Orderly somatotopy in primary motor cortex: does it exist?

A mosaic of whole-body representations in human motor cortex

Understanding how the body is represented in motor cortex is key to understanding how the brain controls movement. The precentral gyrus (PCG) has long been thought to contain largely distinct regions for the arm, leg and face (represented by the "motor homunculus"). However, mounting evidence has begun to reveal a more intermixed, interrelated and broadly tuned motor map. Here, we revisit the motor homunculus using microelectrode array recordings from 20 arrays that broadly sample PCG across 8 individuals, creating a comprehensive map of human motor cortex at single neuron resolution. We found whole-body representations throughout all sampled points of PCG, contradicting traditional leg/arm/face boundaries. We also found two speech-preferential areas with a broadly tuned, orofacial-dominant area in between them, previously unaccounted for by the homunculus. Throughout PCG, movement representations of the four limbs were interlinked, with homologous movements of different limbs (e.g., toe curl and hand close) having correlated representations. Our findings indicate that, while the classic homunculus aligns with each area's preferred body region at a coarse level, at a finer scale, PCG may be better described as a mosaic of functional zones, each with its own whole-body representation.

The dorsal stream of visual processing and action-specific domains in parietal and frontal cortex in primates

This review summarizes our findings obtained from over 15 years of research on parietal-frontal networks involved in the dorsal stream of cortical processing. We have presented considerable evidence for the existence of similar, partially independent, parietal-frontal networks involved in specific motor actions in a number of primates. These networks are formed by connections between action-specific domains representing the same complex movement evoked by electrical microstimulation. Functionally matched domains in the posterior parietal (PPC) and frontal (M1-PMC) motor regions are hierarchically related. M1 seems to be a critical link in these networks, since the outputs of M1 are essential to the evoked behavior, whereas PPC and PMC mediate complex movements mostly via their connections with M1. Thus, lesioning or deactivating M1 domains selectively blocks matching PMC and PPC domains, while having limited impact on other domains. When pairs of domains are stimulated together, domains within the same parietal-frontal network (matching domains) are cooperative in evoking movements, while they are mainly competitive with other domains (mismatched domains) within the same set of cortical areas. We propose that the interaction of different functional domains in each cortical region (as well as in striatum) occurs mainly via mutual suppression. Thus, the domains at each level are in competition with each other for mediating one of several possible behavioral outcomes.

Plasticity of face-hand sensorimotor circuits after a traumatic brachial plexus injury

Background

Interactions between the somatosensory and motor cortices are of fundamental importance for motor control. Although physically distant, face and hand representations are side by side in the sensorimotor cortex and interact functionally. Traumatic brachial plexus injury (TBPI) interferes with upper limb sensorimotor function, causes bilateral cortical reorganization, and is associated with chronic pain. Thus, TBPI may affect sensorimotor interactions between face and hand representations.

Objective

The aim of this study was to investigate changes in hand-hand and face-hand sensorimotor integration in TBPI patients using an afferent inhibition (AI) paradigm.

Method

The experimental design consisted of electrical stimulation (ES) applied to the hand or face followed by transcranial magnetic stimulation (TMS) to the primary motor cortex to activate a hand muscle representation. In the AI paradigm, the motor evoked potential (MEP) in a target muscle is significantly reduced when preceded by an ES at short-latency (SAI) or long-latency (LAI) interstimulus intervals. We tested 18 healthy adults (control group, CG), evaluated on the dominant upper limb, and nine TBPI patients, evaluated on the injured or the uninjured limb. A detailed clinical evaluation complemented the physiological investigation.

Results

Although hand-hand SAI was present in both the CG and the TBPI groups, hand-hand LAI was present in the CG only. Moreover, less AI was observed in TBPI patients than the CG both for face-hand SAI and LAI.

Conclusion

Our results indicate that sensorimotor integration involving both hand and face sensorimotor representations is affected by TBPI.

The evidence against somatotopic organization of function in the primate corticospinal tract

We review the spatial organization of corticospinal outputs from different cortical areas and how this reflects the varied functions mediated by the corticospinal tract. A long-standing question is whether the primate corticospinal tract shows somatotopical organization. Although this has been clearly demonstrated for corticofugal outputs passing through the internal capsule and cerebral peduncle, there is accumulating evidence against somatotopy in the pyramidal tract in the lower brainstem and in the spinal course of the corticospinal tract. Answering the question on somatotopy has important consequences for understanding the effects of incomplete spinal cord injury. Our recent study in the macaque monkey, using high-resolution dextran tracers, demonstrated a great deal of intermingling of fibres originating from primary motor cortex arm/hand, shoulder and leg areas. We quantified the distribution of fibres belonging to these different projections and found no significant difference in their distribution across different subsectors of the pyramidal tract or lateral corticospinal tract, arguing against somatotopy. We further demonstrated intermingling with corticospinal outputs derived from premotor and supplementary motor arm areas. We present new evidence against somatotopy for corticospinal projections from rostral and caudal cingulate motor areas and from somatosensory areas of the parietal cortex. In the pyramidal tract and lateral corticospinal tract, fibres from the cingulate motor areas overlap with each other. Fibres from the primary somatosensory cortex arm area completely overlap those from the leg area. There is also substantial overlap of both these outputs with those from posterior parietal sensorimotor areas. We argue that the extensive intermingling of corticospinal outputs from so many different cortical regions must represent an organizational principle, closely related to its mediation of many different functions and its large range of fibre diameters. The motor sequelae of incomplete spinal injury, such as central cord syndrome and 'cruciate paralysis', include much greater deficits in upper than in lower limb movement. Current teaching and text book explanations of these symptoms are still based on a supposed corticospinal somatotopy or 'lamination', with greater vulnerability of arm and hand versus leg fibres. We suggest that such explanations should now be finally abandoned. Instead, the clinical and neurobiological implications of the complex organization of the corticospinal tract need now to be taken into consideration. This leads us to consider the evidence for a greater relative influence of the corticospinal tract on upper versus lower limb movements, the former best characterized by skilled hand and digit movements.

