Neuronal firing rates and patterns in the globus pallidus internus of patients with cervical dystonia differ from those with Parkinson's disease

Why so slow? Models of parkinsonian bradykinesia

Bradykinesia, or slowness of movement, is a defining feature of Parkinson disease (PD) and a major contributor to the negative effects on quality of life associated with this disorder and related conditions. A dominant pathophysiological model of bradykinesia in PD has existed for approximately 30 years and has been the basis for the development of several therapeutic interventions, but accumulating evidence has made this model increasingly untenable. Although more recent models have been proposed, they also appear to be flawed. In this Perspective, I consider the leading prior models of bradykinesia in PD and argue that a more functionally related model is required, one that considers changes that disrupt the fundamental process of accurate information transmission. In doing so, I review emerging evidence of network level functional connectivity changes, information transfer dysfunction and potential motor code transmission error and present a novel model of bradykinesia in PD that incorporates this evidence. I hope that this model may reconcile inconsistencies in its predecessors and encourage further development of therapeutic interventions.

Cerebellar nuclei cells produce distinct pathogenic spike signatures in mouse models of ataxia, dystonia, and tremor

The cerebellum contributes to a diverse array of motor conditions, including ataxia, dystonia, and tremor. The neural substrates that encode this diversity are unclear. Here, we tested whether the neural spike activity of cerebellar output neurons is distinct between movement disorders with different impairments, generalizable across movement disorders with similar impairments, and capable of causing distinct movement impairments. Using in vivo awake recordings as input data, we trained a supervised classifier model to differentiate the spike parameters between mouse models for ataxia, dystonia, and tremor. The classifier model correctly assigned mouse phenotypes based on single-neuron signatures. Spike signatures were shared across etiologically distinct but phenotypically similar disease models. Mimicking these pathophysiological spike signatures with optogenetics induced the predicted motor impairments in otherwise healthy mice. These data show that distinct spike signatures promote the behavioral presentation of cerebellar diseases.

Bilateral Globus Pallidus Externus Deep Brain Stimulation for the Treatment of Refractory Tourette Syndrome: An Open Clinical Trial

OBJECTIVES

We have previously proposed that Tourette syndrome (TS) is the clinical expression of the hyperactivity of globus pallidus externus (GPe) and various cortical areas. This study was designed to test this hypothesis by verifying the efficacy and safety of bilateral GPe deep brain stimulation (DBS) for treating refractory TS.

MATERIALS AND METHODS

In this open clinical trial, 13 patients were operated on. Target coordinates (center of GPe) were obtained by direct visualization. Physiological mapping was performed with macrostimulation and microrecording. Primary and secondary outcome measures were, respectively, responder and improvement rates of TS and comorbidities, according to pre- and postoperative scores on the following assessment instruments: Yale Global Tic Severity Scale, Yale-Brown Obsessive Compulsive Scale, Beck Depression Inventory/Hamilton Depression Rating Scale, Beck Anxiety Inventory/Hamilton Anxiety Rating Scale, and Concentrated Attention test.

RESULTS

Intraoperative stimulation (100 Hz/5.0V) did not produce any adverse effects or impact on tics. Microrecording revealed bursting cells discharging synchronously with tics in the central part of the dorsal half of GPe. Patients were followed up for a mean of 61.46±48.50 months. Responder rates were 76.9%, 75%, 71.4%, 71.4%, and 85.7%, respectively, for TS, obsessive-compulsive disorder (OCD), depression, anxiety, and attention deficit hyperactivity disorder. Mean improvements among responders in TS, OCD, depression, and anxiety were 77.4%, 74.7%, 89%, and 84.8%, respectively. After starting stimulation, tic improvement was usually delayed, taking up to ten days to manifest. Afterward, it increased over time, usually reaching its maximum at approximately one year postoperatively. The best stimulation parameters were 2.3V to 3.0V, 90 to 120 μsec, and 100 to 150 Hz, and the most effective contacts were the two dorsal ones. Two complications were registered: reversible impairment of previous depression and transient unilateral bradykinesia.

CONCLUSIONS

Bilateral GPe-DBS proved to be low risk and quite effective for treating TS and comorbidities, ratifying the pathophysiological hypothesis that led to this study. Moreover, it compared favorably with DBS of other targets currently in use.

DYT1 dystonia: Neurophysiological properties of the pallidal activity

OBJECTIVES

The aim of this paper is to find the differences in the physiology of the pallidal neurons in DYT1 and non-DYT1 dystonia.

METHODS

We performed microelectrode recording of the single unit activity in both segments of the globus pallidus during stereotactic implantation of electrodes for deep brain stimulation (DBS).

RESULTS

We found a reduced firing rate, reduced burst rate, and increased pause index in both pallidal segments in DYT1. Also, in DYT1 the activity in both pallidal segments was similar, but not so in non-DYT1.

CONCLUSION

The results suggest a common pathological focus for both pallidal segments, located in the striatum. We also speculate that strong striatal influence on GPi and GPe overrides other input sources to the pallidal nuclei causing similarity in neuronal activity.

SIGNIFICANCE

We found significant differences in neuronal activity between DYT1 and non-DYT1 neurons. Our findings shed light on the pathophysiology of DYT-1 dystonia which can be very different from non-DYT1 dystonia and have other efficient treatment tactics.

Influence of data sampling methods on the representation of neural spiking activity in vivo

In vivo single-unit recordings distinguish the basal spiking properties of neurons in different experimental settings and disease states. Here, we examined over 300 spike trains recorded from Purkinje cells and cerebellar nuclei neurons to test whether data sampling approaches influence the extraction of rich descriptors of firing properties. Our analyses included neurons recorded in awake and anesthetized control mice, and disease models of ataxia, dystonia, and tremor. We find that recording duration circumscribes overall representations of firing rate and pattern. Notably, shorter recording durations skew estimates for global firing rate variability toward lower values. We also find that only some populations of neurons in the same mouse are more similar to each other than to neurons recorded in different mice. These data reveal that recording duration and approach are primary considerations when interpreting task-independent single neuron firing properties. If not accounted for, group differences may be concealed or exaggerated.

