Context-dependent modulation of movement-related discharge in the primate globus pallidus

Loss of specificity in Basal Ganglia related movement disorders

The basal ganglia (BG) are a group of interconnected nuclei which play a pivotal part in limbic, associative, and motor functions. This role is mirrored by the wide range of motor and behavioral abnormalities directly resulting from dysfunction of the BG. Studies of normal behavior have found that BG neurons tend to phasically modulate their activity in relation to different behavioral events. In the normal BG, this modulation is highly specific, with each neuron related only to a small subset of behavioral events depending on specific combinations of movement parameters and context. In many pathological conditions involving BG dysfunction and motor abnormalities, this neuronal specificity is lost. Loss of specificity (LOS) manifests in neuronal activity related to a larger spectrum of events and consequently a large overlap of movement-related activation patterns between different neurons. We review the existing evidence for LOS in BG-related movement disorders, the possible neural mechanisms underlying LOS, its effects on frequently used measures of neuronal activity and its relation to theoretical models of the BG. The prevalence of LOS in a many BG-related disorders suggests that neuronal specificity may represent a key feature of normal information processing in the BG system. Thus, the concept of neuronal specificity may underlie a unifying conceptual framework for the BG role in normal and abnormal motor control.

Bicuculline-Induced Chorea Manifests in Focal Rather Than Globalized Abnormalities in the Activation of the External and Internal Globus Pallidus

Chorea is a basal-ganglia (BG) related hyperkinetic movement disorder characterized by irregular continuous involuntary movements. Chorea and related hyperbehavioral disorders may be induced in behaving primates by local microinjections of the GABAA antagonist bicuculline to the globus pallidus externus (GPe). We performed multielectrode extracellular recordings in the GPe and in the globus pallidus internus (GPi) before, during, and after bicuculline microinjections. Bicuculline led to an increase in the firing rate and a change in the firing pattern of GPe neurons. Two types of abnormal neuronal firing patterns were detected in GPe neurons close to the bicuculline microinjection site: continuous high-frequency activity and bistable activity, in which neurons transitioned between high-frequency and complete cessation of firing. Neuronal activity remained uncorrelated within and between the GPe and the GPi, with no evidence for propagation of the focal GPe abnormal activity downstream to the GPi. Despite reduction in the information capacity of bicuculline-affected GPe neurons, the ability to encode behavioral events was maintained. We found similar responses of GPe neurons to bicuculline in vitro in the rat, suggesting a basic cellular mechanism underlying these abnormal firing patterns. These results demonstrate that chorea is associated with focal neuronal changes that are not complemented by global changes in the BG nuclei. This suggests a mechanism of stochastic phasic alteration of BG control leading to the chaotic nature of chorea. Thus rather than imposing a globalized state of cortical excitability, chorea might be associated with changes in internal information processing within the BG.

Basal ganglia contributions to motor control: a vigorous tutor

The roles of the basal ganglia (BG) in motor control are much debated. Many influential hypotheses have grown from studies in which output signals of the BG were not blocked, but pathologically disturbed. A weakness of that approach is that the resulting behavioral impairments reflect degraded function of the BG per se mixed together with secondary dysfunctions of BG-recipient brain areas. To overcome that limitation, several studies have focused on the main skeletomotor output region of the BG, the globus pallidus internus (GPi). Using single-cell recording and inactivation protocols these studies provide consistent support for two hypotheses: the BG modulates movement performance ('vigor') according to motivational factors (i.e. context-specific cost/reward functions) and the BG contributes to motor learning. Results from these studies also add to the problems that confront theories positing that the BG selects movement, inhibits unwanted motor responses, corrects errors on-line, or stores and produces well-learned motor skills.