Biomechanics, motor control and dynamic models of the soft limbs of the octopus and other cephalopods

Muscular hydrostats are organs composed entirely of packed arrays of incompressible muscles and lacking any skeletal support. Found in both vertebrates and invertebrates, they are of great interest for comparative biomechanics from engineering and evolutionary perspectives. The arms of cephalopods (e.g. octopus and squid) are particularly interesting muscular hydrostats because of their flexibility and ability to generate complex behaviors exploiting elaborate nervous systems. Several lines of evidence from octopus studies point to the use of both brain and arm-embedded motor control strategies that have evolved to simplify the complexities associated with the control of flexible and hyper-redundant limbs and bodies. Here, we review earlier and more recent experimental studies on octopus arm biomechanics and neural motor control. We review several dynamic models used to predict the kinematic characteristics of several basic motion primitives, noting the shortcomings of the current models in accounting for behavioral observations. We also discuss the significance of impedance (stiffness and viscosity) in controlling the octopus's motor behavior. These factors are considered in light of several new models of muscle biomechanics that could be used in future research to gain a better understanding of motor control in the octopus. There is also a need for updated models that encompass stiffness and viscosity for designing and controlling soft robotic arms. The field of soft robotics has boomed over the past 15 years and would benefit significantly from further progress in biomechanical and motor control studies on octopus and other muscular hydrostats.

Motor cortex outputs evoked by long-duration microstimulation encode synergistic muscle activation patterns not controlled movement trajectories

The effects of intracortical microstimulation (ICMS) parameters on the evoked electromyographic (EMG) responses and resulting limb movement were investigated. In ketamine-anesthetized cats, paw movement kinematics in 3D and EMG activity from 8 to 12 forelimb muscles evoked by ICMS applied to the forelimb area of the cat motor cortex (MCx) were recorded. The EMG responses evoked by ICMS were also compared to those evoked by focal ictal bursts induced by the iontophoretic ejection of the GABA_A receptor antagonist bicuculline methochloride (BIC) at the same cortical point. The effects of different initial limb starting positions on movement trajectories resulting from long-duration ICMS were also studied. The ICMS duration did not affect the evoked muscle activation pattern (MAP). Short (50 ms) and long (500 ms) stimulus trains activated the same muscles in the same proportions. MAPs could, however, be modified by gradually increasing the stimulus intensity. MAPs evoked by focal ictal bursts were also highly correlated with those obtained by ICMS at the same cortical point. Varying the initial position of the forelimb did not change the MAPs evoked from a cortical point. Consequently, the evoked movements reached nearly the same final end point and posture, with variability. However, the movement trajectories were quite different depending on the initial limb configuration and starting position of the paw. The evoked movement trajectory was most natural when the forelimb lay pendant ~ perpendicular to the ground (i.e., in equilibrium with the gravitational force). From other starting positions, the movements did not appear natural. These observations demonstrate that while the output of the cortical point evokes a seemingly coordinated limb movement from a rest position, it does not specify a particular movement direction or a controlled trajectory from other initial positions.

Moving in on human motor cortex. Characterizing the relationship between body parts with non-rigid population response fields

For cortical motor activity, the relationships between different body part representations is unknown. Through reciprocal body part relationships, functionality of cortical motor areas with respect to whole body motor control can be characterized. In the current study, we investigate the relationship between body part representations within individual neuronal populations in motor cortices, following a 7 Tesla fMRI 18-body-part motor experiment in combination with our newly developed non-rigid population Response Field (pRF) model and graph theory. The non-rigid pRF metrics reveal somatotopic structures in all included motor cortices covering frontal, parietal, medial and insular cortices and that neuronal populations in primary sensorimotor cortex respond to fewer body parts than secondary motor cortices. Reciprocal body part relationships are estimated in terms of uniqueness, clique-formation, and influence. We report unique response profiles for the knee, a clique of body parts surrounding the ring finger, and a central role for the shoulder and wrist. These results reveal associations among body parts from the perspective of the central nervous system, while being in agreement with intuitive notions of body part usage.

Corticospinal excitability and somatosensory information processing of the lower limb muscle during upper limb voluntary or electrically induced muscle contractions

Neural interactions between upper and lower limbs underlie motor coordination in humans. Specifically, upper limb voluntary muscle contraction can facilitate spinal and corticospinal excitability of the lower limb muscles. However, little remains known on the involvement of somatosensory information in arm-leg neural interactions. Here, we investigated effects of voluntary and electrically induced wrist flexion on corticospinal excitability and somatosensory information processing of the lower limbs. In Experiment 1, we measured transcranial magnetic stimulation (TMS)-evoked motor evoked potentials (MEPs) of the resting soleus (SOL) muscle at rest or during voluntary or neuromuscular electrical stimulation (NMES)-induced wrist flexion. The wrist flexion force was matched to 10% of the maximum voluntary contraction (MVC). We found that SOL MEPs were significantly increased during voluntary, but not NMES-induced, wrist flexion, compared to the rest (P < 0.001). In Experiment 2, we examined somatosensory evoked potentials (SEPs) following tibial nerve stimulation under the same conditions. The results showed that SEPs were unchanged during both voluntary and NMES-induced wrist flexion. In Experiment 3, we examined the modulation of SEPs during 10%, 20%, and 30% MVC voluntary wrist flexion. During 30% MVC voluntary wrist flexion, P50-N70 SEP component was significantly attenuated compared to the rest (P = 0.003). Our results propose that the somatosensory information generated by NMES-induced upper limb muscle contractions may have a limited effect on corticospinal excitability and somatosensory information processing of the lower limbs. However, voluntary wrist flexion modulated corticospinal excitability and somatosensory information processing of the lower limbs via motor areas.

Distributed yet compartmentalized neural dynamics of hand actions

In this issue of Neuron,Natraj et al. (2021) demonstrate that finger and hand grasping movements are represented in the human fronto-parietal grasp network in a compartmentalized fashion. The movements are encoded in a distributed network that is preserved across various hand actions. The neural dynamics are specific to particular hand movements, leading to movement-specific submanifolds in the network.