Brain oscillatory dysfunctions in dystonia

Dystonia is a hyperkinetic movement disorder associated with loss of inhibition, abnormal plasticity, dysfunctional sensorimotor integration, and brain oscillatory dysfunctions at cortical and subcortical levels of the central nervous system. Hence, dystonia is considered a network disorder that can, in many cases, be efficiently treated by pallidal deep brain stimulation (DBS). Abnormal oscillatory activity has been identified across the motor circuit of patients with dystonia. Increased low frequency (LF) synchronization in the internal pallidum is the most prominent abnormality. LF oscillations have been associated with the severity of dystonic motor symptoms; they are suppressed by DBS and localized to the clinically most effective stimulation site. Although the origin of these pathologic changes in brain activity needs further clarifications, their characterization will help in adjusting DBS parameters for successful clinical outcome.

Optogenetic Activation of Striatopallidal Neurons Reveals Altered HCN Gating in DYT1 Dystonia

Firing activity of external globus pallidus (GPe) is crucial for motor control and is severely perturbed in dystonia, a movement disorder characterized by involuntary, repetitive muscle contractions. Here, we show that GPe projection neurons exhibit a reduction of firing frequency and an irregular pattern in a DYT1 dystonia model. Optogenetic activation of the striatopallidal pathway fails to reset pacemaking activity of GPe neurons in mutant mice. Abnormal firing is paralleled by alterations in motor learning. We find that loss of dopamine D2 receptor-dependent inhibition causes increased GABA input at striatopallidal synapses, with subsequent downregulation of hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels. Accordingly, enhancing in vivo HCN channel activity or blocking GABA release restores both the ability of striatopallidal inputs to pause ongoing GPe activity and motor coordination deficits. Our findings demonstrate an impaired striatopallidal connectivity, supporting the central role of GPe in motor control and, more importantly, identifying potential pharmacological targets for dystonia.

Effectiveness of Low-Frequency Pallidal Deep Brain Stimulation at 65 Hz in Tourette Syndrome

OBJECTIVES

Pallidal deep brain stimulation (DBS) for refractory Tourette syndrome (TS) is often applied using a high frequency. The effectiveness of low-frequency long-term stimulation is unknown. We aimed to evaluate the clinical efficacy of low-frequency DBS applied to the globus pallidus pars internus (GPi) at 65 Hz for the treatment of TS, with long-term follow-up, to provide data for the optimization of stimulation parameters.

MATERIALS AND METHODS

A total of six patients with refractory TS were implanted with electrodes in the GPi and were assigned to receive low-frequency (65 Hz) DBS programming. Assessments were performed pre-DBS and at 3, 12, and a median of 34 (range 26-48) months post-DBS. The primary outcome was tic severity, as assessed by the Yale Global Tic Severity Scale (YGTSS), and the secondary outcomes were comorbid behavioral disorders, mood, functioning, and quality of life.

RESULTS

We noted significant differences in the YGTSS scores between the baseline and the post-DBS follow-ups (p = 0.01). At the final follow up, four of six (66.6%) patients had a greater than 50% reduction in the YGTSS score, whereas the remaining two patients showed a mild worsening of tic severity. The secondary outcome measures also showed remarkable improvements in associated behavioral disorders, mood, functioning, and quality of life. Stimulation-induced adverse effects were not reported, although a device-related complication (an uncomfortable feeling in the neck) occurred in 1 patient.

CONCLUSIONS

The results of this study indicated that low-frequency DBS represents an effective and practical treatment for refractory TS with comparable efficacy to high-frequency DBS.

Progression of basal ganglia pathology in heterozygous Q175 knock-in Huntington's disease mice

We used behavioral testing and morphological methods to detail the progression of basal ganglia neuron type-specific pathology and the deficits stemming from them in male heterozygous Q175 mice, compared to age-matched WT males. A rotarod deficit was not present in Q175 mice until 18 months, but increased open field turn rate (reflecting hyperkinesia) and open field anxiety were evident at 6 months. No loss of striatal neurons was seen out to 18 months, but ENK+ and DARPP32+ striatal perikarya were fewer by 6 months, due to diminished expression, with further decline by 18 months. No reduction in SP+ striatal perikarya or striatal interneurons was seen in Q175 mice at 18 months, but cholinergic interneurons showed dendrite attenuation by 6 months. Despite reduced ENK expression in indirect pathway striatal perikarya, ENK-immunostained terminals in globus pallidus externus (GPe) were more abundant at 6 months and remained so out to 18 months. Similarly, SP-immunostained terminals from striatal direct pathway neurons were more abundant in globus pallidus internus and substantia nigra at 6 months and remained so at 18 months. FoxP2+ arkypallidal GPe neurons and subthalamic nucleus neurons were lost by 18 months but not prototypical PARV+ GPe neurons or dopaminergic nigral neurons. Our results show that striatal projection neuron abnormalities and behavioral abnormalities reflecting them develop between 2 and 6 months of age in Q175 male heterozygotes, indicating early effects of the HD mutation. The striatal pathologies resemble those in human HD, but are less severe at 18 months than even in premanifest HD.

Electrophysiological signatures predict clinical outcomes after deep brain stimulation of the globus pallidus internus in Meige syndrome

OBJECTIVE

Deep brain stimulation (DBS) of the globus pallidus internus (GPi) has been shown to be a safe and effective alternative therapy for ameliorating medically refractory primary Meige syndrome. However, the associations between DBS target position and surrounding electrophysiological properties as well as patients' clinical outcomes remains largely unknown. In a large number of patients, we investigated electrophysiological features around stimulation targets and explored their roles in predicting clinical outcomes following bilateral GPi-DBS.

METHODS

The locations of DBS active contacts along the long axis of the GPi in a standard space were calculated and compared among three groups with different clinical outcomes. The firing rates of individual neurons within the GPi were calculated for each patient and compared across the three groups.

RESULTS

Compared with the bad group (poor clinical outcome), active contacts in the good group (good clinical outcome) and the best group (best clinical outcome) were located in the more posterior GPi. The average firing rates in the good and best groups were significantly higher than in the bad group, and this difference was pronounced within the ventral GPi. For the bad group, the average firing rates were significantly lower in the ventral than in the dorsal GPi.

