Does the force level modulate the cortical activity during isometric contractions after a cervical spinal cord injury?

Movement-related ERS and connectivity in the gamma frequency decrease with practice

Previous work showed that movements are accompanied by modulation of electroencephalographic (EEG) activity in both beta (13-30 Hz) and gamma (>30 Hz) ranges. The amplitude of beta event-related synchronization (ERS) is not linked to movement characteristics, but progressively increases with motor practice, returning to baseline after a period of rest. Conversely, movement-related gamma ERS amplitude is proportional to movement distance and velocity. Here, high-density EEG was recorded in 51 healthy subjects to investigate whether i) three-hour practice in two learning tasks, one with a motor component and one without, affects gamma ERS amplitude and connectivity during a motor reaching test, and ii) 90-minutes of either sleep or quiet rest have an effect on gamma oscillatory activity. We found that, while gamma ERS was appropriately scaled to the target extent at all testing points, its amplitude decreased after practice, independently of the type of interposed learning, and after both quiet wake and nap, with partial correlations with subjective fatigue scores. During movement execution, connectivity patterns within fronto-parieto-occipital electrodes, over areas associated with attentional networks, decreased after both practice and after 90-minute rest. While confirming the prokinetic nature of movement-related gamma ERS, these findings demonstrated the preservation of gamma ERS scaling to movement velocity with practice, despite constant amplitude reduction. We thus speculate that such decreases, differently from the practice-related increases of beta ERS, are related to reduced attention or working memory mechanisms due to fatigue or a switch of strategy toward automatization of movement execution and do not specifically reflect plasticity phenomena.

Increased intensity of unintended mirror muscle contractions after cervical spinal cord injury is associated with changes in interhemispheric and corticomuscular coherences

Mirror contractions refer to unintended contractions of the contralateral homologous muscles during voluntary unilateral contractions or movements. Exaggerated mirror contractions have been found in several neurological diseases and indicate dysfunction or lesion of the cortico-spinal pathway. The present study investigates mirror contractions and the associated interhemispheric and corticomuscular interactions in adults with spinal cord injury (SCI) - who present a lesion of the cortico-spinal tract - compared to able-bodied participants (AB). Eight right-handed adults with chronic cervical SCI and ten age-matched right-handed able-bodied volunteers performed sets of right elbow extensions at 20% of maximal voluntary contraction. Electromyographic activity (EMG) of the right and left elbow extensors, interhemispheric coherence over cerebral sensorimotor regions evaluated by electroencephalography (EEG) and corticomuscular coherence between signals over the cerebral sensorimotor regions and each extensor were quantified. Overall, results revealed that participants with SCI exhibited (1) increased EMG activity of both active and unintended active limbs, suggesting more mirror contractions, (2) reduced corticomuscular coherence between signals over the left sensorimotor region and the right active limb and increased corticomuscular coherence between the right sensorimotor region and the left unintended active limb, (3) decreased interhemispheric coherence between signals over the two sensorimotor regions. The increased corticomuscular communication and decreased interhemispheric communication may reflect a reduced inhibition leading to increased communication with the unintended active limb, possibly resulting to exacerbated mirror contractions in SCI. Finally, mirror contractions could represent changes of neural and neuromuscular communication after SCI.

Corticomuscular Coherence and Motor Control Adaptations after Isometric Maximal Strength Training

