Thalamic distribution of projection neurons to the primary motor cortex relative to afferent terminal fields from the globus pallidus in the macaque monkey

Structural Plasticity of GABAergic Pallidothalamic Terminals in MPTP-Treated Parkinsonian Monkeys: A 3D Electron Microscopic Analysis

The internal globus pallidus (GPi) is a major source of tonic GABAergic inhibition to the motor thalamus. In parkinsonism, the firing rate of GPi neurons is increased, and their pattern switches from a tonic to a burst mode, two pathophysiological changes associated with increased GABAergic pallidothalamic activity. In this study, we used high-resolution 3D electron microscopy to demonstrate that GPi terminals in the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM), the two main GPi-recipient motor thalamic nuclei in monkeys, undergo significant morphometric changes in parkinsonian monkeys including (1) increased terminal volume in both nuclei; (2) increased surface area of synapses in both nuclei; (3) increased number of synapses/GPi terminals in the CM, but not VApc; and (4) increased total volume, but not number, of mitochondria/terminals in both nuclei. In contrast to GPi terminals, the ultrastructure of putative GABAergic nonpallidal terminals was not affected. Our results also revealed striking morphological differences in terminal volume, number/area of synapses, and volume/number of mitochondria between GPi terminals in VApc and CM of control monkeys. In conclusion, GABAergic pallidothalamic terminals are endowed with a high level of structural plasticity that may contribute to the development and maintenance of the abnormal increase in pallidal GABAergic outflow to the thalamus in the parkinsonian state. Furthermore, the evidence for ultrastructural differences between GPi terminals in VApc and CM suggests that morphologically distinct pallidothalamic terminals from single pallidal neurons may underlie specific physiological properties of pallidal inputs to VApc and CM in normal and diseased states.

Neurophysiological Correlates of Motor and Cognitive Dysfunction in Prodromal and Overt Dementia with Lewy Bodies

Background: The neurophysiological correlates of cognitive and motor symptoms in prodromal and overt dementia with Lewy bodies (DLB) are still to be elucidated. Objective: To evaluate if cognitive and motor features of patients with prodromal and overt DLB are associated with the impairment of specific neurotransmitter circuits, evaluated in vivo with transcranial magnetic stimulation (TMS). Methods: Fifty-one patients with DLB (twenty-five prodromal; twenty-six with dementia) underwent neuropsychological and clinical evaluation, with twenty-five patients having at least one follow-up evaluation. All patients were assessed with TMS at baseline, with protocols assessing cholinergic circuits (short latency afferent inhibition, SAI), GABAergic circuits (short interval intracortical inhibition, SICI), and glutamatergic circuits (intracortical facilitation, ICF). Results: Compared to HC, SICI, ICF, and SAI resulted significantly impaired in both prodromal and overt DLB, with the latter showing a reduced SICI and SAI also compared to prodromal DLB. There was a significant correlation between motor deficits, evaluated with the UPDRS-III, and the impairment of GABAergic (SICI) (r = 0.729, p <  0.001) and glutamatergic (ICF) (r –0.608, p <  0.001) circuits; global cognition, evaluated with the Mini-Mental State Examination, correlated with the impairment of cholinergic (SAI) circuits (r=–0.738, p <  0.001). Worsening of cognitive functions at follow-up was associated with reduced cholinergic functions at baseline (R2  = 0.53%, p <  0.001). Conclusion: These results suggest that motor and cognitive dysfunctions in prodromal and overt DLB depend on specific and independent neurotransmitter circuits.

Cortico-subcortical β burst dynamics underlying movement cancellation in humans

Dominant neuroanatomical models hold that humans regulate their movements via loop-like cortico-subcortical networks, which include the subthalamic nucleus (STN), motor thalamus, and sensorimotor cortex (SMC). Inhibitory commands across these networks are purportedly sent via transient, burst-like signals in the β frequency (15-29 Hz). However, since human depth-recording studies are typically limited to one recording site, direct evidence for this proposition is hitherto lacking. Here, we present simultaneous multi-site recordings from SMC and either STN or motor thalamus in humans performing the stop-signal task. In line with their purported function as inhibitory signals, subcortical β-bursts were increased on successful stop-trials. STN bursts in particular were followed within 50 ms by increased β-bursting over SMC. Moreover, between-site comparisons (including in a patient with simultaneous recordings from SMC, thalamus, and STN) confirmed that β-bursts in STN temporally precede thalamic β-bursts. This highly unique set of recordings provides empirical evidence for the role of β-bursts in conveying inhibitory commands along long-proposed cortico-subcortical networks underlying movement regulation in humans.

Comparative Ultrastructural Analysis of Thalamocortical Innervation of the Primary Motor Cortex and Supplementary Motor Area in Control and MPTP-Treated Parkinsonian Monkeys

The synaptic organization of thalamic inputs to motor cortices remains poorly understood in primates. Thus, we compared the regional and synaptic connections of vGluT2-positive thalamocortical glutamatergic terminals in the supplementary motor area (SMA) and the primary motor cortex (M1) between control and MPTP-treated parkinsonian monkeys. In controls, vGluT2-containing fibers and terminal-like profiles invaded layer II-III and Vb of M1 and SMA. A significant reduction of vGluT2 labeling was found in layer Vb, but not in layer II-III, of parkinsonian animals, suggesting a potential thalamic denervation of deep cortical layers in parkinsonism. There was a significant difference in the pattern of synaptic connectivity in layers II-III, but not in layer Vb, between M1 and SMA of control monkeys. However, this difference was abolished in parkinsonian animals. No major difference was found in the proportion of perforated versus macular post-synaptic densities at thalamocortical synapses between control and parkinsonian monkeys in both cortical regions, except for a slight increase in the prevalence of perforated axo-dendritic synapses in the SMA of parkinsonian monkeys. Our findings suggest that disruption of the thalamic innervation of M1 and SMA may underlie pathophysiological changes of the motor thalamocortical loop in the state of parkinsonism.

