Changes in corticospinal drive to spinal motoneurones following visuo-motor skill learning in humans

We have previously demonstrated an increase in the excitability of the leg motor cortical area in relation to acquisition of a visuo-motor task in healthy humans. It remains unknown whether the interaction between corticospinal drive and spinal motoneurones is also modulated following motor skill learning. Here we investigated the effect of visuo-motor skill training involving the ankle muscles on the coupling between electroencephalographic (EEG) activity recorded from the motor cortex (Cz) and electromyographic (EMG) activity recorded from the left tibialis anterior (TA) muscle in 11 volunteers. Coupling in the time (cumulant density function) and frequency domains (coherence) between EEG–EMG and EMG–EMG activity were calculated during tonic isometric dorsiflexion before and after 32 min of training a visuo-motor tracking task involving the ankle muscles or performing alternating dorsi- and plantarflexion movements without visual feedback. A significant increase in EEG–EMG coherence around 15–35 Hz was observed following the visuo-motor skill session in nine subjects and in only one subject after the control task. Changes in coherence were specific to the trained muscle as coherence for the untrained contralateral TA muscle was unchanged. EEG and EMG power were unchanged following the training. Our results suggest that visuo-motor skill training is associated with changes in the corticospinal drive to spinal motorneurones. Possibly these changes reflect sensorimotor integration processes between cortex and muscle as part of the motor learning process.     

Dynamical changes in corticospinal excitability during imagery of unimanual and bimanual wrist movements in humans: a transcranial magnetic stimulation study

This study explored the dynamical changes in corticospinal excitability during the imagination of cyclical unimanual and bimanual wrist flexion-extension movements. Transcranial magnetic stimulation was applied over the left motor cortex to evoke motor evoked potentials in the right wrist flexor and extensor muscles. Findings provided evidence for increased reciprocal excitability changes during imagery of symmetrical in-phase movements as compared to asymmetrical (anti-phase) or unimanual movements. This suggests that in-phase movements may reinforce whereas anti-phase movements may reduce the temporal representation of the task in the corticospinal motor networks of the brain.     

Cortical excitability and motor task in man: an investigation of the wrist extensor motor area

Task-related changes in the corticospinal excitation of the right extensor carpi radialis (ECR) muscle were investigated in 16 healthy human subjects. The subjects were asked to perform a tonic isometric wrist extension or to clench their hand around a manipulandum, thereby coactivating the antagonistic wrist muscles. At matched levels of background EMG in the ECR muscle, transcranial magnetic stimulation (TMS) was applied through a figure-of-eight coil at 20-30 sites spaced 1 cm apart over the hand area of the left motor cortex. The cortical maps of the representation of the ECR muscle constructed in this way did not change between the two motor tasks. Nevertheless, for all investigated cortical sites TMS evoked a smaller motor evoked potential (MEP) in the ECR muscles during hand clenching than during wrist extension. A similar decrease in the short-latency peak in the poststimulus time histogram (PSTH) of single ECR motor units to TMS during hand clenching was found in seven subjects (number of motor units = 35). In contrast, short-latency peaks in the PSTH evoked by electrical stimulation of the motor cortex had a similar size during the two tasks (number of motor units = 9; two subjects). Already the initial 0.5-1.0 ms of the short-latency peak evoked by TMS was depressed during hand clenching, which suggests that decreased excitability of corticospinal cells with monosynaptic projections onto ECR motor units was involved. This decreased excitability was not explained by increased intracortical inhibition, which was found to be of a similar size during hand clenching and wrist extension. The task-related changes in the efficiency of the motor cortex output are discussed in relation to the function of the wrist antagonist muscles in handling and gripping tasks.     