Shedding new light on the role of the basal ganglia-superior colliculus pathway in eye movements

A large body of work spanning 25+ years provides compelling evidence for the involvement of the basal ganglia-superior colliculus pathway in the initiation of rapid, orienting movements of the eyes, called saccades. The role of this pathway in saccade control is similar to the role of the basal ganglia-thalamic pathway in the control of skeletal movement: a transient cessation in tonic inhibition supplied by the basal ganglia to motor structures releases movements via the direct pathway whereas a transient increase in inhibition by the basal ganglia to motor structures prevents movements via the indirect pathway. In parallel with recent advances in the study and treatment of patients with basal ganglia disease and in animal experiments in the skeletal motor system, the results of studies exploring the role of the basal ganglia-superior colliculus pathway in saccades highlight the need for a revisiting of our understanding of the role of this pathway in saccades. The discovery of many different response profiles of neurons in the substantia nigra pars reticulata of the basal ganglia and in the superior colliculus, coupled with advances in experimental and statistical techniques including sophisticated behavioral procedures and multiple neuron recording and analysis, point toward a role for the basal ganglia-superior colliculus pathway in cognitive events intervening between vision and action, such as memory, target selection and saccade choice and valuation.

Selective activation of striatal fast-spiking interneurons during choice execution

Basal ganglia circuits are essential for the organization and execution of voluntary actions. Within the striatum, fast-spiking interneurons (FSIs) are thought to tightly regulate the activity of medium-spiny projection neurons (MSNs) through feed-forward inhibition, yet few studies have investigated the functional contributions of FSIs in behaving animals. We recorded presumed MSNs and FSIs together with motor cortex and globus pallidus (GP) neurons, in rats performing a simple choice task. MSN activity was widely distributed across the task sequence, especially near reward receipt. By contrast, FSIs showed a coordinated pulse of increased activity as chosen actions were initiated, in conjunction with a sharp decrease in GP activity. Both MSNs and FSIs were direction selective, but neighboring MSNs and FSIs showed opposite selectivity. Our findings suggest that individual FSIs participate in local striatal information processing, but more global disinhibition of FSIs by GP is important for initiating chosen actions while suppressing unwanted alternatives.

Singing-related neural activity distinguishes two putative pallidal cell types in the songbird basal ganglia: comparison to the primate internal and external pallidal segments

The songbird area X is a basal ganglia homolog that contains two pallidal cell types-local neurons that project within the basal ganglia and output neurons that project to the thalamus. Based on these projections, it has been proposed that these classes are structurally homologous to the primate external (GPe) and internal (GPi) pallidal segments. To test the hypothesis that the two area X pallidal types are functionally homologous to GPe and GPi neurons, we recorded from neurons in area X of singing juvenile male zebra finches, and directly compared their firing patterns to neurons recorded in the primate pallidus. In area X, we found two cell classes that exhibited high firing (HF) rates (>60 Hz) characteristic of pallidal neurons. HF-1 neurons, like most GPe neurons we examined, exhibited large firing rate modulations, including bursts and long pauses. In contrast, HF-2 neurons, like GPi neurons, discharged continuously without bursts or long pauses. To test whether HF-2 neurons were the output neurons that project to the thalamus, we next recorded directly from pallidal axon terminals in thalamic nucleus DLM, and found that all terminals exhibited singing-related firing patterns indistinguishable from HF-2 neurons. Our data show that singing-related neural activity distinguishes two putative pallidal cell types in area X: thalamus-projecting neurons that exhibit activity similar to the primate GPi, and non-thalamus-projecting neurons that exhibit activity similar to the primate GPe. These results suggest that song learning in birds and motor learning in mammals use conserved basal ganglia signaling strategies.

