Salience network functional connectivity is spatially heterogeneous across sensorimotor cortex in healthy humans

The salience network is responsive during a range of conditions requiring immediate behavioral responses, including pain processing. Resting-state functional connectivity of the salience network to the sensorimotor cortex is altered in chronic pain. However, little is understood about their fundamental communication in the absence of pain. In this study, we mapped salience network resting-state functional connectivity across sensorimotor cortex in healthy individuals. Using electromyography and task-based functional magnetic resonance imaging (fMRI), we first localized distinct regions-of-interest across sensorimotor cortex in medial (gluteal), intermediate (shoulder), and lateral (hand) areas. We then used resting-state fMRI for two cohorts (primary and replication) of healthy individuals from public repositories to map salience network resting-state functional connectivity across sensorimotor cortex. Both the primary and replication cohorts exhibited significant heterogeneity in salience network resting-state functional connectivity across the sensorimotor regions-of-interest. Using a cortical flatmap to visualize the entire sensorimotor surface, we observed similar heterogeneity in both cohorts. In general, the somatotopic representation of proximal body regions (trunk/face) had higher salience network resting-state functional connectivity compared to distal body regions (upper/lower limbs). We conclude that sensorimotor cortex is spatially heterogeneous in its interaction with the salience network in healthy individuals.

Cortical plasticity in phantom limb pain: A fMRI study on the neural correlates of behavioral clinical manifestations

The neural mechanism of phantom limb pain (PLP) is related to the intense brain reorganization process implicating plasticity after deafferentation mostly in sensorimotor system. There is a limited understanding of the association between the sensorimotor system and PLP. We used a novel task-based functional magnetic resonance imaging (fMRI) approach to (1) assess neural activation within a-priori selected regions-of-interested (motor cortex [M1], somatosensory cortex [S1], and visual cortex [V1]), (2) quantify the cortical representation shift in the affected M1, and (3) correlate these changes with baseline clinical characteristics. In a sample of 18 participants, we found a significantly increased activity in M1 and S1 as well as a shift in motor cortex representation that was not related to PLP intensity. In an exploratory analyses (not corrected for multiple comparisons), they were directly correlated with time since amputation; and there was an association between increased activity in M1 with a lack of itching sensation and V1 activation was negatively correlated with PLP. Longer periods of amputation lead to compensatory changes in sensory-motor areas; and itching seems to be a protective marker for less signal changes. We confirmed that PLP intensity is not associated with signal changes in M1 and S1 but in V1.

Paired-pulse navigated TMS is more effective than single-pulse navigated TMS for mapping upper extremity muscles in brain tumor patients

OBJECTIVE

Single-pulse navigated transcranial magnetic stimulation (sp-nTMS) is used for presurgical motor mapping in patients with motor-eloquent lesions. However, recently introduced paired-pulse nTMS (pp-nTMS) with biphasic pulses could improve motor mapping.

METHODS

Thirty-four patients (mean age: 56.0 ± 12.7 years, 53.0% high-grade glioma) with motor-eloquent lesions underwent motor mapping of upper extremity representations and nTMS-based tractography of the corticospinal tract (CST) by both sp-nTMS and pp-nTMS with biphasic pulses for the tumor-affected hemisphere before resection.

RESULTS

In three patients (8.8%), conventional sp-nTMS did not provide motor-positive points, in contrast to pp-nTMS that was capable of generating motor maps in all patients. Good concordance between pp-nTMS and sp-nTMS in the spatial location of motor hotspots and center of gravity (CoG) as well as for CST tracking was observed, with pp-nTMS leading to similar motor map volumes (585.0 ± 667.8 vs. 586.8 ± 204.2 mm³, p = 0.9889) with considerably lower resting motor thresholds (35.0 ± 8.8 vs. 32.8 ± 7.6% of stimulator output, p = 0.0004).

CONCLUSIONS

Pp-nTMS with biphasic pulses may provide motor maps even in highly demanding cases with tumor-affected motor structures or edema, using lower stimulation intensity compared to sp-nTMS.

SIGNIFICANCE

Pp-nTMS with biphasic pulses could replace standardly used sp-nTMS for motor mapping and may be safer due to lower stimulation intensity.