Changes in Corticospinal Excitability in the Reactions of Forearm Muscles in Humans to Vibration

Transcranial magnetic stimulation was used to study changes in corticospinal excitability during vibration of the flexor and extensor muscles of the wrist in healthy humans. The ratios of muscle stimulation responses to activity levels in these muscles on contraction associated with vibration (the tonic vibratory reflex, TVR) and after vibration of antagonist muscles in isometric conditions (the antagonist vibratory reflex, AVR) were analyzed. The normalized muscle response in the wrist flexors was found to increase by 66% compared with threshold values in the direct vibratory response (TVR), by 75% in the relayed vibratory response (AVR), and by 18% on voluntary contraction. However, increases in the motor response in vibratory responses as compared with those on voluntary contraction did not reach significance, which contrasted with the responses in the forearm flexors. These results are discussed from the point of view that the motor cortex plays different roles in vibratory responses in the distal and proximal muscles.     

The effect of simultaneous contractions of ipsilateral muscles on changes in corticospinal excitability induced by paired associative stimulation (PAS)

Consideration was given to means of increasing the reliability and muscle specificity of paired associative stimulation (PAS) by utilising the phenomenon of crossed-facilitation. Eight participants completed three separate sessions: isometric flexor contractions of the left wrist at 20% of maximum voluntary contraction (MVC) simultaneously with PAS (20 s intervals; 14 min duration) delivered at the right median nerve and left primary motor cortex (M1); isometric contractions at 20% of MVC; and PAS only (14 min). Eight further participants completed two sessions of longer duration PAS (28 min): either alone or in conjunction with flexion contractions of the left wrist. Thirty motor potentials (MEPs) were evoked in the right flexor (rFCR) and extensor (rECR) carpi radialis muscles by magnetic stimulation of left M1 prior to the interventions, immediately post-intervention, and 10 min post-intervention. Both 14 and 28 min of combined PAS and (left wrist flexion) contractions resulted in reliable increases in rFCR MEP amplitude, which were not present in rECR. In the PAS only conditions, 14 min of stimulation gave rise to unreliable increases in MEP amplitudes in rFCR and rECR, whereas 28 min of PAS induced small (unreliable) changes only for rFCR. These results support the conclusion that changes in the excitability of the corticospinal pathway induced by PAS interact with those associated with contraction of the muscles ipsilateral to the site of cortical stimulation. Furthermore, focal contractions applied by the opposite limb increase the extent and muscle specificity of the induced changes in excitability associated with PAS.     

Stability of corticospinal excitability and grip force in intrinsic hand muscles in man over a 24-h period

The maximum voluntary muscle force can vary throughout the day; typically being low in the morning and high in the evening. The nature of this possible variation has been investigated with respect to corticospinal excitability. Six healthy subjects were studied. Maximum voluntary contraction (MVC) in the thenar muscles was measured. In addition, we monitored several indices of corticospinal excitability using electromyographic (EMG) recording and transcranial magnetic stimulation (TMS) of the motor cortex. Motor evoked potentials (MEPs) were recorded while relaxed and at 10% MVC when the silent period was assessed as an index of corticospinal inhibition. Readings were taken every 3 h for 24 h. MVC of the thenar muscles did not change significantly over the 24 h. The mean areas, latencies and durations of MEPs did not show significant changes over the 24-h test period with the muscle relaxed or contracted; however, MEP area did vary between sessions at all stimulus intensities suggesting non-time-of-day-dependent changes in corticospinal excitability. Furthermore, the extent and duration of the silent period seen after the MEP in the contracted muscle did not change significantly over the 24 h of the experiment at any stimulus intensity. These results provide evidence that the MVC force of the thenar muscles and their responses to TMS are stable throughout the course of the day and suggest that, in hand muscles, corticospinal excitability may not be subject to circadian variation.     

