A quantitative study of the distribution of neurons projecting to the precentral motor cortex in the monkey (M. fascicularis)

Circuits in the motor cortex explain oscillatory responses to transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a popular method used to investigate brain function. Stimulation over the motor cortex evokes muscle contractions known as motor evoked potentials (MEPs) and also high-frequency volleys of electrical activity measured in the cervical spinal cord. The physiological mechanisms of these experimentally derived responses remain unclear, but it is thought that the connections between circuits of excitatory and inhibitory neurons play a vital role. Using a spiking neural network model of the motor cortex, we explained the generation of waves of activity, so called 'I-waves', following cortical stimulation. The model reproduces a number of experimentally known responses including direction of TMS, increased inhibition, and changes in strength. Using populations of thousands of neurons in a model of cortical circuitry we showed that the cortex generated transient oscillatory responses without any tuning, and that neuron parameters such as refractory period and delays influenced the pattern and timing of those oscillations. By comparing our network with simpler, previously proposed circuits, we explored the contributions of specific connections and found that recurrent inhibitory connections are vital in producing later waves that significantly impact the production of motor evoked potentials in downstream muscles (Thickbroom, 2011). This model builds on previous work to increase our understanding of how complex circuitry of the cortex is involved in the generation of I-waves.

Disruption of somatosensory cortex impairs motor learning and retention

This study tests for a function of the somatosensory cortex, that, in addition to its role in processing somatic afferent information, somatosensory cortex contributes both to motor learning and the stabilization of motor memory. Continuous theta-burst magnetic stimulation (cTBS) was applied, before force-field training to disrupt activity in either the primary somatosensory cortex, primary motor cortex, or a control zone over the occipital lobe. Tests for retention and relearning were conducted after a 24 h delay. Analysis of movement kinematic measures and force-channel trials found that cTBS to somatosensory cortex disrupted both learning and subsequent retention, whereas cTBS to motor cortex had little effect on learning but possibly impaired retention. Basic movement variables are unaffected by cTBS suggesting that the stimulation does not interfere with movement but instead disrupts changes in the cortex that are necessary for learning. In all experimental conditions, relearning in an abruptly introduced force field, which followed retention testing, showed extensive savings, which is consistent with previous work suggesting that more cognitive aspects of learning and retention are not dependent on either of the cortical zones under test. Taken together, the findings are consistent with the idea that motor learning is dependent on learning-related activity in the somatosensory cortex.NEW & NOTEWORTHY This study uses noninvasive transcranial magnetic stimulation to test the contribution of somatosensory and motor cortex to human motor learning and retention. Continuous theta-burst stimulation is applied before learning; participants return 24 h later to assess retention. Disruption of the somatosensory cortex is found to impair both learning and retention, whereas disruption of the motor cortex has no effect on learning. The findings are consistent with the idea that motor learning is dependent upon learning-related plasticity in somatosensory cortex.

Effects of transcranial direct current stimulation over human motor cortex on cognitive-motor and sensory-motor functions

The primary motor cortex (M1) is broadly acknowledged for its crucial role in executing voluntary movements. Yet, its contributions to cognitive and sensory functions remain largely unexplored. Transcranial direct current stimulation (tDCS) is a noninvasive neurostimulation method that can modify brain activity, thereby enabling the establishment of a causal link between M1 activity and behavior. This study aimed to investigate the online effects of tDCS over M1 on cognitive-motor and sensory-motor functions. Sixty-four healthy participants underwent either anodal or sham tDCS while concurrently performing a set of standardized robotic tasks. These tasks provided sensitive and objective assessments of brain functions, including action selection, inhibitory control, cognitive control of visuomotor skills, proprioceptive sense, and bimanual coordination. Our results revealed that anodal tDCS applied to M1 enhances decision-making capacity in selecting appropriate motor actions and avoiding distractors compared to sham stimulation, suggesting improved action selection and inhibitory control capabilities. Furthermore, anodal tDCS reduces the movement time required to accomplish bimanual movements, suggesting enhanced bimanual performance. However, we found no impact of anodal tDCS on cognitive control of visuomotor skills and proprioceptive sense. This study suggests that augmenting M1 activity via anodal tDCS influences cognitive-motor and sensory-motor functions in a task-dependent manner.

Microstimulation of human somatosensory cortex evokes task-dependent, spatially patterned responses in motor cortex

The primary motor (M1) and somatosensory (S1) cortices play critical roles in motor control but the signaling between these structures is poorly understood. To fill this gap, we recorded - in three participants in an ongoing human clinical trial (NCT01894802) for people with paralyzed hands - the responses evoked in the hand and arm representations of M1 during intracortical microstimulation (ICMS) in the hand representation of S1. We found that ICMS of S1 activated some M1 neurons at short, fixed latencies consistent with monosynaptic activation. Additionally, most of the ICMS-evoked responses in M1 were more variable in time, suggesting indirect effects of stimulation. The spatial pattern of M1 activation varied systematically: S1 electrodes that elicited percepts in a finger preferentially activated M1 neurons excited during that finger's movement. Moreover, the indirect effects of S1 ICMS on M1 were context dependent, such that the magnitude and even sign relative to baseline varied across tasks. We tested the implications of these effects for brain-control of a virtual hand, in which ICMS conveyed tactile feedback. While ICMS-evoked activation of M1 disrupted decoder performance, this disruption was minimized using biomimetic stimulation, which emphasizes contact transients at the onset and offset of grasp, and reduces sustained stimulation.

Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

The Interactions Between Primary Somatosensory and Motor Cortex during Human Grasping Behaviors

Afferent inputs to the primary somatosensory cortex (S1) are differentially processed during precision and power grip in humans. However, it remains unclear how S1 interacts with the primary motor cortex (M1) during these two grasping behaviors. To address this question, we measured short-latency afferent inhibition (SAI), reflecting S1-M1 interactions via thalamo-cortical pathways, using paired-pulse transcranial magnetic stimulation (TMS) during precision and power grip. The TMS coil over the hand representation of M1 was oriented in the posterior-anterior (PA) and anterior-posterior (AP) direction to activate distinct sets of corticospinal neurons. We found that SAI increased during precision compared with power grip when AP, but not PA, currents were applied. Notably, SAI tested in the AP direction were similar during two-digit than five-digit precision grip. The M1 receives movement information from S1 through direct cortico-cortical pathways, so intra-hemispheric S1-M1 interactions using dual-site TMS were also evaluated. Stimulation of S1 attenuated M1 excitability (S1-M1 inhibition) during precision and power grip, while the S1-M1 inhibition ratio remained similar across tasks. Taken together,our findings suggest that distinct neural mechanisms for S1-M1 interactions mediate precision and power grip, presumably by modulating neural activity along thalamo-cortical pathways.