Prodromal local sleep disorders in a rat model of Parkinson's disease cholinopathy, hemiparkinsonism and hemiparkinsonism with cholinopathy

We investigated the prodromal alterations of local sleep, particularly the motor cortical and hippocampal sleep, along with spontaneous locomotor activity in the rat models of Parkinson's disease (PD). We performed our experiments in adult, male Wistar rats, chronically implanted for sleep recording and divided into four experimental groups: the control (implanted controls), the bilateral pedunculopontine tegmental nucleus (PPT) lesions (PD cholinopathy), the unilateral substantia nigra pars compacta (SNpc) lesions (hemiparkinsonism) and the unilateral SNpc/bilateral PPT lesions (hemiparkinsonism with PD cholinopathy). We followed their sleep, basal locomotor activity and spatial habituation for 14 days following the surgical procedures. Severe prodromal local sleep disturbances in the hemiparkinsonian rats were expressed as sleep fragmentation and distinct local NREM/REM EEG microstructure alterations in both the motor cortex and the hippocampus. Alongside the state-unrelated role of the dopaminergic control of theta oscillations and NREM/REM related sigma and beta oscillations, we demonstrated that the REM neurochemical regulatory substrate is particularly important in the dopaminergic control of beta oscillations. In addition, hippocampal prodromal sleep disorders in the hemiparkinsonian rats were expressed as NREM/REM fragmentation and the opposite impact of dopaminergic versus cholinergic control of the NREM delta and beta oscillation amplitudes in the hippocampus, likewise in the motor cortex versus the hippocampus. All these distinct prodromal local sleep disorders and the dopaminergic vs. cholinergic impact on NREM/REM EEG microstructure alterations are of fundamental importance for the further development and follow-up of PD-modifying therapies, and for the identification of patients who are at risk of developing PD.

Participation of ipsilateral cortical descending influences in bimanual wrist movements in humans

There are contralateral and less studied ipsilateral (i), indirect cortical descending projections to motoneurons (MNs). We compared ipsilateral cortical descending influences on MNs of wrist flexors by applying transcranial magnetic stimulation (TMS) over the right primary motor cortex at actively maintained flexion and extension wrist positions in uni- and bimanual tasks in right-handed participants (n = 23). The iTMS response includes a short latency (~ 25 ms) motor evoked potential (iMEP), a silent period (iSP) and a long latency (~ 60 ms) facilitation called rebound (iRB). We also investigated whether the interaction between the two hands while holding an object in a bimanual task involves ipsilateral cortical descending influences. In the unimanual task, iTMS responses in the right wrist flexors were unaffected by changes in wrist position. In the bimanual task with an object, iMEPs in the right wrist flexors were larger when the ipsilateral wrist was in flexion compared to extension. Without the object, only iRB were larger when the ipsilateral wrist was extended. Thus, ipsilateral cortical descending influences on MNs were modulated only in bimanual tasks and depended on how the two hands interacted. It is concluded that the left and right cortices cooperate in bimanual tasks involving holding an object with both hands, with possible involvement of oligo- and poly-synaptic, as well as transcallosal projections to MNs. The possible involvement of spinal and transcortical stretch and cutaneous reflexes in bimanual tasks when holding an object is discussed in the context of the well-established notion that indirect, referent control underlies motor actions.

Transcranial random noise stimulation over the primary motor cortex in PD-MCI patients: a crossover, randomized, sham-controlled study

