Motor Cortex Representation Research Articles

Terminal distribution of the corticospinal projection from the hand/arm region of the primary motor cortex to the cervical enlargement in rhesus monkey

To further our understanding of the corticospinal projection (CSP) from the hand/arm representation of the primary motor cortex (M1), high-resolution anterograde tracing methodology and stereology were used to investigate the terminal distribution of this connection at spinal levels C5 to T1. The highest number of labeled terminal boutons occurred contralaterally (98%) with few ipsilaterally (2%). Contralaterally, labeled boutons were located within laminae I-X, with the densest distribution found in lamina VII and, to a lesser extent, laminae IX and VI. Fewer terminals were found in other contralateral laminae. Within lamina VII, terminal boutons were most prominent in the dorsomedial, dorsolateral, and ventrolateral subsectors. Within lamina IX, the heaviest terminal labeling was distributed dorsally. Ipsilaterally, boutons were found in laminae V-X. The most pronounced distribution occurred in the dorsomedial and ventromedial sectors of lamina VII and fewer labeled boutons were located in other ipsilateral laminae. Segmentally, contralateral lamina VII labeling was highest at levels C5-C7. In contrast, lamina IX labeling was highest at C7-T1 and more widely dispersed among the quadrants at C8-T1. Our findings suggest dominant contralateral influence of the M1 hand/arm CSP, a contralateral innervation pattern in lamina VII supporting Kuypers (1982) conceptual framework of a "lateral motor system," and a projection to lamina IX indicating significant influence on motoneurons innervating flexors acting on the shoulder and elbow rostrally (C5-C7), along with flexors, extensors, abductors and adductors acting on the digits, hand and wrist caudally (C8-T1).

Modulation of transcallosal inhibition by bilateral activation of agonist and antagonist proximal arm muscles

Transcallosal inhibitory interactions between proximal representations in the primary motor cortex remain poorly understood. In this study, we used transcranial magnetic stimulation to examine the ipsilateral silent period (iSP; a measure of transcallosal inhibition) in the biceps and triceps brachii during unilateral and bilateral isometric voluntary contractions. Healthy volunteers performed 10% of maximal isometric voluntary elbow flexion or extension with one arm while the contralateral arm remained at rest or performed 30% of maximal isometric voluntary elbow flexion or extension. The iSP was measured in the arm performing 10% contractions, and electromyographic (EMG) recordings were comparable across conditions. The iSP onset and duration in the biceps and triceps brachii were comparable. In both muscles, the iSP depth and area were increased during bilateral contractions of homologous agonist muscles (extension-extension and flexion-flexion) compared with a unilateral contraction, whereas during bilateral contractions of nonhomologous antagonist muscles (extension-flexion and flexion-extension), the iSP depth and area were decreased compared with a unilateral contraction, and sometimes facilitation of EMG was seen. This effect was never observed during bilateral activation of homologous muscles. The size of responses evoked by cervicomedullary electrical stimulation in the arm that made 10% contractions remained unchanged across conditions. Thus transcallosal inhibition targeting triceps and biceps brachii is upregulated by voluntary contraction of the contralateral agonist muscle and downregulated by voluntary contraction of the contralateral antagonist muscle. We speculate that these reciprocal task-dependent interactions between bilateral flexor and extensor arm regions of the motor cortex may contribute to coupling between the arms during motor behavior.

