Preparation for movement in the cat. II. Unit activity in the basal ganglia and thalamus

Review of electromyography onset detection methods for real-time control of robotic exoskeletons

BACKGROUND

Electromyography (EMG) is a classical technique used to record electrical activity associated with muscle contraction and is widely applied in Biomechanics, Biomedical Engineering, Neuroscience and Rehabilitation Robotics. Determining muscle activation onset timing, which can be used to infer movement intention and trigger prostheses and robotic exoskeletons, is still a big challenge. The main goal of this paper was to perform a review of the state-of-the-art of EMG onset detection methods. Moreover, we compared the performance of the most commonly used methods on experimental EMG data.

METHODS

A total of 156 papers published until March 2022 were included in the review. The papers were analyzed in terms of application domain, pre-processing method and EMG onset detection method. The three most commonly used methods [Single (ST), Double (DT) and Adaptive Threshold (AT)] were applied offline on experimental intramuscular and surface EMG signals obtained during contractions of ankle and knee joint muscles.

RESULTS

Threshold-based methods are still the most commonly used to detect EMG onset. Compared to ST and AT, DT required more processing time and, therefore, increased onset timing detection, when applied on experimental data. The accuracy of these three methods was high (maximum error detection rate of 7.3%), demonstrating their ability to automatically detect the onset of muscle activity. Recently, other studies have tested different methods (especially Machine Learning based) to determine muscle activation onset offline, reporting promising results.

CONCLUSIONS

This study organized and classified the existing EMG onset detection methods to create consensus towards a possible standardized method for EMG onset detection, which would also allow more reproducibility across studies. The three most commonly used methods (ST, DT and AT) proved to be accurate, while ST and AT were faster in terms of EMG onset detection time, especially when applied on intramuscular EMG data. These are important features towards movement intention identification, especially in real-time applications. Machine Learning methods have received increased attention as an alternative to detect muscle activation onset. However, although several methods have shown their capability offline, more research is required to address their full potential towards real-time applications, namely to infer movement intention.

Limb Preference Changes after Focused-Ultrasound Thalamotomy for Tremor

BACKGROUND

Magnetic resonance-guided focused-ultrasound (MRgFUS) thalamotomy is an effective treatment for essential and other tremors. It targets the ventrointermedius (Vim) nucleus, which is the thalamic relay in a proprioceptive pathway, and contains kinesthetic cells. Although MRgFUS thalamotomy reduces some risks associated with more invasive surgeries, it still has side effects, such as balance and gait disturbances; these may be caused by the lesion impacting proprioception.

OBJECTIVES

Our aim was to quantitatively measure the effects of MRgFUS on proprioception and limb use in essential tremor patients. We hypothesized that this thalamotomy alters proprioception, because the sensorimotor Vim thalamus is lesioned.

METHODS

Proprioception was measured using the Kinarm exoskeleton robot in 18 patients. Data were collected pre-operatively, and then 1 day, 3 months, and 1 year after surgery. Patients completed four tasks, assessing motor coordination and postural control, goal-directed movement and bimanual planning, position sense, and kinesthesia.

RESULTS

Immediately after surgery there were changes in posture speed (indicating tremor improvement), and in bimanual hand use, with the untreated limb being preferred. However, these measures returned to pre-operative baseline over time. There were no changes in parameters related to proprioception. None of these measures correlated with lesion size or lesion-overlap with the dentato-rubro-thalamic tract.

CONCLUSIONS

This is the first quantitative assessment of proprioception and limb preference following MRgFUS thalamotomy. Our results suggest that focused-ultrasound lesioning of the Vim thalamus does not degrade proprioception but alters limb preference. This change may indicate a required "relearning" in the treated limb, because the effect is transient. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Neuronal activity reorganization in motor cortex for successful locomotion after a lesion in the ventrolateral thalamus

