Pathological subthalamic nucleus oscillations in PD: Can they be the cause of bradykinesia and akinesia?

Beta-gamma phase-amplitude coupling of scalp electroencephalography during walking preparation in Parkinson's disease differs depending on the freezing of gait

Introduction

Despite using beta oscillations within the subthalamic nucleus as a biomarker of akinesia or rigidity in Parkinson's disease, a specific biomarker for freezing of gait (FOG) remains unclear. Recently, scalp phase-amplitude coupling (PAC) measured through scalp electroencephalography (EEG) has emerged as a promising tool for analyzing brain function. In this study, we examined whether PAC could be a biomarker for FOG.

Methods

We enrolled 11 patients with Parkinson's disease and recorded scalp EEG in preparation for and during gait while simultaneously assessing motor function, including FOG. We investigated changes in cortical PAC during walking with and without FOG and examined its correlation with the postural instability and gait difficulty (PIGD) score.

Results

Patient characteristics were as follows: mean age 59.1 ± 6.9 years, disease duration 13.9 ± 4.1 years, and seven men. Four trials were excluded from the analysis owing to artifacts. In the trials without FOG (n = 18), beta-gamma PAC in the sensorimotor area decreased during gait preparation (p = 0.011; linear mixed-effects model), which was not the case in trials with FOG (n = 6) (p = 0.64; linear mixed-effects model). Using a support vector machine, machine learning of PAC during preparation for walking predicted the presence of FOG with an accuracy of 71.2%. Conversely, PAC increased during walking in trials with FOG (p = 0.0042; linear mixed-effects model), and PAC 20 s after the start of walking was positively correlated with the PIGD score (correlation coefficient = 0.406, p = 0.032; Pearson's rank correlation).

Conclusion

Beta-gamma PAC in the sensorimotor area during preparation for walking differs depending on the emergence of FOG. As gait symptoms worsened, beta-gamma PAC in the sensorimotor area during walking gradually increased. Cortical PAC may be a biomarker for FOG in Parkinson's disease and may lead to the development of strategies to prevent falls in the future.

Circuit-Specific Deep Brain Stimulation Provides Insights into Movement Control

Deep brain stimulation (DBS), a method in which electrical stimulation is delivered to specific areas of the brain, is an effective treatment for managing symptoms of a number of neurological and neuropsychiatric disorders. Clinical access to neural circuits during DBS provides an opportunity to study the functional link between neural circuits and behavior. This review discusses how the use of DBS in Parkinson's disease and dystonia has provided insights into the brain networks and physiological mechanisms that underlie motor control. In parallel, insights from basic science about how patterns of electrical stimulation impact plasticity and communication within neural circuits are transforming DBS from a therapy for treating symptoms to a therapy for treating circuits, with the goal of training the brain out of its diseased state.

Asymmetries of the subthalamic activity in Parkinson's disease: phase-amplitude coupling among local field potentials

The role of brain asymmetries of dopaminergic neurons in motor symptoms of Parkinson's disease is still undefined. Local field recordings from the subthalamic nucleus revealed some neurophysiological biomarkers of the disease: increased beta activity, increased low-frequency activity and high-frequency oscillations. Phase-amplitude coupling coordinates the timing of neuronal activity and allows determining the mechanism for communication within distinct regions of the brain. In this study, we discuss the use of phase-amplitude coupling to assess the differences between the two hemispheres in a cohort of 24 patients with Parkinson's disease before and after levodopa administration. Subthalamic low- (12-20 Hz) and high-beta (20-30 Hz) oscillations were compared with low- (30-45 Hz), medium- (70-100 Hz) and high-frequency (260-360 Hz) bands. We found a significant beta-phase-amplitude coupling asymmetry between left and right and an opposite-side-dependent effect of the pharmacological treatment, which is associated with the reduction of motor symptoms. In particular, high coupling between high frequencies and high-beta oscillations was found during the OFF condition (P < 0.01) and a low coupling during the ON state (P < 0.0001) when the right subthalamus was assessed; exactly the opposite happened when the left subthalamus was considered in the analysis, showing a lower coupling between high frequencies and high-beta oscillations during the OFF condition (P < 0.01), followed by a higher one during the ON state (P < 0.01). Interestingly, these asymmetries are independent of the motor onset side, either left or right. These findings have important implications for neural signals that may be used to trigger adaptive deep brain stimulation in Parkinson's and could provide more exhaustive insights into subthalamic dynamics.

Differences in subthalamic oscillatory activity in the two hemispheres associated with severity of Parkinson's disease

Background

It is well known that motor features of Parkinson's disease (PD) commonly begin on one side of the body and extend to the other side with disease progression. The onset side generally remains more severely affected over the course of the disease. However, the pathophysiology underlying the asymmetry of motor manifestations remains unclear. The purpose of the present study is to examine whether alterations in neuronal activity in the subthalamic nucleus (STN) associate with PD severity.

Methods

Microelectrode recording was performed in the STN during targeting for 30 patients in the treatment of deep brain stimulation. The mean spontaneous firing rate (MSFR), power density spectral analysis, and correlations were calculated. Characteristics of subthalamic oscillatory activity were compared between two hemispheres. UPDRS III scores during "Off" and "On" states were obtained for the body side of initial symptoms (BSIS) and the body side of extended symptoms (BSES).

Results

There were significant differences of MSFR (41.3 ± 11.0 Hz vs 35.2 ± 10.0 Hz) and percentage of ß frequency oscillatory neurons (51.3% vs 34.9%) between BSIS and BSES. The percentage of ß frequency oscillatory neurons correlated with the bradykinesia/rigidity scores for both sides (p < 0.05). In contrast, the percentage of tremor frequency oscillatory neurons was significantly higher in the BSES than that in the BSIS. In particular, these neurons only correlated with the tremor scores of the BSES (p < 0.05).

Conclusion

The results suggest that increased neuronal firing rate and ß frequency oscillatory neurons in the STN are associated with contralateral side motor severity and its progression. Tremor frequency oscillatory neurons are less observed in the STN of the BSIS suggesting that ß oscillatory activity dominates and tremor frequency oscillatory activity reciprocally declines.

The Genetic Etiology of Parkinson's Disease Does Not Robustly Affect Subthalamic Physiology

BACKGROUND

It is unknown whether Parkinson's disease (PD) genetic heterogeneity, leading to phenotypic and pathological variability, is also associated with variability in the unique PD electrophysiological signature. Such variability might have practical implications for adaptive deep brain stimulation (DBS).

OBJECTIVE

The aim of our work was to study the electrophysiological activity in the subthalamic nucleus (STN) of patients with PD with pathogenic variants in different disease-causing genes.

METHODS

Electrophysiological data from participants with negative genetic tests were compared with those from GBA, LRRK2, and PRKN-PD.