Rapid and persistent impairments of the forelimb motor representations following cervical deafferentation in rats

Skilled motor control is regulated by the convergence of somatic sensory and motor signals in brain and spinal motor circuits. Cervical deafferentation is known to diminish forelimb somatic sensory representations rapidly and to impair forelimb movements. Our focus was to determine what effect deafferentation has on the motor representations in motor cortex, knowledge of which could provide new insights into the locus of impairment following somatic sensory loss, such as after spinal cord injury or stroke. We hypothesized that somatic sensory information is important for cortical motor map topography. To investigate this we unilaterally transected the dorsal rootlets in adult rats from C4 to C8 and mapped the forelimb motor representations using intracortical microstimulation, immediately after rhizotomy and following a 2-week recovery period. Immediately after deafferentation we found that the size of the distal representation was reduced. However, despite this loss of input there were no changes in motor threshold. Two weeks after deafferentation, animals showed a further distal representation reduction, an expansion of the elbow representation, and a small elevation in distal movement threshold. These changes were specific to the forelimb map in the hemisphere contralateral to deafferentation; there were no changes in the hindlimb or intact-side forelimb representations. Degradation of the contralateral distal forelimb representation probably contributes to the motor control deficits after deafferentation. We propose that somatic sensory inputs are essential for the maintenance of the forelimb motor map in motor cortex and should be considered when rehabilitating patients with peripheral or spinal cord injuries or after stroke.

IS 13. Mean corticospinal excitability and its variability during decisions for actions

The neural underpinnings that link decision making and action selection remain largely undetermined. It is thought that action selection in motor regions originates from a competitive process that is gradually biased by evidence signals originating in other regions, such as those specialized in value computations. Biases reflecting the evaluation of choice options should thus emerge in the motor system before the decision process is complete. We here review recent evidence that this is indeed the case. Mean changes in cortico-spinal excitability (CSE) for chosen versus unchosen actions distinguish forthcoming choices before completion of the decision process. Additionally, the variability of CSE prior to movement declines contralateral to the responding hand, and more strongly so on fast compared to slow response trials; this is consistent with data in non-human primates. For the non-responding hand, CSE variability also decreased, but only on choice trials and earlier compared to the responding hand, possibly reflecting choice-specific suppression of unselected actions. Internally generated (value-based) decisions thus influence the competition between action representations in motor cortex before the decision process is complete. In addition, the variability of human CSE elicited by TMS over M1 tracks the state of motor preparation, and may reflect the optimization of preparatory population activity. Collectively these results show how the mean changes in CSE and their variability reveal distinct influences of decision processes on regions specified in action preparation and selection.

Rhythmic Whisking Area (RW) in Rat Primary Motor Cortex: An Internal Monitor of Movement-Related Signals?

Vibrissae-related sensorimotor cortex controls whisking movements indirectly via modulation of lower-level sensorimotor loops and a brainstem central pattern generator (CPG). Two different whisker representations in primary motor cortex (vM1) affect whisker movements in different ways. Prolonged microstimulation in RF, a larger anterior subregion of vM1, gives rise to complex face movements and whisker retraction while the same stimulation evokes large-amplitude rhythmic whisker movement in a small caudo-medial area (RW). To characterize the motor cortex representation of explorative whisking movements, here we recorded RW units in head-fixed rats trained to contact a moving object with one whisker. RW single units were found to encode two aspects of whisker movement independently, albeit on slow time scales (hundreds of milliseconds). The first is whisker position. The second consists of speed (absolute velocity), intensity (instantaneous power), and frequency (spectral centroid). The coding for the latter three parameters was tightly correlated and realized by a continuum of RW responses-ranging from a preference of movement to a preference of rest. Information theory analysis indicated that RW spikes carry most information about position and frequency, while intensity and speed are less well represented. Further, investigating multiple and single RW units, we found a lack of phase locking, movement anticipation, and contact-related tactile responses. These findings suggest that RW neither programs detailed whisker trajectories nor initiates them. Nor does it play a role in processing object touch. Its relationship to whisking is thus indirect and may be related to movement monitoring, perhaps using feedback from the CPG.

