Reduction Of Motor Evoked Potentials Amplitudes Research Articles

Mirror neuron activity depending on the content and stage of the observed action: a TMS study.

The firing rate of the mirror neuron system in monkeys decreases systematically with more repetitions. The aim of this study is to investigate whether the activity of the mirror neuron system varies based on the observed movement and the contents of the action, as well as whether there is inhibition in the mirror neuron system when humans observe repeated actions. If inhibition is present, the second question of the study is whether it is related to the organization of the observed action. Fourteen healthy volunteers participated in the study. Transcranial magnetic stimulation was applied to the left primary motor cortex and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous and abductor pollicis brevis muscles while the participants were watching videos specially prepared for the study. There were no significant changes in MEP amplitudes compared to baseline MEPs while observing aimless action. However, while participants watched the repeated action video, the mean MEP amplitude increased at the beginning of the movement, but neither facilitation nor inhibition was detected when the participants watched the phase of grasping the object of the action compared to the baseline MEP amplitude. On the other hand, while participants were watching different activities, an increased MEP amplitude was observed at the beginning of the movement and in the grasping of the object of the action. Additionally, there was no significant reduction in MEP amplitude during any movement stages while observing the repeated action video. The findings of this study suggest that the activation of the mirror neuron system in humans depends on the content and stages of the observed movement. Additionally, there was no inhibition or systematic reduction in MEP amplitudes while watching a repeated action.

Human Theta Burst Stimulation Combined with Subsequent Electroacupuncture Increases Corticospinal Excitability.

Objective Intermittent theta burst stimulation (iTBS) is a widely used noninvasive brain stimulation for the facilitation of corticospinal excitability (CSE). Previous studies have shown that acupuncture applied to acupoints associated with motor function in healthy people can reduce the amplitude of the motor-evoked potentials (MEPs), which reflects the inhibition of CSE. In our work, we wanted to test whether the combination of iTBS and electroacupuncture (EA) would have different effects on CSE in humans. Methods A single-blind sham-controlled crossover design study was conducted on 20 healthy subjects. Subjects received 20 minutes' sham or real EA stimulation immediately after sham or real iTBS. MEPs, short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), cortical silent period (CSP), and central motor conduction time (CMCT) were recorded before each trial, and immediately, 20 minutes, and 40 minutes after the end of stimulation. Results In the sham iTBS group, EA produced a reduction in MEPs amplitude, lasting approximately 40 minutes, while in the real iTBS group, EA significantly increased MEPs amplitude beyond 40 minutes after the end of stimulation. In sham EA group, the recorded MEPs amplitude showed no significant trend over time compared to baseline. Among all experiments, there were no significant changes in SICI, ICF, CSP, CMCT, etc. Conclusion These data indicate that immediate application of EA after iTBS significantly increased corticospinal excitability. This trial was registered in the Chinese Clinical Trial Registry (registration no. ChiCTR1900025348).

The effect of dexmedetomidine on motor-evoked potentials during pediatric posterior spinal fusion surgery: a retrospective case-control study.

Motor-evoked potentials (MEPs) are frequently used in pediatric posterior spinal fusion surgery (PSFS) to detect spinal cord ischemia. Dexmedetomidine is increasingly being used as an adjunct to total intravenous anesthesia, but its effect on MEP amplitude has been variably reported. The purpose of this study was to evaluate the effect of an infusion of dexmedetomidine on the amplitude of MEPs. We performed a retrospective case-control study of 30 pediatric patients who received a 0.5 µg·kg-1·hr-1 infusion of dexmedetomidine, ten patients who received 0.3 µg·kg-1·hr-1 dexmedetomidine, and 30 control patients who did not receive dexmedetomidine during PSFS. Two neurophysiologists reviewed the MEP amplitudes in six muscle groups at three time points: when the patient was turned prone (baseline; T1), one hour after incision (T2), and after exposure of the spine but before insertion of the first screw (T3). In all muscles tested, the mean MEP amplitude was reduced by T3 when dexmedetomidine was infused at 0.5 µg·kg-1·hr-1. The greatest reduction from baseline MEP amplitude was 829 µV (95% confidence interval, 352 to 1230; P < 0.001) seen in first right dorsus interosseous. When dexmedetomidine was infused at 0.3 µg·kg-1·hr-1, there was a significant reduction in MEP amplitude in four of the six muscles tested at T3 compared with the control group. Dexmedetomidine at commonly used infusion rates of 0.3 µg·kg-1·hr-1 or 0.5 µg·kg-1·hr-1 causes a significant decrease in MEP amplitude during pediatric PSFS. We suggest that dexmedetomidine should be avoided in children undergoing PSFS so as not to confuse the interpretation of this important neurophysiological monitor.

Modulating Observation-Execution-Related Motor Cortex Activity by Cathodal Transcranial Direct Current Stimulation.