Mild cognitive impairment (MCI) is a very common non-motor feature of Parkinson’s disease (PD) and the non-amnestic single-domain is the most frequent subtype. Transcranial random noise stimulation (tRNS) is a non-invasive technique, which is capable of enhancing cortical excitability. As the main contributor to voluntary movement control, the primary motor cortex (M1) has been recently reported to be involved in higher cognitive functioning. The aim of this study is to evaluate the effects of tRNS applied over M1 in PD-MCI patients in cognitive and motor tasks. Ten PD-MCI patients, diagnosed according to the Movement Disorder Society, Level II criteria for MCI, underwent active (real) and placebo (sham) tRNS single sessions, at least 1 week apart. Patients underwent cognitive (Digit Span Forward and Backward, Digit Symbol, Visual Search, Letter Fluency, Stroop Test) and motor assessments (Unified Parkinson’s Disease Rating Scale [UPDRS-ME], specific timed trials for bradykinesia, 10-m walk and Timed up and go tests) before and after each session. A significant improvement in motor ability (UPDRS-ME and lateralized scores, ps from 0.049 to 0.003) was observed after real versus sham tRNS. On the contrary, no significant differences were found in other motor tasks and cognitive assessment both after real and sham stimulations. These results confirm that tRNS is a safe and effective tool for improving motor functioning in PD-MCI. Future studies using a multisession tRNS applied over multitargeted brain areas (i.e., dorsolateral prefrontal cortex and M1) are required to clarify the role of tRNS regarding rehabilitative intervention in PD.

Altered motor system function in post-concussion syndrome as assessed via transcranial magnetic stimulation

•

Study examining corticospinal and cortical activity in post-concussion.

•

Reduction in GABAB-mediated inhibition observed.

•

These changes were associated with depression-related symptoms.

Objective

It is unclear why specific individuals incur chronic symptoms following a concussion. This exploratory research aims to identify and characterize any neurophysiological differences that may exist in motor cortex function in post-concussion syndrome (PCS).

Methods

Fifteen adults with PCS and 13 healthy, non-injured adults were tested. All participants completed symptom questionnaires, and transcranial magnetic stimulation (TMS) was used to measure intracortical and transcallosal excitability and inhibition in the dominant motor cortex.

Results

Cortical silent period (p = 0.02, g = 0.96) and ipsilateral silent period (p = 0.04, g = 0.78) were shorter in the PCS group compared to the control group which may reflect reduced GABA-mediated inhibition in PCS. Furthermore, increased corticomotor excitability was observed in the left hemisphere but not the right hemisphere.

Conclusions

These data suggest that persistent neurophysiological differences are present in those with PCS. The exact contributing factors to such changes remain to be investigated by future studies.

Significance

This study provides novel evidence of lasting neurophysiological changes in PCS.

Effects of tDCS on spontaneous spike activity in a healthy ambulatory rat model

BACKGROUND

The neurophysiological effects of transcranial direct current stimulation (tDCS) are typically described with respect to changes in cortical excitability, defined by using transcranial magnetic stimulation pulses to determine changes in motor evoked potentials. However, how individual cortical neurons change firing patterns under the influence of tDCS is largely unknown. While the relatively weak currents produced in the brain by tDCS may not be adequate to directly depolarize neuronal membranes, ongoing neuronal activity, combined with subthreshold changes in membrane polarization might be sufficient to alter the threshold for neural firing.

OBJECTIVES

The purpose of this study was to determine the effects of tDCS on neurophysiological activity in motor cortex of freely moving, healthy rats.

METHODS

In nine healthy, ambulatory rats, each studied under six different stimulation conditions varying in current intensity (maximum current density = 39.8 A/m² at 0.4 mA) and polarity (anodal or cathodal), neural activity was analyzed in response to 20 min of tDCS applied through bone screws insulated from the overlying scalp.

RESULTS

After analysis of 480 multi-unit channels that satisfied a rigid set of neurophysiological criteria, we found no systematic effect of tDCS stimulation condition on firing rate or firing pattern. Restricting the analysis to the most responsive units, subtle, but statistically significant changes occurred only in the highest intensity anodal condition.

CONCLUSIONS

These results confirm that at current densities typically used in human or animal tDCS studies, observed effects of tDCS are likely to occur via mechanisms other than direct neuronal depolarization.