Cortical disinhibition in Parkinson's disease

In Parkinson's disease, striatal dopamine depletion produces profound alterations in the neural activity of the cortico-basal ganglia motor loop, leading to dysfunctional motor output and parkinsonism. A key regulator of motor output is the balance between excitation and inhibition in the primary motor cortex, which can be assessed in humans with transcranial magnetic stimulation techniques. Despite decades of research, the functional state of cortical inhibition in Parkinson's disease remains uncertain. Towards resolving this issue, we applied paired-pulse transcranial magnetic stimulation protocols in 166 patients with Parkinson's disease (57 levodopa-naïve, 50 non-dyskinetic, 59 dyskinetic) and 40 healthy controls (age-matched with the levodopa-naïve group). All patients were studied OFF medication. All analyses were performed with fully automatic procedures to avoid confirmation bias, and we systematically considered and excluded several potential confounding factors such as age, gender, resting motor threshold, EMG background activity and amplitude of the motor evoked potential elicited by the single-pulse test stimuli. Our results show that short-interval intracortical inhibition is decreased in Parkinson's disease compared to controls. This reduction of intracortical inhibition was obtained with relatively low-intensity conditioning stimuli (80% of the resting motor threshold) and was not associated with any significant increase in short-interval intracortical facilitation or intracortical facilitation with the same low-intensity conditioning stimuli, supporting the involvement of cortical inhibitory circuits. Short-interval intracortical inhibition was similarly reduced in levodopa-naïve, non-dyskinetic and dyskinetic patients. Importantly, intracortical inhibition was reduced compared to control subjects also on the less affected side (n = 145), even in de novo drug-naïve patients in whom the less affected side was minimally symptomatic (lateralized Unified Parkinson's Disease Rating Scale part III = 0 or 1, n = 23). These results suggest that cortical disinhibition is a very early, possibly prodromal feature of Parkinson's disease.

Thalamic interactions of cerebellum and basal ganglia

Cerebellum and basal ganglia are reciprocally interconnected with the neocortex via oligosynaptic loops. The signal pathways of these loops predominantly converge in motor areas of the frontal cortex and are mainly segregated on subcortical level. Recent evidence, however, indicates subcortical interaction of these systems. We have reviewed literature that addresses the question whether, and to what extent, projections of main output nuclei of basal ganglia (reticular part of the substantia nigra, internal segment of the globus pallidus) and cerebellum (deep cerebellar nuclei) interact with each other in the thalamus. To this end, we compiled data from electrophysiological and anatomical studies in rats, cats, dogs, and non-human primates. Evidence suggests the existence of convergence of thalamic projections originating in basal ganglia and cerebellum, albeit sparse and restricted to certain regions. Four regions come into question to contain converging inputs: (1) lateral parts of medial dorsal nucleus (MD); (2) parts of anterior intralaminar nuclei and centromedian and parafascicular nuclei (CM/Pf); (3) ventromedial nucleus (VM); and (4) border regions of cerebellar and ganglia terminal territories in ventral anterior and ventral lateral nuclei (VA-VL). The amount of convergences was found to exhibit marked interspecies differences. To explain the rather sparse convergences of projection territories and to estimate their physiological relevance, we present two conceivable principles of anatomical organization: (1) a "core-and-shell" organization, in which a central core is exclusive to one projection system, while peripheral shell regions intermingle and occasionally converge with other projection systems and (2) convergences that are characteristic to distinct functional networks. The physiological relevance of these convergences is not yet clear. An oculomotor network proposed in this work is an interesting candidate to examine potential ganglia and cerebellar subcortical interactions.

New Models of Frontal-Subcortical Skeletomotor Circuit Pathology in Tardive Dyskinesia

Tardive dyskinesia (TD) is a hyperkinetic movement disorder that can occur as a side effect of treatment with antipsychotic medications. Because antipsychotics block the D2 family of dopamine receptors in the striatum, it has long been suspected this blockade contributes to the development of TD. Specifically, increased sensitivity of the dopamine receptors following chronic blockade has been thought to result in abnormal functioning of the frontal-subcortical (FSC) skeletomotor circuit and the symptoms of TD. However, this hypothesis remains unproven. In recent years, substantial research has focused on the basal ganglia and FSC circuits. This research has resulted in the development of the focused selection model of skeletomotor circuit function. This hypothesis provides a compelling model of neurocircuit abnormalities in TD. A greater understanding of the neuropathology of TD may lead to the development of better treatment and prevention strategies for this disorder. Furthermore, this information may contribute to a more complete understanding of normal skeletomotor circuit function and the role of circuit pathology in numerous neuropsychiatric conditions.

The role of the cerebellum in the pathophysiology of Parkinson's disease

Parkinson's disease (PD), the most common neurodegenerative movement disorder, has traditionally been considered a "classic" basal ganglia disease, as the most obvious pathology is seen in the dopaminergic cells in the substantia nigra pars compacta. Nevertheless recent discoveries in anatomical connections linking the basal ganglia and the cerebellum have led to a re-examination of the role of the cerebellum in the pathophysiology of PD. This review summarizes the role of the cerebellum in explaining many curious features of PD: the significant variation in disease progression between individuals; why severity of dopaminergic deficit correlates with many features of PD such as bradykinesia, but not tremor; and why PD subjects with a tremor-predominant presentation tend to have a more benign prognosis. It is clear that the cerebellum participates in compensatory mechanisms associated with the disease and must be considered an essential contributor to the overall pathophysiology of PD.

Deep-Brain Stimulation for Basal Ganglia Disorders

The realization that medications used to treat movement disorders and psychiatric conditions of basal ganglia origin have significant shortcomings, as well as advances in the understanding of the functional organization of the brain, has led to a renaissance in functional neurosurgery, and particularly the use of deep brain stimulation (DBS). Movement disorders are now routinely being treated with DBS of 'motor' portions of the basal ganglia output nuclei, specifically the subthalamic nucleus and the internal pallidal segment. These procedures are highly effective and generally safe. Use of DBS is also being explored in the treatment of neuropsychiatric disorders, with targeting of the 'limbic' basal ganglia-thalamocortical circuitry. The results of these procedures are also encouraging, but many unanswered questions remain in this emerging field. This review summarizes the scientific rationale and practical aspects of using DBS for neurologic and neuropsychiatric disorders.

Basal ganglia neurons dynamically facilitate exploration during associative learning

The basal ganglia (BG) appear to play a prominent role in associative learning, the process of pairing external stimuli with rewarding responses. Accumulating evidence suggests that the contributions of various BG components may be described within a reinforcement learning model, in which a broad repertoire of possible responses to environmental stimuli are evaluated before the most profitable one is chosen. The striatum receives diverse cortical inputs, providing a rich source of contextual information about environmental cues. It also receives projections from midbrain dopaminergic neurons, whose phasic activity reflects a reward prediction error signal. These coincident information streams are well suited for evaluating responses and biasing future actions toward the most profitable response. Still lacking in this model is a mechanistic description of how initial response variability is generated. To investigate this question, we recorded the activity of single neurons in the globus pallidus internus (GPi), the primary BG output nucleus, in nonhuman primates (Macaca mulatta) performing a motor associative learning task. A subset (29%) of GPi neurons showed learning-related effects, decreasing firing during the early stages of learning, then returning to higher baseline rates as associations were mastered. On a trial-by-trial basis, lower firing rates predicted exploratory behavior, whereas higher rates predicted an exploitive response. These results suggest that, during associative learning, BG output is initially permissive, allowing exploration of a variety of responses. Once a profitable response is identified, increased GPi activity suppresses alternative responses, sharpening the response profile and encouraging exploitation of the profitable learned behavior.