Unraveling somatotopic organization in the human brain using machine learning and adaptive supervoxel-based parcellations

In addition to the well-established somatotopy in the pre- and post-central gyrus, there is now strong evidence that somatotopic organization is evident across other regions in the sensorimotor network. This raises several experimental questions: To what extent is activity in the sensorimotor network effector-dependent and effector-independent? How important is the sensorimotor cortex when predicting the motor effector? Is there redundancy in the distributed somatotopically organized network such that removing one region has little impact on classification accuracy? To answer these questions, we developed a novel experimental approach. fMRI data were collected while human subjects performed a precisely controlled force generation task separately with their hand, foot, and mouth. We used a simple linear iterative clustering (SLIC) algorithm to segment whole-brain beta coefficient maps to build an adaptive brain parcellation and then classified effectors using extreme gradient boosting (XGBoost) based on parcellations at various spatial resolutions. This allowed us to understand how data-driven adaptive brain parcellation granularity altered classification accuracy. Results revealed effector-dependent activity in regions of the post-central gyrus, precentral gyrus, and paracentral lobule. SMA, regions of the inferior and superior parietal lobule, and cerebellum each contained effector-dependent and effector-independent representations. Machine learning analyses showed that increasing the spatial resolution of the data-driven model increased classification accuracy, which reached 94% with 1755 supervoxels. Our SLIC-based supervoxel parcellation outperformed classification analyses using established brain templates and random simulations. Occlusion experiments further demonstrated redundancy across the sensorimotor network when classifying effectors. Our observations extend our understanding of effector-dependent and effector-independent organization within the human brain and provide new insight into the functional neuroanatomy required to predict the motor effector used in a motor control task.

The motor deficit of ALS reflects failure to generate muscle synergies for complex motor tasks, not just muscle strength

Customarily the motor deficits that develop in ALS are considered in terms of muscle weakness. Functional rating scales used to assess ALS in terms of functional decline do not measure the deficits when performing complex motor tasks, that make up the human skilled motor repertoire, best exemplified by tasks requiring skilled hand and finger movement. This repertoire depends primarily upon the strength of direct corticomotoneuronal (CM) connectivity from primary motor cortex to the motor units subserving skilled movements. Our review prompts the question: if accumulating evidence suggests involvement of the CM system in the early stages of ALS, what kinds of motor deficit might be expected to result, and is current methodology able to identify such deficits? We point out that the CM system is organized not in "commands" to individual muscles, but rather encodes the building blocks of complex and intricate movements, which depend upon synergy between not only the prime mover muscles, but other muscles that stabilize the limb during skilled movement. Our knowledge of the functional organization of the CM system has come both from invasive studies in non-human primates and from advanced imaging and neurophysiological techniques in humans, some of which are now being applied in ALS. CM pathology in ALS has consequences not only for muscle strength, but importantly in the failure to generate complex motor tasks, often involving elaborate muscle synergies. Our aim is to encourage innovative methodology specifically directed to assessing complex motor tasks, failure of which is likely a very early clinical deficit in ALS.

Simultaneous Recording of Motor Evoked Potentials in Hand, Wrist and Arm Muscles to Assess Corticospinal Divergence

The purpose of this study was to further develop methods to assess corticospinal divergence and muscle coupling using transcranial magnetic stimulation (TMS). Ten healthy right-handed adults participated (7 females, age 34.0 ± 12.9 years). Monophasic single pulses were delivered to 14 sites over the right primary motor cortex at 40, 60, 80 and 100% of maximum stimulator output (MSO), using MRI-based neuronavigation. Motor evoked potentials (MEPs) were recorded simultaneously from 9 muscles of the contralateral hand, wrist and arm. For each intensity, corticospinal divergence was quantified by the average number of muscles that responded to TMS per cortical site, coactivation across muscle pairs as reflected by overlap of cortical representations, and correlation of MEP amplitudes across muscle pairs. TMS to each muscle's most responsive site elicited submaximal MEPs in most other muscles. The number of responsive muscles per cortical site and the extent of coactivation increased with increasing intensity (ANOVA, p < 0.001). In contrast, correlations of MEP amplitudes did not differ across the 60, 80 and 100% MSO intensities (ANOVA, p = 0.34), but did differ across muscle pairs (ANOVA, p < 0.001). Post hoc analysis identified 4 sets of muscle pairs (Tukey homogenous subsets, p < 0.05). Correlations were highest for pairs involving two hand muscles and lowest for pairs that included an upper arm muscle. Correlation of MEP amplitudes may quantify varying levels of muscle coupling. In future studies, this approach may be a biomarker to reveal altered coupling induced by neural injury, neural repair and/or motor learning.

Functional organization of the human primary somatosensory cortex: A stereo-electroencephalography study

OBJECTIVE

The classical homunculus of the human primary somatosensory cortex (S1) established by Penfield has mainly portrayed the functional organization of convexial cortex, namely Brodmann area (BA) 1. However, little is known about the functions in fissural cortex including BA2 and BA3. We aim at drawing a refined and detailed somatosensory homunculus of the entire S1.

METHODS

We recruited 20 patients with drug-resistant focal epilepsy who underwent stereo-electroencephalography for preoperative assessments. Direct electrical stimulation was performed for functional mapping. Montreal Neurological Institute coordinates of the stimulation sites lying in S1 were acquired.

RESULTS

Stimulation of 177 sites in S1 yielded 149 positive sites (84%), most of which were located in the sulcal cortex. The spatial distribution of different body-part representations across the S1 surface revealed that the gross medial-to-lateral sequence of body representations within the entire S1 was consistent with the classical "homunculus". And we identified several unreported body-part representations from the sulcal cortex, such as forehead, deep elbow and wrist joints, and some dorsal body regions.

CONCLUSIONS

Our results reveal general somatotopical characteristics of the entire S1 cortex and differences with the previous works of Penfield.

SIGNIFICANCE

The classical S1 homunculus was extended by providing further refinement and additional detail.

Precise finger somatotopy revealed by focal motor cortex injury

BACKGROUND

Somatotopy is considered the hallmark of the primary motor cortex. While this is fundamentally true for the major body parts (head, upper and lower extremities), evidence supporting the existence of within-limb somatotopy is scarce.

METHOD

We report a young man presenting recurrent ischemic strokes with selective finger weakness in whom serial motor cortex mapping procedures were performed.