Arm muscle activation for static forces in three-dimensional space

1. Muscle activity was related to the direction of a static force at the human wrist. For each muscle the force direction of maximal activity and the directional tuning characteristics were determined. 2. Electromyographic (EMG) activity was recorded from nine superficial elbow and/or shoulder muscles while subjects held the right arm stationary in one of six postures. The direction of the force at the wrist was varied in two orthogonal planes. In each experiment a cable was attached to the subject's wrist, and a constant force magnitude was applied in various directions with the use of a pulley system. 3. The relationship between the averaged EMG level and the force direction was described for each muscle, in each posture, and in each plane. The EMG data were fit with a nonlinear, multiple cosine function, which allowed the identification of one, two, or sometimes three separate cosine peaks. 4. Two-cosine functions often provided the best fit to the EMG data. All nine muscles were best fit with a two-cosine function in at least two of the six postures. Four of the muscles had a second peak of activity in more than one-half of the experimental situations. The second peak was often in a direction that was nearly opposite the direction of the first peak and represented a negative contribution to the total force produced at the wrist ("coactivation"). We suggest that multimodal directional tuning results from the convergence of multiple sources of descending signals onto motoneurons. 5. The mechanical actions of nine elbow and/or shoulder muscles were estimated with the use of published data from a cadaver study by Wood and co-workers. Postural changes in the mechanical actions of muscles were substantial. A 45 degrees rotation of the shoulder, for example, might cause a 30-50 degrees change in the direction of force at the wrist that could be produced by the contraction of a given muscle. The magnitude of these postural changes suggests that arm position is an important determinate of EMG patterns. 6. Postural changes in the direction of maximal EMG activity usually paralleled the postural changes in mechanical pulling direction. Postural changes in EMG amplitudes usually covaried with postural changes in mechanical advantage. 7. The posterior deltoid (PD) was an exception to the general rule of covariation of mechanical actions and EMG activities. Instead of reflecting the muscle's mechanical action, the EMG activity of the PD closely resembled the EMG activity of the medial deltoid (MD).(ABSTRACT TRUNCATED AT 400 WORDS)     

Changes in corticospinal motor excitability induced by non-motor linguistic tasks

The excitability of the corticospinal motor pathways to transcranial magnetic stimulation (TMS) can be differentially modulated by a variety of motor tasks. However, there is emerging evidence that linguistic tasks may alter excitability of the corticospinal motor pathways also. In this study we evaluated the effect of several movement-free, low-level linguistic processes involved in reading and writing on the excitability of the bilateral corticospinal motor pathways in a group of right-handed subjects. The study included two series of tasks, visual searching/matching and imaginal writing/drawing. The tasks were designed to roughly correspond with elemental aspects of the reading and writing, grapheme recognition and grapheme generation, respectively. Each task series included separate blocks with different task targets: letters, digits, semantically easy-to-code (i.e. geometric) shapes, and semantically hard-to-code shapes, as well as control blocks with no task. During task performance, TMS was delivered randomly over the hand area of either the left or right motor cortex and the modulation of the excitability of the corticospinal motor pathways was measured bilaterally through changes of the size of the motor-evoked potential (MEP) induced in the relaxed right and left first dorsal interosseous (FDI) muscles. We found that the size of the MEP in hand muscles increased during visual searching/matching tasks, particularly when targets were letters or geometric shapes, and the increase was significant for the dominant hand (left hemisphere) only. No such consistent effects were seen across subjects during imaginal tasks. This study provides evidence that even the performance of certain low-level linguistic tasks can modulate the excitability of the corticospinal motor pathways, particularly those originating from the left (dominant) hemisphere, despite the absence of overt motor activity. Moreover, in the light of the recently increased awareness of the role of "mirror neurons" in perception, the results suggest that activation of motor circuits used in generation of the written output may be an essential part of the perception of the written material as well. Understanding the patterns of task-dependent changes in excitability of the corticospinal motor pathways will provide insights into the organisation of central nervous system functional networks involved in linguistic processes, and may also be useful for future development of novel approaches to rehabilitation therapy of linguistic and motor functions.     