Neurons in both pallidal segments change their firing properties similarly prior to closure of the eyes

Current anatomical models of the cortico-basal ganglia (BG) network predict reciprocal discharge patterns between the external and internal segments of the globus pallidus (GPe and GPi, respectively), as well as cortical driving of BG activity. However, physiological studies revealing similarity in the transient responses of GPe and GPi neurons cast doubts on these predictions. Here, we studied the discharge properties of GPe, GPi, and primary motor cortex neurons of two monkeys in two distinct states: when eyes are open versus when they are closed. Both pallidal populations exhibited decreased discharge rates in the "eye closed" state accompanied by elevated values of the coefficient of variation (CV) of their interspike interval (ISI) distributions. The pallidal modulations in discharge patterns were partially attributable to larger fractions of longer ISIs in the "eye closed" state. In addition, the pallidal discharge modulations were gradual, starting prior to closing of the eyes. Cortical neurons, as opposed to pallidal neurons, increased their discharge rates steeply on closure of the eyes. Surprisingly, the cortical rate modulations occurred after pallidal modulations. However, as in the pallidum, the CV values of cortical ISI distributions increased in the "eye closed" state, indicating a more bursty discharge pattern in that state. Thus changes in GPe and GPi discharge properties were positively correlated, suggesting that the subthalamic nucleus and/or the striatum constitute the main common driving force for both pallidal segments. Furthermore, the early, unexpected changes in the pallidum are better explained by a subcortical rather than a cortical loop through the BG.

Basal ganglia contribution to the initiation of corrective submovements

We investigated the neural processes, with a focus on subcortical circuits, which govern corrective submovements in visually targeted action. During event-related fMRI, subjects moved a cursor to capture targets presented at varying movement amplitudes. Movements were performed in a rehearsed null and a novel viscous (25% random trials) torque field. Movement error feedback was provided after each trial. The viscous field invoked a significantly larger error at the end of the primary movement. Subjects compensated by producing more corrections than they had in the null condition. Corrective submovements were appropriately scaled such that terminal error was similar between the two conditions. Parametric analysis identified two regions where the BOLD signal correlated with the number of submovements per trial: a cerebellar region similar to the one noted in the task contrast and the contralateral dorsal putamen. A separate parametric analysis identified brain regions where activity correlated with movement amplitude. This identified the same cerebellar region as above, bilateral parietal cortex, and left motor and premotor cortex. Our data indicate that the basal ganglia and cerebellum play complementary roles in regulating ongoing actions when precise updating is required. The basal ganglia have a key role in contextually-based motor decision-making, i.e. for deciding if and when to correct a given movement by initiating corrective submovements, and the cerebellum is more generally involved in amplifying and refining the command signals for movements of different amplitudes.

A parietal-premotor network for movement intention and motor awareness

It is commonly assumed that we are conscious of our movements mainly because we can sense ourselves moving as ongoing peripheral information coming from our muscles and retina reaches the brain. Recent evidence, however, suggests that, contrary to common beliefs, conscious intention to move is independent of movement execution per se. We propose that during movement execution it is our initial intentions that we are mainly aware of. Furthermore, the experience of moving as a conscious act is associated with increased activity in a specific brain region: the posterior parietal cortex. We speculate that movement intention and awareness are generated and monitored in this region. We put forward a general framework of the cognitive and neural processes involved in movement intention and motor awareness.

Functional organization of the basal ganglia: therapeutic implications for Parkinson's disease

