Étude des interactions interhémisphériques entre les représentations des muscles de l'épaule et du tronc dans le cortex moteur primaire.

Après un accident vasculaire cérébral (AVC), 30% des personnes ont une atteinte de la fonction motrice du membre supérieur. Un des mécanismes pouvant intervenir dans la récupération motrice après un AVC est la réorganisation des interactions interhémisphériques. À ce jour, la plupart des études se sont intéressées aux interactions entre les représentations des muscles de la main. Or la réalisation de mouvements de la main nécessite une coordination précise des muscles proximaux de l’épaule et le maintien d’une stabilité assurée par les muscles du tronc. Cependant, il existe peu d’informations sur le contrôle interhémisphérique de ces muscles. Ainsi, l’objectif de cette étude était de caractériser les interactions entre les représentations corticales des muscles proximaux (Deltoïde antérieur (DA)), et axiaux (Erecteur spinal (ES L1)) chez le sujet sain et de les comparer avec les interactions interhémisphériques entre les représentations des muscles distaux (1er interosseux dorsal (FDI)). Deux techniques de stimulation magnétique transcrânienne ont été utilisées pour évaluer ces interactions. La stimulation du cortex moteur ipsilatéral évoque une période de silence ipsilatérale (iSP)-reflétant l’inhibition interhémiphérique-dans le FDI et le DA. Dans ES L1, l’iSP est précédée d’une facilitation. Le paradigme de l’impulsion pairée démontre aussi la présence d’inhibition interhémisphérique dans les trois muscles. Ces résultats suggèrent un patron distinct d’interactions réciproques entre les représentations des muscles distaux, proximaux et axiaux qui peut être expliqué à la fois par des changements d’excitabilité au niveau cortical et sous-cortical. Ces résultats pourraient servir de bases normatives afin d’évaluer les changements survenant suite à un AVC.

Origine des projections sensorimotrices dans des sous-régions du cortex moteur primaire chez le singe capucin.

Certaines études ont démontrés que les connexions entre l’aire prémotrice ventrale (PMv) et la région de la main du cortex moteur primaire (M1) sont distribuées non-uniformément, ciblant des sous-régions spécifiques dans M1. Dans la présente étude nous avons voulu développer ces résultats en étudiant la distribution au sein de M1 des projections corticales issues de PMv, l’aire prémotrice dorsale (PMd), l’aire motrice supplémentaire (SMA) et les aires pariétales 1, 2 et 5. Pour se faire, nous avons combiné des approches électrophysiologiques et anatomiques chez trois singes naïfs du Nouveau Monde (Cebus apella) pour examiner l’organisation et la spécificité topographique des projections corticales dans M1. Nos résultats indiquent que quatre sous-régions à l’intérieur de la région dédiée à la main reçoivent des inputs prédominants de différentes aires sensorimotrices. Ces résultats suggèrent que des sous-régions de M1 puissent avoir des fonctions spécifiques pour le contrôle moteur de la main et des doigts.

Estimulacion magnetica trascraneal en el desempeno motor en enfermedades del sistema nervioso central: Revision Sistematica

Introducción: El uso de la estimulación cerebral no invasiva en procesos de rehabilitación es de gran interés, por cuanto con mediación tecnológica se generan nuevas posibilidades de recuperación motora, a partir de la activación de la corteza cerebral. El objetivo del estudio es establecer la evidencia del uso terapéutico de la EMT, relacionado con el desempeño motor de pacientes con enfermedades del sistema nervioso central. Metodología: Se realizó una revisión sistemática de la literatura. Se incluyeron 10 estudios en el análisis cualitativo que incluyó la evaluación de calidad con la escala de Jadad y del riesgo de sesgo con la herramienta Cochrane. Fueron excluidos 1613 estudios. Se aplicó el protocolo del estudio para la extracción, revisión y validez de los estudios incluidos. Resultados: La evidencia disponible muestra resultados positivos del uso terapéutico de la EMT en el desempeño motor en aspectos como la aceleración, la fuerza de pinza y de agarre, la estabilidad y la fuerza muscular, así como una mejor velocidad de la marcha y una disminución en la frecuencia y severidad de los espasmos. Discusión: La EMT puede constituir una estrategia terapéutica para mejorar el desempeño motor en pacientes con ECV, Lesión Medular y enfermedad de Parkinson, que requiere más investigación por la heterogeneidad de los diseños y medidas de descenlace utilizados, así como por la alta variabilidad interindividual que hace complejo estandarizar los protocolos de su uso terapéutico.

