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

Convergence of inputs from the basal ganglia with layer 5 of motor cortex and cerebellum in mouse motor thalamus

A key to motor control is the motor thalamus, where several inputs converge. One excitatory input originates from layer 5 of primary motor cortex (M1L5), while another arises from the deep cerebellar nuclei (Cb). M1L5 terminals distribute throughout the motor thalamus and overlap with GABAergic inputs from the basal ganglia output nuclei, the internal segment of the globus pallidus (GPi), and substantia nigra pars reticulata (SNr). In contrast, it is thought that Cb and basal ganglia inputs are segregated. Therefore, we hypothesized that one potential function of the GABAergic inputs from basal ganglia is to selectively inhibit, or gate, excitatory signals from M1L5 in the motor thalamus. Here, we tested this possibility and determined the circuit organization of mouse (both sexes) motor thalamus using an optogenetic strategy in acute slices. First, we demonstrated the presence of a feedforward transthalamic pathway from M1L5 through motor thalamus. Importantly, we discovered that GABAergic inputs from the GPi and SNr converge onto single motor thalamic cells with excitatory synapses from M1L5. Separately, we also demonstrate that, perhaps unexpectedly, GABAergic GPi and SNr inputs converge with those from the Cb. We interpret these results to indicate that a role of the basal ganglia is to gate the thalamic transmission of M1L5 and Cb information to cortex.

Convergence of inputs from the basal ganglia with layer 5 of motor cortex and cerebellum in mouse motor thalamus

A key to motor control is the motor thalamus, where several inputs converge. One excitatory input originates from layer 5 of primary motor cortex (M1L5), while another arises from the deep cerebellar nuclei (Cb). M1L5 terminals distribute throughout the motor thalamus and overlap with GABAergic inputs from the basal ganglia output nuclei, the internal segment of the globus pallidus (GPi) and substantia nigra pars reticulata (SNr). In contrast, it is thought that Cb and basal ganglia inputs are segregated. Therefore, we hypothesized that one potential function of the GABAergic inputs from basal ganglia is to selectively inhibit, or gate, excitatory signals from M1L5 in the motor thalamus. Here, we tested this possibility and determined the circuit organization of mouse (both sexes) motor thalamus using an optogenetic strategy in acute slices. First, we demonstrated the presence of a feedforward transthalamic pathway from M1L5 through motor thalamus. Importantly, we discovered that GABAergic inputs from the GPi and SNr converge onto single motor thalamic cells with excitatory synapses from M1L5 and, unexpectedly, Cb as well. We interpret these results to indicate that a role of the basal ganglia is to gate the thalamic transmission of M1L5 and Cb information to cortex.

Micropopulation mapping of the mouse parafascicular nucleus connections reveals diverse input-output motifs

Introduction

In primates, including humans, the centromedian/parafascicular (CM-Pf) complex is a key thalamic node of the basal ganglia system. Deep brain stimulation in CM-Pf has been applied for the treatment of motor disorders such as Parkinson's disease or Tourette syndrome. Rodents have become widely used models for the study of the cellular and genetic mechanisms of these and other motor disorders. However, the equivalence between the primate CM-Pf and the nucleus regarded as analogous in rodents (Parafascicular, Pf) remains unclear.

Methods

Here, we analyzed the neurochemical architecture and carried out a brain-wide mapping of the input-output motifs in the mouse Pf at micropopulation level using anterograde and retrograde labeling methods. Specifically, we mapped and quantified the sources of cortical and subcortical input to different Pf subregions, and mapped and compared the distribution and terminal structure of their axons.

Results

We found that projections to Pf arise predominantly (>75%) from the cerebral cortex, with an unusually strong (>45%) Layer 5b component, which is, in part, contralateral. The intermediate layers of the superior colliculus are the main subcortical input source to Pf. On its output side, Pf neuron axons predominantly innervate the striatum. In a sparser fashion, they innervate other basal ganglia nuclei, including the subthalamic nucleus (STN), and the cerebral cortex. Differences are evident between the lateral and medial portions of Pf, both in chemoarchitecture and in connectivity. Lateral Pf axons innervate territories of the striatum, STN and cortex involved in the sensorimotor control of different parts of the contralateral hemibody. In contrast, the mediodorsal portion of Pf innervates oculomotor-limbic territories in the above three structures.

Discussion

Our data thus indicate that the mouse Pf consists of several neurochemically and connectively distinct domains whose global organization bears a marked similarity to that described in the primate CM-Pf complex.

Loss of Motor Cortical Inputs to the Red Nucleus after CNS Disorders in Nonhuman Primates

The premotor (PM) and primary motor (M1) cortical areas broadcast voluntary motor commands through multiple neuronal pathways, including the corticorubral projection that reaches the red nucleus (RN). However, the respective contribution of M1 and PM to corticorubral projections as well as changes induced by motor disorders or injuries are not known in nonhuman primates. Here, we quantified the density and topography of axonal endings of the corticorubral pathway in RN in intact monkeys, as well as in monkeys subjected to either cervical spinal cord injury (SCI), Parkinson's disease (PD)-like symptoms or primary motor cortex injury (MCI). Twenty adult macaque monkeys of either sex were injected with the biotinylated dextran amine anterograde tracer either in PM or in M1. We developed a semiautomated algorithm to reliably detect and count axonal boutons within the magnocellular and parvocellular (pRN) subdivisions of RN. In intact monkeys, PM and M1 preferentially target the medial part of the ipsilateral pRN, reflecting its somatotopic organization. Projection of PM to the ipsilateral pRN is denser than that of M1, matching previous observations for the corticotectal, corticoreticular, and corticosubthalamic projections (Fregosi et al., 2018, 2019; Borgognon et al., 2020). In all three types of motor disorders, there was a uniform and strong decrease (near loss) of the corticorubral projections from PM and M1. The RN may contribute to functional recovery after SCI, PD, and MCI, by reducing direct cortical influence. This reduction possibly privileges direct access to the final output motor system, via emphasis on the direct corticospinal projection.SIGNIFICANCE STATEMENT We measured the corticorubral projection density arising from the PM or the M1 cortices in adult macaques. The premotor cortex sent denser corticorubral projections than the primary motor cortex, as previously observed for the corticotectal, corticoreticular, and corticosubthalamic projections. The premotor cortex may thus exert more influence than primary motor cortex onto subcortical structures. We next asked whether the corticorubral motor projections undergo lesion-dependent plasticity after either cervical spinal cord injury, Parkinson's disease-like symptoms, or primary motor cortex lesion. In all three types of pathology, there was a strong decrease of the corticorubral motor projection density, suggesting that the red nucleus may contribute to functional recovery after such motor system disorders based on a reduced direct cortical influence.