The basal ganglia (BG) are a highly organized network, where different parts are activated for specific functions and circumstances. The BG are involved in movement control, as well as associative learning, planning, working memory, and emotion. We concentrate on the "motor circuit" because it is the best understood anatomically and physiologically, and because Parkinson's disease is mainly thought to be a movement disorder. Normal function of the BG requires fine tuning of neuronal excitability within each nucleus to determine the exact degree of movement facilitation or inhibition at any given moment. This is mediated by the complex organization of the striatum, where the excitability of medium spiny neurons is controlled by several pre- and postsynaptic mechanisms as well as interneuron activity, and secured by several recurrent or internal BG circuits. The motor circuit of the BG has two entry points, the striatum and the subthalamic nucleus (STN), and an output, the globus pallidus pars interna (GPi), which connects to the cortex via the motor thalamus. Neuronal afferents coding for a given movement or task project to the BG by two different systems: (1) Direct disynaptic projections to the GPi via the striatum and STN. (2) Indirect trisynaptic projections to the GPi via the globus pallidus pars externa (GPe). Corticostriatal afferents primarily act to inhibit medium spiny neurons in the "indirect circuit" and facilitate neurons in the "direct circuit." The GPe is in a pivotal position to regulate the motor output of the BG. Dopamine finely tunes striatal input as well as neuronal striatal activity, and modulates GPe, GPi, and STN activity. Dopaminergic depletion in Parkinson's disease disrupts the corticostriatal balance leading to increased activity the indirect circuit and reduced activity in the direct circuit. The precise chain of events leading to increased STN activity is not completely understood, but impaired dopaminergic regulation of the GPe, GPi, and STN may be involved. The parkinsonian state is characterized by disruption of the internal balance of the BG leading to hyperactivity in the two main entry points of the network (striatum and STN) and excessive inhibitory output from the GPi. Replacement therapy with standard levodopa creates a further imbalance, producing an abnormal pattern of neuronal discharge and synchronization of neuronal firing that sustain the "off" and "on with dyskinesia" states. The effect of levodopa is robust but short-lasting and converts the parkinsonian BG into a highly unstable system, where pharmacological and compensatory effects act in opposing directions. This creates a scenario that substantially departs from the normal physiological state of the BG.

Encoding of probabilistic rewarding and aversive events by pallidal and nigral neurons

Previous studies have rarely tested whether the activity of high-frequency discharge (HFD) neurons of the basal ganglia (BG) is modulated by expectation, delivery, and omission of aversive events. Therefore the full value domain encoded by the BG network is still unknown. We studied the activity of HFD neurons of the globus pallidus external segment (GPe, n=310), internal segment (GPi, n=149), and substantia nigra pars reticulata (SNr, n=145) in two monkeys during a classical conditioning task with cues predicting the probability of food, neutral, or airpuff outcomes. The responses of BG HFD neurons were long-lasting and diverse with coincident increases and decreases in discharge rate. The population responses to reward-related events were larger than the responses to aversive and neutral-related events. The latter responses were similar, except for the responses to actual airpuff delivery. The fraction of responding cells was larger for reward-related events, with better discrimination between rewarding and aversive trials in the responses with an increase rather than a decrease in discharge rate. GPe and GPi single units were more strongly modulated and better reflected the probability of reward- than aversive-related events. SNr neurons were less biased toward the encoding of the rewarding events, especially during the outcome epoch. Finally, the latency of SNr responses to all predictive cues was shorter than the latency of pallidal responses. These results suggest preferential activation of the BG HFD neurons by rewarding compared with aversive events.

Balance of increases and decreases in firing rate of the spontaneous activity of basal ganglia high-frequency discharge neurons

Most neurons in the external and internal segments of the globus pallidus and the substantia nigra pars reticulata (GPe, GPi, and SNr) are characterized by a high-frequency discharge (HFD) rate (50-80 Hz) that, in most GPe neurons, is also interrupted by pauses. Almost all (approximately 90%) of the synaptic inputs to these HFD neurons are GABAergic and inhibitory. Nevertheless, their responses to behavioral events are usually dominated by increases in discharge rate. Additionally, there are no reports of prolonged bursts in the spontaneous activity of these cells that could reflect their disinhibition by GPe pauses. We recorded the spontaneous activity of 385 GPe, GPi, and SNr HFD neurons during a quiet-wakeful state from two monkeys. We developed three complementary methods to quantify the balance of increases and decreases in the spontaneous discharge of HFD neurons and validated them by simulations. Unlike the behavioral evoked responses, the spontaneous activity of pallidal and SNr neurons is not dominated by increases. Moreover, the activity of basal ganglia neurons does not include bursts that could reflect disinhibition by the spontaneous pauses of GPe neurons. These findings suggest that the discharge increase/decrease balance during a quiet-wakeful state better reflects the inhibitory input of the HFD basal ganglia neurons than during responses to behavioral events; however, the GPe pauses are not echoed by comparable bursts either in the GPe or in the output nuclei. Changes in the excitatory drive of these structures (e.g., during behavioral activity) thus may lead to a remarkable change in this balance.