Coding of visual space during motor preparation: approaching objects rapidly modulate corticospinal excitability in hand-centered coordinates

Defensive behaviors, such as withdrawing your hand to avoid potentially harmful approaching objects, rely on rapid sensorimotor transformations between visual and motor coordinates. We examined the reference frame for coding visual information about objects approaching the hand during motor preparation. Subjects performed a simple visuomanual task while a task-irrelevant distractor ball rapidly approached a location either near to or far from their hand. After the distractor ball appearance, single pulses of transcranial magnetic stimulation were delivered over the subject's primary motor cortex, eliciting motor evoked potentials (MEPs) in their responding hand. MEP amplitude was reduced when the ball approached near the responding hand, both when the hand was on the left and the right of the midline. Strikingly, this suppression occurred very early, at 70-80ms after ball appearance, and was not modified by visual fixation location. Furthermore, it was selective for approaching balls, since static visual distractors did not modulate MEP amplitude. Together with additional behavioral measurements, we provide converging evidence for automatic hand-centered coding of visual space in the human brain.

Perceiving time to collision activates the sensorimotor cortex

The survival of many animals hinges upon their ability to avoid collisions with other animals or objects or to precisely control the timing of collisions. Optical expansion provides a compelling impression of object approach and in principle can provide the basis for judgments of time to collision (TTC) [1]. It has been demonstrated that pigeons [2] and houseflies [3] have neural systems that can initiate rapid coordinated actions on the basis of optical expansion. In the case of humans, the linkage between judgments of TTC and coordinated action has not been established at a cortical level. Using functional magnetic resonance imaging (fMRI), we identified superior-parietal and motor-cortex areas that are selectively active during perceptual TTC judgments, some of which are normally involved in producing reach-to-grasp responses. These activations could not be attributed to actual movement of participants. We demonstrate that networks involved in the computational problem of extracting TTC from expansion information have close correspondence with the sensorimotor systems that would be involved in preparing a timed motor response, such as catching a ball or avoiding collision.

Reduced cortico-motor facilitation in a normal sample with high traits of autism

Recent research in social neuroscience proposes a link between mirror neuron system (MNS) and social cognition. The MNS has been proposed to be the neural mechanism underlying action recognition and intention understanding and more broadly social cognition. Pre-motor MNS has been suggested to modulate the motor cortex during action observation. This modulation results in an enhanced cortico-motor excitability reflected in increased motor evoked potentials (MEPs) at the muscle of interest during action observation. Anomalous MNS activity has been reported in the autistic population whose social skills are notably impaired. It is still an open question whether traits of autism in the normal population are linked to the MNS functioning. We measured TMS-induced MEPs in normal individuals with high and low traits of autism as measured by the autistic quotient (AQ), while observing videos of hand or mouth actions, static images of a hand or mouth or a blank screen. No differences were observed between the two while they observed a blank screen. However participants with low traits of autism showed significantly greater MEP amplitudes during observation of hand/mouth actions relative to static hand/mouth stimuli. In contrast, participants with high traits of autism did not show such a MEP amplitude difference between observation of actions and static stimuli. These results are discussed with reference to MNS functioning.

Effect of instructive visual stimuli on neurofeedback training for motor imagery-based braincomputer interface

Event-related desynchronization (ERD) of the electroencephalogram (EEG) from the motor cortex is associated with execution, observation, and mental imagery of motor tasks. Generation of ERD by motor imagery (MI) has been widely used for brain-computer interfaces (BCIs) linked to neuroprosthetics and other motor assistance devices. Control of MI-based BCIs can be acquired by neurofeedback training to reliably induce MI-associated ERD. To develop more effective training conditions, we investigated the effect of static and dynamic visual representations of target movements (a picture of forearms or a video clip of hand grasping movements) during the BCI training. After 4 consecutive training days, the group that performed MI while viewing the video showed significant improvement in generating MI-associated ERD compared with the group that viewed the static image. This result suggests that passively observing the target movement during MI would improve the associated mental imagery and enhance MI-based BCIs skills.

