Independent replication of motor cortex and cervical spinal cord electrical stimulation to promote forelimb motor function after spinal cord injury in rats

Cervical spinal cord injury (SCI) impairs arm and hand function largely by interrupting descending tracts. Most SCI spare some axons at the lesion, including the corticospinal tract (CST), which is critical for voluntary movement. We targeted descending motor connections with paired electrical stimulation of motor cortex and cervical spinal cord in the rat. We sought to replicate the previously published effects of intermittent theta burst stimulation of forelimb motor cortex combined with trans-spinal direct current stimulation placed on the skin over the neck to target the cervical enlargement. We hypothesized that paired stimulation would improve performance in skilled walking and food manipulation (IBB) tasks. Rats received a moderate C4 spinal cord contusion injury (200 kDynes), which ablates the main CST. They were randomized to receive paired stimulation for 10 consecutive days starting 11 days after injury, or no stimulation. Behavior was assessed weekly from weeks 4-7 after injury, and then CST axons were traced. Rats with paired cortical and spinal stimulation achieved significantly better forelimb motor function recovery, as measured by fewer stepping errors on the horizontal ladder task (34 ± 9% in stimulation group vs. 51 ± 18% in control, p = .013) and higher scores on the food manipulation task (IBB, 0-9 score; 7.2 ± 0.8 in stimulated rats vs. 5.2 ± 2.6 in controls, p = .025). The effect size for both tasks was large (Cohen's d = 1.0 and 0.92, respectively). The CST axon length in the cervical spinal cord did not differ significantly between the groups, but there was denser and broader ipsilateral axons distribution distal to the spinal cord injury. The large behavioral effect and replication in an independent laboratory validate this approach, which will be trialed in cats before being tested in people using non-invasive methods.

Neck and trunk representations in the primary motor cortex in rats

[Purpose] The neck and trunk play crucial roles in body movement and are extremely important areas of treatment for physical therapists. However, many aspects of the neural basis of this motor control remain unknown. Therefore, we investigated the distribution and electrophysiological properties of the neck and trunk in the primary motor cortex in rats. [Subjects and Methods] Using intracortical microstimulation, we investigated the somatotopic representation and movements induced by electrical stimulation of the neck and truck areas of the motor cortex in 8 Wistar rats. [Results] We determined that the neck and trunk areas are located separately on the rostral and caudal sides of the motor cortex, respectively. The neck area was significantly larger in size, while the threshold was significantly larger for the trunk area. Stimulation of the neck area with a current higher than the threshold induced movement of the forelimbs, jaw, trunk, and whiskers. However, stimulation of the trunk area did not result in movement in sites other than the trunk. [Conclusion] During movement, the respective activities of the neck and trunk are interdependent. However, due to the separate locations of these areas in the motor cortex, their properties differ greatly.

Contributions of the basal ganglia to action sequence learning and performance

Animals engage in intricately woven and choreographed action sequences that are constructed from trial-and-error learning. The mechanisms by which the brain links together individual actions which are later recalled as fluid chains of behavior are not fully understood, but there is broad consensus that the basal ganglia play a crucial role in this process. This paper presents a comprehensive review of the role of the basal ganglia in action sequencing, with a focus on whether the computational framework of reinforcement learning can capture key behavioral features of sequencing and the neural mechanisms that underlie them. While a simple neurocomputational model of reinforcement learning can capture key features of action sequence learning, this model is not sufficient to capture goal-directed control of sequences or their hierarchical representation. The hierarchical structure of action sequences, in particular, poses a challenge for building better models of action sequencing, and it is in this regard that further investigations into basal ganglia information processing may be informative.

A Surgical Case of Complete Resection of the Focal Cortical and Subcortical Dysplasia in the Motor Cortex

BACKGROUND

Focal cortical dysplasias (FCDs) are highly epileptogenic and frequently associated with medically refractory focal epilepsy. FCDs are frequently located in the frontal lobe, making its complete resection highly challenging when in proximity to the motor cortex.

CASE DESCRIPTION

We report a case of a 25-year-old woman with medically refractory epilepsy secondary to a focal cortical dysplasia in the motor cortex and extending deeply into the subcortical white matter. A detailed presurgical evaluation and invasive electroencephalographic monitoring performed at our epilepsy monitoring unit, along with the use of motor mapping, functional magnetic resonance imaging, diffusion tensor imaging, and the Stealth navigation system resulted in the complete resection of the lesion without a permanent postoperative motor deficit. The patient remained seizure-free at a 63-month follow-up while being maintained on a single antiepileptic drug.

