Inhibition Of Motor Evoked Potentials Research Articles

Validation of a new double-coil TMS method to assess corticospinal excitability bilaterally

Validation of a new double-coil TMS method to assess corticospinal excitability bilaterally

Safety and effects on motor cortex excitability of five cathodal transcranial direct current stimulation sessions in 25 hours

To assess the safety and effects on motor cortex excitability of five cathodal-tDCS sessions (charge density 342.9C/m2) delivered over the dominant motor cortex with a return electrode over the ipsilateral shoulder at increasing time intervals in 25hours. Safety was operatively defined as absence of serious adverse events related to tDCS including brain tissue alterations documentable by magnetic resonance imaging and spectroscopy. Effects on motor cortex excitability were evaluated by motor evoked potential (MEP) amplitude. Thirty-two healthy subjects were enrolled. No serious adverse events occurred. Magnetic resonance imaging and spectroscopy did not show alterations. Inter-individual MEP variability was assessed by the standard error of mean at baseline and subjects were classified on the basis of the ratio between normalized MEPs after the first stimulation compared to baseline. Fifty-six percent of subjects responded with reduction of MEP amplitude, 25% were non-responders and 19% were inverse responders. In responders, MEP suppression was 32% one hour after the end of first cathodal-tDCS, 21% three hours after the second, no longer present with increasing stimulation intervals and 38% two and half hours after the fifth stimulation. Intra-individual inter-sessional reliability in response was high (88-92%). Five cathodal-tDCS sessions in 25hours are safe. Inter-individual variability in MEP suppression is considerable but response to one cathodal-tDCS highly predicts the response to other sessions. Duration of MEP suppression is limited to three hours. These findings should be considered in trials utilizing repeated cathodal-tDCS.

11. Excessive inhibitory visuomotor connections in Parkinson’s disease with freezing of gait

Objectives Freezing of gait (FoG) is an invalidating symptom in patients with Parkinson’s disease (PD); it is defined as a brief, episodic absence or marked reduction of forward progression of the feet despite the intention to walk. The pathophysiology of this phenomenon remains obscure. Visual information are crucial for the initiation and control of movement, therefore an impairment of the visuomotor connections may underlie FoG. We used transcranial magnetic stimulation (TMS) to assess bilateral physiological connections (VMc) between primary visual (V1) and motor (M1) areas in PD with (PD + FoG) and without FoG (PD-FoG).We hypothesized that PD+FoG would show an abnormal response in M1. Methods Twelve PD+FoG were compared with 12 PD-FoG and 12 healthy subjects (HS) of similar age and sex. VMc was assessed bilaterally in resting participants by delivering a conditioning stimulus (CS) over the phosphene hotspot of V1 (intensity 90% phosphene threshold, PT) followed at two random interstimulus intervals (ISIs) (18 and 40 ms) by a test stimulus (TS) over left and right M1 to elicit a motor evoked potential (MEP) of ∼1 mV from the controlateral first dorsal interosseous (FDI). Results In HS, the VMc was reproducible with MEP suppression at ISI of 18 and 40 ms. Similar effects occurred in PD-FoG. PD+FoG behaved differently, since the inhibitory VMc was significantly enhanced in the right, non-dominant hemisphere. Conclusions PD+FoG had an excessive inhibitory response of the right M1 to inputs travelling from V1. Significance This may represent one core factor for the pathophysiology of FoG.