RESULT

Following the first stroke, which largely spared the motor cortex, motor mapping displayed overlap of the motor representations of the hand muscles. The second focal stroke, affecting the lateral part of the hand knob, resulted in selective loss of the first dorsal interosseous muscle motor evoked potentials while sparing those of the adductor digiti minimi muscle. This observation is in apparent contradiction with the first mapping results that suggested complete overlap of motor representations.

DISCUSSION

Our mapping results provide evidence for the existence of very precise within-limb somatotopy and confirm the proposed homuncular order, whereby lateral fingers are represented laterally and medial fingers medially. The discrepancy between the initial and subsequent mapping results is discussed in light of functional organization of the primary motor cortex.

Assessment of Somatosensory Reorganization by Functional Magnetic Resonance Imaging After Hand Replantation

INTRODUCTION

Amputation of the hand is a rare and extremely intense trauma. Replanting and allografting after this type of injury require a major reorganization of the brain. Brain plasticity, though better known in the context of disorders of the central nervous system, is just as indispensable when the extremities are damaged.

MATERIALS AND METHODS

A 17-year-old patient underwent replantation of the nondominant hand after transmetaphyseal amputation after traumatic injury. After 18 days in hospital and subsequent treatment in a physical rehabilitation center, the patient attended clinical and radiology follow-up sessions over the next 2 years.

RESULTS

The management of this patient led to an excellent functional outcome in conjunction with successful social and professional reintegration. Electromyography at 18 months confirmed nerve regrowth. Functional magnetic resonance imaging was done at 2 years to evaluate cerebral plasticity. Motor function, largely dependent on the primary motor area, is aided by the addition of secondary and accessory motor areas for both simple and complex movements. A change in sensory information is stimulation in its own right hemisphere and increases solicitation of the contralateral precentral and postcentral gyrus.

CONCLUSIONS

There seems to be a real reversible dynamic plasticity under the balance of inhibitory and excitatory influences exerted on the cortical neurons. Any disruption of this balance requires the brain to adapt to the new circumstances to reestablish the hand as a functioning part of the body.

How does human motor cortex regulate vocal pitch in singers?

Vocal pitch is used as an important communicative device by humans, as found in the melodic dimension of both speech and song. Vocal pitch is determined by the degree of tension in the vocal folds of the larynx, which itself is influenced by complex and nonlinear interactions among the laryngeal muscles. The relationship between these muscles and vocal pitch has been described by a mathematical model in the form of a set of 'control rules'. We searched for the biological implementation of these control rules in the larynx motor cortex of the human brain. We scanned choral singers with functional magnetic resonance imaging as they produced discrete pitches at four different levels across their vocal range. While the locations of the larynx motor activations varied across singers, the activation peaks for the four pitch levels were highly consistent within each individual singer. This result was corroborated using multi-voxel pattern analysis, which demonstrated an absence of patterned activations differentiating any pairing of pitch levels. The complex and nonlinear relationships between the multiple laryngeal muscles that control vocal pitch may obscure the neural encoding of vocal pitch in the brain.

Robust tactile sensory responses in finger area of primate motor cortex relevant to prosthetic control

OBJECTIVE

Challenges in improving the performance of dexterous upper-limb brain-machine interfaces (BMIs) have prompted renewed interest in quantifying the amount and type of sensory information naturally encoded in the primary motor cortex (M1). Previous single unit studies in monkeys showed M1 is responsive to tactile stimulation, as well as passive and active movement of the limbs. However, recent work in this area has focused primarily on proprioception. Here we examined instead how tactile somatosensation of the hand and fingers is represented in M1.

APPROACH

We recorded multi- and single units and thresholded neural activity from macaque M1 while gently brushing individual finger pads at 2 Hz. We also recorded broadband neural activity from electrocorticogram (ECoG) grids placed on human motor cortex, while applying the same tactile stimulus.

MAIN RESULTS

Units displaying significant differences in firing rates between individual fingers (p  <  0.05) represented up to 76.7% of sorted multiunits across four monkeys. After normalizing by the number of channels with significant motor finger responses, the percentage of electrodes with significant tactile responses was 74.9%  ±  24.7%. No somatotopic organization of finger preference was obvious across cortex, but many units exhibited cosine-like tuning across multiple digits. Sufficient sensory information was present in M1 to correctly decode stimulus position from multiunit activity above chance levels in all monkeys, and also from ECoG gamma power in two human subjects.

SIGNIFICANCE

These results provide some explanation for difficulties experienced by motor decoders in clinical trials of cortically controlled prosthetic hands, as well as the general problem of disentangling motor and sensory signals in primate motor cortex during dextrous tasks. Additionally, examination of unit tuning during tactile and proprioceptive inputs indicates cells are often tuned differently in different contexts, reinforcing the need for continued refinement of BMI training and decoding approaches to closed-loop BMI systems for dexterous grasping.

Detailed somatotopy in primary motor and somatosensory cortex revealed by Gaussian population receptive fields

The relevance of human primary motor cortex (M1) for motor actions has long been established. However, it is still unknown how motor actions are represented, and whether M1 contains an ordered somatotopy at the mesoscopic level. In the current study we show that a detailed within-limb somatotopy can be obtained in M1 during finger movements using Gaussian population Receptive Field (pRF) models. Similar organizations were also obtained for primary somatosensory cortex (S1), showing that individual finger representations are interconnected throughout sensorimotor cortex. The current study additionally estimates receptive field sizes of neuronal populations, showing differences between finger digit representations, between M1 and S1, and additionally between finger digit flexion and extension. Using the Gaussian pRF approach, the detailed somatotopic organization of M1 can be obtained including underlying characteristics, allowing for the in-depth investigation of cortical motor representation and sensorimotor integration.

Movement goals encoded within the cortex and muscle synergies to reduce redundancy pre and post-stroke. The relevance for gait rehabilitation and the prescription of walking-aids. A literature review and scholarly discussion

Current knowledge of neural and neuromuscular processes controlling gait and movement as well as an understanding of how these mechanisms change following stroke is an important basis for the development of effective rehabilitation interventions. To support the translation of findings from basic research into useful treatments in clinical practice, up-to-date neuroscience should be presented in forms accessible to all members of the multidisciplinary team. In this review we discuss aspects of cortical control of gait and movement, muscle synergies as a way of translating cortical commands into specific muscle activity and as an efficient means of reducing neural and musculoskeletal redundancy. We discuss how these mechanisms change following stroke, potential consequences for gait rehabilitation, and the prescription and use of walking-aids as well as areas requiring further research.