Morphological differences in Parkinson's disease with and without rest tremor

BACKGROUND

Rest tremor is a hallmark of Parkinson's disease (PD), but its pathogenesis remains incompletely understood. Nigro-striatal dopamine deficiency correlates best with bradykinesia, but not with tremor. Oscillating neurons in one or multiple localizations within the basal gangliathalamo-cortical loop may cause rest tremor, and an active contribution of the cerebellum and the cerebello-thalamo-cortical projections has been postulated.

OBJECTIVE

To compare the pattern of grey matter volume in PD patients with and without tremor to identify structural correlates of rest tremor.

METHODS

Voxel-based morphometry (VBM) of a high-resolution 3 Tesla, T1-weighted MR images, pre-processed according to an optimized protocol using SPM2, was performed in 24 patients with mild to moderate PD comparing local grey matter volume in patients with (n = 14) and without rest tremor (n = 10).

RESULTS

Grey matter volume is decreased in the right quadrangular lobe and declive of the cerebellum in PD with tremor compared to those without (PFDR < 0.05).

CONCLUSIONS

These results demonstrate for the first time morphological changes in the cerebellum in PD patients with rest tremor and highlight the involvement of the cerebellum and cerebello- thalamo-cortical circuit in the pathogenesis of parkinsonian rest tremor.

Human pallidothalamic and cerebellothalamic tracts: anatomical basis for functional stereotactic neurosurgery

Anatomical knowledge of the structures to be targeted and of the circuitry involved is crucial in stereotactic functional neurosurgery. The present study was undertaken in the context of surgical treatment of motor disorders such as essential tremor (ET) and Parkinson's disease (PD) to precisely determine the course and three-dimensional stereotactic localisation of the cerebellothalamic and pallidothalamic tracts in the human brain. The course of the fibre tracts to the thalamus was traced in the subthalamic region using multiple staining procedures and their entrance into the thalamus determined according to our atlas of the human thalamus and basal ganglia [Morel (2007) Stereotactic atlas of the human thalamus and basal ganglia. Informa Healthcare Inc., New York]. Stereotactic three-dimensional coordinates were determined by sectioning thalamic and basal ganglia blocks parallel to stereotactic planes and, in two cases, by correlation with magnetic resonance images (MRI) from the same brains prior to sectioning. The major contributions of this study are to provide: (1) evidence that the bulks of the cerebellothalamic and pallidothalamic tracts are clearly separated up to their thalamic entrance, (2) stereotactic maps of the two tracts in the subthalamic region, (3) the possibility to discriminate between different subthalamic fibre tracts on the basis of immunohistochemical stainings, (4) correlations of histologically identified fibre tracts with high-resolution MRI, and (5) evaluation of the interindividual variability of the fibre systems in the subthalamic region. This study should provide an important basis for accurate stereotactic neurosurgical targeting of the subthalamic region in motor disorders such as PD and ET.

Thalamic connections of the dorsal and ventral premotor areas in New World owl monkeys

Thalamic connections of two premotor cortex areas, dorsal (PMD) and ventral (PMV), were revealed in New World owl monkeys by injections of fluorescent dyes or wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The injections were placed in the forelimb and eye-movement representations of PMD and in the forelimb representation of PMV as determined by microstimulation mapping. For comparison, injections were also placed in the forelimb representation of primary motor cortex (M1) of two owl monkeys. The results indicate that both PMD and PMV receive dense projections from the ventral lateral (VL) and ventral anterior (VA) thalamus, and sparser projections from the ventromedial (VM), mediodorsal (MD) and intralaminar (IL) nuclei. Labeled neurons in VL were concentrated in the anterior (VLa) and the medial (VLx) nuclei, with only a few labeled cells in the dorsal (VLd) and posterior (VLp) nuclei. In VA, labeled neurons were concentrated in the parvocellular division (VApc) dorsomedial to VLa. Labeled neurons in MD were concentrated in the most lateral and posterior parts of the nucleus. VApc projected more densely to PMD than PMV, especially to rostral PMD, whereas caudal PMD received stronger projections from neurons in VLx and VLa. VLd projected exclusively to PMD, and not to PMV. In addition, neurons labeled by PMD injections tended to be more dorsal in VL, IL, and MD than those labeled by PMV injections. The results indicate that both premotor areas receive indirect inputs from the cerebellum (via VLx, VLd and IL) and globus pallidus (via VLa, VApc, and MD). Comparisons of thalamic projections to premotor and M1 indicate that both regions receive strong projections from VLx and VLa, with the populations of cells projecting to M1 located more laterally in these nuclei. VApc, VLd, and MD project mainly to premotor areas, while VLp projects mainly to M1. Overall, the thalamic connectivity patterns of premotor cortex in New World owl monkeys are similar to those reported for Old World monkeys.

Input-output organization of jaw movement-related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

The Motor Thalamus in Neurosurgery

THE MOTOR THALAMUS is an important target for the treatment of tremor. It receives afferents from the cerebellum, globus pallidus internus, and substantia nigra and projects mainly to the motor cortex, premotor cortex, and supplementary motor area. Various nomenclatures have been proposed to subdivide the motor thalamus, none of which are universally accepted. Both thalamic lesions and high-frequency stimulation ameliorate tremor in diverse pathological conditions. Modern neurophysiological techniques have allowed the recording of the activity of thalamic neurons in patients with different clinical conditions. This has provided a better understanding of the functions of the motor thalamus in humans. The aim of the present article is to briefly review the major anatomic and physiological aspects of the motor thalamus as well as the electrophysiological findings described in humans undergoing surgical procedures.

Neurochemical characterization of the cerebellar-recipient motor thalamic territory in the macaque monkey

Abstract The immunoarchitectonics of the macaque motor thalamus was analysed to look for a possible neurochemical characterization of thalamic territories, which were not definable cytoarchitectonically, associated with different functional pathways. Thalamic sections from 15 macaque monkeys were processed for visualization of calbindin (CB), parvalbumin (PV), calretinin (CR) and SMI-32 immunoreactivity (ir). PV-, CR- and SMI-32ir distributions did not show any clear correlation with known functional subdivisions. In contrast, CBir distribution reliably defined two markedly distinct motor thalamic territories, one characterized by high cell and neuropil CBir (CB-positive territory), the other by very low cell and neuropil CBir (CB-negative territory). These two neurochemically distinct compartments, the CB-negative and the CB-positive territories, appear to correspond to the cerebellar- and basal ganglia-recipient territories, respectively. To verify the possible correspondence of the CB-negative territory with the cerebellar-recipient sector of the motor thalamus, we compared the distribution of cerebello-thalamic projections with the distribution of CBir in two monkeys. The distribution of cerebellar afferent terminals was similar to that reported from previous reports and in line with the notion that in the motor thalamus the cerebellar-recipient territory does not respect cytoarchitectonic boundaries. Comparison with CB immunoarchitecture showed very close correspondence in the motor thalamus between the distribution of the anterograde labeling and the CB-negative territory, suggesting that the CB-negative territory represents the architectonic counterpart of the cerebellar-recipient territory. CB immunostaining may therefore represent a helpful tool for describing the association between thalamocortical projections and the basal ganglia or the cerebellar loops and for establishing possible homologies between the motor thalamus of non-human primates and humans.