CONCLUSIONS

This study suggests that DBS of the posterior GPi may produce better clinical outcomes during primary Meige syndrome treatment and that higher GPi neuronal activity, particularly within the ventral part, can be used as a biomarker to guide DBS electrode implantation during surgery.

Globus pallidus internus activity during simultaneous bilateral microelectrode recordings in status dystonicus

Limited data are available regarding the electrophysiology of status dystonicus (SD). We report simultaneous microelectrode recordings (MERs) from the globus pallidus internus (GPi) of a patient with SD who was treated with bilateral deep brain stimulation (DBS). Mean neuronal discharge rate was of 30.1 ± 10.9 Hz and 38.5 Hz ± 11.1 Hz for the right and left GPi, respectively. On the right side, neuronal electrical activity was completely abolished at the target point, whereas the mean burst index values showed a predominance of bursting and irregular activity along trajectories on both sides. Our data are in line with previous findings of pallidal irregular hypoactivity as a potential electrophysiological marker of dystonia and thus SD, but further electrophysiological studies are needed to confirm our results.

The Association of Dexmedetomidine with Firing Properties in Pallidal Neurons

BACKGROUND

Microelectrode recordings (MERs) are used during deep brain stimulation surgery (DBS) to optimize patient outcomes and provide a unique method of collecting data regarding neurological conditions. However, MERs can be affected by anesthetics such as dexmedetomidine. Little is known about the effects of dexmedetomidine (DEX) on the globus pallidus interna (GPi), a common target for DBS. The primary aim of this study is to investigate the hypothesis that DEX is associated with alterations in GPi MERs.

METHODS

We conducted a retrospective analysis comparing MERs from patients with Parkinson's disease (PD) and dystonia who underwent insertion of DBS of the GPi under DEX sedation with those who went through the same procedure without DEX (No DEX).

RESULTS

Firing rates for GPi neurons in the DEX group were lower (57.44 ± 2.04; mean ± SEM, n = 163 cells) than the No DEX group (69.53 ± 2.06, n = 112 cells, P < 0.0001). Overall, DEX was associated with a greater proportion of GPi cells classified as firing in bursty pattern compared to our No DEX group. (29.41%, n = 153 vs 14.81%, n = 108, P = 0.008). This effect was present for both PD and dystonia patients who underwent the procedure. High doses of DEX were associated with lower firing rates than low doses.

CONCLUSIONS

Our results suggest that DEX is associated with a decrease in GPi firing rates and are associated with an increase in burstiness. Furthermore, these effects are similar between dystonia and PD patients. Lastly, the effects of DEX may differ between high doses and low doses.

Can Pallidal Deep Brain Stimulation Rescue Borderline Dystonia? Possible Coexistence of Functional (Psychogenic) and Organic Components

The diagnosis and treatment of functional movement disorders are challenging for clinicians who manage patients with movement disorders. The borderline between functional and organic dystonia is often ambiguous. Patients with functional dystonia are poor responders to pallidal deep brain stimulation (DBS) and are not good candidates for DBS surgery. Thus, if patients with medically refractory dystonia have functional features, they are usually left untreated with DBS surgery. In order to investigate the outcome of functional dystonia in response to pallidal DBS surgery, we retrospectively included five patients with this condition. Their dystonia was diagnosed as organic by dystonia specialists and also as functional according to the Fahn and Williams criteria or the Gupta and Lang Proposed Revisions. Microelectrode recordings in the globus pallidus internus of all patients showed a cell-firing pattern of bursting with interburst intervals, which is considered typical of organic dystonia. Although their clinical course after DBS surgery was incongruent to organic dystonia, the outcome was good. Our results question the possibility to clearly differentiate functional dystonia from organic dystonia. We hypothesized that functional dystonia can coexist with organic dystonia, and that medically intractable dystonia with combined functional and organic features can be successfully treated by DBS surgery.

Neurophysiological insights in dystonia and its response to deep brain stimulation treatment

Dystonia is a movement disorder characterised by involuntary muscle contractions resulting in abnormal movements, postures and tremor. The pathophysiology of dystonia is not fully understood but loss of neuronal inhibition, excessive sensorimotor plasticity and defective sensory processing are thought to contribute to network dysfunction underlying the disorder. Neurophysiology studies have been important in furthering our understanding of dystonia and have provided insights into the mechanism of effective dystonia treatment with pallidal deep brain stimulation. In this article we review neurophysiology studies in dystonia and its treatment with Deep Brain Stimulation, including Transcranial magnetic stimulation studies, studies of reflexes and sensory processing, and oscillatory activity recordings including local field potentials, micro-recordings, EEG and evoked potentials.

Movement disorders in patients with alternating hemiplegia: "Soft" and "stiff" at the same time

OBJECTIVE

To assess nonparoxysmal movement disorders in ATP1A3 mutation-positive patients with alternating hemiplegia of childhood (AHC).

METHODS

Twenty-eight patients underwent neurologic examination with particular focus on movement phenomenology by a specialist in movement disorders. Video recordings were reviewed by another movement disorders specialist and data were correlated with patients' characteristics.

RESULTS

Ten patients were diagnosed with chorea, 16 with dystonia (nonparoxysmal), 4 with myoclonus, and 2 with ataxia. Nine patients had more than one movement disorder and 8 patients had none. The degree of movement disorder was moderate to severe in 12/28 patients. At inclusion, dystonic patients (n = 16) were older (p = 0.007) than nondystonic patients. Moreover, patients (n = 18) with dystonia or chorea, or both, had earlier disease onset (p = 0.042) and more severe neurologic impairment (p = 0.012), but this did not correlate with genotype. All patients presented with hypotonia, which was characterized as moderate or severe in 16/28. Patients with dystonia or chorea (n = 18) had more pronounced hypotonia (p = 0.011). Bradykinesia (n = 16) was associated with an early age at assessment (p < 0.01). Significant dysarthria was diagnosed in 11/25 cases. A history of acute neurologic deterioration and further regression of motor function, typically after a stressful event, was reported in 7 patients.