Anatomical organization of forebrain circuits in the primate

The primate forebrain is a complex structure. Thousands of connections have been identified between cortical areas, and between cortical and sub-cortical areas. Previous work, however, has suggested that a number of principles can be used to reduce this complexity. Here, we integrate four principles that have been put forth previously, including a nested model of neocortical connectivity, gradients of connectivity between frontal cortical areas and the striatum and thalamus, shared patterns of sub-cortical connectivity between connected posterior and frontal cortical areas, and topographic organization of cortical-striatal-pallidal-thalamocortical circuits. We integrate these principles into a single model that accounts for a substantial amount of connectivity in the forebrain. We then suggest that studies in evolution and development can account for these four principles, by assuming that the ancestral vertebrate pallium was dominated by medial, hippocampal and ventral-lateral, pyriform areas, and at most a small dorsal pallium. The small dorsal pallium expanded massively in the lineage leading to primates. During this expansion, topological, adjacency relationships were maintained between pallial and sub-pallial areas. This maintained topology led to the connectivity gradients seen between cortex, striatum, pallidum, and thalamus.

Characterization and manipulation of Corticothalamic neurons in associative cortices using Syt6-Cre transgenic mice

Corticothalamic interactions between associative cortices and higher order thalamic nuclei are involved in high-cognitive functions such as decision-making and working memory. Corticothalamic neurons (CTn) in the prefrontal cortex and other associative areas have been much less studied than their counterparts in the primary sensory areas. The availability of characterized transgenic tools to study CTn in associative areas will facilitate their study and contribute to overcome the scarcity of data about their properties, network dynamics, and contribution to cognitive functions. Here, we characterized the Syt6-Cre (KI148Gsat/Mmud) transgenic mouse line, by tracking expression of a Cre-mediated reporter. In this line, Cre-reporter is strongly expressed in the prefrontal, motor, cingulate, and retrosplenial cortices, as well as in other brain areas including the cerebellum and the olfactory tubercle. Cortical expression starts embryonically and reaches the adult expression pattern by postnatal day 15. In the cortex, Cre-reporter is expressed by layer 6-CTn and by layer 5-CTn to a lesser extent. We quantified Syt6-Cre⁺ CTn axon varicosities to estimate the distribution and density of putative corticothalamic driver and modulator inputs to thalamic nuclei in the medial, midline, intralaminar, anterior, and motor groups. Also, we characterized the effect of optogenetic stimulation of Syt6-Cre⁺ neurons in the activity of the prefrontal cortex. CTn stimulation in the prefrontal cortex induces an oscillatory activity in the local field potential that resembles the cortical downstates typically observed during slow-wave sleep or quiet wake.

Hierarchical Organization of Corticothalamic Projections to the Pulvinar

Signals from lower cortical visual areas travel to higher-order areas for further processing through cortico-cortical projections, organized in a hierarchical manner. These signals can also be transferred between cortical areas via alternative cortical transthalamic routes involving higher-order thalamic nuclei like the pulvinar. It is unknown whether the organization of transthalamic pathways may reflect the cortical hierarchy. Two axon terminal types have been identified in corticothalamic (CT) pathways: the types I (modulators) and II (drivers) characterized by thin axons with small terminals and by thick axons and large terminals, respectively. In cats, projections from V1 to the pulvinar complex comprise mainly type II terminals, whereas those from extrastriate areas include a combination of both terminals suggesting that the nature of CT terminals varies with the hierarchical order of visual areas. To test this hypothesis, distribution of CT terminals from area 21a was charted and compared with 3 other visual areas located at different hierarchical levels. Results demonstrate that the proportion of modulatory CT inputs increases along the hierarchical level of cortical areas. This organization of transthalamic pathways reflecting cortical hierarchy provides new and fundamental insights for the establishment of more accurate models of cortical signal processing along transthalamic cortical pathways.

Disentangling the influences of multiple thalamic nuclei on prefrontal cortex and cognitive control

The prefrontal cortex (PFC) has a complex relationship with the thalamus, involving many nuclei which occupy predominantly medial zones along its anterior-to-posterior extent. Thalamocortical neurons in most of these nuclei are modulated by the affective and cognitive signals which funnel through the basal ganglia. We review how PFC-connected thalamic nuclei likely contribute to all aspects of cognitive control: from the processing of information on internal states and goals, facilitating its interactions with mnemonic information and learned values of stimuli and actions, to their influence on high-level cognitive processes, attentional allocation and goal-directed behavior. This includes contributions to transformations such as rule-to-choice (parvocellular mediodorsal nucleus), value-to-choice (magnocellular mediodorsal nucleus), mnemonic-to-choice (anteromedial nucleus) and sensory-to-choice (medial pulvinar). Common mechanisms appear to be thalamic modulation of cortical gain and cortico-cortical functional connectivity. The anatomy also implies a unique role for medial PFC in modulating processing in thalamocortical circuits involving other orbital and lateral PFC regions. We further discuss how cortico-basal ganglia circuits may provide a mechanism through which PFC controls cortico-cortical functional connectivity.

Cortical Projection From the Premotor or Primary Motor Cortex to the Subthalamic Nucleus in Intact and Parkinsonian Adult Macaque Monkeys: A Pilot Tracing Study

Besides the main cortical inputs to the basal ganglia, via the corticostriatal projection, there is another input via the corticosubthalamic projection (CSTP), terminating in the subthalamic nucleus (STN). The present study investigated and compared the CSTPs originating from the premotor cortex (PM) or the primary motor cortex (M1) in two groups of adult macaque monkeys. The first group includes six intact monkeys, whereas the second group was made up of four monkeys subjected to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication producing Parkinson’s disease (PD)-like symptoms and subsequently treated with an autologous neural cell ecosystem (ANCE) therapy. The CSTPs were labeled with the anterograde tracer biotinylated dextran amine (BDA), injected either in PM or in M1. BDA-labeled axonal terminal boutons in STN were charted, counted, and then normalized based on the number of labeled corticospinal axons in each monkey. In intact monkeys, the CSTP from PM was denser than that originating from M1. In two PD monkeys, the CSTP originating from PM or M1 were substantially increased, as compared to intact monkeys. In one other PD monkey, there was no obvious change, whereas the last PD monkey showed a decrease of the CSTP originating from M1. Interestingly, the linear relationship between CSTP density and PD symptoms yielded a possible dependence of the CSTP re-organization with the severity of the MPTP lesion. The higher the PD symptoms, the larger the CSTP densities, irrespective of the origin (from both M1 or PM). Plasticity of the CSTP in PD monkeys may be related to PD itself and/or to the ANCE treatment.

Primary motor cortex in Parkinson's disease: Functional changes and opportunities for neurostimulation

Movement abnormalities of Parkinson's disease (PD) arise from disordered neural activity in multiple interconnected brain structures. The planning and execution of movement requires recruitment of a heterogeneous collection of pyramidal projection neurons in the primary motor cortex (M1). The neural representations of movement in M1 single-cell and field potential recordings are directly and indirectly influenced by the midbrain dopaminergic neurons that degenerate in PD. This review examines M1 functional alterations in PD as uncovered by electrophysiological recordings and neurostimulation studies in patients and experimental animal models. Dysfunction of the parkinsonian M1 depends on the severity and/or duration of dopamine-depletion and the species examined, and is expressed as alterations in movement-related firing dynamics; functional reorganisation of local circuits; and changes in field potential beta oscillations. Neurostimulation methods that modulate M1 activity directly (e.g., transcranial magnetic stimulation) or indirectly (subthalamic nucleus deep brain stimulation) improve motor function in PD patients, showing that targeted neuromodulation of M1 is a realistic therapy. We argue that the therapeutic profile of M1 neurostimulation is likely to be greatly enhanced with alternative technologies that permit cell-type specific control and incorporate feedback from electrophysiological biomarkers measured locally.