16. Safety and effects on motor cortex of closely repeated cathodal transcranial direct current stimulations (C-tDCS)

To assess safety and effects of five C-tDCS (charge density 342,857 C/m 2 ) delivered at increasing time intervals in 25 h. Safety was defined as absence of serious adverse events and by magnetic resonance imaging and spectroscopy. Effects on motor cortex excitability were evaluated by motor evoked potential (MEP) amplitudes. Inter-individual MEP variability was calculated by the SEM at baseline and subjects were classified on the basis of the ratio between normalized MEPs after the first stimulation compared to baseline. Thirty-two healthy subjects were enrolled. No serious adverse events occurred. Magnetic resonance imaging and spectroscopy did not show structural and biochemical alterations. Only 56% of subjects responded to cathodal-tDCS with the expected reduction of MEP amplitude, 25% were non-responders and 19% opposite responders. In responders, MEP suppression was 32% one hour after the first cathodal-tDC, 21% three hours after the second, no longer present four hours after the third and 12 h after the fourth cathodal-tDCS. Loss of effect was due to the increasing interval between C-tDCS and not to intervening homeostatic plasticity. Five C-tDCS in 25 h are safe. Interindividual variability on motor cortex excitability and effect duration limited to three hours should be considered in planning therapeutic trials utilizing repeated C-tDCS.

O158 Excessive inhibitory visuomotor connections in parkinson’s disease with freezing of gait

Freezing of gait (FoG) is an invalidating symptom in patients with Parkinson’s disease (PD); it is defined as a brief, episodic absence or marked reduction of forward progression of the feet despite the intention to walk. The pathophysiology of this phenomenon remains obscure. Visual information are crucial for the initiation and control of movement, therefore an impairment of the visuomotor connections may underlie FoG. We used transcranial magnetic stimulation (TMS) to assess bilateral physiological connections (VMc) between primary visual (V1) and motor (M1) areas in PD with (PD + FoG) and without FoG (PD-FoG).We hypothesized that PD+FoG would show an abnormal response in M1. Twelve PD+FoG were compared with 12 PD-FoG and 12 healthy subjects (HS) of similar age and sex. VMc was assessed bilaterally in resting participants by delivering a conditioning stimulus (CS) over the phosphene hotspot of V1 (intensity 90% phosphene threshold, PT) followed at two random interstimulus intervals (ISIs) (18 and 40 ms) by a test stimulus (TS) over left and right M1 to elicit a motor evoked potential (MEP) of ∼1 mV from the controlateral first dorsal interosseous (FDI). In HS, the VMc was reproducible with MEP suppression at ISI of 18 and 40 ms. Similar effects occurred in PD-FoG. PD+FoG behaved differently, since the inhibitory VMc was significantly enhanced in the right, non-dominant hemisphere. PD+FoG had an excessive inhibitory response of the right M1 to inputs travelling from V1. This may represent one core factor for the pathophysiology of FoG.

O207 Safety and effects on motor cortex excitability of five closely repeated cathodal transcranial direct current stimulations

Objective To assess safety and effects of five cathodal-tDCS (charge density 342,857 C/m 2 ) delivered at increasing time intervals in 25 h. Methods Safety was defined as absence of serious adverse events and assessed by magnetic resonance imaging and spectroscopy. Effects on cortical excitability were evaluated by motor evoked potential (MEP) amplitudes. Inter-individual MEP variability was calculated by the standard error of mean at baseline and subjects were classified on the basis of the ratio between normalized MEPs after the first stimulation compared to baseline. Results Thirty-two healthy subjects were enrolled and completed the study. No serious adverse events occurred. Magnetic resonance imaging and spectroscopy did not show structural and biochemical alterations. All subjects reported light to moderate tingling under the cephalic electrode. Reversible mild scalp erythema was observed in 16% of subjects.Only 56% of subjects responded to cathodal-tDCS with reduction of MEP amplitude, 25% were non-responders and 19% opposite responders. In responders, MEP suppression was 32% one hour after the first cathodal-tDC, 21% three hours after the second, no longer present four hours after the third and 12 h after the fourth cathodal-tDCS. MEP suppression reappeared after the fifth stimulation. Discussion and conclusions Five C-tDCS in 25 h are safe and well tolerated. Loss of effect was due to the increasing interval between C-tDCS and not to intervening homeostatic plasticity. Significance Interindividual variability on motor cortex excitability and effect duration limited to three hours should be considered in planning therapeutic trials utilizing repeated C-tDCS.

