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

Motor cortical inactivation impairs corrective submovements in mice performing a hold-still center-out reach task

Holding still and aiming reaches to spatial targets may depend on distinct neural circuits. Using automated homecage training and a sensitive joystick, we trained freely moving mice to contact a joystick, hold their forelimb still, and then reach to rewarded target locations. Mice learned the task by initiating forelimb sequences with clearly resolved submillimeter-scale micromovements followed by millimeter-scale reaches to learned spatial targets. Hundreds of thousands of trajectories were decomposed into millions of kinematic submovements, while photoinhibition was used to test roles of motor cortical areas. Inactivation of both caudal and rostral forelimb areas preserved the ability to produce aimed reaches, but reduced reach speed. Inactivation specifically of contralateral caudal forelimb area (CFA) additionally impaired the ability to aim corrective submovements to remembered locations following target undershoots. Our findings show that motor cortical inactivations reduce the gain of forelimb movements but that inactivation specifically of contralateral CFA impairs corrective movements important for reaching a target location.NEW & NOTEWORTHY To test the role of different cortical areas in holding still and reaching to targets, this study combined home-cage training with optogenetic silencing as mice engaged in a learned center-out-reach task. Inactivation specifically of contralateral caudal forelimb area (CFA) impaired corrective movements necessary to reach spatial targets to earn reward.

Movement-related activity in the internal globus pallidus of the parkinsonian macaque

Although the basal ganglia (BG) plays a central role in the motor symptoms of Parkinson's disease, few studies have investigated the influence of parkinsonism on movement-related activity in the BG. Here, we studied the perimovement activity of neurons in globus pallidus internus (GPi) of non-human primates before and after the induction of parkinsonism by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Neuronal responses were equally common in the parkinsonian brain as seen prior to MPTP and the distribution of different response types was largely unchanged. The slowing of behavioral reaction times and movement durations following the induction of parkinsonism was accompanied by a prolongation of the time interval between neuronal response onset and movement initiation. Neuronal responses were also reduced in magnitude and prolonged in duration after the induction of parkinsonism. Importantly, those two effects were more pronounced among decrease-type responses, and they persisted after controlling for MPTP-induced changes in the trial-by-trial timing of neuronal responses. Following MPTP The timing of neuronal responses also became uncoupled from the time of movement onset and more variable from trial-to-trial. Overall, the effects of MPTP on temporal features of neural responses correlated most consistently with the severity of parkinsonian motor impairments whereas the changes in response magnitude and duration were either anticorrelated with symptom severity or inconsistent. These findings point to a potential previously underappreciated role for abnormalities in the timing of GPi task-related activity in the generation of parkinsonian motor signs.

Why so slow? Models of parkinsonian bradykinesia

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

A neurocomputational view of the effects of Parkinson's disease on speech production

The purpose of this article is to review the scientific literature concerning speech in Parkinson's disease (PD) with reference to the DIVA/GODIVA neurocomputational modeling framework. Within this theoretical view, the basal ganglia (BG) contribute to several different aspects of speech motor learning and execution. First, the BG are posited to play a role in the initiation and scaling of speech movements. Within the DIVA/GODIVA framework, initiation and scaling are carried out by initiation map nodes in the supplementary motor area acting in concert with the BG. Reduced support of the initiation map from the BG in PD would result in reduced movement intensity as well as susceptibility to early termination of movement. A second proposed role concerns the learning of common speech sequences, such as phoneme sequences comprising words; this view receives support from the animal literature as well as studies identifying speech sequence learning deficits in PD. Third, the BG may play a role in the temporary buffering and sequencing of longer speech utterances such as phrases during conversational speech. Although the literature does not support a critical role for the BG in representing sequence order (since incorrectly ordered speech is not characteristic of PD), the BG are posited to contribute to the scaling of individual movements in the sequence, including increasing movement intensity for emphatic stress on key words. Therapeutic interventions for PD have inconsistent effects on speech. In contrast to dopaminergic treatments, which typically either leave speech unchanged or lead to minor improvements, deep brain stimulation (DBS) can degrade speech in some cases and improve it in others. However, cases of degradation may be due to unintended stimulation of efferent motor projections to the speech articulators. Findings of spared speech after bilateral pallidotomy appear to indicate that any role played by the BG in adult speech must be supplementary rather than mandatory, with the sequential order of well-learned sequences apparently represented elsewhere (e.g., in cortico-cortical projections).

Grid cells: the missing link in understanding Parkinson's disease?

The mechanisms underlying Parkinson's disease (PD) are complex and not fully understood, and the box-and-arrow model among other current models present significant challenges. This paper explores the potential role of the allocentric brain and especially its grid cells in several PD motor symptoms, including bradykinesia, kinesia paradoxa, freezing of gait, the bottleneck phenomenon, and their dependency on cueing. It is argued that central hubs, like the locus coeruleus and the pedunculopontine nucleus, often narrowly interpreted in the context of PD, play an equally important role in governing the allocentric brain as the basal ganglia. Consequently, the motor and secondary motor (e.g., spatially related) symptoms of PD linked with dopamine depletion may be more closely tied to erroneous computation by grid cells than to the basal ganglia alone. Because grid cells and their associated central hubs introduce both spatial and temporal information to the brain influencing velocity perception they may cause bradykinesia or hyperkinesia as well. In summary, PD motor symptoms may primarily be an allocentric disturbance resulting from virtual faulty computation by grid cells revealed by dopamine depletion in PD.

Integrated neural dynamics of sensorimotor decisions and actions

Recent theoretical models suggest that deciding about actions and executing them are not implemented by completely distinct neural mechanisms but are instead two modes of an integrated dynamical system. Here, we investigate this proposal by examining how neural activity unfolds during a dynamic decision-making task within the high-dimensional space defined by the activity of cells in monkey dorsal premotor (PMd), primary motor (M1), and dorsolateral prefrontal cortex (dlPFC) as well as the external and internal segments of the globus pallidus (GPe, GPi). Dimensionality reduction shows that the four strongest components of neural activity are functionally interpretable, reflecting a state transition between deliberation and commitment, the transformation of sensory evidence into a choice, and the baseline and slope of the rising urgency to decide. Analysis of the contribution of each population to these components shows meaningful differences between regions but no distinct clusters within each region, consistent with an integrated dynamical system. During deliberation, cortical activity unfolds on a two-dimensional "decision manifold" defined by sensory evidence and urgency and falls off this manifold at the moment of commitment into a choice-dependent trajectory leading to movement initiation. The structure of the manifold varies between regions: In PMd, it is curved; in M1, it is nearly perfectly flat; and in dlPFC, it is almost entirely confined to the sensory evidence dimension. In contrast, pallidal activity during deliberation is primarily defined by urgency. We suggest that these findings reveal the distinct functional contributions of different brain regions to an integrated dynamical system governing action selection and execution.