Orientation-selective and directional deep brain stimulation in swine assessed by functional MRI at 3T

Functional MRI (fMRI) has become an important tool for probing network-level effects of deep brain stimulation (DBS). Previous DBS-fMRI studies have shown that electrical stimulation of the ventrolateral (VL) thalamus can modulate sensorimotor cortices in a frequency and amplitude dependent manner. Here, we investigated, using a swine animal model, how the direction and orientation of the electric field, induced by VL-thalamus DBS, affects activity in the sensorimotor cortex. Adult swine underwent implantation of a novel 16-electrode (4 rows × 4 columns) directional DBS lead in the VL thalamus. A within-subject design was used to compare fMRI responses for (1) directional stimulation consisting of monopolar stimulation in four radial directions around the DBS lead, and (2) orientation-selective stimulation where an electric field dipole was rotated 0°−360° around a quadrangle of electrodes. Functional responses were quantified in the premotor, primary motor, and somatosensory cortices. High frequency electrical stimulation through leads implanted in the VL thalamus induced directional tuning in cortical response patterns to varying degrees depending on DBS lead position. Orientation-selective stimulation showed maximal functional response when the electric field was oriented approximately parallel to the DBS lead, which is consistent with known axonal orientations of the cortico-thalamocortical pathway. These results demonstrate that directional and orientation-selective stimulation paradigms in the VL thalamus can tune network-level modulation patterns in the sensorimotor cortex, which may have translational utility in improving functional outcomes of DBS therapy.

Machine Learning for Neural Decoding

Despite rapid advances in machine learning tools, the majority of neural decoding approaches still use traditional methods. Modern machine learning tools, which are versatile and easy to use, have the potential to significantly improve decoding performance. This tutorial describes how to effectively apply these algorithms for typical decoding problems. We provide descriptions, best practices, and code for applying common machine learning methods, including neural networks and gradient boosting. We also provide detailed comparisons of the performance of various methods at the task of decoding spiking activity in motor cortex, somatosensory cortex, and hippocampus. Modern methods, particularly neural networks and ensembles, significantly outperform traditional approaches, such as Wiener and Kalman filters. Improving the performance of neural decoding algorithms allows neuroscientists to better understand the information contained in a neural population and can help to advance engineering applications such as brain–machine interfaces. Our code package is available at github.com/kordinglab/neural_decoding.

Neck rotation modulates motor-evoked potential duration of proximal muscle cortical representations in healthy adults

Transcranial magnetic stimulation (TMS) produces motor-evoked potentials (MEP) used to infer changes in corticomotor excitability. In humans, neck rotation can probe reticulospinal input on corticomotor output. This study investigated the effect of neck rotation on MEP duration in a proximal and distal upper limb muscle and compared responses between rest and preactivation. Single-pulse TMS to motor cortex was used to evoke MEPs at two stimulus intensities in 18 healthy adults (20-40 years). Surface electromyography recorded MEPs from the non-dominant biceps brachii (BB) and first dorsal interosseous (FDI). Participants were seated with the target muscle at rest or 10% preactivated, and head rotated ipsilateral, contralateral, or in neutral position. The primary outcome was MEP tail, defined as the mean difference in MEP duration between active and rest trials. Secondary outcomes were MEP duration and amplitude. MEP tail was modulated by neck rotation in the proximal BB (P = 0.03) but not distal FDI (P > 0.19), with shorter duration during ipsilateral or contralateral rotation relative to neutral. In a neutral neck position, MEP duration was prolonged by muscle preactivation and higher TMS intensities in the FDI and BB (P < 0.03). Neck rotation attenuated the prolongation of MEP duration during preactivation in the BB, but not the FDI. Neck rotation had no effect on MEP amplitude for either muscle (P > 0.05). Modulation of the late portion of the MEP by rotation of the neck could indicate subcortical projections to alpha-motoneuron pools are stronger in proximal than distal upper limb muscles. These findings may have relevance for using MEP duration as a neural biomarker in neurological diseases.

Optogenetic stimulation of the motor cortex alleviates neuropathic pain in rats of infraorbital nerve injury with/without CGRP knock-down

BACKGROUND

Previous studies have reported that electrical stimulation of the motor cortex is effective in reducing trigeminal neuropathic pain; however, the effects of optical motor cortex stimulation remain unclear.

OBJECTIVE

The present study aimed to investigate whether optical stimulation of the primary motor cortex can modulate chronic neuropathic pain in rats with infraorbital nerve constriction injury.