Focus on motor inhibition as an actor of action selection

Event Abstract Back to Event Focus on motor inhibition as an actor of action selection Caroline Quoilin1*, Fanny Fievez1 and Julie Duque1 1 Université catholique de Louvain, Institute of Neuroscience, Belgium By applying transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) to elicit motor evoked potentials (MEPs) in muscles of the contralateral hand, many studies have shown that action preparation is associated with a transient decrease in the excitability of the corticospinal pathway. So far, such inhibition has been mainly evaluated during instructed-delay choice reaction time (RT) tasks where the correct response (e.g. a left or right index finger response) is indicated by a preparatory cue, but participants are required to withhold their movement until the onset of a go signal. When TMS is applied during this delay period, MEPs are suppressed regardless of whether they are probed in a muscle that is selected or non-selected for the forthcoming response. Furthermore, a MEP suppression has also been reported when the muscle is task-irrelevant, suggesting the operation of an inhibitory process broadly suppressing motor activity. Several hypotheses have been proposed regarding the functional role of this motor inhibition, including the idea that it may serve to assist action selection. Yet, motor inhibition may also help regulate movement initiation, preventing responses from being released prematurely. The goal of the present study was to investigate the contribution of motor inhibition to action selection. To do so, we used a standard (no-delay) RT task in which the cue also served as the go signal, thus eliminating a potential inhibition related to initiation regulation. TMS was applied at several time points after the go signal in different block types. In the “choice blocks”, subjects had to choose between left and right index finger responses, while they had to always respond either with the left or right index finger in the “simple blocks”. MEPs were elicited in relevant (index agonist) and irrelevant (pinky agonist) muscles using a Double-Coil TMS procedure allowing to obtain nearly-simultaneous MEPs from both hands in each trial. The amplitude of MEPs elicited in the selected index finger progressively increased during action preparation, reflecting the recruitment of the corresponding M1 for the forthcoming movement. Surprisingly, MEPs elicited in the homonymous muscle (index agonist) of the resting hand did not show much change during action preparation, regardless of whether this muscle was part of the initial response set (choice blocks) or not (simple blocks). In fact, motor inhibition was only observed in MEPs elicited in non-homonymous irrelevant muscles (pinky agonist), and this effect was comparable in choice and simple blocks. Altogether, these results indicate that preparatory inhibition is tenuous when probed in standard RT tasks, possibly because concurrent excitatory processes tend to increase MEPs in this context. In addition, our findings did not reveal any impact of choice on MEP suppression, questioning the view that inhibition may serve to assist action selection. Keywords: action selection, Transcranial Magnetic Stimulation, motor inhibition, action preparation, motor evoked potential (MEP) Conference: 12th National Congress of the Belgian Society for Neuroscience, Gent, Belgium, 22 May - 22 May, 2017. Presentation Type: Poster Presentation Topic: Sensory and Motor Systems Citation: Quoilin C, Fievez F and Duque J (2019). Focus on motor inhibition as an actor of action selection. Front. Neurosci. Conference Abstract: 12th National Congress of the Belgian Society for Neuroscience. doi: 10.3389/conf.fnins.2017.94.00116 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 15 Mar 2017; Published Online: 25 Jan 2019. * Correspondence: PhD. Caroline Quoilin, Université catholique de Louvain, Institute of Neuroscience, Brussels, Belgium, caroline.quoilin@uclouvain.be Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Caroline Quoilin Fanny Fievez Julie Duque Google Caroline Quoilin Fanny Fievez Julie Duque Google Scholar Caroline Quoilin Fanny Fievez Julie Duque PubMed Caroline Quoilin Fanny Fievez Julie Duque Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Defective interhemispheric inhibition in drug-treated focal epilepsies

