Abnormal oscillatory synchronisation in the motor system leads to impaired movement

Alpha- and beta-band oscillations subserve different processes in reactive control of limb movements

The capacity to rapidly suppress a behavioral act in response to sudden instruction to stop is a key cognitive function. This function, called reactive control, is tested in experimental settings using the stop signal task, which requires subjects to generate a movement in response to a go signal or suppress it when a stop signal appears. The ability to inhibit this movement fluctuates over time: sometimes, subjects can stop their response, and at other times, they can not. To determine the neural basis of this fluctuation, we recorded local field potentials (LFPs) in the alpha (6–12 Hz) and beta (13–35 Hz) bands from the dorsal premotor cortex of two nonhuman primates that were performing the task. The ability to countermand a movement after a stop signal was predicted by the activity of both bands, each purportedly representing a distinct neural process. The beta band represents the level of movement preparation; higher beta power corresponds to a lower level of movement preparation, whereas the alpha band supports a proper phasic, reactive inhibitory response: movements are inhibited when alpha band power increases immediately after a stop signal. Our findings support the function of LFP bands in generating the signatures of various neural computations that are multiplexed in the brain.

Different patterns of local field potentials from limbic DBS targets in patients with major depressive and obsessive compulsive disorder

The role of distinct limbic areas in emotion regulation has been largely inferred from neuroimaging studies. Recently, the opportunity for intracranial recordings from limbic areas has arisen in patients undergoing deep brain stimulation (DBS) for neuropsychiatric disorders including major depressive disorder (MDD) and obsessive compulsive disorder (OCD). Here we test the hypothesis that distinct temporal patterns of local field potential (LFP) activity in the human limbic system reflect disease state and symptom severity in MDD and OCD patients. To this end, we recorded LFPs via implanted DBS electrodes from the bed nucleus of stria terminalis (BNST area) in 12 patients (5 OCD, 7 MDD) and from the subgenual cingulate cortex in 7 MDD patients (CG25 area). We found a distinct pattern of oscillatory activity with significantly higher α-power in MDD compared with OCD in the BNST area (broad α-band 8-14 Hz; P<0.01) and a similar level of α-activity in the CG25 area as in the BNST area in MDD patients. The mean α-power correlated with severity of depressive symptoms as assessed by the Beck depression inventory in MDD (n=14, r=0.55, P=0.042) but not with severity of obsessive compulsive symptoms in OCD. Here we show larger α-band activity in MDD patients compared with OCD recorded from intracranial DBS targets. Our results suggest that α-activity in the limbic system may be a signature of symptom severity in MDD and may serve as a potential state biomarker for closed loop DBS in MDD.

Putting reins on the brain. How the body and environment use it

Radical embodied cognitive neuroscience (RECN) will probably rely on dynamical systems theory (DST) and complex systems theory for methods and formalism. Yet, there have been plenty of non-radical neurodynamicists out there for quite some time. How much of their work fits with radical embodied cognitive science, what do they need RECN for, and what are the inconsistencies between RECN and established neurodynamics that would have to be resolved? This paper is both theoretical hypothesis and review. First, it provides a brief overview of the typical, purely structural considerations why the central nervous systems (CNS) should be treated as a nonlinear dynamical system and what this entails. The reader will learn about the circular causality enclosing brain and behavior and different attempts to formalize this circularity. Then, three different attempts at linking dynamics and theory of brain function are described in more detail and criticized. A fourth method based on ecological psychology could fix some of the issues that the others encounter. It is argued that studying self-organization of the brain without taking its ecological embedding into account is insufficient. Finally, based on existing theoretical work we propose two roles that the CNS has to be fulfilling in order to allow an animal to behave adequately in its niche. In its first role the CNS has to be enslaved easily by patterns of behavior that guide the animal through its environment. In the second role the brain has to flexibly switch among patterns, what can be called the metastable circuit breaker. The relevance of this idea is supported using certain motor symptoms of Parkinson's disease (PD). These symptoms can be explained as consequent to an excessive stability of the (metastable) circuit breaker.

Causes and consequences of degeneration of the dorsal motor nucleus of the vagus nerve in Parkinson's disease

SIGNIFICANCE

Parkinson's disease (PD) is no longer considered merely a movement disorder caused by degeneration of dopamine neurons in the midbrain. It is now recognized as a widespread neuropathological syndrome accompanied by a variety of motor and nonmotor clinical symptoms. As such, any hypothesis concerning PD pathogenesis and pathophysiology must account for the entire spectrum of disease and not solely focus on the dopamine system.

RECENT ADVANCES

Based on its anatomy and the intrinsic properties of its neurons, the dorsal motor nucleus of the vagus nerve (DMV) is uniquely vulnerable to damage from PD. Fibers in the vagus nerve course throughout the gastrointestinal (GI) tract to and from the brainstem forming a close link between the peripheral and central nervous systems and a point of proximal contact between the environment and areas where PD pathology is believed to start. In addition, DMV neurons are under high levels of oxidative stress due to their high level of α-synuclein expression, fragile axons, and specific neuronal physiology. Moreover, several consequences of DMV damage, namely, GI dysfunction and unrestrained inflammation, may propagate a vicious cycle of injury affecting vulnerable brain regions.

CRITICAL ISSUES

Current evidence to suggest the vagal system plays a pivotal role in PD pathogenesis is circumstantial, but given the current state of the field, the time is ripe to obtain direct experimental evidence to better delineate it.

FUTURE DIRECTIONS

Better understanding of the DMV and vagus nerve may provide insight into PD pathogenesis and a neural highway with direct brain access that could be harnessed for novel therapeutic interventions.

