Comparison of cerebellothalamic and pallidothalamic projections in the monkey (Macaca fuscata): a double anterograde labeling study

Reduced reach-related modulation of motor thalamus neural activity in a rat model of Parkinson's disease

Motor thalamus (Mthal) is a key node in the corticobasal ganglia (BG) loop that controls complex, cognitive aspects of movement. In Parkinson's disease (PD), profound alterations in neuronal activity occur in BG nuclei and cortex. Because Mthal is located between these two structures, altered Mthal activity has been assumed to underlie the pathogenesis of PD motor deficits. However, to date, inconsistent changes in neuronal firing rate and pattern have been reported in parkinsonian animals. Moreover, although a distinct firing pattern of Mthal neurons, called low-threshold calcium spike bursts (LTS bursts), is observed in reduced preparations, it remains unknown whether they occur or what their role might be in behaving animals. We recorded Mthal spiking activity in control and unilateral 6-hydroxydopamine lesioned rats performing a skilled forelimb-reaching task. We show for the first time that Mthal firing rate in control rats is modulated in a temporally precise pattern during reach-to-grasp movements, with a peak at the time of the reach-end and troughs just before and after it. We identified LTS-like events on the basis of LTS burst characteristics. These were rare, but also modulated, decreasing in incidence just after reach-end. The inhibitory modulations in firing rate and LTS-like events were abolished in parkinsonian rats. These data confirm that nigrostriatal dopamine depletion is accompanied by profound and specific deficits in movement-related Mthal activity. These changes would severely impair Mthal contributions to motor program development in motor cortex and are likely to be an important factor underlying the movement deficits of PD.

Frequency-dependent functional neuromodulatory effects on the motor network by ventral lateral thalamic deep brain stimulation in swine

Thalamic deep brain stimulation (DBS) is an FDA-approved neurosurgical treatment for medication-refractory essential tremor. Its therapeutic benefit is highly dependent upon stimulation frequency and voltage parameters. We investigated these stimulation parameter-dependent effects on neural network activation by performing functional magnetic resonance imaging (fMRI) during DBS of the ventral lateral (VL) thalamus and comparing the blood oxygenation level-dependent (BOLD) signals induced by multiple stimulation parameter combinations in a within-subject study of swine. Low (10 Hz) and high (130 Hz) frequency stimulation was applied at 3, 5, and 7 V in the VL thalamus of normal swine (n = 5). We found that stimulation frequency and voltage combinations differentially modulated the brain network activity in the sensorimotor cortex, the basal ganglia, and the cerebellum in a parameter-dependent manner. Notably, in the motor cortex, high frequency stimulation generated a negative BOLD response, while low frequency stimulation increased the positive BOLD response. These frequency-dependent differential effects suggest that the VL thalamus is an exemplary target for investigating functional network connectivity associated with therapeutic DBS.

Motor thalamus integration of cortical, cerebellar and basal ganglia information: implications for normal and parkinsonian conditions

Motor thalamus (Mthal) is implicated in the control of movement because it is strategically located between motor areas of the cerebral cortex and motor-related subcortical structures, such as the cerebellum and basal ganglia (BG). The role of BG and cerebellum in motor control has been extensively studied but how Mthal processes inputs from these two networks is unclear. Specifically, there is considerable debate about the role of BG inputs on Mthal activity. This review summarizes anatomical and physiological knowledge of the Mthal and its afferents and reviews current theories of Mthal function by discussing the impact of cortical, BG and cerebellar inputs on Mthal activity. One view is that Mthal activity in BG and cerebellar-receiving territories is primarily "driven" by glutamatergic inputs from the cortex or cerebellum, respectively, whereas BG inputs are modulatory and do not strongly determine Mthal activity. This theory is steeped in the assumption that the Mthal processes information in the same way as sensory thalamus, through interactions of modulatory inputs with a single driver input. Another view, from BG models, is that BG exert primary control on the BG-receiving Mthal so it effectively relays information from BG to cortex. We propose a new "super-integrator" theory where each Mthal territory processes multiple driver or driver-like inputs (cortex and BG, cortex and cerebellum), which are the result of considerable integrative processing. Thus, BG and cerebellar Mthal territories assimilate motivational and proprioceptive motor information previously integrated in cortico-BG and cortico-cerebellar networks, respectively, to develop sophisticated motor signals that are transmitted in parallel pathways to cortical areas for optimal generation of motor programmes. Finally, we briefly review the pathophysiological changes that occur in the BG in parkinsonism and generate testable hypotheses about how these may affect processing of inputs in the Mthal.

Cerebellar networks with basal ganglia: feasibility for tracking cerebello-pallidal and subthalamo-cerebellar projections in the human brain

Neuroanatomical studies using transneuronal virus tracers in macaque monkeys recently demonstrated that substantial interactions exist between basal ganglia and the cerebellum. To what extent these interactions are present in the human brain remains unclear; however, these connections are thought to provide an important framework for understanding cerebellar contributions to the manifestation of basal ganglia disorders, especially with respect to tremor genesis in movement disorders such as Parkinson's disease. Here, we tested the feasibility of assessing these connections in vivo and non-invasively in the human brain with diffusion magnetic resonance imaging and tractography. After developing a standardized protocol for manual segmentation of basal ganglia and cerebellar structures, masks for diffusion tractography were defined based on structural magnetic resonance images. We tested intra- and inter-observer stability and carried out tractography for dentato-pallidal and subthalamo-cerebellar projections. After robustly achieving connection probabilities per tract, the connectivity values and connectional fingerprints were calculated in a group of healthy volunteers. Probabilistic diffusion tractography was applicable to probe the inter-connection of the cerebellum and basal ganglia. Our data confirmed that dentato-thalamo-striato-pallidal and subthalamo-cerebellar connections also exist in the human brain at a level similar to those that were recently suggested by transneuronal tracing studies in non-human primates. Standardized segmentation protocols made these findings reproducible with high stability. We have demonstrated that diffusion tractography in humans in vivo is capable of revealing the structural bases of cerebellar networks with the basal ganglia. These findings support the role of the cerebellum as a satellite system of established cortico-basal ganglia networks in humans.

Frontal-thalamic circuits associated with language

Thalamic nuclei associated with language including the ventral lateral, ventral anterior, intralaminar and mediodorsal form a hub that uniquely receives the output of the basal ganglia and cerebellum, and is connected with frontal (premotor and prefrontal) cortices through two parallel circuits: a thalamic pathway targets the middle frontal cortical layers focally, and the other innervates widely cortical layer 1, poised to recruit other cortices and thalamic nuclei for complex cognitive operations. Return frontal pathways to the thalamus originate from cortical layers 6 and 5. Information through this integrated thalamo-cortical system is gated by the inhibitory thalamic reticular nucleus and modulated by dopamine, representing a specialization in primates. The intricate dialogue of distinct thalamic nuclei with the basal ganglia, cerebellum, and specific dorsolateral prefrontal and premotor cortices associated with language, suggests synergistic roles in the complex but seemingly effortless sequential transformation of cognitive operations for speech production in humans.