Abnormal changes in motor cortical maps in humans with spinal cord injury

KEY POINTS

The functional role of motor cortical reorganization following spinal cord injury (SCI) remains largely unknown. Here, we tested motor maps in a hand muscle at rest and during voluntary contraction of the hand with and without voluntary contraction of a proximal arm muscle. Motor map area in participants with SCI decreased during hand voluntary contraction and further decreased during additional contraction of a proximal arm muscle compared with rest. In contrast, motor map area in controls increased during the same motor tasks. Participants with SCI with more severe sensory deficits in the hand showed larger decreases in motor map area. Ten minutes of hand muscle-tendon vibration increased the motor map area during voluntary contraction in SCI participants. These novel findings suggest that abnormal changes in motor cortical maps during voluntary contraction after SCI can be reshaped by sensory input, knowledge that can have implications for rehabilitation.

ABSTRACT

Motor cortical representations reorganize following cervical spinal cord injury (SCI). The functional role of this reorganization remains largely unknown. Using neuronavigated transcranial magnetic stimulation, we examined motor cortical maps during voluntary contraction in humans with chronic cervical SCI and age-matched controls. We constructed motor maps in the first dorsal interosseous (FDI) muscle at rest and during voluntary contraction of the FDI with and without voluntary contraction of the biceps brachi (BB). The role of sensory input into this reorganization was examined by muscle-tendon vibration. We found that, at rest, motor maps were larger in SCI (22.3 cm2 ) compared with control (12.6 cm2 , P < 0.001) participants. Motor map area increased during voluntary contraction of the FDI (120.7%) and further increased during contraction of the BB (143.9%) compared with rest in control subjects; however, motor map area decreased during voluntary contraction of the FDI (69.5%) and further decreased during contraction of the BB (55.5%) in individuals with SCI. SCI participants with larger decreases in map area during voluntary contraction of the FDI were those with larger sensory deficits in the hand and 10 min of hand muscle-tendon vibration increased motor map area. These results provide the first evidence of abnormal changes in motor cortical maps in humans with chronic SCI during voluntary contraction, suggesting that sensory input can help to reshape this reorganization.

Convergence of forepaw somatosensory and motor cortical projections in the striatum, claustrum, thalamus, and pontine nuclei of cats

The basal ganglia and pontocerebellar systems regulate somesthetic-guided motor behaviors and receive prominent inputs from sensorimotor cortex. In addition, the claustrum and thalamus are forebrain subcortical structures that have connections with somatosensory and motor cortices. Our previous studies in rats have shown that primary and secondary somatosensory cortex (S1 and S2) send overlapping projections to the neostriatum and pontine nuclei, whereas, overlap of primary motor cortex (M1) and S1 was much weaker. In addition, we have shown that M1, but not S1, projects to the claustrum in rats. The goal of the current study was to compare these rodent projection patterns with connections in cats, a mammalian species that evolved in a separate phylogenetic superorder. Three different anterograde tracers were injected into the physiologically identified forepaw representations of M1, S1, and S2 in cats. Labeled fibers terminated throughout the ipsilateral striatum (caudate and putamen), claustrum, thalamus, and pontine nuclei. Digital reconstructions of tracer labeling allowed us to quantify both the normalized distribution of labeling in each subcortical area from each tracer injection, as well as the amount of tracer overlap. Surprisingly, in contrast to our previous findings in rodents, we observed M1 and S1 projections converging prominently in striatum and pons, whereas, S1 and S2 overlap was much weaker. Furthermore, whereas, rat S1 does not project to claustrum, we confirmed dense claustral inputs from S1 in cats. These findings suggest that the basal ganglia, claustrum, and pontocerebellar systems in rat and cat have evolved distinct patterns of sensorimotor cortical convergence.

Motor Recovery After Stroke: From a Vespa Scooter Ride Over the Roman Sampietrini to Focal Muscle Vibration (fMV) Treatment. A 99mTc-HMPAO SPECT and Neurophysiological Case Study

Focal repetitive muscle vibration (fMV) is a safe and well-tolerated non-invasive brain and peripheral stimulation (NIBS) technique, easy to perform at the bedside, and able to promote the post-stroke motor recovery through conditioning the stroke-related dysfunctional structures and pathways. Here we describe the concurrent cortical and spinal plasticity induced by fMV in a chronic stroke survivor, as assessed with 99mTc-HMPAO SPECT, peripheral nerve stimulation, and gait analysis. A 72-years-old patient was referred to our stroke clinic for a right leg hemiparesis and spasticity resulting from a previous (4 years before) hemorrhagic stroke. He reported a subjective improvement of his right leg's spasticity and dysesthesia that occurred after a30-min ride on a Vespa scooter as a passenger over the Roman Sampietrini (i.e., cubic-shaped cobblestones). Taking into account both the patient's anecdote and the current guidelines that recommend fMV for the treatment of post-stroke spasticity, we then decided to start fMV treatment. 12 fMV sessions (frequency 100 Hz; amplitude range 0.2–0.5 mm, three 10-min daily sessions per week for 4 consecutive weeks) were applied over the quadriceps femoris, triceps surae, and hamstring muscles through a specific commercial device (Cro®System, NEMOCOsrl). A standardized clinical and instrumental evaluation was performed before (T0) the first fMV session and after (T1) the last one. After fMV treatment, we observed a clinically relevant motor and functional improvement, as assessed by comparing the post-treatment changes in the score of the Fugl-Meyer assessment, the Motricity Index score, the gait analysis, and the Ashworth modified scale, with the respective minimal detectable change at the 95% confidence level (MDC₉₅). Data from SPECT and peripheral nerve stimulation supported the evidence of a concurrent brain and spinal plasticity promoted by fMV treatment trough activity-dependent changes in cortical perfusion and motoneuron excitability, respectively. In conclusion, the substrate of post-stroke motor recovery induced by fMV involves a concurrently acting multisite plasticity (i.e., cortical and spinal plasticity). In our patient, operant conditioning of both cortical perfusion and motoneuron excitability throughout a month of fMV treatment was related to a clinically relevant improvement in his strength, step symmetry (with reduced limping), and spasticity.

In vivo measurement of widespread synaptic loss in Alzheimer's disease with SV2A PET

Introduction

Synaptic loss is a robust and consistent pathology in Alzheimer's disease (AD) and the major structural correlate of cognitive impairment. Positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A) has emerged as a promising biomarker of synaptic density.