Homologous Muscle Contraction during Unilateral Movement Does Not Show a Dominant Effect on Leg Representation of the Ipsilateral Primary Motor Cortex

Co-activation of homo- and heterotopic representations in the primary motor cortex (M1) ipsilateral to a unilateral motor task has been observed in neuroimaging studies. Further analysis showed that the ipsilateral M1 is involved in motor execution along with the contralateral M1 in humans. Additionally, transcranial magnetic stimulation (TMS) studies have revealed that the size of the co-activation in the ipsilateral M1 has a muscle-dominant effect in the upper limbs, with a prominent decline of inhibition within the ipsilateral M1 occurring when a homologous muscle contracts. However, the homologous muscle-dominant effect in the ipsilateral M1 is less clear in the lower limbs. The present study investigates the response of corticospinal output and intracortical inhibition in the leg representation of the ipsilateral M1 during a unilateral motor task, with homo- or heterogeneous muscles. We assessed functional changes within the ipsilateral M1 and in corticospinal outputs associated with different contracting muscles in 15 right-handed healthy subjects. Motor tasks were performed with the right-side limb, including movements of the upper and lower limbs. TMS paradigms were measured, consisting of short-interval intracortical inhibition (SICI) and recruitment curves (RCs) of motor evoked potentials (MEPs) in the right M1, and responses were recorded from the left rectus femoris (RF) and left tibialis anterior (TA) muscles. TMS results showed that significant declines in SICI and prominent increases in MEPs of the left TA and left RF during unilateral movements. Cortical activations were associated with the muscles contracting during the movements. The present data demonstrate that activation of the ipsilateral M1 on leg representation could be increased during unilateral movement. However, no homologous muscle-dominant effect was evident in the leg muscles. The results may reflect that functional coupling of bilateral leg muscles is a reciprocal movement.

Functional Topography of Cortical Thumb Movement Representations in Human Primary Motor Cortex

Intracortical microstimulation and single cell recordings in non-human primates showed that both, muscles and movements are represented in primary motor cortex (M1). This was also suggested in humans using electrophysiological and neuroimaging techniques. Transcranial magnetic stimulation (TMS) thus far was used to study motor cortical muscle representations, but data on movement representations in man are scarce. Therefore, we used TMS of M1 to evoke directional (flexion, extension) thumb movements and generated topographic movement maps (TMS-mov-map) in fourteen healthy individuals. TMS-mov-maps were related to the individual voluntary motor output (Vol-mov, directional thumb acceleration). Conventional TMS-motor evoked potentials (TMS-MEP) were simultaneously recorded from the prime movers (flexor and extensor pollicis brevis) and also compared to the voluntary motor output. Remarkably stable topographic maps were generated with both, thumb flexion and extension being multiply represented, overlapped and interspersed. Thumb Flexion was usually more robust than extension. TMS-mov-maps rather than TMS-MEP-maps seemed to better reflect the individual voluntary motor output. The findings emphasize existing models of multiple, overlapping finger movement representations in human M1, and indicate that this model also adapts to antagonistic thumb movements. The results suggest that investigating topographic movement maps may be an interesting adjunct in studying human motor cortical topography and its relevance for motor control.

Functional magnetic resonance imaging of chronic dysarthric speech after childhood brain injury: reliance on a left-hemisphere compensatory network