Low Frequency Cortico-Muscular Coherence During Voluntary Rapid Movements of the Wrist Joint

Rapid voluntary point-to-point wrist tracking movements are generated by the co-operative action of a large number of wrist muscles activated in a stereotypic pattern. This pattern is composed of a two burst sequence occurring in synergist and antagonist muscles. The time course and duration of these bursts are relatively fixed but the burst magnitude in any one muscle varies in relation to the direction of movement and the preferred directional tuning characteristic of the muscle. This creates a highly adaptive method for generating fast movements to different positions in space. In this study we have examined the extent to which this adaptive burst behaviour can be associated with activity changes occurring in the contralateral motor cortex. Time dependent coherence estimates were obtained from simultaneous recordings of the electroencephalogram (EEG) made from the contralateral sensorimotor cortex and the electromyogram (EMG) from various wrist flexor and extensor muscles during fast point-to-point wrist tracking movements. Using the onset of movement as a trigger, event related coherence estimates reveal the presence of short lasting periods of low frequency (<12 Hz) coherence during the execution of fast wrist movements. The onset and duration of the periods of low frequency coherence vary with direction of movement and the temporal burst profile of a particular muscle's EMG activity. It is therefore likely that a significant low frequency activation of the motor cortex plays a part in the generation of the EMG burst patterns that underpin rapid point-to-point movements of the human wrist.     

Combined effect of motor imagery and peripheral nerve electrical stimulation on the motor cortex

Although motor imagery enhances the excitability of the corticospinal tract, there are no peripheral afferent inputs during motor imagery. In contrast, peripheral nerve electrical stimulation (ES) can induce peripheral afferent inputs; thus, a combination of motor imagery and ES may enhance the excitability of the corticospinal tract compared with motor imagery alone. Moreover, the level of stimulation intensity may also be related to the modulation of the excitability of the corticospinal tract during motor imagery. Here, we evaluated whether a combination of motor imagery and peripheral nerve ES influences the excitability of the corticospinal tract and measured the effect of ES intensity on the excitability induced during motor imagery. The imagined task was a movement that involved touching the thumb to the little finger, whereas ES involved simultaneous stimulation of the ulnar and median nerves at the wrist. Two different ES intensities were used, one above the motor threshold and another above the sensory threshold. Further, we evaluated whether actual movement with afferent input induced by ES modulates the excitability of the corticospinal tract as well as motor imagery. We found that a combination of motor imagery and ES enhanced the excitability of the motor cortex in the thenar muscle compared with the other condition. Furthermore, we established that the modulation of the corticospinal tract was related to ES intensity. However, we found that the excitability of the corticospinal tract induced by actual movement was enhanced by peripheral nerve ES above the sensory threshold.     

The relative activation of muscles during isometric contractions and low-velocity movements against a load

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, this direction was different for isometric contractions and for movements. Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.     

Increase in flexor but not extensor corticospinal motor outputs following ischemic nerve block

Human motor cortex is capable of rapid and long-lasting reorganization, evident globally, as shifts in body part representations, and at the level of individual muscles as changes in corticospinal excitability. Representational shifts provide an overview of how various body parts reorganize relative to each other but do not tell us whether all muscles in a given body part reorganize in the same manner and to the same extent. Transcranial magnetic stimulation (TMS) provides information about individual muscles and can therefore inform us about the uniformity of plastic changes within a body part. We used TMS to investigate changes in corticospinal excitability of forearm flexors and extensors after inflation of a tourniquet around the wrist. Motor evoked potential (MEP) amplitudes and input/output (I/O) curves were obtained from wrist flexors and extensors simultaneously before and during block. TMS was delivered to the optimal site for eliciting MEPs in flexors in experiment 1, extensors in experiment 2, and both flexors and extensors in experiment 3. In all experiments flexor MEP amplitude increased during block while extensor MEP amplitude showed no systematic change, and the slope of flexor but not extensor I/O curves increased. Flexor H-reflex amplitude normalized to maximal M wave showed negligible changes during block, suggesting that the increase in corticospinal excitability in the flexors cannot be completely explained by increased excitability at the spinal cord level. These findings show that forearm flexors and extensors differ in their potential for plastic changes, highlight the importance of investigating how experimentally induced plasticity affects anatomically close, but functionally distinct, muscle groups, and suggest that rehabilitation interventions aiming to alter cortical organization should consider the differential sensitivity of various muscle groups to plasticity processes.     