Pallidal Activity Related to Posture and Movement during Reaching in the Cat

We tested the hypothesis that the pallidum contributes to the control of both posture and movement. We recorded neuronal activity from the pallidum in a task in which male cats reached forward from a standing posture to depress a lever. In agreement with previous studies, we found that a majority of pallidal cells (91/116, 78%), including neurons in both the entopeduncular nucleus and the globus pallidus, showed significant modulation of their activity during reaching with the contralateral limb. Mostly different populations of cells were active during the transport (flexion) and lever press (extension) phase of the task. Most cells showed dynamic patterns of activity related to the movement. However, a modest proportion of modulated cells (18/91, 20%) showed properties consistent with a contribution to the control of anticipatory postural responses, whereas a further 10% showed activity consistent with a contribution to postural support during the movement. Although some cells that showed modified activity only during reaches with the contralateral forelimb, many cells (65/91, 71%) were also activated during reaches with the ipsilateral forelimb. This was particularly true for cells related to the lever press, many of which discharged similarly during reaches of either limb. This suggests a context-dependent control of movement and posture in which the same muscles are used for different functions during contralateral and ipsilateral reach. Comparison with the results from recordings made previously from the motor cortex and the pontomedullary reticular formation in the same task show more similarities with the former than the latter.SIGNIFICANCE STATEMENT Pathologic changes in basal ganglia function frequently lead to problems with postural stability and gait initiation. Here, we show that some neurons in one of the output regions of the basal ganglia, the pallidum, show discharge activity compatible with a contribution to postural control. At the same time, we note that such cells are a minority in this region with most cells being related to movement rather than posture. We also show that many neurons are active during movements of both the contralateral and ipsilateral limbs, sometimes with identical discharge patterns. We suggest that this indicates a context-dependent regulation of movement and posture in the pallidum.

Premotor Ramping of Thalamic Neuronal Activity Is Modulated by Nigral Inputs and Contributes to Control the Timing of Action Release

The ventromedial (VM)/ventro-anterior-lateral (VAL) motor thalamus is a key junction within the brain circuits sustaining normal and pathologic motor control functions and decision-making. In this area of thalamus, on one hand, the inhibitory nigro-thalamic pathway provides a main output from the basal ganglia, and, on the other hand, motor thalamo-cortical loops are involved in the maintenance of ramping preparatory activity before goal-directed movements. To better understand the nigral impact on thalamic activity, we recorded electrophysiological responses from VM/VAL neurons while male and female mice were performing a delayed right/left decision licking task. Analysis of correct (corr) and error trials revealed that thalamic ramping activity was stronger for premature licks (impulsive action) and weaker for trials with no licks [omission (omi)] compared with correct trials. Suppressing ramping activity through optogenetic activation of nigral terminals in the motor thalamus during the delay epoch of the task led to a reduced probability of impulsive action and an increased amount of omissions trials. We propose a parsimonious model explaining our data and conclude that a thalamic ramping mechanism contributes to the control of proper timing of action release and that inhibitory nigral inputs are sufficient to interrupt this mechanism and modulate the amount of motor impulsivity in this task.SIGNIFICANCE STATEMENT Coordinated neural activity in motor circuits is essential for correct movement preparation and execution, and even slight imbalances in neural processing can lead to failure in behavioral tasks or motor disorders. Here we focused on how failure to regulate the control of activity balance in the motor thalamus can be implicated in impulsive action release or omissions to act, through an activity ramping mechanism that is required for proper action release. Using optogenetic activation of inhibitory basal ganglia terminals in motor thalamus we show that basal ganglia input is well positioned to control this ramping activity and determine the timing of action initiation.

Basal Ganglia Output Has a Permissive Non-Driving Role in a Signaled Locomotor Action Mediated by the Midbrain

The basal ganglia are important for movement and reinforcement learning. Using mice of either sex, we found that the main basal ganglia GABAergic output in the midbrain, the substantia nigra pars reticulata (SNr), shows movement-related neural activity during the expression of a negatively reinforced signaled locomotor action known as signaled active avoidance; this action involves mice moving away during a warning signal to avoid a threat. In particular, many SNr neurons deactivate during active avoidance responses. However, whether SNr deactivation has an essential role driving or regulating active avoidance responses is unknown. We found that optogenetic excitation of SNr or striatal GABAergic fibers that project to an area in the pedunculopontine tegmentum (PPT) within the midbrain locomotor region abolishes signaled active avoidance responses, while optogenetic inhibition of SNr cells (mimicking the SNr deactivation observed during an active avoidance behavior) serves as an effective conditioned stimulus signal to drive avoidance responses by disinhibiting PPT neurons. However, preclusion of SNr deactivation, or direct inhibition of SNr fibers in the PPT, does not impair the expression of signaled active avoidance, indicating that SNr output does not drive the expression of a signaled locomotor action mediated by the midbrain. Consistent with a permissive regulatory role, SNr output provides information about the state of the ongoing action to downstream structures that mediate the action.SIGNIFICANCE STATEMENT During signaled active avoidance behavior, subjects move away to avoid a threat when directed by an innocuous sensory stimulus. Excitation of GABAergic cells in the substantia nigra pars reticulata (SNr), the main output of the basal ganglia, blocks signaled active avoidance, while inhibition of SNr cells is an effective stimulus to drive active avoidance. Interestingly, many SNr cells inhibit their firing during active avoidance responses, suggesting that SNr inhibition could be driving avoidance responses by disinhibiting downstream areas. However, interfering with the modulation of SNr cells does not impair the behavior. Thus, SNr may regulate the active avoidance movement in downstream areas that mediate the behavior, but does not drive it.