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

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

Interhemispheric modulations of motor outputs by the rostral and caudal forelimb areas in rats

In rats, forelimb movements are evoked from two cortical regions, the caudal and rostral forelimb areas (CFA and RFA, respectively). These areas are densely interconnected and RFA induces complex and powerful modulations of CFA outputs. CFA and RFA also have interhemispheric connections, and these areas from both hemispheres send projections to common targets along the motor axis, providing multiple potential sites of interactions for movement production. Our objective was to characterize how CFA and RFA in one hemisphere can modulate motor outputs of the opposite hemisphere. To do so, we used paired-pulse protocols with intracortical microstimulation techniques (ICMS), while recording electromyographic (EMG) activity of forelimb muscles in sedated rats. A subthreshold conditioning stimulation was applied in either CFA or RFA in one hemisphere simultaneously or before a suprathreshold test stimulation in either CFA or RFA in the opposite hemisphere. Both CFA and RFA tended to facilitate motor outputs with short (0–2.5 ms) or long (20–35 ms) delays between the conditioning and test stimuli. In contrast, they tended to inhibit motor outputs with intermediate delays, in particular 10 ms. When comparing the two areas, we found that facilitatory effects from RFA were more frequent and powerful than the ones from CFA. In contrast, inhibitory effects from CFA on its homolog were more frequent and powerful than the ones from RFA. Our results demonstrate that interhemispheric modulations from CFA and RFA share some similarities but also have clear differences that could sustain specific functions these cortical areas carry for the generation of forelimb movements.

NEW & NOTEWORTHY We show that caudal and rostral forelimb areas (CFA and RFA) have distinct effects on motor outputs from the opposite hemisphere, supporting that they are distinct nodes in the motor network of rats. However, the pattern of interhemispheric modulations from RFA has no clear equivalent among premotor areas in nonhuman primates, suggesting they contribute differently to the generation of ipsilateral hand movements. Understanding these interspecies differences is important given the common use of rodent models in motor control and recovery studies.

Structural plasticity of GABAergic and glutamatergic networks in the motor thalamus of parkinsonian monkeys

In the primate thalamus, the parvocellular ventral anterior nucleus (VApc) and the centromedian nucleus (CM) receive GABAergic projections from the internal globus pallidus (GPi) and glutamatergic inputs from motor cortices. In this study, we used electron microscopy to assess potential structural changes in GABAergic and glutamatergic microcircuits in the VApc and CM of MPTP-treated parkinsonian monkeys. The intensity of immunostaining for GABAergic markers in VApc and CM did not differ between control and parkinsonian monkeys. In the electron microscope, three major types of terminals were identified in both nuclei: (a) vesicular glutamate transporter 1 (vGluT1)-positive terminals forming asymmetric synapses (type As), which originate from the cerebral cortex, (b) GABAergic terminals forming single symmetric synapses (type S1), which likely arise from the reticular nucleus and GABAergic interneurons, and (c) GABAergic terminals forming multiple symmetric synapses (type S2), which originate from GPi. The density of As terminals outnumbered that of S1 and S2 terminals in VApc and CM of control and parkinsonian animals. No significant change was found in the abundance and synaptic connectivity of S1 and S2 terminals in VApc or CM of MPTP-treated monkeys, while the prevalence of "As" terminals in VApc of parkinsonian monkeys was 51.4% lower than in controls. The cross-sectional area of vGluT1-positive boutons in both VApc and CM of parkinsonian monkeys was significantly larger than in controls, but their pattern of innervation of thalamic cells was not altered. Our findings suggest that the corticothalamic system undergoes significant synaptic remodeling in the parkinsonian state.

Ipsilateral corticotectal projections from the primary, premotor and supplementary motor cortical areas in adult macaque monkeys: a quantitative anterograde tracing study

The corticotectal projection from cortical motor areas is one of several descending pathways involved in the indirect control of spinal motoneurons. In non-human primates, previous studies reported that cortical projections to the superior colliculus (SC) originated from the premotor cortex (PM) and the primary motor cortex, whereas no projection originated from the supplementary motor area (SMA). The aim of the present study was to investigate and compare the properties of corticotectal projections originating from these three cortical motor areas in intact adult macaques (n = 9). The anterograde tracer biotinylated dextran amine was injected into one of these cortical areas in each animal. Individual axonal boutons, both en passant and terminaux, were charted and counted in the different layers of the ipsilateral SC. The data confirmed the presence of strong corticotectal projections from the PM. A new observation was that strong corticotectal projections were also found to originate from the SMA (its proper division). The corticotectal projection from the primary motor cortex was quantitatively less strong than that from either the premotor or SMAs. The corticotectal projection from each motor area was directed mainly to the deep layer of the SC, although its intermediate layer was also a consistent target of fairly dense terminations. The strong corticotectal projections from non-primary motor areas are in position to influence the preparation and planning of voluntary movements.

Astrocytes modulate thalamic sensory processing via mGlu2 receptor activation

Astrocytes possess many of the same signalling molecules as neurons. However, the role of astrocytes in information processing, if any, is unknown. Using electrophysiological and imaging methods, we report the first evidence that astrocytes modulate neuronal sensory inhibition in the rodent thalamus.

We found that mGlu2 receptor activity reduces inhibitory transmission from the thalamic reticular nucleus to the somatosensory ventrobasal thalamus (VB): mIPSC frequencies in VB slices were reduced by the Group II mGlu receptor agonist LY354740, an effect potentiated by mGlu2 positive allosteric modulator (PAM) LY487379 co-application (30 nM LY354740: 10.0 ± 1.6% reduction; 30 nM LY354740 & 30 μM LY487379: 34.6 ± 5.2% reduction).

We then showed activation of mGlu2 receptors on astrocytes: astrocytic intracellular calcium levels were elevated by the Group II agonist, which were further potentiated upon mGlu2 PAM co-application (300 nM LY354740: ratio amplitude 0.016 ± 0.002; 300 nM LY354740 & 30 μM LY487379: ratio amplitude 0.035 ± 0.003).

We then demonstrated mGlu2-dependent astrocytic disinhibition of VB neurons in vivo: VB neuronal responses to vibrissae stimulation trains were disinhibited by the Group II agonist and the mGlu2 PAM (LY354740: 156 ± 12% of control; LY487379: 144 ± 10% of control). Presence of the glial inhibitor fluorocitrate abolished the mGlu2 PAM effect (91 ± 5% of control), suggesting the mGlu2 component to the Group II effect can be attributed to activation of mGlu2 receptors localised on astrocytic processes within the VB.

Gating of thalamocortical function via astrocyte activation represents a novel sensory processing mechanism. As this thalamocortical circuitry is important in discriminative processes, this demonstrates the importance of astrocytes in synaptic processes underlying attention and cognition.

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Thalamic inhibition is mediated by both neuronal and astrocytic mechanisms.