Divergence and convergence of thalamocortical projections to premotor and supplementary motor cortex: a multiple tracing study in the macaque monkey

The premotor cortex of macaque monkeys is currently subdivided into at least six different subareas on the basis of structural, hodological and physiological criteria. To determine the degree of divergence/convergence of thalamocortical projections to mesial [supplementary motor area (SMA)-proper and pre-SMA] and lateral (PMd-c, PMd-r, PMv-c and PMv-r) premotor (PM) subareas, quantitative analyses were performed on the distribution of retrograde labelling after multiple tracer injections in the same animal. The results demonstrate that all PM and SMA subareas receive common inputs from several thalamic nuclei, but the relative contribution of these nuclei to thalamocortical projections differs. The largest difference occurs between subareas of SMA, with much greater contribution from the mediodorsal (MD) and area X, and a smaller contribution from the ventral lateral anterior (VLa) and ventral part of the ventral lateral posterior (VLpv) to pre-SMA than to SMA-proper. In PM, differences between subareas are less pronounced; in particular, all receive a significant contribution from MD, the ventral anterior (VApc) and area X. However, there are clear gradients, such as increasing projections from MD to rostral, from VLa and VLpv to caudal, and from dorsal VLp (VLpd) to dorsal premotor subareas. Intralaminar nuclei provide widespread projections to all premotor subareas. The degree of overlap between thalamocortical projections varies among different PM and SMA subareas and different sectors of the thalamus. These variations, which correspond to different origin and topography of thalamocortical projections, are discussed in relation to functional organizations at thalamic and cortical levels.

Input–output organization of the rostral part of the dorsal premotor cortex, with special reference to its corticostriatal projection

Until recently, little was known about the rostral part of the dorsal premotor cortex (PMdr). In the present study, somatotopical representations of the PMdr were electrophysiologically identified in the macaque monkey, and the distribution of corticostriatal input from the forelimb region of the PMdr was analyzed in relation to its thalamocortical and intracortical (with the frontal lobe) connections. Results have revealed that (1) the forelimb is represented predominantly in the PMdr, while only a few sites representing other body parts are distributed as embedded within the forelimb representation; (2) the corticostriatal input zone is located in the striatal cell bridges and their surroundings; (3) the cells of origin of the thalamocortical projections to the PMdr are located mainly in the parvicellular division of the ventroanterior nucleus, the oral divison of the ventrolateral nucleus, area X, the caudal divison of the ventrolateral nucleus, the mediodorsal nucleus, and the intralaminar nuclear group; (4) the PMdr is interconnected primarily with higher-order motor-related areas and dorsal area 46. These data indicate that the input-output pattern of the PMdr resembles those of the presupplementary motor area and the rostral cingulate motor area, and that the PMdr may play critical roles in higher-order motor functions.

Thalamocortical and intracortical connections of monkey cingulate motor areas

Although there has been an increasing interest in motor functions of the cingulate motor areas, data concerning their input organization are still limited. To address this issue, the patterns of thalamic and cortical inputs to the rostral (CMAr), dorsal (CMAd), and ventral (CMAv) cingulate motor areas were investigated in the macaque monkey. Tracer injections were made into identified forelimb representations of these areas, and the distributions of retrogradely labeled neurons were analyzed in the thalamus and the frontal cortex. The cells of origin of thalamocortical projections to the CMAr were located mainly in the parvicellular division of the ventroanterior nucleus and the oral division of the ventrolateral nucleus (VLo). On the other hand, the thalamocortical neurons to the CMAd/CMAv were distributed predominantly in the VLo and the oral division of the ventroposterolateral nucleus-the caudal division of the ventrolateral nucleus. Additionally, many neurons in the intralaminar nuclear group were seen to project to the cingulate motor areas. Except for their well-developed interconnections, the corticocortical projections to the CMAr and CMAd/CMAv were also distinctively preferential. Major inputs to the CMAr arose from the presupplementary motor area and the dorsal premotor cortex, whereas inputs to the CMAd/CMAv originated not only from these areas but also from the supplementary motor area and the primary motor cortex. The present results indicate that the CMAr and the caudal cingulate motor area (involving both the CMAd and the CMAv) are characterized by distinct patterns of thalamocortical and intracortical connections, reflecting their functional differences.

Pathophysiology of Parkinson's disease: the MPTP primate model of the human disorder

The striatum is viewed as the principal input structure of the basal ganglia, while the internal pallidal segment (GPi) and the substantia nigra pars reticulata (SNr) are output structures. Input and output structures are linked via a monosynaptic "direct" pathway and a polysynaptic "indirect" pathway involving the external pallidal segment (GPe) and the subthalamic nucleus (STN). According to current schemes, striatal dopamine (DA) enhances transmission along the direct pathway (via D1 receptors), and reduces transmission over the indirect pathway (via D2 receptors). DA also acts on receptors in GPe, GPi, SNr, and STN. Electrophysiologic and other studies in primates rendered parkinsonian by treatment with the dopaminergic neurotoxin MPTP have demonstrated a reduction of neuronal activity of GPe and an increase of neuronal discharge in STN, GPi. and SNr. These findings are compatible with the view that striatal DA loss results in increased activity over the indirect pathway. Prominent bursting, oscillatory discharge patterns, and increased synchronization of neighboring neurons are found throughout the basal ganglia. These may result from changes in the activity of local circuits (e.g., the GPe-STN "pacemaker") or from more global abnormalities of the basal ganglia-thalamocortical network. These findings have been replicated in human patients undergoing microelectrode-guided stereotactic procedures targeted at GPi or STN. PET studies in patients with Parkinson's disease have lent further support to the proposed circuit abnormalities. The current models of basal ganglia function have recently been criticized. For instance, the strict separation of direct and indirect pathways and the segregation of D1 and D2 receptors have been questioned, and the almost complete absence of motor side effects of pallidal or thalamic lesions in human patients and animals is inconsistent. These results suggest that changes in discharge patterns and synchronization between basal ganglia neurons, abnormal network interactions, and compensatory mechanisms are at least as important in the pathophysiology of parkinsonism as changes in discharge rates in individual basal ganglia nuclei. Lesions of GPi or STN are effective in treating parkinsonism, because they reduce or abolish abnormal basal ganglia output, enabling remaining circuits to function more normally.