Neural activity during a simple reaching task in macaques is counter to gating and rebound in basal ganglia-thalamic communication

Task-related activity in the ventral thalamus, a major target of basal ganglia output, is often assumed to be permitted or triggered by changes in basal ganglia activity through gating- or rebound-like mechanisms. To test those hypotheses, we sampled single-unit activity from connected basal ganglia output and thalamic nuclei (globus pallidus-internus [GPi] and ventrolateral anterior nucleus [VLa]) in monkeys performing a reaching task. Rate increases were the most common peri-movement change in both nuclei. Moreover, peri-movement changes generally began earlier in VLa than in GPi. Simultaneously recorded GPi-VLa pairs rarely showed short-time-scale spike-to-spike correlations or slow across-trials covariations, and both were equally positive and negative. Finally, spontaneous GPi bursts and pauses were both followed by small, slow reductions in VLa rate. These results appear incompatible with standard gating and rebound models. Still, gating or rebound may be possible in other physiological situations: simulations show how GPi-VLa communication can scale with GPi synchrony and GPi-to-VLa convergence, illuminating how synchrony of basal ganglia output during motor learning or in pathological conditions may render this pathway effective. Thus, in the healthy state, basal ganglia-thalamic communication during learned movement is more subtle than expected, with changes in firing rates possibly being dominated by a common external source.

Inactivation of endopeduncular nucleus impaired fear conditioning and hippocampal synaptic plasticity in rats

The internal globus pallidus (GPi) is one part of basal ganglion nucleuses which play fundamental role in motor function. Recent studies indicated that GPi could modulate emotional processing and learning, but the possible mechanism remains still unknown. In this study, the effects of endopeduncular nucleus (EP, a rodent homolog of GPi) on fear conditioning were tested in rats. GABA_A receptor agonist muscimol was bilaterally delivered into the EP 15 min before or immediately after fear conditioning in rats. We found that EP inactivation impaired the acquisition but not consolidation of fear memory in rats. Furthermore, the long-term potentiation (LTP) in hippocampal CA1 area was impaired, and the learning related phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit 1 (GluA1) at the Ser845 site in hippocampus was decreased in muscimol treated group. These results demonstrated that dysfunction of EP impaired hippocampal dependent learning and memory in rats.

A natural history of skills

The dorsal pallium (a.k.a. cortex in mammals) makes a loop circuit with the basal ganglia and the thalamus known to control and adapt behavior but the who's who of the functional roles of these structures is still debated. Influenced by the Triune brain theory that was proposed in the early sixties, many current theories propose a hierarchical organization on the top of which stands the cortex to which the subcortical structures are subordinated. In particular, habits formation has been proposed to reflect a switch from conscious on-line control of behavior by the cortex, to a fully automated subcortical control. In this review, we propose to revalue the function of the network in light of the current experimental evidence concerning the anatomy and physiology of the basal ganglia-cortical circuits in vertebrates. We briefly review the current theories and show that they could be encompassed in a broader framework of skill learning and performance. Then, after reminding the state of the art concerning the anatomical architecture of the network and the underlying dynamic processes, we summarize the evolution of the anatomical and physiological substrate of skill learning and performance among vertebrates. We then review experimental evidence supporting for the hypothesis that the development of automatized skills relies on the BG teaching cortical circuits and is actually a late feature linked with the development of a specialized cortex or pallium that evolved in parallel in different taxa. We finally propose a minimal computational framework where this hypothesis can be explicitly implemented and tested.

A system-level mathematical model of Basal Ganglia motor-circuit for kinematic planning of arm movements

In this paper, a novel system-level mathematical model of the Basal Ganglia (BG) for kinematic planning, is proposed. An arm composed of several segments presents a geometric redundancy. Thus, selecting one trajectory among an infinite number of possible ones requires overcoming redundancy, according to some kinds of optimization. Solving this optimization is assumed to be the function of BG in planning. In the proposed model, first, a mathematical solution of kinematic planning is proposed for movements of a redundant arm in a plane, based on minimizing energy consumption. Next, the function of each part in the model is interpreted as a possible role of a nucleus of BG. Since the kinematic variables are considered as vectors, the proposed model is presented based on the vector calculus. This vector model predicts different neuronal populations in BG which is in accordance with some recent experimental studies. According to the proposed model, the function of the direct pathway is to calculate the necessary rotation of each joint, and the function of the indirect pathway is to control each joint rotation considering the movement of the other joints. In the proposed model, the local feedback loop between Subthalamic Nucleus and Globus Pallidus externus is interpreted as a local memory to store the previous amounts of movements of the other joints, which are utilized by the indirect pathway. In this model, activities of dopaminergic neurons would encode, at short-term, the error between the desired and actual positions of the end-effector. The short-term modulating effect of dopamine on Striatum is also modeled as cross product. The model is simulated to generate the commands of a redundant manipulator. The performance of the model is studied for different reaching movements between 8 points in a plane. Finally, some symptoms of Parkinson's disease such as bradykinesia and akinesia are simulated by modifying the model parameters, inspired by the dopamine depletion.

The Basal Ganglia Do Not Select Reach Targets but Control the Urgency of Commitment

Prominent theories of decision making suggest that the basal ganglia (BG) play a causal role in deliberation between action choices. An alternative hypothesis is that deliberation occurs in cortical regions, while the BG control the speed-accuracy trade-off (SAT) between committing to a choice versus continuing to deliberate. Here, we test these hypotheses by recording activity in the internal and external segments of the globus pallidus (GPi/GPe) while monkeys perform a task dissociating the process of deliberation, the moment of commitment, and adjustment of the SAT. Our data suggest that unlike premotor and motor cortical regions, pallidal output does not contribute to the process of deliberation but instead provides a time-varying signal that controls the SAT and reflects the growing urgency to commit to a choice. Once a target is selected by cortical regions, GP activity confirms commitment to the decision and invigorates the subsequent movement.

Voluntary saccade inhibition deficits correlate with extended white-matter cortico-basal atrophy in Huntington's disease

The ability to inhibit automatic versus voluntary saccade commands in demanding situations can be impaired in neurodegenerative diseases such as Huntington's disease (HD). These deficits could result from disruptions in the interaction between basal ganglia and the saccade control system. To investigate voluntary oculomotor control deficits related to the cortico-basal circuitry, we evaluated early HD patients using an interleaved pro- and anti-saccade task that requires flexible executive control to generate either an automatic response (look at a peripheral visual stimulus) or a voluntary response (look away from the stimulus in the opposite direction). The impairments of HD patients in this task are mainly attributed to degeneration in the striatal medium spiny neurons leading to an over-activation of the indirect-pathway thorough the basal ganglia. However, some studies have proposed that damage outside the indirect-pathway also contribute to executive and saccade deficits. We used the interleaved pro- and anti-saccade task to study voluntary saccade inhibition deficits, Voxel-based morphometry and Tract-based spatial statistic to map cortico-basal ganglia circuitry atrophy in HD. HD patients had voluntary saccade inhibition control deficits, including increased regular-latency anti-saccade errors and increased anticipatory saccades. These deficits correlated with white-matter atrophy in the inferior fronto-occipital fasciculus, anterior thalamic radiation, anterior corona radiata and superior longitudinal fasciculus. These findings suggest that cortico-basal ganglia white-matter atrophy in HD, disrupts the normal connectivity in a network controlling voluntary saccade inhibitory behavior beyond the indirect-pathway. This suggests that in vivo measures of white-matter atrophy can be a reliable marker of the progression of cognitive deficits in HD.