METHODS

Animals were randomly divided into a trigeminal neuralgia group, a sham group, and a control group. Trigeminal neuropathic pain was generated via constriction of the infraorbital nerve and animals were treated via selective inhibition of calcitonin gene-related peptide in the trigeminal ganglion. We assessed alterations in behavioral responses in the pre-stimulation, stimulation, and post-stimulation conditions. In vivo extracellular recordings were obtained from the ventral posteromedial nucleus of the thalamus, and viral and α-CGRP expression were investigated in the primary motor cortex and trigeminal ganglion, respectively.

RESULTS

We found that optogenetic stimulation significantly improved pain behaviors in the trigeminal neuralgia animals and it provided more significant improvement with inhibited α-CGRP state than active α-CGRP state. Electrophysiological recordings revealed decreases in abnormal thalamic firing during the stimulation-on condition.

CONCLUSION

Our findings suggest that optical motor cortex stimulation can alleviate pain behaviors in a rat model of trigeminal neuropathic pain. Transmission of trigeminal pain signals can be modulated via knock-down of α-CGRP and optical motor cortex stimulation.

Automatic identification of atypical clinical fMRI results

Purpose

Functional MRI is not routinely used for neurosurgical planning despite potential important advantages, due to difficulty of determining quality. We introduce a novel method for objective evaluation of fMRI scan quality, based on activation maps. A template matching analysis (TMA) is presented and tested on data from two clinical fMRI protocols, performed by healthy controls in seven clinical centers. Preliminary clinical utility is tested with data from low-grade glioma patients.

Methods

Data were collected from 42 healthy subjects from seven centers, with standardized finger tapping (FT) and verb generation (VG) tasks. Copies of these “typical” data were deliberately analyzed incorrectly to assess feasibility of identifying them as “atypical.” Analyses of the VG task administered to 32 tumor patients assessed sensitivity of the TMA method to anatomical abnormalities.

Results

TMA identified all atypical activity maps for both tasks, at the cost of incorrectly classifying 3.6 (VG)–6.5% (FT) of typical maps as atypical. For patients, the average TMA was significantly higher than atypical healthy scans, despite localized anatomical abnormalities caused by a tumor.

Conclusion

This study supports feasibility of TMA for objective identification of atypical activation patterns for motor and verb generation fMRI protocols. TMA can facilitate the use and evaluation of clinical fMRI in hospital settings that have limited access to fMRI experts. In a clinical setting, this method could be applied to automatically flag fMRI scans showing atypical activation patterns for further investigation to determine whether atypicality is caused by poor scan data quality or abnormal functional topography.

Spatial extent of cortical motor hotspot in navigated transcranial magnetic stimulation

BACKGROUND

Motor mapping with navigated transcranial magnetic stimulation (nTMS) requires defining a "hotspot", a stimulation site consistently producing the highest-amplitude motor-evoked potentials (MEPs). The exact location of the hotspot is difficult to determine, and the spatial extent of high-amplitude MEPs usually remains undefined due to MEP variability and the spread of the TMS-induced electric field (E-field). Therefore, here we aim to define the hotspot as a sub-region of a motor map.

NEW METHOD

We analyzed MEP amplitude distributions in motor mappings of 30 healthy subjects in two orthogonal directions on the motor cortex. Based on the widths of these distributions, the hotspot extent was estimated as an elliptic area. In addition, E-field distributions induced by motor map edge stimulations were simulated for ten subjects, and the E-field attenuation was analyzed to obtain another estimate for hotspot extent.

RESULTS

The median MEP-based hotspot area was 13 mm² (95% confidence interval (CI) = [10, 18] mm²). The mean E-field-based hotspot area was 26 mm² (95% CI = [13, 38] mm²).

COMPARISON WITH EXISTING METHODS

In contrast to the conventional hotspot, the new definition considers its spatial extent, indicating the most easily excited area where subsequent nTMS stimuli should be targeted for maximal response. The E-field-based hotspot provides an estimate for the extent of cortical structures where the E-field is close to its maximum.

CONCLUSIONS

The nTMS hotspot should be considered as an area rather than a single qualitatively defined spot due to MEP variability and E-field spread.