CONCLUSIONS

Despite the relatively limited number of patients and the cross-sectional nature of the study, this detailed categorization of movement disorders in patients with AHC offers valuable insight into their precise characterization. Further longitudinal studies on this topic are needed.

Parkinsonism Differently Affects the Single Neuronal Activity in the Primary and Supplementary Motor Areas in Monkeys: An Investigation in Linear and Nonlinear Domains

The changes in neuronal firing activity in the primary motor cortex (M1) and supplementary motor area (SMA) were compared in monkeys rendered parkinsonian by treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The neuronal dynamic was characterized using mathematical tools defined in different frameworks (rate, oscillations or complex patterns). Then, and for each cortical area, multivariate and discriminate analyses were further performed on these features to identify those important to differentiate between the normal and the pathological neuronal activity. Our results show a different order in the importance of the features to discriminate the pathological state in each cortical area which suggests that the M1 and the SMA exhibit dissimilarities in their neuronal alterations induced by parkinsonism. Our findings highlight the need for multiple mathematical frameworks to best characterize the pathological neuronal activity related to parkinsonism. Future translational studies are warranted to investigate the causal relationships between cortical region-specificities, dominant pathological hallmarks and symptoms.

Local field potential dynamics in the primate cortex in relation to parkinsonism reveled by machine learning: A comparison between the primary motor cortex and the supplementary area

The present study compares the cortical local field potentials (LFPs) in the primary motor cortex (M1) and the supplementary motor area (SMA) of non-human primates rendered Parkinsonian with administration of dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The dynamic of the LFPs was investigated under several mathematical frameworks and machine learning was used to discriminate the recordings based on these features between healthy, parkinsonian with off-medication and parkinsonian with on-medication states. The importance of each feature in the discrimination process was further investigated. The dynamic of the LFPs in M1 and SMA was affected regarding its variability (time domain analysis), oscillatory activities (frequency domain analysis) and complex patterns (non-linear domain analysis). Machine learning algorithms achieved accuracy near 0.90 for comparisons between conditions. The TreeBagger algorithm provided best accuracy. The relative importance of these features differed with the cortical location, condition and treatment. Overall, the most important features included beta oscillation, fractal dimension, gamma oscillation, entropy and asymmetry of amplitude fluctuation. The importance of features in discriminating between normal and pathological states, and on- or off-medication states depends on the pair-comparison and it is region-specific. These findings are discussed regarding the refinement of current models for movement disorders and the development of on-demand therapies.

Real-time machine learning classification of pallidal borders during deep brain stimulation surgery

OBJECTIVE

Deep brain stimulation (DBS) of the internal segment of the globus pallidus (GPi) in patients with Parkinson's disease and dystonia improves motor symptoms and quality of life. Traditionally, pallidal borders have been demarcated by electrophysiological microelectrode recordings (MERs) during DBS surgery. However, detection of pallidal borders can be challenging due to the variability of the firing characteristics of neurons encountered along the trajectory. MER can also be time-consuming and therefore costly. Here we show the feasibility of real-time machine learning classification of striato-pallidal borders to assist neurosurgeons during DBS surgery.

APPROACH

An electrophysiological dataset from 116 trajectories of 42 patients consisting of 11 774 MER segments of background spiking activity in five classes of disease was used to train the classification algorithm. The five classes included awake Parkinson's disease patients, as well as awake and lightly anesthetized genetic and non-genetic dystonia patients. A machine learning algorithm was designed to provide prediction of the striato-pallidal borders, based on hidden Markov models (HMMs) and the L₁-distance measure in normalized root mean square (NRMS) and power spectra of the MER. We tested its performance prospectively against the judgment of three electrophysiologists in the operating rooms of three hospitals using newly collected data.

MAIN RESULTS

The awake and the light anesthesia dystonia classes could be merged. Using MER NRMS and spectra, the machine learning algorithm was on par with the performance of the three electrophysiologists across the striatum-GPe, GPe-GPi, and GPi-exit transitions for all disease classes.

SIGNIFICANCE

Machine learning algorithms enable real-time GPi navigation systems to potentially shorten the duration of electrophysiological mapping of pallidal borders, while ensuring correct pallidal border detection.

Toward adaptive deep brain stimulation for dystonia

The presence of abnormal neural oscillations within the cortico-basal ganglia-thalamo-cortical (CBGTC) network has emerged as one of the current principal theories to explain the pathophysiology of movement disorders. In theory, these oscillations can be used as biomarkers and thereby serve as a feedback signal to control the delivery of deep brain stimulation (DBS). This new form of DBS, dependent on different characteristics of pathological oscillations, is called adaptive DBS (aDBS), and it has already been applied in patients with Parkinson's disease. In this review, the authors summarize the scientific research to date on pathological oscillations in dystonia and address potential biomarkers that might be used as a feedback signal for controlling aDBS in patients with dystonia.

Dystonia and dopamine: From phenomenology to pathophysiology

A line of evidence suggests that the pathophysiology of dystonia involves the striatum, whose activity is modulated among other neurotransmitters, by the dopaminergic system. However, the link between dystonia and dopamine appears complex and remains unclear. Here, we propose a physiological approach to investigate the clinical and experimental data supporting a role of the dopaminergic system in the pathophysiology of dystonic syndromes. Because dystonia is a disorder of motor routines, we first focus on the role of dopamine and striatum in procedural learning. Second, we consider the phenomenology of dystonia from every angle in order to search for features giving food for thought regarding the pathophysiology of the disorder. Then, for each dystonic phenotype, we review, when available, the experimental and imaging data supporting a connection with the dopaminergic system. Finally, we propose a putative model in which the different phenotypes could be explained by changes in the balance between the direct and indirect striato-pallidal pathways, a process critically controlled by the level of dopamine within the striatum. Search strategy and selection criteria References for this article were identified through searches in PubMed with the search terms « dystonia », « dopamine", « striatum », « basal ganglia », « imaging data », « animal model », « procedural learning », « pathophysiology », and « plasticity » from 1998 until 2018. Articles were also identified through searches of the authors' own files. Only selected papers published in English were reviewed. The final reference list was generated on the basis of originality and relevance to the broad scope of this review.