BackgroundFocal epilepsies (FEs) arise from a lateralized network, while in generalized epilepsies (GEs) there is a bilateral involvement from the outset. Intuitively, the corpus callosum is the anatomical substrate for interhemispheric spread. ObjectiveWe used transcranial magnetic stimulation (TMS) to explore whether there are any physiological differences in the corpus callosum of drug-treated patients with FE and those with genetic GE (GGE), compared to healthy subjects (HS). MethodsTMS was used to measure the interhemispheric inhibition (IHI) from right-to-left primary motor cortex (M1) and viceversa in 16 patients with FE, 17 patients with GGE and 17 HS. A conditioning stimulus (CS) was given to one M1 10 and 50 ms before a test stimulus delivered to the contralateral M1. Motor evoked potentials (MEPs) were analysed both as a function of the side of stimulation and of the epileptic focus (left-right). ResultsIn HS, IHI was reproducible with suppression of MEPs at ISIs of 10 and 50 ms. Similar effects occurred in GGE patients. FE patients behaved differently, since IHI was significantly reduced bilaterally. When FE patients were stratified according to the side of their epileptic focus, the long-ISI IHI (=50 ms) appeared to be defective only when the CS was applied over the “focal” hemisphere. ConclusionsFE patients had a defective inhibitory response of contralateral M1 to inputs travelling from the “focal” hemisphere that was residual to the drug action. Whilst IHI changes would not be crucial for the GGE pathophysiology, they may represent one key factor for the contralateral spread of focal discharges, and seizure generalization.

Motor cortical plasticity in extrinsic hand muscles is determined by the resting thresholds of overlapping representations

Knowledge of the properties that govern the effectiveness of transcranial magnetic stimulation (TMS) interventions is critical to clinical application. Extrapolation to clinical populations has been limited by high inter-subject variability and a focus on intrinsic muscles of the hand in healthy populations. Therefore, the current study assessed variability of continuous theta burst stimulation (cTBS), a patterned TMS protocol, across an agonist–antagonist pair of extrinsic muscles of the hand. Secondarily, we assessed whether concurrent agonist contraction could enhance the efficacy of cTBS. Motor evoked potentials (MEP) were simultaneously recorded from the agonist flexor (FCR) and antagonist extensor (ECR) carpi radialis before and after cTBS over the FCR hotspot. cTBS was delivered with the FCR relaxed (cTBS-Relax) or during isometric wrist flexion (cTBS-Contract). cTBS-Relax suppressed FCR MEPs evoked from the FCR hotspot. However, the extent of FCR MEP suppression was strongly correlated with the relative difference between FCR and ECR resting motor thresholds. cTBS-Contract decreased FCR suppression but increased suppression of ECR MEPs elicited from the FCR hotspot. The magnitude of ECR MEP suppression following cTBS-Contract was independent of the threshold-amplitude relationships observed with cTBS-Relax. Contraction alone had no effect confirming the effect of cTBS-Contract was driven by the interaction between neuromuscular activity and cTBS. Interactions across muscle representations should be taken into account when predicting cTBS outcomes in healthy and clinical populations. Contraction during cTBS may be a useful means of focusing aftereffects when differences in baseline excitability across overlapping agonist–antagonist cortical representations may mitigate the inhibitory effect of cTBS.

Short-latency afferent inhibition determined by the sensory afferent volley.

Short-latency afferent inhibition (SAI) is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential (MEP) by the cortical arrival of a somatosensory afferent volley. It remains unknown whether the magnitude of SAI reflects changes in the sensory afferent volley, similar to that observed for somatosensory evoked potentials (SEPs). The present study investigated stimulus-response relationships between sensory nerve action potentials (SNAPs), SAI, and SEPs and their interrelatedness. Experiment 1 (n = 23, age 23 ± 1.5 yr) investigated the stimulus-response profile for SEPs and SAI in the flexor carpi radialis muscle after stimulation of the mixed median nerve at the wrist using ∼25%, 50%, 75%, and 100% of the maximum SNAP and at 1.2× and 2.4× motor threshold (the latter equated to 100% of the maximum SNAP). Experiment 2 (n = 20, age 23.1 ± 2 yr) probed SEPs and SAI stimulus-response relationships after stimulation of the cutaneous digital nerve at ∼25%, 50%, 75%, and 100% of the maximum SNAP recorded at the elbow. Results indicate that, for both nerve types, SAI magnitude is dependent on the volume of the sensory afferent volley and ceases to increase once all afferent fibers within the nerve are recruited. Furthermore, for both nerve types, the magnitudes of SAI and SEPs are related such that an increase in excitation within somatosensory cortex is associated with an increase in the magnitude of afferent-induced MEP inhibition.