Methods

We measured SV2A binding in 34 participants with early AD and 19 cognitively normal (CN) participants using [¹¹C]UCB‐J PET and a cerebellar reference region for calculation of the distribution volume ratio.

Results

We observed widespread reductions of SV2A binding in medial temporal and neocortical brain regions in early AD compared to CN participants. These reductions were largely maintained after correction for volume loss and were more extensive than decreases in gray matter volume.

Conclusion

We were able to measure widespread synaptic loss due to AD using [¹¹C]UCB‐J PET. Future studies will continue to evaluate the utility of SV2A PET for tracking AD progression and for monitoring potential therapies.

Cerebellar outputs contribute to spontaneous and movement-related activity in the motor cortex of monkeys

Cerebellar outputs originate from the dentate nucleus (DN), project to the primary motor cortex (M1) via the motor thalamus, control M1 activity, and play an essential role in coordinated movements. However, it is unclear when and how the cerebellar outputs contribute to M1 activity. To address this question, we examined the response of M1 neurons to electrical stimulation of the DN and M1 activity during performance of arm-reaching tasks. Based on response patterns to DN stimulation, M1 neurons were classified into facilitation-, suppression-, and no-response-types. During tasks, not only facilitation- and suppression-type M1 neurons, but also no response-type M1 neurons increased or decreased their firing rates in relation to arm reaching movements. However, the firing rates of facilitation- and suppression-type neurons were higher than those of no-response-type neurons during both inter-trial intervals and arm reaching movements. These results imply that cerebellar outputs contribute to both spontaneous and movement-related activity in the M1, which help to maintain muscle tones and execute coordinated movements, although other inputs also contribute to movement-related activity. Pharmacological inactivation of the DN supports this notion, in that DN inactivation reduced both spontaneous firing rates and movement-related activity in the M1.

Neuromuscular electrical stimulation-promoted plasticity of the human brain

The application of neuromuscular electrical stimulation (NMES) to paretic limbs has demonstrated utility for motor rehabilitation following brain injury. When NMES is delivered to a mixed peripheral nerve, typically both efferent and afferent fibres are recruited. Muscle contractions brought about by the excitation of motor neurons are often used to compensate for disability by assisting actions such as the formation of hand aperture, or by preventing others including foot drop. In this context, exogenous stimulation provides a direct substitute for endogenous neural drive. The goal of the present narrative review is to describe the means through which NMES may also promote sustained adaptations within central motor pathways, leading ultimately to increases in (intrinsic) functional capacity. There is an obvious practical motivation, in that detailed knowledge concerning the mechanisms of adaptation has the potential to inform neurorehabilitation practice. In addition, responses to NMES provide a means of studying CNS plasticity at a systems level in humans. We summarize the fundamental aspects of NMES, focusing on the forms that are employed most commonly in clinical and experimental practice. Specific attention is devoted to adjuvant techniques that further promote adaptive responses to NMES thereby offering the prospect of increased therapeutic potential. The emergent theme is that an association with centrally initiated neural activity, whether this is generated in the context of NMES triggered by efferent drive or via indirect methods such as mental imagery, may in some circumstances promote the physiological changes that can be induced through peripheral electrical stimulation.

Supplementary motor area plays a causal role in automatic inhibition of motor responses

BACKGROUND

The masked-priming paradigm is used to test unconscious inhibitory processes of the brain. A tendency towards responses that are incompatible with the prime, designated as negative compatibility effect (NCE), emerges when the perception of a priming visual stimulus is "masked" afterwards. This effect presumably stems from a subliminal inhibitory process against the masked-prime. Prior lesions as well as activation studies suggest a key role of SMA in this effect.

OBJECTIVE

This study was conducted to elucidate a causal role of SMA in the subliminal response inhibition represented by the NCE.

METHODS

Using a repeated-measures pre-post design with a group of healthy people, physiological measures (resting and active motor thresholds and motor evoked potential (MEP) amplitude) and behavioral ones (choice reaction time (CRT), positive compatibility effect (PCE) and NCE) were obtained before and after three quadripulse stimulation (QPS), namely sham, M1-QPS, and SMA-QPS, on different days. CRT and PCE served as indices for different aspects of motor execution.

RESULTS

Motor thresholds were not altered after any QPS, although the M1-QPS increased MEP amplitude. Neither CRT nor PCE was altered significantly after QPS protocols. NCE was abolished after the SMA-QPS.

CONCLUSIONS

Abolished NCE after the SMA-QPS in the absence of MEP changes suggests that (1) SMA plays a cardinal role in the NCE, and (2) the network involved in NCE is different from that of MEP generation.

Short-Term Effects of Focal Muscle Vibration on Motor Recovery After Acute Stroke: A Pilot Randomized Sham-Controlled Study

Repetitive focal muscle vibration (rMV) is known to promote neural plasticity and long-lasting motor recovery in chronic stroke patients. Those structural and functional changes within the motor network underlying motor recovery occur in the very first hours after stroke. Nonetheless, to our knowledge, no rMV-based studies have been carried out in acute stroke patients so far, and the clinical benefit of rMV in this phase of stroke is yet to be determined. The aim of this randomized double-blind sham-controlled study is to investigate the short-term effect of rMV on motor recovery in acute stroke patients. Out of 22 acute stroke patients, 10 were treated with the rMV (vibration group-VG), while 12 underwent the sham treatment (control group-CG). Both treatments were carried out for 3 consecutive days, starting within 72 h of stroke onset; each daily session consisted of three 10-min treatments (for each treated limb), interspersed with a 1-min interval. rMV was delivered using a specific device (Cro®System, NEMOCO srl, Italy). The transducer was applied perpendicular to the target muscle's belly, near its distal tendon insertion, generating a 0.2-0.5 mm peak-to-peak sinusoidal displacement at a frequency of 100 Hz. All participants also underwent a daily standard rehabilitation program. The study protocol underwent local ethics committee approval (ClinicalTrial.gov NCT03697525) and written informed consent was obtained from all of the participants. With regard to the different pre-treatment clinical statuses, VG patients showed significant clinical improvement with respect to CG-treated patients among the NIHSS (p < 0.001), Fugl-Meyer (p = 0.001), and Motricity Index (p < 0.001) scores. In addition, when the upper and lower limb scales scores were compared between the two groups, VG patients were found to have a better clinical improvement at all the clinical end points. This study provides the first evidence that rMV is able to improve the motor outcome in a cohort of acute stroke patients, regardless of the pretreatment clinical status. Being a safe and well-tolerated intervention, which is easy to perform at the bedside, rMV may represent a valid complementary non-pharmacological therapy to promote motor recovery in acute stroke patients.