Thalamic stroke leads to ataxia if the cerebellum-receiving ventrolateral thalamus (VL) is affected. The compensation mechanisms for this deficit are not well understood, particularly the roles that single neurons and specific neuronal subpopulations outside the thalamus play in recovery. The goal of this study was to clarify neuronal mechanisms of the motor cortex involved in mitigation of ataxia during locomotion when part of the VL is inactivated or lesioned. In freely ambulating cats, we recorded the activity of neurons in layer V of the motor cortex as the cats walked on a flat surface and horizontally placed ladder. We first reversibly inactivated ∼10% of the VL unilaterally using glutamatergic transmission antagonist CNQX and analyzed how the activity of motor cortex reorganized to support successful locomotion. We next lesioned 50%-75% of the VL bilaterally using kainic acid and analyzed how the activity of motor cortex reorganized when locomotion recovered. When a small part of the VL was inactivated, the discharge rates of motor cortex neurons decreased, but otherwise the activity was near normal, and the cats walked fairly well. Individual neurons retained their ability to respond to the demand for accuracy during ladder locomotion; however, most changed their response. When the VL was lesioned, the cat walked normally on the flat surface but was ataxic on the ladder for several days after lesion. When ladder locomotion normalized, neuronal discharge rates on the ladder were normal, and the shoulder-related group was preferentially active during the stride's swing phase.NEW & NOTEWORTHY This is the first analysis of reorganization of the activity of single neurons and subpopulations of neurons related to the shoulder, elbow, or wrist, as well as fast- and slow-conducting pyramidal tract neurons in the motor cortex of animals walking before and after inactivation or lesion in the thalamus. The results offer unique insights into the mechanisms of spontaneous recovery after thalamic stroke, potentially providing guidance for new strategies to alleviate locomotor deficits after stroke.

The neurobiology of human fear generalization: meta-analysis and working neural model

Fear generalization to stimuli resembling a conditioned danger-cue (CS+) is a fundamental dynamic of classical fear-conditioning. Despite the ubiquity of fear generalization in human experience and its known pathogenic contribution to clinical anxiety, neural investigations of human generalization have only recently begun. The present work provides the first meta-analysis of this growing literature to delineate brain substrates of conditioned fear-generalization and formulate a working neural model. Included studies (K = 6, N = 176) reported whole-brain fMRI results and applied generalization-gradient methodology to identify brain activations that gradually strengthen (positive generalization) or weaken (negative generalization) as presented stimuli increase in CS+ resemblance. Positive generalization was instantiated in cingulo-opercular, frontoparietal, striatal-thalamic, and midbrain regions (locus coeruleus, periaqueductal grey, ventral tegmental area), while negative generalization was implemented in default-mode network nodes (ventromedial prefrontal cortex, hippocampus, middle temporal gyrus, angular gyrus) and amygdala. Findings are integrated within an updated neural account of generalization centering on the hippocampus, its modulation by locus coeruleus and basolateral amygdala, and the excitation of threat- or safety-related loci by the hippocampus.

Basal Ganglia Circuits for Action Specification

Behavior is readily classified into patterns of movements with inferred common goals-actions. Goals may be discrete; movements are continuous. Through the careful study of isolated movements in laboratory settings, or via introspection, it has become clear that animals can exhibit exquisite graded specification to their movements. Moreover, graded control can be as fundamental to success as the selection of which action to perform under many naturalistic scenarios: a predator adjusting its speed to intercept moving prey, or a tool-user exerting the perfect amount of force to complete a delicate task. The basal ganglia are a collection of nuclei in vertebrates that extend from the forebrain (telencephalon) to the midbrain (mesencephalon), constituting a major descending extrapyramidal pathway for control over midbrain and brainstem premotor structures. Here we discuss how this pathway contributes to the continuous specification of movements that endows our voluntary actions with vigor and grace.

Basal ganglia output to the thalamus: still a paradox

The basal ganglia (BG)-recipient thalamus controls motor output but it remains unclear how its activity is regulated. Several studies report that thalamic activation occurs via disinhibition during pauses in the firing of inhibitory pallidal inputs from the BG. Other studies indicate that thalamic spiking is triggered by pallidal inputs via post-inhibitory 'rebound' calcium spikes. Finally excitatory cortical inputs can drive thalamic activity, which becomes entrained, or time-locked, to pallidal spikes. We present a unifying framework where these seemingly distinct results arise from a continuum of thalamic firing 'modes' controlled by excitatory inputs. We provide a mechanistic explanation for paradoxical pallidothalamic coactivations observed during behavior that raises new questions about what information is integrated in the thalamus to control behavior.

Signals from the ventrolateral thalamus to the motor cortex during locomotion

The activity of the motor cortex during locomotion is profoundly modulated in the rhythm of strides. The source of modulation is not known. In this study we examined the activity of one of the major sources of afferent input to the motor cortex, the ventrolateral thalamus (VL). Experiments were conducted in chronically implanted cats with an extracellular single-neuron recording technique. VL neurons projecting to the motor cortex were identified by antidromic responses. During locomotion, the activity of 92% of neurons was modulated in the rhythm of strides; 67% of cells discharged one activity burst per stride, a pattern typical for the motor cortex. The characteristics of these discharges in most VL neurons appeared to be well suited to contribute to the locomotion-related activity of the motor cortex. In addition to simple locomotion, we examined VL activity during walking on a horizontal ladder, a task that requires vision for correct foot placement. Upon transition from simple to ladder locomotion, the activity of most VL neurons exhibited the same changes that have been reported for the motor cortex, i.e., an increase in the strength of stride-related modulation and shortening of the discharge duration. Five modes of integration of simple and ladder locomotion-related information were recognized in the VL. We suggest that, in addition to contributing to the locomotion-related activity in the motor cortex during simple locomotion, the VL integrates and transmits signals needed for correct foot placement on a complex terrain to the motor cortex.