Interactions between cortical rhythms and spiking activity of single basal ganglia neurons in the normal and parkinsonian state

In order to evaluate the specific interactions between cortical oscillations and basal ganglia-spiking activity under normal and parkinsonian conditions, we examined the relationship between frontal cortex electroencephalographic (EEG) signals and simultaneously recorded neuronal activity in the internal and external segments of the pallidum or the subthalamic nucleus (STN) in 3 rhesus monkeys. After we made recordings in the normal state, hemiparkinsonism was induced with intracarotid injections of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in one animal, followed by additional recordings. Spiking activity in the pallidum and STN was associated with significant shifts in the level of EEG synchronization. We also found that the spectral power of beta- and gamma-band EEG rhythms covaried positively before the basal ganglia spikes but did not covary or covaried negatively thereafter. In parkinsonism, changes in cortical synchronization and phase coherence were reduced in EEG segments aligned to STN spikes, whereas both were increased in data segments aligned to pallidal spikes. Spiking-related changes in beta/gamma-band covariance were reduced. The findings indicate that basal ganglia and cortex interact in the processing of cortical rhythms that contain oscillations across a broad range of frequencies and that this interaction is severely disrupted in parkinsonism.

Visuo-vestibular Influences on the Moving Platform Locomotor Aftereffect

After walking onto a moving platform subjects experience a locomotor aftereffect (LAE), including a self-generated stumble, when walking again onto a stationary platform. Thus this LAE affords examination of the role of vestibular input during an internally generated postural challenge. The experiments involved walking onto the stationary sled (BEFORE trials), walking onto the moving sled (MOVING), and a second set of stationary trials (AFTER). We investigated 9 bilateral labyrinthine defective subjects (LDS) and 13 age-matched normal controls (NC) with eyes open. We repeated the experiment in 5 NC and 5 LDS but this time the AFTER trials were performed twice, first eyes closed and then on eye reopening. During MOVING trials, LDS were considerably unstable, thus confirming the established role of the vestibular system during externally imposed postural perturbations. During AFTER trials, both groups experienced an aftereffect with eyes open and closed, shown as higher approach gait velocity, a forward trunk overshoot, and increased leg EMG. However, there were no significant group differences due to the fact that stopping the forward trunk overshoot was accomplished by anticipatory EMG bursts. On eye reopening the aftereffect reemerged, significantly larger in LDS than that in NC. The lack of group differences in AFTER trials suggests that when facing internally generated postural perturbations, as in this adaptation process, the CNS relies less on vestibular feedback and more on anticipatory mechanisms. Reemergence of the aftereffect on eye reopening indicates the existence of a feedforward visuo-contextual mechanism for locomotor learning, which is adaptively enhanced in the absence of vestibular function.

Testing basal ganglia motor functions through reversible inactivations in the posterior internal globus pallidus

To test current hypotheses on the contribution of the basal ganglia (BG) to motor control, we examined the effects of muscimol-induced inactivations in the skeletomotor region of the internal globus pallidus (sGPi) on visually directed reaching. Injections were made in two monkeys trained to perform four out-and-back reaching movements in quick succession toward four randomly selected target locations. Following sGPi inactivations the following occurred. 1) Peak velocity and acceleration were decreased in nearly all sessions, whereas movement duration lengthened inconsistently. 2) Reaction times were unaffected on average, although minor changes were observed in several individual sessions. 3) Outward reaches showed a substantial hypometria that correlated closely with bradykinesia, but directional accuracy was unaffected. 4) Endpoint accuracy was preserved for the slow visually guided return movements. 5) No impairments were found in the rapid chaining of out-and-back movements, in the selection or initiation of four independent reaches in quick succession or in the quick on-line correction of initially misdirected reaches. 6) Inactivation-induced reductions in the magnitude of movement-related muscle activity (EMG) correlated with the severity of slowing and hypometria. There was no evidence for inactivation-induced alterations in the relative timing of EMG bursts, excessive cocontraction, or impaired suppression of antagonist EMG. Therefore disconnecting the BG motor pathway consistently produced bradykinesia and hypometria, but seldom affected movement initiation time, feedback-mediated guidance, the capacity to produce iterative reaches, or the ability to abruptly reverse movement direction. These results are discussed with reference to the idea that the BG motor loop may regulate energetic expenditures during movement (i.e., movement "vigor").

