Acceleration deficit in patients with cerebellar lesions. A study of kinematic and EMG-parameters in fast wrist movements

Disentangling acute motor deficits and adaptive responses evoked by the loss of cerebellar output

Cerebellar patients exhibit a broad range of impairments when performing voluntary movements. However, the sequence of events leading to these deficits and the distinction between primary and compensatory processes remain unclear. We addressed this question by reversibly blocking cerebellar outflow in monkeys performing a planar reaching task. We found that the reduced hand velocity observed under cerebellar block is driven by a combination of a general decrease in muscle torque and a spatially tuned reduction in velocity, particularly pronounced in movements involving inter-joint interactions. The time course of these two processes was examined using repeated movements to the same target under cerebellar block. We found that the reduced velocity was driven by an acute onset of weakness superimposed on a gradually emergent strategy aimed to minimize passive inter-joint interactions. Finally, although the reduced velocity affected movements to all targets, it could not explain the enhanced motor noise observed under cerebellar block, which manifested as decomposed and variable trajectories. Our results suggest that cerebellar deficits lead to motor impairments through a loss of muscle strength and altered motor control strategy to compensate for the impaired control of limb dynamics. However, the loss of feedforward control also leads to increased motor noise, which cannot be strategically eliminated.

Significance Statement

Our study examined the impact of cerebellar dysfunction on motor control by reversibly blocking the cerebellar output in monkeys. Under cerebellar block, movements initially slowed due to acute-onset muscle weakness. Beyond this primary deficit, there was a secondary, seemingly strategic, slowing of movements aimed at mitigating inter-joint interactions associated with rapid, ballistic movements. Finally, during the cerebellar block we observed movement variability increased independently of the reduced velocity, likely reflecting errors in movement planning. Taken together, these findings highlight the role of cerebellar information in motor control and delineate the sequence of processes following cerebellar dysfunction that culminate in a broad range of motor impairments.

Emerging concepts on bradykinesia in non-parkinsonian conditions

BACKGROUND AND PURPOSE

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease. However, clinical and experimental studies indicate that bradykinesia may also be observed in various neurological diseases not primarily characterized by parkinsonism. These conditions include hyperkinetic movement disorders, such as dystonia, chorea, and essential tremor. Bradykinesia may also be observed in patients with neurological conditions that are not seen as "movement disorders," including those characterized by the involvement of the cerebellum and corticospinal system, dementia, multiple sclerosis, and psychiatric disorders.

METHODS

We reviewed clinical reports and experimental studies on bradykinesia in non-parkinsonian conditions and discussed the major findings.

RESULTS

Bradykinesia is a common motor abnormality in non-parkinsonian conditions. From a pathophysiological standpoint, bradykinesia in neurological conditions not primarily characterized by parkinsonism may be explained by brain network dysfunction.

CONCLUSION

In addition to the pathophysiological implications, the present paper highlights important terminological issues and the need for a new, more accurate, and more widely used definition of bradykinesia in the context of movement disorders and other neurological conditions.

Consensus paper: Decoding the Contributions of the Cerebellum as a Time Machine. From Neurons to Clinical Applications