A mobility gradient in the organization of vertebrate movement: The perception of movement through symbolic language

Ordinary language can prevent us from seeing the organization of whole-animal movement. This may be why the search for behavioral homologies has not been as fruitful as the founders of ethology had hoped. The Eshkol-Wachman (EW) movement notational system can reveal shared movement patterns that are undetectable in the kinds of informal verbal descriptions of the same behaviors that are in current use. Rules of organization that are common to locomotor development, agonistic and exploratory behavior, scent marking, play, and dopaminergic drug-induced stereotypies in a variety of vertebrates suggest that behavior progresses along a “mobility gradient” from immobility to increasing complexity and unpredictability. A progression in the opposite direction, with decreasing spatial complexity and increased stereotypy, occurs under the influence of the nonselective dopaminergic drugs apomorphine and amphetamine and partly also the selective dopamine agonist quinpirole. The behaviors associated with the mobility gradient appear to be mediated by a family of basal ganglia-thalamocortical circuits and their descending output stations. Because the small number of rules underlying the mobility gradient account for a large variety of behaviors, they may be related to the specific functional demands on these neurological systems. The EW system and the mobility gradient model should prove useful to ethologists and neurobiologists.

Supplementary motor area structure and function: Review and hypotheses

Though its existence has been known for well over 30 years, only recently has the supplementary motor area (SMA) and its role in the cortical organization of movement come to be examined in detail by neuroscientists. Evidence from a wide variety of investigational perspectives is reviewed in an attempt to synthesize a conceptual framework for understanding SMA function. It is suggested that the SMA has an important role to play in the intentional process whereby internal context influences the elaboration of action. It may be viewed as phylogenetically older motor cortex, derived from anterior cingulate periarchicortical limbic cortex, which, as a key part of a medial premotor system, is crucial in the “programming” and fluent execution of extended action sequences which are “projectional” in that they rely on model-based prediction. This medial system can be distinguished from a lateral premotor system postulated to have evolved over phylogeny from a different neural source. An anatomico-physiologic model of the medial premotor system is proposed which embodies the principles of cyclicity and reentrance in the process of selecting those neural components to become active in conjunction with the performance of a particular action. The postulated dynamic action of this model in the microgenesis of a discrete action is outlined. It is concluded that although there is a great deal to be learned about the SMA, a convergence of current evidence can be identified. Such evidence suggests that the SMA plays an important role in the development of the intention-to-act and the specification and elaboration of action through its mediation between medial limbic cortex and primary motor cortex.

Event‐related beta desynchronization in human subthalamic nucleus correlates with motor performance

Although the basal ganglia play an important role in self-generated movement, their involvement in externally paced voluntary movement is less clear. We recorded local field potentials (LFPs) from the region of the subthalamic nuclei of eight patients with Parkinson's disease during the performance of a warned reaction time task in which an imperative cue instructed the subject to move or not to move. In 'go' trials, LFP activity in the beta frequency band ( approximately 20 Hz) decreased prior to movement, with an onset latency that strongly correlated with mean reaction time across patients. This was followed by a late post-movement increase in beta power. In contrast, in 'nogo' trials the beta power drop following imperative signals was prematurely terminated compared with go trials and reversed into an early beta power increase. These differences were manifest as power increases when go trials were subtracted from nogo trials. In six patients these relative beta power increases in nogo-go difference trials were of shorter latency than the respective reaction time. The findings suggest that, firstly, the subthalamic nucleus is involved in the preparation of externally paced voluntary movements in humans and, secondly, the degree of synchronization of subthalamic nucleus activity in the beta band may be an important determinant of whether motor programming and movement initiation is favoured or suppressed.