Food deprivation and nicotine correct akinesia and freezing in Na(+) -leak current channel (NALCN)-deficient strains of Caenorhabditis elegans

Mutations in various genes adversely affect locomotion in model organisms, and thus provide valuable clues about the complex processes that control movement. In Caenorhabditis elegans, loss-of-function mutations in the Na(+) leak current channel (NALCN) and associated proteins (UNC-79 and UNC-80) cause akinesia and fainting (abrupt freezing of movement during escape from touch). It is not known how defects in the NALCN induce these phenotypes or if they are chronic and irreversible. Here, we report that akinesia and freezing are state-dependent and reversible in NALCN-deficient mutants (nca-1;nca-2, unc-79 and unc-80) when additional cation channels substitute for this protein. Two main measures of locomotion were evaluated: spontaneous movement (traversal of >2 head lengths during a 5 second observation period) and the touch-freeze response (movement greater than three body bends in response to tail touch). Food deprivation for as little as 3 min stimulated spontaneous movement and corrected the touch-freeze response. Conversely, food-deprived animals that moved normally in the absence of bacteria rapidly reverted to uncoordinated movement when re-exposed to food. The effects of food deprivation were mimicked by nicotine, which suggested that acetylcholine mediated the response. Nicotine appeared to act on interneurons or motor neurons rather than directly at the neuromuscular junction because levamisole, which stimulates muscle contraction, did not correct movement. Neural circuits have been proposed to account for the effects of food deprivation and nicotine on spontaneous movement and freezing. The NALCN may play an unrecognized role in human movement disorders characterized by akinesia and freezing gait.

High-frequency oscillations in Parkinson's disease: spatial distribution and clinical relevance

The pathophysiology of Parkinson's disease (PD) has been related to excessive beta band oscillations in the basal ganglia. Recent recordings from the subthalamic nucleus of PD patients showed that beta oscillations show strong cross-frequency coupling with high-frequency oscillations (>200 Hz). However, little is known about the characteristics and functional properties of these oscillations. We studied the spatial distribution of high-frequency oscillations and their relation to PD motor symptoms. We included 10 PD patients in medication OFF who underwent implantation of deep brain stimulation (DBS) electrodes. Intraoperative five-channel microelectrode recordings were performed at 9 to 10 recording sites within the subthalamic nucleus and its immediate surroundings. We found a focal spatial distribution of high-frequency oscillations with highest power 2 mm below the dorsolateral border of the subthalamic nucleus. Within the subthalamic nucleus, power peaked slightly anterior to the DBS target site. In addition, contralateral akinesia/rigidity scores were negatively correlated with high-frequency oscillation power. Our results demonstrate a focal origin of high-frequency oscillations within the subthalamic nucleus and provide further evidence for their functional association with motor state.

Hypersynchrony despite pathologically reduced beta oscillations in patients with Parkinson's disease: a pharmaco-magnetoencephalography study

Parkinson's disease (PD) is a progressive debilitating neurodegenerative disorder clinically manifest by motor, posture and gait abnormalities. Human neurophysiological studies recording local field potentials within the subthalamic nucleus and scalp-based electroencephalography have shown pathological beta synchrony throughout the basal ganglia-thalamic-cortical motor network in PD. Notably, suppression of this pathological beta synchrony by dopamine replacement therapy or deep-brain stimulation has been associated with improved motor function. However, due to the invasive nature of these studies, it remains unknown whether this "pathological beta" is actually stronger than that observed in healthy demographically matched controls. We used magnetoencephalography to investigate neuronal synchrony and oscillatory amplitude in the beta range and lower frequencies during the resting state in patients with PD and a matched group of patients without neurological disease. Patients with PD were studied both in the practically defined drug "OFF" state, and after administration of dopamine replacements. We found that beta oscillatory amplitude was reduced bilaterally in the primary motor regions of unmedicated patients with PD compared with controls. Administration of dopaminergic medications significantly increased beta oscillatory activity, thus having a normalizing effect. Interestingly, we also found significantly stronger beta synchrony (i.e., hypersynchrony) between the primary motor regions in unmedicated patients with PD compared with controls, and that medication reduced this coupling which is in agreement with the intraoperative studies. These results are consistent with the known functionality of the basal ganglia-thalamic-cortical motor circuit and the likely consequences of beta hypersynchrony in the subthalamic nucleus of patients with PD.

EEG beta suppression and low gamma modulation are different elements of human upright walking

Cortical involvement during upright walking is not well-studied in humans. We analyzed non-invasive electroencephalographic (EEG) recordings from able-bodied volunteers who participated in a robot-assisted gait-training experiment. To enable functional neuroimaging during walking, we applied source modeling to high-density (120 channels) EEG recordings using individual anatomy reconstructed from structural magnetic resonance imaging scans. First, we analyzed amplitude differences between the conditions, walking and upright standing. Second, we investigated amplitude modulations related to the gait phase. During active walking upper μ (10-12 Hz) and β (18-30 Hz) oscillations were suppressed [event-related desynchronization (ERD)] compared to upright standing. Significant β ERD activity was located focally in central sensorimotor areas for 9/10 subjects. Additionally, we found that low γ (24-40 Hz) amplitudes were modulated related to the gait phase. Because there is a certain frequency band overlap between sustained β ERD and gait phase related modulations in the low γ range, these two phenomena are superimposed. Thus, we observe gait phase related amplitude modulations at a certain ERD level. We conclude that sustained μ and β ERD reflect a movement related state change of cortical excitability while gait phase related modulations in the low γ represent the motion sequence timing during gait. Interestingly, the center frequencies of sustained β ERD and gait phase modulated amplitudes were identified to be different. They may therefore be caused by different neuronal rhythms, which should be taken under consideration in future studies.

Activity parameters of subthalamic nucleus neurons selectively predict motor symptom severity in Parkinson's disease

Parkinson's disease (PD) is a heterogeneous disorder that leads to variable expression of several different motor symptoms. While changes in firing rate, pattern, and oscillation of basal ganglia neurons have been observed in PD patients and experimental animals, there is limited evidence linking them to specific motor symptoms. Here we examined this relationship using extracellular recordings of subthalamic nucleus neurons from 19 PD patients undergoing surgery for deep brain stimulation. For each patient, ≥ 10 single units and/or multi-units were recorded in the OFF medication state. We correlated the proportion of neurons displaying different activities with preoperative Unified Parkinson's Disease Rating Scale subscores (OFF medication). The mean spectral power at sub-beta frequencies and percentage of units oscillating at beta frequencies were positively correlated with the axial and limb rigidity scores, respectively. The percentage of units oscillating at gamma frequency was negatively correlated with the bradykinesia scores. The mean intraburst rate was positively correlated with both bradykinesia and axial scores, while the related ratio of interspike intervals below/above 10 ms was positively correlated with these symptoms and limb rigidity. None of the activity parameters correlated with tremor. The grand average of all the significantly correlated subthalamic nucleus activities accounted for >60% of the variance of the combined bradykinetic-rigid and axial scores. Our results demonstrate that the occurrence of alterations in the rate and pattern of basal ganglia neurons could partly underlie the variability in parkinsonian phenotype.