A new approach for estimation of spiketrain patterns in basal ganglia

The pathophysiological model of dystonia proposed that in addition to reduced firing rate in the internal pallidum, changes in the pattern may also play a role in disease manifestation. While common methods for patterns separation depends on arbitrary spiketrain parameters, we considered the new method for neural patterns based on spike density histograms and hierarchical clustering of real datasets. We used the single unit activity recordings from the globus pallidus external (GPe) and the globus pallidus internal (GPi) from 10 cervical dystonia (CD), 7 segmental dystonia (SD) and 8 generalized dystonia (GD) patients undergoing deep brain stimulation surgery. Using novel method, we separated three patterns of activity: burst, burst-like and tonic. Using this separation, we revealed the differences both in firing rate and pattern distribution between dystonia patients. We have shown the suitability of the proposed method for pattern clusterization on real data and assume that further application of this method would facilitate more detailed study of the neural activity in the basal ganglia and the search for neurophysiological biomarkers of movement disorders.

A review of basal ganglia circuits and physiology: Application to deep brain stimulation

INTRODUCTION

Drawing on the seminal work of DeLong, Albin, and Young, we have now entered an era of basal ganglia neuromodulation. Understanding, re-evaluating, and leveraging the lessons learned from neuromodulation will be crucial to facilitate an increased and improved application of neuromodulation in human disease.

METHODS

We will focus on deep brain stimulation (DBS) - the most common form of basal ganglia neuromodulation - however, similar principles can apply to other neuromodulation modalities. We start with a brief review of DBS for Parkinson's disease, essential tremor, dystonia, and Tourette syndrome. We then review hallmark studies on basal ganglia circuits and electrophysiology resulting from decades of experience in neuromodulation. The organization and content of this paper follow Dr. Okun's Lecture from the 2018 Parkinsonism and Related Disorders World Congress.

RESULTS

Information gained from neuromodulation has led to an expansion of the basal ganglia rate model, an enhanced understanding of nuclei dynamics, an emerging focus on pathological oscillations, a revision of the tripartite division of the basal ganglia, and a redirected focus toward individualized symptom-specific stimulation. Though there have been many limitations of the basal ganglia "box model," the construct provided the necessary foundation to advance the field. We now understand that information in the basal ganglia is encoded through complex neural responses that can be reliably measured and used to infer disease states for clinical translation.

CONCLUSIONS

Our deepened understanding of basal ganglia physiology will drive new neuromodulation strategies such as adaptive DBS or cell-specific neuromodulation through the use of optogenetics.

Eltoprazine prevents levodopa-induced dyskinesias by reducing causal interactions for theta oscillations in the dorsolateral striatum and substantia nigra pars reticulate

Oscillatory activities within basal ganglia (BG) circuitry in L-DOPA induced dyskinesia (LID), a condition that occurs in patients with Parkinson disease (PD), are not well understood. The aims of this study were firstly to investigate oscillations in main BG input and output structures-the dorsolateral striatum (dStr) and substantia nigra pars reticulata (SNr), respectively- including the direction of oscillation information flow, and secondly to investigate the effects of 5-HT1A/B receptor agonism with eltoprazine on oscillatory activities and abnormal involuntary movements (AIMs) characteristic. To this end, we conducted local field potential (LFP) electrophysiology in the dStr and SNr of LID rats simultaneous with AIM scoring. The LFP data were submitted to power spectral density, coherence, and partial Granger causality analyses. AIM data were analyzed relative to simultaneous oscillatory activities, with and without eltoprazine. We obtained four major findings. 1) Theta band (5-8 Hz) oscillations were enhanced in the dStr and SNr of LID rats. 2) Theta power correlated with AIM scores in the 180-min period after the last LID-inducing L-DOPA injection, but not with daily summed AIM scores during LID development. 3) Oscillatory information flowed from the dStr to the SNr. 4) Chronic eltoprazine reduced BG theta activity in LID rats and normalized information flow directionality, relative to that in LID rats not given eltoprazine. These results indicate that dStr activity plays a determinative role in the causal interactions of theta oscillations and that serotonergic inhibition may suppress dyskinesia by reducing dStr-SNr theta activity and restoring theta network information flow.

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.

Long-Lasting Electrophysiological After-Effects of High-Frequency Stimulation in the Globus Pallidus: Human and Rodent Slice Studies

Deep-brain stimulation (DBS) of the globus pallidus pars interna (GPi) is a highly effective therapy for movement disorders, yet its mechanism of action remains controversial. Inhibition of local neurons because of release of GABA from afferents to the GPi is a proposed mechanism in patients. Yet, high-frequency stimulation (HFS) produces prolonged membrane depolarization mediated by cholinergic neurotransmission in endopeduncular nucleus (EP, GPi equivalent in rodent) neurons. We applied HFS while recording neuronal firing from an adjacent electrode during microelectrode mapping of GPi in awake patients (both male and female) with Parkinson disease (PD) and dystonia. Aside from after-suppression and no change in neuronal firing, high-frequency microstimulation induced after-facilitation in 38% (26/69) of GPi neurons. In neurons displaying after-facilitation, 10 s HFS led to an immediate decrease of bursting in PD, but not dystonia patients. Moreover, the changes of bursting patterns in neurons with after-suppression or no change after HFS, were similar in both patient groups. To explore the mechanisms responsible, we applied HFS in EP brain slices from rats of either sex. As in humans, HFS in EP induced two subtypes of after-excitation: excitation or excitation with late inhibition. Pharmacological experiments determined that the excitation subtype, induced by lower charge density, was dependent on glutamatergic transmission. HFS with higher charge density induced excitation with late inhibition, which involved cholinergic modulation. Therefore HFS with different charge density may affect the local neurons through multiple synaptic mechanisms. The cholinergic system plays a role in mediating the after-facilitatory effects in GPi neurons, and because of their modulatory nature, may provide a basis for both the immediate and delayed effects of GPi-DBS. We propose a new model to explain the mechanisms of DBS in GPi.SIGNIFICANCE STATEMENT Deep-brain stimulation (DBS) in the globus pallidus pars interna (GPi) improves Parkinson disease (PD) and dystonia, yet its mechanisms in GPi remain controversial. Inhibition has been previously described and thought to indicate activation of GABAergic synaptic terminals, which dominate in GPi. Here we report that 10 s high-frequency microstimulation induced after-facilitation of neural firing in a substantial proportion of GPi neurons in humans. The neurons with after-facilitation, also immediately reduced their bursting activities after high-frequency stimulation in PD, but not dystonia patients. Based on these data and further animal experiments, a mechanistic hypothesis involving glutamatergic, GABAergic, and cholinergic synaptic transmission is proposed to explain both short- and longer-term therapeutic effects of DBS in GPi.