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Group II mGlu receptor (mGlu2/3) activation can modulate this thalamic inhibition.

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Thalamic astrocytes can be activated upon mGlu2 receptor stimulation.

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This process may enable relevant activity to be discerned from background noise.

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Targeting astrocytic mGlu2 receptors may therefore affect attention and cognition.

Effects of Optogenetic Activation of Corticothalamic Terminals in the Motor Thalamus of Awake Monkeys

The role of the corticothalamic projection in the ventral motor thalamus remains poorly understood. Therefore, we studied the electrophysiological responses of neurons in the basal ganglia and cerebellar receiving-territories of the motor thalamus (BGMT and CbMT, respectively) using optogenetic activation of corticothalamic projections in awake rhesus macaques. After injections of viral vectors carrying the excitatory opsins ChR2 or C1V1 into the primary motor and premotor cortices of two monkeys, we used optrodes to light activate opsin-expressing neurons in cortex or their terminals in the thalamus while simultaneously recording the extracellular activity of neurons in the vicinity of the stimulation sites. As expected, light activation of opsins in the cerebral cortex evoked robust, short-latency increases in firing of cortical neurons. In contrast, light stimulation of corticothalamic terminals induced small-amplitude, long-latency increases and/or decreases of activity in thalamic neurons. In postmortem material, opsins were found to be expressed in cell bodies and dendrites of cortical neurons and along their corticothalamic projections. At the electron microscopic level, opsin labeling was confined to unmyelinated preterminal axons and small terminals that formed asymmetric synapses with dendrites of projection neurons or GABAergic interneurons in BGMT and CbMT and with neurons in the reticular thalamic nucleus. The morphological features of the transfected terminals, along with the long latency and complex physiological responses of thalamic neurons to their activation, suggest a modulatory role of corticothalamic afferents upon the primate ventral motor thalamus.

SIGNIFICANCE STATEMENT

This study provides the first analysis of the physiological effects of cortical inputs on the activity of neurons in the primate ventral motor thalamus using light activation of opsin-containing corticothalamic terminals in awake monkeys. We found that selective light activation of corticothalamic terminals in contact with distal dendrites of thalamocortical neurons and GABAergic interneurons elicits complex patterns of slowly developing excitatory and inhibitory effects in thalamic neurons of the basal ganglia- and cerebellar-receiving regions of the motor thalamus. Our observations suggest a modulatory (instead of a "driver") role of the corticothalamic system in the primate ventral motor thalamus.

The role of tactile afference in shaping motor behaviour and implications for prosthetic innovation

The present review focusses on how tactile somatosensory afference is encoded and processed, and how this information is interpreted and acted upon in terms of motor control. We relate the fundamental workings of the sensorimotor system to the rehabilitation of amputees using modern prosthetic interventions. Our sense of touch is central to our everyday lives, from allowing us to manipulate objects accurately to giving us a sense of self-embodiment. There are a variety of specialised cutaneous mechanoreceptive afferents, which differ in terms of type and density according to the skin site. In humans, there is a dense innervation of our hands, which is reflected in their vast over-representation in somatosensory and motor cortical areas. We review the accumulated evidence from animal and human studies about the precise interplay between the somatosensory and motor systems, which is highly integrated in many brain areas and often not separable. The glabrous hand skin provides exquisite, discriminative detail about touch, which is useful for refining movements. When these signals are disrupted, such as through injury or amputation, the consequences are considerable. The development of sensory feedback in prosthetics offers a promising avenue for the full integration of a missing body part. Real-time touch feedback from motor intentions aids in grip control and the ability to distinguish different surfaces, even introducing the possibility of pleasure in artificial touch. Thus, our knowledge from fundamental research into sensorimotor interactions should be used to develop more realistic and integrative prostheses.

Long-range connectomics

Decoding neural algorithms is one of the major goals of neuroscience. It is generally accepted that brain computations rely on the orchestration of neural activity at local scales, as well as across the brain through long-range connections. Understanding the relationship between brain activity and connectivity is therefore a prerequisite to cracking the neural code. In the past few decades, tremendous technological advances have been achieved in connectivity measurement techniques. We now possess a battery of tools to measure brain activity and connections at all available scales. A great source of excitement are the new in vivo tools that allow us to measure structural and functional connections noninvasively. Here, we discuss how these new technologies may contribute to deciphering the neural code.

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.

Pathways for emotions and attention converge on the thalamic reticular nucleus in primates

How do emotional events readily capture our attention? To address this question we used neural tracers to label pathways linking areas involved in emotional and attentional processes in the primate brain (Macaca mulatta). We report that a novel pathway from the amygdala, the brain's emotional center, targets the inhibitory thalamic reticular nucleus (TRN), a key node in the brain's attentional network. The amygdalar pathway formed unusual synapses close to cell bodies of TRN neurons and had more large and efficient terminals than pathways from the orbitofrontal cortex and the thalamic mediodorsal nucleus, which similarly innervated extensive TRN sites. The robust amygdalar pathway provides a mechanism for rapid shifting of attention to emotional stimuli. Acting synergistically, pathways from the amygdala and orbitofrontal cortex provide a circuit for purposeful assessment of emotional stimuli. The different pathways to TRN suggest distinct mechanisms of attention to external and internal stimuli that may be differentially disrupted in anxiety and mood disorders and may be selectively targeted for therapeutic interventions.

Cerebral causes and consequences of parkinsonian resting tremor: a tale of two circuits?

Tremor in Parkinson's disease has several mysterious features. Clinically, tremor is seen in only three out of four patients with Parkinson's disease, and tremor-dominant patients generally follow a more benign disease course than non-tremor patients. Pathophysiologically, tremor is linked to altered activity in not one, but two distinct circuits: the basal ganglia, which are primarily affected by dopamine depletion in Parkinson's disease, and the cerebello-thalamo-cortical circuit, which is also involved in many other tremors. The purpose of this review is to integrate these clinical and pathophysiological features of tremor in Parkinson's disease. We first describe clinical and pathological differences between tremor-dominant and non-tremor Parkinson's disease subtypes, and then summarize recent studies on the pathophysiology of tremor. We also discuss a newly proposed ‘dimmer-switch model’ that explains tremor as resulting from the combined actions of two circuits: the basal ganglia that trigger tremor episodes and the cerebello-thalamo-cortical circuit that produces the tremor. Finally, we address several important open questions: why resting tremor stops during voluntary movements, why it has a variable response to dopaminergic treatment, why it indicates a benign Parkinson's disease subtype and why its expression decreases with disease progression.

Review of motor and phantom-related imagery

This review presents a summary of recent efforts in understanding the systems of the brain involved in motor imagery. Motor imagery likely involves many cortical regions in its generation, but in action may also involve subcortical structures. The parietal lobe seems to be particularly important, as demonstrated by brain imaging studies and patients with lesions of this region. Brain activity correlated with imagery may be related to an efference copy used to compare with peripheral sensory signals for the correction of movement. Amputees with phantom representations have also provided valuable information in this field, as they demonstrate cortical reorganization, which also alters imagery of the missing limb. The following summary explores the recent difficult and challenging studies used to tease out motor imagery in man.

