The study of patients to infer normal brain function has a long tradition in neurology and psychology. More recently, the motor system has been subject to quantitative and computational characterization. The pur- pose of this review is to argue that the lesion approach and theoretical motor control can mutually inform each other. Specifically, one may identify distinct motor control pro- cesses from computational models and map them onto specific deficits in patients. Here we review some of the impairments in motor control, motor learning and higher- order motor control in patients with lesions of the corti- cospinal tract, the cerebellum, parietal cortex, the basal ganglia, and the medial temporal lobe. We attempt to explain some of these impairments in terms of computa- tional ideas such as state estimation, optimization, prediction, cost, and reward. We suggest that a function of the cerebellum is system identification: to build internal models that predict sensory outcome of motor commands and correct motor commands through internal feedback. A function of the parietal cortex is state estimation: to inte- grate the predicted proprioceptive and visual outcomes with sensory feedback to form a belief about how the commands affected the states of the body and the envi- ronment. A function of basal ganglia is related to optimal control: learning costs and rewards associated with sensory states and estimating the ''cost-to-go'' during execution of a motor task. Finally, functions of the primary and the premotor cortices are related to implementing the optimal control policy by transforming beliefs about proprioceptive and visual states, respectively, into motor commands.

/pdf/a-computational-neuroanatomy-for-motor-control-3zgpp9nlc9.pdf

A computational neuroanatomy for motor control

The cerebellum is an essential part of the neural network involved in adapting goal-directed arm movements. This adaptation might rely on two distinct signals: a sensory prediction error or a motor correction. Sensory prediction errors occur when an initial motor command is generated but the predicted sensory consequences do not match the observed values. In some tasks, these sensory errors are monitored and result in on-line corrective motor output as the movement progresses. Here we asked whether cerebellum-dependent adaptation of reaching relies on sensory or on-line motor corrections. Healthy controls and people with hereditary cerebellar ataxia reached during a visuomotor perturbation in two conditions: "shooting" movements without on-line corrections and "pointing" movements that allowed for on-line corrections. Sensory (i.e., visual) errors were available in both conditions. Results showed that the addition of motor corrections did not influence adaptation in control subjects, suggesting that only sensory errors were needed for learning. Cerebellar subjects were comparably impaired in both adaptation conditions relative to controls, despite abnormal and inconsistent on-line motor correction. Specifically, poor on-line motor corrections were unrelated to cerebellar subjects' adaptation deficit (i.e., adaptation did not worsen), further suggesting that only sensory prediction errors influence this process. Therefore adaptation to visuomotor perturbations depends on the cerebellum and is driven by the mismatch between predicted and actual sensory outcome of motor commands.

/pdf/sensory-prediction-errors-drive-cerebellum-dependent-1p06ibw5tx.pdf

Sensory Prediction Errors Drive Cerebellum-Dependent Adaptation of Reaching

Adaptation to a novel visuomotor transformation has revealed important principles regarding learning and memory. Computational and behavioral studies have suggested that acquisition and retention of a new visuomotor transformation are distinct processes. However, this dissociation has never been clearly shown. Here, participants made fast reaching movements while unexpectedly a 30-degree visuomotor transformation was introduced. During visuomotor adaptation, subjects received cerebellar, primary motor cortex (M1) or sham anodal transcranial direct current stimulation (tDCS), a noninvasive form of brain stimulation known to increase excitability. We found that cerebellar tDCS caused faster adaptation to the visuomotor transformation, as shown by a rapid reduction of movement errors. These findings were not present with similar modulation of visual cortex excitability. In contrast, tDCS over M1 did not affect adaptation, but resulted in a marked increase in retention of the newly learnt visuomotor transformation. These results show a clear dissociation in the processes of acquisition and retention during adaptive motor learning and demonstrate that the cerebellum and primary motor cortex have distinct functional roles. Furthermore, they show that is possible to enhance cerebellar function using tDCS.