The characteristics of pallidal low-frequency and beta bursts could help implementing adaptive brain stimulation in the parkinsonian and dystonic internal globus pallidus

INTRODUCTION

Adaptive deep brain stimulation (aDBS) has been applied in Parkinson's disease (PD), based on the presence of brief high-amplitude beta (13-35 Hz) oscillation bursts in the subthalamic nucleus (STN), which correlate with symptom severity. Analogously, average low-frequency (LF) oscillatory power (4-12 Hz) in the internal globus pallidus (GPi) correlates with dystonic symptoms and might be a suitable physiomarker for aDBS in dystonia. Characterization of pallidal bursts could facilitate the implementation of aDBS in the GPi of PD and dystonia patients.

OBJECTIVE AND METHODS

We aimed to describe the bursting behaviour of LF and beta oscillations in a cohort of five GPi-DBS PD patients and compare their amplitude and length with those of a cohort of seven GPi-DBS dystonia, and six STN-DBS PD patients (n electrodes = 34). Furthermore, we used the information obtained to set up aDBS and test it in the GPi of both a dystonia and a PD patient (n = 2), using either LF (dystonia) or beta oscillations (PD) as feedback signals.

RESULTS

LF and beta oscillations in the dystonic and parkinsonian GPi occur as phasic, short-lived bursts, similarly to the parkinsonian STN. The amplitude profile of such bursts, however, differed significantly. Dystonia showed higher LF burst amplitudes, while PD presented higher beta burst amplitudes. Burst characteristics in the parkinsonian GPi and STN were similar. Furthermore, aDBS applied in the GPi was feasible and well tolerated in both diseases.

CONCLUSION

Pallidal LF and beta burst amplitudes have different characteristics in PD and dystonia. The presence of increased burst amplitudes could be employed as feedback for GPi-aDBS.

ATP1A3-related disorders: An update

Alternating Hemiplegia of Childhood (AHC), Rapid-onset Dystonia Parkinsonism (RDP) and CAPOS syndrome (cerebellar ataxia, areflexia, pes cavus, optic atrophy, and sensorineural hearing loss) are three distinct, yet partially overlapping clinical syndromes that have long been thought to be allelic disorders. From 2004 to 2012, both autosomal dominant and de novo mutations in ATP1A3 have been detected in patients affected by these three conditions. Growing evidence suggests that AHC, RDP and CAPOS syndrome are part of a large and continuously expanding clinical spectrum and share some recurrent clinical features, such as abrupt-onset, asymmetric anatomical distribution and the presence of triggering factors, which are highly suggestive of ATP1A3 mutations. In this review, we will highlight the main clinical and genetic features of ATP1A3-related disorders focussing on shared and distinct features that can be helpful in clinical practice to individuate mutation carriers.

Rethinking status dystonicus

Status dystonicus is a movement disorder emergency that has been a source of controversy in terms of terminology, phenomenology, and management since it was first described in 1982. Here we argue that the current use of the term status dystonicus falls well short of the precision needed for either clinical or academic use. We performed a critical review on this topic, describing possible pathophysiological mechanisms and areas of uncertainties. This review also addresses the problems derived by the extreme clinical heterogeneity of this condition, as the lack of an objective criterion useful for the definition, or the fact that status dystonicus may present not only in the context of a known dystonic syndrome. We propose a new possible definition that includes not only dystonia but also other hyperkinetic movements in the wide range of movement disorders that can be seen during an episode. The new definition keeps the term status dystonicus and highlights the fact that this is a medical emergency based on the impairment of bulbar and/or respiratory function requiring hospital admission as the principal feature. Furthermore, the new definition should not consider as necessary unspecific features as patient's condition at baseline, the distribution of dystonia, occurrence of systemic symptoms such as fever or laboratory findings. We hope that this proposal will stimulate the debate on this subject among our peers, further developing a clinical and pathophysiological understanding of status dystonicus. © 2017 International Parkinson and Movement Disorder Society.

Single-unit activity of the anterior Globus pallidus internus in Tourette patients and posterior Globus pallidus internus in dystonic patients

OBJECTIVES

Our goal was to provide a detailed analysis of neurons' electrophysiological activity recorded in sub-territories of Globus pallidus internus (GPi) used as Deep Brain Stimulation (DBS) targets for these clinical conditions to potentially assist electrode targeting.

METHODS

We used intra-operative microelectrode recording during stereotactic neurosurgery to guide implantation of DBS lead.

RESULTS

Units in the medial anterior part of GPi of 7 Tourette's syndrome patients under general anesthesia were firing at mean and median rate of 32.1 and 21 Hz respectively (n = 101), with 45% of spikes fired during bursts and 21.3 bursts per minute. In the latero-posterior part of GPi of 7 dystonic patients under local anesthesia the mean and median activity were 46.1 and 30.6 Hz respectively (n = 27), and a mean of 21.7 bursts per minute was observed, with 30% of all spikes occurring during these bursts.

CONCLUSION

Units activity pattern - slow-regular, fast-irregular or fast-regular were present in different proportions between the two targets.

SIGNIFICANCE

The electrophysiological characteristics of the medial-anterior part of GPi and its latero-posterior portion can be used to assist DBS electrode targeting and also support the refinement of pathophysiological models of Tourette's syndrome and Dystonia.