RESULTS

We analyzed data from 93 STN trajectories (GBA-PD: 28, LRRK2-PD: 22, PARK-PD: 10, idiopathic PD: 33) of 52 individuals who underwent DBS surgery. Characteristics of β oscillatory activity in the dorsolateral motor part of the STN were similar for patients with negative genetic tests and for patients with different forms of monogenic PD.

CONCLUSIONS

The genetic heterogeneity in PD is not associated with electrophysiological differences. Therefore, similar adaptive DBS algorithms would be applicable to genetically heterogeneous patient populations. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Movement-related beta ERD and ERS abnormalities in neuropsychiatric disorders

Movement-related oscillations in the beta range (from 13 to 30 Hz) have been observed over sensorimotor areas with power decrease (i.e., event-related desynchronization, ERD) during motor planning and execution followed by an increase (i.e., event-related synchronization, ERS) after the movement's end. These phenomena occur during active, passive, imaged, and observed movements. Several electrophysiology studies have used beta ERD and ERS as functional indices of sensorimotor integrity, primarily in diseases affecting the motor system. Recent literature also highlights other characteristics of beta ERD and ERS, implying their role in processes not strictly related to motor function. Here we review studies about movement-related ERD and ERS in diseases characterized by motor dysfunction, including Parkinson's disease, dystonia, stroke, amyotrophic lateral sclerosis, cerebral palsy, and multiple sclerosis. We also review changes of beta ERD and ERS reported in physiological aging, Alzheimer's disease, and schizophrenia, three conditions without overt motor symptoms. The review of these works shows that ERD and ERS abnormalities are present across the spectrum of the examined pathologies as well as development and aging. They further suggest that cognition and movement are tightly related processes that may share common mechanisms regulated by beta modulation. Future studies with a multimodal approach are warranted to understand not only the specific topographical dynamics of movement-related beta modulation but also the general meaning of beta frequency changes occurring in relation to movement and cognitive processes at large. Such an approach will provide the foundation to devise and implement novel therapeutic approaches to neuropsychiatric disorders.

The immunology of Parkinson's disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder which affects 6.1 million people worldwide. The neuropathological hallmarks include the loss of dopaminergic neurons in the substantia nigra, the presence of Lewy bodies and Lewy neurites caused by α-synuclein aggregation, and neuroinflammation in the brain. The prodromal phase happens years before the onset of PD during which time many patients show gastro-intestinal symptoms. These symptoms are in support of Braak’s theory and model where pathological α‐synuclein propagates from the gut to the brain. Importantly, immune responses play a determinant role in the pathogenesis of Parkinson’s disease. The innate immune responses triggered by microglia can cause neuronal death and disease progression. In addition, T cells infiltrate into the brains of PD patients and become involved in the adaptive immune responses. Interestingly, α‐synuclein is associated with both innate and adaptive immune responses by directly interacting with microglia and T cells. Here, we give a detailed review of the immunobiology of Parkinson’s disease, focusing on the role α-synuclein in the gut-brain axis hypothesis, the innate and adaptive immune responses involved in the disease, and current treatments.

Transcranial Magnetic Stimulation in the Treatment of Neurological Diseases

Transcranial Magnetic Stimulation (TMS) has widespread use in research and clinical application. For psychiatric applications, such as depression or OCD, repetitive TMS protocols (rTMS) are an established and globally applied treatment option. While promising, rTMS is not yet as common in treating neurological diseases, except for neurorehabilitation after (motor) stroke and neuropathic pain treatment. This may soon change. New clinical studies testing the potential of rTMS in various other neurological conditions appear at a rapid pace. This can prove challenging for both practitioners and clinical researchers. Although most of these neurological applications have not yet received the same level of scientific/empirical scrutiny as motor stroke and neuropathic pain, the results are encouraging, opening new doors for TMS in neurology. We here review the latest clinical evidence for rTMS in pioneering neurological applications including movement disorders, Alzheimer's disease/mild cognitive impairment, epilepsy, multiple sclerosis, and disorders of consciousness.

Subthalamic burst firing: A pathophysiological target in Parkinson's disease

Understanding the pathophysiological mechanism of Parkinson's disease (PD) in the subthalamic nucleus (STN) has become a critical issue since deep brain stimulation (DBS) in this region has been proven as an effective treatment for this disease. The STN possesses a special ability to switch from the spike to the burst firing mode in response to dopamine deficiency in parkinsonism, and this STN burst is considered an electrophysiological signature of the cortico-basal ganglia circuit in the brains of PD patients. This review focuses on the role of STN burst firing in the pathophysiology of PD and during DBS. Here, we review existing literature on how STN bursts originate and the specific factors affecting their formation; how STN burst firing causes motor symptoms in PD and how interventions can rescue these symptoms. Finally, the similarities and differences between the two electrophysiological hallmarks of PD, STN burst firing and beta-oscillation, are discussed. STN burst firing should be considered as a pathophysiological target in PD during treatment with DBS.

Influence of inter-electrode distance on subthalamic nucleus local field potential recordings in Parkinson's disease

OBJECTIVES

To evaluate spectra and their correlations with clinical symptoms of local field potentials (LFP) acquired from wide- and close-spaced contacts (i.e. between contacts 0-3 or LFP03, and contacts 1-2 or LFP12 respectively) on the same DBS electrode within the subthalamus (STN) in Parkinson's disease (PD), before and after levodopa administration.

METHODS

LFP12 and LFP03 were recorded from 20 PD patients. We evaluated oscillatory power, local and switched phase-amplitude coupling (l- and Sw-PAC) and correlation with motor symptoms (UPDRSIII).

RESULTS

Before levodopa, both LFP03 and LFP12 power in the α band inversely correlated with UPDRSIII. Differences between contacts were found in the low-frequency bands power. After levodopa, differences in UPDRSIII were associated to changes in LFP03 low-β and LFP12 HFO (high frequency oscillations, 250-350 Hz) power, while a modulation of the low-β power and an increased β-LFO (low frequency oscillations, 15-45 Hz) PAC was found only for LFP12.

CONCLUSION

This study reveals differences in spectral pattern between LFP12 and LFP03 before and after levodopa administration, as well as different correlations with PD motor symptoms.

SIGNIFICANCE

Differences between LFP12 and LFP03 may offer an opportunity for optimizing adaptive deep brain stimulation (aDBS) protocols for PD. LFP12 can be used to detect β-HFO coupling and β power (i.e. bradykinesia), while LFP03 are optimal for low frequency oscillations (dyskinesias).