Mechanism of parkinsonian neuronal oscillations in the primate basal ganglia: some considerations based on our recent work

Accumulating evidence suggests that abnormal neuronal oscillations in the basal ganglia (BG) contribute to the manifestation of parkinsonian symptoms. In this article, we would like to summarize our recent work on the mechanism underlying abnormal oscillations in the parkinsonian state and discuss its significance in pathophysiology of Parkinson’s disease. We recorded neuronal activity in the BG of parkinsonian monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Systemic administration of L-DOPA alleviated parkinsonian motor signs and decreased abnormal neuronal oscillations (8–15 Hz) in the internal (GPi) and external (GPe) segments of the globus pallidus and the subthalamic nucleus (STN). Inactivation of the STN by muscimol (GABA_A receptor agonist) injection also ameliorated parkinsonian signs and suppressed GPi oscillations. The blockade of glutamatergic inputs to the STN by local microinjection of a mixture of 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (glutamatergic NMDA receptor antagonist) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (glutamatergic AMPA/kainate receptor antagonist) suppressed neuronal oscillations in the STN. STN oscillations were also attenuated by the blockade of GABAergic neurotransmission from the GPe to the STN by muscimol inactivation of the GPe. These results suggest that cortical glutamatergic inputs to the STN and reciprocal GPe-STN interconnections are both important for the generation and amplification of the oscillatory activity of GPe and STN neurons in the parkinsonian state. The oscillatory activity in the STN is subsequently transmitted to the GPi and may contribute to manifestation of parkinsonian symptoms.

Models of vocal learning in the songbird: Historical frameworks and the stabilizing critic

Birdsong is a form of sensorimotor learning that involves a mirror-like system that activates with both song hearing and production. Early models of song learning, based on behavioral measures, identified key features of vocal plasticity, such as the requirements for memorization of a tutor song and auditory feedback during song practice. The concept of a comparator, which compares the memory of the tutor song to auditory feedback, featured prominently. Later models focused on linking anatomically-defined neural modules to behavioral concepts, such as the comparator. Exploiting the anatomical modularity of the songbird brain, localized lesions illuminated mechanisms of the neural song system. More recent models have integrated neuronal mechanisms identified in other systems with observations in songbirds. While these models explain multiple aspects of song learning, they must incorporate computational elements based on unknown biological mechanisms to bridge the motor-to-sensory delay and/or transform motor signals into the sensory domain. Here, I introduce the stabilizing critic hypothesis, which enables sensorimotor learning by (1) placing a purely sensory comparator afferent of the song system and (2) endowing song system disinhibitory interneuron networks with the capacity both to bridge the motor-sensory delay through prolonged bursting and to stabilize song segments selectively based on the comparator signal. These proposed networks stabilize an otherwise variable signal generated by both putative mirror neurons and a cortical-basal ganglia-thalamic loop. This stabilized signal then temporally converges with a matched premotor signal in the efferent song motor cortex, promoting spike-timing-dependent plasticity in the premotor circuitry and behavioral song learning.

An EEMD-IVA framework for concurrent multidimensional EEG and unidimensional kinematic data analysis

Joint blind source separation (JBSS) is a means to extract common sources simultaneously found across multiple datasets, e.g., electroencephalogram (EEG) and kinematic data jointly recorded during reaching movements. Existing JBSS approaches are designed to handle multidimensional datasets, yet to our knowledge, there is no existing means to examine common components that may be found across a unidimensional dataset and a multidimensional one. In this paper, we propose a simple, yet effective method to achieve the goal of JBSS when concurrent multidimensional EEG and unidimensional kinematic datasets are available, by combining ensemble empirical mode decomposition (EEMD) with independent vector analysis (IVA). We demonstrate the performance of the proposed method through numerical simulations and application to data collected from reaching movements in Parkinson's disease. The proposed method is a promising JBSS tool for real-world biomedical signal processing applications.

Oscillatory Activity in Basal Ganglia and Motor Cortex in an Awake Behaving Rodent Model of Parkinson's Disease

Exaggerated beta range (15-30 Hz) oscillatory activity is observed in the basal ganglia of Parkinson's disease (PD) patients during implantation of deep brain stimulation electrodes. This activity has been hypothesized to contribute to motor dysfunction in PD patients. However, it remains unclear how these oscillations develop and how motor circuits become entrained into a state of increased synchronization in this frequency range after loss of dopamine. It is also unclear whether this increase in neuronal synchronization actually plays a significant role in inducing the motor symptoms of this disorder. The hemiparkinsonian rat has emerged as a useful model for investigating relationships between loss of dopamine, increases in oscillatory activity in motor circuits and behavioral state. Chronic recordings from these animals show exaggerated activity in the high beta/low gamma range (30-35 Hz) in the dopamine cell-lesioned hemisphere. This activity is not evident when the animals are in an inattentive rest state, but it can be stably induced and monitored in the motor cortex and basal ganglia when they are engaged in an on-going activity such as treadmill walking. This review discusses data obtained from this animal model and the implications and limitations of this data for obtaining further insight into the significance of beta range activity in PD.

Effects of antidromic and orthodromic activation of STN afferent axons during DBS in Parkinson's disease: a simulation study

Recent studies suggest that subthalamic nucleus (STN)-Deep Brain Stimulation (DBS) may exert at least part of its therapeutic effect through the antidromic suppression of pathological oscillations in the cortex in 6-OHDA treated rats and in parkinsonian patients. STN-DBS may also activate STN neurons by initiating action potential propagation in the orthodromic direction, similarly resulting in suppression of pathological oscillations in the STN. While experimental studies have provided strong evidence in support of antidromic stimulation of cortical neurons, it is difficult to separate relative contributions of antidromic and orthodromic effects of STN-DBS. The aim of this computational study was to examine the effects of antidromic and orthodromic activation on neural firing patterns and beta-band (13-30 Hz) oscillations in the STN and cortex during DBS of STN afferent axons projecting from the cortex. High frequency antidromic stimulation alone effectively suppressed simulated beta activity in both the cortex and STN-globus pallidus externa (GPe) network. High frequency orthodromic stimulation similarly suppressed beta activity within the STN and GPe through the direct stimulation of STN neurons driven by DBS at the same frequency as the stimulus. The combined effect of both antidromic and orthodromic stimulation modulated cortical activity antidromically while simultaneously orthodromically driving STN neurons. While high frequency DBS reduced STN beta-band power, low frequency stimulation resulted in resonant effects, increasing beta-band activity, consistent with previous experimental observations. The simulation results indicate effective suppression of simulated oscillatory activity through both antidromic stimulation of cortical neurons and direct orthodromic stimulation of STN neurons. The results of the study agree with experimental recordings of STN and cortical neurons in rats and support the therapeutic potential of stimulation of cortical neurons.