Direct optic nerve pulvinar connections defined by diffusion MR tractography in humans: implications for photophobia

The pathway that underlies exacerbation of migraine headache by light has not been elucidated in the human brain but has recently been reported in a rodent model. We employ diffusion weighted imaging and probabilistic tractography to map connectivity of direct pathways from the optic nerve to the pulvinar implicated with whole-body allodynia during migraine. Nine healthy subjects were recruited to the study and underwent scanning on a 3T magnet. We were able to define well-known image-forming (optic nerve -> lateral geniculate -> visual cortex) as well as a less known nonimage forming visual pathway from the optic chiasm to the pulvinar, and from the pulvinar to several associative cortical brain regions. Such pathway may allow photic signals to converge on a thalamic region we described recently to be selectively activated during migraine headache. Consistent with physiological and anatomical studies in rats, the data provide an anatomical substrate for exacerbation of migraine headache by light in the human.

Demonstration of motor imagery movement and phantom movement-related neuronal activity in human thalamus

Functional imaging studies show that motor imagery activates multiple structures in the human forebrain. We now show that phantom movements in an amputee and imagined movements in intact individuals elicit responses from neurons in several human thalamic nuclei. These include the somatic sensory nucleus receiving input from the periphery (ventral caudal), and the motor nuclei receiving input from the cerebellum [ventral intermediate (Vim)] and the basal ganglia [ventral oral posterior (Vop)]. Seven neurons in the amputee showed phantom movement-related activity (three Vim, two Vop, and two ventral caudal). In addition, seven neurons in a group of three controls showed motor imagery-related activity (four Vim and three Vop). These studies were performed during single neuron recording sessions in patients undergoing therapeutic treatment of phantom pain, tremor, and chronic pain conditions by thalamic stimulation. The activity of neurons in these sensory and motor nuclei, respectively, may encode the expected sensory consequences and the dynamics of planned movements.

Triadic synaptic interactions of large corticothalamic terminals in non-lemniscal thalamic nuclei of the cat auditory system

Large corticothalamic (CT) terminals, presumed to originate from cortical layer 5 pyramidal cells, are distributed predominantly in non-specific thalamic nuclei in mammals. In the auditory system, little is known about whether these CT projections participate in the synaptic aggregation referred to as the triad. We studied synaptic interactions of these terminals with neuronal elements in one of the auditory non-lemniscal thalamic nuclei, the dorsal nucleus of the medial geniculate complex (MGC), in cats. After injections of an anterograde tracer in the primary auditory cortex, areas containing labeled large terminals were examined using an electron microscope. It was revealed that a fraction of large CT terminals participated in complicated synaptic arrangements: labeled terminals making synaptic contacts with vesicle-free dendrites, probably of thalamic principal neurons, and/or vesicle-filled neuronal profiles, probably of presynaptic dendrites (PSDs) of interneurons. In reconstructions or even in single sections, we found that these synaptic connections participated in triadic arrangements. Thus, PSDs postsynaptic to the labeled CT terminals were in turn presynaptic to the vesicle-free dendrites.

Involvement of the subthalamic nucleus in engagement with behaviourally relevant stimuli

In this study we investigate how the basal ganglia (BG) may process the behavioural relevance of environmental cues by recording local field potentials (LFPs) in the subthalamic nucleus of patients with Parkinson's disease who had undergone implantation of electrodes for deep brain stimulation. Fourteen patients were recorded as they performed a paradigm dissociating warning cue presentation from programming related to execution of specific tasks. Target and non-target warning cues of differing behavioural relevance were contrasted, and we evaluated if warning cue-evoked activities varied according to whether the eventual task to be performed was motor or cognitive and whether patients were receiving or withdrawn from dopaminergic therapy. Warning cues evoked a complex temporal sequence of activities with three epochs over the 760 ms following the onset of the warning cue. In contrast to the initial evoked LFP, evoked activities over two later periods were significantly influenced by behavioural relevance and by treatment state. The early activity was likely related to the initial orientating of attention induced by a novel target, while the delayed responses in our paradigm may reflect processing related to the non-motor resource implications of cues. The results suggest that the BG are intimately involved in the evaluation of changes in the environment and of their behavioural significance. The latter process is partly modulated by dopamine. Weakness in this function might contribute to the behavioural impairment that can follow BG lesions and surgery.

The thalamocortical projection systems in primate: an anatomical support for multisensory and sensorimotor interplay

Multisensory and sensorimotor integrations are usually considered to occur in superior colliculus and cerebral cortex, but few studies proposed the thalamus as being involved in these integrative processes. We investigated whether the organization of the thalamocortical (TC) systems for different modalities partly overlap, representing an anatomical support for multisensory and sensorimotor interplay in thalamus. In 2 macaque monkeys, 6 neuroanatomical tracers were injected in the rostral and caudal auditory cortex, posterior parietal cortex (PE/PEa in area 5), and dorsal and ventral premotor cortical areas (PMd, PMv), demonstrating the existence of overlapping territories of thalamic projections to areas of different modalities (sensory and motor). TC projections, distinct from the ones arising from specific unimodal sensory nuclei, were observed from motor thalamus to PE/PEa or auditory cortex and from sensory thalamus to PMd/PMv. The central lateral nucleus and the mediodorsal nucleus project to all injected areas, but the most significant overlap across modalities was found in the medial pulvinar nucleus. The present results demonstrate the presence of thalamic territories integrating different sensory modalities with motor attributes. Based on the divergent/convergent pattern of TC and corticothalamic projections, 4 distinct mechanisms of multisensory and sensorimotor interplay are proposed.

Anti-Nogo-A antibody treatment enhances sprouting of corticospinal axons rostral to a unilateral cervical spinal cord lesion in adult macaque monkey

After injury, regrowth of axons in mammalian adult central nervous system is highly limited. However, in monkeys subjected to unilateral cervical lesion (C7-C8 level), neutralization of an important neurite outgrowth inhibitor, Nogo-A, stimulated axonal sprouting caudal to the lesion, accompanied by enhanced functional recovery of manual dexterity, compared with lesioned monkeys treated with a control antibody (Freund et al. [2006] Nat. Med. 12:790-792). The present study aimed at comparing the same two groups of monkeys for axonal sprouting rostral to the cervical lesion. The corticospinal tract was labeled by injecting the anterograde tracer biotinylated dextran amine into the contralesional motor cortex. The corticospinal axons were interrupted at the level of the lesion, accompanied by retrograde axonal degeneration (axon dieback), reflected by the presence of terminal retraction bulbs. The number of terminal retraction bulbs was lower in anti-Nogo-A antibody treated monkeys, and, when present, they were found closer to the lesion than in control-antibody treated monkeys. Compared with control antibody treated monkeys, the anti-Nogo-A antibody treated monkeys exhibited an increased cumulated axon arbor length and a higher number of axon arbors going in the medial direction from the white to the gray matter. Higher in the cervical cord (at C5 level), the anti-Nogo-A treatment enhanced the number of corticospinal fibers crossing the midline, suggesting axonal sprouting. Thus, the anti-Nogo-A antibody treatment enhanced axonal sprouting rostral to the cervical lesion; some of these fibers grew around the lesion and into the caudal spinal segments. These processes paralleled the observed improved functional recovery.