On neuromechanical approaches for the study of biological and robotic grasp and manipulation

Biological and robotic grasp and manipulation are undeniably similar at the level of mechanical task performance. However, their underlying fundamental biological vs. engineering mechanisms are, by definition, dramatically different and can even be antithetical. Even our approach to each is diametrically opposite: inductive science for the study of biological systems vs. engineering synthesis for the design and construction of robotic systems. The past 20 years have seen several conceptual advances in both fields and the quest to unify them. Chief among them is the reluctant recognition that their underlying fundamental mechanisms may actually share limited common ground, while exhibiting many fundamental differences. This recognition is particularly liberating because it allows us to resolve and move beyond multiple paradoxes and contradictions that arose from the initial reasonable assumption of a large common ground. Here, we begin by introducing the perspective of neuromechanics, which emphasizes that real-world behavior emerges from the intimate interactions among the physical structure of the system, the mechanical requirements of a task, the feasible neural control actions to produce it, and the ability of the neuromuscular system to adapt through interactions with the environment. This allows us to articulate a succinct overview of a few salient conceptual paradoxes and contradictions regarding under-determined vs. over-determined mechanics, under- vs. over-actuated control, prescribed vs. emergent function, learning vs. implementation vs. adaptation, prescriptive vs. descriptive synergies, and optimal vs. habitual performance. We conclude by presenting open questions and suggesting directions for future research. We hope this frank and open-minded assessment of the state-of-the-art will encourage and guide these communities to continue to interact and make progress in these important areas at the interface of neuromechanics, neuroscience, rehabilitation and robotics.

Novel imaging techniques to study the functional organization of the human brain

Despite more than a century of investigation into the cortical organization of motor function, the existence of motor somatotopy is still debated. We review functional magnetic resonance imaging (fMRI) studies examining motor somatotopy in the cerebral cortex. In spite of a substantial overlap of representations corresponding to different body parts, especially in non-primary motor cortices, geographic approaches are capable of revealing somatotopic ordering. From the iconic homunculus in the contralateral primary cortex to the subtleties of ipsilateral somatotopy and its relations with lateralization, we outline potential reasons for the lack of segregation between motor representations. Among these are the difficulties in distinguishing activity that arises from multiple muscular effectors, the need for flexible motor control and coordination of complex movements through functional integration and artefacts in fMRI. Methodological advances with regard to the optimization of experimental design and fMRI acquisition protocols as well as improvements in spatial registration of images and indices aiming at the quantification of the degree of segregation between different functional representations are inspected. Additionally, we give some hints as to how the functional organization of motor function might be related to various anatomical landmarks in brain morphometry.

Representation of individual forelimb muscles in primary motor cortex

Stimulus-triggered averaging (StTA) of forelimb muscle electromyographic (EMG) activity was used to investigate individual forelimb muscle representation within the primary motor cortex (M1) of rhesus macaques with the objective of determining the extent of intra-areal somatotopic organization. Two monkeys were trained to perform a reach-to-grasp task requiring multijoint coordination of the forelimb. EMG activity was simultaneously recorded from 24 forelimb muscles including 5 shoulder, 7 elbow, 5 wrist, 5 digit, and 2 intrinsic hand muscles. Microstimulation (15 µA at 15 Hz) was delivered throughout the movement task and individual stimuli were used as triggers for generating StTAs of EMG activity. StTAs were used to map the cortical representations of individual forelimb muscles. As reported previously (Park et al. 2001), cortical maps revealed a central core of distal muscle (wrist, digit, and intrinsic hand) representation surrounded by a horseshoe-shaped proximal (shoulder and elbow) muscle representation. In the present study, we found that shoulder and elbow flexor muscles were predominantly represented in the lateral branch of the horseshoe whereas extensors were predominantly represented in the medial branch. Distal muscles were represented within the core distal forelimb representation and showed extensive overlap. For the first time, we also show maps of inhibitory output from motor cortex, which follow many of the same organizational features as the maps of excitatory output.NEW & NOTEWORTHY While the orderly representation of major body parts along the precentral gyrus has been known for decades, questions have been raised about the possible existence of additional more detailed aspects of somatotopy. In this study, we have investigated this question with respect to muscles of the arm and show consistent features of within-arm (intra-areal) somatotopic organization. For the first time we also show maps of how inhibitory output from motor cortex is organized.

Unilateral nasal obstruction affects motor representation development within the face primary motor cortex in growing rats

Postnatal growth is influenced by genetic and environmental factors. Nasal obstruction during growth alters the electromyographic activity of orofacial muscles. The facial primary motor area represents muscles of the tongue and jaw, which are essential in regulating orofacial motor functions, including chewing and jaw opening. This study aimed to evaluate the effect of chronic unilateral nasal obstruction during growth on the motor representations within the face primary motor cortex (M1). Seventy-two 6-day-old male Wistar rats were randomly divided into control (n = 36) and experimental (n = 36) groups. Rats in the experimental group underwent unilateral nasal obstruction after cauterization of the external nostril at 8 days of age. Intracortical microstimulation (ICMS) mapping was performed when the rats were 5, 7, 9, and 11 wk old in control and experimental groups (n = 9 per group per time point). Repeated-measures multivariate ANOVA was used for intergroup and intragroup statistical comparisons. In the control and experimental groups, the total number of positive ICMS sites for the genioglossus and anterior digastric muscles was significantly higher at 5, 7, and 9 wk, but there was no significant difference between 9 and 11 wk of age. Moreover, the total number of positive ICMS sites was significantly smaller in the experimental group than in the control at each age. It is possible that nasal obstruction induced the initial changes in orofacial motor behavior in response to the altered respiratory pattern, which eventually contributed to face-M1 neuroplasticity.NEW & NOTEWORTHY Unilateral nasal obstruction in rats during growth periods induced changes in arterial oxygen saturation (SpO₂) and altered development of the motor representation within the face primary cortex. Unilateral nasal obstruction occurring during growth periods may greatly affect not only respiratory function but also craniofacial function in rats. Nasal obstruction should be treated as soon as possible to avoid adverse effects on normal growth, development, and physiological functions.