Cognitive signals in the primate motor thalamus predict saccade timing

We often generate movements without any external event that immediately triggers them. How the brain decides the timing of self-initiated movements remains unclear. Previous studies suggest that the basal ganglia-thalamocortical pathways play this role, but the subcortical signals that determine movement timing have not been identified. The present study reports that a subset of thalamic neurons predicts the timing of self-initiated saccadic eye movements. When monkeys made a saccade in response to the fixation point (FP) offset in the traditional memory saccade task, neurons in the ventrolateral and the ventroanterior nuclei of the thalamus exhibited a gradual buildup of activity that peaked around the most probable time of the FP offset; however, neither the timing nor the magnitude of neuronal activity correlated with saccade latencies, suggesting that the brain is unlikely to have used this information to decide the times of saccades in the traditional memory saccade task. In contrast, when monkeys were required to make a self-timed saccade within a fixed time interval after an external cue, the same neurons again exhibited a strong buildup of activity that preceded saccades by several hundred milliseconds, showing a close correlation between the times of neuronal activity and the times of self-initiated saccades. The results suggest that neurons in the motor thalamus carry subjective time information, which is used by cortical networks to determine the timing of self-initiated saccades.

Distinct changes in evoked and resting globus pallidus activity in early and late Parkinson's disease experimental models

The main clinical manifestations of Parkinson's disease are caused by alterations of basal ganglia activity that are tied in with the progressive loss of mesencephalic dopaminergic neurons. Recent theoretical and modeling studies have suggested that changes in resting neuronal activity occurred later in the course of the disease than those evoked by phasic cortical input. However, there is no empirical support for this proposal. Here we report a marked increase in the responsiveness of globus pallidus neurons to electrical motor cortex stimulation, in the absence of noticeable changes in resting activity, in anesthetized rats that had consistently shown a deficit in forelimb use during behavioral testing before the experiments, and had approximately 45% dopamine neurons spared in the substantia nigra. Pallidal neurons were also over-responsive to motor cortex stimulation and lost spatial selectivity for cortical inputs in rats with extensive nigrostriatal damage. After partial lesions, over-responsiveness was mainly due to an increased proportion of neurons showing excitatory responses, while extensive lesions led to an increased likelihood of inhibitory responding neurons. Changes in resting neuronal activity, comprising pauses disrupting tonic discharge, occurred across different global brain states, including an activated condition which shares similarities with natural patterns of cortical activity seen in awake states and rapid eye-movement sleep, but only after massive nigrostriatal degeneration. These results suggest that a loss of functional segregation and an abnormal temporal encoding of phasic cortical inputs by globus pallidus neurons may contribute to inducing early motor impairment in Parkinson's disease.