Involvement of Subcortical Structures in the Preparation of Self-Paced Movement

Abstract Although it has long been suggested that the basal ganglia and thalamus are involved in movement planning and preparation, there was little direct evidence in humans to support this hypothesis. Deep brain stimulation (DBS) is a well-established treatment for movement disorders such as Parkinson's disease, tremor, and dystonia. In patients undergoing DBS surgery, we recorded simultaneously from scalp contacts and from electrodes surgically implanted in the subthalamic nucleus (STN) of 13 patients with Parkinson's disease and in the “cerebellar” thalamus of 5 patients with tremor. The aim of our studies was to assess the role of the cortico-basal ganglia-thalamocortical loop through the STN and the cerebello-thalamocortical circuit through the “cerebellar” thalamus in movement preparation. The patients were asked to perform self-paced wrist extension movements. All subjects showed a cortical readiness potential (RP) with onset ranging between 1.5 to 2s before the onset of movement. Subcortical RPs were recorded in 11 of 13 with electrodes in the STN and in 4 of 5 patients with electrodes in the thalamus. The onset time of the STN and thalamic RPs were not significantly different from the onset time of the scalp RP. The STN and thalamic RPs were present before both contralateral and ipsilateral hand movements. Postoperative MRI studies showed that contacts with maximum RP amplitude generally were inside the target nucleus. These findings indicate that both the basal ganglia and the cerebellar circuits participate in movement preparation in parallel with the cortex.

Motivational views of reinforcement: implications for understanding the behavioral functions of nucleus accumbens dopamine

Although the Skinnerian 'Empirical Law of Effect' does not directly consider the fundamental properties of stimuli that enable them to act as reinforcers, such considerations are critical for determining if nucleus accumbens dopamine systems mediate reinforcement processes. Researchers who have attempted to identify the critical characteristics of reinforcing stimuli or activities have generally arrived at an emphasis upon motivational factors. A thorough review of the behavioral literature indicates that, across several different investigators offering a multitude of theoretical approaches, motivation is seen by many as being fundamental to the process of reinforcement. The reinforcer has been described as a goal, a commodity, an incentive, or a stimulus that is being approached, self-administered, attained or preserved. Reinforcers also have been described as activities that are preferred, deprived or in some way being regulated. It is evident that this 'motivational' or 'regulatory' view of reinforcement has had enormous influence over the hypothesis that DA directly mediates 'reward' or 'reinforcement' processes. Indeed, proponents of the DA/reward hypothesis regularly cite motivational theorists and employ their language. Nevertheless, considerable evidence indicates that low/moderate doses of DA antagonists, and depletions of DA in nucleus accumbens, can suppress instrumental responding for food while, at the same time, these conditions leave fundamental aspects of reinforcement (i.e. primary or unconditioned reinforcement; primary motivation or primary incentive properties of natural reinforcers) intact. Several complex features of the literature on dopaminergic involvement in reinforcement are examined below, and it is argued that the assertions that DA mediates 'reward' or 'reinforcement' are inaccurate and grossly oversimplified. Thus, it appears as though it is no longer tenable to assert that drugs of abuse are simply turning on the brain's natural 'reward system'. In relation to the hypothesis that DA systems are involved in 'wanting', but not 'liking', it is suggested in the present review that 'wanting' has both directional aspects (e.g. appetite to consume food) and activational aspects (e.g. activation for initiating and sustaining instrumental actions; tendency to work for food). The present paper reviews findings in support of the hypothesis that low doses of DA antagonists and accumbens DA depletions do not impair appetite to consume food, but do impair activational aspects of motivation. This suggestion is consistent with the studies showing that low doses of DA antagonists and accumbens DA depletions alter the relative allocation of instrumental responses, making the animals less likely to engage in instrumental responses that have a high degree of work-related response costs. In addition, this observation is consistent with studies demonstrating that accumbens DA depletions make rats highly sensitive to ratio requirements on operant schedules. Although accumbens DA is not seen as directly mediating appetite to consume food, principles of behavioral economics indicate that accumbens DA could be involved in the elasticity of demand for food in terms of the tendency to pay work-related response costs. Future research must focus upon how specific aspects of task requirements (i.e. ratio requirements, intermittence of reinforcement, temporal features of response requirements, dependence upon conditioned stimuli) interact with the effects of accumbens DA depletions, and which particular factors determine sensitivity to the effects of DA antagonism or depletion.

Movement-related potentials in the basal ganglia: a SEEG readiness potential study

OBJECTIVES

The brain potentials preceding and accompanying self-paced acral limb movements (Bereitschaftspotential/readiness potential (RP) paradigm) were studied in 12 patients.

METHODS

Intracranial electrodes were implanted in order to explore intractable epilepsy. The electrodes were introduced into sites corresponding to the electroclinical characteristics of each patient's epileptic seizures. In 7 patients, several depth electrodes were implanted orthogonally, in the temporal, fronto-orbital and prefrontal cortex. In 4 patients, subdural strip electrodes were used for the exploration of the fronto-temporal convexity. There were no RPs recorded in these areas. No contacts were placed in the central region known to generate cortical RP. In all the patients, one or two diagonal electrodes passed through or touched the basal ganglia to reach the amygdala and the hippocampus. The putamen was explored in 11 patients (in 3 of them bilaterally); the caudate head was explored in two patients, and the pallidum was explored in two patients.