Thalamocortical and the dual pattern of corticothalamic projections of the posterior parietal cortex in macaque monkeys

The corticothalamic projection includes a main, modulatory projection from cortical layer VI terminating with small endings whereas a less numerous, driving projection from layer V forms giant endings. Such dual pattern of corticothalamic projections is well established in rodents and cats for many cortical areas. In non-human primates (monkeys), it has been reported for the primary sensory cortices (A1, V1, S1), the motor and premotor cortical areas and, in the parietal lobe, also for area 7. The present study aimed first at refining the cytoarchitecture parcellation of area 5 into the sub-areas PE and PEa and, second, establishing whether area 5 also exhibits this dual pattern of corticothalamic projection and what is its precise topography. To this aim, the tracer biotinylated dextran amine (BDA) was injected in area PE in one monkey and in area PEa in a second monkey. Area PE sends a major projection terminating with small endings to the thalamic lateral posterior nucleus (LP), ventral posterior lateral nucleus (VPL), medial pulvinar (PuM) and, but fewer, to ventral lateral posterior nucleus, dorsal division (VLpd), central lateral nucleus (CL) and center median nucleus (CM), whereas giant endings formed restricted terminal fields in LP, VPL and PuM. For area PEa, the corticothalamic projection formed by small endings was found mainly in LP, VPL, anterior pulvinar (PuA), lateral pulvinar (PuL), PuM and, to a lesser extent, in ventral posterior inferior nucleus (VPI), CL, mediodorsal nucleus (MD) and CM. Giant endings originating from area PEa formed restricted terminal fields in LP, VPL, PuA, PuM, MD and PuL. Furthermore, the origin of the thalamocortical projections to areas PE and PEa was established, exhibiting clusters of neurons in the same thalamic nuclei as above, in other words predominantly in the caudal thalamus. Via the giant endings CT projection, areas PE and PEa may send feedforward, transthalamic projections to remote cortical areas in the parietal, temporal and frontal lobes contributing to polysensory and sensorimotor integration, relevant for visual guidance of reaching movements for instance.

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

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

Prefrontal projections to the thalamic reticular nucleus form a unique circuit for attentional mechanisms

The inhibitory thalamic reticular nucleus (TRN) intercepts and modulates all corticothalamic and thalamocortical communications. Previous studies showed that projections from sensory and motor cortices originate in layer VI and terminate as small boutons in central and caudal TRN. Here we show that prefrontal projections to TRN in rhesus monkeys have a different topographic organization and mode of termination. Prefrontal cortices projected mainly to the anterior TRN, at sites connected with the mediodorsal, ventral anterior, and anterior medial thalamic nuclei. However, projections from areas 46, 13, and 9 terminated widely in TRN and colocalized caudally with projections from temporal auditory, visual, and polymodal association cortices. Population analysis and serial EM reconstruction revealed two distinct classes of corticoreticular terminals synapsing with GABA/parvalbumin-positive dendritic shafts of TRN neurons. Most labeled boutons from prefrontal axons were small, but a second class of large boutons was also prominent. This is in contrast to the homogeneous small TRN terminations from sensory cortices noted previously and in the present study, which are thought to arise exclusively from layer VI. The two bouton types were often observed on the same axon, suggesting that both prefrontal layers V and VI could project to TRN. The dual mode of termination suggests a more complex role of prefrontal input in the functional regulation of TRN and gating of thalamic output back to the cortex. The targeting of sensory tiers of TRN by specific prefrontal areas may underlie attentional regulation for the selection of relevant sensory signals and suppression of distractors.

Callosal connections of dorsal versus ventral premotor areas in the macaque monkey: a multiple retrograde tracing study

Background

The lateral premotor cortex plays a crucial role in visually guided limb movements. It is divided into two main regions, the dorsal (PMd) and ventral (PMv) areas, which are in turn subdivided into functionally and anatomically distinct rostral (PMd-r and PMv-r) and caudal (PMd-c and PMv-c) sub-regions. We analyzed the callosal inputs to these premotor subdivisions following 23 injections of retrograde tracers in eight macaque monkeys. In each monkey, 2–4 distinct tracers were injected in different areas allowing direct comparisons of callosal connectivity in the same brain.

Results

Based on large injections covering the entire extent of the corresponding PM area, we found that each area is strongly connected with its counterpart in the opposite hemisphere. Callosal connectivity with the other premotor areas, the primary motor cortex, prefrontal cortex and somatosensory cortex varied from one area to another. The most extensive callosal inputs terminate in PMd-r and PMd-c, with PMd-r strongly connected with prefrontal cortex. Callosal inputs to PMv-c are more extensive than those to PMv-r, whose connections are restricted to its counterpart area. Quantitative analysis of labelled cells confirms these general findings, and allows an assessment of the relative strength of callosal inputs.

Conclusion

PMd-r and PMv-r receive their strongest callosal inputs from their respective counterpart areas, whereas PMd-c and PMv-c receive strong inputs from heterotopic areas as well (namely from PMd-r and PMv-r, respectively). Finally, PMd-r stands out as the lateral premotor area with the strongest inputs from the prefrontal cortex, and only the PMd-c and PMv-c receive weak callosal inputs from M1.

Dual axonal terminations from the retrosplenial and visual association cortices in the laterodorsal thalamic nucleus of the rat

Light and electron microscopic tracing studies were conducted to assess the synaptic organization in the laterodorsal thalamic nucleus (LD) of the rat and the laminar origins of corticothalamic terminals from the retrosplenial and visual association cortices to LD. A survey of the general ultrastructure of LD revealed at least three types of presynaptic terminals identified on the basis of size, synaptic vesicle morphology, and synaptic membrane specializations: (1) small axon terminals with round synaptic vesicles (SR), which accounted for the majority of terminal profiles and made asymmetric synaptic contacts predominantly with small dendritic shafts and spines; (2) large axon terminals with round synaptic vesicles (LR), which formed asymmetric synaptic contacts mainly with large dendritic shafts; and (3) small to medium-size axon terminals with pleomorphic synaptic vesicles (SMP), which symmetrically synapsed with a wide range of postsynaptic structures from cell bodies to small dendrites. Synaptic glomeruli were identified, whereas no presynaptic dendrites were found. To characterize and identify corticothalamic terminals arising from the retrosplenial and visual association cortices that project to LD, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into these cortices. Axons anterogradely labeled with WGA-HRP ended in both SR and LR terminals. On the other hand, dextran-tetramethylrhodamine injected into LD as a retrograde fluorescent tracer labeled large pyramidal cells of layer V as well as small round or multiform cells of layer VI in the retrosplenial and visual association cortices. These findings provide the possibility that corticothalamic terminations from cortical neurons in layer V end as LR terminals, while those from neurons in layer VI end as SR boutons.