CONCLUSION

A detailed presurgical evaluation, accurate mapping of the functional and dysplastic cortex, and a well-planned tailored and complete surgical resection of the cortical dysplasia can result in a favorable outcome with relatively little risk of postoperative neurologic deficit.

Ventral tegmental area connections to motor and sensory cortical fields in humans

In humans, sensorimotor cortical areas receive relevant dopaminergic innervation—although an anatomic description of the underlying fiber projections is lacking so far. In general, dopaminergic projections towards the cortex originate within the ventral tegmental area (VTA) and are organized in a meso-cortico-limbic system. Using a DTI-based global tractography approach, we recently characterized the superolateral branch of the medial forebrain bundle (slMFB), a prominent pathway providing dopaminergic (and other transmitters) innervation for the pre-frontal cortex (Coenen et al., NeuroImage Clin 18:770–783, 2018). To define the connections between VTA and sensory–motor cortical fields that should contain dopaminergic fibers, we use the slMFB as a key structure to lead our fiber selection procedure: using a similar tracking-seed and tractography algorithm, we describe a dorsal extension of this slMFB that covers sensorimotor fields that are dorsally appended to pre-frontal cortical areas. This “motorMFB”, that connects the VTA to sensorimotor cortical fields, can be further segregated into three sub-bundles with a seed-based fiber-selection strategy: A PFC bundle that is attendant to the pre-frontal cortex, passes the lateral VTA, runs through the border zone between the posterior and lateral ventral thalamic nucleus, and involves the pre- and postcentral gyrus. An MB bundle that is attendant to the mammillary bodies runs directly through the medial VTA, passes the lateral ventral thalamic nucleus, and involves the pre- and postcentral gyrus as well as the supplementary motor area (SMA) and the dorsal premotor cortex (dPMC). Finally, a BC bundle that is attendant to the brainstem and cerebellum runs through the lateral VTA, passes the anterior ventral thalamic nucleus, and covers the SMA, pre-SMA, and the dPMC. We, furthermore, included a fiber tracking of the well-defined dentato-rubro-thalamic tract (DRT) that is known to lie in close proximity with respect to fiber orientation and projection areas. As expected, the tract is characterized by a decussation at the ponto-mesencephal level and a projection covering the superior-frontal and precentral cortex. In addition to the physiological role of these particular bundles, the physiological and pathophysiological impact of dopaminergic signaling within sensorimotor cortical fields becomes discussed. However, some limitations have to be taken into account in consequence of the method: the transmitter content, the directionality, and the occurrence of interposed synaptic contacts cannot be specified.

The motor system's [modest] contribution to speech perception

Recent evidence suggests that the motor system may have a facilitatory role in speech perception during noisy listening conditions. Studies clearly show an association between activity in auditory and motor speech systems, but also hint at a causal role for the motor system in noisy speech perception. However, in the most compelling "causal" studies performance was only measured at a single signal-to-noise ratio (SNR). If listening conditions must be noisy to invoke causal motor involvement, then effects will be contingent on the SNR at which they are tested. We used articulatory suppression to disrupt motor-speech areas while measuring phonemic identification across a range of SNRs. As controls, we also measured phoneme identification during passive listening, mandible gesturing, and foot-tapping conditions. Two-parameter (threshold, slope) psychometric functions were fit to the data in each condition. Our findings indicate: (1) no effect of experimental task on psychometric function slopes; (2) a small effect of articulatory suppression, in particular, on psychometric function thresholds. The size of the latter effect was 1 dB (~5% correct) on average, suggesting, at best, a minor modulatory role of the speech motor system in perception.

