Florent Lebon

Generic inhibition of the selected movement and constrained inhibition of nonselected movements during response preparation

Previous studies have identified two inhibitory mechanisms that operate during action selection and preparation. One mechanism, competition resolution, is manifest in the inhibition of the nonselected response and attributed to competition between candidate actions. The second mechanism, impulse control, is manifest in the inhibition of the selected response and is presumably invoked to prevent premature response. To identify constraints on the operation of these two inhibitory mechanisms, we manipulated the effectors used for the response alternatives, measuring changes in corticospinal excitability with motor-evoked potentials to TMS. Inhibition of the selected response (impulse control) was independent of the task context, consistent with a model in which this form of inhibition is automatically triggered as part of response preparation. In contrast, inhibition of the nonselected response (competition resolution) was context-dependent. Inhibition of the nonselected response was observed when the response alternatives involved movements of the upper limbs but was absent when one response alternative involved an upper limb and the other involved a lower limb. Interestingly, competition resolution for pairs of upper limbs did not require homologous effectors, observed when a left index finger response was pitted with either a nonhomologous right index finger movement or a right arm movement. These results argue against models in which competition resolution is viewed as a generic or fully flexible process, as well as models based on strong anatomical constraints. Rather, they are consistent with models in which inhibition for action selection is constrained by the similarity between the potential responses, perhaps reflecting an experience-dependent mechanism sensitive to the past history of competitive interactions.

Neural plasticity during motor learning with motor imagery practice: Review and perspectives

In the last decade, many studies confirmed the benefits of mental practice with motor imagery. In this review we first aimed to compile data issued from fundamental and clinical investigations and to provide the key-components for the optimization of motor imagery strategy. We focused on transcranial magnetic stimulation studies, supported by brain imaging research, that sustain the current hypothesis of a functional link between cortical reorganization and behavioral improvement. As perspectives, we suggest a model of neural adaptation following mental practice, in which synapse conductivity and inhibitory mechanisms at the spinal level may also play an important role.

Motor imagery and cortico-spinal excitability: A review

Abstract Motor imagery (MI) has received a lot of interest during the last decades as its chronic or acute use has demonstrated several effects on improving sport performances or skills. The development of neuroimagery techniques also helped further our understanding of the neural correlates underlying MI. While some authors showed that MI, motor execution and action observation activated similar motor cortical regions, transcranial magnetic stimulation (TMS) studies brought great insights on the role of the primary motor cortex and on the activation of the cortico-spinal pathway during MI. After defining MI and describing the TMS technique, a short report of MI activities only at cortical level is provided. Then, a main focus on the specificities of cortico-spinal modulations during MI, investigated by TMS, is provided. Finally, a brief overview of sub-cortical mechanisms gives importance to the activation of peripheral neural structures during MI.

Does a Mental Training Session Induce Neuromuscular Fatigue

UNLABELLED Mental training, as physical training, enhances muscle strength. Whereas the repetition of maximal voluntary contractions (MVC) induces neuromuscular fatigue, the effect of maximal imagined contractions (MIC) on neuromuscular fatigue remains unknown. Here, we investigated neuromuscular alterations after a mental training session including MIC, a physical training session including MVC, and a combined training session including both MIC and MVC of the elbow flexor muscles. METHODS Ten participants performed 80 MIC (duty cycle, 5-s MIC and 10-s rest), 80 MVC (identical duty cycle), or 80 MVC and 80 MIC (5-s MVC, 2-s rest, 5-s MIC, and 3-s rest) in three separate sessions. MVC torque was assessed five times over the course of the training and 10 min after the end of the training in the three protocols. Central activation ratio (CARc), reflecting central fatigue, and corticospinal excitability, at rest and during MIC, were estimated using transcranial magnetic stimulation. RESULTS Both the physical training and the combined training induced an approximately 40% drop of MVC torque, accompanied with an approximately 10% decrease of CARc without significant difference between the two sessions. On the contrary, the repetition of MIC did not reduce maximal force production capacity and did not alter CARc. Corticospinal excitability was always facilitated during MIC compared with that during rest, ensuring that the participants imagined the desired movement. CONCLUSIONS These results suggested that one session of mental training alone or combined with physical training do not induce (additional) neuromuscular fatigue despite the repetitive activation of the corticospinal track. Motor imagery may be added to physical practice to increase the total workload without exacerbating neuromuscular fatigue.