The importance of being agranular: a comparative account of visual and motor cortex

The agranular cortex is an important landmark-anatomically, as the architectural flag of mammalian motor cortex, and historically, as a spur to the development of theories of localization of function. But why, exactly, do agranularity and motor function go together? To address this question, it should be noted that not only does motor cortex lack granular layer four, it also has a relatively thinner layer three. Therefore, it is the two layers which principally constitute the ascending pathways through the sensory (granular) cortex that have regressed in motor cortex: simply stated, motor cortex does not engage in serial reprocessing of incoming sensory data. But why should a granular architecture not be demanded by the downstream relay of motor instructions through the motor cortex? The scant anatomical evidence available regarding laminar patterns suggests that the pathways from frontal and premotor areas to the primary motor cortex actually bear a greater resemblance to the descending, or feedback connections of sensory cortex that avoid the granular layer. The action of feedback connections is generally described as "modulatory" at a cellular level, or "selective" in terms of systems analysis. By contrast, ascending connections may be labelled "driving" or "instructive". Where the motor cortex uses driving inputs, they are most readily identified as sensory signals instructing the visual location of targets and the kinaesthetic state of the body. Visual signals may activate motor concepts, e.g. "mirror neurons", and the motor plan must select the appropriate muscles and forces to put the plan into action, if the decision to move is taken. This, perhaps, is why "driving" motor signals might be inappropriate-the optimal selection and its execution are conditional upon both kinaesthetic and motivational factors. The argument, summarized above, is constructed in honour of Korbinian Brodmann's centenary, and follows two of the fundamental principles of his school of thought: that uniformities in cortical structure, and development imply global conservation of some aspects of function, whereas regional variations in architecture can be used to chart the "organs" of the cortex, and perhaps to understand their functional differences.

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.

Differences in projection patterns between large and small corticothalamic terminals

We injected tracer into wide regions of visual cortex in the cat to produce retrograde and orthograde labeling in the thalamus, chiefly in the lateral geniculate nucleus and lateral posterior-pulvinar complex (LP-Pulvinar). We used the electron microscope to measure the sizes of orthogradely labeled terminals in thalamus and used these measurements to help determine whether the terminals were "RL" (large, presumed excitatory) or "RS" (small, presumed excitatory). We also distinguished reciprocal regions, which were zones of corticothalamic feedback defined by the presence of many retrogradely labeled cell bodies and orthogradely labeled terminals, from nonreciprocal regions, which were zones of feedforward corticothalamic projections defined by the presence of orthogradely labeled terminals alone. The lateral geniculate nucleus, a reciprocal region, had retrogradely labeled cell bodies as well as labeled RS terminals. Likewise, reciprocal regions in LP-Pulvinar were dominated by labeled RS terminals. In contrast, nonreciprocal regions were dominated by labeled RL terminals. Based on other evidence of corticothalamic projections that RL and RS terminals derive, respectively, from layer 5 and layer 6, we suggest the same relationship here, leading to the conclusion that the corticothalamic input from layer 6 is largely feedback, whereas that from layer 5 is largely feedforward. This finding lends credence to a recent hypothesis that layer 5 corticothalamic axons represent the afferent limb of a cortico-thalamo-cortical pathway that is critical for corticocortical communication.

The dual pattern of corticothalamic projection of the primary auditory cortex in macaque monkey

The distribution and terminal morphology of the corticothalamic projection originating from the primary auditory cortex (A1) were established in a macaque monkey, using the anterograde (and retrograde) tracer biotinylated dextran amine. A dense corticothalamic projection from A1 was found in the ventral (vMGB) and dorsal (dMGB) divisions of the medial geniculate body and, to a lesser extent, in the medial division (mMGB), the posterior thalamic nucleus (PO) and the suprageniculate nucleus. Most terminal boutons were small (<1 microm), except some large boutons (2-6 microm) located in PO and vMGB. The data demonstrate that the corticothalamic projection from A1 in primate consists of two types of terminals (small and giant endings) in line with previous observations in rat and cat. Retrogradely labeled thalamocortical neurons formed clusters generally overlapping the corticothalamic terminal fields.

Reevaluation of the primary motor cortex connections with the thalamus in primates

Six injections (approximately 1 mm in diameter) of biotinylated dextran amine (BDA) were placed in different locations of the primary motor cortex of the rhesus monkey. Anterograde and retrograde labeling patterns in the thalamus were charted and individual labeled axons traced in continuous serial sections. Both anterograde and retrograde labeling in the thalamus was extensive, spanning several millimeters mediolaterally and including ventral lateral, ventral anterior, centromedian, and centrolateral nuclei. Paracentral, mediodorsal, lateral posterior, and medial pulvinar nuclei were also labeled. Two basic types of corticothalamic axons were identified: small to medium-width, type 1 axons that formed large terminal fields with small boutons, and thick, type 2 axons that formed small terminal fields with large boutons. Within each group, subtypes were identified based on specific features of the axons and terminals: two subtypes of type 1 axons and four subtypes of type 2 axons. The results revealed multiple modes of corticothalamic connectivity: sparsely distributed type 1 axons, dense plexuses of type 1 axons, type 2 axon terminal fields either singly or in clusters, and mixed plexuses of type 1 and type 2 axons. Only some cells in the plexuses were retrogradely labeled; some plexuses did not contain any labeled neurons, and many retrogradely labeled neurons were in the regions devoid of anterograde labeling. These connectivity patterns differed between thalamic nuclei. The results revealed much more complex relationships between M1 and thalamus than were previously thought to exist. It is suggested that this connectivity is neither of exclusively a feedback nature nor perfectly reciprocal but is subserved by a multitude of channels, most likely originating from different populations of cortical neurons, and feeding into a variety of functionally different neuronal networks, with each processing specific information.

Dissociation of spatial-, object-, and sound-coding neurons in the mediodorsal nucleus of the primate thalamus

The mediodorsal nucleus (MD) is the thalamic gateway to the prefrontal cortex, an area of the brain associated with spatial and object working memory functions. We have recorded single-neuron activities from the MD nucleus in monkeys trained to perform spatial tasks with peripheral visual stimuli and a nonspatial task with foveally presented pictures of objects and faces-tasks identical to those we have previously used to map regional specializations in the dorso- and ventro-lateral prefrontal cortex, respectively. We found that MD neurons exhibited categorical specificity-either responding selectively to locations in the spatial tasks or preferentially to specific representations of faces and objects in the nonspatial task. Spatially tuned neurons were located in parts of the MD connected with the dorsolateral prefrontal cortex while neurons responding to the identity of stimuli mainly occupied more ventral positions in the nucleus that has its connections with the inferior prefrontal convexity. Neuronal responses to auditory stimuli were also examined, and vocalization sensitive neurons were found in more posterior portions of the MD. We conclude that MD neurons are dissociable by their spatial and nonspatial coding properties in line with their cortical connections and that the principle of information segregation in cortico-cortical pathways extends to the "association" nuclei of the thalamus.