The cerebral oscillatory network of parkinsonian resting tremor

Data from experiments in MPTP monkeys as well as from invasive and non-invasive recordings in patients with Parkinson's disease suggest an abnormal synchronization of neuronal activity in the generation of resting tremor in Parkinson's disease. In six patients with tremor-dominant idiopathic Parkinson's disease, we recorded simultaneously surface electromyograms (EMGs) of hand muscles, and brain activity with a whole-head magnetoencephalography (MEG) system. Using a recently developed analysis tool (Dynamic Imaging of Coherent Sources; DICS), we determined cerebro-muscular and cerebro-cerebral coherence as well as the partial coherence between cerebral areas and muscle, and localized coherent sources within the individual MRI scans. The phase lag between the EMG and cerebral activity was determined by means of a Hilbert transform of both signals. After overnight withdrawal from medication, patients showed typical Parkinson's disease resting tremor (4-6 Hz). This tremor was associated with strong coherence between the EMG of forearm muscles and activity in the contralateral primary motor cortex (M1) at tremor frequency but also at double tremor frequency. Phase lags between M1 activity and EMG were between 15 and 25 ms (M1 activity leading) at single, but also at double tremor frequency, corresponding well to the corticomuscular conduction time. Furthermore, significant coherence was observed between M1 and medial wall areas (cingulate/supplementary motor area; CMA/SMA), lateral premotor cortex (PM), diencephalon, secondary somatosensory cortex (SII), posterior parietal cortex (PPC) and the contralateral cerebellum at single tremor and, even stronger at double tremor frequency. Spectra of coherence between thalamic activity and cerebellum as well as several brain areas revealed additional broad peaks around 20 Hz. Power spectral analysis of activity in all central areas indicated the strongest frequency components at double tremor frequency. Partial coherence analysis and the calculation of phase shifts revealed a strong bidirectional coupling between the EMG and diencephalic activity and a direct afferent coupling between the EMG and SII and the PPC. In contrast, the cerebellum, SMA/CMA and PM show little evidence for direct coupling with the peripheral EMG but seem to be connected with the periphery via other cerebral areas (e.g. M1). In summary, our results demonstrate tremor-related oscillatory activity within a cerebral network, with abnormal coupling in a cerebello-diencephalic-cortical loop and cortical motor (M1, SMA/CMA, PM) and sensory (SII, PPC) areas contralateral to the tremor hand. The main frequency of cerebro-cerebral coupling corresponds to double the tremor frequency.

Neurochemical organization of the human basal ganglia: anatomofunctional territories defined by the distributions of calcium-binding proteins and SMI-32

The distribution of the calcium-binding proteins calbindin-D28K (CB), parvalbumin (PV) and calretinin (CR), and of the nonphosphorylated neurofilament protein (with SMI-32) was investigated in the human basal ganglia to identify anatomofunctional territories. In the striatum, gradients of neuropil immunostaining define four major territories: The first (T1) includes all but the rostroventral half of the putamen and is characterized by enhanced matriceal PV and SMI-32 immunoreactivity (-ir). The second territory (T2) encompasses most part of the caudate nucleus (Cd) and rostral putamen (PuT), which show enhanced matriceal CB-ir. The third and fourth territories (T3 and T4) comprise rostroventral parts of Cd and PuT characterized by complementary patch/matrix distributions of CB- and CR-ir, and the accumbens nucleus (Acb), respectively. The latter is separated into lateral (prominently enhanced in CB-ir) and medial (prominently enhanced in CR-ir) subdivisions. In the pallidum, parallel gradients also delimit four territories, T1 in the caudal half of external (GPe) and internal (GPi) divisions, characterized by enhanced PV- and SMI-32-ir; T2 in their rostral half, characterized by enhanced CB-ir; and T3 and T4 in their rostroventral pole and in the subpallidal area, respectively, both expressing CB- and CR-ir but with different intensities. The subthalamic nucleus (STh) shows contrasting patterns of dense PV-ir (sparing only the most medial part) and low CB-ir. Expression of CR-ir is relatively low, except in the medial, low PV-ir, part of the nucleus, whereas SMI-32-ir is moderate across the whole nucleus. The substantia nigra is characterized by complementary patterns of high neuropil CB- and SMI-32-ir in pars reticulata (SNr) and high CR-ir in pars compacta (SNc) and in the ventral tegmental area (VTA). The compartmentalization of calcium-binding proteins and SMI-32 in the human basal ganglia, in particular in the striatum and pallidum, delimits anatomofunctional territories that are of significance for functional imaging studies and target selection in stereotactic neurosurgery.

Axon trajectories in local circuits of the primary motor cortex in the macaque monkey (Macaca fuscata)

The intrinsic trajectories and terminal arbors of two axons and one horizontal axon collateral within the primary motor cortex (M1) were studied in the macaque monkey using injections of biotinylated dextran amine (BDA) into the putative primary forelimb motor cortex, and two-dimensional (2-D) reconstruction of the individually labeled axons and collateral. (1) A long collateral of the main axon from a large pyramidal cell in layer Vb of the putative forelimb area on the anterior bank of the central sulcus coursed horizontally anteriorly for 3 mm and formed a terminal arbor in layer III of M1. (2) The main axon of a pyramidal cell in layer IIIa+b of the putative forelimb area on the precentral gyrus descended into the white matter and then entered the anterior bank of the central sulcus to form a terminal arbor in layers III and V. (3) The main axon of a pyramidal cell in layer IIIc of the putative forelimb area on the precentral gyrus descended and bifurcated in the white matter. One branch entered the anterior bank of the central sulcus to form a terminal field in layer VI. These results indicate that some local axons and horizontal axon collaterals arising from M1 reach their single targets within M1 to form single terminal fields.

Delayed onset of hemidystonia and hemiballismus following head injury: a clinicopathological correlation. Case report

The authors report the case of a young man who suffered multiple injuries in a motor vehicle accident, the most significant of which arose in the brain, creating an unusual clinical syndrome. After experiencing an initial coma for several days, the patient was found to have a right-sided homonymous hemianopsia and a right hemiparesis, which was more marked at the shoulder and was accompanied by preservation of finger movement. Dystonic movements appeared 2 months later and progressed, along with increased spasticity on volition, to severe uncontrolled arm movements at 2 years postinjury. This motor disorder continued to worsen during the following 6 years prior to the patient's death. At autopsy, the left side of the brain was observed to have marked atrophy of the optic tract, a partial lesion of the posterior portion of the medial segment of the globus pallidus (GP), and a reduction in the size of the internal capsule at the level of the GP, suggesting impaired circulation to these areas at the time of injury. The isolated lesion of the internal segment of the GP was the presumed cause of the dystonia, acting through an alteration in thalamic inhibition. The atrophic subthalamic nucleus was the probable cause of the hemiballismus. The authors speculate that this and other delayed and progressive features of this case were the result of an active, but disordered, adaptive process that failed to compensate and, instead, caused even greater problems than the original injury.