Modulation of excitatory but not inhibitory synaptic inputs in the mouse primary motor cortex in the late phase of motor learning

Motor skill training induces functional and structural changes in the primary motor cortex. New dendritic spines are formed with training and the horizontal connections in the layer II/III area of the primary motor cortex are strengthened. Here we investigated the functional synaptic properties of pyramidal neurons following motor skill training. We trained mice on a single forelimb-reaching task for five days and performed whole cell recordings from layer II/III pyramidal neurons in the forelimb representation area of the primary motor cortex in the ipsilateral (untrained) and contralateral (trained) hemispheres in acute brain slices. Success rate in the forelimb-reaching task rapidly improved over the first 3 days and stabilized on subsequent days. After five days of training, a time at which learning has peaked and synaptic strengthening with field potential recordings show enhancement, we observed an increase in mEPSC frequency while increases in mEPSC amplitudes was only observed in 20% of the cells. Increase in excitatory synaptic properties were correlated with improved motor skill. Measurement of miniature IPSC (mIPSC) after five days of training showed no difference in either frequency or amplitude between the trained and untrained hemispheres. Our present results indicate dynamic changes in excitatory but not inhibitory synapses in M1 layer II/III pyramidal neurons at the late stages of motor skill learning.

Superconditioning TMS for examining upper motor neuron function in MND

We used transcranial magnetic stimulation (TMS) of motor cortex, including a novel four-pulse superconditioning (TMSsc) paradigm, in repeated examinations of motor-evoked potentials (MEPs) in eight subjects with motor neuron disease (MND), including seven with amyotrophic lateral sclerosis (ALS). The goals were: (1) to look for evidence of cortical hyperexcitability, including a reduction in short-interval intracortical inhibition (SICI); and (2) to examine the utility of using TMSsc for quantifying upper motor neuron function during MND progression. Testing of abductor pollicis brevis (APB) and tibialis anterior (TA) muscles bilaterally was carried out every 3 months in MND subjects for up to 2 years; results were compared to those from a cohort of 15 control subjects. Measures of SICI were not significantly different between control and MND subjects for either APB or TA muscles. Other measures of cortical excitability, including TMS threshold and MEP amplitude, were consistent with lowered cortical excitability in MND subjects. Certain combinations of superconditioning TMS were capable of causing stronger inhibition or facilitation of MEPs compared to dual-pulse TMS, for both APB and TA target muscles. Moreover, there were multiple cases in which target muscles unresponsive to strong single-pulse TMS, whether at rest or when tested with an active contraction, showed an MEP in response to TMSsc optimized for facilitation. Our findings suggest that a multi-faceted neurophysiologic protocol for examining upper motor neuron function in MND subjects might benefit from inclusion of TMSsc testing.

Norepinephrine and serotonin are required for vagus nerve stimulation directed cortical plasticity

Vagus nerve stimulation (VNS) paired with forelimb training drives robust, specific reorganization of movement representations in the motor cortex. This effect is hypothesized to be mediated by VNS-dependent engagement of neuromodulatory networks. VNS influences activity in the locus coeruleus (LC) and dorsal raphe nucleus (DRN), but the involvement of these neuromodulatory networks in VNS-directed plasticity is unknown. We tested the hypothesis that cortical norepinephrine and serotonin are required for VNS-dependent enhancement of motor cortex plasticity. Rats were trained on a lever pressing task emphasizing proximal forelimb use. Once proficient, all rats received a surgically implanted vagus nerve cuff and cortical injections of either immunotoxins to deplete serotonin or norepinephrine, or vehicle control. Following surgical recovery, rats received half second bursts of 0.8 mA or sham VNS after successful trials. After five days of pairing intracortical microstimulation (ICMS) was performed in the motor cortex contralateral to the trained limb. VNS paired with training more than doubled cortical representations of proximal forelimb movements. Depletion of either cortical norepinephrine or serotonin prevented this effect. The requirement of multiple neuromodulators is consistent with earlier studies showing that these neuromodulators regulate synaptic plasticity in a complimentary fashion.

The effects of conditioning startling acoustic stimulation (SAS) on the corticospinal motor system: a SAS-TMS study