RESULTS

RP with a clear amplitude gradient was present in all explored structures, however a phase reversal was never observed. RP was observed in the caudate in all recordings, and in the pallidum in one patient. It was recorded in the putamen in 8 out of the 11 explored patients. RPs were displayed contralaterally to the movement 9 times in 13 explorations, and ipsilaterally 4 times in 9 explorations. The shape of RP resembled the RP shape in the cortex and on the scalp. Movement accompanying potentials (MAPs) were also present in all 3 explored structures. The electrophysiological characteristics of MAP differed from RP, indicating separate generators. In the basal ganglia, RPs preceded the onset of movement by 500-1500 ms, at an average of 1080 (+/-330) ms. It seems that the RP in the basal ganglia starts slightly later than the RP in the motor cortices. That should be definitely demonstrated in patients with simultaneous recordings from cortical and subcortical structures. RP and MAP were displayed synchronously in the cortex and in the basal ganglia during most of the premovement period, as well as during the execution of movement. RP generators were reported by several authors in other deeply located structures, i.e. in the thalamus and in the brain-stem.

CONCLUSIONS

Based on all these recordings, we presume that the RPs recorded on the scalp are generated simultaneously in several cortical as well as subcortical structures.

Involvement of GABA neurons in allylnitrile-induced dyskinesia

Nitriles are a class of compounds with potential relevance to human health. Allylnitrile, one of nitriles, induces persistent behavioral abnormalities in mice. To explore what type of neuronal system is involved in these behavioral abnormalities, five neuronal markers, gamma-aminobutyric acid (GABA), tyrosine hydroxylase, serotonin, the serotonin transporter and choline acetyltransferase were immunohistochemically examined within various brain structures in allylnitrile and vehicle-treated mice. Allylnitrile induced changes in the immunolabelling of GABA in the medial habenula, interpeduncular nucleus, substantia nigra, dorsal raphe nucleus and median raphe nucleus; the amount of immunolabelling decreased in all of these brain structures except the medial habenula at 2 days postdosing, and increased in all of these structures at 14 days postdosing. Allylnitrile also induced changes in the amount of immunolabelling of tyrosine hydroxylase in the arcuate nucleus, substantia nigra pars compacta, locus coeruleus and caudoventrolateral reticular nucleus at either 2 or 14 days postdosing, depending on the structures. No immunohistochemical change was seen for serotonin, serotonin transporter and choline acetyltransferase. The present results suggest that the GABAergic systems through the medial habenula-interpeduncular nucleus-ascending raphe nuclei relay and through the substantia nigra may be involved in allylnitrile-induced behavioral abnormalities.

Parkinson's disease impairs the ability to change set quickly

We tested the hypothesis that basal ganglia dysfunction in Parkinson's disease impairs the ability to quickly change set. The ability to change set was inferred by measuring the change in the amplitude of automatic gastrocnemius or tibialis anterior muscle responses in standing subjects: (1) when the direction of a surface perturbation changed from a backward translation to a toes up rotation; and (2) when subjects were instructed to 'give' or 'resist' while responding to the translations and rotations. In experiment 1, a change in sensorimotor set was assessed by the suppression of gastrocnemius responses to toes up rotations following a series of backward translations. Unlike healthy young and older subjects, Parkinson subjects did not change sensorimotor set immediately to the first rotation, but needed several rotations to change their responses. When required to alternate their responses between backward translations and toes up rotations, Parkinson subjects showed a smaller amplitude change in gastrocnemius responses. In experiment 2, Parkinson subjects had more difficulty in using cognitive set to modify their responses, especially when instructed to 'resist' the perturbations. A small number of healthy older subjects also had difficulties changing set quickly, but to a lesser extent than the Parkinson subjects. Levodopa medication did not improve the Parkinson subjects' ability to change set quickly. These results suggest that the basal ganglia, which are affected in Parkinson's disease, are critical neural substrates in the ability to change set quickly.

Behavior-related changes in the activity of substantia nigra pars reticulata neurons in freely moving rats