Roles of Multiple Globus Pallidus Territories of Monkeys and Humans in Motivation, Cognition and Action: An Anatomical, Physiological and Pathophysiological Review

The globus pallidus (GP) communicates with widespread cortical areas that support various functions, including motivation, cognition and action. Anatomical tract-tracing studies revealed that the anteroventral GP communicates with the medial prefrontal and orbitofrontal cortices, which are involved in motivational control; the anterodorsal GP communicates with the lateral prefrontal cortex, which is involved in cognitive control; and the posterior GP communicates with the frontal motor cortex, which is involved in action control. This organization suggests that distinct subdivisions within the GP play specific roles. Neurophysiological studies examining GP neurons in monkeys during behavior revealed that the types of information coding performed within these subdivisions differ greatly. The anteroventral GP is characterized by activities related to motivation, such as reward seeking and aversive avoidance; the anterodorsal GP is characterized by activity that reflects cognition, such as goal decision and action selection; and the posterior GP is characterized by activity associated with action preparation and execution. Pathophysiological studies have shown that GABA-related substances or GP lesions result in abnormal activity in the GP, which causes site-specific behavioral and motor symptoms. The present review article discusses the anatomical organization, physiology and pathophysiology of the three major GP territories in nonhuman primates and humans.

The Cognitive Role of the Globus Pallidus interna; Insights from Disease States

The motor symptoms of both Parkinson's disease and focal dystonia arise from dysfunction of the basal ganglia, and are improved by pallidotomy or deep brain stimulation of the Globus Pallidus interna (GPi). However, Parkinson's disease is associated with a greater degree of basal ganglia-dependent learning impairment than dystonia. We attempt to understand this observation in terms of a comparison of the electrophysiology of the output of the basal ganglia between the two conditions. We use the natural experiment offered by Deep Brain Stimulation to compare GPi local field potential responses in subjects with Parkinson's disease compared to subjects with dystonia performing a forced-choice decision-making task with sensory feedback. In dystonic subjects, we found that auditory feedback was associated with the presence of high gamma oscillations nestled on a negative deflection, morphologically similar to sharp wave ripple complexes described in human rhinal cortex. These were not present in Parkinson's disease subjects. The temporal properties of the high gamma burst were modified by incorrect trial performance compared to correct trial performance. Both groups exhibited a robust low frequency response to 'incorrect' trial performance in dominant GPi but not non-dominant GPi at theta frequency. Our results suggest that cellular processes associated with striatum-dependent memory function may be selectively impaired in Parkinson's disease even if dopaminergic drugs are administered, but that error detection mechanisms are preserved.

Why Don't We Move Slower? The Value of Time in the Neural Control of Action

To want something now rather than later is a common attitude that reflects the brain's tendency to value the passage of time. Because the time taken to accomplish an action inevitably delays task achievement and reward acquisition, this idea was ported to neural movement control within the "cost of time" theory. This theory provides a normative framework to account for the underpinnings of movement time formation within the brain and the origin of a self-selected pace in human and animal motion. Then, how does the brain exactly value time in the control of action? To tackle this issue, we used an inverse optimal control approach and developed a general methodology allowing to squarely sample infinitesimal values of the time cost from experimental motion data. The cost of time underlying saccades was found to have a concave growth, thereby confirming previous results on hyperbolic reward discounting, yet without making any prior assumption about this hypothetical nature. For self-paced reaching, however, movement time was primarily valued according to a striking sigmoidal shape; its rate of change consistently presented a steep rise before a maximum was reached and a slower decay was observed. Theoretical properties of uniqueness and robustness of the inferred time cost were established for the class of problems under investigation, thus reinforcing the significance of the present findings. These results may offer a unique opportunity to uncover how the brain values the passage of time in healthy and pathological motor control and shed new light on the processes underlying action invigoration.

SIGNIFICANCE STATEMENT

Movement time is a fundamental characteristic of neural motor control, but the principles underlying its formation remain little known. This work addresses that question within the inverse optimal control framework where the challenge is to uncover what optimality criterion underlies a system's behavior. Here we rely on the "cost of time" theory that finds its roots into the brain's tendency to discount the actual value of future reward. It asserts that the time elapsed until action completion entails a cost, thereby making slow moves nonoptimal. By means of a thorough theoretical analysis, the present article shows how to sample the infinitesimal values of the time cost without prior assumption about its hypothetical nature and emphasizes its sigmoidal shape for reaching.

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

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

Primary motor cortex of the parkinsonian monkey: altered encoding of active movement

Abnormalities in the movement-related activation of the primary motor cortex (M1) are thought to be a major contributor to the motor signs of Parkinson's disease. The existing evidence, however, variably indicates that M1 is under-activated with movement, overactivated (due to a loss of functional specificity) or activated with abnormal timing. In addition, few models consider the possibility that distinct cortical neuron subtypes may be affected differently. Those gaps in knowledge were addressed by studying the extracellular activity of antidromically-identified lamina 5b pyramidal-tract type neurons (n = 153) and intratelencephalic-type corticostriatal neurons (n = 126) in the M1 of two monkeys as they performed a step-tracking arm movement task. We compared movement-related discharge before and after the induction of parkinsonism by administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and quantified the spike rate encoding of specific kinematic parameters of movement using a generalized linear model. The fraction of M1 neurons with movement-related activity declined following MPTP but only marginally. The strength of neuronal encoding of parameters of movement was reduced markedly (mean 29% reduction in the coefficients from the generalized linear model). This relative decoupling of M1 activity from kinematics was attributable to reductions in the coefficients that estimated the spike rate encoding of movement direction (-22%), speed (-40%), acceleration (-49%) and hand position (-33%). After controlling for MPTP-induced changes in motor performance, M1 activity related to movement itself was reduced markedly (mean 36% hypoactivation). This reduced activation was strong in pyramidal tract-type neurons (-50%) but essentially absent in corticostriatal neurons. The timing of M1 activation was also abnormal, with earlier onset times, prolonged response durations, and a 43% reduction in the prevalence of movement-related changes beginning in the 150-ms period that immediately preceded movement. Overall, the results are consistent with proposals that under-activation and abnormal timing of movement-related activity in M1 contribute to parkinsonian motor signs but are not consistent with the idea that a loss of functional specificity plays an important role. Given that pyramidal tract-type neurons form the primary efferent pathway that conveys motor commands to the spinal cord, the dysfunction of movement-related activity in pyramidal tract-type neurons is likely to be a central factor in the pathophysiology of parkinsonian motor signs.