Influence of Delay Period Duration on Inhibitory Processes for Response Preparation

In this study, we examined the dynamics of inhibitory preparatory processes, using a delayed response task in which a cue signaled a left or right index finger (Experiment 1) or hand (Experiment 2) movement in advance of an imperative signal. In Experiment 1, we varied the duration of the delay period (200, 500, and 900 ms). When transcranial magnetic stimulation (TMS) was applied 100 ms before the imperative, motor evoked potentials (MEPs) elicited in the first dorsal interosseous were strongly inhibited. For delays of 500 ms or longer, this inhibition was greater when the targeted muscle was selected compared with when it was not selected. In contrast, the magnitude of inhibition just after the cue was inversely related to the duration of the delay period, and the difference between the selected and nonselected conditions was attenuated. In Experiment 2, TMS and peripheral nerve stimulation procedures were used during a 300-ms delay period. MEPs in the flexor carpi radialis for both selected and nonselected conditions were inhibited, but without any change in the H-reflex. Taken together, these results reveal the dual influence of temporal constraints associated with anticipation and urgency on inhibitory processes recruited during response preparation.

New evidence of corticospinal network modulation induced by motor imagery.

Motor imagery (MI) is the mental simulation of movement, without the corresponding muscle contraction. Whereas the activation of cortical motor areas during MI is established, the involvement of spinal structures is still under debate. We used original and complementary techniques to probe the influence of MI on spinal structures. Amplitude of motor-evoked potentials (MEPs), cervico-medullary-evoked potentials (CMEPs), and Hoffmann (H)-reflexes of the flexor carpi radialis (FCR) muscle and of the triceps surae muscles was measured in young, healthy subjects at rest and during MI. Participants were asked to imagine maximal voluntary contraction of the wrist and ankle, while the targeted limb was fixed (static condition). We confirmed previous studies with an increase of FCR MEPs during MI compared with rest. Interestingly, CMEPs, but not H-reflexes, also increased during MI, revealing a possible activation of subcortical structures. Then, to investigate the effect of MI on the spinal network, we used two techniques: 1) passive lengthening of the targeted muscle via an isokinetic dynamometer and 2) conditioning of H-reflexes with stimulation of the antagonistic nerve. Both techniques activate spinal inhibitory presynaptic circuitry, reducing the H-reflex amplitude at rest. In contrast, no reduction of H-reflex amplitude was observed during MI. These findings suggest that MI has modulatory effects on the spinal neuronal network. Specifically, the activation of low-threshold spinal structures during specific conditions (lengthening and H-reflex conditioning) highlights the possible generation of subliminal cortical output during MI.

A prolonged motor imagery session alter imagined and actual movement durations: Potential implications for neurorehabilitation

Mental practice with motor imagery improves motor performance, for example reducing the duration of goal-directed movements. However, it is not known whether an experimental session involving prolonged sequences of motor imagery induces mental fatigue and alters motor and mental performances. In this study, participants imagined 100 point-to-point arm movements combined with actual pointing movements every 10 or 50 imagined movements. Participants reported a subjective feeling of mental fatigue after imagining 100 pointing movements. When participants performed actual movements every 50 imagined movements, the duration of both actual and imagined movements increased at the end of the protocol. On the contrary, no change in actual and imagined movement duration was observed when participants performed actual movements every 10 imagined movements. These results suggested that the repetition of many imagined movements induced mental fatigue and altered the mental simulation and the actual execution processes of the movement. However, the regular execution of actual movements seemed to counteract the negative effect of mental fatigue as both actual and imagined movement duration remained constant with actual trials inserted between mental rehearsals. We suggest that during training or rehabilitation programs, actual movements should be executed and/or imagined movement duration should be controlled to avoid the negative effects of mental fatigue on motor performance.