Elite competitive swimmers exhibit higher motor cortical inhibition and superior sensorimotor skills in a water environment

Motor skill learning leads to task-related contextual behavioral changes that are underpinned by neuroplastic cortical reorganization. Short-term training induces environment-related contextual behavioral changes and neuroplastic changes in the primary motor cortex (M1). However, it is unclear whether environment-related contextual behavioral changes persist after long-term training and how cortical plastic changes are involved in behavior. To address these issues, we examined 14 elite competitive swimmers and 14 novices. We hypothesized that the sensorimotor skills of swimmers would be higher in a water environment than those of novices, and the recruitment of corticospinal and intracortical projections would be different between swimmers and novices. We assessed joint angle modulation performance as a behavioral measure and motor cortical excitation and inhibition using transcranial magnetic stimulation (TMS) at rest and during the tasks that were performed before, during, and after water immersion (WI). Motor cortical inhibition was measured with short-interval intracortical inhibition and long-interval intracortical inhibition by a paired-pulse TMS paradigm. We found that 1) the sensorimotor skills of swimmers who underwent long-term training in a water environment were superior and robustly unchanged compared with those of novices with respect to baseline on land, during WI, on land post-WI and 2) intracortical inhibition in water environments was increased in swimmers but was decreased in non-swimmers at rest compared to that on land; however, the latter alterations in intracortical inhibition in water environment were insufficient to account for the superior sensorimotor skills of swimmers. In conclusion, we demonstrate that environment-related contextual behavioral and neural changes occur even with long-term training experience.

TMS coil orientation and muscle activation influence lower limb intracortical excitability

INTRODUCTION

Previous research with transcranial magnetic stimulation (TMS) indicates that coil orientation (TMS current direction) and muscle activation state (rest or active) modify corticospinal and intracortical excitability of upper limb muscles. However, the extent to which these factors influence corticospinal and intracortical excitability of lower limb muscles is unknown. This study aimed to examine how variations in coil orientation and muscle activation affect corticospinal and intracortical excitability of tibialis anterior (TA), a lower leg muscle.

METHODS

In 21 young (21.6 ± 3.3 years, 11 female) adults, TMS was administered to the motor cortical representation of TA in posterior-anterior (PA) and mediolateral (ML) orientations at rest and during muscle activation. Single-pulse TMS measures of motor evoked potential amplitude, in addition to resting and active motor thresholds, were used to index corticospinal excitability, whereas paired-pulse TMS measures of short-interval intracortical inhibition (SICI) and facilitation (SICF), and long-interval intracortical inhibition (LICI), were used to assess excitability of intracortical circuits.

RESULTS

For single-pulse TMS, motor thresholds and test TMS intensity were lower for ML stimulation (all P < 0.05). In a resting muscle, ML TMS produced greater SICI (P < 0.001) and less SICF (both P < 0.05) when compared with PA TMS. In contrast, ML TMS in an active muscle resulted in reduced SICI but increased SICF (both P ≤ 0.001) when compared with PA TMS.

CONCLUSION

TMS coil orientation and muscle activation influence measurements of intracortical excitability recorded in the tibialis anterior, and are therefore important considerations in TMS studies of lower limb muscles.

Optical imaging reveals functional domains in primate sensorimotor cortex

Motor cortex (M1) and somatosensory cortex (S1) are central to arm and hand control. Efforts to understand encoding in M1 and S1 have focused on temporal relationships between neural activity and movement features. However, it remains unclear how the neural activity is spatially organized within M1 and S1. Optical imaging methods are well-suited for revealing the spatio-temporal organization of cortical activity, but their application is sparse in monkey sensorimotor cortex. Here, we investigate the effectiveness of intrinsic signal optical imaging (ISOI) for measuring cortical activity that supports arm and hand control in a macaque monkey. ISOI revealed spatial domains that were active in M1 and S1 in response to instructed reaching and grasping. The lateral M1 domains overlapped the hand representation and contained a population of neurons with peak firing during grasping. In contrast, the medial M1 domain overlapped the arm representation and a population of neurons with peak firing during reaching. The S1 domain overlapped the hand representations of areas 1 and 2 and a population of neurons with peak firing upon hand contact with the target. Our single unit recordings indicate that ISOI domains report the locations of spatial clusters of functionally related neurons. ISOI is therefore an effective tool for surveilling the neocortex for “hot zones” of activity that supports movement. Combining the strengths of ISOI with other imaging modalities (e.g., fMRI, 2-photon) and with electrophysiological methods can open new frontiers in understanding the spatio-temporal organization of cortical signals involved in movement control.