A startling acoustic stimulus (SAS) could cause transient effects on the primary motor cortex and its descending tracts after habituation of reflex responses. In the literature, there is evidence that the effects of SAS depend on the status of M1 excitability and delivery time of SAS. In this study, we aimed to comprehensively investigate the effects of SAS on the excitability of primary motor cortex. Eleven healthy subjects participated in this study. Transcranial magnetic stimulation (TMS) was delivered to the hot spot for left biceps at rest and during isometric right elbow flexion (10, 30, and 60% of their maximum voluntary contraction, MVC). There were three SAS conditions: (1) No SAS; (2) SAS was delivered 50 ms prior to TMS (SAS50); (3) SAS 90 ms prior to TMS (SAS90). For each subject, the induced MEP amplitude was normalized to the largest response at rest with No SAS. Two-way ANOVAs (4 force levels × 3 SAS conditions) with repeated measures were used to determine the differences under different conditions. For the MEP amplitude, there were significant force level effect and FORCE LEVEL × SAS interactions. Specifically, the MEP amplitude increased with force level. Furthermore, post hoc analysis showed that the MEP amplitude reduced during SAS50 and SAS90 compared to No SAS only at rest. Our results provide evidence that a conditioning SAS causes a transient suppression of the corticospinal excitability at rest when it is delivered 50 ms and 90 ms prior to TMS. However, a conditioning SAS has no effect when the corticospinal excitability is already elevated with an external visual target.

Corticospinal excitability for flexor carpi radialis decreases with baroreceptor unloading during intentional co-contraction with opposing forearm muscles

Concurrent activation of antagonistic muscles (co-contraction) is used for stiffening a joint, whereas its neural control under hemodynamic stress (e.g., posture change, high gravity, and hemorrhage) is unknown. Corticospinal excitability during co-contraction may be altered with baroreceptor unloading due to potential modulations in spinal and/or inhibitory pathways (e.g., disynaptic group I inhibition and GABA-mediated intracortical inhibition). The purpose of this study was to understand the effect of baroreceptor unloading on corticospinal excitability during co-contraction in humans. Motor evoked potential and cortical silent period in a wrist flexor muscle were examined using transcranial magnetic stimulation in two groups of healthy young adults. All subjects performed isometric contraction of the wrist flexors (flexion) and co-contraction of the wrist flexors and extensors (co-contraction). Spinal disynaptic inhibition was also assessed with the ratio of H-reflex responses to unconditioned and conditioned electrical stimulations of the peripheral nerves for the muscles. In one of the groups, baroreflex unloading was induced by applying lower body negative pressure. There was no significant effect of baroreflex unloading on cortical silent period or H-reflex measure of disynaptic inhibition. With baroreflex unloading, motor evoked potential area in the flexor carpi radialis was decreased during co-contraction but not during flexion. The results indicated that baroreceptor unloading decreases corticospinal excitability during co-contraction of antagonistic muscles, apparently by influencing neural pathways that were not probed with cortical silent period or spinal disynaptic inhibition.

Large-scale changes in cortical dynamics triggered by repetitive somatosensory electrical stimulation

Background

Repetitive somatosensory electrical stimulation (SES) of forelimb peripheral nerves is a promising therapy; studies have shown that SES can improve motor function in stroke subjects with chronic deficits. However, little is known about how SES can directly modulate neural dynamics. Past studies using SES have primarily used noninvasive methods in human subjects. Here we used electrophysiological recordings from the rodent primary motor cortex (M1) to assess how SES affects neural dynamics at the level of single neurons as well as at the level of mesoscale dynamics.

Methods

We performed acute extracellular recordings in 7 intact adult Long Evans rats under ketamine-xylazine anesthesia while they received transcutaneous SES. We recorded single unit spiking and local field potentials (LFP) in the M1 contralateral to the stimulated arm. We then compared neural firing rate, spike-field coherence (SFC), and power spectral density (PSD) before and after stimulation.

Results

Following SES, the firing rate of a majority of neurons changed significantly from their respective baseline values. There was, however, a diversity of responses; some neurons increased while others decreased their firing rates. Interestingly, SFC, a measure of how a neuron’s firing is coupled to mesoscale oscillatory dynamics, increased specifically in the δ-band, also known as the low frequency band (0.3- 4 Hz). This increase appeared to be driven by a change in the phase-locking of broad-spiking, putative pyramidal neurons. These changes in the low frequency range occurred without a significant change in the overall PSD.

Conclusions

Repetitive SES significantly and persistently altered the local cortical dynamics of M1 neurons, changing both firing rates as well as the SFC magnitude in the δ-band. Thus, SES altered the neural firing and coupling to ongoing mesoscale dynamics. Our study provides evidence that SES can directly modulate cortical dynamics.