A Double-Coil TMS Method to Assess Corticospinal Excitability Changes at a Near-Simultaneous Time in the Two Hands during Movement Preparation.

Background: Many previous transcranial magnetic stimulation (TMS) studies have investigated corticospinal excitability changes occurring when choosing which hand to use for an action, one of the most frequent decision people make in daily life. So far, these studies have applied single-pulse TMS eliciting motor-evoked potential (MEP) in one hand when this hand is either selected or non-selected. Using such method, hand choices were shown to entail the operation of two inhibitory mechanisms, suppressing MEPs in the targeted hand either when it is non-selected (competition resolution, CR) or selected (impulse control, IC). However, an important limitation of this “Single-Coil” method is that MEPs are elicited in selected and non-selected conditions during separate trials and thus those two settings may not be completely comparable. Moreover, a more important problem is that MEPs are computed in relation to the movement of different hands. The goal of the present study was to test a “Double-Coil” method to evaluate IC and CR preceding the same hand responses by applying Double-Coil TMS over the two primary motor cortices (M1) at a near-simultaneous time (1 ms inter-pulse interval).Methods: MEPs were obtained in the left (MEPLEFT) and right (MEPRIGHT) hands while subjects chose between left and right hand key-presses in blocks using a Single-Coil or a Double-Coil method; in the latter blocks, TMS was either applied over left M1 first (TMSLRM1 group, n = 12) or right M1 first (TMSRLM1 group, n = 12).Results: MEPLEFT were suppressed preceding both left (IC) and right (CR) hand responses whereas MEPRIGHT were only suppressed preceding left (CR) but not right (IC) hand responses. This result was observed regardless of whether Single-Coil or Double-Coil TMS was applied in the two subject groups. However, in the TMSLRM1 group, the MEP suppression was attenuated in Double-Coil compared to Single-Coil blocks for both IC and CR, when probed with MEPLEFT (elicited by the second pulse).Conclusions: Although Double-Coil TMS may be a reliable method to assess bilateral motor excitability provided that a RM1-LM1 pulse order is used, further experiments are required to understand the reduced MEPLEFT changes in Double-Coil blocks when the LM1-RM1 pulse order was used.

ID 394 – Polarity independent suppression of long-term associative plasticity in the human SMA–M1 network by simultaneous tDCS

Introduction Excitability and connectivity of the supplementary motor area (SMA) and primary motor cortex (M1) are important for motor rehabilitation after stroke. Previously, we demonstrated that paired associative stimulation of SMA and M1 (SMA–M1-PAS) by dual coil transcranial magnetic stimulation (TMS) may induce STDP-like plasticity in this network. Here, we tested the influence of transcranial direct current stimulation (tDCS) on SMA–M1 plasticity. Methods 15 healthy right-handed men participated in this pseudo-randomized, double-blinded, crossover study. Subjects received simultaneously two blocks of neuronavigated SMA–before-M1 PAS and anodal, cathodal or sham tDCS over the left M1 in three different sessions. Motor-evoked potentials (MEP) quantified changes in SMA–M1-Plasticity. Results SMA–M1-PAS resulted in an LTP-like MEP amplitude increase 60% of the subjects in the sham tDCS condition (‘responders’), but LTD-like MEP decreases in 15 subjects (‘non-responders’). In 7 of the 9 responders, the LTP-like MEP increase was suppressed or even turned into MEP depression both in the anodal and cathodal tDCS condition. In contrast, in all 6 non-responders anodal and cathodal tDCS led to less MEP suppression or even turned it into facilitation. Conclusion Our results demonstrate a negative interaction between tDCS-induced excitability modulation and PAS-induced associative plasticity in the human SMA–M1 network, independent of tDCS-polarity.

Inter- and intra-subject variability of motor cortex plasticity following continuous theta-burst stimulation.

