Marie-Claude Hepp-Reymond

EMG activation patterns during force production in precision grip

Electromyographic (EMG) activity was examined in six normal subjects, producing low isometric forces between thumb and index finger in a visually guided step-tracking task. Target forces ranged between 0.5 and 3.0 N. EMG activity of all 15 muscles acting on thumb or index finger was screened with simultaneous recordings of up to 8 muscles. Linear regression was applied to quantify the EMG activity as a function of force. The intrinsic muscles and the long flexors of the index finger had a tight relation to force, as indicated by the high correlation coefficient, as did the adductor and short flexor of the thumb. In contrast, the long extensors of the index finger did not show force-related activity. The other muscles, including the long flexor and extensor of the thumb, had varying,on average moderate, correlations to force. The slope of the regression lines, a measure for the amount of EMG modulation with increasing force, revealed the same trends. Thus the majority of the intrinsic muscles were as closely related to force as the long flexors, suggesting a more important role in production of low isometric forces in the grip than previously believed, perhaps even a primary role. Systematic interindividual differences were rarely observed. Analysis of the trialby-trial variability of EMG activity revealed that for most muscles the observed scatter was produced by varying background activity and was not a random fluctuation of relative increases in activity from one force level to the next.

Relation of activity in precentral cortical neurons to force and rate of force change during isometric contractions of finger muscles

SummaryThe activity of single neurons within the hand area of the precentral motor cortex of primates was recorded during the performance of a maintained precision grip between the thumb and forefinger. The finger opposition forces were exerted against a strain gauge which allowed force changes to be studied under near isometric conditions. Task performance required the generation of a force ramp (the dynamic phase) and thereafter the maintenance of a stable force for one second (the static phase). Intracortical stimulation through the recording electrode was used to verify that the recordings were made from the appropriate somatotopographic area of the motor cortex.From a total of 221 recorded neurons, 76 were found to be either activated or deactivated during performance of the task. Among the 51 activated neurons, most discharged at higher frequencies during the dynamic phase, than during the static phase. The discharge of some of these neurons could be related to both force (F) and rate of force change (df/dt) whereas certain others could only be correlated with df/dt. The change in discharge frequency for these neurons generally occurred prior to the onset of EMG activity. Eight neurons were more active during maintained force than during the force ramp. The discharge frequency could not be correlated with df/dt and only one showed a significant positive relation to force. The change in discharge frequency for these neurons either coincided or occurred after the onset of EMG activity.

Listing's law for eye, head and arm movements and their synergistic control

SummaryWe have recorded eye, head, and upper arm rotations in five healthy human subjects using the three-dimensional search coil technique. Our measurements show that the coordination of eye and head movements during gaze shifts within ± 25 deg relative to the forward direction is organized by restricting the rotatory trajectories of the two systems to almost parallel planes. These so-called “Listing planes” for eye-in-space and head-in-space rotations are workspace-oriented, not body-fixed. Eye and head trajectories in their respective planes are closely related in direction and amplitude. For pointing or grasping, the rotatory trajectories of the arm are also restricted to a workspace-oriented Listing plane. During visually guided movements, arm follows gaze, and the nine-dimensional rotatory configuration space for eye-head-arm-synergies (three degrees of freedom for each system) is reduced to a two-dimensional plane in the space of quaternion vectors.

Gamma-range corticomuscular coherence during dynamic force output

The beta-range synchronization between cortical motor and muscular activity as revealed by EEG/MEG-EMG coherence has been extensively investigated for steady-state motor output. However, there is a lack of information on the modulation of the corticomuscular coherence in conjunction with dynamic force output. We addressed this question comparing the EEG-EMG coherence and the cortical motor spectral power in eight healthy subjects in a visuomotor task, in which the subjects exerted a steady-state or periodically modulated dynamic isometric force output with their right-index finger to keep a visual cursor within a target zone. In the static condition, significant coherence was confined to the beta-range. In the dynamic condition, the most distinct coherence occurred in the gamma-range and the significant beta-range coherence was strikingly reduced. The cortical motor power in the beta-range during dynamic force output was decreased, whereas the power in the gamma-range remained without significant change. We conclude that during dynamic force the corticospinal oscillation mode of the sensorimotor system shifts towards higher (principally gamma) frequencies for the rapid integration of the visual and somatosensory information required to produce the appropriate motor command.