Motor cortical plasticity in schizophrenia: A meta-analysis of Transcranial Magnetic Stimulation - Electromyography studies

BACKGROUND

Several lines of investigations converge upon aberrant synaptic plasticity as a potential pathophysiological characteristic of schizophrenia. In vivo experiments using neuromodulatory perturbation techniques like Transcranial Magnetic and Direct Current Stimulation (TMS & tDCS) have been increasingly used to measure 'motor cortical plasticity' in schizophrenia. A systematic quantification of cortical plasticity and its moderators in schizophrenia is however lacking.

METHOD

The PubMed/MEDLINE database was searched for studies up to December 31st, 2017 that examined case-control experiments comparing neuromodulation following single-session of TMS or tDCS. The primary outcome was the standardized mean difference for differential changes in motor evoked potential (MEP) amplitudes measured with single-pulse TMS (MEP Δ) between patients and healthy subjects following TMS or tDCS. After examining heterogeneity, meta-analyses were performed using fixed effects models.

RESULTS

A total of 16 datasets comparing cortical plasticity (MEP Δ) between 189 schizophrenia patients and 187 healthy controls were included in the meta-analysis. Patients demonstrated diminished MEP Δ with effect sizes (Cohen's d) ranging from 0.66 (LTP-like plasticity) to 0.68 (LTD-like plasticity). Heterosynaptic plasticity studies demonstrated a greater effect size (0.79) compared to homosynaptic plasticity studies (0.62), though not significant (P = 0.43). Clinical, perturbation protocol- and measurement-related factors, and study quality did not significantly moderate the aberrant plasticity demonstrated in schizophrenia.

CONCLUSIONS

Schizophrenia patients demonstrate diminished LTP- and LTD-like motor cortical plasticity, which is not influenced by the various clinical and experimental protocol related confounders. These consistent findings should encourage the use of perturbation-based biomarkers to characterize illness trajectories and treatment response.

Multimodal Surgical Treatment of High-Grade Gliomas in the Motor Area: The Impact of the Combination of Navigated Transcranial Magnetic Stimulation and Fluorescein-Guided Resection

BACKGROUND

Fluorescein-guided surgery of high-grade gliomas (HGGs) increases the extent of tumor resection but its efficacy has been questioned, especially for tumors located close to functional networks. In these cases, navigated transcranial magnetic stimulation (nTMS) may be used to plan and guide a safe resection. The aim of this study was to assess the impact of these techniques combined with intraoperative neurophysiologic mapping (IONM) to achieve the maximal safe resection of tumors located in the motor area.

METHODS

We collected data of patients operated using a multimodal combination of sodium fluorescein-guided resection, nTMS motor planning, and IONM for HGGs in the motor area. The nTMS planning accuracy, extent of resection, and postoperative motor and functional status were compared with a matched control group of patients with HGG operated on only by IONM-guided resection.

RESULTS

Forty-one patients treated by multimodal approach (group A) and 41 controls (group B) were included. The nTMS-based planning reliably identified the tumor/motor pathway spatial relationship (accuracy, 92.68%). We obtained in group A versus controls a higher gross total resection rate (73.17% vs. 51.22%; P = 0.04), and a reduction of cases with new permanent motor deficits (9.75% vs. 29.27%; P = 0.04) or Karnofsky Performance Status worsening (12.19% vs. 31.71%; P = 0.03).

CONCLUSIONS

This study supports the role of the combination of sodium fluorescein-guided resection and nTMS-based planning for surgery of HGGs close to the motor pathway. This multimodal approach in combination with IONM may lead to customized preoperative planning, increased extent of resection, and improved functional outcome, compared with standard IONM-guided surgery.

Time in the motor cortex: Motor evoked potentials track foreperiod duration without concurrent movement

Transcranial magnetic stimulation (TMS) allows for the monitoring of motor cortex dynamics in preparation for response. Using this method, it has previously been shown that motor evoked potentials (MEPs) are suppressed as a response approaches. In the current article, we applied TMS while participants either relaxed or contracted their first dorsal interosseous muscle. We varied the time at which TMS was applied, however, unlike previous studies, no participant response was required. Using this method, we provide evidence that MEPs systematically decrease with the duration of the trial, while inhibition is not similarly affected. Further, we found some evidence that MEPs are inversely proportional to the duration of the prior trial. These findings have ramifications for other research interested in the application of TMS, especially when used across multiple possible points in a trial. Further, this finding shows a role for the motor cortex in timing more broadly.