Disrupting neuronal transmission: mechanism of DBS?

Applying high-frequency stimulation (HFS) to deep brain structure, known as deep brain stimulation (DBS), has now been recognized an effective therapeutic option for a wide range of neurological and psychiatric disorders. DBS targeting the basal ganglia thalamo-cortical loop, especially the internal segment of the globus pallidus (GPi), subthalamic nucleus (STN) and thalamus, has been widely employed as a successful surgical therapy for movement disorders, such as Parkinson’s disease, dystonia and tremor. However, the neurophysiological mechanism underling the action of DBS remains unclear and is still under debate: does DBS inhibit or excite local neuronal elements? In this review, we will examine this question and propose the alternative interpretation: DBS dissociates inputs and outputs, resulting in disruption of abnormal signal transmission.

The functional role of beta oscillations in Parkinson's disease

Modulations of beta oscillations (13-30 Hz) during normal motor control suggest that they may act to promote current motor set at the expense of new movements. These oscillations are greatly enhanced in Parkinson's disease (PD) and there is strong correlative evidence linking beta activity at rest and beta changes in response to treatment with bradykinesia and rigidity. Some evidence that this link may be mechanistically important or causal comes from studies in which either cortical or subcortical sites have been stimulated in the beta frequency range causing modest but significant slowing of movements. However, recent trials in which high frequency deep brain stimulation (DBS) has only been delivered during periods of elevated beta activity have demonstrated major clinical effects that even exceed those of standard continuous high frequency DBS. These studies suggest that beta activity may be both causally and quantitatively important in the motor impairment of PD, and demonstrate how improvements in the understanding of the pathophysiology of PD can lead to enhanced therapeutic interventions in this condition.

Beta oscillations and their functional role in movement perception

Neuronal oscillations refer to periodic changes of neuronal activity. A prominent neuronal oscillation, especially in sensorimotor areas, is the beta-frequency-band (∼ 13–29 Hz). Sensorimotor beta oscillations are predominantly linked to motor-related functions such as preparation and/or execution of movements. In addition, beta oscillations have been suggested to play a role in long-range communication between multiple brain areas. In this review, we assess different studies that show that sensorimotor beta oscillations are additionally involved in the visual perception and imagery of biological movements. We propose that sensorimotor beta oscillations reflect a mechanism of attempted matching to internally stored representations of movements. We additionally, provide evidence that beta oscillations play a role for the integration of visual and sensorimotor areas to a functional network that incorporates perceptual components at specific spatial-temporal profiles and transmits information across different areas.

Expectation modulates the effect of deep brain stimulation on motor and cognitive function in tremor-dominant Parkinson's disease

Expectation contributes to placebo and nocebo responses in Parkinson's disease (PD). While there is evidence for expectation-induced modulations of bradykinesia, little is known about the impact of expectation on resting tremor. Subthalamic nucleus (STN) deep brain stimulation (DBS) improves cardinal PD motor symptoms including tremor whereas impairment of verbal fluency (VF) has been observed as a potential side-effect. Here we investigated how expectation modulates the effect of STN-DBS on resting tremor and its interaction with VF. In a within-subject-design, expectation of 24 tremor-dominant PD patients regarding the impact of STN-DBS on motor symptoms was manipulated by verbal suggestions (positive [placebo], negative [nocebo], neutral [control]). Patients participated with (MedON) and without (MedOFF) antiparkinsonian medication. Resting tremor was recorded by accelerometry and bradykinesia of finger tapping and diadochokinesia were assessed by a 3D ultrasound motion detection system. VF was quantified by lexical and semantic tests. In a subgroup of patients, the effect of STN-DBS on tremor was modulated by expectation, i.e. tremor decreased (placebo response) or increased (nocebo response) by at least 10% as compared to the control condition while no significant effect was observed for the overall group. Interestingly, nocebo responders in MedON were additionally characterized by significant impairment in semantic verbal fluency. In contrast, bradykinesia was not affected by expectation. These results indicate that the therapeutic effect of STN-DBS on tremor can be modulated by expectation in a subgroup of patients and suggests that tremor is also among the parkinsonian symptoms responsive to placebo and nocebo interventions. While positive expectations enhanced the effect of STN-DBS by further decreasing the magnitude of tremor, negative expectations counteracted the therapeutic effect and at the same time exacerbated a side-effect often associated with STN-DBS. The present findings underscore the potency of patients' expectation and its relevance for therapeutic outcomes.

Levodopa reinstates connectivity from prefrontal to premotor cortex during externally paced movement in Parkinson's disease

Dopamine deficiency affects functional integration of activity in distributed neural regions. It has been suggested that lack of dopamine induces disruption of neural interactions between prefrontal and premotor areas, which might underlie impairment of motor control observed in patients with Parkinson's disease (PD). In this study we recorded cortical activity with high-density electroencephalography in 11 patients with PD as a pathological model of dopamine deficiency, and 13 healthy control subjects. Participants performed repetitive extension-flexion movements of their right index finger, which were externally paced at a rate of 0.5 Hz. This required participants to align their movement velocity to the slow external pace. Patients were studied after at least 12-hour withdrawal of dopaminergic medication (OFF state) and after intake of the dopamine precursor levodopa (ON state) in order to examine oscillatory coupling between prefrontal and premotor areas during respectively low and high levels of dopamine. In 10 patients and 12 control participants multiple source beamformer analysis yielded task-related activation of a contralateral cortical network comprising prefrontal cortex (PFC), lateral premotor cortex (lPM), supplementary motor area (SMA) and primary motor cortex (M1). Dynamic causal modelling was used to characterize task-related oscillatory coupling between prefrontal and premotor cortical areas. Healthy participants showed task-induced coupling from PFC to SMA, which was modulated within the γ-band. In the OFF state, PD patients did not express any frequency-specific coupling between prefrontal and premotor areas. Application of levodopa reinstated task-related coupling from PFC to SMA, which was expressed as high-β-γ coupling. Additionally, strong within-frequency γ-coupling as well as cross-frequency θ-γ coupling was observed from PFC to lPM. Enhancement of this cross-frequency θ-γ coupling after application of levodopa was positively correlated with individual improvement in motor function. The results demonstrate that dopamine deficiency impairs the ability to establish oscillatory coupling between prefrontal and premotor areas during an externally paced motor task. Application of extrinsic dopamine in PD patients reinstates physiological prefrontal-premotor coupling and additionally induces within- and cross-frequency coupling from prefrontal to premotor areas, which is not expressed in healthy participants.