Revisiting the "Paradox of Stereotaxic Surgery": Insights Into Basal Ganglia-Thalamic Interactions

Basal ganglia dysfunction is implicated in movement disorders including Parkinson Disease, dystonia, and choreiform disorders. Contradicting standard "rate models" of basal ganglia-thalamic interactions, internal pallidotomy improves both hypo- and hyper-kinetic movement disorders. This "paradox of stereotaxic surgery" was recognized shortly after rate models were developed, and is underscored by the outcomes of deep brain stimulation (DBS) for movement disorders. Despite strong evidence that DBS activates local axons, the clinical effects of lesions and DBS are nearly identical. These observations argue against standard models in which GABAergic basal ganglia output gates thalamic activity, and raise the question of how lesions and stimulation can have similar effects. These paradoxes may be resolved by considering thalamocortical loops as primary drivers of motor output. Rather than suppressing or releasing cortex via motor thalamus, the basal ganglia may modulate the timing of thalamic perturbations to cortical activity. Motor cortex exhibits rotational dynamics during movement, allowing the same thalamocortical perturbation to affect motor output differently depending on its timing with respect to the rotational cycle. We review classic and recent studies of basal ganglia, thalamic, and cortical physiology to propose a revised model of basal ganglia-thalamocortical function with implications for basic physiology and neuromodulation.

Desflurane and sevoflurane differentially affect activity of the subthalamic nucleus in Parkinson's disease

BACKGROUND

Desflurane and sevoflurane are commonly used during inhalational anaesthesia, but few studies have investigated their effects on deep cerebral neuronal activity. In addition, the association between subthalamic nucleus (STN) neurophysiology and general anaesthesia induced by volatile anaesthetics are not yet identified. This study aimed to identify differences in neurophysiological characteristics of the STN during comparable minimal alveolar concentration (MAC) desflurane and sevoflurane anaesthesia for deep brain stimulation (DBS) in patients with Parkinson's disease.

METHODS

Twelve patients with similar Parkinson's disease severity received desflurane (n=6) or sevoflurane (n=6) during DBS surgery. We obtained STN spike firing using microelectrode recording at 0.5-0.6 MAC and compared firing rate, power spectral density, and coherence.

RESULTS

Neuronal firing rate was lower with desflurane (47.4 [26.7] Hz) than with sevoflurane (63.9 [36.5] Hz) anaesthesia (P<0.001). Sevoflurane entrained greater gamma oscillation power than desflurane (62.9% [0.9%] vs 57.0% [1.5%], respectively; P=0.002). There was greater coherence in the theta band of the desflurane group compared with the sevoflurane group (13% vs 6%, respectively). Anaesthetic choice did not differentially influence STN mapping accuracy or the clinical outcome of DBS electrode implantation.

CONCLUSIONS

Desflurane and sevoflurane produced distinct neurophysiological profiles in humans that may be associated with their analgesic and hypnotic actions.

EEG and MEG primers for tracking DBS network effects

Deep brain stimulation (DBS) is an effective treatment method for a range of neurological and psychiatric disorders. It involves implantation of stimulating electrodes in a precisely guided fashion into subcortical structures and, at a later stage, chronic stimulation of these structures with an implantable pulse generator. While the DBS surgery makes it possible to both record brain activity and stimulate parts of the brain that are difficult to reach with non-invasive techniques, electroencephalography (EEG) and magnetoencephalography (MEG) provide complementary information from other brain areas, which can be used to characterize brain networks targeted through DBS. This requires, however, the careful consideration of different types of artifacts in the data acquisition and the subsequent analyses. Here, we review both the technical issues associated with EEG/MEG recordings in DBS patients and the experimental findings to date. One major line of research is simultaneous recording of local field potentials (LFPs) from DBS targets and EEG/MEG. These studies revealed a set of cortico-subcortical coherent networks functioning at distinguishable physiological frequencies. Specific network responses were linked to clinical state, task or stimulation parameters. Another experimental approach is mapping of DBS-targeted networks in chronically implanted patients by recording EEG/MEG responses during stimulation. One can track responses evoked by single stimulation pulses or bursts as well as brain state shifts caused by DBS. These studies have the potential to provide biomarkers for network responses that can be adapted to guide stereotactic implantation or optimization of stimulation parameters. This is especially important for diseases where the clinical effect of DBS is delayed or develops slowly over time. The same biomarkers could also potentially be utilized for the online control of DBS network effects in the new generation of closed-loop stimulators that are currently entering clinical use. Through future studies, the use of network biomarkers may facilitate the integration of circuit physiology into clinical decision making.

Non-invasive brain stimulation for Parkinson's disease: Clinical evidence, latest concepts and future goals: A systematic review

Parkinson's disease (PD) is becoming a major public-health issue in an aging population. Available approaches to treat advanced PD still have limitations; new therapies are needed. The non-invasive brain stimulation (NIBS) may offer a complementary approach to treat advanced PD by personalized stimulation. Although NIBS is not as effective as the gold-standard levodopa, recent randomized controlled trials show promising outcomes in the treatment of PD symptoms. Nevertheless, only a few NIBS-stimulation paradigms have shown to improve PD's symptoms. Current clinical recommendations based on the level of evidence are reported in Table 1 through Table 3. Furthermore, novel technological advances hold promise and may soon enable the non-invasive stimulation of deeper brain structures for longer periods.

Cortico-Striatal Oscillations Are Correlated to Motor Activity Levels in Both Physiological and Parkinsonian Conditions

Oscillatory neural activity in the cortico-basal ganglia-thalamocortical (CBGTC) loop is associated with the motor state of a subject, but also with the availability of modulatory neurotransmitters. For example, increased low-frequency oscillations in Parkinson’s disease (PD) are related to decreased levels of dopamine and have been proposed as biomarkers to adapt and optimize therapeutic interventions, such as deep brain stimulation. Using neural oscillations as biomarkers require differentiating between changes in oscillatory patterns associated with parkinsonism vs. those related to a subject’s motor state. To address this point, we studied the correlation between neural oscillatory activity in the motor cortex and striatum and varying degrees of motor activity under normal and parkinsonian conditions. Using rats with bilateral or unilateral 6-hydroxydopamine lesions as PD models, we correlated the motion index (MI)—a measure based on the physical acceleration of the head of rats—to the local field potential (LFP) oscillatory power in the 1–80 Hz range. In motor cortices and striata, we observed a robust correlation between the motion index and the oscillatory power in two main broad frequency ranges: a low-frequency range [5.0–26.5 Hz] was negatively correlated to motor activity, whereas a high-frequency range [35.0–79.9 Hz] was positively correlated. We observed these correlations in both normal and parkinsonian conditions. In addition to these general changes in broad-band power, we observed a more restricted narrow-band oscillation [25–40 Hz] in dopamine-denervated hemispheres. This oscillation, which seems to be selective to the parkinsonian state, showed a linear frequency dependence on the concurrent motor activity level. We conclude that, independently of the parkinsonian condition, changes in broad-band oscillatory activities of cortico-basal ganglia networks (including changes in the relative power of low- and high-frequency bands) are closely correlated to ongoing motions, most likely reflecting he operations of these neural circuits to control motor activity. Hence, biomarkers based on neural oscillations should focus on specific features, such as narrow frequency bands, to allow differentiation between parkinsonian states and physiological movement-dependent circuit modulation.