Time perception is an essential element of conscious and subconscious experience, coordinating our perception and interaction with the surrounding environment. In recent years, major technological advances in the field of neuroscience have helped foster new insights into the processing of temporal information, including extending our knowledge of the role of the cerebellum as one of the key nodes in the brain for this function. This consensus paper provides a state-of-the-art picture from the experts in the field of the cerebellar research on a variety of crucial issues related to temporal processing, drawing on recent anatomical, neurophysiological, behavioral, and clinical research.The cerebellar granular layer appears especially well-suited for timing operations required to confer millisecond precision for cerebellar computations. This may be most evident in the manner the cerebellum controls the duration of the timing of agonist-antagonist EMG bursts associated with fast goal-directed voluntary movements. In concert with adaptive processes, interactions within the cerebellar cortex are sufficient to support sub-second timing. However, supra-second timing seems to require cortical and basal ganglia networks, perhaps operating in concert with cerebellum. Additionally, sensory information such as an unexpected stimulus can be forwarded to the cerebellum via the climbing fiber system, providing a temporally constrained mechanism to adjust ongoing behavior and modify future processing. Patients with cerebellar disorders exhibit impairments on a range of tasks that require precise timing, and recent evidence suggest that timing problems observed in other neurological conditions such as Parkinson's disease, essential tremor, and dystonia may reflect disrupted interactions between the basal ganglia and cerebellum.The complex concepts emerging from this consensus paper should provide a foundation for further discussion, helping identify basic research questions required to understand how the brain represents and utilizes time, as well as delineating ways in which this knowledge can help improve the lives of those with neurological conditions that disrupt this most elemental sense. The panel of experts agrees that timing control in the brain is a complex concept in whom cerebellar circuitry is deeply involved. The concept of a timing machine has now expanded to clinical disorders.

A loss of a velocity-duration trade-off impairs movement precision in patients with cerebellar degeneration

Current theories discussing the role of the cerebellum have been consistently pointing towards the concept of motor learning. The unavailability of a structure for motor learning able to use information on past errors to change future movements should cause consistent metrical deviations and an inability to correct them; however, it should not boost "motor noise." However, dysmetria, a loss of endpoint precision and an increase in endpoint variability ("motor noise") of goal-directed movements is the central aspect of cerebellar ataxia. Does the prevention of dysmetria or "motor noise" by the healthy cerebellum tell us anything about its normal function? We hypothesize that the healthy cerebellum is able to prevent dysmetria by adjusting movement duration such as to compensate changes in movement velocity. To address this question, we studied fast goal-directed index finger movements in patients with global cerebellar degeneration and in healthy subjects. We demonstrate that healthy subjects are able to maintain endpoint precision despite continuous fluctuations in movement velocity because they are able to adjust the overall movement duration in a fully compensatory manner ("velocity-duration trade-off"). We furthermore provide evidence that this velocity-duration trade-off accommodated by the healthy cerebellum is based on a priori information on the future movement velocity. This ability is lost in cerebellar disease. We suggest that the dysmetria observed in cerebellar patients is a direct consequence of the loss of a cerebellum-based velocity-duration trade-off mechanism that continuously fine-tunes movement durations using information on the expected velocity of the upcoming movement.

Kinematics of arm joint rotations in cerebellar and unskilled subjects associated with the inability to throw fast

Cerebellar subjects and unskilled throwers cannot produce fast arm movements when throwing. We investigated the arm movement kinematics associated with this lack of skill. Cerebellar subjects and matched controls, and skilled throwers throwing with their skilled (dominant) and unskilled (nondominant) arms, were instructed to make slow, medium, and fast 3-D overarm throws from a sitting position. Only the fast throws were analyzed in detail. Joint motions were computed from angular positions of arm segments recorded with search coils. When throwing, both the cerebellar group and the unskilled-arm group had slower arm movements, and slower elbow extension and wrist flexion velocities than their reference groups. They also had similar magnitudes of many kinematic parameters, e.g., both cerebellar and unskilled groups had similar elbow extension and wrist flexion velocities. Compared to their reference groups, both the cerebellar and unskilled-arm groups also had a smaller elbow extension acceleration, a smaller shoulder adduction deceleration, and the absence of a large elbow extension deceleration before ball release. Similar decreases in joint velocities and in joint accelerations and decelerations in the cerebellar and unskilled groups are consistent with the idea that the absence of the skill of throwing fast in both groups is associated with an inability to exploit interaction torques.