Preparation and execution of teeth clenching and foot muscle contraction influence on corticospinal hand-muscle excitability

Contraction of a muscle modulates not only the corticospinal excitability (CSE) of the contracting muscle but also that of different muscles. We investigated to what extent the CSE of a hand muscle is modulated during preparation and execution of teeth clenching and ipsilateral foot dorsiflexion either separately or in combination. Hand-muscle CSE was estimated based on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and recorded from the first dorsal interosseous (FDI) muscle. We found higher excitability during both preparation and execution of all the motor tasks than during mere observation of a fixation cross. As expected, the excitability was greater during the execution phase than the preparation one. Furthermore, both execution and preparation of combined motor tasks led to higher excitability than individual tasks. These results extend our current understanding of the neural interactions underlying simultaneous contraction of muscles in different body parts.

Intracortical Brain-Machine Interfaces Advance Sensorimotor Neuroscience

Brain-machine interfaces (BMIs) decode brain activity to control external devices. Over the past two decades, the BMI community has grown tremendously and reached some impressive milestones, including the first human clinical trials using chronically implanted intracortical electrodes. It has also contributed experimental paradigms and important findings to basic neuroscience. In this review, we discuss neuroscience achievements stemming from BMI research, specifically that based upon upper limb prosthetic control with intracortical microelectrodes. We will focus on three main areas: first, we discuss progress in neural coding of reaches in motor cortex, describing recent results linking high dimensional representations of cortical activity to muscle activation. Next, we describe recent findings on learning and plasticity in motor cortex on various time scales. Finally, we discuss how bidirectional BMIs have led to better understanding of somatosensation in and related to motor cortex.

Revealing humans' sensorimotor functions with electrical cortical stimulation

Direct electrical stimulation (DES) of the human brain has been used by neurosurgeons for almost a century. Although this procedure serves only clinical purposes, it generates data that have a great scientific interest. Had DES not been employed, our comprehension of the organization of the sensorimotor systems involved in movement execution, language production, the emergence of action intentionality or the subjective feeling of movement awareness would have been greatly undermined. This does not mean, of course, that DES is a gold standard devoid of limitations and that other approaches are not of primary importance, including electrophysiology, modelling, neuroimaging or psychophysics in patients and healthy subjects. Rather, this indicates that the contribution of DES cannot be restricted, in humans, to the ubiquitous concepts of homunculus and somatotopy. DES is a fundamental tool in our attempt to understand the human brain because it represents a unique method for mapping sensorimotor pathways and interfering with the functioning of localized neural populations during the performance of well-defined behavioural tasks.

Evolution of posterior parietal cortex and parietal-frontal networks for specific actions in primates

Posterior parietal cortex (PPC) is an extensive region of the human brain that develops relatively late and is proportionally large compared with that of monkeys and prosimian primates. Our ongoing comparative studies have led to several conclusions about the evolution of this posterior parietal region. In early placental mammals, PPC likely was a small multisensory region much like PPC of extant rodents and tree shrews. In early primates, PPC likely resembled that of prosimian galagos, in which caudal PPC (PPCc) is visual and rostral PPC (PPCr) has eight or more multisensory domains where electrical stimulation evokes different complex motor behaviors, including reaching, hand-to-mouth, looking, protecting the face or body, and grasping. These evoked behaviors depend on connections with functionally matched domains in premotor cortex (PMC) and motor cortex (M1). Domains in each region compete with each other, and a serial arrangement of domains allows different factors to influence motor outcomes successively. Similar arrangements of domains have been retained in New and Old World monkeys, and humans appear to have at least some of these domains. The great expansion and prolonged development of PPC in humans suggest the addition of functionally distinct territories. We propose that, across primates, PMC and M1 domains are second and third levels in a number of parallel, interacting networks for mediating and selecting one type of action over others.

[The motor organization of cerebral cortex and the role of the mirror neuron system. Clinical impact for rehabilitation]

The basic characteristics of Penfield homunculus (somatotopy and unique representation) have been questioned. The existence of a defined anatomo-functional organization within different segments of the same region is controversial. The presence of multiple motor representations in the primary motor area and in the parietal lobe interconnected by parieto-frontal circuits, which are widely overlapped, form a complex organization. Both features support the recovery of functions after brain injury. Regarding the movement organization, it is possible to yield a relevant impact through the understanding of actions and intentions of others, which is mediated by the activation of mirror-neuron systems. The implementation of cognitive functions (observation, image of the action and imitation) from the acute treatment phase allows the activation of motor representations without having to perform the action and it plays an important role in learning motor patterns.

Functional somatotopy revealed across multiple cortical regions using a model of complex motor task

The primary motor cortex (M1) possesses a functional somatotopic structure-representations of adjacent within-limb joints overlap to facilitate coordination while maintaining discrete centers for individuated movement. We examined whether similar organization exists across other sensorimotor cortices. Twenty-four right-handed healthy subjects underwent functional magnetic resonance imaging (fMRI) while tracking complex targets with flexion/extension at right finger, elbow and ankle separately. Activation related to each joint at false discovery rate of 0.005 served as its representation across multiple regions. Within each region, we identified the center of mass (COM) for each representation, and the overlap between the representations of within-limb (finger and elbow) and between-limb joints (finger and ankle). Somatosensory (S1) and premotor cortices (PMC) demonstrated greater distinction of COM and minimal overlap for within- and between-limb representations. In contrast, M1 and supplementary motor area (SMA) showed more integrative somatotopy with higher sharing for within-limb representations. Superior and inferior parietal lobule (SPL and IPL) possessed both types of structure. Some clusters exhibited extensive overlap of within- and between-limb representations, while others showed discrete COMs for within-limb representations. Our results help to infer hierarchy in motor control. Areas such as S1 may be associated with individuated movements, while M1 may be more integrative for coordinated motion; parietal associative regions may allow switch between both modes of control. Such hierarchy creates redundant opportunities to exploit in stroke rehabilitation. The use of complex rather than traditionally used simple movements was integral to illustrating comprehensive somatotopic structure; complex tasks can potentially help to understand cortical representation of skill and learning-related plasticity.