Relative Contribution of Magnetic Resonance Imaging, Microelectrode Recordings, and Awake Test Stimulation in Final Lead Placement during Deep Brain Stimulation Surgery of the Subthalamic Nucleus in Parkinson's Disease

INTRODUCTION

For deep brain stimulation (DBS) surgery of the subthalamic nucleus (STN) in Parkinson's disease (PD), many centers employ visualization of the nucleus on magnetic resonance imaging (MRI), intraoperative microelectrode recordings (MER), and test stimulation in awake patients. The value of these steps is a subject for ongoing debate. In the current study, we determined the relative contribution of MRI targeting, multitrack MER, and awake test stimulation in final lead placement during STN DBS surgery for PD.

METHODS

Data on PD patients undergoing MRI-targeted STN DBS surgery with three-channel MER and awake test stimulation between February 2010 and January 2014 were analyzed to determine in which MER trajectory final leads were implanted and why this tract was chosen.

RESULTS

Seventy-six patients underwent implantation of 146 DBS leads. In 92% of the STN, the final leads were implanted in one of the three planned channels. In 6%, additional channels were needed. In 2%, surgery was aborted before final lead implantation due to anxiety or fatigue. The final leads were implanted in the channels with the longest STN MER signal trajectory in 60% of the STN (38% of the bilaterally implanted patients). This was the central channel containing the MRI target in 39% of the STN (18% bilaterally). The most frequently noted reasons why another channel than the central channel was chosen for final lead placement were (1) a lower threshold for side effects (54%) and (2) no or a too short trajectory of the STN MER signal (40%) in the central channel. The latter reason correlated with larger 2D (x and y) errors in our stereotactic method.

CONCLUSIONS

STN DBS leads were often not implanted in the MRI-planned trajectory or in the trajectory with the longest STN MER signal. Thresholds for side effects during awake test stimulation were decisive for final target selection in the majority of patients.

Bipolar oscillations between positive and negative mood states in a computational model of Basal Ganglia

Bipolar disorder is characterized by mood swings-oscillations between manic and depressive states. The swings (oscillations) mark the length of an episode in a patient's mood cycle (period), and can vary from hours to years. The proposed modeling study uses decision making framework to investigate the role of basal ganglia network in generating bipolar oscillations. In this model, the basal ganglia system performs a two-arm bandit task in which one of the arms (action responses) leads to a positive outcome, while the other leads to a negative outcome. We explore the dynamics of key reward and risk related parameters in the system while the model agent receives various outcomes. Particularly, we study the system using a model that represents the fast dynamics of decision making, and a module to capture the slow dynamics that describe the variation of some meta-parameters of fast dynamics over long time scales. The model is cast at three levels of abstraction: (1) a two-dimensional dynamical system model, that is a simple two variable model capable of showing bistability for rewarding and punitive outcomes; (2) a phenomenological basal ganglia model, to extend the implications from the reduced model to a cortico-basal ganglia setup; (3) a detailed network model of basal ganglia, that incorporates detailed cellular level models for a more realistic understanding. In healthy conditions, the model chooses positive action and avoids negative one, whereas under bipolar conditions, the model exhibits slow oscillations in its choice of positive or negative outcomes, reminiscent of bipolar oscillations. Phase-plane analyses on the simple reduced dynamical system with two variables reveal the essential parameters that generate pathological 'bipolar-like' oscillations. Phenomenological and network models of the basal ganglia extend that logic, and interpret bipolar oscillations in terms of the activity of dopaminergic and serotonergic projections on the cortico-basal ganglia network dynamics. The network's dysfunction, specifically in terms of reward and risk sensitivity, is shown to be responsible for the pathological bipolar oscillations. The study proposes a computational model that explores the effects of impaired serotonergic neuromodulation on the dynamics of the cortico basal ganglia network, and relates this impairment to abstract mood states (manic and depressive episodes) and oscillations of bipolar disorder.

Cardinal Motor Features of Parkinson's Disease Coexist with Peak-Dose Choreic-Type Drug-Induced Dyskinesia

Background:

Clinical and anecdotal observations propose that patients with Parkinson’s disease (PD) may show drug-induced dyskinesia (DID) concomitantly with cardinal motor features. However, the extent of the concomitant presence of DID and cardinal features remains to be determined.

Objectives:

This cross-sectional study measured peak-dose choreic-type DID in a quantitative manner in patients diagnosed with PD, and determined whether symptoms such as tremor, bradykinesia, rigidity, postural instability or freezing of gait (FoG) were still detectable in these patients.

Methods:

89 patients diagnosed with PD were recruited and assessed using a combination of quantitative measures using inertial measurement units to capture DID, tremor, bradykinesia, and FoG. Clinical evaluations were also used to assess rigidity and postural instability. Motor symptoms of PD were assessed 3 times during the testing period, and a series of activities of daily living were repeated twice, in between clinical tests, during which the level of DID was quantified. Peak-dose was identified as the period during which patients had the highest levels of DID. Levels of tremor, rigidity, bradykinesia, postural instability, and FoG were used to determine the percentage of patients showing these motor symptoms simultaneously with DID.

Results:

72.4% of patients tested presented with measurable DID during the experiment. Rest, postural and kinetic tremor (12.7% , 38.1% , and 15.9% respectively), bradykinesia (28.6% ), rigidity (55.6% ), postural instability (71.4% ) and FoG (9.5% ) were detected simultaneously with DID.

Conclusions:

PD symptomatology remains present in patients showing peak-dose choreic-type DID, illustrating the challenge facing physicians when trying to avoid dyskinesia while attempting to alleviate motor symptoms.

Toward a closed-loop deep brain stimulation in Parkinson's disease using local field potential in parkinsonian rat model

Deep brain stimulation (DBS) is an invasive method used for treating Parkinson's disease in its advanced stages. Nowadays, the initial adjustment of DBS parameters and their automatic matching proportion to the progression of the disease is viewed as one of the research areas discussed by the researchers, which is called closed-loop DBS. Various studies were conducted regarding finding the signal(s) which reflects different symptoms of the disease. Local Field Potential (LFP) is one of the signals that is suitable for using as feedback, because it can be recorded by the same implemented electrodes for stimulation. The present study aimed to identify the distinguishing features of patients from healthy individuals using LFP signals.

METHODS

In the present study, LFP was recorded from the rats in sham and parkinsonian model groups. After evaluating the signals in the frequency domain, sixty-six features were extracted from power spectral density of LFPs. The features were classified by Support Vector Machine (SVM) to determine the ability of features for separating parkinsonian rats from healthy ones. Finally, the most effective features were selected for distinguishing between the sham and parkinsonian model groups using a genetic algorithm.