A single-cell analysis of intrinsic connectivity in the rat globus pallidus

GABAergic neurons of the globus pallidus (GP) play critical roles in basal ganglia function by virtue of their widespread axonal projections to all parts of the basal ganglia. They also possess local axon collaterals. In view of the importance of GABAergic inputs in sculpting neuronal activity, we quantitatively characterized the local axon collaterals of individual GP neurons by in vivo recording, juxtacellular labeling, reconstruction, and light and electron microscopic analysis in the rat. All labeled GP neurons had similar firing properties and gave rise to local axon collaterals, the main synaptic targets of which were perikarya and primary dendrites. The neurons could be divided into two populations; neurons located within approximately 100 microm of the striatopallidal border ("outer" neurons), which possess a mean of 264 local axonal boutons, and those located approximately 100 microm or more from the striatopallidal border ("inner" neurons), which possess a mean of 581 local axonal boutons. The local axon collaterals gave rise to arborizations close to, or within, the parent dendritic field and arborizations located caudal, medial, and ventral to the parent neuron. The qualitative and quantitative differences in the connectivity of neurons located in the outer and inner regions of the GP underlie a complex microcircuitry that follows an asymmetric rostral to caudal organization. These data suggest that the GP should no longer be considered as an homogeneous relay nucleus that simply transmits striatal information to the subthalamic nucleus and basal ganglia output nuclei, but rather as a structure that can perform complex computations within its borders.

Striatal dysfunction increases basal ganglia output during motor cortex activation in parkinsonian rats

During movement, inhibitory neurons in the basal ganglia output nuclei show complex modulations of firing, which are presumptively driven by corticostriatal and corticosubthalamic input. Reductions in discharge should facilitate movement by disinhibiting thalamic and brain stem nuclei while increases would do the opposite. A proposal that nigrostriatal dopamine pathway degeneration disrupts trans-striatal pathways' balance resulting in sustained overactivity of basal ganglia output nuclei neurons and Parkinson's disease clinical signs is not fully supported by experimental evidence, which instead shows abnormal synchronous oscillatory activity in animal models and patients. Yet, the possibility that variation in motor cortex activity drives transient overactivity in output nuclei neurons in parkinsonism has not been explored. In Sprague-Dawley rats with 6-hydroxydopamine (6-OHDA)-induced nigrostriatal lesions, approximately 50% substantia nigra pars reticulata (SNpr) units show abnormal cortically driven slow oscillations of discharge. Moreover, these units selectively show abnormal responses to motor cortex stimulation consisting in augmented excitations of an odd latency, which overlapped that of inhibitory responses presumptively mediated by the trans-striatal direct pathway in control rats. Delivering D1 or D2 dopamine agonists into the striatum of parkinsonian rats by reverse microdialysis reduced these abnormal excitations but had no effect on pathological oscillations. The present study establishes that dopamine-deficiency related changes of striatal function contribute to producing abnormally augmented excitatory responses to motor cortex stimulation in the SNpr. If a similar transient overactivity of basal ganglia output were driven by motor cortex input during movement, it could contribute to impeding movement initiation or execution in Parkinson's disease.

Functional anatomy of motor urgency

This PET H(2)(15)O study uses a reaching task to determine the neural basis of the unconscious motor speed up observed in the context of urgency in healthy subjects. Three conditions were considered: self-initiated (produce the fastest possible movement toward a large plate, when ready), externally-cued (same as self-initiated but in response to an acoustic cue) and temporally-pressing (same as externally-cued with the plate controlling an electromagnet that prevented a rolling ball from falling at the bottom of a tilted ramp). Results show that: (1) Urgent responses (Temporally-pressing versus Externally-cued) engage the left parasagittal and lateral cerebellar hemisphere and the sensorimotor cortex (SMC) bilaterally; (2) Externally-driven responses (Externally-cued versus Self-initiated) recruit executive areas within the contralateral SMC; (3) Volitional responses (Self-initiated versus Externally-cued) involve prefrontal cortical areas. These observations are discussed with respect to the idea that neuromuscular energy is set to a submaximal threshold in self-determined situations. In more challenging tasks, this threshold is raised and the first answer of the nervous system is to optimize the response of the lateral (i.e. crossed) corticospinal tract (contralateral SMC) and ipsilateral cerebellum. In a second step, the anterior (i.e. uncrossed) corticospinal tract (ipsilateral SMC) and the contralateral cerebellum are recruited. This recruitment is akin to the strategy observed during recovery in patients with brain lesions.