Cerebellar networks with the cerebral cortex and basal ganglia

The dominant view of cerebellar function has been that it is exclusively concerned with motor control and coordination. Recent findings from neuroanatomical, behavioral, and imaging studies have profoundly changed this view. Neuroanatomical studies using virus transneuronal tracers have demonstrated that cerebellar output reaches vast areas of the neocortex, including regions of prefrontal and posterior parietal cortex. Furthermore, it has recently become clear that the cerebellum is reciprocally connected with the basal ganglia, which suggests that the two subcortical structures are part of a densely interconnected network. Taken together, these findings elucidate the neuroanatomical substrate for cerebellar involvement in non-motor functions mediated by the prefrontal and posterior parietal cortex, as well as in processes traditionally associated with the basal ganglia.

Biology of Parkinson's disease: pathogenesis and pathophysiology of a multisystem neurodegenerative disorder

Parkinson's disease (PD) is the second most common movement disorder. The characteristic motor impairments - bradykinesia, rigidity, and resting tremor - result from degenerative loss of midbrain dopamine (DA) neurons in the substantia nigra, and are responsive to symptomatic treatment with dopaminergic medications and functional neurosurgery. PD is also the second most common neurodegenerative disorder. Viewed from this perspective, PD is a disorder of multiple functional systems, not simply the motor system, and of multiple neurotransmitter systems, not merely that of DA. The characteristic pathology - intraneuronal Lewy body inclusions and reduced numbers of surviving neurons - is similar in each of the targeted neuron groups, suggesting a common neurodegenerative process. Pathological and experimental studies indicate that oxidative stress, proteolytic stress, and inflammation figure prominently in the pathogenesis of PD. Yet, whether any of these mechanisms plays a causal role in human PD is unknown, because to date we have no proven neuroprotective therapies that slow or reverse disease progression in patients with PD. We are beginning to understand the pathophysiology of motor dysfunction in PD, but its etiopathogenesis as a neurodegenerative disorder remains poorly understood.

Mapping motor representations in the human cerebellum

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

Contrasting connectivity of the ventralis intermedius and ventralis oralis posterior nuclei of the motor thalamus demonstrated by probabilistic tractography

BACKGROUND

Targeting of the motor thalamus for the treatment of tremor has traditionally been achieved by a combination of anatomical atlases and neuroimaging, intraoperative clinical assessment, and physiological recordings.

OBJECTIVE

To evaluate whether thalamic nuclei targeted in tremor surgery could be identified by virtue of their differing connections with noninvasive neuroimaging, thereby providing an extra factor to aid successful targeting.

METHODS

Diffusion tensor tractography was performed in 17 healthy control subjects using diffusion data acquired at 1.5-T magnetic resonance imaging (60 directions, b value = 1000 s/mm, 2 × 2 × 2-mm³ voxels). The ventralis intermedius (Vim) and ventralis oralis posterior (Vop) nuclei were identified by a stereotactic neurosurgeon, and these sites were used as seeds for probabilistic tractography. The expected cortical connections of these nuclei, namely the primary motor cortex (M1) and contralateral cerebellum for the Vim and M1, the supplementary motor area, and dorsolateral prefrontal cortex for the Vop, were determined a priori from the literature.

RESULTS

Tractogram signal intensity was highest in the dorsolateral prefrontal cortex and supplementary motor area after Vop seeding (P < .001, Wilcoxon signed-rank tests). High intensity was seen in M1 after seeding of both nuclei but was greater with Vim seeding (P < .001). Contralateral cerebellar signal was highest with Vim seeding (P < .001).

CONCLUSION

Probabilistic tractography can depict differences in connectivity between intimate nuclei within the motor thalamus. These connections are consistent with published anatomical studies; therefore, tractography may provide an important adjunct in future targeting in tremor surgery.

Unravelling cerebellar pathways with high temporal precision targeting motor and extensive sensory and parietal networks

Increasing evidence has implicated the cerebellum in providing forward models of motor plants predicting the sensory consequences of actions. Assuming that cerebellar input to the cerebral cortex contributes to the cerebro-cortical processing by adding forward model signals, we would expect to find projections emphasising motor and sensory cortical areas. However, this expectation is only partially met by studies of cerebello-cerebral connections. Here we show that by electrically stimulating the cerebellar output and imaging responses with functional magnetic resonance imaging, evoked blood oxygen level-dependant activity is observed not only in the classical cerebellar projection target, the primary motor cortex, but also in a number of additional areas in insular, parietal and occipital cortex, including sensory cortical representations. Further probing of the responses reveals a projection system that has been optimized to mediate fast and temporarily precise information. In conclusion, both the topography of the stimulation effects and its emphasis on temporal precision are in full accordance with the concept of cerebellar forward model information modulating cerebro-cortical processing.

Differentiated participation of thalamocortical subnetworks in slow/spindle waves and desynchronization

During sleep, the electroencephalogram exhibits synchronized slow waves that desynchronize when animals awaken [desynchronized states (DSs)]. During slow-wave states, the membrane potentials of cortical neurons oscillate between discrete depolarized states ("Up states") and periods of hyperpolarization ("Down states"). To determine the role of corticothalamic loops in generating Up/Down oscillations in rats, we recorded unit activities of layer 5 (L5) corticothalamic (CTh) cells in the frontal cortex, neurons in the thalamic reticular nucleus, and basal ganglia- and cerebellum-linked thalamic relay nuclei, while simultaneously monitoring the local cortical field potential to identify slow-wave/spindle oscillations and desynchronization. We found that (1) some basal ganglia-linked and reticular thalamic cells fire preferentially near the beginning of Up states; (2) thalamic cells fire more selectively at a given Up-state phase than do CTh cells; (3) CTh and thalamic cells exhibit different action potential timings within spindle cycles; and (4) neurons exhibit different firing characteristics when comparing their activity during Up states and DSs. These data demonstrate that cortico-thalamo-cortical subnetworks are temporally differentiated during slow and spindle oscillations, that the basal ganglia-linked thalamic nuclei are closely related with Up-state initiation, and that Up states and DSs are distinguished as different depolarization states of neurons within the network.