Hysteresis in corticospinal excitability during gradual muscle contraction and relaxation in humans

Many studies have demonstrated that the firing behavior of single motor units varies in a nonlinear manner to the exerted torque during gradual muscle contraction and relaxation. However, it is unclear whether corticospinal excitability has such a hysteresis-like feature. In this study, we examined corticospinal excitability using transcranial magnetic stimulation (TMS) during gradual muscle contraction and relaxation for torque regulation in elbow flexor muscles. Eight healthy male subjects performed two different isometric elbow flexion tasks, namely, sinusoidal and tonic torque exertion tasks. In the sinusoidal task, the subjects sinusoidally increased and decreased the isometric elbow flexion torque (range of 0–15% of maximum voluntary contraction) at three different frequencies (0.33, 0.17, and 0.08 Hz). For each ascending (contraction: CON) and descending (relaxation: REL) period of the exerted torque, a single TMS was applied at 5 phases. In the tonic task, the elbow flexion torque was tonically exerted at 7 levels in a similar range as that in the sinusoidal task. EMG activities were recorded from the agonists, the biceps brachii (BB) and brachioradialis (BRD) muscles, and an antagonist, the triceps brachii (TB) muscle. The results demonstrated that the EMG activities of both the agonists and antagonist were larger in the CON period than the REL period, even when the exerted torque was the same. However, there were no significant differences in EMG activation profiles among the different frequencies of contraction. In BB and BRD, the motor-evoked potential (MEP) elicited by the TMS was also greater in the CON period than in the REL period. This CON-REL difference of MEP amplitudes was still observed when corrections were made for the increased EMG activities; that is, the MEP amplitudes to the identical EMG activities were greater in the CON period than in the REL period, and this phenomenon was more pronounced at higher frequencies. In addition, the degree to which sinusoidal MEPs exceeded tonic MEPs in the CON period and were smaller than tonic MEPs in the REL period became more pronounced at higher frequencies. On the other hand, there were significant correlations between the BB and BRD MEP amplitudes and the rate of change of elbow flexion/extension torque. These results indicate that corticospinal excitability during muscle contraction and relaxation has a neural hysteresis to the muscle activity, i.e., spinal motoneuronal activity, according to the rate of change of the exerted torque, i.e., muscle tension. This suggests that corticospinal excitability modulation depends not only on concurrent spinal motoneuronal activity and muscle tension but also on the time-series pattern of their changes during muscle contraction and relaxation.     

Preparatory activity in motor cortex reflects learning of local visuomotor skills

In humans, learning to produce correct visually guided movements to adapt to new sensorimotor conditions requires the formation of an internal model that represents the new transformation between visual input and the required motor command. When the new environment requires adaptation to directional errors, learning generalizes poorly to untrained locations and directions, indicating that such learning is local. Here we replicated these behavioral findings in rhesus monkeys using a visuomotor rotation task and simultaneously recorded neuronal activity. Specific changes in activity were observed only in a subpopulation of cells in the motor cortex with directional properties corresponding to the locally learned rotation. These changes adhered to the dynamics of behavior during learning and persisted between learning and relearning of the same rotation. These findings suggest a neural mechanism for the locality of newly acquired sensorimotor tasks and provide electrophysiological evidence for their retention in working memory.     