Preservation of motor programs in paraplegics as demonstrated by attempted and imagined foot movements

Execution and imagination of movement activate distinct neural circuits, partially overlapping in premotor and parietal areas, basal ganglia and cerebellum. Can long-term deafferented/deefferented patients still differentiate attempted from imagined movements? The attempted execution and motor imagery network of foot movements have been investigated in nine chronic complete spinal cord-injured (SCI) patients using fMRI. Thorough behavioral assessment showed that these patients were able to differentiate between attempted execution and motor imagery. Supporting the outcome of the behavioral assessment, fMRI disclosed specific patterns of activation for movement attempt and for motor imagery. Compared with motor execution data of healthy controls, movement attempt in SCI patients revealed reduced primary motor cortex activation at the group level, although activation was found in all single subjects with a high variability. Further comparisons with healthy subjects revealed that during attempt and motor imagery, SCI patients show enhanced activation and recruitment of additional regions in the parietal lobe and cerebellum that are important in sensorimotor integration. These findings reflect central plastic changes due to altered input and output and suggest that SCI patients may require additional cognitive resources to perform these tasks that may be one and the same phenomenon, or two versions of the same phenomenon, with quantitative differences between the two. Nevertheless, the retained integrity of movement attempt and motor imagery networks in SCI patients demonstrates that chronic paraplegics can still dispose of the full motor programs for foot movements and that therefore, attempted and imagined movements should be integrated in rehabilitative strategies.

Impact of a Weekly Dance Class on the Functional Mobility and on the Quality of Life of Individuals with Parkinson’s Disease

Individuals with Parkinson’s disease (PD) mainly suffer from motor impairments which increase the risk of falls and lead to a decline of quality of life. Several studies investigated the long-term effect of dance for people with PD. The aims of the present study were to investigate (i) the short-term effects of dance (i.e., the effect immediately after the dance class) on motor control in individuals with PD and (ii) the long-term effects of 8 months of participation in the weekly dance class on the quality of life of the PD patients and their caregivers. The dance lessons took place in a ballet studio and were led by a professional dancer. Eleven people with moderate to severe PD (58–85 years old) were subjected to a motor and quality of life assessments. With respect to the motor assessments the unified Parkinson disease rating scale III (UPDRS III), the timed up and go test (TUG), and the Semitandem test (SeTa) before and after the dance class were used. With respect to the quality of life and well-being we applied quality of life scale (QOLS) as well as the Westheimer questionnaire. Additionally, we asked the caregivers to fill out the Questionnaire for caregivers. We found a significant beneficial short-term effect for the total score of the UPDRS motor score. The strongest improvements were in rigidity scores followed by significant improvements in hand movements, finger taps, and facial expression. No significant changes were found for TUG and for SeTa. The results of the questionnaires showed positive effects of the dance class on social life, health, body-feeling and mobility, and on everyday life competences of the PD patients. Beneficial effect was also found for the caregivers. The findings demonstrate that dance has beneficial effect on the functional mobility of individuals with PD. Further, dance improves the quality of life of the patients and their caregivers. Dance may lead to better therapeutic strategies as it is engaging and enjoyable.