Corticomotor excitability during precision motor tasks

The aim of this preliminary study was to investigate motor cortex (cortical) excitability between a similar fine visuomotor task of varying difficulty. Ten healthy adults (three female, seven male; 20–45 years of age) participated in the study. Participants were instructed to perform a fine visuomotor task by statically abducting their first index finger against a force transducer which displayed the level of force (represented as a marker) on a computer monitor. This marker was to be maintained between two stationary bars, also displayed on the computer monitor. The level of difficulty was increased by amplifying the position of the marker, making the task more difficult to control. Cortical measures of motor evoked potential (MEP) and silent period (SP) duration in first dorsal interosseous (FDI) muscle were obtained using transcranial magnetic stimulation (TMS) while the participant maintained the “easy” or “difficult” static task. An 11.8% increase in MEP amplitude was observed when subjects undertook the “difficult” task, but no differences in MEP latency or SP duration. The results from this preliminary study suggest that cortical excitability increases reflect the demand required to perform tasks requiring greater precision with suggestions for further research discussed.

Corticospinal excitability following short-term motor imagery training of a strength task

Motor imagery and actual movement engage similar neural structures, however, whether they produce similar training-related corticospinal adaptations has yet to be established. The aim of this study was to compare changes in strength and corticospinal excitability following short-term motor imagery strength training and short-term strength training. Transcranial magnetic stimulation (TMS) was applied over the contralateral motor cortex (M1) to elicit motor-evoked potentials in the dominant biceps brachii muscle prior to and following 3-week strength training using actual bicep curls or motor imagery of bicep curls. The strength training (n = 6) and motor imagery (n = 6) groups underwent three supervised training sessions per week for 3 weeks. Participants completed four sets of six to eight repetitions (actual or imagined) at a training load of 80% of their one-repetition maximum. The control group (n = 6) were required to maintain their current level of physical activity. Both training groups exhibited large performance gains in strength (p < 0.001; strength training 39% improvement, imagery 16% improvement), which were significantly different between groups (p = 0.027). TMS revealed that the performance improvements observed in both imagery and strength training were accompanied by increases in corticospinal excitability (p < 0.001), however, these differences were not significantly different between groups (p = 0.920). Our findings suggest that both strength training and motor imagery training utilised similar neural substrates within the primary M1, however, strength training resulted in greater gains in strength than motor imagery strength training. This difference in strength increases may be attributed to adaptations during strength training that are not confined to the primary M1. These findings have theoretical implications for functional equivalent views of motor imagery as well as important therapeutic implications.

Corticomotor responses to attentionally demanding motor performance: a mini-review

Increased attentional demand has been shown to reduce motor performance, leading to increases in accidents, particularly in elderly populations. While these deficits have been well documented behaviorally, their cortical correlates are less well known. Increased attention has been shown to affect activity in prefrontal regions of the cortex. However there have been varying results within past research investigating corticomotor regions, mediating motor performance. This mini-review initially discusses past behavioral research, before moving to studies investigating corticomotor areas in response to changes in attention. Recent dual task studies have revealed a possible decline in the ability of older, but not younger, adults to activate inhibitory processes within the motor cortex, which may be correlated with poor motor performance, and thus accidents. A reduction in cortical inhibition may be caused by neurodegeneration within prefrontal regions of the cortex with age, rendering older adults less able to allocate attention to corticomotor regions.

Formation of cortical plasticity in older adults following tDCS and motor training

Neurodegeneration accompanies the process of natural aging, reducing the ability to perform functional daily activities. Transcranial direct current stimulation (tDCS) alters neuronal excitability and motor performance; however its beneficial effect on the induction of primary motor cortex (M1) plasticity in older adults is unclear. Moreover, little is known as to whether the tDCS electrode arrangement differentially affects M1 plasticity and motor performance in this population. In a double-blinded, cross-over trial, we compared unilateral, bilateral and sham tDCS combined with visuomotor tracking, on M1 plasticity and motor performance of the non-dominant upper limb, immediately post and 30 min following stimulation. We found (a) unilateral and bilateral tDCS decreased tracking error by 12–22% at both time points; with sham decreasing tracking error by 10% at 30 min only, (b) at both time points, motor evoked potentials (MEPs) were facilitated (38–54%) and short-interval intracortical inhibition was released (21–36%) for unilateral and bilateral conditions relative to sham, (c) there were no differences between unilateral and bilateral conditions for any measure. These findings suggest that tDCS modulated elements of M1 plasticity, which improved motor performance irrespective of the electrode arrangement. The results provide preliminary evidence indicating that tDCS is a safe non-invasive tool to preserve or improve neurological function and motor control in older adults.