Deep brain stimulation in Parkinson's disease

For the last 50 years, levodopa has been the cornerstone of Parkinson's disease management. However, a majority of patients develop motor complications a few years after therapy onset. Deep brain stimulation has been approved by the FDA as an adjunctive treatment in Parkinson disease, especially aimed at controlling these complications. However, the exact mechanism of action of deep brain stimulation, the best nucleus to target as well as the best timing for surgery are still debatable. We here provide an in-depth and critical review of the current literature on this topic.

Deep brain stimulation in Parkinson's disease

For the last 50 years, levodopa has been the cornerstone of Parkinson's disease management. However, a majority of patients develop motor complications a few years after therapy onset. Deep brain stimulation has been approved by the FDA as an adjunctive treatment in Parkinson disease, especially aimed at controlling these complications. However, the exact mechanism of action of deep brain stimulation, the best nucleus to target as well as the best timing for surgery are still debatable. We here provide an in-depth and critical review of the current literature on this topic.

The anatomy of the caudal zona incerta in rodents and primates

The caudal zona incerta is the target of a recent modification of established procedures for deep brain stimulation (DBS) for Parkinson's disease and tremor. The caudal zona incerta contains a number of neuronal populations that are distinct in terms of their cytoarchitecture, connections, and pattern of immunomarkers and is located at a position where a number of major tracts converge before turning toward their final destination in the forebrain. However, it is not clear which of the anatomical features of the region are related to its value as a target for DBS. This paper has tried to identify features that distinguish the caudal zona incerta of rodents (mouse and rat) and primates (marmoset, rhesus monkey, and human) from the remainder of the zona incerta. We studied cytoarchitecture, anatomical relationships, the pattern of immunomarkers, and gene expression in both of these areas. We found that the caudal zona incerta has a number of histological and gene expression characteristics that distinguish it from the other subdivisions of the zona incerta. Of particular note are the sparse population of GABA neurons and the small but distinctive population of calbindin neurons. We hope that a clearer appreciation of the anatomy of the region will in the end assist the interpretation of cases in which DBS is used in human patients.

Lateralized alpha-band cortical networks regulate volitional modulation of beta-band sensorimotor oscillations

Sensorimotor rhythms (SMRs) are oscillatory brain activities in the α- and β-bands across the sensorimotor regions of the brain. Each frequency band has its own specific function. The α-band oscillations are closely related to intrinsic cortical networks, whereas oscillations in the β-band are relevant for the information transfer between the cortex and periphery, as well as for visual and proprioceptive feedback. This study aimed to investigate the interaction between these two frequency bands, under the premise that the regional modulation of β-band power is linked to a cortical network in the α-band. We therefore designed a procedure to maximize the modulation of β-band activity over the sensorimotor cortex by combining kinesthetic motor-imagery with closed-loop haptic feedback. The cortical network activity during this procedure was estimated via the phase slope index in electroencephalographic recordings. Analysis of effective connectivity within the α-band network revealed an information flow between the precentral (premotor and primary motor), postcentral (primary somatosensory) and parietal cortical areas. The range of β-modulation was connected to a reduction of an ipsilateral sensorimotor and parietal α-network and, consequently, to a lateralization of this network to the contralateral side. These results showed that regional sensorimotor oscillatory activity in the β-band was regulated by cortical coupling of distant areas in the α-band.

Critical involvement of the motor cortex in the pathophysiology and treatment of Parkinson's disease

This review examines the involvement of the motor cortex in Parkinson's disease (PD), a debilitating movement disorder typified by degeneration of dopamine cells of the substantia nigra. While much of PD research has focused on the caudate/putamen, many aspects of motor cortex function are abnormal in PD patients and in animal models of PD, implicating motor cortex involvement in disease symptoms and their treatment. Herein, we discuss several lines of evidence to support this hypothesis. Dopamine depletion alters regional metabolism in the motor cortex and also reduces interneuron activity, causing a breakdown in intracortical inhibition. This leads to functional reorganization of motor maps and excessive corticostriatal synchrony when movement is initiated. Recent work suggests that electrical stimulation of the motor cortex provides a clinical benefit for PD patients. Based on extant research, we identify a number of unanswered questions regarding the motor cortex in PD and argue that a better understanding of the contribution of the motor cortex to PD symptoms will facilitate the development of novel therapeutic approaches.

Globus pallidus internus oscillatory activity is related to movement speed

Local field potentials (LFPs) recorded from deep brain stimulation electrodes implanted in the globus pallidus internus (GPi) of patients with hyperkinetic movement disorders (dystonia and Tourette's syndrome) have shown desynchronized activity at 8-20 Hz and synchronized activity at 30-90 Hz during voluntary movements. However, the impact of the speed of the motor task on these frequency shifts is still unclear. In the current study, we recorded LFPs bilaterally from the GPi in seven patients with hyperkinetic movement disorders during normal/slow and fast horizontal line drawing movements as well as during rest. In comparison with rest, the low beta band showed a significant decrease in power during the motor tasks. Low beta power was more suppressed with increasing speed of the movement on the contralateral side. In contrast, a significant increase in power was induced by movements in the high beta and gamma bands on the contralateral side. When comparing slow and fast movements, the power of the low gamma frequency band was significantly elevated on the contralateral side with fast movements. In conclusion, an increase in movement speed changes the power of GPi oscillations by means of a reduction of the activity in the low beta band and an elevation of activity in the gamma band. The current study yields new insights into the physiological mechanism of GPi during the execution of the motor task at low and high speed.