Temporary inactivation in the primate motor thalamus during visually triggered and internally generated limb movements

To better understand the contribution of cerebellar- and basal ganglia-receiving areas of the thalamus [ventral posterolateral nucleus, pars oralis (VPLo), area X, ventral lateral nucleus, pars oralis (VLo), or ventral anterior nucleus, pars parvicellularis (VApc)] to movements based on external versus internal cues, we temporarily inactivated these individual nuclei in two monkeys trained to make visually triggered (VT) and internally generated (IG) limb movements. Infusions of lignocaine centered within VPLo caused hemiplegia during which movements of the contralateral arm rarely were performed in either task for a short period of time ( approximately 5-30 min). When VT responses were produced, they had prolonged reaction times and movement times and a higher incidence of trajectory abnormalities compared with responses produced during the preinfusion baseline period. In contrast, those IG responses that were produced remained relatively normal. Infusions centered within area X never caused hemiplegia. The only deficits observed were an increase in reaction time and movement amplitude variability and a higher incidence of trajectory abnormalities during VT trials. Every other aspect of both the VT and IG movements remained unchanged. Infusions centered within VLo reduced the number of movements attempted during each block of trials. This did not appear to be due to hemiplegia, however, as voluntary movements easily could be elicited outside of the trained tasks. The other main deficit resulting from inactivation of VLo was an increased reaction time in the VT task. Finally, infusions centered within VApc caused IG movements to become slower and smaller in amplitude, whereas VT movements remained unchanged. Control infusions with saline did not cause any consistent deficits. This pattern of results implies that VPLo and VLo play a role in the production of movements in general regardless of the context under which they are performed. They also suggest that VPLo contributes more specifically to the execution of movements that are visually triggered and guided, whereas area X contributes specifically to the initiation of such movements. In contrast, VApc appears to play a role in the execution of movements based on internal cues. These results are consistent with the hypothesis that specific subcircuits within the cerebello- and basal ganglio-thalamo-cortical systems preferentially contribute to movements based on external versus internal cues.

Neuronal activity in the primate motor thalamus during visually triggered and internally generated limb movements

Single-unit recordings were made from the basal-ganglia- and cerebellar-receiving areas of the thalamus in two monkeys trained to make arm movements that were either visually triggered (VT) or internally generated (IG). A total of 203 neurons displaying movement-related changes in activity were examined in detail. Most of these cells (69%) showed an increase in firing rate in relation to the onset of movement and could be categorized according to whether they fired in the VT task exclusively, in the IG task exclusively, or in both tasks. The proportion of cells in each category was found to vary between each of the cerebellar-receiving [oral portion of the ventral posterolateral nucleus (VPLo) and area X] and basal-ganglia-receiving [oral portion of the ventral lateral nucleus (VLo) and parvocellular portion of the ventral anterior nucleus (VApc)] nuclei that were examined. In particular, in area X the largest group of cells (52%) showed an increase in activity during the VT task only, whereas in VApc the largest group of cells (53%) fired in the IG task only. In contrast to this, relatively high degree of task specificity, in both VPLo and VLo the largest group of cells ( approximately 55%) burst in relation to both tasks. Of the cells that were active in both tasks, a higher proportion were preferentially active in the VT task in VPLo and area X, and the IG task in VLo and VApc. In addition, cells in all four nuclei became active earlier relative to movement onset in the IG task compared with the VT task. These results demonstrate that functional distinctions do exist in the cerebellar- and basal-ganglia-receiving portions of the primate motor thalamus in relation to the types of cues used to initiate and control movement. These distinctions are most clear in area X and VApc, and are much less apparent in VPLo and VLo.

Origin of thalamic inputs to the primary, premotor, and supplementary motor cortical areas and to area 46 in macaque monkeys: a multiple retrograde tracing study

The origin of thalamic inputs to distinct motor cortical areas was established in five monkeys to determine whether the motor areas receive inputs from a common thalamic nucleus and the extent to which the territories of origin overlap. To not rely on the rough definition of cytoarchitectonic boundaries in the thalamus, monkeys were subjected to multiple injections of tracers (four to seven) in the primary (M1), premotor (PM), and supplementary (SMA) motor cortical areas and in area 46. The cortical areas were distributed into five groups, each receiving inputs from a specific set of thalamic nuclei: 1) M1; 2) SMA-proper and the caudal part of the dorsal PM (PMdc); 3) the rostral and caudal parts of the ventral PM (PMvr and PMvc); 4) the rostral part of the dorsal PM (PMdr); and 5) the superior and inferior parts of area 46 (area 46sup and area 46inf). A major degree of overlap was obtained for the origins of the thalamocortical projections directed to areas 46inf and 46sup and for those terminating in SMA-proper and PMdc. PMvc and PMvr received inputs from adjacent and/or common thalamic regions. In contrast, the degree of overlap between M1 and SMA was smaller. The projection to M1 shared relatively limited zones of origin with the projections directed to PM. Thalamic inputs to the motor cortical areas (M1, SMA, PMd, and PMv), in general, were segregated from those directed to area 46, except in the mediodorsal nucleus, in which there was clear overlap of the territories sending projections to area 46, SMA-proper, and PMdc.

The organization of cerebellar and basal ganglia outputs to primary motor cortex as revealed by retrograde transneuronal transport of herpes simplex virus type 1

We used retrograde transneuronal transport of herpes simplex virus type 1 to map the origin of cerebellar and basal ganglia "projections" to leg, arm, and face areas of the primary motor cortex (M1). Four to five days after virus injections into M1, we observed many densely labeled neurons in localized regions of the output nuclei of the cerebellum and basal ganglia. The largest numbers of these neurons were found in portions of the dentate nucleus and the internal segment of the globus pallidus (GPi). Smaller numbers of labeled neurons were found in portions of the interpositus nucleus and the substantia nigra pars reticulata. The distribution of neuronal labeling varied with the cortical injection site. For example, within the dentate, neurons labeled from leg M1 were located rostrally, those from face M1 caudally, and those from arm M1 at intermediate levels. In each instance, labeled neurons were confined to approximately the dorsal third of the nucleus. Within GPi, neurons labeled from leg M1 were located in dorsal and medial regions, those from face M1 in ventral and lateral regions, and those from arm M1 in intermediate regions. These results demonstrate that M1 is the target of somatotopically organized outputs from both the cerebellum and basal ganglia. Surprisingly, the projections to M1 originate from only 30% of the volume of the dentate and <15% of GPi. Thus, the majority of the outputs from the cerebellum and basal ganglia are directed to cortical areas other than M1.