Systematic changes in motor cortex cell activity with arm posture during directional isometric force generation

We report here the activity of 96 cells in primate primary motor cortex (MI) during exertion of isometric forces at the hand in constant spatial directions, while the hand was at five to nine different spatial locations on a plane. The discharge of nearly all cells varied significantly with both hand location and the direction of isometric force before and during force-ramp generation as well as during static force-hold. In addition, nearly all cells displayed changes in the variation of their activity with force direction at different hand locations. This change in relationship was often expressed in part as a change in the cell's directional tuning at different hand locations. Cell directional tuning tended to shift systematically with hand location even though the direction of static force output at the hand remained constant. These directional effects were less pronounced before the onset of force output than after force onset. Cells also often showed planar modulations of discharge level with hand location. Sixteen proximal arm muscles showed similar effects, reflecting how hand location-dependent biomechanical factors altered their task-related activity. These findings indicate that MI single-cell activity does not covary exclusively with the level and direction of net force output at the hand and provides further evidence that MI contributes to the transformation between extrinsic and intrinsic representations of motor output during isometric force production.     

Effects of motor preparation and spatial attention on corticospinal excitability in a delayed-response paradigm

The preparation of motor responses during the delay period of an instructed delay task is associated with sustained neural firing in the primate premotor cortex. It remains unclear how and when such preparation-related premotor activity influences the motor output system. In this study, we tested modulation of corticospinal excitability using single-pulse transcranial magnetic stimulation (TMS) during a delayed-response task. At the beginning of the delay interval participants were either provided with no information, spatial attentional information concerning location but not identity of an upcoming imperative stimulus, or information regarding the upcoming response. Behavioral data indicate that participants used all information available to them. Only when information concerning the upcoming response was provided did corticospinal excitability show differential modulation for the effector muscle compared to other task-unrelated muscles. We conclude that modulation of corticospinal excitability reflects specific response preparation, rather than non-specific event preparation.     

Asymmetrical modulation of corticospinal excitability in the contracting and resting contralateral wrist flexors during unilateral shortening, lengthening and isometric contractions

Unilateral isometric muscle contractions increase motor-evoked potentials (MEPs) produced by transcranial magnetic stimulation not only in the contracting muscle but also in the resting contralateral homologous muscle. Corticospinal excitability in the M1 contralateral to the contracting muscle changes depending on the type of muscle contraction. Here, we investigated the possibility that corticospinal excitability in M1 ipsilateral to the contracting muscle is modulated in a contraction-type-dependent manner. To this end, we evaluated MEPs in the resting left flexor carpi radialis (FCR) during unilateral shortening, lengthening, and isometric muscle contractions of the right wrist flexors at 10, 20, and 30% of maximal isometric contraction force. To compare the effects of different unilateral contractions on MEPs between the contracting and resting sides, MEPs in the right FCR were recorded on two separate days. In a separate experiment, we investigated the contraction specificity of the crossed effect at the spinal level by recording H-reflexes from the resting left FCR during contraction of the right wrist flexors. The results showed that MEPs in the contracting right FCR were the smallest during lengthening contraction. By contrast, MEPs in the resting left FCR were the largest during lengthening contraction, whereas the H-reflex was similar in the resting left FCR during the three types of muscle contraction. These results suggest that different types of unilateral muscle contraction asymmetrically modulate MEP size in the resting contralateral homologous muscle and in the contracting muscle and that this regulation occurs at the supraspinal level.     

Modulation of corticospinal excitability dependent upon imagined force level

Motor imagery is defined as the mental execution of a movement without any muscle activity. In the present study, corticospinal excitability was assessed by motor evoked potentials (MEPs) when the subjects imagined isometric elbow flexion at various force levels. Electromyography was recorded from the right brachioradialis, the biceps brachii and the triceps brachii muscles. First, the maximum voluntary contraction (MVC) of elbow flexion was recorded in each subject. Subjects practiced performing 10, 30 and 60 % MVC using visual feedback. After the practice, MEPs were recorded during the imagery of elbow flexion with the forces of 10, 30 and 60 % MVC without any feedback. After the MEPs recording, we assigned subjects to reproduce the actual elbow flexion force at 10, 30 and 60 % MVC. The MEPs amplitudes in the brachioradialis and biceps brachii in the 60 % MVC condition were significantly greater than those in the 10 % MVC condition (p < 0.05). These findings suggest that the enhancement of corticospinal excitability during motor imagery is associated with an increase in imagined force level.     