Unilateral pyramidotomy in monkeys: Effect on force and speed of a conditioned precision grip

Summary Cynomolgus monkeys were trained to perform a conditioned finger grip on a microtransducer. The monkeys were then subjected to unilateral transection of one bulbar pyramid and the performance of the grip was again tested after retraining. In the first experiment involving 4 monkeys, the force required for delivery of the reinforcement was varied from 100 to 700 g. The electromyographic activity of forearm muscles and the pressure exerted on the transducer were recorded. In a second experiment involving one monkey, reaction time was measured with respect to the electromyographic latency and the response latency. After pyramidotomy, use of the contralateral hand was obviously impaired in all animals, and one to two weeks of retraining were necessary for the relearning of the motor task at the lowest force threshold. There was a gradual improvement of the performance and a partial recovery was noted even in monkeys with a total or nearly total lesion of the pyramidal tract, though some residual deficits were observed for up to 6 months after pyramidotomy. Those monkeys trained to exert force at increasing thresholds, though they performed successfully at the highest threshold (700 g), showed a delayed building up of force as evidenced by the increased EMG summation time and the flat slope of the force curve versus time. For the animal trained in the reaction time situation, the response time almost doubled after destruction of about 75% of the pyramidal tract fibers. However an equivalent increase of the EMG latencies was not observed. It was therefore concluded that execution rather than initiation of the contraction was delayed.

Effects of unilateral and bilateral pyramidotomy on a conditioned rapid precision grip in monkeys (Macaca fascicularis).

SummaryTwo monkeys (Macaca fascicularis) were trained to perform fast conditioned fingergrip on a small transducer. When performance was stabilized to the shortest possible reaction time, the pyramidal tract was transected on one side in one monkey, bilaterally in another. Retraining was resumed 1–3 weeks after surgery. Response latency as well as electromyographic latency and summation time were measured before and up to several months after pyramidotomy.The data show that pyramidotomy induced a long-lasting slowing in the performance of the fingergrip. This slowing was due mainly to a delay in the execution of the movement. However, a short-lasting significant delay of the onset of the EMG activity preceding the movement shows that not only the execution but also the initiation contributes to the increase of the mean response latency. The deficits were more severe and of longer duration in the monkey with bilateral pyramidotomy, especially the delay in the onset of the EMG activity. The mechanisms underlying these deficits and the role of the pyramidal tract in rapid movements are discussed, specifically in consideration of the possible function of the ipsilateral pyramidal tract.

Modulation of human corticomuscular beta‐range coherence with low‐level static forces

Although corticomuscular synchronization in the beta range (15–30 Hz) was shown to occur during weak steady‐state contractions, an examination of low‐level forces around 10% of the maximum voluntary contraction (MVC) is still missing. We addressed this question by investigating coherence between electroencephalogram (EEG) and electromyogram (EMG) as well as cortical spectral power during a visuomotor task. Eight healthy right‐handed subjects compensated isometrically static forces at a level of 4% and 16% of MVC with their right index finger. While 4% MVC was accompanied by low coherence values in the middle to high beta frequency range (25–30 Hz), a significant increase of coherence mainly confined to low beta frequencies (19–20 Hz) was observed with force of 16% MVC. Furthermore, this increase was associated with better performance, as reflected in decreased relative error in force during 16% MVC. We additionally show that periods of good motor performance within each condition were associated with higher values of EEG–EMG coherence and spectral power. In conclusion, our results suggest a role for beta‐range corticomuscular coherence in effective sensorimotor integration, thus stabilizing corticospinal communication.

Identification of multiple nonprimary motor cortical areas with simple movements

The human cortex reportedly contains at least five nonprimary motor areas: in the frontolateral convexity, the dorsal and ventral premotor cortex (PMd and PMv), and in the frontomesial wall, the presupplementary and supplementary motor areas (pre-SMA and SMA), and the rostral, dorsal and ventral cingulate areas (CMAr, CMAd, and CMAv). Activation of these regions in neuroimaging studies has been generally associated either with the performance of complex motor tasks or with reorganization occurring with motor recovery in the presence of pathology. Recent evidence from neuroimaging studies suggests that the same areas are activated with well controlled simple movements in healthy subjects providing support to the observation that their contribution may be more quantitative rather than exclusively specific to a certain aspect of motor behaviour. An important consequence of this observation is that activation of multiple nonprimary motor areas during simple motor tasks should not be considered unique to patients with upper or lower motoneuron lesions but rather as a normal physiological process.