Reorganizing the intrinsic functional architecture of the human primary motor cortex during rest with non-invasive cortical stimulation

The primary motor cortex (M1) is the main effector structure implicated in the generation of voluntary movements and is directly involved in motor learning. The intrinsic horizontal neuronal connections of M1 exhibit short-term and long-term plasticity, which is a strong substrate for learning-related map reorganization. Transcranial direct current stimulation (tDCS) applied for few minutes over M1 has been shown to induce relatively long-lasting plastic alterations and to modulate motor performance. Here we test the hypothesis that the relatively long-lasting synaptic modification induced by tDCS over M1 results in the alteration of associations among populations of M1 neurons which may be reflected in changes of its functional architecture. fMRI resting-state datasets were acquired immediately before and after 10 minutes of tDCS during rest, with the anode/cathode placed over the left M1. For each functional dataset, grey-matter voxels belonging to Brodmann area 4 (BA4) were labelled and afterwards BA4 voxel-based synchronization matrices were calculated and thresholded to construct undirected graphs. Nodal network parameters which characterize the architecture of functional networks (connectivity degree, clustering coefficient and characteristic path-length) were computed, transformed to volume maps and compared before and after stimulation. At the dorsolateral-BA4 region cathodal tDCS boosted local connectedness, while anodal-tDCS enhanced long distance functional communication within M1. Additionally, the more efficient the functional architecture of M1 was at baseline, the more efficient the tDCS-induced functional modulations were. In summary, we show here that it is possible to non-invasively reorganize the intrinsic functional architecture of M1, and to image such alterations.

Increased occlusal vertical dimension induces cortical plasticity in the rat face primary motor cortex

Previous studies have demonstrated that functional plasticity in the primary motor cortex (M1) is related to motor-skill learning and changes in the environment. Increased occlusal vertical dimension (iOVD) may modulate mastication, such as in the masticatory cycle, and the firing properties of jaw-muscle spindles. However, little is known about the changes in motor representation within the face primary motor cortex (face-M1) after iOVD. The purpose of the present study was to determine the effect of iOVD on the face-M1 using intracortical microstimulation (ICMS). In an iOVD group, the maxillary molars were built-up by 2mm with acrylic. The electromyographic (EMG) activities from the left (LAD) and right (RAD) anterior digastric (AD), masseter and genioglossus (GG) muscles elicited by ICMS within the right face-M1 were recorded 1, 2 and 8 weeks after iOVD. IOVD was associated with a significant increase in the number of sites within the face-M1 from which ICMS evoked LAD and/or GG EMG activities, as well as a lateral shift in the center of gravity of the RAD and LAD muscles at 1 and 2 weeks, but not at 8 weeks. These findings suggest that a time-dependent neuroplastic change within the rat face-M1 occurs in association with iOVD. This may be related to the animal's ability to adapt to a change in the oral environment.

Cortical plasticity during motor learning and recovery after ischemic stroke

The motor system has the ability to adapt to environmental constraints and injury to itself. This adaptation is often referred to as a form of plasticity allowing for livelong acquisition of new movements and for recovery after stroke. We are not sure whether learning and recovery work via same or similar neural mechanisms. But, all these processes require widespread changes within the matrix of the brain. Here, basic mechanisms of these adaptations on the level of cortical circuitry and networks are reviewed. We focus on the motor cortices because their role in learning and recovery has been investigated more thoroughly than other brain regions.

A new perspective on the organization of an invertebrate brain

The concept of 'embodiment' and its implications for the evolution of cognitive capacities is emerging as a major issue in biology. Invertebrates have immensely diverse nervous structures and body plans, revealing the variety of solutions evolved by animals living successfully in all kinds of niches. Among invertebrates, the octopus is a special case because of its high cognitive abilities and a uniquely flexible body and manoeuvrable arms with virtually infinite degrees of freedom. Here we discuss how the octopus embodiment may be considered a 'key' to the development of its neural organisation and cognitive abilities.

Brain Hodotopy: From Esoteric Concept to Practical Surgical Applications

BACKGROUND:

The traditional neurosurgical approach to cerebral lesions is based on the classic view of a rigid brain organization in fixed “eloquent” areas. However, this method is brought into discussion by the conceptual and methodological advances in neurosciences that provide a more dynamic representation of the anatomo-functional distribution of the human central nervous system (CNS).

OBJECTIVE AND METHODS:

We review the relevant literature concerning the main features of the modern CNS representation and their implications in neurosurgical practice.

RESULTS:

The CNS is an integrated, wide, plastic network made up of cortical functional epicenters, “topic organization,” connected by both short-local and large-scale white matter fibers, ie, “hodological organization.” According to this model, called hodotopic, brain function results from parallel streams of information dynamically modulated within an interactive, multimodal, and widely distributed circuit. The application of this framework, which can be studied by combining preoperative, intraoperative, and postoperative mapping techniques, enables the neurosurgeon exploration of the individual anatomo-functional architecture, including neurocognitive and emotional aspects. Thus, it is possible to adapt the surgical approach specifically to each patient and to each lesion according to the individual organization. Several experiences demonstrate the possibility of removing regions traditionally considered inoperable without inducing permanent deficits and the potential use of these areas as a safe passage to deeper territories.

CONCLUSION:

We advocate the more systematic integration of a hodotopical view of the CNS to improve the surgical indications and planning for brain lesions, with the goal of optimizing both the extent of resection and functional outcome.