Severe and persistent speech disorder, dysarthria, may be present for life after brain injury in childhood, yet the neural correlates of this chronic disorder remain elusive. Although abundant literature is available on language reorganization after lesions in childhood, little is known about the capacity of motor speech networks to reorganize after injury. Here, we examine the structural and functional neural correlates associated with chronic dysarthria after childhood-onset traumatic brain injury. Forty-nine participants aged 12 years 3 months to 24 years 11 months were recruited to the study: (i) a group with chronic dysarthria (n = 17); matched for age and sex with two control groups of (ii) healthy control subjects (n = 17); and (iii) individuals without dysarthria after traumatic brain injury (n = 15). A high-resolution 3D T(1)-weighted whole-brain data set was acquired for voxel-based morphometry analyses of group differences in grey matter. Functional magnetic resonance imaging was used to localize activation associated with speaking single words (baseline: listening to words). Group differences on voxel-based morphometry revealed widespread grey matter reductions in the dysarthric group compared with healthy control subjects, including in numerous speech motor regions bilaterally, such as the cerebellum, the basal ganglia and primary motor cortex representation of the articulators. Relative to the non-dysarthric traumatic brain injury group, individuals with dysarthria showed reduced grey matter bilaterally in the ventral sensorimotor cortex, but this reduction was concomitant with increased functional activation only in the left-hemisphere cluster during speech. Finally, increased recruitment of Broca's area (Brodmann area 45, pars triangularis) but not its right homologue, correlated with better speech outcome, suggesting that this 'higher-level' area may be more critically involved with production when associated motor speech regions are damaged. We suggest that the bilateral morphological abnormalities within cortical speech networks in childhood prevented reorganization of speech function from the left- to right-hemisphere. Rather, functional reorganization involved over-recruitment of left-hemisphere motor regions, a reorganization method that was only partly relatively effective, given the presence of persisting yet mild speech deficits. The bilateral structural abnormalities found to limit functional reorganization here, may also be critical to poor speech prognosis for populations with congenital, degenerative or acquired neurological disorders throughout the lifespan.

Complex Movement Topography and Extrinsic Space Representation in the Rat Forelimb Motor Cortex as Defined by Long-Duration Intracortical Microstimulation

Electrical stimulation of the motor cortex in the rat can evoke complex forelimb multi-joint movements, including movement of limb and paw. In this study, these movements have been quantified in terms of 3D displacement and kinematic variables of two markers positioned on the wrist and middle digits (limb and paw movement, respectively). Electrical microstimulation was applied to the motor cortex using a pulse train of 500 ms duration. Movements were measured using a high-resolution 3D optical system. Five classes of limb movements (abduction, adduction, extension, retraction, elevation) and four classes of paw movements (opening, closure, opening/closure sequence, supination) were described according to their kinematics. A consistent topography of these classes of movements was presented across the motor cortex together with a topography of spatial locations to which the paw was directed. In about one-half of cortical sites, a specific pattern of limb-paw movement combination did exist. Four categories of limb-paw movements resembling behavioral repertoire were identified: reach-shaping, reach-grasp sequence, bring-to-body, and hold-like movement. Overall, the forelimb motor region included: (1) a large caudal forelimb area dominated by reach-shaping movement representation; (2) a small rostral area containing reach-grasp sequence and bring-to-body movement representation; and (3) a more lateral portion where hold-like movement was represented. These results support the view that, in rats, the motor cortex controls forelimb movements at a relatively complex level and suggest that the orderly representation of complex movements and their dynamics/kinematics emerge from the principles of forelimb motor cortex organization.

Comparison of Functional Recovery of Manual Dexterity after Unilateral Spinal Cord Lesion or Motor Cortex Lesion in Adult Macaque Monkeys

In relation to mechanisms involved in functional recovery of manual dexterity from cervical cord injury or from motor cortical injury, our goal was to determine whether the movements that characterize post-lesion functional recovery are comparable to original movement patterns or do monkeys adopt distinct strategies to compensate the deficits depending on the type of lesion? To this aim, data derived from earlier studies, using a skilled finger task (the modified Brinkman board from which pellets are retrieved from vertical or horizontal slots), in spinal cord and motor cortex injured monkeys were analyzed and compared. Twelve adult macaque monkeys were subjected to a hemi-section of the cervical cord (n = 6) or to a unilateral excitotoxic lesion of the hand representation in the primary motor cortex (n = 6). In addition, in each subgroup, one half of monkeys (n = 3) were treated for 30 days with a function blocking antibody against the neurite growth inhibitory protein Nogo-A, while the other half (n = 3) represented control animals. The motor deficits, and the extent and time course of functional recovery were assessed. For some of the parameters investigated (wrist angle for horizontal slots and movement types distribution for vertical slots after cervical injury; movement types distribution for horizontal slots after motor cortex lesion), post-lesion restoration of the original movement patterns (“true” recovery) led to a quantitatively better functional recovery. In the motor cortex lesion groups, pharmacological reversible inactivation experiments showed that the peri-lesion territory of the primary motor cortex or re-arranged, spared domain of the lesion zone, played a major role in the functional recovery, together with the ipsilesional intact premotor cortex.