Weak but Critical Links between Primary Somatosensory Centers and Motor Cortex during Movement

Motor performance is improved by stimulation of the agonist muscle during movement. However, related brain mechanisms remain unknown. In this work, we perform a functional magnetic resonance imaging (fMRI) study in 21 healthy subjects under three different conditions: (1) movement of right ankle alone; (2) movement and simultaneous stimulation of the agonist muscle; or (3) movement and simultaneous stimulation of a control area. We constructed weighted brain networks for each condition by using functional connectivity. Network features were analyzed using graph theoretical approaches. We found that: (1) the second condition evokes the strongest and most widespread brain activations (5147 vs. 4419 and 2320 activated voxels); and (2) this condition also induces a unique network layout and changes hubs and the modular structure of the brain motor network by activating the most “silent” links between primary somatosensory centers and the motor cortex, particularly weak links from the thalamus to the left primary motor cortex (M1). Significant statistical differences were found when the strength values of the right cerebellum (P < 0.001) or the left thalamus (P = 0.006) were compared among the three conditions. Over the years, studies reported a small number of projections from the thalamus to the motor cortex. This is the first work to present functions of these pathways. These findings reveal mechanisms for enhancing motor function with somatosensory stimulation, and suggest that network function cannot be thoroughly understood when weak ties are disregarded.

Effect of Range and Angular Velocity of Passive Movement on Somatosensory Evoked Magnetic Fields

To clarify characteristics of each human somatosensory evoked field (SEF) component following passive movement (PM), PM1, PM2, and PM3, using high spatiotemporal resolution 306-channel magnetoencephalography and varying PM range and angular velocity. We recorded SEFs following PM under three conditions [normal range-normal velocity (NN), small range-normal velocity (SN), and small range-slow velocity (SS)] with changing movement range and angular velocity in 12 participants and calculated the amplitude, equivalent current dipole (ECD) location, and the ECD strength for each component. All components were observed in six participants, whereas only PM1 and PM3 in the other six. Clear response deflections at the ipsilateral hemisphere to PM side were observed in seven participants. PM1 amplitude was larger under NN and SN conditions, and mean ECD location for PM1 was at primary motor area. PM3 amplitude was larger under SN condition and mean ECD location for PM3 under SS condition was at primary somatosensory area. PM1 amplitude was dependent on the angular velocity of PM, suggesting that PM1 reflects afferent input from muscle spindle, whereas PM3 amplitude was dependent on the duration. The ECD for PM3 was located in the primary somatosensory cortex, suggesting that PM3 reflects cutaneous input. We confirmed the hypothesis for locally distinct generators and characteristics of each SEF component.

Cortical Mechanisms of Central Fatigue and Sense of Effort

The purpose of this study was to investigate cortical mechanisms upstream to the corticospinal motor neuron that may be associated with central fatigue and sense of effort during and after a fatigue task. We used two different isometric finger abduction protocols to examine the effects of muscle activation and fatigue the right first dorsal interosseous (FDI) of 12 participants. One protocol was intended to assess the effects of muscle activation with minimal fatigue (control) and the other was intended to elicit central fatigue (fatigue). We hypothesized that high frequency repetitive transcranial magnetic stimulation (rTMS) of the supplementary motor area (SMA) would hasten recovery from central fatigue and offset a fatigue-induced increase in sense of effort by facilitating the primary motor cortex (M1). Constant force-sensation contractions were used to assess sense of effort associated with muscle contraction. Paired-pulse TMS was used to assess intracortical inhibition (ICI) and facilitation (ICF) in the active M1 and interhemispheric inhibitory (IHI) was assessed to determine if compensation occurs via the resting M1. These measures were made during and after the muscle contraction protocols. Corticospinal excitability progressively declined with fatigue in the active hemisphere. ICF increased at task failure and ICI was also reduced at task failure with no changes in IHI found. Although fatigue is associated with progressive reductions in corticospinal excitability, compensatory changes in inhibition and facilitation may act within, but not between hemispheres of the M1. rTMS of the SMA following fatigue enhanced recovery of maximal voluntary force and higher levels of ICF were associated with lower sense of effort following stimulation. rTMS of the SMA may have reduced the amount of upstream drive required to maintain motor output, thus contributing to a lower sense of effort and increased rate of recovery of maximal force.

Reversible Deactivation of Motor Cortex Reveals Functional Connectivity with Posterior Parietal Cortex in the Prosimian Galago (Otolemur garnettii)

We examined the functional macrocircuitry of frontoparietal networks in the neocortex of prosimian primates (Otolemur garnettii) using a microfluidic thermal regulator to reversibly deactivate selected regions of motor cortex (M1). During deactivation of either forelimb or mouth/face movement domains within M1, we used long-train intracortical microstimulation techniques to evoke movements from the rostral division of posterior parietal cortex (PPCr). We found that deactivation of M1 movement domains in most instances abolished movements evoked in PPCr. The most common effect of deactivating M1 was to abolish evoked movements in a homotopic domain in PPCr. For example, deactivating M1 forelimb lift domains resulted in loss of evoked movement in forelimb domains in PPCr. However, at some sites, we also observed heterotopic effects; deactivating a specific domain in M1 (e.g., forelimb lift) resulted in loss of evoked movement in a different movement domain in PPCr (e.g., hand-to-mouth or eye-blink). At most sites examined in PPCr, rewarming M1 resulted in a reestablishment of the baseline movement at the same amplitude as that observed before cooling. However, at some sites, reactivation did not result in a return to baseline movement or to the full amplitude of the baseline movement. We discuss our findings in the context of frontoparietal circuits and how they may subserve a repertoire of ecologically relevant behaviors.

SIGNIFICANCE STATEMENT

The posterior parietal cortex (PPC) of primates integrates sensory information used to guide movements. Different modules within PPC and motor cortex (M1) appear to control various motor behaviors (e.g., reaching, defense, and feeding). How these modules work together may vary across species and may explain differences in dexterity and even the capacity for tool use. We investigated the functional connectivity of these modules in galagos, a prosimian primate with relatively simple frontoparietal circuitry. By deactivating a reaching module in M1, we interfered with the function of similar PPC modules and occasionally unrelated PPC modules as well (e.g., eye blink). This circuitry in galagos, therefore, is more complex than in nonprimates, indicating that it has been altered with the expansion of primate PPC.