Desynchronization and rebound of beta oscillations during conscious and unconscious local neuronal processing in the macaque lateral prefrontal cortex

Accumulating evidence indicates that control mechanisms are not tightly bound to conscious perception since both conscious and unconscious information can trigger control processes, probably through the activation of higher-order association areas like the prefrontal cortex. Studying the modulation of control-related prefrontal signals in a microscopic, neuronal level during conscious and unconscious neuronal processing, and under control-free conditions could provide an elementary understanding of these interactions. Here we performed extracellular electrophysiological recordings in the macaque lateral prefrontal cortex (LPFC) during monocular physical alternation (PA) and binocular flash suppression (BFS) and studied the local scale relationship between beta (15-30 Hz) oscillations, a rhythmic signal believed to reflect the current sensory, motor, or cognitive state (status-quo), and conscious or unconscious neuronal processing. First, we show that beta oscillations are observed in the LPFC during resting state. Both PA and BFS had a strong impact on the power of this spontaneous rhythm with the modulation pattern of beta power being identical across these two conditions. Specifically, both perceptual dominance and suppression of local neuronal populations in BFS were accompanied by a transient beta desynchronization followed by beta activity rebound, a pattern also observed when perception occurred without any underlying visual competition in PA. These results indicate that under control-free conditions, at least one rhythmic signal known to reflect control processes in the LPFC (i.e., beta oscillations) is not obstructed by local neuronal, and accordingly perceptual, suppression, thus being independent from temporally co-existing conscious and unconscious local neuronal representations. Future studies could reveal the additive effects of motor or cognitive control demands on prefrontal beta oscillations during conscious and unconscious processing.

Basal ganglia modulation of thalamocortical relay in Parkinson's disease and dystonia

Basal ganglia dysfunction has being implied in both Parkinson's disease and dystonia. While these disorders probably involve different cellular and circuit pathologies within and beyond basal ganglia, there may be some shared neurophysiological pathways. For example, pallidotomy and pallidal Deep Brain Stimulation (DBS) are used in symptomatic treatment of both disorders. Both conditions are marked by alterations of rhythmicity of neural activity throughout basal ganglia-thalamocortical circuits. Increased synchronized oscillatory activity in beta band is characteristic of Parkinson's disease, while different frequency bands, theta and alpha, are involved in dystonia. We compare the effect of the activity of GPi, the output nuclei of the basal ganglia, on information processing in the downstream neural circuits of thalamus in Parkinson's disease and dystonia. We use a data-driven computational approach, a computational model of the thalamocortical (TC) cell modulated by experimentally recorded data, to study the differences and similarities of thalamic dynamics in dystonia and Parkinson's disease. Our analysis shows no substantial differences in TC relay between the two conditions. Our results suggest that, similar to Parkinson's disease, a disruption of thalamic processing could also be involved in dystonia. Moreover, the degree to which TC relay fidelity is impaired is approximately the same in both conditions. While Parkinson's disease and dystonia may have different pathologies and differ in the oscillatory content of neural discharge, our results suggest that the effect of patterning of pallidal discharge is similar in both conditions. Furthermore, these results suggest that the mechanisms of GPi DBS in dystonia may involve improvement of TC relay fidelity.

Dopaminergic therapy in Parkinson's disease decreases cortical beta band coherence in the resting state and increases cortical beta band power during executive control

It is not yet well understood how dopaminergic therapy improves cognitive and motor function in Parkinson's disease (PD). One possibility is that it reduces the pathological synchronization within and between the cortex and basal ganglia, thus improving neural communication. We tested this hypothesis by recording scalp electroencephalography (EEG) in PD patients when On and Off medication, during a brief resting state epoch (no task), and during performance of a stop signal task that is thought to engage two partially overlapping (or different) frontal-basal-ganglia circuits. For resting state EEG, we measured pair-wise coherence between scalp electrodes in several frequency bands. Consistent with previous studies, in the Off medication state, those patients with the greatest clinical impairment had the strongest coherence, especially in the beta band, indicating pathological over-synchronization. Dopaminergic medication reduced this coherence. For the stop signal task, On vs. Off medication increased beta band power over right frontal cortex for successful stopping and over bilateral sensorimotor cortex for going, especially for those patients who showed greater clinical improvement. Thus, medication reduced pathological coherence in beta band at rest and increased task related beta power for two potentially dissociable cortico-basal ganglia circuits. These results support the hypothesis that dopaminergic medication in PD improves neural communication both at rest and for executive and motor function.

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EEG measured in PD while On/Off medication during rest and an executive control task.

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Dopaminergic therapy reduces pathological locking jointly with clinical improvement.

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Medication increases beta power during successful stopping over right frontal cortex.

Reaching to proprioceptively defined targets in Parkinson's disease: effects of deep brain stimulation therapy

Deep brain stimulation of the subthalamic nucleus (STN DBS) provides a unique window into human brain function since it can reversibly alter the functioning of specific brain circuits. Basal ganglia-cortical circuits are thought to be excessively noisy in patients with Parkinson's disease (PD), based in part on the lack of specificity of proprioceptive signals in basal ganglia-thalamic-cortical circuits in monkey models of the disease. PD patients are known to have deficits in proprioception, but the effects are often subtle, with paradigms typically restricted to one or two joint movements in a plane. Moreover, the effects of STN DBS on proprioception are virtually unexplored. We tested the following hypotheses: first, that PD patients will show substantial deficits in unconstrained, multi-joint proprioception, and, second, that STN DBS will improve multi-joint proprioception. Twelve PD patients with bilaterally implanted electrodes in the subthalamic nucleus and 12 age-matched healthy subjects were asked to position the left hand at a location that was proprioceptively defined in 3D space with the right hand. In a second condition, subjects were provided visual feedback during the task so that they were not forced to rely on proprioception. Overall, with STN DBS switched off, PD patients showed significantly larger proprioceptive localization errors, and greater variability in endpoint localizations than the control subjects. Visual feedback partially normalized PD performance, and demonstrated that the errors in proprioceptive localization were not simply due to a difficulty in executing the movements or in remembering target locations. Switching STN DBS on significantly reduced localization errors from those of control subjects when patients moved without visual feedback relative to when they moved with visual feedback (when proprioception was not required). However, this reduction in localization errors without vision came at the cost of increased localization variability.