It's not just the basal ganglia: Cerebellum as a target for dystonia therapeutics

Dystonia is a common movement disorder that devastates the lives of many patients, but the etiology of this disorder remains poorly understood. Dystonia has traditionally been considered a disorder of the basal ganglia. However, growing evidence suggests that the cerebellum may be involved in certain types of dystonia, raising several questions. Can different types of dystonia be classified as either a basal ganglia disorder or a cerebellar disorder? Is dystonia a network disorder that involves the cerebellum and basal ganglia? If dystonia is a network disorder, how can we target treatments to alleviate symptoms in patients? A recent study by Chen et al, using the pharmacological mouse model of rapid-onset dystonia parkinsonism, has provided some insight into these important questions. They showed that the cerebellum can directly modulate basal ganglia activity through a short latency cerebello-thalamo-basal ganglia pathway. Further, this article and others have provided evidence that in some cases, aberrant cerebello-basal ganglia communication can be involved in dystonia. In this review we examine the evidence for the involvement of the cerebellum and cerebello-basal ganglia interactions in dystonia. We conclude that there is ample evidence to suggest that the cerebellum plays a role in some dystonias, including the early-onset primary torsion dystonia DYT1 and that further studies examining the role of this brain region and its interaction with the basal ganglia in dystonia are warranted. © 2017 International Parkinson and Movement Disorder Society.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Deep Brain Stimulation for Movement Disorders of Basal Ganglia Origin: Restoring Function or Functionality?

Deep brain stimulation (DBS) is highly effective for both hypo- and hyperkinetic movement disorders of basal ganglia origin. The clinical use of DBS is, in part, empiric, based on the experience with prior surgical ablative therapies for these disorders, and, in part, driven by scientific discoveries made decades ago. In this review, we consider anatomical and functional concepts of the basal ganglia relevant to our understanding of DBS mechanisms, as well as our current understanding of the pathophysiology of two of the most commonly DBS-treated conditions, Parkinson's disease and dystonia. Finally, we discuss the proposed mechanism(s) of action of DBS in restoring function in patients with movement disorders. The signs and symptoms of the various disorders appear to result from signature disordered activity in the basal ganglia output, which disrupts the activity in thalamocortical and brainstem networks. The available evidence suggests that the effects of DBS are strongly dependent on targeting sensorimotor portions of specific nodes of the basal ganglia-thalamocortical motor circuit, that is, the subthalamic nucleus and the internal segment of the globus pallidus. There is little evidence to suggest that DBS in patients with movement disorders restores normal basal ganglia functions (e.g., their role in movement or reinforcement learning). Instead, it appears that high-frequency DBS replaces the abnormal basal ganglia output with a more tolerable pattern, which helps to restore the functionality of downstream networks.

Neuronal Entropy-Rate Feature of Entopeduncular Nucleus in Rat Model of Parkinson's Disease

The function of the nigro-striatal pathway on neuronal entropy in the basal ganglia (BG) output nucleus, i.e. the entopeduncular nucleus (EPN) was investigated in the unilaterally 6-hyroxydopamine (6-OHDA)-lesioned rat model of Parkinson's disease (PD). In both control subjects and subjects with 6-OHDA lesion of dopamine (DA) the nigro-striatal pathway, a histological hallmark for parkinsonism, neuronal entropy in EPN was maximal in neurons with firing rates ranging between 15 and 25 Hz. In 6-OHDA lesioned rats, neuronal entropy in the EPN was specifically higher in neurons with firing rates above 25 Hz. Our data establishes that the nigro-striatal pathway controls neuronal entropy in motor circuitry and that the parkinsonian condition is associated with abnormal relationship between firing rate and neuronal entropy in BG output nuclei. The neuronal firing rates and entropy relationship provide putative relevant electrophysiological information to investigate the sensory-motor processing in normal condition and conditions such as movement disorders.

Long-term increase in coherence between the basal ganglia and motor cortex after asphyxial cardiac arrest and resuscitation in developing rats

BACKGROUND

The basal ganglia are vulnerable to injury during cardiac arrest. Movement disorders are a common morbidity in survivors. Yet, neuronal motor network changes post-arrest remain poorly understood.

METHODS

We compared function of the motor network in adult rats that, during postnatal week 3, underwent 9.5 min of asphyxial cardiac arrest (n = 9) or sham intervention (n = 8). Six months after injury, we simultaneously recorded local field potentials (LFP) from the primary motor cortex (MCx) and single neuron firing and LFP from the rat entopeduncular nucleus (EPN), which corresponds to the primate globus pallidus pars interna. Data were analyzed for firing rates, power, and coherence between MCx and EPN spike and LFP activity.

RESULTS

Cardiac arrest survivors display chronic motor deficits. EPN firing rate is lower in cardiac arrest survivors (19.5 ± 2.4 Hz) compared with controls (27.4 ± 2.7 Hz; P < 0.05). Cardiac arrest survivors also demonstrate greater coherence between EPN single neurons and MCx LFP (3-100 Hz; P < 0.001).

CONCLUSIONS

This increased coherence indicates abnormal synchrony in the neuronal motor network after cardiac arrest. Increased motor network synchrony is thought to be antikinetic in primary movement disorders. Characterization of motor network synchrony after cardiac arrest may help guide management of post-hypoxic movement disorders.

Biochemical mechanisms of pallidal deep brain stimulation in X-linked dystonia parkinsonism

OBJECTIVE

Invasive techniques such as in-vivo microdialysis provide the opportunity to directly assess neurotransmitter levels in subcortical brain areas.

METHODS

Five male Filipino patients (mean age 42.4, range 34-52 years) with severe X-linked dystonia-parkinsonism underwent bilateral implantation of deep brain leads into the internal part of the globus pallidus (GPi). Intraoperative microdialysis and measurement of gamma aminobutyric acid and glutamate was performed in the GPi in three patients and globus pallidus externus (GPe) in two patients at baseline for 25/30 min and during 25/30 min of high-frequency GPi stimulation.