Dual morphology and topography of the corticothalamic terminals originating from the primary, supplementary motor, and dorsal premotor cortical areas in macaque monkeys

In the motor, somatosensory, and auditory systems of rodents and cats, the corticothalamic connection is composed of a main projection formed by small endings and a minor projection terminating with giant endings. To establish whether the corticothalamic projection originating from motor cortical areas in primates exhibits the same duality, the anterograde tracer biotinylated dextran amine was injected in eight macaque monkeys in the primary motor (M1; n = 3), the supplementary motor (SMA; n = 3) and the dorsal premotor (PMd; n = 2) cortical areas to label corticothalamic axons. The corticothalamic projection originating from these three motor cortical areas was characterized by the presence of axon terminals constituting the same two types of endings, observed both as boutons en passant and terminaux. The population of small endings exhibited a mean cross-sectional maximum diameter of 0.95 microm (S.D. = 0.23), a range of diameters not overlapping that of giant endings (mean diameter = 3.46 microm, S.D. = 0.74 microm). Topographically, the giant endings originating from M1 were located in the same thalamic nucleus (ventroposterolateral nucleus, oral part) in which the small endings were found. In contrast, the giant endings originating from SMA and PMd were located in a thalamic nucleus (mediodorsal nucleus) distinct from the main termination zone formed by small endings. Along the rostrocaudal axis, the giant endings were distributed in a restricted zone, irrespective of the origin of the projection (M1, SMA, PMd). The dual morphology of corticothalamic endings, previously found in rodents and cats, is present in the motor system of subhuman primates for both primary and nonprimary motor cortical areas.

Reevaluation of ipsilateral corticocortical inputs to the orofacial region of the primary motor cortex in the macaque monkey

An anatomical approach to possible areas in the cerebral cortex involved in somatic motor behavior is to analyze the cortical areas containing neurons that connect directly to the primary motor cortex (MI). To define the cortical areas related to orofacial movements, we examined the distribution of cortical neurons that send their axons to the orofacial region of the MI in the macaque monkey. Injections of retrograde tracers into the electrophysiologically identified orofacial region of the MI revealed that labeled neurons were distributed in the following cortical areas: the orbital cortex (area 12), insular cortex, frontoparietal operculum (including the deep part of the cortical masticatory area and the secondary somatosensory cortex), ventral division of the premotor cortex (especially in its lateral part), orofacial region of the supplementary motor area, rostral division of the cingulate motor area (CMA), and CMA on the ventral bank. A number of labeled neurons were also seen in the MI around the injection sites and in the parietal cortex (including the primary somatosensory cortex and area 7b). No labeled neurons were found in the dorsal division of the premotor cortex. Fluorescent retrograde double labeling further revealed virtually no overlap of distribution between cortical neurons projecting to the orofacial and forelimb regions of the MI. Based on the present results, we discuss the functional diversity of the cortical areas related to orofacial motor behavior and the somatotopical organization in the premotor areas of the frontal cortex.

Changes in human motor cortex excitability induced by dopaminergic and anti-dopaminergic drugs

Transcranial magnetic stimulation was used to probe the acute effect of a single oral dose of various dopaminergic (levodopa, selegiline, bromocriptine) and antidopaminergic drugs (sulpiride, haloperidol) on motor cortex excitability in healthy volunteers. Motor threshold, intracortical inhibition and intracortical facilitation were tested in the abductor digiti minimi muscle. The latter two parameters were studied in a conditioning-test paired stimulus paradigm. The principal findings were an increase in intracortical inhibition by bromocriptine, and, conversely, a decrease in intracortical inhibition and an increase in intracortical facilitation by haloperidol. Effects peaked at delays consistent with the pharmacokinetics of the two drugs and were fully reversible. In conclusion, dopamine receptor agonists and antagonists can be considered inverse modulators of motor cortex excitability: the former enhance inhibition while the latter reduce it. The relation of the present findings to current models of motor excitability abnormalities in movement disorders will be discussed.

Multiarchitectonic and stereotactic atlas of the human thalamus

To improve anatomical definition and stereotactic precision of thalamic targets in neurosurgical treatments of chronic functional disorders, a new atlas of the human thalamus has been developed. This atlas is based on multiarchitectonic parcellation in sections parallel or perpendicular to the standard intercommissural reference plane. The calcium-binding proteins parvalbumin (PV), calbindin D-28K (CB), and calretinin (CR) were used as neurochemical markers to further characterize thalamic nuclei and delimit subterritories of functional significance for stereotactic explorations. Their overall distribution reveals a subcompartmentalization of thalamic nuclei into several groups. Predominant PV immunostaining characterizes primary somatosensory, visual and auditory nuclei, the ventral lateral posterior nucleus, reticular nucleus (R), and to a lesser degree also, lateral part of the centre median nucleus, and anterior, lateral, and inferior divisions of the pulvinar complex. In contrast, CB immunoreactivity is prevalent in medial thalamic nuclei (intralaminar and midline), the posterior complex, ventral posterior inferior nucleus, the ventral lateral anterior nucleus, ventral anterior, and ventral medial nuclei. The complementary distributions of PV and CB appear to correlate with distinct lemniscal and spinothalamic somatosensory pathways and to cerebellar and pallidal motor territories, respectively. Calretinin, while overlapping with CB in medial thalamic territories, is also expressed in R and limbic associated anterior group nuclei that contain little or no CB. Preliminary analysis indicates that interindividual nuclear variations cannot easily be taken into account by standardization procedures. Nevertheless, some corrections in antero-posterior coordinates in relation to different intercommissural distances are proposed.

Pallidal afferent territory of the Macaca mulatta thalamus: neuronal and synaptic organization of the VAdc

Ventral anterior thalamic nucleus pars densicellularis (VAdc) as delineated earlier (Ilinsky and Kultas-Ilinsky [1987] J. Comp. Neurol. 262:331-364) was analyzed by using qualitative and quantitative neuroanatomical techniques. Projection neurons (PN), retrogradely labeled with wheat germ agglutinin conjugated horseradish peroxidase from the cortex, were small to medium in size (mean area, 312 microm2) with numerous primary dendrites displaying a tufted branching pattern. Local circuit neurons (LCN), immunoreactive for gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase, were small (mean area, 110 microm2), and gave off few dendrites. Two subpopulations of GABA positive boutons (F1 type) were distinguished: large (mean area, 2.6 microm2) terminals with symmetric synapses containing few pleomorphic vesicles and numerous mitochondria densely covered proximal PN sites; smaller F1 boutons with a slightly different morphology contacted mostly distal PN dendrites. Two subpopulations of terminals containing round vesicles and forming asymmetric synapses were distinguished by bouton size (mean areas, 0.4 microm2 and 1.6 microm2, respectively). These targeted mainly distal PN dendrites, but some synapsed proximally next to large F1 boutons. On distal dendrites, representatives of both types were labeled from the cortex. The density of boutons with symmetric and asymmetric synapses (the number of boutons per 100 microm of PN membrane length) was 3.3:0.2 on primary, 2.5:1.2 on secondary, and 0.8:12 on distal dendrites. The numerical density of synapses formed by presynaptic LCN dendrites on all PN levels was 20 to 40 times less than that of axon terminals at the same sites. Afferent input to LCN from boutons of all types, including that from 50% of labeled cortical boutons, mainly targeted distal dendrites. Overall, the findings suggest that PN in VAdc receive massive inhibitory input proximally intermingled with some presumably excitatory input, and that LCN contribution to PN inhibition is modest.