Distal forelimb representations in primary motor cortex are redistributed after forelimb restriction: a longitudinal study in adult squirrel monkeys

Primary motor cortex (M1) movement representations reflect acquired motor skills. Representations of muscles and joints used in a skilled task expand. However, it is unknown whether motor restriction in healthy individuals results in complementary reductions in M1 representations. With the use of intracortical microstimulation techniques in squirrel monkeys, detailed maps of movement representations in M1 were derived before and up to 35 wk after restriction of the preferred distal forelimb (DFL) by use of a soft cast. Although total DFL area and movement threshold remained constant, casting resulted in a redistribution of digit and wrist/forearm representations. Digit representations progressively decreased, whereas wrist/forearm representations progressively increased in areal extent. In three of four monkeys, hand preference returned to normal by the end of the postcast recovery period, and postrecovery maps demonstrated reversal of restriction-induced changes. However, in one monkey, a chronic motor impairment occurred in the casted limb. Rehabilitation via a forced-use paradigm resulted in recovery in use and skill of the impaired limb, as well as restoration of normal motor maps. These results demonstrate that plasticity in motor representations can be induced by training or restricting movements of the limb. Physiological changes induced by restriction appear to be reversible, even in the case of adverse motor outcomes. The respective contributions of both disuse and lost motor skills are discussed. These results have relevance for clinical conditions requiring forelimb casting as well as interpreting the differential effects of injury and disuse that are necessarily intertwined after cortical injury, as occurs in stroke.

Effect of transcranial magnetic stimulation protocol on recruitment curve parameters

Introduction: The construction of transcranial magnetic stimulation (TMS) recruitment curves (RCs) involves stimulation over the target muscle representation in the primary motor cortex, with stimulus intensities beginning below an individual's active motor threshold (AMT), and increasing in even increments until a plateau is observed in the amplitude of the motor evoked potential [MEP]. A sigmoidal equation is then fitted to the data, which provides four key parameters including minimum, slope, stimulus intensity at which MEP amplitude is 50% of maximum (V50), and plateau value (maximum).

Motor Recovery after Spinal Cord Injury Enhanced by Strengthening Corticospinal Synaptic Transmission

The corticospinal tract is an important target for motor recovery after spinal cord injury (SCI) in animals and humans. Voluntary motor output depends on the efficacy of synapses between corticospinal axons and spinal motoneurons, which can be modulated by the precise timing of neuronal spikes. Using noninvasive techniques, we developed tailored protocols for precise timing of the arrival of descending and peripheral volleys at corticospinal-motoneuronal synapses of an intrinsic finger muscle in humans with chronic incomplete SCI. We found that arrival of presynaptic volleys prior to motoneuron discharge enhanced corticospinal transmission and hand voluntary motor output. The reverse order of volley arrival and sham stimulation did not affect or decreased voluntary motor output and electrophysiological outcomes. These findings are the first demonstration that spike timing-dependent plasticity of residual corticospinal-motoneuronal synapses provides a mechanism to improve motor function after SCI. Modulation of residual corticospinal-motoneuronal synapses may present a novel therapeutic target for enhancing voluntary motor output in motor disorders affecting the corticospinal tract.