The effect of experimental pain on short-interval intracortical inhibition with multi-locus transcranial magnetic stimulation

Chronic neuropathic pain is known to alter the primary motor cortex (M1) function. Less is known about the normal, physiological effects of experimental neurogenic pain on M1. The objective of this study is to determine how short-interval intracortical inhibition (SICI) is altered in the M1 representation area of a muscle exposed to experimental pain compared to SICI of another muscle not exposed to pain. The cortical representation areas of the right abductor pollicis brevis (APB) and biceps brachii (BB) muscles of 11 subjects were stimulated with a multi-locus transcranial magnetic stimulation device while the resulting motor-evoked potentials (MEPs) were recorded with electromyography. Single- and paired-pulse TMS was administered in seven conditions, including one with the right hand placed in cold water. The stimulation intensity for the conditioning pulses in the paired-pulse examination was 80% of the resting motor threshold (RMT) of the stimulated site and 120% of RMT for both the test and single pulses. The paired-pulse MEP amplitudes were normalized with the mean amplitude of the single-pulse MEPs of the same condition and muscle. SICI was compared between conditions. After the cold pain, the normalized paired-pulse MEP amplitudes decreased in APB, but not in BB, indicating that SICI was potentially increased only in the cortical area of the muscle subjected to pain. These data suggest that SICI is increased in the M1 representation area of a hand muscle shortly after exposure to pain has ended, which implies that short-lasting pain can alter the inhibitory balance in M1.

Motor cortex and pain control: exploring the descending relay analgesic pathways and spinal nociceptive neurons in healthy conscious rats

Motor cortex stimulation (MCS) is an effective therapy for refractory neuropathic pain. MCS increases the nociceptive threshold in healthy rats via endogenous opioids, inhibiting thalamic nuclei and activating the periaqueductal gray. It remains unclear how the motor cortex induces top-down modulation of pain in the absence of persistent pain. Here, we investigated the main nuclei involved in the descending analgesic pathways and the spinal nociceptive neurons in rats that underwent one session of MCS and were evaluated with the paw pressure nociceptive test. The pattern of neuronal activation in the dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), locus coeruleus (LC), and dorsal horn of the spinal cord (DHSC) was assessed by immunoreactivity (IR) for Egr-1 (a marker of activated neuronal nuclei). IR for serotonin (5HT) in the DRN and NRM, tyrosine hydroxylase (TH) in the LC, and substance P (SP) and enkephalin (ENK) in the DHSC was also evaluated. MCS increased the nociceptive threshold of the animals; this increase was accompanied by activation of the NRM, while DRN activation was unchanged. However, cortical stimulation induced an increase in 5HT-IR in both serotonergic nuclei. MCS did not change the activation pattern or TH-IR in the LC, and it inhibited neuronal activation in the DHSC without altering SP or ENK-IR. Taken together, our results suggest that MCS induces the activation of serotonergic nuclei as well as the inhibition of spinal neurons, and such effects may contribute to the elevation of the nociceptive threshold in healthy rats. These results allow a better understanding of the circuitry involved in the antinociceptive top-down effect induced by MCS under basal conditions, reinforcing the role of primary motor cortex in pain control.

A hierarchy of corticospinal plasticity in human hand and forearm muscles

Key points

Pairing stimulation of a finger flexor or extensor muscle at the motor point with transcranial magnetic stimulation (TMS) of the motor cortex generated plastic changes in motor output.

Increases in output were greater in intrinsic hand muscles than in the finger flexor. No changes occurred in the finger extensor. This gradient was seen irrespective of which muscle was stimulated paired with transcranial magnetic stimulation.

Intermittent theta‐burst stimulation also produced increases in output, although these were similar across muscles.

We suggest that intrinsic hand and flexor muscles have a higher potential to show plasticity than extensors, although only when plasticity is induced by sensory input. This may relate to differences seen in recovery of function in these muscles after injury, such as post‐stroke.