The potential of non-invasive brain stimulation (NIBS) for studying, and inducing, functionally relevant neuroplasticity is dependent on protocols that can induce lasting, robust and reliable effects. A current limiting factor is the large inter- and intra-subject variability in NIBS-induced neuroplastic responses. There has been some study of inter-subject response variability and factors that contribute to it; however, intra-subject response variability has, so far, received little investigation. By testing participants on multiple occasions we aimed to (1) compare inter- and intra-subject variability of neuroplastic responses induced by continuous theta-burst stimulation (cTBS); (2) determine whether the transcranial magnetic stimulation (TMS) intensity used to measure cTBS-induced neuroplastic responses contributes to response variability; (3) determine whether assessment of factors known to influence response variability can be used to explain some of the variability in cTBS-induced neuroplastic responses across experimental sessions. In three separate experimental sessions, motor-evoked potential (MEP) input-output (IO) curves were obtained before and after cTBS, and questionnaire-based assessments of physical activity and perceived stress were obtained. cTBS-induced MEP suppression was greatest at the upper end of the IO curve (150-180% resting motor threshold; RMT) and most consistent across subjects and across experimental sessions when assessed with a TMS intensity of 150% RMT. The magnitude of cTBS-induced MEP suppression evoked at 150% RMT correlated with self-reported perceived stress, but not with self-reported physical activity. The most reliable TMS intensity to probe cTBS-induced long-term depression (LTD)-like neuroplastic responses is 150% RMT. This is unlikely to simply be a ceiling effect and, we suggest, may be due to changes in the descending volley evoked at higher stimulus intensities. The perceived stress scale appears to be sufficiently sensitive to measure the influence of subject stress on LTD-like neuroplastic responses.

P149. Suppression of LTP-like associative plasticity in the human SMA–M1 network by simultaneous tDCS

Introduction Excitability and connectivity in the human motor network comprising the supplementary motor area (SMA) and primary motor cortex (M1) are important for voluntary movement generation and rehabilitation of lost motor function after stroke. Previously, we demonstrated that paired associative stimulation of SMA and M1 (SMA–M1-PAS) by dual coil transcranial magnetic stimulation (TMS) may induce LTP-like plasticity in this network (Arai et al., 2011. J Neurosci 31:15376–83). Here, we tested the influence of simultaneous modulation of general network excitability by transcranial direct current stimulation (tDCS) on associative SMA–M1 plasticity. Methods 12 healthy right-handed male subjects took part in this pseudo-randomized,single-blinded, crossover study. Subjects received two blocks (spaced by 5 min) of 50 pairs of neuronavigated SMA-before-M1 PAS at an interstimulus interval of 6 ms and intertrial interval of 5 s. Simultaneously we applied anodal, cathodal, or sham tDCS over the left M1 in three different sessions, at least 1 week apart. Associative SMA–M1 plasticity was quantified by changes in (i) motor-evoked potentials (MEP) in the right first dorsal interosseus muscle, and (ii) SMA-to-M1 connectivity (ratio of SMA-conditioned/unconditioned MEPs), at baseline versus for up to two hours after the intervention. Results SMA–M1-PAS resulted in an expected LTP-like MEP amplitude increase in 9 of 12 subjects in the sham tDCS condition. In 7 of these 9 SMA–M1-PAS responders, the LTP-like MEP amplitude increase was suppressed or even turned into depression both in the anodal and cathodal tDCS condition. In contrast, in all three non-responders anodal and cathodal tDCS led to less MEP suppression or even turned it into facilitation. Likewise, in 7 (anodal tDCS) and 6 (cathodal tDCS) of the 8 subjects who showed a facilitation of SMA-to-M1 connectivity in the sham tDCS condition this connectivity was reduced in the anodal and cathodal tDCS conditions, whereas it was less reduced or even facilitated in 2 (anodal tDCS) and all (cathodal tDCS) of the 4 subjects showing a reduction of SMA-to-M1 connectivity in the sham condition. Conclusion Our results demonstrate a reversing interaction between general network excitability modulation by tDCS and associative plasticity in the human SMA–M1 network, independent of the polarity of tDCS.

Distinct influence of hand posture on cortical activity during human grasping.