Expression and Cellular Distribution of P-Glycoprotein and Breast Cancer Resistance Protein in Amyotrophic Lateral Sclerosis Patients

For amyotrophic lateral sclerosis (ALS), achieving and maintaining effective drug levels in the brain is challenging due to the activity of ATP-binding cassette (ABC) transporters which efflux drugs that affect drug exposure and response in the brain. We investigated the expression and cellular distribution of the ABC transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) using immunohistochemistry in spinal cord (SC), motor cortex, and cerebellum from a large cohort of genetically well characterized ALS patients (n = 25) and controls (n = 14). The ALS group included 17 sporadic (sALS) and 8 familial (fALS) patients. Strong P-gp expression was observed in endothelial cells in both control and ALS specimens. Immunohistochemical analysis showed higher P-gp expression in reactive astroglial cells in both gray (ventral horn) and white matter of the SC, as well as in the motor cortex of all ALS patients, as compared with controls. BCRP expression was higher in glia in the SC and in blood vessels and glia in the motor cortex of ALS patients, as compared with controls. P-gp and BCRP immunoreactivity did not differ between sALS and fALS cases. The upregulation of both ABC transporters in the brain may explain multidrug resistance in ALS patients and has implications for the use of both approved and experimental therapeutics.

Dissecting two distinct interneuronal networks in M1 with transcranial magnetic stimulation

Interactions from both inhibitory and excitatory interneurons are necessary components of cortical processing that contribute to the vast amount of motor actions executed by humans daily. As transcranial magnetic stimulation (TMS) over primary motor cortex is capable of activating corticospinal neurons trans-synaptically, studies over the past 30 years have provided how subtle changes in stimulation parameters (i.e., current direction, pulse width, and paired-pulse) can elucidate evidence for two distinct neuronal networks that can be probed with this technique. This article provides a brief review of some fundamental studies demonstrating how these networks have separable excitatory inputs to corticospinal neurons. Furthermore, the findings of recent investigations will be discussed in detail, illustrating how each network's sensitivity to different brain states (i.e., rest, movement preparation, and motor learning) is dissociable. Understanding the physiological characteristics of each network can help to explain why interindividual responses to TMS exist, while also providing insights into the role of these networks in various human motor behaviors.

Cortical visuomotor interactions in Freezing of Gait: A TMS approach

OBJECTIVES

Altered cortical visuomotor integration has been involved in the pathophysiology of freezing of gait (FoG) in parkinsonism. The aim of this study was to assess the connections between the primary visual (V1) and motor (M1) areas with a paired-pulse, twin-coil transcranial magnetic stimulation (TMS) technique in patients with FoG.

METHODS

Twelve Parkinson's disease (PD) patients suffering from levodopa-responsive-FoG (off-FoG) were compared with 12 PD patients without FoG and 12 healthy subjects of similar age/sex. In the "off" condition, visuomotor connections (VMCs) were assessed bilaterally. A conditioning stimulus over the V1 phosphene hotspot was followed at interstimulus intervals (ISIs) of 18 and 40ms by a test stimulus over M1, to elicit motor evoked potentials (MEPs) in the contralateral first dorsal interosseous muscle.

RESULTS

Significant (P<0.01), bilateral effects due to VMCs were detected in all three groups, consisting of a MEP suppression at ISI 18 and 40ms. However, in PD patients with FoG, the MEP suppression was significantly (P<0.05) enhanced, both at ISI 18-40ms, in comparison with the other two groups. The phenomenon was limited to the right hemisphere.

CONCLUSIONS

PD patients with FoG showed an excessive inhibitory response of the right M1 to inputs travelling from V1 at given ISIs. Right-sided alterations of the cortical visuomotor integration may contribute to the pathophysiology of FoG.