As one of the primary targets of the striatum, the substantia nigra pars reticulata (SNr) has been hypothesized to play a role in normal motor behavior. Specifically, inhibition of usually high, tonic SNr output is predicted to correlate with motor activation. While support for this has come primarily from electrophysiological studies in primates performing goal-directed movements, we tested this hypothesis in rats behaving in an open-field arena. SNr single-unit activity was recorded during spontaneous bouts of open-field behavior (e.g., head and body movements, locomotion) and after rats were given D-amphetamine (1.0 mg/kg, s.c.), which reliably increases motor activity and elevates the firing of motor-related striatal neurons. Prior to drug administration, SNr neurons had either regular, slightly irregular or irregular firing patterns when animals rested quietly. During movement, some inhibitions were observed, but the majority ( approximately 79%) of analyzed units increased firing by as much as 38%. Regardless of the predrug behavioral response of the cell, amphetamine strongly inhibited firing rate ( approximately 90% below nonmovement baseline) and changed firing pattern such that all cells fired irregularly. Subsequent injection with the dopamine antagonist haloperidol (1.0 mg/kg, s.c.) reversed amphetamine-induced inhibitions in all tested cells, which supports a role for dopamine in this effect. These results suggest that the pattern of striatal activity established by amphetamine, which may be critical for determining the drug-induced behavioral pattern, is represented in the SNr regardless of the predrug behavioral response of the cell.

The basal ganglia: focused selection and inhibition of competing motor programs

The basal ganglia comprise several nuclei in the forebrain, diencephalon, and midbrain thought to play a significant role in the control of posture and movement. It is well recognized that people with degenerative diseases of the basal ganglia suffer from rigidly held abnormal body postures, slowing of movement, involuntary movements, or a combination of these a abnormalities. However, it has not been agreed just what the basal ganglia contribute to normal movement. Recent advances in knowledge of the basal ganglia circuitry, activity of basal ganglia neurons during movement, and the effect of basal ganglia lesions have led to a new hypothesis of basal ganglia function. The hypothesis states that the basal ganglia do not generate movements. Instead, when voluntary movement is generated by cerebral cortical and cerebellar mechanisms, the basal ganglia act broadly to inhibit competing motor mechanisms that would otherwise interfere with the desired movement. Simultaneously, inhibition is removed focally from the desired motor mechanisms to allow that movement to proceed. Inability to inhibit competing motor programs results in slow movements, abnormal postures and involuntary muscle activity.

The behavioral neurochemistry of motivation: methodological and conceptual issues in studies of the dynamic activity of nucleus accumbens dopamine

Considerable experimental and clinical evidence links forebrain dopamine (DA) systems to the performance of motor activities and to motivational processes. Much of the support for this conclusion was obtained from studies utilizing lesions or drugs to manipulate aspects of central dopaminergic function. Although such experiments yield important information concerning the behavioral consequences of interference with DA systems in brain, they do not demonstrate any relation between the dynamic activity of DA neurons and the level or type of motor function exhibited by the organism. This review discusses the emerging field of behavioral neurochemistry, and provides an overview of recent studies investigating the relation between nucleus accumbens DA release and behavior. Particular emphasis is placed upon current research involving microdialysis, voltammetry and electrophysiology. These different methods are viewed as complementary techniques for investigating the activity of DA systems in behaving animals. Evidence indicates that DA activity is most reliably activated by stimuli that trigger instrumental behavior and during the preparatory or instrumental phase of motivated behavior. The effects of consummatory reactions to positive reinforcers are somewhat equivocal; with food consumption, dialysis studies have yielded inconsistent results, while some voltammetric and electrophysiological studies have shown that DA activity in accumbens or ventral tegmental area actually decreases during consumption of food reinforcement. Moreover, the responsiveness of accumbens DA activity during behavioral stimulation is not unique to appetitive conditions, as several studies have shown that aversive or stressful conditions also stimulate accumbens DA release or metabolism. It is reasonable to suggest at this time that accumbens DA neurons are activated by a variety of different motivational conditions, but that the consequence of that activation is to modulate the behavioral reactivity of the organism. This type of function is seen as representing an area of overlap between motor and motivational processes.

Alterations in pallidal neuronal responses to peripheral sensory and striatal stimulation in symptomatic and recovered parkinsonian cats

The spontaneous activity, responses to peripheral sensory and ipsilateral caudate nucleus stimulation of globus pallidus (GP) and entopeduncular nucleus (ENTO) neurons were studied in cats while normal, symptomatic for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced parkinsonism, and when spontaneously recovered from gross parkinsonian motor deficits. Administration of MPTP resulted in parkinsonian motor symptoms that spontaneously recovered approximately 4-6 weeks after the MPTP administration. Post-mortem dopamine levels in recovered animals was approximately 95% below levels previously measured in normal animals. In symptomatic animals, the mean spontaneous firing rate for GP units was decreased by 50% and increased by 55% for ENTO units recorded. Spontaneous firing rates for GP and ENTO units in recovered cats were not significantly different from those observed in normal cats. In normal cats, 31.4% of GP and 29% of ENTO units tested responded to tactile stimulation of the face. Only 12.2% of GP and 13% of ENTO units responded to such stimulation in parkinsonian animals while the responses were generally less specific (larger receptive fields, more bilateral receptive fields, and more responses to multiple stimulation types) than normal. In recovered cats GP and ENTO responses resembled those observed in normal cats. There was no difference in the overall percentage of pallidal units responding to striatal stimulation across the 3 experimental conditions. There was, however, an increase in the percentage of units responding with complex response sequences (i.e. decrease in activity followed by an increase in activity) in symptomatic animals as compared to normal and recovered animals. The results suggest that loss of striatal dopamine in parkinsonian animals has profound effects on the sensory responsiveness of GP and ENTO neurons and that these effects coincide with the appearance of and recovery from parkinsonian motor deficits. These data further support the notion that sensory information processing by the basal ganglia may play an important role in influencing motor output.