Projections of the claustrum to the primary motor, premotor, and prefrontal cortices in the macaque monkey

The claustrum is interconnected with the frontal lobe, including the motor cortex, prefrontal cortex, and cingulate cortex. The goal of the present study was to assess whether the claustral projections to distinct areas within the frontal cortex arise from separate regions within the claustrum. Multiple injections of tracers were performed in 15 macaque monkeys, aimed toward primary motor area (M1), pre-supplementary motor area (pre-SMA), SMA-proper, rostral (PMd-r) and caudal (PMd-c) parts of the dorsal premotor cortex (PM), rostral (PMv-r) and caudal (PMv-c) parts of the ventral PM, and superior and inferior parts of area 46. The distribution of retrogradely labeled neurons showed no clear segregation along the rostrocaudal axis of the claustrum; they were usually located along the entire anteroposterior extent of the claustrum. For all motor cortical areas, there was a general trend of the labeled neurons to occupy the dorsal and intermediate parts of the claustrum along the dorsoventral axis. The same territories were labeled after injection in area 46, but in addition numerous labeled neurons were found in the most ventral part of the claustrum. At higher resolution, however, there was clear evidence that the territories projecting to pre-SMA and SMA-proper formed separate, interdigitating, clusters along the dorsoventral axis. A comparable local segregation was observed for the two subdivisions of area 46, whereas there was more local overlap among the subareas of PM. The projections from the claustrum to the multiple subareas of the motor cortex and to area 46 arise from largely overlapping territories, with, however, some degree of local segregation.

Origins of callosal projections to the supplementary motor area (SMA): a direct comparison between pre-SMA and SMA-proper in macaque monkeys

The two subdivisions of the supplementary motor area (SMA), the pre-SMA (rostrally) and SMA-proper (caudally), exhibit distinct functional properties and clear differences with respect to their connectivity with the spinal cord, the thalamus, and other homolateral motor cortical areas. The goal of the present study was to establish in monkeys whether these subdivisions also differ with regard to their callosal connectivity. Two fluorescent retrograde tracers (Fast Blue and Diamidino Yellow) were injected in each animal, one in the pre-SMA and the second in the SMA-proper. Tracer injections in the pre-SMA or in SMA-proper resulted in significant numbers of labeled neurons in the opposite SMA, premotor cortex (PM), cingulate motor areas (CMA), and cingulate gyrus. Labeled neurons in M1 were rare, being observed only after injection in the SMA-proper. The two subdivisions of the SMA differed in the proportion of labeled neurons found across areas providing their callosal inputs. The SMA-proper receives about half of its callosal inputs from its counterpart in the other hemisphere (42-65% across monkeys). A comparable proportion of neurons was found in the pre-SMA after injection in the opposite pre-SMA (32-47%). The pre-SMA receives more callosal inputs from the rostral halves of the dorsal PM, the ventral PM, and the CMA than from their caudal halves. In addition, the pre-SMA, but not the SMA-proper, receives callosal inputs from the prefrontal cortex. The SMA-proper receives more callosal inputs from the caudal halves of the dorsal PM and ventral PM than from their rostral halves. The two subdivisions of the SMA receive callosal inputs from the same cortical areas (except the prefrontal cortex and M1), but they differ with respect to the quantitative contribution of each area of origin. In conclusion, quantitative data now support the notion that pre-SMA receives more transcallosal inputs than the SMA-proper.

Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system

All neocortical areas receive thalamic inputs. Some thalamocortical pathways relay information from ascending pathways (first order thalamic relays) and others relay information from other cortical areas (higher order thalamic relays), thus serving a role in corticocortical communication. Most, possibly all, afferents reaching thalamus, ascending and cortical, are branches of axons that innervate lower (motor) centers, so that thalamocortical pathways can be viewed generally as monitors of ongoing motor instructions. In terms of numbers, the thalamic relay is dominated by synapses that modulate the relay functions. One of the roles of these modulatory pathways is to change the transfer of information through the thalamus, in accord with current attentional demands. Other roles remain to be explored. These modulatory functions can be expected to act on corticocortical communication in addition to their action on ascending pathways.

Connections of higher order visual relays in the thalamus: a study of corticothalamic pathways in cats

Axonal markers injected into layers 5 and 6 of cortical areas 17, 18, or 19 labeled axons going to the lateral geniculate nucleus (LGN), the lateral part of the lateralis posterior nucleus (LPl), and pulvinar (P). Area 19 sends fine axons (type 1, Guillery [1966] J Comp Neurol 128:21-50) to LGN, LPl, and P, and thicker, type 2 axons to LPl and P. Areas 17 and 18 send type 1 axons to LGN, and a few type 1, but mainly type 2 axons to LPl and P. Type 1 and 2 axons from a single small cortical locus distribute to distinct, generally nonoverlapping parts of LP and P; type 1 axons have a broader distribution than type 2 axons. Type 2 axons, putative drivers of thalamic relay cells (Sherman and Guillery [1998] Proc Natl Acad Sci USA 95:7121-7126; Sherman and Guillery [2001] Exploring the thalamus. San Diego: Academic Press), supply small terminal arbors (100- to 200-microm diameter) in LPl and P, and then continue into the midbrain. Each thalamic type 2 arbor contains two terminal types. One, at the center of the arbor, is complex and multilobulated; the other, with a more peripheral distribution, is simpler and may contribute to adjacent arbors. Type 2 arbors from a single injection are scattered around and along "isocortical columns" in LPl, (i.e., columns that represent cells having connections to a common cortical locus). Evidence is presented that the connections and consequently the functional properties of cells in LP change along these isocortical columns. Type 2 driver afferents from a single cortical locus can, thus, be seen as representing functionally distinct, parallel pathways from cortex to thalamus.

Thalamic terminal morphology and distribution of single corticothalamic axons originating from layers 5 and 6 of the cat motor cortex

We investigated the axonal morphology of single corticothalamic (CT) neurons of the motor cortex (Mx) in the cat thalamus, using a neuronal tracer, biotinylated dextran amine (BDA). After localized injection of BDA into the Mx, labeled CT axons were found ipsilaterally in the thalamic reticular nucleus (TRN), the ventroanterior-ventrolateral complex (VA-VL), the central lateral nucleus (CL), the central medial nucleus, and the centromedian nucleus, but with the primary focus in the VA-VL. The terminals in the VA-VL formed a large laminar cluster, which extended approximately in parallel with the internal medullary lamina. The laminar organization mirrored morphologic features of single CT axons. We reconstructed the trajectories of 25 single CT axons that arose from layer V (16 axons) or layer VI (9 axons) and terminated in the VA-VL. Terminals of single CT axons that originated from both layer V and layer VI were confined within a laminar structure about 700 microm thick, suggesting the existence of laminar input organization in the VA-VL. Otherwise, the two groups of the CT axons showed contrasting features. All of the CT axons derived from layer VI gave rise to a few short collaterals to the TRN and then formed extensive arborization with numerous small, drumstick-like terminals in the VA-VL. On the other hand, the CT axons arising from layer V gave rise to collaterals whose main axons descended into the cerebral peduncle. Each collateral projected to the VA-VL or CL without projection to the TRN and formed a few small clusters of giant terminals. The two groups of CT neurons in the same cortical column had convergent rather than segregated termination in the VA-VL. However, the terminals of layer VI CT neurons were distributed diffusely and widely in the VA-VL, whereas the terminals of layer V CT neurons were much more focused and surrounded by the terminals of the former group. These contrasting features of the two types of CT projections appear to represent their different functional roles in the generation of motor commands and control of movements in the Mx.