The Neural Specificity of Movement Preparation During Actual and Imagined Movements

Current theories consider motor imagery, the mental representation of action, to have considerable functional overlap with the processes involved in actual movement preparation and execution. To test the neural specificity of motor imagery, we conducted a series of 3 experiments using transcranial magnetic stimulation (TMS). We compared changes in corticospinal excitability as people prepared and implemented actual or imagined movements, using a delayed response task in which a cue indicated the forthcoming response. TMS pulses, used to elicit motor-evoked responses in the first dorsal interosseous muscle of the right hand, were applied before and after an imperative signal, allowing us to probe the state of excitability during movement preparation and implementation. Similar to previous work, excitability increased in the agonist muscle during the implementation of an actual or imagined movement. Interestingly, preparing an imagined movement engaged similar inhibitory processes as that observed during actual movement, although the degree of inhibition was less selective in the imagery conditions. These changes in corticospinal excitability were specific to actual/imagined movement preparation, as no modulation was observed when preparing and generating images of cued visual objects. Taken together, inhibition is a signature of how actions are prepared, whether they are imagined or actually executed.

Motor imagery training: Kinesthetic imagery strategy and inferior parietal fMRI activation

Motor imagery (MI) is the mental simulation of action frequently used by professionals in different fields. However, with respect to performance, well‐controlled functional imaging studies on MI training are sparse. We investigated changes in fMRI representation going along with performance changes of a finger sequence (error and velocity) after MI training in 48 healthy young volunteers. Before training, we tested the vividness of kinesthetic and visual imagery. During tests, participants were instructed to move or to imagine moving the fingers of the right hand in a specific order. During MI training, participants repeatedly imagined the sequence for 15 min. Imaging analysis was performed using a full‐factorial design to assess brain changes due to imagery training. We also used regression analyses to identify those who profited from training (performance outcome and gain) with initial imagery scores (vividness) and fMRI activation magnitude during MI at pre‐test (MIpre). After training, error rate decreased and velocity increased. We combined both parameters into a common performance index. FMRI activation in the left inferior parietal lobe (IPL) was associated with MI and increased over time. In addition, fMRI activation in the right IPL during MIpre was associated with high initial kinesthetic vividness. High kinesthetic imagery vividness predicted a high performance after training. In contrast, occipital activation, associated with visual imagery strategies, showed a negative predictive value for performance. Our data echo the importance of high kinesthetic vividness for MI training outcome and consider IPL as a key area during MI and through MI training.

Neural mechanisms of strength increase after one-week motor imagery training

Abstract The neural mechanisms explaining strength increase following mental training by motor imagery (MI) are not clearly understood. While gains are mostly attributed to cortical reorganization, the sub-cortical adaptations have never been investigated. The present study investigated the effects of MI training on muscle force capacity and the related spinal and supraspinal mechanisms. Eighteen young healthy participants (mean age: 22.5 ± 2.6) took part in the experiment. They were distributed into two groups: a control group (n = 9) and an MI training group (n = 9). The MI group performed seven consecutive sessions (one per day) of imagined maximal isometric plantar flexion (4 blocks of 25 trials per session). The control group did not engage in any physical or mental training. Both groups were tested for the isometric maximal plantar flexion torque (MVC) and the rate of torque development (RTD) before and after the training session. In addition, soleus and medial gastrocnemius spinal and supraspinal adaptations were assessed through the recording of H-reflexes and V-waves, with electrical stimulations of the posterior tibial nerve evoked at rest and during MVC, respectively. After one week, only the MI training group increased both plantar flexion MVC and RTD. The enhancement of muscle torque capacity was accompanied by significant increase of electromyographic activity and V-wave during MVC and of H-reflex at rest. The increased cortical descending neural drive and the excitability of spinal networks at rest could explain the greater RTD and MVC after one week of MI training.