We recently showed that subcortical circuits contribute to control the gain of motor cortical inputs to spinal motoneurons during precision grip of a small object. Here, we examine whether the involvement of the motor cortex could be revealed by grasping with different hand postures. Using noninvasive cortical, cervicomedullary, and peripheral nerve stimulation we examined in humans motor-evoked potentials (MEPs) and the activity in intracortical circuits (suppression of voluntary electromyography) and spinal motoneurons (F-waves) in intrinsic hand muscles when grasping a 6 mm cylinder with the index finger and thumb while the hand was held in the neutral position or during full pronation and supination. We demonstrate that the size of cortically evoked MEPs in the first dorsal interosseous, but not in the abductor pollicis brevis and abductor digit minimi muscles, was reduced to a similar extent during grasping with the hand pronated or supinated compared with the neutral position. Notably, the suppression of MEPs was present from the MEP onset, suggesting that indirect corticospinal pathways were less likely to be involved than direct connections. There was less intracortical inhibition targeting the first dorsal interosseous during hand pronation and supination compared with neutral and this negatively correlated with changes in MEP size. In contrast, cervicomedullary MEPs and F-waves remained unchanged across conditions, as did MEPs evoked during unopposed weak flexion of the index finger. Our findings reveal a distinct influence of the posture of the hand on the activity of cortical pathways controlling different hand muscles during grasping.

Gamma-band connectivity is reduced in schizophrenia following prefrontal TMS and during working memory

s / Brain Stimulation 8 (2015) 360e377 365 NIBS is large interand intra-subject variability in NIBS-induced neuroplastic responses. We investigated (1) the reproducibility of neuroplastic responses induced by continuous theta-burst stimulation (cTBS) and (2) factors that can influence an individual’s neuroplasticity response to cTBS. In 18 subjects, motor evoked potential (MEP) input-output (I/O) curves were obtained before and after cTBS (3 stimuli at 50 Hz every 200 msec for 40 sec) and measures of physical activity levels, stress levels, and sleep patterns were obtained in three separate experimental sessions. MEP suppression was observed following cTBS at the upper end of the IO curve (150 e 180% RMT). Significant linear relationships were evident between (1) cTBS-induced MEP suppression (probed at 150% RMT) and perceived stress and (2) cTBS-induced MEP suppression (probed at the stimulus intensity that evoked a MEPw50% of the maximal MEP; SI50) and the interval between experimental sessions (in days). The current data suggest (1) the most reliable TMS intensity to probe cTBS-induced neuroplastic responses is 150% RMT; (2) the perceived stress scale is sufficiently sensitive to show the positive relationship between stress and long-term depression-like plasticity; (3) short intervals between experimental sessions administering NIBS protocols is associated with increased NIBS-induced plasticity responses.

A randomized controlled comparison of neuro-navigated unilateral vs sequential bilateral rTMS for treatment resistant depression

s / Brain Stimulation 8 (2015) 360e377 365 NIBS is large interand intra-subject variability in NIBS-induced neuroplastic responses. We investigated (1) the reproducibility of neuroplastic responses induced by continuous theta-burst stimulation (cTBS) and (2) factors that can influence an individual’s neuroplasticity response to cTBS. In 18 subjects, motor evoked potential (MEP) input-output (I/O) curves were obtained before and after cTBS (3 stimuli at 50 Hz every 200 msec for 40 sec) and measures of physical activity levels, stress levels, and sleep patterns were obtained in three separate experimental sessions. MEP suppression was observed following cTBS at the upper end of the IO curve (150 e 180% RMT). Significant linear relationships were evident between (1) cTBS-induced MEP suppression (probed at 150% RMT) and perceived stress and (2) cTBS-induced MEP suppression (probed at the stimulus intensity that evoked a MEPw50% of the maximal MEP; SI50) and the interval between experimental sessions (in days). The current data suggest (1) the most reliable TMS intensity to probe cTBS-induced neuroplastic responses is 150% RMT; (2) the perceived stress scale is sufficiently sensitive to show the positive relationship between stress and long-term depression-like plasticity; (3) short intervals between experimental sessions administering NIBS protocols is associated with increased NIBS-induced plasticity responses.

Inhibition during response preparation is sensitive to response complexity.