Movement representation in the primary motor cortex and its contribution to generalizable EMG predictions

It is well known that discharge of neurons in the primary motor cortex (M1) depends on end-point force and limb posture. However, the details of these relations remain unresolved. With the development of brain-machine interfaces (BMIs), these issues have taken on practical as well as theoretical importance. We examined how the M1 encodes movement by comparing single-neuron and electromyographic (EMG) preferred directions (PDs) and by predicting force and EMGs from multiple neurons recorded during an isometric wrist task. Monkeys moved a cursor from a central target to one of eight peripheral targets by exerting force about the wrist while the forearm was held in one of two postures. We fit tuning curves to both EMG and M1 activity measured during the hold period, from which we computed both PDs and the change in PD between forearm postures (ΔPD). We found a unimodal distribution of these ΔPDs, the majority of which were intermediate between the typical muscle response and an unchanging, extrinsic coordinate system. We also discovered that while most neuron-to-EMG predictions generalized well across forearm postures, end-point force measured in extrinsic coordinates did not. The lack of force generalization was due to musculoskeletal changes with posture. Our results show that the dynamics of most of the recorded M1 signals are similar to those of muscle activity and imply that a BMI designed to drive an actuator with dynamics like those of muscles might be more robust and easier to learn than a BMI that commands forces or movements in external coordinates.

Mapping the hand, foot and face representations in the primary motor cortex — Retest reliability of neuronavigated TMS versus functional MRI

Functional magnetic resonance imaging (fMRI) is a frequently used non-invasive mapping technique for investigating the human motor system. Recently, neuronavigated transcranial magnetic stimulation (nTMS) has been established as an alternative approach. We here compared the test-retest reliability of both mapping techniques with regard to the cortical representations of the hand, leg, face and tongue areas. Ten healthy subjects were examined three times (intervals: 3-5days/21-35days) with fMRI and nTMS. Motor-evoked potentials were recorded from the abductor pollicis brevis, plantaris, mentalis and the tongue muscles. The same muscles were activated in an fMRI motor task. Euclidean distances (ED) between hotspots and centers of gravity (CoG) were computed for the respective somatotopic representations. Furthermore, spatial reliability was tested by intersession overlap volumes (OV) and voxel-wise intraclass correlations (ICC). Feasibility of fMRI was 100% for all body parts and sessions. In contrast, nTMS was feasible in all sessions and subjects only for the hand area, while mappings of the foot (90%), face (70%) and tongue representations (40%) remained incomplete in several subjects due to technical constraints and co-stimulation artifacts. On average, the mean ED of the hotspots was better for fMRI (6.2±1.1mm) compared to nTMS (10.8±1.9mm) while stability of CoG was similar for both methods. Peak voxel reliability (ICC) was high for both methods (>0.8), and there was no influence of inter-session intervals. In contrast, the reliability of mapping the spatial extent of the hand, foot, lips and tongue representations was poor to moderate for both fMRI and nTMS (OVs and ICC<50%). Especially nTMS mappings of the face and tongue areas yielded poor reliability estimates. Both methods are highly reliable when mapping the core region of a given target muscle, especially for the hand representation area. In contrast, mapping the spatial extent of a cortical representation area was only little reliable for both nTMS and fMRI. In summary, fMRI was better suited when mapping motor representations of the head, while nTMS showed equal reliability for mapping the hand and foot representation areas. Hence, both methods may well complement each other.

Similarities in the Neural Control of the Shoulder and Elbow Joints Belie Their Structural Differences

Movement of the hand in three dimensional space is primarily controlled by the orientation of the shoulder and elbow complexes. Due to discrepancies in proprioceptive acuity, overlap in motor cortex representation and grossly different anatomies between these joints, we hypothesized that there would be differences in the accuracy of aimed movements between the two joints. Fifteen healthy young adults were tested under four conditions – shoulder motion with the elbow constrained and unconstrained, and elbow motion with the shoulder constrained and unconstrained. End point target locations for each joint were set to coincide with joint excursions of 10, 20 or 30 degrees of either the shoulder or elbow joint. Targets were presented in a virtual reality environment. For the constrained condition, there were no significant differences in angular errors between the two joints, suggesting that the central nervous system represents linked segment models of the limb in planning and controlling movements. For the unconstrained condition, although angle errors were higher, hand position errors remained the same as those of the constrained trials. These results support the idea that the CNS utilizes abundant degrees of freedom to compensate for the potentially different contributions to end-point errors introduced by each joint.