Identification of motor areas of the cat cerebral cortex based on studies of cortical stimulation and corticospinal connections

The location and topography of motor areas in the cat cerebral cortex were studied by electrical stimulation of the cortex in five animals, and by the injection of retrograde tracers into the spinal cord of four animals. Movements evoked by intracortical microstimulation (ICMS) of the anterior, posterior and lateral sigmoid gyri, both banks of the cruciate sulcus and the dorsal bank of the presylvian sulcus were observed in anaesthetized cats. Fluorescent tracers (Fast Blue and/or Diamadino Yellow) were injected into the lateral funiculus in the second cervical segment, into the gray matter of cervical segments C3-T1 and/or into the gray matter of lumbar segments L2-S1. Contraction of the contralateral forelimb, hindlimb or facial muscles was observed following electrical stimulation of several cytoarchitectonic areas: 4 gamma, 4 delta, 6a alpha, 6a gamma, and 3a. These findings suggested representations of contralateral forelimb and hindlimb movements in areas 4 gamma and 4 delta, and of the contralateral forelimb muscles in areas 6a alpha and 6a gamma. Corticospinal neurons were located in all the above cytoarchitectonic areas as well as in areas 3b, 1, 2, 2pri, and 5. Large numbers of neurons were labeled in areas 4 gamma and 4 delta, and moderate labeling was observed in areas 6a gamma and 6a alpha. Corticospinal neurons projecting to cervical and lumbar segments were located in areas 4 gamma and 4 delta, while those projecting only to cervical segments were detected in areas 6a alpha and 6a gamma. Based on these findings it is proposed that within the motor cortex of the cat there are representations of limb movements in several cytoarchitectonic subdivisions. Many of these representations may be candidate secondary motor areas.

Functional properties of primate putamen neurons during the categorization of tactile stimuli

We used psychometric techniques and neurophysiological recordings to study the role of the putamen in somesthetic perception. Four monkeys were trained to categorize the speed of moving tactile stimuli. Animals performed a task in which one of two target switches had to be pressed with the right hand to indicate whether the speed of probe movement across the glabrous skin of the left, restrained hand was low or high. During the task we recorded the activity of neurons in the putamen contralateral (right) and ipsilateral (left) to the stimulated hand. We found different types of neuronal responses, all present in the right and left putamen. Some neurons responded during the stimulus period, others responded during the hand-arm movement used to indicate categorization, and others responded during both of these periods. The responses of many neurons did not vary either with the speed of the stimuli or in relation to the categorization process. In contrast, neurons of a particular type responded differentially: their activity reflected whether stimulus speed was low or high. These differential responses occurred during the stimulus and hand-arm motion periods. A number of the nondifferential and differential neurons were studied when the same stimuli used in the categorization task were delivered passively. Few neurons with nondifferential discharges, and none of the differential neurons, responded in this condition. In a visually cued control task we studied the possibility that the differential responses were associated with the intention to press or with the trajectory of the hand to one of the target switches. In this condition, a light turned on instructed the animal which target switch to press for a reward. Very few neurons in both hemispheres maintained the differential responses observed during the categorization task. Those neurons that discharged selectively for low or high speeds were analyzed quantitatively to produce a measure comparable with the psychometric function. The thresholds of the resulting neurometric curves for the neuronal populations were very similar to the psychometric thresholds. The activity of a large fraction of these neurons could be used to accurately predict whether the stimulus speed was low or high. The results indicate that the putamen, both contralateral and ipsilateral to the stimulated hand, contains neurons that discharge in response to the somesthetic stimuli during the categorization task. Those neurons that respond irrespective of the stimulus speed appear to be involved in the general sensorimotor behavior of the animal during the execution of the task. The results suggest that the putamen may play a role in bimanual tasks. The recording of neurons in the right and left putamen whose activities correlate with the speed categories suggests that this region of the basal ganglia, in addition to its role in motor functions, is also involved in the animal's decision process.

Neural Networks and Motor Control

Motor control is accomplished by the cooperative interaction of many brain networks, among which the motor cortex holds a central place. This article reviews some of the structural and functional properties of neurons of the motor cortical network, some principles of connectivity with other motor networks, the handling of spatial information regarding reaching movements, and some ideas on how motor cortical commands could be translated to muscle activations by spinal motor networks. Finally, I review recent neural network modeling studies of motor cortical ensemble operations.

New concepts of the supplementary motor area

The supplementary motor area, although traditionally defined as a single motor area, is now viewed as including at least three different areas that can be distinguished anatomically and physiologically. The differential use of these three areas for various motor behaviors has been the subject of recent studies that are beginning to provide novel concepts of the functional differentiation of each area.

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.

Overlapping corticostriatal projections from the supplementary motor area and the primary motor cortex in the macaque monkey: an anterograde double labeling study

The purpose of the present study was to determine if the cortical efferents from homologous body regions of the supplementary motor area (SMA) and the primary motor cortex (MI) project to separate or to overlapping regions in the striatum. In order to investigate the dual corticostriatal projections, we employed an anterograde double labeling paradigm in which two tracers could be simultaneously detected in the same histological section. Prior to the injections, the forelimb representation in the two cortical motor areas was identified by using intracortical microstimulation in four Japanese monkeys (Macaca fuscata). Multiple injections of biotinylated dextran amine (BDA) were made into the forelimb regions of MI and wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into the arm region of the SMA. In additional animals, the tracers were reversed such that BDA was injected into the SMA and WGA-HRP was injected into the MI. The tissue was processed sequentially using different chromogens in order to visualize both tracers in a single section. We analyzed the distribution of the ipsilateral anterograde label. The striatal labeling from each cortical area basically consisted of a wide band of patchy dense labeling interrupted by lighter labeling. The SMA striatal projections were located mainly within the putamen, distributing from the level of the anterior commissure to the most posterior extent of the putamen. At an intermediate level, the label spread obliquely from the ventrolateral edge of the putamen dorsomedially as far as the lateral edge of the caudate nucleus. The label from the MI was observed in comparable portions of the putamen, although the SMA projections were shifted more anterior and dorsomedial to the MI projections and the heaviest projections from the SMA and the MI were separately located. On the basis of the double anterograde labeling technique, we found considerable overlap mainly in the central portion of the putamen from the SMA and MI forelimb representation. These results suggest that the homologous body regions of the SMA and MI send widespread, and substantially overlapping projections, to portions of the striatum.