Strength training (ST) induces corticomuscular adaptations leading to enhanced strength. ST alters the agonist and antagonist muscle activations, which changes the motor control, i.e., force production stability and accuracy. This study evaluated the alteration of corticomuscular communication and motor control through the quantification of corticomuscular coherence (CMC) and absolute (AE) and variable error (VE) of the force production throughout a 3 week Maximal Strength Training (MST) intervention specifically designed to strengthen ankle plantarflexion (PF). Evaluation sessions with electroencephalography, electromyography, and torque recordings were conducted pre-training, 1 week after the training initiation, then post-training. Training effect was evaluated over the maximal voluntary isometric contractions (MVIC), the submaximal torque production, AE and VE, muscle activation, and CMC changes during submaximal contractions at 20% of the initial and daily MVIC. MVIC increased significantly throughout the training completion. For submaximal contractions, agonist muscle activation decreased over time only for the initial torque level while antagonist muscle activation, AE, and VE decreased over time for each torque level. CMC remained unaltered by the MST. Our results revealed that neurophysiological adaptations are noticeable as soon as 1 week post-training. However, CMC remained unaltered by MST, suggesting that central motor adaptations may take longer to be translated into CMC alteration.

Beta band frequency differences between motor and frontal cortices in reaching movements

Movement is associated with power changes over sensory-motor areas in different frequency ranges, including beta (15-30 Hz). In fact, beta power starts decreasing before the movement onset (event-related desynchronization, ERD) and rebounds after its end (event-related synchronization, ERS). There is increasing evidence that beta modulation depth (measured as ERD-ERS difference) increases with practice in a planar reaching task, suggesting that this measure may reflect plasticity processes. In the present work, we analyzed beta ERD, ERS and modulation depth in healthy subjects to determine their changes over three regions of interest (ROIs): right and left sensorimotor and frontal areas, during a reaching task with the right arm. We found that ERD, ERS and modulation depth increased with practice with lower values over the right sensory-motor area. Timing of peak ERD and ERS were similar across ROIs, with ERS peak occurring earlier in later sets. Finally, we found that beta ERS of the frontal ROI involved higher beta range (23-29 Hz) than the sensory-motor ROIs (15-18 Hz). Altogether these results suggest that practice in a reaching task is associated with modification of beta power and timing. Additionally, beta ERS may have different functional meaning in the three ROIs, as suggested by the involvement of upper and lower beta bands in the frontal and sensorimotor ROIs, respectively.

Beta Modulation Depth Is Not Linked to Movement Features

Beta power over the sensorimotor areas starts decreasing just before movement execution (event-related desynchronization, ERD) and increases post-movement (event-related synchronization, ERS). In this study, we determined whether the magnitude of beta ERD, ERS and modulation depth are linked to movement characteristics, such as movement length and velocity. Brain activity was recorded with a 256-channels EEG system in 35 healthy subjects performing fast, uncorrected reaching movements to targets located at three distances. We found that the temporal profiles of velocity were bell-shaped and scaled to the appropriate target distance. However, the magnitude of beta ERD, ERS and modulation depth, as well as their timing, did not significantly change and were not related to movement features.

Motor-related cortical activity after cervical spinal cord injury: multifaceted EEG analysis of isometric elbow flexion contractions

Electroencephalographic (EEG) studies have well established that motor cortex (M1) activity ~20 Hz decreases during muscular contraction and increases as soon as contraction stops, which are known as event-related desynchronization (ERD) and event-related synchronization (ERS), respectively. ERD is supposed to reflect M1 activation, sending information to recruited muscles, while the process underlying ERS is interpreted either as active cortical inhibition or as processing of sensory inputs. Investigation of the process behind ERD/ERS in people with spinal cord injury (SCI) would be particularly relevant since their M1 remains effective despite decreased sensorimotor abilities. In this study, we recorded net joint torque and EEG in 6 participants with cervical SCI and 8 healthy participants who performed isometric elbow flexion at 3 force levels. Multifaceted EEG analysis was introduced to assess ERD/ERS according to their amplitude, frequency range and duration. The results revealed that net joint torque increased with the required force level for all participants and time to contraction inhibition was longer in the SCI group. At the cortical level, ERD/ERS frequency ranges increased with the required force level in all participants, indicating that the modulation of cortical activity with force level is preserved after SCI. However, ERS amplitude decreased only in SCI participants, which may be linked to delayed contraction inhibition. All in all, cortical modulation of frequency range and amplitude could reflect two different kinds of neural communication.