Dopaminergic control of motivation and reinforcement learning: a closed-circuit account for reward-oriented behavior

Humans and animals take actions quickly when they expect that the actions lead to reward, reflecting their motivation. Injection of dopamine receptor antagonists into the striatum has been shown to slow such reward-seeking behavior, suggesting that dopamine is involved in the control of motivational processes. Meanwhile, neurophysiological studies have revealed that phasic response of dopamine neurons appears to represent reward prediction error, indicating that dopamine plays central roles in reinforcement learning. However, previous attempts to elucidate the mechanisms of these dopaminergic controls have not fully explained how the motivational and learning aspects are related and whether they can be understood by the way the activity of dopamine neurons itself is controlled by their upstream circuitries. To address this issue, we constructed a closed-circuit model of the corticobasal ganglia system based on recent findings regarding intracortical and corticostriatal circuit architectures. Simulations show that the model could reproduce the observed distinct motivational effects of D1- and D2-type dopamine receptor antagonists. Simultaneously, our model successfully explains the dopaminergic representation of reward prediction error as observed in behaving animals during learning tasks and could also explain distinct choice biases induced by optogenetic stimulation of the D1 and D2 receptor-expressing striatal neurons. These results indicate that the suggested roles of dopamine in motivational control and reinforcement learning can be understood in a unified manner through a notion that the indirect pathway of the basal ganglia represents the value of states/actions at a previous time point, an empirically driven key assumption of our model.

Oscillatory beta activity mediates neuroplastic effects of motor cortex stimulation in humans

Continuous theta burst stimulation (cTBS) is a repetitive transcranial magnetic stimulation protocol that can inhibit human motor cortex (M1) excitability and impair movement for ≤ 1 h. While offering valuable insights into brain function and potential therapeutic benefits, these neuroplastic effects are highly variable between individuals. The source of this variability, and the electrophysiological mechanisms underlying the inhibitory after-effects, are largely unknown. In this regard, oscillatory activity at beta frequency (15-35 Hz) is of particular interest as it is elevated in motor disorders such as Parkinson's disease and modulated during the generation of movements. Here, we used a source-level magnetoencephalography approach to investigate the hypothesis that the presence of neuroplastic effects following cTBS is associated with concurrent changes in oscillatory M1 beta activity. M1 cortices were localized with a synthetic aperture magnetometry beamforming analysis of visually cued index finger movements. Virtual electrode analysis was used to reconstruct the spontaneous and movement-related oscillatory activity in bilateral M1 cortices, before and from 10 to 45 min after cTBS. We demonstrate that 40 s of cTBS applied over left M1 reduced corticospinal excitability in the right index finger of 8/16 participants. In these responder participants only, cTBS increased the power of the spontaneous beta oscillations in stimulated M1 and delayed reaction times in the contralateral index finger. No further changes were observed in the latency or power of movement-related beta oscillations. These data provide insights into the electrophysiological mechanisms underlying cTBS-mediated impairment of motor function and demonstrate the association between spontaneous oscillatory beta activity in M1 and the inhibition of motor function.

Oscillations and the basal ganglia: motor control and beyond

Oscillations form a ubiquitous feature of the central nervous system. Evidence is accruing from cortical and sub-cortical recordings that these rhythms may be functionally important, although the precise details of their roles remain unclear. The basal ganglia share this predilection for rhythmic activity which, as we see in Parkinson's disease, becomes further enhanced in the dopamine depleted state. While certain cortical rhythms appear to penetrate the basal ganglia, others are transformed or blocked. Here, we discuss the functional association of oscillations in the basal ganglia and their relationship with cortical activity. We further explore the neural underpinnings of such oscillatory activity, including the important balance to be struck between facilitating information transmission and limiting information coding capacity. Finally, we introduce the notion that synchronised oscillatory activity can be broadly categorised as immutability promoting rhythms that reinforce incumbent processes, and mutability promoting rhythms that favour novel processing.

Beta-band amplitude oscillations in the human internal globus pallidus support the encoding of sequence boundaries during initial sensorimotor sequence learning

Sequential behavior characterizes both simple everyday tasks, such as getting dressed, and complex skills, such as music performance. The basal ganglia (BG) play an important role in the learning of motor sequences. To study the contribution of the human BG to the initial encoding of sequence boundaries, we recorded local field potentials in the sensorimotor area of the internal globus pallidus (GPi) during the early acquisition of sensorimotor sequences in patients undergoing deep brain stimulation for dystonia. We demonstrated an anticipatory modulation of pallidal beta-band neuronal oscillations that was specific to sequence boundaries, as compared to within-sequence elements, and independent of both the movement parameters and the initiation/termination of ongoing movement. The modulation at sequence boundaries emerged with training, in parallel with skill learning, and correlated with the degree of long-range temporal correlations (LRTC) in the dynamics of ongoing beta-band amplitude oscillations. The implication is that LRTC of beta-band oscillations in the sensorimotor GPi might facilitate the emergence of beta power modulations by the sequence boundaries in parallel with sequence learning. Taken together, the results reveal the oscillatory mechanisms in the human BG that contribute at an initial learning phase to the hierarchical organization of sequential behavior as reflected in the formation of boundary-delimited representations of action sequences.

An entropy-based model for basal ganglia dysfunctions in movement disorders

During this last decade, nonlinear analyses have been used to characterize the irregularity that exists in the neuronal data stream of the basal ganglia. In comparison to linear parameters for disparity (i.e., rate, standard deviation, and oscillatory activities), nonlinear analyses focus on complex patterns that are composed of groups of interspike intervals with matching lengths but not necessarily contiguous in the data stream. In light of recent animal and clinical studies, we present a review and commentary on the basal ganglia neuronal entropy in the context of movement disorders.

Selective effects of dopamine depletion and L-DOPA therapy on learning-related firing dynamics of striatal neurons

Despite evidence that dopamine neurotransmission in the striatum is critical for learning as well as for movement control, little is yet known about how the learning-related dynamics of striatal activity are affected by dopamine depletion, a condition faced in Parkinson's disease. We made localized intrastriatal 6-hydroxydopamine lesions in rats and recorded within the dopamine-depleted sensorimotor striatal zone and its contralateral correspondent as the animals learned a conditional maze task. Rather than producing global, nonspecific elevations in firing rate across the task, the dopamine depletion altered striatal projection neuron activity and fast-spiking interneuron activity selectively, with sharply task-specific and cell type-specific effects, and often, with learning-stage selective effects as well. Striatal projection neurons with strong responses during the maze runs had especially elevated responsiveness during the maze runs. Projection neurons that, instead, fired most strongly before maze running showed elevated pre-start firing rates, but not during maze running, as learning progressed. The intrastriatal dopamine depletion severely affected the learning-related patterning of fast-spiking interneuron ensembles, especially during maze running and after extended training. Remarkably, L-DOPA treatment almost entirely reversed the depletion-induced elevations in pre-run firing of the projection neurons, and elevated their responses around start and end of maze runs. By contrast, L-DOPA failed to normalize fast-spiking interneuron activity. Thus the effects of striatal dopamine depletion and restoration on striatal activity are highly dependent not only on cell type, as previously shown, but also on the behavioral activity called for and the state of behavioral learning achieved.