Cerebellar processing of sensory inputs primes motor cortex plasticity

Plasticity of the human primary motor cortex (M1) has a critical role in motor control and learning. The cerebellum facilitates these functions using sensory feedback. We investigated whether cerebellar processing of sensory afferent information influences the plasticity of the primary motor cortex (M1). Theta-burst stimulation protocols (TBS), both excitatory and inhibitory, were used to modulate the excitability of the posterior cerebellar cortex and to condition an ongoing M1 plasticity. M1 plasticity was subsequently induced in 2 different ways: by paired associative stimulation (PAS) involving sensory processing and TBS that exclusively involves intracortical circuits of M1. Cerebellar excitation attenuated the PAS-induced M1 plasticity, whereas cerebellar inhibition enhanced and prolonged it. Furthermore, cerebellar inhibition abolished the topography-specific response of PAS-induced M1 plasticity, with the effects spreading to adjacent motor maps. Conversely, cerebellar excitation had no effect on the TBS-induced M1 plasticity. This demonstrates the key role of the cerebellum in priming M1 plasticity, and we propose that it is likely to occur at the thalamic or olivo-dentate nuclear level by influencing the sensory processing. We suggest that such a cerebellar priming of M1 plasticity could shape the impending motor command by favoring or inhibiting the recruitment of several muscle representations.

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.

Cognitive consilience: primate non-primary neuroanatomical circuits underlying cognition

Interactions between the cerebral cortex, thalamus, and basal ganglia form the basis of cognitive information processing in the mammalian brain. Understanding the principles of neuroanatomical organization in these structures is critical to understanding the functions they perform and ultimately how the human brain works. We have manually distilled and synthesized hundreds of primate neuroanatomy facts into a single interactive visualization. The resulting picture represents the fundamental neuroanatomical blueprint upon which cognitive functions must be implemented. Within this framework we hypothesize and detail 7 functional circuits corresponding to psychological perspectives on the brain: consolidated long-term declarative memory, short-term declarative memory, working memory/information processing, behavioral memory selection, behavioral memory output, cognitive control, and cortical information flow regulation. Each circuit is described in terms of distinguishable neuronal groups including the cerebral isocortex (9 pyramidal neuronal groups), parahippocampal gyrus and hippocampus, thalamus (4 neuronal groups), basal ganglia (7 neuronal groups), metencephalon, basal forebrain, and other subcortical nuclei. We focus on neuroanatomy related to primate non-primary cortical systems to elucidate the basis underlying the distinct homotypical cognitive architecture. To display the breadth of this review, we introduce a novel method of integrating and presenting data in multiple independent visualizations: an interactive website (http://www.frontiersin.org/files/cognitiveconsilience/index.html) and standalone iPhone and iPad applications. With these tools we present a unique, annotated view of neuroanatomical consilience (integration of knowledge).

Glutamic acid decarboxylase isoform 65 immunoreactivity in the motor thalamus of humans and monkeys: γ-aminobutyric acidergic connections and nuclear delineations

The neurotransmitter γ-aminobutyric acid (GABA) plays an important role in the motor thalamic nuclei. This report analyzes the distribution of the GABA-producing enzyme glutamic acid decarboxylase isoform 65 (GAD65), stained with monoclonal antibody, in human and rhesus monkey thalami and compares it with staining patterns of some widely used cytoskeletal and calcium binding protein markers. GAD65 immunoreactivity distinctly labeled two systems: fibers and terminals of basal ganglia thalamic afferents and local circuit neurons, revealing fine features of GABAergic circuitry in the human thalamus. Gross distribution patterns of GAD65 were identical in human and rhesus monkey thalami. The area displaying specific staining of large-caliber beaded fibers coincided with nigro- and pallidothalamic afferent territories previously identified in monkeys with anterograde tracers. Accordingly, a similarly stained region in the human thalamus was considered basal ganglia territory. Except for cytoarchitecture, no specific markers differentiating between the nigro- and pallidothalamic projection zones within this territory were found. GAD65 staining in the cerebellar afferent territory reflected organization of its local circuit neuron network, distinguishing it from adjacent nuclei. Specific GAD65 staining pattern and negative calcium binding protein immunoreactivity identify the cerebellar afferent territory in humans. It is subdivided further into ventral and dorsal regions based on the cytoskeletal protein SMI31 staining pattern. The nuclear outlines revised according to the results are compared with those of Hassler (Schaltenbrand G and Bailey P [1959] Einfuhrung in die stereotaktishen Operationen mit einem Atlas des menschlichen Gehirns, vol 3. Stuttgart: Thieme) and discussed in light of the ongoing controversy regarding delineations of the motor thalamic nuclei in humans.

Probing basal ganglia functions by saccade eye movements

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

The cerebellum and basal ganglia are interconnected

The cerebellum and the basal ganglia are major subcortical nuclei that control multiple aspects of behavior largely through their interactions with the cerebral cortex. Discrete multisynaptic loops connect both the cerebellum and the basal ganglia with multiple areas of the cerebral cortex. Interactions between these loops have traditionally been thought to occur mainly at the level of the cerebral cortex. Here, we review a series of recent anatomical studies in nonhuman primates that challenge this perspective. We show that the anatomical substrate exists for substantial interactions between the cerebellum and the basal ganglia. Furthermore, we discuss how these pathways may provide a useful framework for understanding cerebellar contributions to the manifestation of two prototypical basal ganglia disorders, Parkinson's disease and dystonia.

Involvement of the human ventrolateral thalamus in olfaction

It is widely assumed that the thalamus is not involved in olfaction. The ventrolateral thalamus is, however, closely connected to the contralateral cerebellum, which is involved in the sense of smell based on findings from functional imaging studies and findings of olfactory deficits in patients with cerebellar disease. We hypothesized that olfactory deficits following lesions of the ventrolateral thalamus may be similar to olfactory deficits following cerebellar lesions. Fifteen patients with a focal thalamic lesion involving the ventrolateral thalamus were examined and compared to 15 patients with a focal cerebellar lesion and 15 healthy controls. A detailed olfactory test ("Sniffin' Sticks") was used to assess different olfactory functions separately for each nostril. In the group of patients with a lesion of the ventrolateral thalamus, an impairment of the odor threshold was found at the ipsilateral nostril, consistent with the unilateral orientation of the olfactory system in the telencephalon. In the group of patients with a cerebellar lesion, an olfactory deficit at the contralesional nostril emerged. In controls, no significant side difference was found. The involvement of the ventrolateral thalamus in olfaction is comparable to that of the cerebellum in respect to odor threshold. Further study is needed to assess if these findings are related to an impairment of an olfactomotor loop. Present evidence for this hypothesis is indirect. Effects were subclinical as none of the patients reported olfactory disturbance. The results suggest that the cerebello-thalamic axis plays an adjuvant role in olfaction.