Neostriatal modulation of motor cortex excitability

The influence of the basal ganglia motor loop on motor cortex function was examined by pharmacologically altering neostriatal activity while monitoring the electrical stimulation thresholds for eliciting movements of the ipsilateral and contralateral motor cortex in ketamine anesthetized rats. Repeated unilateral intraneostriatal infusions (1-3) of the glutamate agonist, kainic acid (0.1 microliter, 75 ng), or glutamate (0.3 microliter, 1.65 micrograms) reliably increased ipsilateral but not contralateral cortical thresholds. Single infusions of kainic acid (0.3 microliter, 150 or 225 ng) elevated ipsilateral cortical thresholds for 30-45 min; with glutamate (0.3 microliter, 1.65 micrograms), the change lasted less than 10 min. Antidromically identified striatonigral projection neurons (n = 8) located approximately 500 microM from the infusion cannula, showed either increased firing (n = 4) for less than 10 min following glutamate infusion or no change from their non-firing state (n = 4). Non-antidromically activated neurons (n = 3) were all excited by the infusion, although an interval of inhibition preceded or followed the excitation in two cases. Infusions (0.3 microliter) of inhibitory agents (GABA, 31 and 310 ng; muscimol 34.2 ng; and DNQX 34.2 ng) did not alter cortical threshold, nor did saline vehicle. Lesion of the ventrolateral but not ventromedial thalamic nucleus prevented the modulation of cortical thresholds following intraneostriatal infusion of 225 ng kainic acid. Thus the neostriatal alteration of cortical thresholds indicates a modulation of cortical excitability via thalamic projections and not the outcome of competing descending cortical and neonstriatal influences converging on motorneurons. These results suggest that tonic feedforward modulation of the motor cortex and the pyramidal tract by the basal ganglia can be inhibitory.

Basal ganglia intrinsic circuits and their role in behavior

There have been significant recent advances in the understanding of basal ganglia circuitry and its role in behavior. Important areas of work in the past year include, firstly, the role of striatal neurons in early phases of movement and, secondly, further characterization of the intrinsic circuitry with emphasis on the importance of the subthalamic nucleus and its connections. A conceptual model of basal ganglia inhibition of competing motor programs is discussed.

Primate basal ganglia activity in a precued reaching task: preparation for movement

Single cell activity was recorded from the primate putamen, caudate nucleus, and globus pallidus during a precued reaching movement task. Two monkeys were trained to touch one of several target knobs mounted in front of them after an LED was lighted on the correct target. A precue was presented prior to this target "go cue" by a randomly varied delay interval, giving the animals partial or complete advance information about the target for the movement task. The purpose of this design was to examine neuronal activity in the major structures of the basal ganglia during the preparation phase of limb movements when varying amounts of advance information were provided to the animals. The reaction times were shortest with complete precues, intermediate with partial precues, and longest with precues containing no information, demonstrating that the animals used precue information to prepare partly or completely for the reaching movement before the target go cue was given. Changes in activity were seen in the basal ganglia during the preparatory period in 30% of neurons in putamen, 31% in caudate nucleus, and 27% in globus pallidus. Preparatory changes were stronger and more closely linked to the time of movement initiation in putamen than in caudate nucleus. Although the amount of information contained in the precues had no significant effect on preparatory activity preceding the target go cue, a directional selectivity during this period was observed for a subset of neurons with preparatory changes (15% in putamen, 11% in caudate nucleus, 14% in globus pallidus) when the precue contained information about the upcoming direction of movement. A smaller subset of neurons showed selectivity for the preparation of movement amplitude. A larger number of preparatory changes showed selectivity for the direction or amplitude of movement following the target go cue than in the delay period before the cue. The intensity of preparatory changes in activity in many cases depended on the length of the delay interval preceding the target go cue. Even following the target go cue, the intensity of the preparatory changes in activity continued to be significantly influenced by the length of the preceding delay interval for 11% of changes in putamen, 8% in caudate nucleus, and 18% in globus pallidus. This finding suggests that preparatory activity in the basal ganglia takes part in a process termed motor readiness. Behaviorally, this process was seen as a shortening of reaction time regardless of precue information for trials in which the delay interval was long and the animals showed an increased readiness to move.(ABSTRACT TRUNCATED AT 400 WORDS)