Ordered motor-unit firing behavior in acute cerebellar stroke

It is known that at any given force level, the lower-threshold motor units generally fire at greater rates than the higher-threshold units during isometric tasks of extremity muscles. In addition to this hierarchical arrangement, firing rates of motor units fluctuate in unison with nearly no time delay; an observation that has led to the concept of common drive, a basic motoneuronal rule. Although it is established that the cerebellum plays a critical function in motor control, its role in the genesis, triggering, selection, and monitoring of motor-unit firing pattern discharges during isometric tasks is unknown. We applied an electromyographic (EMG) decomposition technique, known as precision decomposition, to accurately identify motor-unit firing times from the EMG signal recorded from the first dorsal interosseous muscle to unravel the features of motor-unit firings in three patients presenting a unilateral cerebellar stroke and exhibiting an acute cerebellar syndrome. We observed ataxic isometric force during visually guided abduction of the index finger on the affected side. However, the hierarchical response of individual motor units was spared. Furthermore, acute cerebellar ataxia was not associated with a loss of the common drive.

Pathophysiology of spastic paresis. I: Paresis and soft tissue changes

Spastic paresis follows chronic disruption of the central execution of volitional command. Motor function in patients with spastic paresis is subjected over time to three fundamental insults, of which the last two are avoidable: (1) the neural insult itself, which causes paresis, i.e., reduced voluntary motor unit recruitment; (2) the relative immobilization of the paretic body part, commonly imposed by the current care environment, which causes adaptive shortening of the muscles left in a shortened position and joint contracture; and (3) the chronic disuse of the paretic body part, which is typically self-imposed in most patients. Chronic disuse causes plastic rearrangements in the higher centers that further reduce the ability to voluntarily recruit motor units, i.e., that aggravate baseline paresis. Part I of this review focuses on the pathophysiology of the first two factors causing motor impairment in spastic paresis: the vicious cycle of paresis-disuse-paresis and the contracture in soft tissues.

Coordination of oral and laryngeal movements in the perceptually fluent speech of adults who stutter

This work investigated whether stuttering and nonstuttering adults differ in the coordination of oral and laryngeal movements during the production of perceptually fluent speech. This question was addressed by completing correlation analyses that extended previous acoustic studies by others as well as inferential analyses based on the within-subject central tendency and variability of acoustic and physiological indices of oral-laryngeal control and coordination. Stuttering and nonstuttering adults produced the target /p/ as the medial consonant in C(1)V(1)#C(2)V(2)C(3) sequences (C = consonant; V = vowel or diphthong; # = word boundary) embedded in utterances differing in length and location of the target movements. No between-groups differences were found for across- or within-subject correlations between acoustic measures of stop gap and voice onset time (VOT). However, the acoustic data did show longer durations for devoicing interval and VOT in the stuttering versus nonstuttering individuals, in the absence of a difference for a proportional measure specifically reflecting oral-laryngeal relative timing. Analyses of combined kinematic and electroglottographic data revealed that the stuttering individuals' speech was also characterized by (a) longer durations from bilabial closing movement onset and peak velocity to V(1) vocal fold vibration offset and (b) greater within-subject variability for dependent variables that were physiological indices of devoicing interval and VOT, but again no between-groups differences were found for specific indices of oral-laryngeal relative timing. Overall, findings suggest that, for the production of voiceless bilabial stops in perceptually fluent speech, stuttering and nonstuttering adults differ in the duration of intervals defined by events within as well as across the oral and laryngeal subsystems, but the groups show similar patterns of relative timing for the involved oral and laryngeal movements.

Deficits in phasic muscle force generation explain insufficient compensation for interaction torque in cerebellar patients

A simple paradigm was used to investigate how patients with cerebellar lesions cope with the need to correct for joint interactions during a multi-joint movement. Normal subjects and patients with cerebellar degeneration performed fast unconstrained elbow flexions with the instruction to voluntarily fixate the shoulder joint. Angular kinematics and inverse dynamics analyses were performed. A susceptibility index quantified how strong-concomitant shoulder-motion depended on interactions from the elbow. Amplitudes of involuntary shoulder movements increased with elbow movement speed and were generally larger in patients. Susceptibility indices were smaller in patients, indicating a more variable compensatory response, however, increased with elbow movement speed. We conclude that patients were significantly less able to 'tune' their postural stabilizing response to match interaction torques. However, the velocity dependence of this effect points to a deficit in generating normal levels of phasic torque.