Responses of ventral posterior thalamus neurons to three-dimensional vestibular and optic flow stimulation

Multisensory neurons tuned to both vestibular and visual motion (optic flow) signals are found in several cortical areas in the dorsal visual stream. Here we examine whether such convergence occurs subcortically in the macaque thalamus. We searched the ventral posterior nuclei, including the anterior pulvinar, as well as the ventro-lateral and ventral posterior lateral nuclei, areas that receive vestibular signals from brain stem and deep cerebellar nuclei. Approximately a quarter of cells responded to three-dimensional (3D) translational and/or rotational motion. More than half of the responsive cells were convergent, thus responded during both rotation and translation. The preferred axes of translation/rotation were distributed throughout 3D space. The majority of the neurons were excited, but some were inhibited, during rotation/translation in darkness. Only a couple of neurons were multisensory being tuned to both vestibular and optic flow stimuli. We conclude that multisensory vestibular/optic flow neurons, which are commonly found in cortical visual and visuomotor areas, are rare in the ventral posterior thalamus.

Effects of deep brain stimulation of the cerebellothalamic pathways on the sense of smell

The cerebellum and the motor thalamus, connected by cerebellothalamic pathways, are traditionally considered part of the motor-control system. Yet, functional imaging studies and clinical studies including patients with cerebellar disease suggest an involvement of the cerebellum in olfaction. Additionally, there are anecdotal clinical reports of olfactory disturbances elicited by electrical stimulation of the motor thalamus and its neighbouring subthalamic region. Deep brain stimulation (DBS) targeting the cerebellothalamic pathways is an effective treatment for essential tremor (ET), which also offers the possibility to explore the involvement of cerebellothalamic pathways in the sense of smell. This may be important for patient care given the increased use of DBS for the treatment of tremor disorders. Therefore, 21 none-medicated patients with ET treated with DBS (13 bilateral, 8 unilateral) were examined with "Sniffin' Sticks," an established and reliable method for olfactory testing. Patients were studied either with DBS switched on and then off or in reversed order. DBS impaired odor threshold and, to a lesser extent, odor discrimination. These effects were sub-clinical as none of the patients reported changes in olfactory function. The findings, however, demonstrate that olfaction can be modulated in a circumscribed area of the posterior (sub-) thalamic region. We propose that the impairment of the odor threshold with DBS is related to effects on an olfacto-motor loop, while disturbed odor discrimination may be related to effects of DBS on short-term memory.

Cerebellum: connections and functions

In addition to its role in motor control, reflex adaptation, and motor learning, three sorts of evidence have been put forward to support the idea that the cerebellum may also be involved in cognition. Patients with cerebellar lesions are reported to have deficits in performing one or another cognitive task. The cerebellum is often seen to be activated when normal subjects perform such tasks. There are connections to and from areas of the prefrontal cortex that may be involved in cognition. In this paper, we review the anatomical evidence to support the claim. We suggest that there are only minor connections with cognitive areas of the cerebral cortex and that some of the imaging evidence may reflect the cerebellum's role in the control of eye movements rather than cognition.

Subcortical Interactions Between Somatosensory Stimuli of Different Modalities and Their Temporal Profile

Interactions between inputs of different sensory modality occur along the sensory pathway, including the thalamus. However, the temporal profile of such interaction has not been fully studied. In eight patients who had been implanted an intrathalamic electrode for deep brain stimulation as symptomatic treatment of tremor, we investigated the interactions between mechanical taps and electrical nerve stimuli. Somatosensory evoked potentials (SEPs) were recorded from Erb's point, cervical spinal cord, nucleus ventrointermedialis of the thalamus, and parietal cortex. A handheld electronic reflex hammer was used to deliver a mechanical tap to the skin overlying the first dorsal interosseous muscle and to trigger an ipsilateral digital median nerve electrical stimulus time-locked to the mechanical tap with a variable delay of 0 to 50 ms. There were significant time-dependent interactions between the two sensory volleys at the subcortical level. Thalamic SEPs were decreased in amplitude at interstimulus intervals (ISIs) from 10 to 40 ms with maximum effect at 20 ms (−42.8 ± 10.5%; P < 0.001). A similar decrease was also seen in the number and frequency of the high-frequency components of thalamic SEPs (−25 ± 4%). A smaller reduction (−18.1 ± 5.8%; P < 0.001) was present in upper cervical response at ISI = 20 ms. There were no changes in peripheral responses. Cortical SEPs were almost completely absent in some subjects at ISIs from 20 to 50 ms. There were no changes in SEP latencies. Our results indicate that significant time-dependent interactions between sensory volleys occur at the subcortical level. These observations provide further insight into the physiological mechanisms underlying afferent gating between sensory volleys of different modality.

Retrograde analysis of the cerebellar projections to the posteroventral part of the ventral lateral thalamic nucleus in the macaque monkey

The organization of cerebellothalamic projections was investigated in macaque monkeys using injections of retrograde tracers (cholera toxin B and fluorescent dextrans) in the posteroventral part of the ventrolateral thalamic nucleus (VLpv), the main source of thalamic inputs to the primary motor cortex. Injections that filled all of VLpv labeled abundant neurons that were inhomogeneously distributed among many unlabeled cells in the deep cerebellar nuclei (DCbN). Single large pressure injections made in face-, forelimb-, or hindlimb-related portions of VLpv using physiological guidance labeled numerous neurons that were broadly dispersed within a coarse somatotopographic anteroposterior (foot to face) gradient in the dentate and interposed nuclei. Small iontophoretic injections labeled fewer neurons with the same somatotopographic gradient, but strikingly, the labeled neurons in these cases were as broadly dispersed as in cases with large injections. Simultaneous injections of multiple tracers in VLpv (one tracer per somatic region with no overlap between injections) confirmed the general somatotopography but also demonstrated clearly the overlapping distributions and the close intermingling of neurons labeled with different tracers. Significantly, very few neurons (<2%) were double-labeled. This organizational pattern contrasts with the concept of a segregated "point-to-point" somatotopy and instead resembles the complex patterns that have been observed throughout the motor pathway. These data support the idea that muscle synergies are represented anatomically in the DCbN by a general somatotopography in which intermingled neurons and dispersed but selective connections provide the basis for plastic, adaptable movement coordination of different parts of the body. Indexing terms:

Structural correlates of efficient GABAergic transmission in the basal ganglia-thalamus pathway

Giant inhibitory terminals with multiple synapses, the counterparts of excitatory "detonator" or "driver" terminals, have not been described in the forebrain. Using three-dimensional reconstructions of electron microscopic images, we quantitatively characterize a GABAergic pathway that establishes synaptic contacts exclusively via multiple synapses. Axon terminals of the nigrothalamic pathway formed, on average, 8.5 synapses on large-diameter dendrites and somata of relay cells in the ventromedial nucleus of the rat thalamus. All synapses of a given terminal converged on a single postsynaptic element. The vast majority of the synapses established by a single terminal were not separated by astrocytic processes. Nigrothalamic terminals in the macaque monkey showed the same ultrastructural features both in qualitative and quantitative terms (the median number of synapse per target was also 8.5). The individual synapses were closely spaced in both species. The nearest-neighbor synaptic distances were 169 nm in the rat and 178 nm in the monkey. The average number of synapses within 0.75 microm from any given synapse was 3.8 in the rat and 3.5 in the monkey. The arrangement of synapses described in this study creates favorable conditions for intersynaptic spillover of GABA among the multiple synapses of a single bouton, which can result in larger charge transfer. This could explain faithful and efficient GABAergic signal transmission in the nigrothalamic pathway in the healthy condition and during Parkinson's disease. In addition, our structural data suggest that the rodent nigrothalamic pathway can be a valid model of the primate condition, when the mechanism of GABAergic transmission is studied.