RESULTS

The results indicated that the frequency domain features of LFP signals from rats have capacity of using them as a feedback for closed-loop DBS. The accuracy of the Support Vector Machine classification using all 66 features was 80.42% which increased to 84.41% using 38 features selected by genetic algorithm. The proposed method not only increase the accuracy, but it also reduce computation by decreasing the number of the effective features. The results indicate the significant capacity of the proposed method for identifying the effective high-frequency features to control the closed-loop DBS.

CONCLUSIONS

The ability of using LFP signals as feedback in closed-loop DBS was shown by extracting useful information in frequency bands below and above 100 Hz regarding LFP signals of parkinsonian rats and sham ones. Based on the results, features at frequencies above 100 Hz were more powerful and robust than below 100 Hz. The genetic algorithm was used for optimizing the classification problem.

Dichotomy between motor and cognitive functions of midbrain cholinergic neurons

Cholinergic neurons of the pedunculopontine nucleus (PPN) are interconnected with all the basal ganglia structures, as well as with motor centers in the brainstem and medulla. Recent theories put into question whether PPN cholinergic neurons form part of a locomotor region that directly regulates the motor output, and rather suggest a modulatory role in adaptive behavior involving both motor and cognitive functions. In support of this, experimental studies in animals suggest that cholinergic neurons reinforce actions by signaling reward prediction and shape adaptations in behavior during changes of environmental contingencies. This is further supported by clinical studies proposing that decreased cholinergic transmission originated in the PPN is associated with impaired sensorimotor integration and perseverant behavior, giving rise to some of the symptoms observed in Parkinson's disease and progressive supranuclear palsy. Altogether, the evidence suggests that cholinergic neurons of the PPN, mainly through their interactions with the basal ganglia, have a leading role in action control.

Non-Linear Dynamical Analysis of Resting Tremor for Demand-Driven Deep Brain Stimulation

Parkinson's Disease (PD) is currently the second most common neurodegenerative disease. One of the most characteristic symptoms of PD is resting tremor. Local Field Potentials (LFPs) have been widely studied to investigate deviations from the typical patterns of healthy brain activity. However, the inherent dynamics of the Sub-Thalamic Nucleus (STN) LFPs and their spatiotemporal dynamics have not been well characterized. In this work, we study the non-linear dynamical behaviour of STN-LFPs of Parkinsonian patients using ε -recurrence networks. RNs are a non-linear analysis tool that encodes the geometric information of the underlying system, which can be characterised (for example, using graph theoretical measures) to extract information on the geometric properties of the attractor. Results show that the activity of the STN becomes more non-linear during the tremor episodes and that ε -recurrence network analysis is a suitable method to distinguish the transitions between movement conditions, anticipating the onset of the tremor, with the potential for application in a demand-driven deep brain stimulation system.

Bidirectional dopamine modulation of excitatory and inhibitory synaptic inputs to subthalamic neuron subsets containing α4β2 or α7 nAChRs

The subthalamic nucleus (STN) possesses microcircuits distinguished by subtypes of nicotinic acetylcholine receptors (nAChRs). Although dysfunction of the STN is well-known in Parkinson's disease, there is still little information about whether dopamine differentially modulates excitatory and inhibitory synaptic inputs to STN neurons expressing different nAChR subtypes. To address this issue, we performed brain slice patch-clamp recordings on STN neurons, while we pharmacologically manipulated dopaminergic inputs. In STN neuron subsets containing either α4β2 or α7 nAChRs, D₁ and D₂ receptors respectively enhanced and inhibited spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) and firing rates. The elevation of dopamine levels resulted in diverse regulations of synaptic transmission in these two neuron subsets, and interestingly, the dopamine regulation of sIPSCs significantly correlated with that of sEPSCs. Surprisingly, depletion of dopamine either by reserpine treatment or by unilateral 6-OHDA lesion of nigrostriatal dopaminergic neurons did not alter synaptic inputs to STN neurons, but STN neurons in the 6-OHDA-lesioned side exhibited hyperactivity. In summary, dopamine regulated both GABAergic and glutamatergic synaptic inputs to STN neuron subsets containing either α4β2 or α7 nAChRs, forming a balancing machinery to control neuronal activity. In parkinsonian mice, postsynaptic mechanisms may exist and contribute to the hyperactivity of STN neurons.

Functional imaging correlates of akinesia in Parkinson's disease: Still open issues

Akinesia is a major manifestation of Parkinson's disease (PD) related to difficulties or failures of willed movement to occur. Akinesia is still poorly understood and is not fully alleviated by standard therapeutic strategies. One reason is that the area of the clinical concept has blurred boundaries referring to confounded motor symptoms. Here, we review neuroimaging studies which, by providing access to finer-grained mechanisms, have the potential to reveal the dysfunctional brain processes that account for akinesia. It comes out that no clear common denominator could be identified across studies that are too heterogeneous with respect to the clinical/theoretical concepts and methods used. Results reveal, however, that various abnormalities within but also outside the motor and dopaminergic pathways might be associated with akinesia in PD patients. Notably, numerous yet poorly reproducible neural correlates were found in different brain regions supporting executive control by means of resting-state or task-based studies. This includes for instance the dorsolateral prefrontal cortex, the inferior frontal cortex, the supplementary motor area, the medial prefrontal cortex, the anterior cingulate cortex or the precuneus. This observation raises the issue of the multidimensional nature of akinesia. Yet, other open issues should be considered conjointly to drive future investigations. Above all, a unified terminology is needed to allow appropriate association of behavioral symptoms with brain mechanisms across studies. We adhere to a use of the term akinesia restricted to dysfunctions of movement initiation, ranging from delayed response to freezing or even total abolition of movement. We also call for targeting more specific neural mechanisms of movement preparation and action triggering with more sophisticated behavioral designs/event-related neurofunctional analyses. More work is needed to provide reliable evidence, but answering these still open issues might open up new prospects, beyond dopaminergic therapy, for managing this disabling symptom.

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No clear picture of the neural bases of PD akinesia can be drawn from the literature.

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Akinesia should be disentangled from bradykinesia and hypokinesia.

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Movement initiation dysfunctions may arise from both motor and executive disorders.

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Future neuroimaging studies should probe more specific neurocognitive processes.

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Future studies should look beyond the dopaminergic basal-ganglia circuitry.

Longer β oscillatory episodes reliably identify pathological subthalamic activity in Parkinsonism

BACKGROUND

The efficacy of deep brain stimulation (DBS) - primarily of the subthalamic nucleus (STN) - for advanced Parkinson's disease (PD) is commonly attributed to the suppression of pathological synchronous β oscillations along the cortico-thalamo-basal ganglia network. Conventional continuous high-frequency DBS indiscriminately influences pathological and normal neural activity. The DBS protocol would therefore be more effective if stimulation was only applied when necessary (closed-loop adaptive DBS).