Signaling patterns of globus pallidus internal segment neurons during forearm rotation

We recorded extracellular single unit discharges of globus pallidus internal segment (GPi) neurons in monkeys performing a visually driven forearm rotation movement task in order to quantify how discharge patterns changed in relation to kinematic parameters. Subjects grasped a handle that rotated about its axis while facing a video screen displaying visual targets. Continuous visual feedback of handle rotation position was provided. Monkeys generated forearm rotation movements of +/-35 degrees and +/-70 degrees amplitude in order to align the cursor and targets. Trial records were aligned to forearm rotation onset in order to compare the discharge patterns that were associated with movements of different amplitudes, velocities, and directions. In addition, we quantified the depth of modulation of neuronal discharge associated with movements generated in two different task phases. Comparisons of discharge patterns were made between the visually guided, rewarded phase ("cued movements") and the self-paced, unrewarded phase that returned the monkey to the task start position ("return movements") by quantifying the goodness of fit between neuronal discharge during cued and return movements. Our analyses revealed no systematic relationship between the depth of modulation of GPi neurons and forearm rotation amplitude, direction, or velocity. Furthermore, comparisons between the two behavioral contexts revealed a systematic attenuation of modulation that could not be attributed to differences in movement velocity. Collectively, these findings suggest that the GPi neurons that we studied were not significantly involved in mediating movement kinematics, but may have instead been instrumental in the processing of information about the behavioral context during which movements were generated.

Statistical properties of pauses of the high-frequency discharge neurons in the external segment of the globus pallidus

The neurons of many basal ganglia nuclei, including the external and internal globus pallidus (GPe and GPi, respectively) and the substantia nigra pars reticulata (SNr) are characterized by their high-frequency (50-100 spikes/s) tonic discharge (HFD). However, the high firing rate of GPe neurons is interrupted by long pauses. We studied the extracellularly recorded spiking activity of 212 well-isolated HFD GPe and 52 GPi/SNr neurons from five monkeys during different states of behavioral activity. An algorithm that maximizes the surprise function was used to detect pauses and pauser cells ("pausers"). Only 6% of the GPi/SNr neurons versus as many as 56% of the GPe neurons were classified as pausers. The GPe average pause duration equals 0.62 s. The interpause intervals follow a Poissonian distribution with a frequency of 13 pauses/minute. No linear relationship was found between pause parameters (duration or frequency) and the firing rate of the cell. Pauses were preceded by various changes in firing rate but not dominantly by a decrease. The average amplitude and duration of the spike waveform was modulated only after the pause but not before it. Pauses of pairs of cells that were recorded simultaneously were not correlated. The probability of GPe cells to pause spontaneously was extremely variable among monkeys (30-90%) and inversely related to the degree of the monkey's motor activity. These findings suggest that spontaneous GPe pauses are related to low-arousal periods and are generated by a process that is independent of the discharge properties of the cells.

"Paradoxical kinesis" is not a hallmark of Parkinson's disease but a general property of the motor system

Although slowness of movement is a typical feature of Parkinson's disease (PD), it has been suggested that severely disabled patients remained able to produce normal motor responses in the context of urgent or externally driven situations. To investigate this phenomenon (often designated "paradoxical kinesis"), we required PD patients and healthy subjects to press a large switch under three main conditions: Self Generated, produce the fastest possible movement; External Cue, same as Self Generated but in response to an acoustic cue; Urgent External Cue, same as External Cue with the switch controlling an electromagnet that prevented a ball falling at the bottom of a tilted ramp. Task difficulty was equalized for the two experimental groups. Results showed that external cues and urgent conditions decreased movement duration (Urgent External Cue < External Cue < Self Generated) and reaction time (Urgent External Cue < External Cue). The amount of reduction was identical in PD patients and healthy subjects. These observations show that paradoxical kineses are not a hallmark of PD or a byproduct of basal ganglia dysfunctions, but a general property of the motor system.