Vestibular signals in primate thalamus: properties and origins

Vestibular activation is found in diverse cortical areas. To characterize the pathways and types of signals supplied to cortex, we recorded responses to rotational and/or translational stimuli in the macaque thalamus. Few cells responded to rotation alone, with most showing convergence between semicircular canal and otolith signals. During sinusoidal rotation, thalamic responses lead head velocity by approximately 30 degrees on average at frequencies between 0.01-4 Hz. During translation, neurons encoded combinations of linear acceleration and velocity. In general, thalamic responses were similar to those recorded in the vestibular and cerebellar nuclei using identical testing paradigms, but differed from those of vestibular afferents. Thalamic responses represented a biased continuum: most cells more strongly encoded translation and fewer cells modulated primarily in response to net gravitoinertial acceleration. Responsive neurons were scattered within a large area that included regions of the ventral posterior and ventral lateral nuclei, and so were not restricted to the known vestibular nuclei projection zones. To determine the origins of these responses, a retrograde tracer was injected into a dorsolateral thalamic site where rotation/translation-sensitive cells were encountered. This injection labeled neurons in the rostral contralateral anterior interposed and fastigial nuclei, but did not label cells within the vestibular nuclei. Examination of thalamic terminations after tracer injections into the cerebellar and vestibular nuclei indicated that most vestibular responsive units fall within the thalamic terminal zones of these nuclei. Thus, vestibular signals, which are supplied to the thalamus from both vestibular and cerebellar nuclei, are positioned for distribution to widespread cortical areas.

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.

Kinematic analysis of thalamic versus subthalamic neurostimulation in postural and intention tremor

Deep brain stimulation of the thalamus (thalamic DBS) is an established therapy for medically intractable essential tremor and tremor caused by multiple sclerosis. In both disorders, motor disability results from complex interaction between kinetic tremor and accompanying ataxia with voluntary movements. In clinical studies, the efficacy of thalamic DBS has been thoroughly assessed. However, the optimal anatomical target structure for neurostimulation is still debated and has never been analysed in conjunction with objective measurements of the different aspects of motor impairment. In 10 essential tremor and 11 multiple sclerosis patients, we analysed the effect of thalamic DBS through each contact of the quadripolar electrode on the contralateral tremor rating scale, accelerometry and kinematic measures of reach-to-grasp-movements. These measures were correlated with the anatomical position of the stimulating electrode in stereotactic space and in relation to nuclear boundaries derived from intraoperative microrecording. We found a significant impact of the stereotactic z-coordinate of stimulation contacts on the TRS, accelerometry total power and spatial deviation in the deceleration and target period of reach-to-grasp-movements. Most effective contacts clustered within the subthalamic area (STA) covering the posterior Zona incerta and prelemniscal radiation. Stimulation within this region led to a mean reduction of the lateralized tremor rating scale by 15.8 points which was significantly superior to stimulation within the thalamus (P < 0.05, student's t-test). STA stimulation resulted in reduction of the accelerometry total power by 99%, whereas stimulation at the ventral thalamic border (68%) or within the thalamus proper (2.5%) was significantly less effective (P < 0.01). Concomitantly, STA stimulation led to a significantly higher increase of tremor frequency and decrease in EMG synchronization compared to stimulation within the thalamus proper (P < 0.001). In reach-to-grasp movements, STA stimulation reduced the spatial variability of the movement path in the deceleration period by 28.9% and in the target period by 58.4%, whereas stimulation within the thalamus was again significantly less effective (P < 0.05), with a reduction in the deceleration period between 6.5 and 21.8% and in the target period between 1.2 and 11.3%. An analysis of the nuclear boundaries from intraoperative microrecording confirmed the anatomical impression that most effective electrodes were located within the STA. Our data demonstrate a profound effect of deep brain stimulation of the thalamic region on tremor and ataxia in essential tremor and tremor caused by multiple sclerosis. The better efficacy of stimulation within the STA compared to thalamus proper favours the concept of a modulation of cerebello-thalamic projections underlying the improvement of these symptoms.

Distribution of the dopamine innervation in the macaque and human thalamus

We recently defined the thalamic dopaminergic system in primates; it arises from numerous dopaminergic cell groups and selectively targets numerous thalamic nuclei. Given the central position of the thalamus in subcortical and cortical interplay, and the functional relevance of dopamine neuromodulation in the brain, detailing dopamine distribution in the thalamus should supply important information. To this end we performed immunohistochemistry for dopamine and the dopamine transporter in the thalamus of macaque monkeys and humans to generate maps, in the stereotaxic coronal plane, of the distribution of dopaminergic axons. The dopamine innervation of the thalamus follows the same pattern in both species and is most dense in midline limbic nuclei, the mediodorsal and lateral posterior association nuclei, and in the ventral lateral and ventral anterior motor nuclei. This distribution suggests that thalamic dopamine has a prominent role in emotion, attention, cognition and complex somatosensory and visual processing, as well as in motor control. Most thalamic dopaminergic axons are thin and varicose and target both the neuropil and small blood vessels, suggesting that, besides neuronal modulation, thalamic dopamine may have a direct influence on microcirculation. The maps provided here should be a useful reference in future experimental and neuroimaging studies aiming at clarifying the role of the thalamic dopaminergic system in health and in conditions involving brain dopamine, including Parkinson's disease, drug addiction and schizophrenia.

The thalamic connections of motor, premotor, and prefrontal areas of cortex in a prosimian primate (Otolemur garnetti)

Connections of motor areas in the frontal cortex of prosimian galagos (Otolemur garnetti) were determined by injecting tracers into sites identified by microstimulation in the primary motor area (M1), dorsal premotor area (PMD), ventral premotor area (PMV), supplementary motor area (SMA), frontal eye field (FEF), and granular frontal cortex. Retrogradely labeled neurons for each injection were related to architectonically defined thalamic nuclei. Nissl, acetylcholinesterase, cytochrome oxidase, myelin, parvalbumin, calbindin, and Cat 301 preparations allowed the ventral anterior and ventral lateral thalamic regions, parvocellular and magnocellular subdivisions of ventral anterior nucleus, and anterior and posterior subdivisions of ventral lateral nucleus of monkeys to be identified. The results indicate that each cortical area receives inputs from several thalamic nuclei, but the proportions differ. M1 receives major inputs from the posterior subdivision of ventral lateral nucleus while premotor areas receive major inputs from anterior parts of ventral lateral nucleus (the anterior subdivision of ventral lateral nucleus and the anterior portion of posterior subdivision of ventral lateral nucleus). PMD and SMA have connections with more dorsal parts of the ventral lateral nucleus than PMV. The results suggest that galagos share many subdivisions of the motor thalamus and thalamocortical connection patterns with simian primates, while having less clearly differentiated subdivisions of the motor thalamus.