OBJECTIVES AND METHODS

Our study aimed to identify a reliable biomarker of the pathological neuronal activity in parkinsonism that could be used as a trigger for adaptive DBS. To this end, we examined the oscillatory features of paired spiking activities recorded in three distinct nodes of the basal ganglia network of 2 African green monkeys before and after induction of parkinsonism (by MPTP intoxication).

RESULTS

Parkinsonism-related basal ganglia β oscillations consisted of synchronized time-limited episodes, rather than a continuous stretch, of β oscillatory activity. Episodic basal ganglia β oscillatory activity, although prolonged in parkinsonism, was not necessarily pathological given that short β episodes could also be detected in the healthy state. Importantly, prolongation of the basal ganglia β episodes was more pronounced than their intensification in the parkinsonian state-especially in the STN. Hence, deletion of longer β episodes was more effective than deletion of stronger β episodes in reducing parkinsonian STN synchronized oscillatory activity.

CONCLUSIONS

Prolonged STN β episodes are pathological in parkinsonism and can be used as optimal trigger for future adaptive DBS applications. © 2018 International Parkinson and Movement Disorder Society.

Ketamine induced converged synchronous gamma oscillations in the cortico-basal ganglia network of nonhuman primates

N-methyl-d-aspartate (NMDA) antagonists are widely used in anesthesia, pain management, and schizophrenia animal model studies, and recently as potential antidepressants. However, the mechanisms underlying their anesthetic, psychotic, cognitive, and emotional effects are still elusive. The basal ganglia (BG) integrate input from different cortical domains through their dopamine-modulated connections to achieve optimal behavior control. NMDA antagonists have been shown to induce gamma oscillations in human EEG recordings and in rodent cortical and BG networks. However, network relations and implications to the primate brain are still unclear. We recorded local field potentials (LFPs) simultaneously from the primary motor cortex (M1) and the external globus pallidus (GPe) of four vervet monkeys (26 sessions, 97 and 76 cortical and pallidal LFPs, respectively) before and after administration of ketamine (NMDA antagonist, 10 mg/kg im). Ketamine induced robust, spontaneous gamma (30-50 Hz) oscillations in M1 and GPe. These oscillations were initially modulated by ultraslow oscillations (~0.3 Hz) and were highly synchronized within and between M1 and the GPe (mean coherence magnitude = 0.76, 0.88, and 0.41 for M1-M1, GPe-GPe, and M1-GPe pairs). Phase differences were distributed evenly around zero with broad and very narrow distribution for the M1-M1 and GPe-GPe pairs (-3.5 ± 31.8° and -0.4 ± 6.0°), respectively. The distribution of M1-GPe phase shift was skewed to the left with a mean of -18.4 ± 20.9°. The increased gamma coherence between M1 and GPe, two central stages in the cortico-BG loops, suggests a global abnormal network phenomenon with a unique spectral signature, which is enabled by the BG funneling architecture.NEW & NOTEWORTHY This study is the first to show spontaneous gamma oscillations under NMDA antagonist in nonhuman primates. These oscillations appear in synchrony in the cortex and the basal ganglia. Phase analysis refutes the confounding effects of volume conduction and supports the funneling and amplifying architecture of the cortico-basal ganglia loops. These results suggest an abnormal network phenomenon with a unique spectral signature that could account for pathological mental and neurological states.

Studying Brain Circuit Function with Dynamic Causal Modeling for Optogenetic fMRI

Defining the large-scale behavior of brain circuits with cell type specificity is a major goal of neuroscience. However, neuronal circuit diagrams typically draw upon anatomical and electrophysiological measurements acquired in isolation. Consequently, a dynamic and cell-type-specific connectivity map has never been constructed from simultaneous measurements across the brain. Here, we introduce dynamic causal modeling (DCM) for optogenetic fMRI experiments-which uniquely allow cell-type-specific, brain-wide functional measurements-to parameterize the causal relationships among regions of a distributed brain network with cell type specificity. Strikingly, when applied to the brain-wide basal ganglia-thalamocortical network, DCM accurately reproduced the empirically observed time series, and the strongest connections were key connections of optogenetically stimulated pathways. We predict that quantitative and cell-type-specific descriptions of dynamic connectivity, as illustrated here, will empower novel systems-level understanding of neuronal circuit dynamics and facilitate the design of more effective neuromodulation therapies.

Deep brain stimulation of the subthalamic nucleus in obsessive-compulsive disorder: Neuroanatomical and pathophysiological considerations

Obsessive-compulsive disorder (OCD) is among the most disabling chronic psychiatric disorders and has a significant negative impact on multiple domains of quality of life. For patients suffering from severe refractory OCD, deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been applied. Reviewing the literature of the last years we believe that through its central position within the cortico-basal ganglia-thalamocortical circuits, the STN has a coordinating role in decision-making and action-selection mechanisms. Dysfunctional information-processing at the level of the STN is responsible for some of the core symptoms of OCD. Research confirms an electrophysiological dysfunction in the associative and limbic (non-motor) parts of the STN. Compared to Parkinson׳s disease patients, STN neurons in OCD exhibit a lower firing rate, less frequent but longer bursts, increased burst activity in the anterior ventromedial area, an asymmetrical left-sided burst distribution, and a predominant oscillatory activity in the δ-band. Moreover, there is direct evidence for the involvement of the STN in both checking behavior and OCD symptoms, which are both related to changes in electrophysiological activity in the non-motor STN. Through a combination of mechanisms, DBS of the STN seems to interrupt the disturbed information-processing, leading to a normalization of connectivity within the cortico-basal ganglia-thalamocortical circuits and consequently to a reduction in symptoms. In conclusion, based on the STN׳s strategic position within cortico-basal ganglia-thalamocortical circuits and its involvement in action-selection mechanisms that are responsible for some of the core symptoms of OCD, the STN is a mechanism-based target for DBS in OCD.

A new bio-inspired stimulator to suppress hyper-synchronized neural firing in a cortical network

Hyper-synchronous neural oscillations are the character of several neurological diseases such as epilepsy. On the other hand, glial cells and particularly astrocytes can influence neural synchronization. Therefore, based on the recent researches, a new bio-inspired stimulator is proposed which basically is a dynamical model of the astrocyte biophysical model. The performance of the new stimulator is investigated on a large-scale, cortical network. Both excitatory and inhibitory synapses are also considered in the simulated spiking neural network. The simulation results show that the new stimulator has a good performance and is able to reduce recurrent abnormal excitability which in turn avoids the hyper-synchronous neural firing in the spiking neural network. In this way, the proposed stimulator has a demand controlled characteristic and is a good candidate for deep brain stimulation (DBS) technique to successfully suppress the neural hyper-synchronization.