Into the Island: A new technique of non-invasive cortical stimulation of the insula

We describe a new neuronavigation-guided technique to target the posterior-superior insula (PSI) using a cooled-double-cone coil for deep cortical stimulation. Despite the analgesic effects brought about by repetitive transcranial magnetic stimulation (TMS) to the primary motor and prefrontal cortices, a significant proportion of patients remain symptomatic. This encouraged the search for new targets that may provide stronger pain relief. There is growing evidence that the posterior insula is implicated in the integration of painful stimuli in different pain syndromes and in homeostatic thermal integration. The primary motor cortex representation of the lower leg was used to calculate the motor threshold and thus, estimate the intensity of PSI stimulation. Seven healthy volunteers were stimulated at 10 Hz to the right PSI and showed subjective changes in cold perception. The technique was safe and well tolerated. The right posterior-superior insula is worth being considered in future studies as a possible target for rTMS stimulation in chronic pain patients.

Corticomuscular coupling in human locomotion: muscle drive or gait control?

An intact gait is one of the most important and commonly used human motor functions. It is an automated repetitive motor pattern that does not require constant conscious control. Therefore its generation has been attributed mainly to spinal and subcortical regions of the central nervous system. An involvement of the motor cortex in gait control has only recently emerged mainly from animal studies (see Petersen et al. 2012). However, the bipedal upright gait of the human is unique. The electroencephalogram (EEG) allows non-invasive measurement of synchronous rhythmic activity in populations of cortical neurons in man. By simultaneously recording the peripheral muscle activity (electromyogram (EMG)) during movements, it is possible to look for cortical rhythmic activity that is correlated (coherent) with the peripheral muscle activity and thus plays a role in cortical control of movements. Such corticomuscular coherence analysis is widely applied in studies of the human cortical motor system (e.g. Witham et al. 2011), but has not been applied during gait, largely due to movement and muscle artefacts in EEG recordings during walking. In a recent issue of The Journal of Physiology, Petersen et al. (2012) show that such EEG–EMG coupling studies are possible in subjects walking on a treadmill if advanced processing techniques are used for artefact removal. They demonstrate beta-band (13–40 Hz) coherence between artefact-cleansed EEG and EMG during late swing and early stance phase and show a predominantly corticomuscular flow of information in this frequency band. This is the first direct evidence of a cortical involvement in muscle activation during human gait. Their findings suggest that the complex cyclic leg motor behaviour during gait is under cortical control at least intermittently. However, as mentioned by the authors, it is not possible to assess the importance of this cortical drive for gait control. EEG–EMG coherence in the beta-band is only shown in the phase of the gait cycle when the anterior tibial muscle is activated isometrically before heel strike. This coherence was even stronger during isometric contractions performed independently of gait, as has been shown for many upper and lower limb muscles previously (e.g. Witham et al. 2011). Its function is still under debate. The coherence disappears in dynamic isotonic movements and there is not only cortico-muscular interaction but also feedback from muscle to cortex in this frequency band. Thus it has been argued that the beta-band coherence may fulfil other functions, such as calibrating and stabilizing the corticospinal system after dynamic movements (Witham et al. 2011). This may also be the role of the EEG-coherent EMG-activity in the late swing phase and its direct relation to gait itself may be questioned. How could the EEG and EMG data be used to search for cortical activity related to the repetitive sequence of movements during gait? As illustrated in Fig. 1C of the paper by Petersen et al., the repetitive activation of the anterior tibial muscle can be viewed as rhythmic bursts, reflecting the individual cadence (rhythm) of gait and so if the cortex is directly involved in controlling gait, this rhythm should be represented cortically. Thus there should be EEG activity that is coherent with the lower frequency of these muscle bursts. For other rhythmic motor behaviours of the hand and also the feet, this approach revealed similar cortical representations in the primary motor cortex of the studied limb (Raethjen et al. 2008) and advanced methods for coherent source analysis have even shown that the subcortical parts of the central motor network are involved (Pollok et al. 2004). EMG-coherence of subcortical parts of the motor network has not been unambiguously shown for the higher frequency (e.g. 15–30 Hz) band for which the representation is mainly cortical (Schoffelen et al. 2008; Muthuraman et al. 2012). Figure 1 Hypothesized dual corticomuscular interaction during gait Taking the novel results of Petersen et al. and these considerations together, one may speculate that there is a dual involvement of the cortex in the oscillatory control of repetitive automated movements such as gait: an intermittent localized drive at 15–30 Hz during each short isometric muscle contraction (burst), and an additional ongoing activity controlling the movement rhythm and involving large parts of the central motor network (see Fig. 1). Whereas the former is shown by Petersen et al. (2012), further work is needed to observe the latter and a possible connection between the two. Comparisons between different upper and lower limb repetitive motor patterns may also reveal further gait-specific cortical network components, e.g. cortical locomotor regions of man.