Sensory Plasticity in Human Motor Learning

There is accumulating evidence from behavioral, neurophysiological, and neuroimaging studies that the acquisition of motor skills involves both perceptual and motor learning. Perceptual learning alters movements, motor learning, and motor networks of the brain. Motor learning changes perceptual function and the sensory circuits of the brain. Here, we review studies of both human limb movement and speech that indicate that plasticity in sensory and motor systems is reciprocally linked. Taken together, this points to an approach to motor learning in which perceptual learning and sensory plasticity have a fundamental role.

Different Patterns of Cortical Inputs to Subregions of the Primary Motor Cortex Hand Representation in Cebus apella

The primary motor cortex (M1) plays an essential role in the control of hand movements in primates and is part of a complex cortical sensorimotor network involving multiple premotor and parietal areas. In a previous study in squirrel monkeys, we found that the ventral premotor cortex (PMv) projected mainly to 3 regions within the M1 forearm representation [rostro-medial (RM), rostro-lateral (RL), and caudo-lateral (CL)] with very few caudo-medial (CM) projections. These results suggest that projections from premotor areas to M1 are not uniform, but rather segregated into subregions. The goal of the present work was to study how inputs from diverse areas of the ipsilateral cortical network are organized within the M1 hand representation. In Cebus apella, different retrograde neuroanatomical tracers were injected in 4 subregions of the hand area of M1 (RM, RL, CM, and CL). We found a different pattern of input to each subregion of M1. RM receives inputs predominantly from dorsal premotor cortex, RL from PMv, CM from area 5, and CL from area 2. These results support that the M1 hand representation is composed of several subregions, each part of a unique cortical network.

Learning to Estimate Dynamical State with Probabilistic Population Codes

Tracking moving objects, including one’s own body, is a fundamental ability of higher organisms, playing a central role in many perceptual and motor tasks. While it is unknown how the brain learns to follow and predict the dynamics of objects, it is known that this process of state estimation can be learned purely from the statistics of noisy observations. When the dynamics are simply linear with additive Gaussian noise, the optimal solution is the well known Kalman filter (KF), the parameters of which can be learned via latent-variable density estimation (the EM algorithm). The brain does not, however, directly manipulate matrices and vectors, but instead appears to represent probability distributions with the firing rates of population of neurons, “probabilistic population codes.” We show that a recurrent neural network—a modified form of an exponential family harmonium (EFH)—that takes a linear probabilistic population code as input can learn, without supervision, to estimate the state of a linear dynamical system. After observing a series of population responses (spike counts) to the position of a moving object, the network learns to represent the velocity of the object and forms nearly optimal predictions about the position at the next time-step. This result builds on our previous work showing that a similar network can learn to perform multisensory integration and coordinate transformations for static stimuli. The receptive fields of the trained network also make qualitative predictions about the developing and learning brain: tuning gradually emerges for higher-order dynamical states not explicitly present in the inputs, appearing as delayed tuning for the lower-order states.

A basic task for animals is to track objects—predators, prey, even their own limbs—as they move through the world. Because the position estimates provided by the senses are not error-free, higher levels of performance can be, and are, achieved when the velocity and acceleration, as well as the position, of the object are taken into account. Likewise, tracking of limbs under voluntary control can be improved by considering the motor command that is (partially) responsible for its trajectory. Engineers have built tools to solve precisely these problems, and even to learn dynamical features of the object to be tracked. How does the brain do it? We show how artificial networks of neurons can learn to solve this task, simply by trying to become good predictive models of their incoming data—as long as some of those data are the activities of the neurons themselves at a fixed time delay, while the remainder (imperfectly) report the current position. The tracking scheme the network learns to use—keeping track of past positions; the corresponding receptive fields; and the manner in which they are learned, provide predictions for brain areas involved in tracking, like the posterior parietal cortex.

Neuronal activity in somatosensory cortex related to tactile exploration

The very light contact forces (∼0.60 N) applied by the fingertips during tactile exploration reveal a clearly optimized sensorimotor strategy. To investigate the cortical mechanisms involved with this behavior, we recorded 230 neurons in the somatosensory cortex (S1), as two monkeys scanned different surfaces with the fingertips in search of a tactile target without visual feedback. During the exploration, the monkeys, like humans, carefully controlled the finger forces. High-friction surfaces offering greater tangential shear force resistance to the skin were associated with decreased normal contact forces. The activity of one group of neurons was modulated with either the normal or tangential force, with little or no influence from the orthogonal force component. A second group responded to kinetic friction or the ratio of tangential to normal forces rather than responding to a specific parameter, such as force magnitude or direction. A third group of S1 neurons appeared to respond to particular vectors of normal and tangential force on the skin. Although 45 neurons correlated with scanning speed, 32 were also modulated by finger forces, suggesting that forces on the finger should be considered as the primary parameter encoding the skin compliance and that finger speed is a secondary parameter that co-varies with finger forces. Neurons (102) were also tested with different textures, and the activity of 62 of these increased or decreased in relation to the surface friction.

Understanding the role of the primary somatosensory cortex: Opportunities for rehabilitation

Emerging evidence indicates impairments in somatosensory function may be a major contributor to motor dysfunction associated with neurologic injury or disorders. However, the neuroanatomical substrates underlying the connection between aberrant sensory input and ineffective motor output are still under investigation. The primary somatosensory cortex (S1) plays a critical role in processing afferent somatosensory input and contributes to the integration of sensory and motor signals necessary for skilled movement. Neuroimaging and neurostimulation approaches provide unique opportunities to non-invasively study S1 structure and function including connectivity with other cortical regions. These research techniques have begun to illuminate casual contributions of abnormal S1 activity and connectivity to motor dysfunction and poorer recovery of motor function in neurologic patient populations. This review synthesizes recent evidence illustrating the role of S1 in motor control, motor learning and functional recovery with an emphasis on how information from these investigations may be exploited to inform stroke rehabilitation to reduce motor dysfunction and improve therapeutic outcomes.

Patterns of cortical input to the primary motor area in the marmoset monkey

In primates the primary motor cortex (M1) forms a topographic map of the body, whereby neurons in the medial part of this area control movements involving trunk and hindlimb muscles, those in the intermediate part control movements involving forelimb muscles, and those in the lateral part control movements of facial and other head muscles. This topography is accompanied by changes in cytoarchitectural characteristics, raising the question of whether the anatomical connections also vary between different parts of M1. To address this issue, we compared the patterns of cortical afferents revealed by retrograde tracer injections in different locations within M1 of marmoset monkeys. We found that the entire extent of this area is unified by projections from the dorsocaudal and medial subdivisions of premotor cortex (areas 6DC and 6M), from somatosensory areas 3a, 3b, 1/2, and S2, and from posterior parietal area PE. While cingulate areas projected to all subdivisions, they preferentially targeted the medial part of M1. Conversely, the ventral premotor areas were preferentially connected with the lateral part of M1. Smaller but consistent inputs originated in frontal area 6DR, ventral posterior parietal cortex, the retroinsular cortex, and area TPt. Connections with intraparietal, prefrontal, and temporal areas were very sparse, and variable. Our results demonstrate that M1 is unified by a consistent pattern of major connections, but also shows regional variations in terms of minor inputs. These differences likely reflect requirements for control of voluntary movement involving different body parts.