/pdf/dissociating-the-roles-of-the-cerebellum-and-motor-cortex-4y7ent1cmg.pdf

Dissociating the Roles of the Cerebellum and Motor Cortex during Adaptive Learning: The Motor Cortex Retains What the Cerebellum Learns

Summary Central post-stroke pain (CPSP) is a neuropathic pain syndrome that can occur after a cerebrovascular accident. This syndrome is characterised by pain and sensory abnormalities in the body parts that correspond to the brain territory that has been injured by the cerebrovascular lesion. The presence of sensory loss and signs of hypersensitivity in the painful area in patients with CPSP might indicate the dual combination of deafferentation and the subsequent development of neuronal hyperexcitability. The exact prevalence of CPSP is not known, partly owing to the difficulty in distinguishing this syndrome from other pain types that can occur after stroke (such as shoulder pain, painful spasticity, persistent headache, and other musculoskeletal pain conditions). Future prospective studies with clear diagnostic criteria are essential for the proper collection and processing of epidemiological data. Although treatment of CPSP is difficult, the most effective approaches are those that target the increased neuronal hyperexcitability.

https://www.thelancet.com/pdfs/journals/laneur/PIIS1474-4422(09)70176-0.pdf

Central post-stroke pain: clinical characteristics, pathophysiology, and management.

Cerebral angiography was inaugurated by Egas Rfoniz of Lisbon in 1926 and has now become one of the standard diagnostic methods of cerebral lesions. Lima’s book is the first systematic treatise on this subject published by the Portuguese school in the English language. The author has been associated as neurosurgeon with RIoniz for more than twenty years, and the combined material of these two investigators comprises more than two thousand angiographies, a numbcr unsurpassed anywhere. Lima corroborates, amplifies and in some rases revises Moniz’ fundamental work which had bcen published previously in the French and German literature. The subject matter is covered in $ell organized and critical fashion, but unfortunately too little attention is paid to the numerous publications on angiography which appeared in Scandinavian, American and German literature, especially after the war. After reviewing the history of angiography, its technique is discussed. Lima advocates surgical exposure of the common carotid artery low a t the neck in preference to the percutaneous technique now practiced in many centers. He performs carotid angiography bilaterally in one sitting under local anesthesia and obtains routinely three lateral films for each injection (the first film to show the arterial phase, the second the early venous phase, and the third the late venous phase). AP projections are rarely used, and stereoscopic films never taken. The next chapter deals in detail with the normal arteriogram and phlebogram obtained by vertebral and carotid injection. Then follows a description of the typical displacements and deformities of cerebral vessels caused by intracranial tumors and, hydrocephalus. I n the next section the angioarchitectonic pattern of cerebral tumors is described. This chapter is particularly interesting and informative since Lima has done much original work on the circulation of meningiomas. The typical double blood supply of meningiomas through branches of the external and internal carotid arteries is discussed, and also the relation of ,angiographic pictures to modification of the circulation time is analyzed. The chapter on cerebral aneurysm is relatively brief but vascular malformations are discussed at length. Angiography offers an important aid i n the diagnosis of cerebral vascular occlusion. Special attention is given to the not uncommon thrombosis of the cervical portion of the carotid artery. This condition produces a characteristic clinical syndrome and can be easily recognized by low cervical carotid angiography.

The Cerebral Cortex of Man

The mechanism of essential tremor (ET) is unclear. Animal models of tremor and functional imaging studies in ET predict that the cerebellum and a cerebellar recipient thalamic nucleus (ventral intermediate, Vim) should exhibit oscillatory activity during rest and during tremor due to abnormal olivo-cerebellar activity. Physiologic responses of 152 single neurons were recorded during awake mapping of the ventral thalamus in seven patients with ET prior to thalamotomy. During postural tremor, spectral cross-correlation analysis demonstrated that 51% of the neurons studied exhibited a concentration of power at tremor frequency that was correlated with electromyography, i.e., tremor neurons. During rest, thalamic neurons did not exhibit tremor-frequency activity. Among the three thalamic nuclei surveyed, Vim had a significantly higher proportion of tremor neurons than did the principal somatic sensory nucleus (ventral caudal, Vc) or a pallidal recipient thalamic nucleus (ventral oral posterior, Vop). Neurons related to active movement (voluntary neurons) had significantly greater tremor-related activity than did nonvoluntary neurons. These findings are not consistent with a model of continuous olivo-cerebellar driving of the motor cortex through thalamic connections. Instead ET may be facilitated by motor circuits that enable tremor-related thalamic activity during voluntary movement. Additionally, a subgroup of tremor neurons with proprioceptive inputs were identified that may allow sensory feedback to access the central tremor network.