RESULTS

While the gamma-aminobutyric acid concentration increased in the GPi during high frequency stimulation (231 ± 102% in comparison to baseline values), a decrease was observed in the GPe (22 ± 10%). Extracellular glutamate levels largely remained unchanged.

CONCLUSIONS

Pallidal microdialysis is a promising intraoperative monitoring tool to better understand pathophysiological implications in movement disorders and therapeutic mechanisms of high frequency stimulation. The increased inhibitory tone of GPi neurons and the subsequent thalamic inhibition could be one of the key mechanisms of GPi deep brain stimulation in dystonia. Such a mechanism may explain how competing (dystonic) movements can be suppressed in GPi/thalamic circuits in favour of desired motor programs.

Intraoperative neurophysiology in deep brain surgery for psychogenic dystonia

Psychogenic dystonia is a challenging entity to diagnose and treat because little is known about its pathophysiology. We describe two cases of psychogenic dystonia who underwent deep brain stimulation when thought to have organic dystonia. The intraoperative microelectrode recordings in globus pallidus internus were retrospectively compared with those of five patients with known DYT1 dystonia using spontaneous discharge parameters of rate and bursting, as well as movement-related discharges. Our data suggest that simple intraoperative neurophysiology measures in single subjects do not differentiate psychogenic dystonia from DYT1 dystonia.

Mechanism of Deep Brain Stimulation: Inhibition, Excitation, or Disruption?

Deep brain stimulation (DBS), applying high-frequency electrical stimulation to deep brain structures, has now provided an effective therapeutic option for treatment of various neurological and psychiatric disorders. DBS targeting the internal segment of the globus pallidus, subthalamic nucleus, and thalamus is used to treat symptoms of movement disorders, such as Parkinson’s disease, dystonia, and tremor. However, the mechanism underlying the beneficial effects of DBS remains poorly understood and is still under debate: Does DBS inhibit or excite local neuronal elements? In this short review, we would like to introduce our recent work on the physiological mechanism of DBS and propose an alternative explanation: DBS dissociates input and output signals, resulting in the disruption of abnormal information flow through the stimulation site.

Pallidal activity in myoclonus dystonia correlates with motor signs

BACKGROUND

Myoclonus-dystonia related to epsilon-sarcoglycan gene mutations is characterized by myoclonic jerks and mild to moderate dystonia. The role of basal ganglia dysfunction in the pathogenesis is unknown.

METHODS

Pallidal neuronal activity was recorded in six myoclonus-dystonia and six primary generalized dystonia patients operated on for internal globus pallidus deep brain stimulation.

RESULTS

In myoclonus-dystonia patients compared with primary-dystonia patients, internal pallidum neurons showed higher burst frequency, lower mean burst, and pause durations. External pallidum neurons showed higher mean pause frequency. Oscillatory activity was present in 33% and 35% of internal pallidum neurons in myoclonus-dystonia and primary-dystonia patients, respectively, predominantly in the theta frequency band (3-8 Hz). In myoclonus-dystonia patients with more severe myoclonus, internal pallidum neurons exhibited a higher bursting activity with high intraburst frequency and lower oscillatory activity frequency.

CONCLUSIONS

Myoclonus-dystonia appears to be related to specific changes in internal pallidum activity, leading to disruption in striato-pallido-thalamo-cortical circuits. © 2015 International Parkinson and Movement Disorder Society.

Postural stability under globus pallidus internus stimulation for dystonia

OBJECTIVE

Deep brain stimulation of the globus pallidus internus (GPi-DBS) is an efficient and safe treatment for medically refractory dystonia. However, recent studies reported gait problems, falls and bradykinesia in patients after the DBS procedure. The aim of this study was to quantify the effect of GPi-DBS on postural performance in patients with cranio-cervical dystonia.

METHODS

Thirteen patients with focal/segmental dystonia and GPi-DBS participated in the study. We performed two postural tests (pull test and push and release test) in on- and off-stimulation conditions and recorded the movements of the patients with inertial sensors.

RESULTS

Under stimulation patients exhibited a higher number of steps (p=0.015), reduced first step length (p=0.011) and lower stepping velocity (p=0.001), compared to off stimulation. We observed a higher number of steps in the push and release test compared to the pull test (p=0.038). The interaction between stimulation condition and test type was significant (p=0.027).

CONCLUSIONS

The velocity and amplitude of postural reactions are compromised by GPi-DBS in patients with cranio-cervical dystonia.

SIGNIFICANCE

This information corresponds to patient's reports of falls and postural instability after GPi-DBS. Pre-operatively, patients should be informed about the possibility of the occurrence of such phenomena.

Bursting activity of substantia nigra pars reticulata neurons in mouse parkinsonism in awake and anesthetized states

Electrophysiological changes in basal ganglia neurons are hypothesized to underlie motor dysfunction in Parkinson's disease (PD). Previous results in head-restrained MPTP-treated non-human primates have suggested that increased bursting within the basal ganglia and related thalamic and cortical areas may be a hallmark of pathophysiological activity. In this study, we investigated whether there is increased bursting in substantia nigra pars reticulata (SNpr) output neurons in anesthetized and awake, head-restrained unilaterally lesioned 6-OHDA mice when compared to control mice. Confirming previous studies, we show that there are significant changes in the firing rate and pattern in SNpr neuron activity under urethane anesthesia. The regular firing pattern of control urethane-anesthetized SNpr neurons was not present in the 6-OHDA-lesioned group, as the latter neurons instead became phase locked with cortical slow wave activity (SWA). Next, we examined whether such robust electrophysiological changes between groups carried over to the awake state. SNpr neurons from both groups fired at much higher frequencies in the awake state than in the anesthetized state and surprisingly showed only modest changes between awake control and 6-OHDA groups. While there were no differences in firing rate between groups in the awake state, an increase in the coefficient of variation (CV) was observed in the 6-OHDA group. Contrary to the bursting hypothesis, this increased CV was not due to changes in bursting but was instead due to a mild increase in pausing. Together, these results suggest that differences in SNpr activity between control and 6-OHDA lesioned mice may be strongly influenced by changes in network activity during different arousal and behavioral states.