Abstract

The ability of the motor system to show plastic change underlies skill learning and also permits recovery after injury. One puzzling observation is that, after stroke, upper limb flexor muscles show good recovery but extensors remain weak, with this being a major contributor to residual disability. We hypothesized that there might be differences in potential for plasticity across hand and forearm muscles. In the present study, we investigated this using two protocols based on transcranial magnetic brain stimulation (TMS) in healthy human subjects. Baseline TMS responses were recorded from two intrinsic hand muscles: flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). In the first study, paired associative stimulation (PAS) was delivered by pairing motor point stimulation of FDS or EDC with TMS. Responses were then remeasured. Increases were greatest in the hand muscles, smaller in FDS and non‐significant in EDC, irrespective of whether stimulation of FDS or EDC was used. In the second study, intermittent theta‐burst rapid rate TMS was applied instead of PAS. In this case, all muscles showed similar increases in TMS responses. We conclude that the potential to show plastic changes in motor cortical output has the gradient: hand muscles > flexors > extensors. However, this was only seen in a protocol that requires integration of sensory input (PAS) and not when plasticity was induced purely by cortical stimulation (rapid rate TMS). This observation may relate to why functional recovery tends to favour flexor and hand muscles over extensors.

Pairing stimulation of a finger flexor or extensor muscle at the motor point with transcranial magnetic stimulation (TMS) of the motor cortex generated plastic changes in motor output.

Increases in output were greater in intrinsic hand muscles than in the finger flexor. No changes occurred in the finger extensor. This gradient was seen irrespective of which muscle was stimulated paired with transcranial magnetic stimulation.

Intermittent theta‐burst stimulation also produced increases in output, although these were similar across muscles.

We suggest that intrinsic hand and flexor muscles have a higher potential to show plasticity than extensors, although only when plasticity is induced by sensory input. This may relate to differences seen in recovery of function in these muscles after injury, such as post‐stroke.

Use of illicit amphetamines is associated with long-lasting changes in hand circuitry and control

OBJECTIVE

The study aim was to determine if use of illicit amphetamines or ecstasy is associated with abnormal excitability of the corticomotoneuronal pathway and manipulation of novel objects with the hand.

METHODS

Three groups of adults aged 18-50 years were investigated: individuals with a history of illicit amphetamine use, individuals with a history of ecstasy use but minimal use of other stimulants, and non-drug users. Transcranial magnetic stimulation was delivered to the motor cortex and the electromyographic response (motor evoked potential; MEP) was recorded from a contralateral hand muscle. Participants also gripped and lifted a novel experimental object consisting of two strain gauges and an accelerometer.

RESULTS

Resting MEP amplitude was larger in the amphetamine group (6M, 6F) than the non-drug and ecstasy groups (p < 0.005) in males but not females. Overestimation of grip force during manipulation of a novel object was observed in the amphetamine group (p = 0.020) but not the ecstasy group.

CONCLUSIONS

History of illicit amphetamine use, in particular methamphetamine, is associated with abnormal motor cortical and/or corticomotoneuronal excitability in males and abnormal manipulation of novel objects in both males and females.

SIGNIFICANCE

Abnormal excitability and hand function is evident months to years after cessation of illicit amphetamine use.

Combined endogenous and exogenous disinhibition of intracortical circuits augments plasticity induction in the human motor cortex

BACKGROUND

Motor imagery (MI) engages cortical areas in the human brain similar to motor practice. Corticospinal excitability (CSE) is facilitated during but not after MI practice. We hypothesized that lasting CSE changes could be achieved by associatively pairing this endogenous modulation with exogenous stimulation of the same intracortical circuits.

METHODS

We combined MI with a disinhibition protocol (DIS) targeting intracortical circuits by paired-pulse repetitive transcranial magnetic stimulation in one main and three subsequent experiments. The follow-up experiments were applied to increase effects, e.g., by individualizing inter-stimulus intervals, adding neuromuscular stimulation and expanding the intervention period. CSE was captured during (online) and after (offline) the interventions via input-output changes and cortical maps of motor evoked potentials. A total of 35 healthy subjects (mean age 26.1 ± 2.6 years, 20 females) participated in this study.

RESULTS

A short intervention (48 stimuli within ∼90s) increased CSE. This plasticity developed rapidly, was associative (with MI_on, but not MI_off or REST) and persisted beyond the intervention period. Follow-up experiments revealed the relevance of individualizing inter-stimulus intervals and of consistent inter-burst periods for online and offline effects, respectively. Expanding this combined MI/DIS intervention to 480 stimuli amplified the sustainability of CSE changes. When concurrent neuromuscular electrical stimulation was applied, the plasticity induction was cancelled.