Three-tesla magnetic resonance imaging of the ventrolateral thalamus: a correlative anatomical description

Object

Surgery for tremor targets the ventrolateral nuclei of the thalamus. An initial radiological estimation of this target can be further refined through intraoperative physiological confirmation. Direct visualization of these nuclei has not yet been described. The improved signal-to-noise ratio associated with 3-tesla (3T) magnetic resonance (MR) imaging makes increased spatial resolution possible, which may aid in the identification of subtle morphological features. This study was conducted to describe the anatomy of the nuclei and fiber projections within the ventral thalamus by using 3T MR imaging.

Methods

Using a commercially available 3T MR unit, the authors obtained images of a formalin-fixed, paraffin-embedded brain. Slices with a 2-mm thickness and 0.2-mm gap were obtained parallel to the anterior commissure–posterior commissure (AC–PC) line. The brain was then sectioned through the cerebral hemispheres to obtain tissue slices encompassing the same levels. Adjacent 10-μm paraffin sections from the middle of each level were stained with Luxol fast blue and cresyl violet. The MR image and histological sections at the level of the AC–PC line were then compared in detail. In a separate study, the human thalamus was scanned in vivo using 3T and 1.5T MR imaging for anatomical comparison.

Conclusions

The anatomy of the nuclei and fiber projections within the ventrolateral thalamus in humans can be described using 3T MR imaging. The findings were reproducible in vivo with 3T but not 1.5T MR imaging. Additional studies are needed to confirm the accuracy of this observation for clinical purposes.

Contralateral cerebellar damage impairs imperative planning but not updating of aimed arm movements in humans

The specific motor control processes supported by the cerebellum and impaired with cerebellar damage remain unclear. The cerebellum has been implicated in both planning and updating of accurate movements. Previously, we used a statistical model to parcel aiming performance that was constrained by a timed-response paradigm into contributions attributed to a specified plan and feedforward updating. Here, we apply this procedure to determine the putative role of the cerebellum in planning and updating goal-directed aiming by comparing the performance of subjects with unilateral cerebellar stroke to controls. Subjects rapidly moved to targets in predictable or unpredictable conditions and cerebellar subjects used the contralesional limb to control for ipsilesional motor execution deficits. Displacement-derived movement velocity was used in the statistical model to determine the effect of planning and updating on accuracy. Compared to controls, the cerebellar group demonstrated errors in final position that were primarily determined by planning deficits. This finding is manifest in four ways: Cerebellar subjects (1) were less accurate than controls in both predictable and unpredictable conditions; (2) they showed minimal benefit from increased preparation time for target amplitude specification; (3) with ample time to plan direction, wrong direction response frequency was greater; and (4) final position was minimally determined by the plan. Because these deficits were found contralesional to the moving limb, the cerebellum's role in planning is not lateralized to one hemisphere but rather our findings suggest that cerebellar output affects motor planning for both upper limbs. Indeed, a lesion analysis showed that the dentate nucleus, an area implicated in planning motor strategies and the primary cerebellar output nucleus, was the only common region affected by our patient group with contralateral cerebellar strokes.

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.

Activity Properties and Location of Neurons in the Motor Thalamus That Project to the Cortical Motor Areas in Monkeys

The activity of neurons in the motor nuclei of the thalamus that project to the cortical motor areas (the primary motor cortex, the ventral and dorsal premotor cortex, and the supplementary motor area) was investigated in monkeys that were performing a task in which wrist extension and flexion movements were instructed by visuospatial cues before the onset of movement. Movement was triggered by a visual, auditory, or somatosensory stimulus. Thalamocortical neurons were identified by a spike collision, and exhibited 2 distinct types of task-related activity: 1) a sustained change in activity during the instructed preparation period in response to the instruction cues (set-related activity); and 2) phasic changes in activity during the reaction and movement time periods (movement-related activity). A number of set- and moment-related neurons exhibited direction selectivity. Most movement-related neurons were similarly active, irrespective of the different sensory modalities of the cue for movement. These properties of neuronal activity were similar, regardless of their target cortical motor areas. There were no significant differences in the antidromic latencies of neurons that projected to the primary and nonprimary motor areas. These results suggest that the thalamocortical neurons play an important role in the preparation for, and initiation and execution of, the movements, but are less important than neurons of the nonprimary cortical motor areas in modality-selective sensorimotor transformation. It is likely that such transformations take place within the nonprimary cortical motor areas, but not through thalamocortical information channels.

A functional MRI study of automatic movements in patients with Parkinson's disease

Patients with Parkinson's disease have great difficulty performing learned movements automatically. The neural contribution to the problem has not been identified. In the current study, we used functional magnetic resonance imaging (fMRI) to investigate the underlying neural mechanisms of movement automaticity in Parkinson's disease patients. Fifteen patients with Parkinson's disease were recruited. Three patients were finally excluded because they could not achieve automaticity. The remaining 12 patients were aged from 52 to 67 years, with a mean age of 61.2 years. Controls included 14 age-matched normal subjects. The subjects were asked to practise four tasks, including two self-initiated, self-paced sequences of finger movements with different complexity until they could perform the tasks automatically. Two dual tasks were used to evaluate automaticity. For dual tasks, subjects performed a visual letter-counting task simultaneously with the sequential movements. Twelve normal subjects performed all sequences automatically. All patients performed sequences correctly; 12 patients could perform the simpler sequence automatically; and only 3 patients could perform the more complex sequence automatically. fMRI results showed that for both groups, sequential movements activated similar brain regions before and after automaticity was achieved. No additional activity was observed in the automatic condition. In normal subjects, many areas had reduced activity at the automatic stage, whereas in patients, only the bilateral superior parietal lobes and left insular cortex were less activated. Patients had greater activity in the cerebellum, premotor area, parietal cortex, precuneus and prefrontal cortex compared with normal subjects while performing automatic movements. We conclude that Parkinson's disease patients can achieve automaticity after proper training, but with more difficulty. Our study is the first to demonstrate that patients with Parkinson's disease require more brain activity to compensate for basal ganglia dysfunction in order to perform automatic movements.

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.

