Lateralized inhibition of symmetric contractions is associated with motor, attentional and executive processes

Spectral properties of physiological mirror activity: an investigation of frequency features and common input between homologous muscles

During unilateral contractions, muscular activation can be detected in both active and resting limbs. In healthy populations, the latter is referred to as physiological mirror activity (pMA). The study of pMA holds implications for clinical applications as well as the understanding of bilateral motor control. However, the underlying mechanisms of pMA remain to be fully resolved. A commonality of prevailing explanatory approaches is the concept of shared neural input. With this study, we, therefore, aimed to investigate neural input in the form of multiple analyses of surface electromyography (sEMG) recordings in the frequency domain. For this purpose, 14 healthy, right-handed males aged 18–35 years were recruited. All participants performed a pinch-force task with the dominant hand in a blockwise manner. In total, 9 blocks of 5 contractions each were completed at 80% of maximum force output. Muscle activity was recorded via sEMG of the first dorsal interosseous muscle of the active and resting hand. We analyzed (1) spectral features as well as (2) intermuscular coherence (IMC). Our results demonstrate a blockwise increase in median frequency, mean frequency, and peak frequency in both hands. Frequency ratio analyses revealed a higher low-frequency component in the resting hand. Although we were able to demonstrate IMC on an individual level, results varied greatly and grand-averaged IMC failed to reach significance. Taken together, our findings imply an overlap of spectral properties between active and passive hands during repeated unilateral contractions. Combined with evidence from previous studies, this suggests a common neural origin between active and resting hands during unilateral contractions possibly resulting from a reduction in interhemispheric inhibition due to high force demands. Nevertheless, the exploratory nature of this study necessitates the classification of our results through follow-up studies.

Temporal Dynamics of Corticomuscular Coherence Reflects Alteration of the Central Mechanisms of Neural Motor Control in Post-Stroke Patients

The neural control of muscular activity during a voluntary movement implies a continuous updating of a mix of afferent and efferent information. Corticomuscular coherence (CMC) is a powerful tool to explore the interactions between the motor cortex and the muscles involved in movement realization. The comparison of the temporal dynamics of CMC between healthy subjects and post-stroke patients could provide new insights into the question of how agonist and antagonist muscles are controlled related to motor performance during active voluntary movements. We recorded scalp electroencephalography activity, electromyography signals from agonist and antagonist muscles, and upper limb kinematics in eight healthy subjects and seventeen chronic post-stroke patients during twenty repeated voluntary elbow extensions and explored whether the modulation of the temporal dynamics of CMC could contribute to motor function impairment. Concomitantly with the alteration of elbow extension kinematics in post-stroke patients, dynamic CMC analysis showed a continuous CMC in both agonist and antagonist muscles during movement and highlighted that instantaneous CMC in antagonist muscles was higher for post-stroke patients compared to controls during the acceleration phase of elbow extension movement. In relation to motor control theories, our findings suggest that CMC could be involved in the online control of voluntary movement through the continuous integration of sensorimotor information. Moreover, specific alterations of CMC in antagonist muscles could reflect central command alterations of the selectivity in post-stroke patients.

Behavioural and cerebral asymmetries of mirror movements are specific to rhythmic task and related to higher attentional and executive control

Mirror movements (MM) refer to the involuntary movements or contractions occurring in homologous muscles contralateral to the unilateral voluntary movements. This behavioural manifestation increases in elderly. In right-handed adults, some studies report asymmetry in MM production, with greater MM in the right dominant hand during voluntary movements of the left non-dominant hand than the opposite. However, other studies report contradictory results, suggesting that MM asymmetry could depend on the characteristics of the task. The present study investigates the behavioural asymmetry of MM and its associated cerebral correlates during a rhythmic task and a non-rhythmic task using low-force contractions (i.e., 25 % MVC). We determined the quantity and the intensity of MM using electromyography (EMG) and cerebral correlates through electroencephalography (EEG) in right-handed healthy young and middle-aged adults during unimanual rhythmic vs. non-rhythmic tasks. Overall, results revealed (1) behavioural asymmetry of MM specific to the rhythmic task and irrespective of age, (2) cerebral asymmetry of motor activations specific to the rhythmic task and irrespective of age and (3) greater attentional and executive activations in the rhythmic task compared to the non-rhythmic task. In line with our hypotheses, behavioural and cerebral motor asymmetries of MM seem to be specific to the rhythmic task. Results are discussed in terms of cognitive-motor interactions: greater attentional and executive control required in the rhythmic tasks could contribute to the increased occurrence of involuntary movements in both young and middle-aged adults.

Voluntary Inhibition of Physiological Mirror Activity: An EEG-EMG Study

Physiological mirror activity (pMA), observed in healthy human adults, describes the involuntary co-activation of contralateral homologous muscles during unilateral limb movements. Here we provide novel evidence, using neuromuscular measurements (electromyography; EMG), that the amplitude of pMA can be voluntarily inhibited during unilateral isometric contractions of intrinsic hand muscles after informing human participants (10 male, 10 female) about its presence and establishing a basic understanding of pMA mechanisms through a standardized protocol. Importantly, significant suppression of pMA was observed immediately after participants were asked to inhibit it, despite the absence of any online feedback during task execution and without special training. Moreover, we observed that the decrease of pMA was specifically accompanied by an increase in relative frontal δ power recorded with electroencephalography (EEG). Correlation analysis further revealed an inverse association between the individual amplitude of pMA and frontal δ power that reached significance once participants started to inhibit. Taken together, these results suggest that δ power in frontal regions might reflect executive processes exerting inhibitory control over unintentional motor output, in this case pMA. Our results provide an initial reference point for the development of therapeutic applications related to the neurorehabilitation of involuntary movements which could be realized through the suppression of pMA observed in the elderly before it would fully manifest in undesirable overt movement patterns.

Spastic co-contraction is directly associated with altered cortical beta oscillations after stroke

OBJECTIVE

Spastic co-contraction is a motor-disabling form of muscle overactivity occurring after a stroke, contributing to a limitation in active movement and a certain level of motor impairment. The cortical mechanisms underlying spastic co-contraction remain to be more fully elucidated, the present study aimed to investigate the role of the cortical beta oscillations in spastic co-contraction after a stroke.

METHOD

We recruited fifteen post-stroke participants and nine healthy controls. The participants were asked to perform active elbow extensions. In the study, multimodal analysis was performed to combine the evaluation of three-dimensional elbow kinematics, the elbow muscles electromyographic activations, and the cortical oscillatory activity.

RESULTS

The movement-related beta desynchronization was significantly decreased in post-stroke participants compared to healthy participants. We found a significant correlation between the movement-related beta desynchronization and the elbow flexors activation during the active elbow extension in post-stroke participants. When compared to healthy participants, post-stroke participants exhibited significant alterations in the elbow kinematics and greater muscle activation levels.

CONCLUSIONS

Cortical beta oscillation alterations may reflect an important neural mechanism underlying spastic co-contraction after a stroke.

SIGNIFICANCE

Measuring the cortical oscillatory activity could be useful to further characterize neuromuscular plasticity induced by recovery or therapeutic interventions.