Patterns of Cortical Synchronization in Isolated Dystonia Compared With Parkinson Disease

IMPORTANCE

Isolated dystonia and Parkinson disease (PD) are disorders of the basal gangliothalamocortical network. They have largely distinct clinical profiles, but both disorders respond to deep brain stimulation (DBS) in the same subcortical targets using similar stimulation paradigms, suggesting pathophysiologic overlap. We hypothesized that, similar to PD, isolated dystonia is associated with elevated cortical neuronal synchronization.

OBJECTIVE

To investigate the electrophysiologic characteristics of the sensorimotor cortex arm-related area using a temporary subdural electrode strip in patients with isolated dystonia and PD undergoing DBS implantation in the awake state.

DESIGN, SETTING, AND PARTICIPANTS

An observational study recruited patients scheduled for DBS at the University of California, San Francisco and the San Francisco Veterans Affairs Medical Center. Data were collected from May 1, 2008, through April 1, 2015. Findings are reported for 22 patients with isolated cervical or segmental dystonia (8 with [DYST-ARM] and 14 without [DYST] arm symptoms) and 14 patients with akinetic rigid PD. Data were analyzed from November 1, 2014, through May 1, 2015.

MAIN OUTCOMES AND MEASURES

Cortical local field potentials, power spectral density, and phase-amplitude coupling (PAC).

RESULTS

Among our 3 groups that together included 36 patients, cortical PAC was present in primary motor and premotor arm-related areas for all groups, but the DYST group was less likely to exhibit increased PAC (P = .008). Similar to what has been shown for patients with PD, subthalamic DBS reversibly decreased PAC in a subset of patients with dystonia who were studied before and during intraoperative test stimulation (n = 4). At rest, broadband gamma (50-200 Hz) power in the primary motor cortex was greater in the DYST-ARM and PD groups compared with the DYST group, whereas alpha (8-13 Hz) and beta (13-30 Hz) power was comparable in all 3 groups. During movement, the DYST-ARM group had impaired beta and low gamma desynchronization in the primary motor cortex.

CONCLUSIONS AND RELEVANCE

Isolated dystonia and PD have physiologic overlap with respect to high levels of motor cortex synchronization and reduction of cortical synchronization by subthalamic DBS, providing an explanation for their similar therapeutic response to basal ganglia stimulation.

Characteristics of subthalamic oscillatory activity in parkinsonian akinetic-rigid type and mixed type

OBJECTIVE

To explore neurons with β oscillatory activity in the subthalamic nucleus (STN) in relation to parkinsonian motor signs.

METHODS

We studied 27 patients with Parkinson's disease (PD) who underwent electrode implantation for STN deep brain stimulation. Thirteen patients were classified as akinetic-rigid (AR) type and 14 patients were classified as mixed type. Microelectrode recording was performed in the STN and the electromyogram (EMG) was simultaneously recorded. Single-unit and spectral analyses were performed. Coherence analysis was used to explore the relationship between β oscillatory activity and EMG activity. Unpaired t-test and chi-square were used to compare the differences between the two PD types.

RESULTS

Of 130 neurons identified in the AR type, 43.8% were β oscillatory neurons (mean: 21.3 ± 6.87 Hz, βFB) and 0.8% were tremor frequency oscillatory neurons (4-6 Hz, TFB). Of 102 neurons identified in the mixed type, 19.6% were β oscillatory neurons and 26.5% were TFB oscillatory neurons. There was a significant difference in proportion of neurons with βFB and TFB oscillations between the two PD groups. Additionally, 12% of the βFB oscillatory neurons were coherent with limb EMG of the AR type, but there was no coherence in the mixed type. Most oscillatory neurons were localized in the dorsal portion of the STN.

CONCLUSION

The STN βFB oscillatory neurons correlate with parkinsonian rigidity-bradykinesia. The high proportion of βFB oscillatory neurons found in the AR type of PD is indirect evidence for their importance in generating motor impairment.

Nicotinic receptor subtype-selective circuit patterns in the subthalamic nucleus

The glutamatergic subthalamic nucleus (STN) exerts control over motor output through nuclei of the basal ganglia. High-frequency electrical stimuli in the STN effectively alleviate motor symptoms in movement disorders, and cholinergic stimulation boosts this effect. To gain knowledge about the mechanisms of cholinergic modulation in the STN, we studied cellular and circuit aspects of nicotinic acetylcholine receptors (nAChRs) in mouse STN. We discovered two largely divergent microcircuits in the STN; these are regulated in part by either α4β2 or α7 nAChRs. STN neurons containing α4β2 nAChRs (α4β2 neurons) received more glutamatergic inputs, and preferentially innervated GABAergic neurons in the substantia nigra pars reticulata. In contrast, STN neurons containing α7 nAChRs (α7 neurons) received more GABAergic inputs, and preferentially innervated dopaminergic neurons in the substantia nigra pars compacta. Interestingly, local electrical stimuli excited a majority (79%) of α4β2 neurons but exerted strong inhibition in 58% of α7 neurons, indicating an additional diversity of STN neurons: responses to electrical stimulation. Chronic exposure to nicotine selectively affects α4β2 nAChRs in STN: this treatment increased the number of α4β2 neurons, upregulated α4-containing nAChR number and sensitivity, and enhanced the basal firing rate of α4β2 neurons both ex vivo and in vivo. Thus, chronic nicotine enhances the function of the microcircuit involving α4β2 nAChRs. This indicates chronic exposure to nicotinic agonist as a potential pharmacological intervention to alter selectively the balance between these two microcircuits, and may provide a means to inhibit substantia nigra dopaminergic neurons.

Optogenetics and deep brain stimulation neurotechnologies

Brain neural network is composed of densely packed, intricately wired neurons whose activity patterns ultimately give rise to every behavior, thought, or emotion that we experience. Over the past decade, a novel neurotechnique, optogenetics that combines light and genetic methods to control or monitor neural activity patterns, has proven to be revolutionary in understanding the functional role of specific neural circuits. We here briefly describe recent advance in optogenetics and compare optogenetics with deep brain stimulation technology that holds the promise for treating many neurological and psychiatric disorders.