Origins of multisynaptic projections from the basal ganglia to the forelimb region of the ventral premotor cortex in macaque monkeys

The ventral premotor cortex (PMv), occupying the ventral aspect of area 6 in the frontal lobe, has been implicated in action planning and execution based on visual signals. Although the PMv has been characterized by cortico-cortical connections with specific subregions of the parietal and prefrontal cortical areas, a topographical input/output organization between the PMv and the basal ganglia (BG) still remains elusive. In the present study, retrograde transneuronal labelling with the rabies virus was employed to identify the origins of multisynaptic projections from the BG to the PMv. The virus was injected into the forelimb region of the PMv, identified in the ventral aspect of the genu of the arcuate sulcus, in macaque monkeys. The survival time after the virus injection was set to allow either the second- or third-order neuron labelling across two or three synapses. The second-order neurons were observed in the ventral portion (primary motor territory) and the caudodorsal portion (higher-order motor territory) of the internal segment of the globus pallidus. Subsequently, the third-order neurons were distributed in the putamen caudal to the anterior commissure, including both the primary and the higher-order motor territories, and in the ventral striatum (limbic territory). In addition, they were found in the dorsolateral portion (motor territory) and ventromedial portion (limbic territory) of the subthalamic nucleus, and in the external segment of the globus pallidus including both the limbic and motor territories. These findings indicate that the PMv receives diverse signals from the primary motor, higher-order motor and limbic territories of the BG.

Contribution of the basal ganglia to spoken language: is speech production like the other motor skills?

Two of the roles assigned to the basal ganglia in spoken language parallel very well their contribution to motor behaviour: (1) their role in sequence processing, resulting in syntax deficits, and (2) their role in movement "vigor," leading to "hypokinetic dysarthria" or "hypophonia." This is an additional example of how the motor system has served the emergence of high-level cognitive functions, such as language.

Dissociable contribution of the parietal and frontal cortex to coding movement direction and amplitude

To reach for an object, we must convert its spatial location into an appropriate motor command, merging movement direction and amplitude. In humans, it has been suggested that this visuo-motor transformation occurs in a dorsomedial parieto-frontal pathway, although the causal contribution of the areas constituting the “reaching circuit” remains unknown. Here we used transcranial magnetic stimulation (TMS) in healthy volunteers to disrupt the function of either the medial intraparietal area (mIPS) or dorsal premotor cortex (PMd), in each hemisphere. The task consisted in performing step-tracking movements with the right wrist towards targets located in different directions and eccentricities; targets were either visible for the whole trial (Target-ON) or flashed for 200 ms (Target-OFF). Left and right mIPS disruption led to errors in the initial direction of movements performed towards contralateral targets. These errors were corrected online in the Target-ON condition but when the target was flashed for 200 ms, mIPS TMS manifested as a larger endpoint spreading. In contrast, left PMd virtual lesions led to higher acceleration and velocity peaks—two parameters typically used to probe the planned movement amplitude—irrespective of the target position, hemifield and presentation condition; in the Target-OFF condition, left PMd TMS induced overshooting and increased the endpoint dispersion along the axis of the target direction. These results indicate that left PMd intervenes in coding amplitude during movement preparation. The critical TMS timings leading to errors in direction and amplitude were different, namely 160–100 ms before movement onset for mIPS and 100–40 ms for left PMd. TMS applied over right PMd had no significant effect. These results demonstrate that, during motor preparation, direction and amplitude of goal-directed movements are processed by different cortical areas, at distinct timings, and according to a specific hemispheric organization.

Reassessing models of basal ganglia function and dysfunction

The basal ganglia are a series of interconnected subcortical nuclei. The function and dysfunction of these nuclei have been studied intensively in motor control, but more recently our knowledge of these functions has broadened to include prominent roles in cognition and affective control. This review summarizes historical models of basal ganglia function, as well as findings supporting or conflicting with these models, while emphasizing recent work in animals and humans directly testing the hypotheses generated by these models.

Movement-related discharge in the macaque globus pallidus during high-frequency stimulation of the subthalamic nucleus

Deep brain stimulation (DBS) of the subthalamic nucleus (STN-DBS) has largely replaced ablative therapies for Parkinson's disease. Because of the similar efficacies of the two treatments, it has been proposed that DBS acts by creating an "informational lesion," whereby pathologic neuronal firing patterns are replaced by low-entropy, stimulus-entrained firing patterns. The informational lesion hypothesis, in its current form, states that DBS blocks the transmission of all information from the basal ganglia, including both pathologic firing patterns and normal, task-related modulations in activity. We tested this prediction in two healthy rhesus macaques by recording single-unit spiking activity from the globus pallidus (232 neurons) while the animals completed choice reaction time reaching movements with and without STN-DBS. Despite strong effects of DBS on the activity of most pallidal cells, reach-related modulations in firing rate were equally prevalent in the DBS-on and DBS-off states. This remained true even when the analysis was restricted to cells affected significantly by DBS. In addition, the overall form and timing of perimovement modulations in firing rate were preserved between DBS-on and DBS-off states in the majority of neurons (66%). Active movement and DBS had largely additive effects on the firing rate of most neurons, indicating an orthogonal relationship in which both inputs contribute independently to the overall firing rate of pallidal neurons. These findings suggest that STN-DBS does not act as an indiscriminate informational lesion but rather as a filter that permits task-related modulations in activity while, presumably, eliminating the pathological firing associated with parkinsonism.