Double representation of the wrist and elbow in human motor cortex

Movements of the fingers, hand and arm involve overlapping neural representations in primary motor cortex (M1). Monkey M1 exhibits a core-surround organisation in which cortical representation of the hand and fingers is surrounded by representations of the wrist, elbow and shoulder. A potentially homologous organisation in human M1 has only been observed in a single study, a functional MRI (fMRI) study by [J.D. Meier, T.N. Aflalo, S. Kastner & M.S. Graziano.(2008) J Neurophysiol, 100(4), 1800-1812]. The results of their study suggested a double representation of the wrist in human M1, an unprecedented finding. Our purpose was to document and simultaneously provide evidence that would extend the presence of double representation of the wrist to that of the elbow. Using fMRI, we observed somatotopic maps in M1 and the supplementary motor area (SMA), the only other cortical area that showed robust within-limb somatotopy during self-timed finger, wrist and elbow movements. We observed double wrist and elbow representation that bracketed finger fMRI responses in M1 and the SMA. Our results show that the cortical locations of these double representations are well predicted by local cortical anatomy. Double representation of the wrist and elbow is important because it violates the traditional somatotopic progression in M1 but it is consistent with the representation of synergistic movements involving adjacent effectors.

Suppression of premotor cortex disrupts motor coding of peripersonal space

Peripersonal space (PPS) representation depends on the activity of a fronto-parietal network including the premotor cortex (PMc) and the posterior parietal cortex (PPc). PPS representation has a direct effect on the motor system: a stimulus activating the PPS around the hand modulates the excitability of hand representation in the primary motor cortex. However, to date, direct information about the involvement of the PMc-PPc network in the motor mapping of sensory events occurring within PPS is lacking. To address this issue, we used a 'perturb-and-measure' paradigm based on the combination of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) techniques. Cathodal tDCS was applied to transiently suppress neural activity in PMc, PPc and primary visual cortex (V1; serving as an active control site); single-pulse TMS was used to induce motor-evoked potentials (MEPs) from hand muscles and so to measure the excitability of the hand motor representation. MEPs were compared when a sound was presented either near the hand or at a distance. In experimental sessions performed after sham-tDCS and after tDCS over the control area V1, we found a spatially dependent modulation of the hand motor representation: sounds presented near the hand induced an inhibitory motor response as compared to sounds presented far apart. Critically, this effect was selectively abolished after tDCS suppression of neural activity in PMc, but not when perturbing the activity of PPc. These findings suggest that PMc has a critical role in mapping sensory representations of space onto the motor system.