muscle contraction exceed the decrease in muscle tone and excitability threshold caused by stimulation of D1 receptors.

The effect of exercise training in improving motor performance and corticomotor excitability in people with early Parkinson's disease

OBJECTIVES

To obtain preliminary data on the effects of high-intensity exercise on functional performance in people with Parkinson's disease (PD) relative to exercise at low and no intensity and to determine whether improved performance is accompanied by alterations in corticomotor excitability as measured through transcranial magnetic stimulation (TMS).

DESIGN

Cohort (prospective), randomized controlled trial.

SETTING

University-based clinical and research facilities.

PARTICIPANTS

Thirty people with PD, within 3 years of diagnosis with Hoehn and Yahr stage 1 or 2.

INTERVENTIONS

Subjects were randomized to high-intensity exercise using body weight-supported treadmill training, low-intensity exercise, or a zero-intensity education group. Subjects in the 2 exercise groups completed 24 exercise sessions over 8 weeks. Subjects in the zero-intensity group completed 6 education classes over 8 weeks.

MAIN OUTCOME MEASURES

Unified Parkinson's Disease Rating Scales (UPDRS), biomechanic analysis of self-selected and fast walking and sit-to-stand tasks; corticomotor excitability was assessed with cortical silent period (CSP) durations in response to single-pulse TMS.

RESULTS

A small improvement in total and motor UPDRS was observed in all groups. High-intensity group subjects showed postexercise increases in gait speed, step and stride length, and hip and ankle joint excursion during self-selected and fast gait and improved weight distribution during sit-to-stand tasks. Improvements in gait and sit-to-stand measures were not consistently observed in low- and zero-intensity groups. The high-intensity group showed lengthening in CSP.

CONCLUSIONS

The findings suggest the dose-dependent benefits of exercise and that high-intensity exercise can normalize corticomotor excitability in early PD.

Motor intracortical inhibition in PD: L-DOPA modulation of high-frequency rTMS effects

Dopaminergic drugs and deep brain stimulation restore cortical inhibition in Parkinson disease (PD) patients. High-frequency rTMS was also found to increase cortical inhibition in PD but its therapeutic effect is still controversial. Here we hypothesize that, if dopaminergic drugs reverse to normal cortical excitability in M1, the effect of high-frequency (hf)-rTMS in PD patients could depend on whether they are in a medicated or unmedicated state. The present study aims to explore the lasting effects of sub-threshold hf rTMS trains over M1 on cortical inhibition in patients with "on" and without "off" L-DOPA treatment. Fourteen PD patients were examined twice while "on" and "off" medication. In both conditions, a paired-pulse paradigm was used to evaluate short intracortical inhibition (SICI) and long intracortical inhibition (LICI) that were evaluated before and after hf rTMS trains applied on the motor cortex. The results were compared with those obtained from normal controls. In baseline condition, SICI and LICI were significantly reduced in "off" compared to "on" patients and controls. hf-rTMS over the motor cortex significantly increased SICI and LICI in "off" medication PD patients. Magnetic stimulation proved to be ineffective when the same patients were in "on" state. The results showed that hf-rTMS affected intracortical inhibition (ICI) only in unmedicated patients. By restoring cortical inhibitory circuits dopaminergic drugs, normalize the excitability changes in M1 subsequent to motor rTMS. Whether patients are in a medicated or an unmedicated state would therefore appear to be critical for rTMS effects in PD patients. If a positive correlation exists between increased cortical inhibition and clinical improvement, hf-rTMS during the "off" state could be regarded as a potential add-on treatment to reduce the need of L-dopa and thus delay the adverse effects of its chronic use.