Induction of cortical plastic changes in wrist muscles by paired associative stimulation in healthy subjects and post-stroke patients

It has been shown on hand muscles in normal subjects that paired associative stimulation (PAS) combining peripheral nerve stimulation and transcranial magnetic stimulation (TMS) induces lasting changes in cortical motor excitability (Stefan et al., Brain 123 (Pt3):572–584, 2000). Because the motor recovery of distal upper limb and particularly wrist extension in post-stroke patients is one of the major rehabilitation challenge, we investigate here the effect of one session of paired associative stimulation on the excitability of the corticospinal projection to extensor carpi radialis (ECR) muscle (motor evoked potential size) before and after PAS in 17 healthy subjects and in two patients 5 months after stroke. The time course, the topographical specificity, changes in rest motor threshold (RMT), short intracortical inhibition and intracortical facilitation (SICI and ICF), the respective role of cutaneous and muscular afferents and the effect of a prolonged peripheral stimulation alone were also studied in normal subjects. Using a protocol derived from that of Ridding et al. J Physiol 537:623–631 (2001), PAS was able to induce lasting changes in the excitability of corticospinal projection to wrist muscles in healthy subjects and in the two post-stroke patients studied. Electrophysiological features of these plastic changes were similar to those previously observed in hand muscles: rapid evolution, 30–60 min duration, reversibility, relative topographical specificity and associative dependence suggesting an LTP-like mechanism. A contribution of cutaneous afferents in inducing PAS effects was also demonstrated. The decrease in ECR RMT after PAS observed in patients and in healthy subjects was an unexpected result because it has not been previously reported in the hand muscles of healthy subjects. However, it has been observed in dystonic patients (Quartarone et al., Brain 126:2586–2596, 2003). This suggests that other mechanisms like changes in membrane excitability could be involved in ECR facilitation after PAS. Further studies performed on patients using daily repeated PAS protocols and showing a functional improvement in hand motor function will be necessary to confirm that this technique could be relevant in motor rehabilitation, at least for some selected patients.     

Motor cortex neural correlates of output kinematics and kinetics during isometric-force and arm-reaching tasks

We recorded the activity of 132 proximal-arm-related neurons in caudal primary motor cortex (M1) of two monkeys while they generated either isometric forces against a rigid handle or arm movements with a heavy movable handle, in the same eight directions in a horizontal plane. The isometric forces increased in monotonic fashion in the direction of the force target. The forces exerted against the handle in the movement task were more complex, including an initial accelerating force in the direction of movement followed by a transient decelerating force opposite to the direction of movement as the hand approached the target. EMG activity of proximal-arm muscles reflected the difference in task dynamics, showing directional ramplike activity changes in the isometric task and reciprocally tuned "triphasic" patterns in the movement task. The apparent instantaneous directionality of muscle activity, when expressed in hand-centered spatial coordinates, remained relatively stable during the isometric ramps but often showed a large transient shift during deceleration of the arm movements. Single-neuron and population-level activity in M1 showed similar task-dependent changes in temporal pattern and instantaneous directionality. The momentary dissociation of the directionality of neuronal discharge and movement kinematics during deceleration indicated that the activity of many arm-related M1 neurons is not coupled only to the direction and speed of hand motion. These results also demonstrate that population-level signals reflecting the dynamics of motor tasks and of interactions with objects in the environment are available in caudal M1. This task-dynamics signal could greatly enhance the performance capabilities of neuroprosthetic controllers.