Neural mechanism of activity spread in the cat motor cortex and its relation to the intrinsic connectivity

Motor cortical points are linked by intrinsic horizontal connections having a recurrent network topology. However, it is not known whether neural activity can propagate over the area covered by these intrinsic connections and whether there are spatial anisotropies of synaptic strength, as opposed to synaptic density. Moreover, the mechanisms by which activity spreads have yet to be determined. To address these issues, an 8 × 8 microelectrode array was inserted in the forelimb area of the cat motor cortex (MCx). The centre of the array had a laser etched hole ∼500 μm in diameter. A microiontophoretic pipette, with a tip diameter of 2-3 μm, containing bicuculline methiodide (BIC) was inserted in the hole and driven to a depth of 1200-1400 μm from the cortical surface. BIC was ejected for ∼2min from the tip of the micropipette with positive direct current ranging between 20 and 40 nA in different experiments. This produced spontaneous nearly periodic bursts (0.2-1.0 Hz) of multi-unit activity in a radius of about 400 μm from the tip of the micropipette. The bursts of neural activity spread at a velocity of 0.11-0.24 ms⁻¹ (mean=0.14 mm ms⁻¹, SD=0.05)with decreasing amplitude.The area activated was on average 7.22 mm² (SD=0.91 mm²), or ∼92% of the area covered by the recording array. The mode of propagation was determined to occur by progressive recruitment of cortical territory, driven by a central locus of activity of some 400 μm in radius. Thus, activity did not propagate as a wave. Transection of the connections between the thalamus and MCx did not significantly alter the propagation velocity or the size of the recruited area, demonstrating that the bursts spread along the routes of intrinsic cortical connectivity. These experiments demonstrate that neural activity initiated within a small motor cortical locus (≤ 400 μm in radius) can recruit a relatively large neighbourhood in which a variety of muscles acting at several forelimb joints are represented. These results support the hypothesis that the MCx controls the forelimb musculature in an integrated and anticipatory manner based on a recurrent network topology

Decoding complete reach and grasp actions from local primary motor cortex populations

How the activity of populations of cortical neurons generates coordinated multijoint actions of the arm, wrist, and hand is poorly understood. This study combined multielectrode recording techniques with full arm motion capture to relate neural activity in primary motor cortex (M1) of macaques (Macaca mulatta) to arm, wrist, and hand postures during movement. We find that the firing rate of individual M1 neurons is typically modulated by the kinematics of multiple joints and that small, local ensembles of M1 neurons contain sufficient information to reconstruct 25 measured joint angles (representing an estimated 10 functionally independent degrees of freedom). Beyond showing that the spiking patterns of local M1 ensembles represent a rich set of naturalistic movements involving the entire upper limb, the results also suggest that achieving high-dimensional reach and grasp actions with neuroprosthetic devices may be possible using small intracortical arrays like those already being tested in human pilot clinical trials.

What do brain lesions tell us about theories of embodied semantics and the human mirror neuron system?

Recent work has been mixed with respect to the notion of embodied semantics, which suggests that processing linguistic stimuli referring to motor-related concepts recruits the same sensorimotor regions of cortex involved in the execution and observation of motor acts or the objects associated with those acts. In this study, we asked whether lesions to key sensorimotor regions would preferentially impact the comprehension of stimuli associated with the use of the hand, mouth or foot. Twenty-seven patients with left-hemisphere strokes and 10 age- and education-matched controls were presented with pictures and words representing objects and actions typically associated with the use of the hand, mouth, foot or no body part at all (i.e., neutral). Picture/sound pairs were presented simultaneously, and participants were required to press a space bar only when the item pairs matched (i.e., congruent trials). We conducted two different analyses: 1) we compared task performance of patients with and without lesions in several key areas previously implicated in the putative human mirror neuron system (i.e., Brodmann areas 4/6, 1/2/3, 21 and 44/45), and 2) we conducted Voxel-based Lesion-Symptom Mapping analyses (VLSM; Bates et al., 2003) to identify additional regions associated with the processing of effector-related versus neutral stimuli. Processing of effector-related stimuli was associated with several regions across the left hemisphere, and not solely with premotor/motor or somatosensory regions. We also did not find support for a somatotopically-organized distribution of effector-specific regions. We suggest that, rather than following the strict interpretation of homuncular somatotopy for embodied semantics, these findings support theories proposing the presence of a greater motor-language network which is associated with, but not restricted to, the network responsible for action execution and observation.

On the nature of the intrinsic connectivity of the cat motor cortex: evidence for a recurrent neural network topology

The details and functional significance of the intrinsic horizontal connections between neurons in the motor cortex (MCx) remain to be clarified. To further elucidate the nature of this intracortical connectivity pattern, experiments were done on the MCx of three cats. The anterograde tracer biocytin was ejected iontophoretically in layers II, III, and V. Some 30-50 neurons within a radius of approximately 250 microm were thus stained. The functional output of the motor cortical point at which biocytin was injected, and of the surrounding points, was identified by microstimulation and electromyographic recordings. The axonal arborizations of the stained neurons were traced under camera lucida. The axon collaterals were extensive, reaching distances of <or=7 mm from the injection site. More importantly, the axonal branches were studded all along their course with boutons. The vast majority of boutons formed synaptic contacts on the target cells as identified by electron microscopy. The majority of these boutons made asymmetric (type I, excitatory) synapses mainly on dendritic spines. The bouton density decreased approximately monotonically with distance from the center of the injection. Cluster analysis, lagged covariance analysis, and eigenvalue decomposition showed the bouton distribution map to be unimodal. Superposition of the synaptic bouton distribution map and the motor output map revealed that motor cortical neurons don't make point-to-point connections but rather bind together the representations of a variety of muscles within a large neighborhood. This recurrent-network type connectivity strongly supports the hypothesis that the MCx controls the musculature in an integrated manner.

Nature of motor control: perspectives and issues

Four perspectives on motor control provide the framework for developing a comprehensive theory of motor control in biological systems. The four perspectives, of decreasing orthodoxy, are distinguished by their sources of inspiration: neuroanatomy, robotics, self-organization, and ecological realities. Twelve major issues that commonly constrain (either explicitly or implicitly) the understanding of the control and coordination of movement are identified and evaluated within the framework of the four perspectives. The issues are as follows: (1) Is control strictly neural? (2) Is there a divide between planning and execution? (3) Does control entail a frequently involved knowledgeable executive? (4) Do analytical internal models mediate control? (5) Is anticipation necessarily model dependent? (6) Are movements preassembled? (7) Are the participating components context independent? (8) Is force transmission strictly myotendinous? (9) Is afference a matter of local linear signaling? (10) Is neural noise an impediment? (11) Do standard variables (of mechanics and physiology) suffice? (12) Is the organization of control hierarchical?