Comparison of learning-related neuronal activity in the dorsal premotor cortex and striatum

Previous studies have reported learning-related changes in neuronal activity during conditional visuomotor learning, also known as arbitrary sensorimotor mapping, conditional visual discrimination, and symbolic or endogenous mapping. Qualitatively similar observations have been reported for the dorsal premotor cortex, the supplementary eye field, the prefrontal cortex, the hippocampus, the striatum and the globus pallidus. The fact that cells in both the dorsal premotor cortex (PMd) and the basal ganglia show changes in activity during associative learning enables a test of the hypothesis that cortex and basal ganglia function in distributed architectures known as cortical-basal ganglionic modules or 'loops'. We reasoned that if these loops represent functional entities, as proposed, then learning-related changes in activity should occur simultaneously in both the cortical and striatal nodes of a loop. The present results confirmed this prediction; as monkeys learned conditional visuomotor associations, neurons in the premotor cortex and associated parts of the putamen changed their rates at approximately the same time. For the largest number of neurons, the evolution in neural activity occurred in close correspondence to the monkeys' learning curves. As a population, however, learning-related changes in activity continued after the monkeys reached an asymptote in performance.

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.

Cortical convergence from different frequency domains in the cat primary auditory cortex

Primary auditory cortex (AI) has a tonotopic map consisting of orderly isofrequency (IF) bands, and cortical connections are commonly supposed to link domains preferring similar characteristic frequencies (CFs) within AI and in auditory association cortex. The interaction of different frequency channels, however, has not fully been understood in terms of anatomical substrates. Here, by injecting two anterograde tracers in different frequency domains of cat AI, without overlap of the injection cores, we attempted to relate the anatomical mapping of cortical outputs to physiologically defined fields in the auditory cortex. Consistent with previous studies, patches of labeled axon terminals were oriented largely along the IF axis. In regions distant from the injection sites, however, terminal patches were divergent in distribution. This divergence resulted in a complex geometry of partial overlap of projections originating from the two injection sites. The relative extent of the overlap tended to vary depending on the distance between the two injection sites. Physiological mapping for tonotopy across auditory fields revealed that projectional overlap was characteristic of dorsal AI and the dorsoposterior field and, to a lesser extent, in the secondary auditory field. Considering the differences in frequency representation in different AI IF bands, the anatomical convergence of projections tuned to different CFs could contribute to the spectral integration of sound components. Furthermore, the different extent of convergence in the functionally distinct fields might reflect field-specific processing of acoustic signals.

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.

Tremor-related activity of neurons in the 'motor' thalamus: changes in firing rate and pattern in the MPTP vervet model of parkinsonism

The pathophysiology of parkinsonian tremor remains a matter of debate with two opposing hypotheses proposing a peripheral and a central origin, respectively. A central origin of tremor could arise either from a rhythmic activity of the internal segment of the globus pallidus (GPi) or from a structure such as the thalamus, outside the basal ganglia. In this study, single-unit recordings were performed in three 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated monkeys within the GPi and within three territories of the motor thalamus (delimited by their afferent inputs from the GPi, the substantia nigra and the cerebellum, respectively). For each recorded neuron, we compared the variations in firing rate and pattern in tremor and no tremor periods. Tremor either occurred spontaneously or was induced by external stimulation. When the animals entered into a tremor period we observed: (i) an increase in the mean firing rate in about half of the recorded neurons of the motor thalamus; and (ii), a change from an irregular to a rhythmic discharge within the range of tremor frequency (5-7 Hz) in about 10% of the recorded neurons of the motor thalamus (pallidal and cerebellar territories) and the GPi. Most of the thalamic neurons that exhibited a rhythmic discharge during tremor were found to be sensitive to external stimulation. Because the changes in firing rate occurred predominantly in the motor thalamus and not in the GPi, and because a fast rhythmic discharge of 10-15 Hz was frequently observed in the GPi and not in the motor thalamus, we conclude that thalamic activity is not a simple reproduction of basal ganglia output. Moreover, we suggest that thalamic processing of basal ganglia outputs could participate in the genesis of tremor, and that this thalamic processing could be influenced by sensory inputs and/or changes in attentional level elicited by external stimulation.

Kinaesthetic neurons in thalamus of humans with and without tremor

Increased afferent input may alter receptive field sizes, properties and somatotopographic representation in the cortex. Changes in the motor thalamus may also occur as a result of altered afferent input. Such plasticity has been implicated in both sensory and movement disorders. Using tremor as a model of augmented afferent input to kinaesthetic/deep neurons representing the shaking limbs, we studied the representation and properties of these neurons in human thalamus in patients with resting tremor (RestTr) from Parkinson's disease, patients with action- or posture-induced tremor (ActionTr), and patients without tremor (NoTr). Data were collected during stereotactic thalamotomy or insertion of deep brain stimulators for relief of pain or movement disorder. Using microelectrode recording, 58 kinaesthetic neurons responding to wrist and/or elbow movement were studied by mapping the receptive field, carefully isolating each joint during testing. There were no significant differences in the proportions of single and multijoint responsive neurons in the different patient groups (RestTr, ActionTr and NoTr). The borders between tactile-cutaneous, deep-kinaesthetic and voluntary cell representations in the thalamus were mapped in 74 patients and compared between the different tremor groups. A significant difference in kinaesthetic representation was found: both the RestTr and ActionTr groups had a significantly greater kinaesthetic representation than the NoTr patients. There was an expansion of kinaesthetic representation in patients with chronic increased afferent drive from tremor, without alteration in RF size. No decrease in tactile representation was found, suggesting that the increase in kinaesthetic representation does not occur at the expense of tactile representation. These data suggest that plasticity can occur at the thalamic level in humans and may contribute to the pathogenesis of tremor.

Somatosensory input to the ventrolateral thalamic region in the macaque monkey: potential substrate for parkinsonian tremor

In the present study, we determined the anatomic relationships between somatosensory and motor pathways within ventrolateral (VL) thalamic nuclei of the motor thalamus of macaque monkeys. In labeling experiments, four macaque monkeys (Macaca mulatta) received injections of biotinylated dextran amine and wheat germ agglutinin conjugated to horseradish peroxidase into the cerebellar nuclei or internal segment of the globus pallidus and cervical segments of the spinal cord, respectively. Each tracer was visualized in brain sections by sequentially using a different chromogen. Labeled terminals were plotted and superimposed on adjacent brain sections processed for Nissl substance, acetylcholinesterase, and the antigens for calbindin and Cat-301 to reveal thalamic nuclei. The labeled cerebellar terminals were distributed throughout the posterior VL (VLp), whereas the labeled pallidothalamic terminals were concentrated in the anterior VL and the ventral anterior nucleus. The spinothalamic input was directed mostly to the ventral posterior complex and cells just caudal to it. In addition, the patches of spinothalamic terminations intermingled and partly overlapped with the cerebellothalamic, but not with the pallidothalamic terminations within VLp. The regions of overlap of somatosensory and cerebellar inputs within the VLp of the present study appear to correspond to the reported locations of the tremor-related cells in parkinsonian patients. Thus, the overlapping spinothalamic and cerebellar inputs may provide a substrate for the altered activity of motor thalamic neurons in such patients.