Neuromagnetic evidence of abnormal movement-related beta desynchronization in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder associated with debilitating motor, posture, and gait abnormalities. Human studies recording local field potentials within the subthalamic nucleus and scalp-based electroencephalography have shown pathological beta synchronization throughout the cortical–basal ganglia motor network in PD. Suppression of such pathological beta synchronization has been associated with improved motor function, which may explain the effectiveness of deep-brain stimulation. We used magnetoencephalography (MEG) to investigate neural population-level beta responses, and other oscillatory activity, during a motor task in unmedicated patients with PD and a matched group of healthy adults. MEG is a noninvasive neurophysiological technique that permits the recording of oscillatory activity during movement planning, execution, and termination phases. Each of these phases was independently examined using beamforming to distinguish the brain areas and movement phases, where pathological oscillations exist during motor control. Patients with PD exhibited significantly diminished beta desynchronization compared with controls prior to and during movement, which paralleled reduced alpha desynchronization. This study is the first to systematically investigate neural oscillatory responses in PD during distinct stages of motor control (e.g. planning, execution, and termination) and indicates that these patients have significant difficulty suppressing cortical beta synchronization during movement planning, which may contribute to their diminished movement capacities.

Control of layer 5 pyramidal cell spiking by oscillatory inhibition in the distal apical dendrites: a computational modeling study

The distal apical dendrites of layer 5 pyramidal neurons receive cortico-cortical and thalamocortical top-down and feedback inputs, as well as local recurrent inputs. A prominent source of recurrent inhibition in the neocortical circuit is somatostatin-positive Martinotti cells, which preferentially target distal apical dendrites of pyramidal cells. These electrically coupled cells can fire synchronously at various frequencies, including over a relatively slow range (5∼30 Hz), thereby imposing oscillatory inhibition on the pyramidal apical tuft dendrites. We examined how such distal oscillatory inhibition influences the firing of a biophysically detailed layer 5 pyramidal neuron model, which reproduced the spatiotemporal properties of sodium, calcium, and N-methyl-D-aspartate receptor spikes found experimentally. We found that oscillatory synchronization strongly influences the impact of distal inhibition on the pyramidal cell firing. Whereas asynchronous inhibition largely cancels out the facilitatory effects of distal excitatory inputs, inhibition oscillating synchronously at around 10∼20 Hz allows distal excitation to drive axosomatic firing, as if distal inhibition were absent. Underlying this is a switch from relatively infrequent burst firing to single spike firing at every period of the inhibitory oscillation. This phenomenon depends on hyperpolarization-activated cation current-dependent membrane potential resonance in the dendrite, but also, in a novel manner, on a cooperative amplification of this resonance by N-methyl-D-aspartate-receptor-driven dendritic action potentials. Our results point to a surprising dependence of the effect of recurrent inhibition by Martinotti cells on their oscillatory synchronization, which may control not only the local circuit activity, but also how it is transmitted to and decoded by downstream circuits.

Failure of delayed feedback deep brain stimulation for intermittent pathological synchronization in Parkinson's disease

Suppression of excessively synchronous beta-band oscillatory activity in the brain is believed to suppress hypokinetic motor symptoms of Parkinson's disease. Recently, a lot of interest has been devoted to desynchronizing delayed feedback deep brain stimulation (DBS). This type of synchrony control was shown to destabilize the synchronized state in networks of simple model oscillators as well as in networks of coupled model neurons. However, the dynamics of the neural activity in Parkinson's disease exhibits complex intermittent synchronous patterns, far from the idealized synchronous dynamics used to study the delayed feedback stimulation. This study explores the action of delayed feedback stimulation on partially synchronized oscillatory dynamics, similar to what one observes experimentally in parkinsonian patients. We employ a computational model of the basal ganglia networks which reproduces experimentally observed fine temporal structure of the synchronous dynamics. When the parameters of our model are such that the synchrony is unphysiologically strong, the feedback exerts a desynchronizing action. However, when the network is tuned to reproduce the highly variable temporal patterns observed experimentally, the same kind of delayed feedback may actually increase the synchrony. As network parameters are changed from the range which produces complete synchrony to those favoring less synchronous dynamics, desynchronizing delayed feedback may gradually turn into synchronizing stimulation. This suggests that delayed feedback DBS in Parkinson's disease may boost rather than suppress synchronization and is unlikely to be clinically successful. The study also indicates that delayed feedback stimulation may not necessarily exhibit a desynchronization effect when acting on a physiologically realistic partially synchronous dynamics, and provides an example of how to estimate the stimulation effect.

Thalamic gamma oscillations correlate with reaction time in a Go/noGo task in patients with essential tremor

Intracerebral recordings of neuronal activity in patients undergoing deep brain stimulation have revealed characteristic movement-related desynchronization at frequencies <30 Hz and increased activity in the gamma band (~30-100 Hz) in the basal ganglia and thalamus. Thalamic gamma activity is also found during arousal. Here, we explore oscillatory gamma band activity recorded from the ventralis intermedius nucleus of the thalamus during motor performance in a Go/noGo task in 10 patients with essential tremor after implantation of deep brain stimulation electrodes. We show that movement-related gamma activity is lateralized to the nucleus contralateral to the moved side similar to previous findings in the globus pallidus internus and the subthalamic nucleus. The onset of contralateral gamma band synchronization following imperative Go cues is positively correlated with reaction time. Remarkably, baseline levels of gamma activity shortly before the Go cue correlated with the reaction times. Here, faster responses occurred in patients with higher levels of pre-cue gamma activity. Our findings support the role of gamma activity as a physiological prokinetic activity in the motor system. Moreover, we suggest that subtle fluctuations in pre-cue gamma band activity may have an impact on task performance and may index arousal-related states.