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

Anticipating the course of an action: evidence from corticospinal excitability

Background

Anticipatory planning, the ability to anticipate future perceptual-motor demands of a goal-oriented action sequence, is essential for flexible, purposeful behavior. Once an action goal has been defined, movement details necessary to achieve that goal can be selected. Here, we investigate if anticipatory planning takes place even when multi-step actions are being carried out. How, we may ask, are the cerebral circuits involved in movement selection influenced by anticipated object-center task demands? Transcranial magnetic stimulation (TMS) was used to investigate how changes in corticospinal excitability (CSE) are dependent on anticipated task variables of intended future actions. Specifically, single- and paired-pulse TMS was used to evaluate corticospinal excitability during the action selection phase preparatory to grasp execution.

Results

We found that during the premovement phase, there is an object- and muscle-specific modulation in the intrinsic hand muscle that will be used during a forthcoming grasping action. Depending on whether the participants were instructed to perform a single- or double-step movement sequence, modulation of the corticospinal output to the appropriate hand muscles was dependent on what object was to be grasped and what type of movement was being prepared. No modulation in excitability was observed during one-step movements.

Conclusions

Anticipation of intended task demands plays an important role in controlling multi- step actions during which ongoing behavior may need to be adjusted. This finding supports the notion that the cortico-cortical mechanism involving movement planning is specific for an object’s properties as well as for the goal of the movement sequence.

Cortical connectivity suggests a role in limb coordination for macaque area PE of the superior parietal cortex

In macaques, superior parietal lobule area 5 has been described as occupying an extensive region, which includes the caudal half of the postcentral convexity as well as the medial bank of the intraparietal sulcus. Modern neuroanatomical methods have allowed the identification of various areas within this region. In the present study, we investigated the corticocortical afferent projections of one of these subdivisions, area PE. Our results demonstrate that PE, defined as a single architectonic area that contains a topographic map of the body, forms specific connections with somatic and motor fields. Thus, PE receives major afferents from parietal areas, mainly area 2, PEc, several areas in the medial bank of the intraparietal sulcus, opercular areas PGop/PFop, and the retroinsular area, frontal afferents from the primary motor cortex, the supplementary motor area, and the caudal subdivision of dorsal premotor cortex, as well as afferents from cingulate areas PEci, 23, and 24. The presence and relative strength of these connections depend on the location of injection sites, so that lateral PE receives preferential input from anterior sectors of the medial bank of intraparietal sulcus and from the ventral premotor cortex, whereas medial PE forms denser connections with area PEc and motor fields. In contrast with other posterior parietal areas, there are no projections to PE from occipital or prefrontal cortices. Overall, the sensory and motor afferents to PE are consistent with functions in goal-directed movement but also hint at a wider variety of motor coordination roles.

Predictions not commands: active inference in the motor system

The descending projections from motor cortex share many features with top-down or backward connections in visual cortex; for example, corticospinal projections originate in infragranular layers, are highly divergent and (along with descending cortico-cortical projections) target cells expressing NMDA receptors. This is somewhat paradoxical because backward modulatory characteristics would not be expected of driving motor command signals. We resolve this apparent paradox using a functional characterisation of the motor system based on Helmholtz's ideas about perception; namely, that perception is inference on the causes of visual sensations. We explain behaviour in terms of inference on the causes of proprioceptive sensations. This explanation appeals to active inference, in which higher cortical levels send descending proprioceptive predictions, rather than motor commands. This process mirrors perceptual inference in sensory cortex, where descending connections convey predictions, while ascending connections convey prediction errors. The anatomical substrate of this recurrent message passing is a hierarchical system consisting of functionally asymmetric driving (ascending) and modulatory (descending) connections: an arrangement that we show is almost exactly recapitulated in the motor system, in terms of its laminar, topographic and physiological characteristics. This perspective casts classical motor reflexes as minimising prediction errors and may provide a principled explanation for why motor cortex is agranular.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.

Increased primary motor cortical excitability by a single-pulse transcranial magnetic stimulation over the supplementary motor area

The supplementary motor area (SMA) is a secondary motor area that is involved in various complex hand movements. In animal studies, short latency and probably direct excitatory inputs from SMA to the primary motor cortex (M1) have been established. Although human imaging studies revealed functional connectivity between SMA and M1, its electrophysiological nature has been less studied. This study explored the connection between SMA and M1 in humans using a single-pulse transcranial magnetic stimulation (TMS) over SMA. First, TMS over SMA did not alter the corticospinal tract excitability measured by the size of motor evoked potential elicited by single-pulse TMS over M1. Next, we measured short-interval intracortical facilitation (SICF), which reflects the function of a facilitatory circuit within M1, with or without a single-pulse TMS over SMA. When the intensity of the second pulse in the SICF paradigm (S2) was as weak as 1.0 active motor threshold for a hand muscle, SMA stimulation significantly enhanced the SICF. Furthermore, this enhancement by SMA stimulation was spatially confined and had a limited time window. On the other hand, SMA stimulation did not alter short-interval intracortical inhibition or contralateral silent period duration, which reflects the function of an inhibitory circuit mediated by gamma-aminobutyric acid A (GABA(A)) or GABA(B) receptors, respectively. We conclude that a single-pulse TMS over SMA modulates a facilitatory circuit within M1.

Writing's shadow: corticospinal activation during letter observation

We can recognize handwritten letters despite the variability among writers. One possible strategy is exploiting the motor memory of orthography. By using TMS, we clarified the excitatory and inhibitory neural circuits of the motor corticospinal pathway that might be activated during the observation of handwritten letters. During experiments, participants looked at the handwritten or printed single letter that appeared in a random order. The excitability of the left and right primary motor cortex (M1) was evaluated by motor-evoked potentials elicited by single-pulse TMS. Short interval intracortical inhibition (SICI) of the left M1 was evaluated using paired-pulse TMS. F waves were measured for the right ulnar nerve. We found significant reduction of corticospinal excitability only for the right hand at 300-400 msec after each letter presentation without significant changes in SICI. This suppression is likely to be of supraspinal origin, because of no significant alteration in F-wave amplitudes. These findings suggest that the recognition of handwritten letters may include the implicit knowledge of "writing" in M1. The M1 activation associated with that process, which has been shown in previous neuroimaging studies, is likely to reflect the active suppression of the corticospinal excitability.