On the challenges and mechanisms of embodied decisions

Neurophysiological studies of decision-making have focused primarily on elucidating the mechanisms of classic economic decisions, for which the relevant variables are the values of expected outcomes and action is simply the means of reporting the selected choice. By contrast, here we focus on the particular challenges of embodied decision-making faced by animals interacting with their environment in real time. In such scenarios, the choices themselves as well as their relative costs and benefits are defined by the momentary geometry of the immediate environment and change continuously during ongoing activity. To deal with the demands of embodied activity, animals require an architecture in which the sensorimotor specification of potential actions, their valuation, selection and even execution can all take place in parallel. Here, we review behavioural and neurophysiological data supporting a proposed brain architecture for dealing with such scenarios, which we argue set the evolutionary foundation for the organization of the mammalian brain.

Multifaceted aspects of chunking enable robust algorithms

Sequence production tasks are a standard tool to analyze motor learning, consolidation, and habituation. As sequences are learned, movements are typically grouped into subsets or chunks. For example, most Americans memorize telephone numbers in two chunks of three digits, and one chunk of four. Studies generally use response times or error rates to estimate how subjects chunk, and these estimates are often related to physiological data. Here we show that chunking is simultaneously reflected in reaction times, errors, and their correlations. This multimodal structure enables us to propose a Bayesian algorithm that better estimates chunks while avoiding overfitting. Our algorithm reveals previously unknown behavioral structure, such as an increased error correlations with training, and promises a useful tool for the characterization of many forms of sequential motor behavior.

Axonal and synaptic failure suppress the transfer of firing rate oscillations, synchrony and information during high frequency deep brain stimulation

High frequency deep brain stimulation (DBS) of the subthalamic nucleus (STN) is a widely used treatment for Parkinson's disease, but its effects on neural activity in basal ganglia circuits are not fully understood. DBS increases the excitation of STN efferents yet decouples STN spiking patterns from the spiking patterns of STN synaptic targets. We propose that this apparent paradox is resolved by recent studies showing an increased rate of axonal and synaptic failures in STN projections during DBS. To investigate this hypothesis, we combine in vitro and in vivo recordings to derive a computational model of axonal and synaptic failure during DBS. Our model shows that these failures induce a short term depression that suppresses the synaptic transfer of firing rate oscillations, synchrony and rate-coded information from STN to its synaptic targets. In particular, our computational model reproduces the widely reported suppression of parkinsonian β oscillations and synchrony during DBS. Our results support the idea that short term depression is a therapeutic mechanism of STN DBS that works as a functional lesion by decoupling the somatic spiking patterns of STN neurons from spiking activity in basal ganglia output nuclei.

A common optimization principle for motor execution in healthy subjects and parkinsonian patients

Recent research on Parkinson's disease (PD) has emphasized that parkinsonian movement, although bradykinetic, shares many attributes with healthy behavior. This observation led to the suggestion that bradykinesia in PD could be due to a reduction in motor motivation. This hypothesis can be tested in the framework of optimal control theory, which accounts for many characteristics of healthy human movement while providing a link between the motor behavior and a cost/benefit trade-off. This approach offers the opportunity to interpret movement deficits of PD patients in the light of a computational theory of normal motor control. We studied 14 PD patients with bilateral subthalamic nucleus (STN) stimulation and 16 age-matched healthy controls, and tested whether reaching movements were governed by similar rules in these two groups. A single optimal control model accounted for the reaching movements of healthy subjects and PD patients, whatever the condition of STN stimulation (on or off). The choice of movement speed was explained in all subjects by the existence of a preset dynamic range for the motor signals. This range was idiosyncratic and applied to all movements regardless of their amplitude. In PD patients this dynamic range was abnormally narrow and correlated with bradykinesia. STN stimulation reduced bradykinesia and widened this range in all patients, but did not restore it to a normal value. These results, consistent with the motor motivation hypothesis, suggest that constrained optimization of motor effort is the main determinant of movement planning (choice of speed) and movement production, in both healthy and PD subjects.

Mapping motor representations in the human cerebellum

The cerebellum is a major motor structure. However, in humans, its efferent topographical organization remains controversial and indirectly inferred from neuroimaging and animal studies. Even central questions such as 'Can we evoke limb movements by stimulating the cerebellar cortex?' have no clear answer. To address this issue, we electrically stimulated the posterior cerebellum of 20 human patients undergoing surgery for tumours located outside this structure (e.g. pineal gland, quadrigeminal plate). Stimulation, delivered at a 60-Hz frequency for 2 s, evoked focal (single-joint) ipsilateral movements. Different regions were associated with the production of head (vermal lobule VI), face/mouth (hemispheric lobule VI) and lower-limb (hemispheric lobules VIIb-IX) responses. Upper-limb representations were more widely distributed. They intermingled with face/mouth representations in the superior posterior cerebellum (hemispheric lobule VI) and lower-limb representations in the inferior posterior cerebellum (hemispheric lobules VIIb-IX). No intra- or inter-limb somatotopy was found in these areas. Functionally, upper-limb (face/mouth movements) and upper limb-lower limb postural coordinations are major elements of our motor repertoire. Representation of these pairs of segments in common regions might favour the production of integrated motor behaviours. The intermediate region of the posterior cerebellum (hemispheric lobule VII and vermal lobules VII-VIII) was mostly silent. Latency results in conjunction with previous electrophysiological evidence in animals suggest that electrically evoked motor responses were not mediated by a cortical route but rather by brainstem structures. The potential role of this descending efferent pathway for fine motor control is discussed.