Motor learning in unilateral cerebral palsy and the influence of corticospinal tract reorganization

Cerebral Palsy (CP) is a complex neurological disorder, characterized by congenital motor disability associated with behaviour, perception and cognition disorders. The sensorimotor impairments represent the main hallmark of the disease, significantly impacting the quality of life. So far, few studies have investigated motor learning abilities in CP and their association with the plastic reorganization of the motor system remains largely unknown. The present proof-of-principle study explored explicit motor sequence learning in children with unilateral CP and different patterns of motor system reorganization (bilateral, ipsilateral, contralateral). Children with unilateral CP, and a group of age-matched typically developing (TD) children, underwent a sequential finger tapping task, performed with the affected hand by children with CP and with the non-dominant hand by TD children. The pattern of corticospinal tract projections in hemiparetic patients was assessed by single-pulse Transcranial Magnetic Stimulation (TMS). Results showed the presence of finger dexterity impairments in children with unilateral CP presenting with a bilateral or an ipsilateral control of the affected (trained) hand, as compared to TD children. Conversely, motor sequence learning was impaired in unilateral CP with ipsilateral or contralateral corticospinal reorganization, but not in the case of a bilateral control of the paretic hand. These preliminary findings, although referred to small clinical samples, suggest that unilateral control of the paretic upper-limb, from the ipsilateral or the contralateral motor cortex, may not be sufficient to develop typical motor learning with the affected hand, which seems to require a bilateral representation in the motor cortex. This evidence has potential implications for fine motor skills rehabilitation in CP.

Effects of anodal transcranial direct current stimulation on motor evoked potentials variability in humans

Motor evoked potentials (MEPs) obtained from transcranial magnetic stimulation (TMS) allow corticospinal excitability (CSE) to be measured in the human primary motor cortex (M1). CSE responses to transcranial direct current stimulation (tDCS) protocols are highly variable. Here, we tested the reproducibility and reliability of individual MEPs following a common anodal tDCS protocol. In this study, 32 healthy subjects received anodal tDCS stimulation over the left M1 for three durations (tDCS‐T5, tDCS‐T10, and tDCS‐T20 min) on separate days in a crossover‐randomized order. After the resting motor threshold (RMT) was determined for the contralateral first dorsal interosseous muscle, 15 single pulses 4–8 sec apart at an intensity of 120% RMT were delivered to the left M1 to determine the baseline MEP amplitude at T₀, T₅, T₁₀, T₂₀, T₃₀, T₄₀, T₅₀, and T₆₀ min after stimulation for each durations. During TMS delivery, 3D images of the participant's cortex and hot spot were visualized for obtaining MEPs from same position. Our findings revealed that there was a significant MEPs improvement at T₀ (P = 0.01) after 10 min of anodal stimulation. After the 20‐min stimulation duration, MEPs differed specifically at T_0, T_5, T₃₀ min (P < 0.05). This indicates that tDCS is a promising tool to improve MEPs. Our observed variability in response to the tDCS protocol is consistent with other noninvasive brain stimulation studies.

The role of spinal reciprocal inhibition and intracortical inhibition in functional recovery from stroke

Spinal reciprocal inhibition (RI) and intracortical inhibition are important physiological mechanisms for voluntary movement control and functional recovery of voluntary movement in patients with stroke. Spasticity, which impairs motor performance, is one of the major manifestations of stroke. RI may be involved in reducing spasticity. This might allow finger extension, and, therefore, better hand function by reducing co-contraction with finger extensors. One potential mechanism of functional reorganization of the motor cortex is that pre-existing masking pathways are unmasked by decreased intracortical inhibition. The inhibitory neurotransmitter GABA plays an important role in this process. Changes in RI might be mediated through unmasking of cortical pathways through decreased inhibition, with the neurotransmitter GABA. These changes can be assessed using short-latency intracortical inhibition (SICI) and RI. Functional recovery in the chronic phase of stroke induced by rehabilitation was accompanied by SICI and spinal RI changes. Cortical reorganization and spinal plasticity might play important roles in functional recovery induced by rehabilitation, even in patients with chronic severe hemiparesis. This review aims to provide a focused overview of neuroplasticity of spinal RI and intracortical inhibition associated with functional motor recovery from stroke.