CONCLUSIONS

This novel associative stimulation protocol augmented plasticity induction in the human motor cortex within a remarkably short period of time and in the absence of active movements. The combination of endogenous and exogenous disinhibition of intracortical circuits may provide a therapeutic backdoor when active movements are no longer possible, e.g., for hand paralysis after stroke.

Motor corticospinal excitability: a novel facet of pain modulation?

Introduction

Increase in excitability of the primary motor cortex (M1) is associated with pain inhibition by analgesics, which is, in turn, associated with the psychophysical antinociceptive pain modulation profile. However, the relationship between neurophysiological M1 excitability and psychophysical pain modulation has not yet been explored.

Objectives

We aim to study these relationships in healthy subjects.

Methods

Forty-one young healthy subjects (22 women) underwent a wide battery of psychophysical testing that included conditioned pain modulation (CPM) and pain temporal summation, and a transcranial magnetic stimulation neurophysiological assessment of the motor corticospinal excitability, including resting motor threshold, motor-evoked potentials (MEPs), and cortical silent period.

Results

Increased motor corticospinal excitability in 2 parameters was associated with more efficient CPM: (1) higher MEP amplitude (r = -0.574; P _{_Bonferroni} = 0.02) and (2) longer MEP duration (r = -0.543; P _{_Bonferroni} = 0.02). The latter also correlated with the lower temporal summation magnitude (r = -0.421; P = 0.007); however, on multiplicity adjustment, significance was lost.

Conclusions

Increased corticospinal excitability of the primary motor cortex is associated with more efficient inhibitory pain modulation as assessed by CPM, in healthy subjects. Motor-evoked potential amplitude and duration may be considered as an additional, objective and easy to measure parameter to allow for better individual assessment of pain modulation profile.

Surgery for epilepsy in the primary motor cortex: A critical review

Surgical resection of the epileptogenic zone within the frontal lobe can be a very effective treatment for medically refractory epilepsy originating from this area. While much of the frontal lobe consists of highly eloquent tissue, surgery is not necessarily contraindicated as long as the epileptogenic zone is well-localized and the tissue resected is limited. Resection of the primary motor cortex was described by Victor Horsley in the 19th century and was used frequently in the early 20th century for a variety of neurological disorders including epilepsy; improvements in surgical techniques and mapping has led to a resurgence of its use in the past few decades. Although many surgeons are hesitant to resect tissue adjacent to the primary hand area based on fears of new motor deficits, there is extensive evidence that focal resections are well-tolerated over the long-term with residual weakness that is fairly mild: some patients experience postoperative weakness, including hemiparesis, but a stereotypical recovery of strength from proximal to distal muscles occurs over months, and only one quarter will have a permanent neurologic deficit, usually consisting of difficulty with fine motor movements. The main alternative to surgical resection is subpial transection, characterized by a small decrease in postoperative deficits and significantly worse seizure outcomes. The treatment of patients with seizures originating from this region requires a solid understanding of the structural and functional anatomy of the frontal lobe.

Direction of TDCS current flow in human sensorimotor cortex influences behavioural learning

Background

Recent studies have shown that neurophysiological outcomes of transcranial direct current stimulation (TDCS) are influenced by current flow in brain regions between the electrodes, and in particular the orientation of current flow relative to the cortical surface.

Objective

We asked whether the directional effects of TDCS on physiological measures in the motor system would also be observed on motor behaviours.

Methods

We applied TDCS during the practice of a ballistic movement task to test whether it affected learning or the retention of learning 48 h later. TDCS electrodes were oriented perpendicular to the central sulcus and two current orientations were used (posterior-anterior, TDCS_PA; and anterior-posterior, TDCS_AP). Transcranial magnetic stimulation (TMS) was used to assess whether changes in corticospinal excitability reflected any behavioural changes.

Results

Directional TDCS_AP impaired the retention of learning on the ballistic movement task compared to TDCS_PA and a sham condition. Although TDCS_PA had no effect on learning or retention, it blocked the typical increase in corticospinal excitability after a period of motor practice.

Conclusions

Our results extend on previous reports of TDCS producing directionally specific changes in neurophysiological outcomes by showing that current direction through a cortical target also impacts upon behavioural outcomes. In addition, changes in corticospinal excitability after a period of motor practice are not causally linked to behavioural learning.

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TDCS current direction influences neurophysiological outcomes.

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We show that it also influences behavioural outcomes.

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Behavioural learning is not linked to changes in corticospinal excitability.