Cytoarchitecture and cortical connections of the anterior cingulate and adjacent somatomotor fields in the rhesus monkey

The cytoarchitecture and cortical connections of the anterior cingulate, medial and dorsal premotor, and precentral region are investigated using the Nissl and NeuN staining methods and the fluorescent retrograde tract tracing technique. There is a gradual stepwise laminar change in the cytoarchitectonic organization from the proisocortical anterior cingulate region, through the lower and upper banks of the cingulate sulcus, to the dorsolateral isocortical premotor and precentral motor regions of the frontal lobe. These changes are characterized by a gradational emphasis on the lower stratum layers (V and VI) in the proisocortical cingulate region to the upper stratum layers (II and III) in the premotor and precentral motor region. This is accompanied by a progressive widening of layers III and VI, a poorly delineated border between layers III and V and a sequential increase in the size of layer V neurons culminating in the presence of giant Betz cells in the precentral motor region. The overall patterns of corticocortical connections paralleled the sequential changes in cytoarchitectonic organization. The proisocortical areas have connections with cingulate motor, supplementary motor, premotor and precentral motor areas on the one hand and have widespread connections with the frontal, parietal, temporal and multimodal association cortex and limbic regions on the other. The dorsal premotor areas have connections with the proisocortical areas including cingulate motor areas and supplementary motor area on the one hand, and premotor and precentral motor cortex on the other. Additionally, this region has significant connections with posterior parietal cortex and limited connections with prefrontal, limbic and multimodal regions. The precentral motor cortex also has connections with the proisocortical areas and premotor areas. Its other connections are limited to the somatosensory regions of the parietal lobe. Since the isocortical motor areas on the dorsal convexity mediate voluntary motor function, their close connectional relationship with the cingulate areas form a pivotal limbic-motor interface that could provide critical sources of cognitive, emotional and motivational influence on complex motor function.

Transcranial Magnetic Mapping of the Short-Latency Modulations of Corticospinal Activity from the Ipsilateral Hemisphere during Rest

Skilled hand function relies heavily on the integrity of the primary motor cortex (M1) and on a web of cortico-cortical connections projecting onto it. We used a novel explorative paradigm to map the origin of cortico-M1 pathways assessed by dual transcranial magnetic stimulation (TMS) in three healthy participants. Subthreshold conditioning TMS (cTMS) was delivered over a grid of ≈100 spots. Covering the left hemisphere, and was followed by suprathreshold test (tTMS) delivered over the ipsilateral M1. Grid points were tested eight times, with inter-stimulus intervals between cTMS and tTMS of 4 and 7 ms. Participants were asked to stay relaxed with no particular task. Motor evoked potentials (MEPs) from cTMS + tTMS were normalized to MEPs from tTMS alone and were compared to the value expected from tTMS alone using t-statistics. The t-values from each grid point were then used to plot statistical maps. Several foci of significant cortico-M1 interactions were found in the dorsal–medial frontal cortex, in the ventral frontal cortex, in the superior and inferior parietal lobules and in the parietal operculum. The majority of active foci had inhibitory effects on corticospinal excitability. The spatial location of the network of different subjects overlapped but with some anatomical variation of single foci. TMS statistical mapping during the resting state revealed a complex inhibitory cortical network. The explorative approach to TMS as a brain mapping tool produced results that are self-standing in single subjects overcoming inter-individual variability of cortical active sites.

Multiple parietal-frontal pathways mediate grasping in macaque monkeys

The nodes of a parietal-frontal pathway that mediates grasping in primates are in anterior intraparietal area (AIP) and ventral premotor cortex (PMv). Nevertheless, multiple somatosensory and motor representations of the hand, in parietal and frontal cortex, respectively, suggest that additional pathways remain unrealized. We explored this possibility in macaque monkeys by injecting retrograde tracers into grasp zones identified in primary motor cortex (M1), PMv, and area 2 with long train electrical stimulation. The M1 grasp zone was densely connected with other frontal cortex motor regions. The remainder of the connections originated from somatosensory areas 3a and second somatosensory cortex/parietal ventral area (S2/PV), and from the medial bank and fundus of the intraparietal sulcus (IPS). The PMv grasp zone was also densely connected with frontal cortex motor regions, albeit to a lesser extent than the M1 grasp zone. The remainder of the connections originated from areas S2/PV and aspects of the inferior parietal lobe such as PF, PFG, AIP, and the tip of the IPS. The area 2 grasp zone was densely connected with the hand representations of somatosensory areas 3b, 1, and S2/PV. The remainder of the connections was with areas 3a and 5 and the medial bank and fundus of the IPS. Connections with frontal cortex were relatively weak and concentrated in caudal M1. Thus, the three grasp zones may be nodes of parallel parietal-frontal pathways. Differential points of origin and termination of each pathway suggest varying functional specializations. Direct and indirect connections between those parietal-frontal pathways likely coordinate their respective functions into an accurate grasp.

Morphological alteration and reduction of MAP2-immunoreactivity in pyramidal neurons of cerebral cortex in a rat model of focal cortical compression

Subdural hematoma causes cortical damage including brain tissue disruption, often resulting in neuronal dysfunction and neurological impairment. The aim of the present study was to identify the relationship between cerebral compression and neuronal injury. In this report, we investigated time-dependent morphological alterations within layers II, III, and V pyramidal neurons in the cerebral cortex, using Golgi-Cox staining and immunohistochemistry for microtubule-associated protein 2 (MAP2) in a rat model of focal cortical compression. An acryl pole was used to experimentally induce chronic cerebral compression by continuous pressure on the cortical surface. Changes in cellular morphology were examined at five survival time periods: 12 h and 1, 2, 3, and 4 weeks. The Golgi-Cox method revealed time-dependent alterations in dendritic length of apical and basilar dendrites of pyramidal neurons. The number of dendritic branch segments and spines of basilar dendrites were decreased in cells in layers II, III, and V. Immunohistochemical staining for MAP2 revealed changes in the intracellular distribution of immunoreactive materials. A significant reduction in MAP2 immunostaining was found in nerve cell bodies and apical dendrites of ipsilateral cortical neurons. The number of MAP2-immunoreactive neurons was significantly decreased at 12 h compared with the contralateral cerebral cortex in the same animal. Dendritic changes in layers II, III, and V pyramidal neurons were accompanied by reductions in intracellular MAP2-immunoreactive materials. The present results suggest that cortical compression causes alteration of cellular morphology as a consequence of injury, and that these morphological changes may be related to reductions in MAP2-immunoreactive materials.