Different types of cerebellar hypometria associated with a distinct topography of the lesion in cerebellum

We recorded ballistic wrist flexion movements in fifteen cerebellar patients exhibiting hypometria. The movement and the associated agonist and antagonist EMG activities were analysed. On the basis of the topography of the cerebellar lesion, our patients were divided into three groups. In the first group including five patients, lesions involved the efferent dentato-thalamo-cortical pathway and hypometria was associated with an imbalance between the rate of rise of the agonist EMG activity and the rate of rise of the antagonist EMG activity. In the three patients of group II, lesions were located at the level of the middle cerebellar peduncle, disrupting the crossed ponto-cerebellar projections. In these patients, the intensity of the agonist EMG activity was reduced and the duration of the antagonist EMG activity was increased. In the third group including seven patients presenting either a diffuse cerebellar atrophy or a stroke involving a large parenchymatous area, the agonist-antagonist EMG pattern showed a prolongation of the duration of the antagonist burst. Our results show that discrete mechanisms of cerebellar hypometria are associated with different anatomical lesions.

Shift from hypermetria to hypometria in an aberrant recovery following cerebellar infarction

Cerebellar hypermetria, a classical sign designating the overshoot when the patient attempts to reach rapidly an aimed target, is associated with an imbalance between timing and/or intensity of agonist and antagonist EMG activities. Recovery of hypermetria following a cerebellar ischemia or hemorrhage has been demonstrated to take place in a multistage process, but aberrant recovery following a cerebellar stroke has not been described previously. We report an 85-year-old woman presenting an abnormal recovery following a cerebellar infarction. We identified three successive stages. At stage 1, fast wrist flexion movements were severely hypermetric and associated with three EMG defects: a delayed onset latency of antagonist EMG activity, a reduction of intensity of the agonist EMG activity and a depression of intensity of antagonist EMG activity. At stage 2, movements were characterized by terminal oscillations around the target and the onset latency of the antagonist activity had returned to normal. At stage 3, movements were markedly hypometric, the intensity of the antagonist EMG activity had returned to normal, while the intensity of the agonist EMG activity remained abnormally low. This case illustrates an abnormal reprogramming of the EMG triphasic pattern, resulting in the shift from severe hypermetria to severe hypometria.

Effects of TRH on ballistic wrist movements in cerebellar cortical atrophy: improvement of two genuine deficiencies but not of the major one

Thyrotropin-releasing hormone (TRH) has been claimed to improve cerebellar ataxia in patients with idiopathic sporadic cerebellar cortical atrophy (CCA). We analysed the effects of intravenous administration of TRH (1 mg) in ballistic wrist flexions movements in 10 healthy subjects and in eight patients with CCA. The associated agonist and antagonist electromyographic (EMG) activities were recorded. In healthy subjects, TRH did modify neither the movement amplitudes, nor the intensity of the agonist and antagonist EMG activities. Before TRH administration, patients with CCA exhibited a hypermetria which was associated with a delayed onset of the antagonist activity. Moreover, the intensity of EMG activity in both the agonist and the antagonist muscles were reduced. After TRH, the hypermetria and the delayed onset latencies of the antagonist EMG activities were unchanged but the intensity of both the agonist and the antagonist EMG activities increased. TRH could increase the magnitude of agonist and antagonist EMG activities in patients with CCA by exerting an excitatory effect directly on motoneurons or by modulating at the supraspinal level the central commands to agonist and antagonist motoneuron pools. Copyright Rapid Science Ltd