https://www.physiology.org/doi/pdf/10.1152/jn.00527.2004

Posture-related oscillations in human cerebellar thalamus in essential tremor are enabled by voluntary motor circuits.

We review the techniques of physiological localization of the site for ventralis intermedius (Vim) thalamotomy or implantation of Vim-deep brain stimulation (DBS) for treatment of parkinsonian, essential, and intention tremor. Both microelectrode and semi-microelectrode techniques are reviewed. We believe the use of microelectrode and semi-microelectrode recordings in combination with Radiological landmarks provide the most accurate localization of the target. In addition to recording, microstimulation of subcortical structures such as Vim and thalamic nucleus ventralis caudal through the microelectrode may improve physiological identification by altering the tremor and evoking somatic sensations, respectively. Microelectrode recording provides the highest resolution picture of the target site at a cost of increased time to locate the target. We also review the relationship between thalamic neuronal firing and electromyographic activity during tremor. Implications of these results for the mechanisms for parkinsonian, essential, and intention tremors are discussed.

Intraoperative microelectrode and semi-microelectrode recording during the physiological localization of the thalamic nucleus ventral intermediate.

Strokes and other forms of injury to the central nervous system cause changes in function because of the injuries themselves and indirectly because injuries cause expression of neural plasticity. Studies in humans undergoing neurosurgical procedures for implantation of electrodes for deep brain stimulation and for making lesions in the brain have contributed understanding of both normal and abnormal functions of the somatic sensory system. This chapter will specifically discuss the reorganization of the ventral caudal (Vc) sensory nucleus of the thalamus that occurs in connection with pain conditions after strokes and spinal cord injuries. It is shown that pain is associated with expression of neural plasticity that alters maps of noxious and innocuous stimulation in the thalamus and affect processing of sensory information. Results from studies of neural activity in the thalamus in humans will be compared with results from animal studies.

Plasticity of pain-related neuronal activity in the human thalamus.

The mechanism of parkinsonian tremor may involve a central oscillator, peripheral feedback to the central nervous system (CNS), or both. The thalamus or the globus pallidus is the most likely site for a central oscillator and would be predicted to generate thalamic tremor-related activity characterized, respectively, by calcium spike-associated bursts and by maximal tremor-related activity in the pallidal relay nucleus of thalamus. Thalamic spike trains demonstrate neither of these characteristics. However, cross-correlation, latency, and transfer function analysis indicate that sensory feedback is a critical element in the relationship between thalamic activity and parkinsonian tremor. Therefore, thalamic spike train activity is most consistent with parkinsonian tremor being mediated by peripheral inputs involved in either an unstable reflex loop or sensory modulation of a central oscillator.

The role of the thalamus and basal ganglia in parkinsonian tremor

Microelectrode studies of single units in the human thalamus during stereotactic surgery offer a unique opportunity to study the organization and plasticity of the sensory thalamus. In this review the authors present results using single-unit microelectrode recording in the mapping of human sensory thalamus in a variety of patients. First they outline the overall organization of the human sensory thalamus, including both somatosensory and pain pathways. They also show that the sensory maps for receptive and projection fields can be altered during pathologic states such as amputation and spinal transection. Additionally, the sensory maps show plasticity during states with abnormal patterns of motor activity, like dystonia. Lastly, they discuss the processing of painful and emotionally laden sensory experiences through the thalamus. The physiologic results of thalamic pain processing are discussed in relation to the sensory-limbic model of pain. The studies reviewed demonstrate the spectrum of stimulus processing and plasticity of both painful and nonpainful signals by the human thalamus.

Microelectrode studies of normal organization and plasticity of human somatosensory thalamus.

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