Testing the contributions of striatal dopamine loss to the genesis of parkinsonian signs

The diverse and independently-varying signs of Parkinson's disease (PD) are often attributed to one simple mechanism: degeneration of the dopaminergic innervation of the posterolateral striatum. However, growing recognition of the dopamine (DA) loss and other pathology in extra-striatal brain regions has led to uncertainty whether loss of DA in the striatum is sufficient to cause parkinsonian signs. We tested this hypothesis by infusing cis-flupenthixol (cis-flu; a broad-spectrum D1/D2 receptor antagonist) into different regions of the macaque putamen (3 hemispheres of 2 monkeys) while the animal performed a visually-cued choice reaction time task in which visual cues indicated the arm to reach with and the peripheral target to contact to obtain food reward. Following reward delivery, the animal was required to self-initiate release of the peripheral target and return of the chosen hand to its home position (i.e., without the benefit of external sensory cues or immediate rewards). Infusions of cis-flu at 15 of 26 sites induced prolongations of reaction time (9 of 15 cases), movement duration (6 cases), and/or dwell time of the hand at the peripheral target (8 cases). Dwell times were affected more severely (+95%) than visually-triggered reaction times or movement durations (+25% and +15%, respectively). Specifically, the animal's hand often 'froze' at the peripheral target for up to 25-s, similar to the akinetic freezing episodes observed in PD patients. Across injections, slowing of self-initiation did not correlate in severity with prolongations of visually-triggered reaction time or movement duration, although the latter two were correlated with each other. Episodes of slowed self-initiation appeared primarily in the arm contralateral to the injected hemisphere and were not associated with increased muscle co-contraction or global alterations in behavioral state (i.e., inattention or reduced motivation), consistent with the idea that these episodes reflected a fundamental impairment of movement initiation. We found no evidence for an anatomic topography within the putamen for the effects elicited. We conclude that acute focal blockade of DA transmission in the putamen is sufficient to induce marked akinesia-like impairments. Furthermore, different classes of impairments can be induced independently, suggesting that specific parkinsonian signs have unique pathophysiologic substrates.

Scaling of movement is related to pallidal γ oscillations in patients with dystonia

Neuronal synchronization in the gamma (γ) band is considered important for information processing through functional integration of neuronal assemblies across different brain areas. Movement-related γ synchronization occurs in the human basal ganglia where it is centered at ~70 Hz and more pronounced contralateral to the moved hand. However, its functional significance in motor performance is not yet well understood. Here, we assessed whether event-related γ synchronization (ERS) recorded from the globus pallidus internus in patients undergoing deep brain stimulation for medically intractable primary focal and segmental dystonia might code specific motor parameters. Pallidal local field potentials were recorded in 22 patients during performance of a choice-reaction-time task. Movement amplitude of the forearm pronation-supination movements was parametrically modulated with an angular degree of 30°, 60°, and 90°. Only patients with limbs not affected by dystonia were tested. A broad contralateral γ band (35-105 Hz) ERS occurred at movement onset with a maximum reached at peak velocity of the movement. The pallidal oscillatory γ activity correlated with movement parameters: the larger and faster the movement, the stronger was the synchronization in the γ band. In contrast, the event-related decrease in beta band activity was similar for all movements. Gamma band activity did not change with movement direction and did not occur during passive movements. The stepwise increase of γ activity with movement size and velocity suggests a role of neuronal synchronization in this frequency range in basal ganglia control of the scaling of ongoing movements.

Probing basal ganglia functions by saccade eye movements

The basal ganglia (BG) are a group of subcortical structures involved in diverse functions, such as motor, cognition and emotion. However, the BG do not control these functions directly, but rather modulate functional processes occurring in structures outside the BG. The BG form multiple functional loops, each of which controls different functions with similar architectures. Accordingly, to understand the modulatory role of the BG, it is strategic to uncover the mechanisms of signal processing within specific functional loops that control simple neural circuits outside the BG, and then extend the knowledge to other BG loops. The saccade control system is one of the best-understood neural circuits in the brain. Furthermore, sophisticated saccade paradigms have been used extensively in clinical research in patients with BG disorders as well as in basic research in behaving monkeys. In this review, we describe recent advances of BG research from the viewpoint of saccade control. Specifically, we account for experimental results from neuroimaging and clinical studies in humans based on the updated knowledge of BG functions derived from neurophysiological experiments in behaving monkeys by taking advantage of homologies in saccade behavior. It has become clear that the traditional BG network model for saccade control is too limited to account for recent evidence emerging from the roles of subcortical nuclei not incorporated in the model. Here, we extend the traditional model and propose a new hypothetical framework to facilitate clinical and basic BG research and dialogue in the future.

Parallel contributions of cerebellar, striatal and M1 mechanisms to motor sequence learning

When learning a new motor sequence, we must execute the correct order of movements while simultaneously optimizing sensorimotor parameters such as trajectory, timing, velocity and force. Neurophysiological studies in animals and humans have identified the major brain regions involved in sequence learning, including the motor cortex (M1), basal ganglia (BG) and cerebellum. Current models link these regions to different stages of learning (early vs. late) or different components of performance (spatial vs. sensorimotor). At the same time, research in motor control has given rise to the concept that internal models at different levels of the motor system may contribute to learning. The goal of this review is to develop a new framework for motor sequence learning that combines stage and component models within the context of internal models. To do this, we review behavioral and neuroimaging studies in humans and neurophysiological studies in animals. Based on this evidence, we present a model proposing that sequence learning is underwritten by parallel, interacting processes, including internal model formation and sequence representation, that are instantiated in specific cerebellar, BG or M1 mechanisms depending on task demands and the stage of learning. The striatal system learns predictive stimulus-response associations and is critical for motor chunking. The role of the cerebellum is to acquire the optimal internal model for sequence performance in a particular context, and to contribute to error correction and control of on-going movement. M1 acts to store the representation of a learned sequence, likely as part of a distributed network including the parietal lobe and premotor cortex.

Role of the medial part of the intraparietal sulcus in implementing movement direction

The contribution of the posterior parietal cortex (PPC) to visually guided movements has been originally inferred from observations made in patients suffering from optic ataxia. Subsequent electrophysiological studies in monkeys and functional imaging data in humans have corroborated the key role played by the PPC in sensorimotor transformations underlying goal-directed movements, although the exact contribution of this structure remains debated. Here, we used transcranial magnetic stimulation (TMS) to interfere transiently with the function of the left or right medial part of the intraparietal sulcus (mIPS) in healthy volunteers performing visually guided movements with the right hand. We found that a "virtual lesion" of either mIPS increased the scattering in initial movement direction (DIR), leading to longer trajectory and prolonged movement time, but only when TMS was delivered 100-160 ms before movement onset and for movements directed toward contralateral targets. Control experiments showed that deficits in DIR consequent to mIPS virtual lesions resulted from an inappropriate implementation of the motor command underlying the forthcoming movement and not from an inaccurate computation of the target localization. The present study indicates that mIPS plays a causal role in implementing specifically the direction vector of visually guided movements toward objects situated in the contralateral hemifield.