Striatal neuronal activity during the initiation and execution of hand movements made in response to visual and vibratory cues

Recordings were obtained from 146 neurons in the neostriatum of rhesus monkeys while they performed wrist movements in response to visual and vibratory cues. Of these, 75 putamena nd 29 caudate neurons exhibited changes in firing rate that were temporally related to the onset of the wrist movements and that began prior to movement onset. This premovement activity (PMA) usually was directionally specific, in that the magnitude or direction of change in firing rates was different during flexion trials as compared to trials involving wrist extension. PMA onset usually preceded movement onset by more than 100 ms and in most instances preceded the average onset of task-related changes in electromyographic (EMG) activity in muscles of the wrist and forelimb. For most neurons, the changes in neuronal activity that began prior to movement were maintained during movement execution. However, approximately one-third of the neurons that exhibited PMA changed their firing rate in the opposite direction, relative to their PMA and to their baseline rate of activity, once the movement began. Several other neurons either exhibited PMA only or they altered their discharge rates during movement execution but did not exhibit PMA. These observations suggest that, despite the close temporal relationship between the onset of PMA and the onset of wrist movement, the neuronal mechanisms mediating the PMA may differ from those that occur during movement execution. The PMA onset of neostriatal neurons occurred earlier in visually cued than in vibratory cued trials. These differences were statistically significant only for flexion trials, however, in which movements were made against a load and in the same direction as the palmar vibratory stimulus. For trials involving wrist extension, PMA onsets for visually cued as compared with vibratory cued trials were not statistically different. These findings contrast with data obtained previously from somatosensory cortical neurons during performance of the same behavioral task. On average, PMA in the putamen began earlier, relative to movement onset, than it did in the somatosensory cortex. Moreover, in the somatosensory cortex, PMA onset occurred earlier in vibratory cued than in visually cued trials, irrespective of movement direction (Nelson 1988; Nelson and Douglas 1989). For putamen neurons, but not for caudate or cortical neurons, the onset of PMA also occurred significantly earlier during extension trials than flexion trials, irrespective of the modality of the "go-cue".(ABSTRACT TRUNCATED AT 400 WORDS)

Role of primate basal ganglia and frontal cortex in the internal generation of movements. III. Neuronal activity in the supplementary motor area

This study is a part of a project investigating neuronal activity in the basal ganglia and frontal cortex and describes externally and internally induced preparatory activity in the supplementary motor area (SMA), which forms a closed neuronal loop with the striatum. Monkeys made self-initiated arm reaching movements toward a constant target in the absence of phasic external stimuli. In separate blocks of trials, animals performed in a delayed go no-go task in which an instruction cue prepared for subsequent movement or no-movement to a trigger stimulus. A total of 328 neurons were tested in the delay task. Of these, 91 responded transiently to the instruction light with a median latency of 262 ms. Three quarters of these responses were restricted to the instruction preparing for arm movement, as opposed to withholding it, and thus may be involved in movement preparation processes. Sustained activation during the instruction-trigger interval was found for 67 neurons and occurred nearly exclusively in movement trials. Activation usually increased gradually after the cue and ended abruptly upon movement onset and thus could be related to the setting and maintenance of processes underlying the preparation of movement. Time-locked responses to the trigger stimulus were found in 38 neurons and were usually restricted to movement trials (median latency 80 ms). Activity time-locked to movement execution occurred in 67 neurons, beginning up to 252 ms before movement onset. A total of 266 neurons were tested with self-initiated arm movements. Of these, 43 showed premovement activity beginning 610-3030 ms before movement onset (median 1430 ms). The activity increased slowly and reached its peak at 370 ms before movement onset. It ended before movement onset or continued until the arm began to move or reached the target. This activity appears to reflect neuronal processes related to the internal generation of movements. Two thirds of activations preceding self-initiated movements occurred in neurons not activated before externally instructed movements, suggesting a selectivity for the internal generation process. Activity related to the execution of self-initiated movements occurred in 67 neurons: it began during and up to 420 ms before movement onset and was usually not associated with premovement activity. Most of these neurons were also activated with stimulus-triggered movements, suggesting a lack of selectivity for the execution of self-initiated movements. In comparison with the striatum, more SMA neurons showed preparatory activity preceding externally instructed movements (transient 27% vs 16%, sustained 20% vs 12%) and self-initiated movements (16% vs 11%).(ABSTRACT TRUNCATED AT 400 WORDS)