Posterior parietal cortex projections to the ventral lateral and some association thalamic nuclei in Macaca mulatta

The study focused on projections from the posterior parietal cortex (PPC) to the ventral lateral thalamic nucleus (VL) and three thalamic association nuclei, mediodorsal (MD), lateral posterior (LP) and pulvinar. For light microscopic analysis small biotinylated dextran amine (BDA) or biocytin injections were placed in midrostral and dorsal portions of the inferior parietal lobule (IPL), respectively. The distribution of anterograde and retrograde labeling was charted, and representative axons and terminal fields were reconstructed in the sagittal plane to examine their features. Two types of fibers were identified--those of thin diameter forming diffuse terminal fields with small boutons, and thick fibers forming focal terminal fields with large boutons. Area PFG injection of BDA resulted in labeling of both types of fibers in LP, MD, and pulvinar, whereas only fibers of the first type were found in VL. Biocytin injection in area Opt resulted in preferential labeling of large fibers terminating in LP and pulvinar. Further electron microscopic analysis of labeled boutons in VL and LP, following a large wheat germ agglutinin conjugated horseradish peroxidase injection in the middle of IPL, confirmed the existence of small and large corticothalamic boutons and their different termination sites: the small boutons formed synapses on distal dendrites while the large boutons were found close to somata of thalamocortical projection neurons, on the dendrites of local circuit neurons and in complex synaptic arrangements, such as glomeruli. The results demonstrate that projections from small loci of the PPC to functionally and connectionally different thalamic nuclei differ anatomically, implying a different functional impact on these diverse targets.

Nigral and pallidal inputs to functionally segregated thalamostriatal neurons in the centromedian/parafascicular intralaminar nuclear complex in monkey

In primates, thalamostriatal projections from the centromedian (CM) and parafascicular (Pf) nuclei are strong and organized according to a strict pattern of functional connectivity with various regions of the striatal complex. In turn, the CM/Pf complex receives a substantial innervation from the internal globus pallidus (GPi). In this study, we demonstrate that the substantia nigra pars reticulata (SNr) also provides a massive input to Pf in monkeys. These pallidothalamic and nigrothalamic projections provide routes whereby information can flow in functional loops between the basal ganglia and the intralaminar nuclear group. To understand better the anatomical organization and the degree of functional specificity of these loops, we combined retrograde and anterograde labeling methods from functionally defined regions of the striatum and GPi/SNr to determine the relationships between thalamostriatal neurons and basal ganglia afferents. Together with previous studies, our data suggest the existence of tightly connected functional circuits between the basal ganglia and the CM/Pf in primates: 1) A "sensorimotor" circuit links together the medial two-thirds of CM, the postcommissural putamen, and the ventrolateral part of the caudal GPi; 2) a "limbic" circuit involves the rostral one-third of Pf, the ventral striatum, and the rostromedial pole of GPi; and 3) an "associative"circuit exists between the caudal two-thirds of Pf, the caudate nucleus, and the SNr. An additional "associative" circuit that involves the caudate-receiving territory of GPi (dorsal one-third), the dorsolateral Pf (Pfdl), and the precommissural putamen was also disclosed. In conclusion, findings of this study provide additional evidence for the high degree of functional specificity of the thalamostriatal system through which CM/Pf may provide attention-specific sensory information important for conditional responses to the primate striatum.

Optimal location of thalamotomy lesions for tremor associated with Parkinson disease: a probabilistic analysis based on postoperative magnetic resonance imaging and an integrated digital atlas

OBJECT

Renewed interest in stereotactic neurosurgery for movement disorders has led to numerous reports of clinical outcomes associated with different treatment strategies. Nevertheless, there is a paucity of autopsy and imaging data that can be used to describe the optimal size and location of lesions or the location of implantable stimulators. In this study the authors correlated the clinical efficacy of stereotactic thalamotomy for tremor with precise anatomical localization by using postoperative magnetic resonance (MR) imaging and an integrated deformable digital atlas of subcortical structures.

METHODS

Thirty-one lesions were created by stereotactic thalamotomy in 25 patients with tremor-dominant Parkinson disease. Lesion volume and configuration were evaluated by reviewing early postoperative MR images and were correlated with excellent, good, or fair tremor outcome categories. To allow valid comparisons of configurations of lesions with respect to cytoarchitectonic thalamic boundaries, the MR image obtained in each patient was nonlinearly deformed into a standardized MR imaging space, which included an integrated atlas of the basal ganglia and thalamus. The volume and precise location of lesions associated with different clinical outcomes were compared using nonparametric statistical methods. Probabilistic maps of lesions in each tremor outcome category were generated and compared. Statistically significant differences in lesion location between excellent and good. and excellent and fair outcome categories were demonstrated. On average, lesions associated with excellent outcomes involved thalamic areas located more posteriorly than sites affected by lesions in the other two outcome groups. Subtraction analysis revealed that lesions correlated with excellent outcomes necessarily involved the interface of the nucleus ventralis intermedius (Vim; also known as the ventral lateral posterior nucleus [VLp]) and the nucleus ventrocaudalis (Vc; also known as the ventral posterior [VP] nucleus). Differences in lesion volume among outcome groups did not achieve statistical significance.

CONCLUSIONS

Anatomical evaluation of lesions within a standardized MR image-atlas integrated reference space is a useful method for determining optimal lesion localization. The results of an analysis of probabilistic maps indicates that optimal relief of tremor is associated with lesions involving the Vim (VLp) and the anterior Vc (VP).

The ventral lateral posterior nucleus of the thalamus in schizophrenia: a post-mortem study

The ventral lateral posterior thalamic nucleus (VLp) is an integral part of both the cerebello-thalamocortical and the basal ganglia-thalamocortical circuit. Although both circuits are thought to be involved in the pathophysiology of schizophrenia, the VLp has not yet been examined in schizophrenia. Using stereological techniques in the brains of eight patients with schizophrenia and in eight age- and sex-matched controls, we measured the nuclear volume of the VLp and obtained estimates of the total number of neurons in this nucleus. Whole brain volume did not differ between the schizophrenia group and the control group and was not correlated to the volume of the right VLp or left VLp. The volume (minus sign25%) and the total neuron number (minus sign27%) of the left VLp were significantly reduced in the schizophrenia group. There were no significant differences in the nuclear volume, neuron density and total neuron number in the right VLp between the schizophrenia group and the control group. There were no significant correlations between length of illness and the nuclear volume, neuron density and total neuron number of the left and right VLp. The present results suggest that the total neuron number of the left VLp is reduced in the schizophrenia group, which may reflect disturbed cerebello-thalamocortical and basal ganglia-thalamocortical circuits in this disease.