The rotenone-induced rat model of Parkinson's disease: behavioral and electrophysiological findings

Exposure to rotenone leads to parkinsonian features, such as loss of dopaminergic neurons in the substantia nigra and motor impairment, however, the validity of this model has recently been questioned. In rodent and monkey models of Parkinson's disease (PD) abnormal neuronal activity in the basal ganglia motor loop has been described, with hyperactivity of the subthalamic nucleus (STN) similar to that found in PD. The present study aims at providing new and more specific evidence for the validity of the rotenone rat model of PD by examining whether neuronal activity in the STN is altered. Male Sprague Dawley rats were treated with rotenone injections (2.5mg/kg bodyweight intraperitoneally) for 60 days. Behavioral analysis showed an impairment in the rotarod and hanging wire test in the rotenone group (p<0.05), accompanied by a decline in tyrosine hydroxylase immunoreactive neurons in the nigro-striatal region (p<0.001). Thereafter, single unit (SU) activities and local field potentials were recorded in the STN in urethane anesthetized rats. The SU analysis revealed a higher neuronal discharge rate (p<0.001), more bursts per minute (p=0.006) and a higher oscillatory activity (p=0.008) in the STN of rotenone treated rats. Spectral analysis showed an increase of relative beta power in the STN as well as in the motor cortex. We found electrophysiological key features of PD pathology and pathophysiology in the STN of rotenone treated rats. Therefore, the rotenone-induced rat model of PD deserves further attention since it covers more aspects than dopamine depletion and implies the reproducibility of PD specific features.

Time and frequency-dependent modulation of local field potential synchronization by deep brain stimulation

High-frequency electrical stimulation of specific brain structures, known as deep brain stimulation (DBS), is an effective treatment for movement disorders, but mechanisms of action remain unclear. We examined the time-dependent effects of DBS applied to the entopeduncular nucleus (EP), the rat homolog of the internal globus pallidus, a target used for treatment of both dystonia and Parkinson's disease (PD). We performed simultaneous multi-site local field potential (LFP) recordings in urethane-anesthetized rats to assess the effects of high-frequency (HF, 130 Hz; clinically effective), low-frequency (LF, 15 Hz; ineffective) and sham DBS delivered to EP. LFP activity was recorded from dorsal striatum (STR), ventroanterior thalamus (VA), primary motor cortex (M1), and the stimulation site in EP. Spontaneous and acute stimulation-induced LFP oscillation power and functional connectivity were assessed at baseline, and after 30, 60, and 90 minutes of stimulation. HF EP DBS produced widespread alterations in spontaneous and stimulus-induced LFP oscillations, with some effects similar across regions and others occurring in a region- and frequency band-specific manner. Many of these changes evolved over time. HF EP DBS produced an initial transient reduction in power in the low beta band in M1 and STR; however, phase synchronization between these regions in the low beta band was markedly suppressed at all time points. DBS also enhanced low gamma synchronization throughout the circuit. With sustained stimulation, there were significant reductions in low beta synchronization between M1-VA and STR-VA, and increases in power within regions in the faster frequency bands. HF DBS also suppressed the ability of acute EP stimulation to induce beta oscillations in all regions along the circuit. This dynamic pattern of synchronizing and desynchronizing effects of EP DBS suggests a complex modulation of activity along cortico-BG-thalamic circuits underlying the therapeutic effects of GPi DBS for conditions such as PD and dystonia.

Information processing in the primate basal ganglia during sensory-guided and internally driven rhythmic tapping

Gamma (γ) and beta (β) oscillations seem to play complementary functions in the cortico-basal ganglia-thalamo-cortical circuit (CBGT) during motor behavior. We investigated the time-varying changes of the putaminal spiking activity and the spectral power of local field potentials (LFPs) during a task where the rhythmic tapping of monkeys was guided by isochronous stimuli separated by a fixed duration (synchronization phase), followed by a period of internally timed movements (continuation phase). We found that the power of both bands and the discharge rate of cells showed an orderly change in magnitude as a function of the duration and/or the serial order of the intervals executed rhythmically. More LFPs were tuned to duration and/or serial order in the β- than the γ-band, although different values of preferred features were represented by single cells and by both bands. Importantly, in the LFPs tuned to serial order, there was a strong bias toward the continuation phase for the β-band when aligned to movements, and a bias toward the synchronization phase for the γ-band when aligned to the stimuli. Our results suggest that γ-oscillations reflect local computations associated with stimulus processing, whereas β-activity involves the entrainment of large putaminal circuits, probably in conjunction with other elements of CBGT, during internally driven rhythmic tapping.

Cortico-muscular coupling and motor performance are modulated by 20 Hz transcranial alternating current stimulation (tACS) in Parkinson's disease

Parkinson’s disease (PD) is associated with pathologically altered oscillatory activity. While synchronized oscillations between 13 and 30 Hz are increased within a cortico-subcortical network, cortico-muscular coupling (CMC) is decreased. The present study aims at investigating the effect of non-invasive transcranial alternating current stimulation (tACS) of the primary motor cortex (M1) on motor symptoms and motor-cortical oscillations in PD. In 10 PD patients and 10 healthy control subjects, static isometric contraction, dynamic fast finger tapping, and diadochokinesia of the more severely affected hand were investigated prior to and shortly after tACS of the contralateral M1 at 10 Hz vs. 20 Hz vs. sham. During isometric contraction, neuromagnetic activity was recorded using magnetoencephalography. 20 Hz tACS attenuated beta band CMC during isometric contraction and amplitude variability during finger tapping in PD patients but not in healthy control subjects. 10 Hz tACS yielded no significant after-effects. The present data suggest that PD is associated with pathophysiological alterations which abet a higher responsiveness toward frequency-specific tACS – possibly due to pathologically altered motor-cortical oscillatory synchronization at frequencies between 13 and 30 Hz.

Non-linear dynamics in parkinsonism

Over the last 30 years, the functions (and dysfunctions) of the sensory-motor circuitry have been mostly conceptualized using linear modelizations which have resulted in two main models: the “rate hypothesis” and the “oscillatory hypothesis.” In these two models, the basal ganglia data stream is envisaged as a random temporal combination of independent simple patterns issued from its probability distribution of interval interspikes or its spectrum of frequencies respectively. More recently, non-linear analyses have been introduced in the modelization of motor circuitry activities, and they have provided evidences that complex temporal organizations exist in basal ganglia neuronal activities. Regarding movement disorders, these complex temporal organizations in the basal ganglia data stream differ between conditions (i.e., parkinsonism, dyskinesia, healthy control) and are responsive to treatments (i.e., l-DOPA, deep brain stimulation). A body of evidence has reported that basal ganglia neuronal entropy (a marker for complexity/irregularity in time series) is higher in hypokinetic state. In line with these findings, an entropy-based model has been recently formulated to introduce basal ganglia entropy as a marker for the alteration of motor processing and a factor of motor inhibition. Importantly, non-linear features have also been identified as a marker of condition and/or treatment effects in brain global signals (EEG), muscular activities (EMG), or kinetic of motor symptoms (tremor, gait) of patients with movement disorders. It is therefore warranted that the non-linear dynamics of motor circuitry will contribute to a better understanding of the neuronal dysfunctions underlying the spectrum of parkinsonian motor symptoms including tremor, rigidity, and hypokinesia.