Investigating striatal function through cell-type-specific manipulations

The striatum integrates convergent input from the cortex, thalamus, and midbrain, and has a powerful influence over motivated behavior via outputs to downstream basal ganglia nuclei. Although the anatomy and physiology of distinct classes of striatal neurons have been intensively studied, the specific functions of these cell subpopulations have been more difficult to address. Recently, application of new methodologies for perturbing activity and signaling in different cell types in vivo has begun to allow direct tests of the causal roles of striatal neurons in behavior.

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.

The arbitration-extension hypothesis: a hierarchical interpretation of the functional organization of the Basal Ganglia

Based on known anatomy and physiology, we present a hypothesis where the basal ganglia motor loop is hierarchically organized in two main subsystems: the arbitration system and the extension system. The arbitration system, comprised of the subthalamic nucleus, globus pallidus, and pedunculopontine nucleus, serves the role of selecting one out of several candidate actions as they are ascending from various brain stem motor regions and aggregated in the centromedian thalamus or descending from the extension system or from the cerebral cortex. This system is an action-input/action-output system whose winner-take-all mechanism finds the strongest response among several candidates to execute. This decision is communicated back to the brain stem by facilitating the desired action via cholinergic/glutamatergic projections and suppressing conflicting alternatives via GABAergic connections. The extension system, comprised of the striatum and, again, globus pallidus, can extend the repertoire of responses by learning to associate novel complex states to certain actions. This system is a state-input/action-output system, whose organization enables it to encode arbitrarily complex Boolean logic rules using striatal neurons that only fire given specific constellations of inputs (Boolean AND) and pallidal neurons that are silenced by any striatal input (Boolean OR). We demonstrate the capabilities of this hierarchical system by a computational model where a simulated generic “animal” interacts with an environment by selecting direction of movement based on combinations of sensory stimuli, some being appetitive, others aversive or neutral. While the arbitration system can autonomously handle conflicting actions proposed by brain stem motor nuclei, the extension system is required to execute learned actions not suggested by external motor centers. Being precise in the functional role of each component of the system, this hypothesis generates several readily testable predictions.

Inactivation of the putamen selectively impairs reward history-based action selection

Behavioral decisions and actions are directed to achieve specific goals and to obtain rewards and escape punishments. Previous studies involving the recording of neuronal activity suggest the involvement of the cerebral cortex, basal ganglia, and midbrain dopamine system in these processes. The value signal of the action options is represented in the striatum, updated by reward prediction errors, and used for selecting higher-value actions. However, it remains unclear whether dysfunction of the striatum leads to impairment of value-based action selection. The present study examined the effect of inactivation of the putamen via local injection of the GABA(A) receptor agonist muscimol in monkeys engaged in a manual reward-based multi-step choice task. The monkeys first searched a reward target from three alternatives, based on the previous one or two choices and their outcomes, and obtained a large reward; they then earned an additional reward by choosing the last rewarded target. Inactivation of the putamen impaired the ability of monkeys to make optimal choices during third trial in which they were required to choose a target different from those selected in the two previous trials by updating the values of the three options. The monkeys normally changed options if the last choice resulted in small reward (lose-shift) and stayed with the last choice if it resulted in large reward (win-stay). Task start time and movement time during individual trials became longer after putamen inactivation. But monkeys could control the motivation level depending on the reward value of individual trial types before and after putamen inactivation. These results support a view that the putamen is involved selectively and critically in neuronal circuits for reward history-based action selection.

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.

High but not low frequency stimulation of both the globus pallidus and the entopeduncular nucleus reduces 'compulsive' lever-pressing in rats

The anti-compulsive effects of high and low frequency stimulation (LFS, HFS) of the entopeduncular nucleus and globus pallidus (the rat's equivalent, respectively, of the primate's internal and external segments of the globus pallidus) were assessed in the signal attenuation rat model of obsessive-compulsive disorder (OCD). HFS, but not LFS, of the two nuclei exerted an anti-compulsive effect, suggesting that HFS of either segment of the globus pallidus may provide an additional therapeutic strategy for OCD.

Motor sequences and the basal ganglia: kinematics, not habits

Despite a lack of definitive evidence, it is frequently proposed that the basal ganglia (BG) motor circuit plays a critical role in the storage and execution of movement sequences (or motor habits). To test this hypothesis directly, we inactivated the sensorimotor territory of the globus pallidus internus (sGPi, the main BG motor output) in two monkeys trained to perform overlearned and random sequences of four out-and-back reaching movements directed to visual targets. Infusion of muscimol (a GABA(A) agonist) into sGPi caused dysmetria and slowing of individual movements, but these impairments were virtually identical for overlearned and random sequences. The fluid predictive execution of learned sequences and the animals' tendency to reproduce the sequence pattern in random trials was preserved following pallidal blockade. These results suggest that the BG motor circuit contributes to motor execution, but not to motor sequencing or the storage of overlearned serial skills.

Nucleus accumbens and effort-related functions: behavioral and neural markers of the interactions between adenosine A2A and dopamine D2 receptors

Nucleus accumbens dopamine (DA) is a critical component of the brain circuitry regulating work output in reinforcement-seeking behavior and effort-related choice behavior. Moreover, there is evidence of an interaction between DA D(2) and adenosine A(2A) receptor function. Systemic administration of adenosine A(2A) antagonists reverses the effects of D(2) antagonists on tasks that assess effort related choice. The present experiments were conducted to determine if nucleus accumbens is a brain locus at which adenosine A(2A) and DA D(2) antagonists interact to regulate effort-related choice behavior. A concurrent fixed ratio 5 (FR5)/chow feeding procedure was used; with this procedure, rats can choose between completing an FR5 lever-pressing requirement for a preferred food (i.e., high carbohydrate operant pellets) or approaching and consuming a freely available food (i.e., standard rodent chow). Rats trained with this procedure spend most of their time pressing the lever for the preferred food, and eat very little of the concurrently available chow. Intracranial injections of the selective DA D(2) receptor antagonist eticlopride (1.0, 2.0, 4.0 microg) into nucleus accumbens core, but not a dorsal control site, suppressed FR5 lever-pressing and increased consumption of freely available chow. Either systemic or intra-accumbens injections of the adenosine A(2A) receptor antagonist MSX-3 reversed these effects of eticlopride on effort-related choice. Intra-accumbens injections of eticlopride also increased local expression of c-Fos immunoreactivity, and this effect was attenuated by co-administration of MSX-3. Adenosine and DA systems interact to regulate instrumental behavior and effort-related processes, and nucleus accumbens is an important locus for this interaction. These findings may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, anergia and fatigue.