Reorganization of the primary motor cortex of adult macaque monkeys after sensory loss resulting from partial spinal cord injuries

Long-term injuries to the dorsal columns of the spinal cord at cervical levels result in large-scale somatotopic reorganization of the somatosensory areas of the cortex and the ventroposterior nucleus of the thalamus. As a result of this reorganization, intact inputs from the face expand into the deafferented hand representations. Dorsal column injuries also result in permanent deficits in the use of digits for precision grip and a loss of fractionated movements of the digits. We determined whether the chronic loss of sensory inputs and the behavioral deficits caused by lesions of the dorsal columns in adult macaque monkeys affect organization of the motor cortex. The results show that, in the primary motor cortex, intracortical microstimulation evokes extension-flexion movements of the thumb at significantly fewer sites compared with the normal monkeys. There is a corresponding increase in the adduction-abduction movements. Furthermore, there is a significant increase in the thresholds of the currents required to evoke movements of the digits. Thus, long-term sensory loss in adult monkeys does not change the overall topography of the movement representation in the motor cortex but results in changes in the details of movement representations.

Stretch sensitive reflexes as an adaptive mechanism for maintaining limb stability

The often studied stretch reflex is fundamental to the involuntary control of posture and movement. Nevertheless, there remains controversy regarding its functional role. Many studies have demonstrated that stretch reflexes can be modulated in a task appropriate manner. This review focuses on modulation of the long-latency stretch reflex, thought to be mediated, at least in part, by supraspinal pathways. For example, this component of the stretch reflex increases in magnitude during interactions with compliant environments, relative to its sensitivity during interactions with rigid environments. This suggests that reflex sensitivity increases to augment limb stability when that stability is not provided by the environment. However, not all results support the stabilizing role of stretch reflexes. Some studies have demonstrated that involuntary responses within the time period corresponding to the long-latency reflex can destabilize limb posture. We propose that this debate stems from the fact that multiple perturbation-sensitive pathways can contribute to the long-latency stretch reflex and that these pathways have separate functional roles. The presented studies suggest that neural activity occurring within the period normally ascribed to the long-latency stretch reflex is highly adaptable to current task demands and possibly should be considered more intelligent than "reflexive".

Long-Term Effects on Cortical Excitability and Motor Recovery Induced by Repeated Muscle Vibration in Chronic Stroke Patients

Background. Muscle vibration modifies corticomotor excitability in healthy subjects and reduces muscle tonus in stroke patients. Objective. This study examined whether repeated muscle vibration (rMV) applied over the flexor carpi radialis (FCR) and biceps brachii (BB) can induce long-lasting changes, using transcranial magnetic stimulation (TMS), in patients with chronic stroke. Methods. Thirty hemiparetic patients who offered at least minimal wrist and elbow isometric voluntary contractions were randomly assigned to either an experimental group, which received rMV in addition to physiotherapy (rMV + PT), or a control group that underwent PT alone. The following parameters of the FCR, BB, and extensor digitorum communis (EDC) were measured through TMS before, and 1 hour, 1 week, and 2 weeks after the end of intervention: resting motor threshold (RMT), map area, map volume, short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF). Muscle tonus and motor function were assessed on the same day as TMS. Results. Pre–post analysis revealed a reduction in RMT and an increase in motor map areas occurred in the vibrated muscles only in the rMV + PT group, with an increase in map volumes of all muscles. Moreover, SICI increased in the flexors and decreased in the extensor. These neurophysiological changes lasted for at least 2 weeks after the end of rMV + PT and paralleled the reduction in spasticity and increase in motor function. A significant correlation was found between the degree of spasticity and the amount of intracortical inhibition. Conclusion. rMV with PT may be used as a nonpharmacological intervention in the neurorehabilitation of mild to moderate hemiparesis.

The human mirror neuron system: a link between action observation and social skills

The discovery of the mirror neuron system (MNS) has led researchers to speculate that this system evolved from an embodied visual recognition apparatus in monkey to a system critical for social skills in humans. It is accepted that the MNS is specialized for processing animate stimuli, although the degree to which social interaction modulates the firing of mirror neurons has not been investigated. In the current study, EEG mu wave suppression was used as an index of MNS activity. Data were collected while subjects viewed four videos: (1) Visual White Noise: baseline, (2) Non-interacting: three individuals tossed a ball up in the air to themselves, (3) Social Action, Spectator: three individuals tossed a ball to each other and (4) Social Action, Interactive: similar to video 3 except occasionally the ball would be thrown off the screen toward the viewer. The mu wave was modulated by the degree of social interaction, with the Non-interacting condition showing the least suppression, followed by the Social Action, Spectator condition and the Social Action, Interactive condition showing the most suppression. These data suggest that the human MNS is specialized not only for processing animate stimuli, but specifically stimuli with social relevance.

A Contribution of Area 5 of the Posterior Parietal Cortex to the Planning of Visually Guided Locomotion: Limb-Specific and Limb-Independent Effects

We tested the hypothesis that area 5 of the posterior parietal cortex (PPC) contributes to the planning of visually guided gait modifications. We recorded 121 neurons from the PPC of two cats during a task in which cats needed to process visual input to step over obstacles attached to a moving treadmill belt. During unobstructed locomotion, 64/121 (53%) of cells showed rhythmic activity. During steps over the obstacles, 102/121 (84%) of cells showed a significant change of their activity. Of these, 46/102 were unmodulated during the control task. We divided the 102 task-related cells into two groups on the basis of their discharge when the limb contralateral to the recording site was the first to pass over the obstacle. One group (41/102) was characterized by a brief, phasic discharge as the lead forelimb passed over the obstacle (Step-related cells). These cells were recorded primarily from area 5a. The other group (61/102) showed a progressive increase in activity prior to the onset of the swing phase in the modified limb and frequently diverged from control at least one step cycle before the gait modification (Step-advanced cells). Most of these cells were recorded in area 5b. In both groups, some cells maintained a fixed relationship to the activity of the contralateral forelimb regardless of which limb was the first to pass over the obstacle (limb-specific cells), whereas others changed their phase of activity so that they were always related to activity of the first limb to pass over the obstacle, either contralateral or ipsilateral (limb-independent cells). Limb-independent cells were more common among the Step-advanced cell population. We suggest that both populations of cells contribute to the gait modification and that the discharge characteristics of the Step-advanced cells are compatible with a contribution to the planning of the gait modification.

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.