Motor system excitability is transiently suppressed during the preparation of movement. This preparatory inhibition is hypothesized to facilitate response selection and initiation. Given that demands on selection and initiation processes increase with movement complexity, we hypothesized that complexity would influence preparatory inhibition. To test this hypothesis, we probed corticospinal excitability during a delayed-response task in which participants were cued to prepare right- or left-hand movements of varying complexity. Single-pulse transcranial magnetic stimulation was applied over right primary motor cortex to elicit motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) of the left hand. MEP suppression was greater during the preparation of responses involving coordination of the FDI and adductor digiti minimi relative to easier responses involving only the FDI, independent of which hand was cued to respond. In contrast, this increased inhibition was absent when the complex responses required sequential movements of the two muscles. Moreover, complexity did not influence the level of inhibition when the response hand was fixed for the trial block, regardless of whether the complex responses were performed simultaneously or sequentially. These results suggest that preparatory inhibition contributes to response selection, possibly by suppressing extraneous movements when responses involve the simultaneous coordination of multiple effectors.

Dissociating the influence of response selection and task anticipation on corticospinal suppression during response preparation

Motor behavior requires selecting between potential actions. The role of inhibition in response selection has frequently been examined in tasks in which participants are engaged in some advance preparation prior to the presentation of an imperative signal. Under such conditions, inhibition could be related to processes associated with response selection, or to more general inhibitory processes that are engaged in high states of anticipation. In Experiment 1, we manipulated the degree of anticipatory preparation. Participants performed a choice reaction time task that required choosing between a movement of the left or right index finger, and used transcranial magnetic stimulation (TMS) to elicit motor evoked potentials (MEPs) in the left hand agonist. In high anticipation blocks, a non-informative cue (e.g., fixation marker) preceded the imperative; in low anticipation blocks, there was no cue and participants were required to divide their attention between two tasks to further reduce anticipation. MEPs were substantially reduced before the imperative signal in high anticipation blocks. In contrast, in low anticipation blocks, MEPs remained unchanged before the imperative signal but showed a marked suppression right after the onset of the imperative. This effect occurred regardless of whether the imperative had signalled a left or right hand response. After this initial inhibition, left MEPs increased when the left hand was selected and remained suppressed when the right hand was selected. We obtained similar results in Experiment 2 except that the persistent left MEP suppression when the left hand was not selected was attenuated when the alternative response involved a non-homologous effector (right foot). These results indicate that, even in the absence of an anticipatory period, inhibitory mechanisms are engaged during response selection, possibly to prevent the occurrence of premature and inappropriate responses during a competitive selection process.

Long-latency, inhibitory spinal pathway to ankle flexors activated by homonymous group 1 afferents

Inhibitory feedback from sensory pathways is important for controlling movement. Here, we characterize, for the first time, a long-latency, inhibitory spinal pathway to ankle flexors that is activated by low-threshold homonymous afferents. To examine this inhibitory pathway in uninjured, healthy participants, we suppressed motor-evoked potentials (MEPs), produced in the tibialis anterior (TA), by a prior stimulation to the homonymous common peroneal nerve (CPN). The TA MEP was suppressed by a triple-pulse stimulation to the CPN, applied 40, 50, and 60 ms earlier and at intensities of 0.5-0.7 times motor threshold (average suppression of test MEP was 33%). Whereas the triple-pulse stimulation was below M-wave and H-reflex threshold, it produced a long-latency inhibition of background muscle activity, approximately 65-115 ms after the CPN stimulation, a time period that overlapped with the test MEP. However, not all of the MEP suppression could be accounted for by this decrease in background muscle activity. Evoked responses from direct activation of the corticospinal tract, at the level of the brain stem or thoracic spinal cord, were also suppressed by low-threshold CPN stimulation. Our findings suggest that low-threshold muscle and cutaneous afferents from the CPN activate a long-latency, homonymous spinal inhibitory pathway to TA motoneurons. We propose that inhibitory feedback from spinal networks, activated by low-threshold homonymous afferents, helps regulate the activation of flexor motoneurons by the corticospinal tract.