The aim of this randomized sham-controlled study was to examine the impact of cathodal transcranial direct current stimulation (ctDCS) of the primary motor cortex (M1) during movement observation on subsequent execution-related motor cortex activity. Thirty healthy participants received sham or real ctDCS (1 mA) over the left M1 for 10 minutes, respectively. The participants observed a video showing repeated button pressing tasks of the right hand during the sham or real ctDCS, followed by performance of these tasks by the right hand. Motor-evoked potentials (MEP) were recorded from the resting right first dorsal interosseous muscle before movement observation during the sham or real ctDCS, immediately after observation of actions, and after subsequent movement execution. The results of the ANOVA showed a significant main effect on the group (F1,28 = 4.60, p = 0.041) and a significant interaction between time and the group (F2,56 = 5.34, p = 0.008). As revealed by respective post hoc tests, ctDCS induced a significant reduction of MEP amplitudes in connection with movement observation (p = 0.026, Cohen’s d = 0.861) and after subsequent movement execution (p = 0.018, Cohen’s d = 0.914) in comparison with the sham stimulation. It is concluded that ctDCS during movement observation was effective in terms of modulating motor cortex excitability. Moreover, it subsequently influenced execution-related motor cortex activity. This indicates a possible application for rehabilitative treatment in syndromes with pathologically enhanced cortical activity.

Titrating the neuroplastic effects of cathodal transcranial direct current stimulation (tDCS) over the primary motor cortex

Transcranial direct current stimulation (tDCS) non-invasively induces polarity-dependent excitability alterations in the human motor cortex lasting for more than an hour after stimulation. Clinical applications with encouraging results have been reported in several pilot studies, but the optimal stimulation protocols remain to be determined. This is also important because the efficacy and directionality of tDCS effects follow non-linear rules regarding neuroplastic effects for the stimulation parameters duration and intensity. In this study, we systemically explored the association between tDCS, these parameters and induced after-effects on motor cortex excitability. Cathodal tDCS was applied in four different intensities (sham, 1, 2 and 3 mA) and three durations (15, 20 and 30 mins) in 16 young healthy subjects and the after-effects were monitored with TMS-induced motor evoked potentials (MEP) until the next day evening after stimulation. The results of the repeated measures ANOVA conducted to disentangle the effects of tDCS intensity and duration show a main effect of intensity in which 1 mA and 3 mA stimulation induced a reduction of MEP amplitudes, but 2 mA resulted in excitability enhancement. The results of a secondary ANOVA conducted to compare if active stimulation effects differ from those of sham stimulation revealed a significant main effect of tDCS condition in which 1 mA–15 min, 1 mA–30 min and 3 mA–20 min cathodal tDCS induced LTD-like plasticity, while LTP-like plasticity was observed after 2 mA–20 min stimulation. Our study thus provides further insights on the dependency of tDCS -induced neuroplasticity from the stimulation parameters, and therefore delivers crucial information for future applications.

Decreased Corticospinal Excitability after the Illusion of Missing Part of the Arm.

Previous studies on body ownership illusions have shown that under certain multimodal conditions, healthy people can experience artificial body-parts as if they were part of their own body, with direct physiological consequences for the real limb that gets ‘substituted.’ In this study we wanted to assess (a) whether healthy people can experience ‘missing’ a body-part through illusory ownership of an amputated virtual body, and (b) whether this would cause corticospinal excitability changes in muscles associated with the ‘missing’ body-part. Forty right-handed participants saw a virtual body from a first person perspective but for half of them the virtual body was missing a part of its right arm. Single pulse transcranial magnetic stimulation was applied before and after the experiment to left and right motor cortices. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) and the extensor digitorum communis (EDC) of each hand. We found that the stronger the illusion of amputation and arm ownership, the more the reduction of MEP amplitudes of the EDC muscle for the contralateral sensorimotor cortex. In contrast, no association was found for the EDC amplitudes in the ipsilateral cortex and for the FDI amplitudes in both contralateral and ipsilateral cortices. Our study provides evidence that a short-term illusory perception of missing a body-part can trigger inhibitory effects on corticospinal pathways and importantly in the absence of any limb deafferentation or disuse.

Single Limb Fatigue Compromises The Ability To Generate Force In The Opposite Limb

Performing strong voluntary contractions with a single limb produces unintended muscle activity in the homologous muscle of the opposite resting limb. The mechanisms underlying these crossed-effects are likely to involve supraspinal centres, as neural activity in the active motor cortex is known to spread within and between motor cortices during repeated muscle contractions. Although strong contractions result in unintended activity in the resting opposite limb, it is unknown how brief contractions or fatigue inducing contractions affect the maximum force generating ability of the opposite limb. PURPOSE: To determine how intermittent maximal contractions of a single limb influences the ability to produce force in the opposite limb. METHODS: Twelve healthy participant (age: 23 ± 3.4 yr) were recruited into the study. Motor evoked potentials (MEP), EMG of the first dorsal interosseous (FDI), and index finger abduction forces were obtained before, during, and after a single limb intermittent maximal contraction protocol. The contraction sets consisted of four 4 s contractions separated by 2 s of rest for the dominant limb- followed a 10 s period where MEP, FDI EMG and force data were collected for the non-dominant limb. Contractions were continued until MVC reduced by 50%. RESULTS: In unfatigued muscles, only small increases in unintended activity occurred when the opposite limb was maximally contracting (∼5% increase in force with no changes in MEP amplitude). After one contraction set was performed by the dominant limb, the capacity to generate force in the non-dominant limb was significantly reduced by 15% which corresponded to a 20% reduction in MEP amplitude. Maximal force output for both limbs significantly reduced across the duration of contraction sets, where maximum force output declined by 52% for the exercising limb and 36% for the contralateral limb when the criteria for termination was achieved. CONCLUSIONS: Crossed-effects were evident across all conditions; however fatiguing contractions were most effective in eliciting responses in the contralateral limb. Given that repeated single limb contractions reduced the maximal force generating capacity in the opposite limb, it appears that inhibitory mechanisms were activated on the opposite side of the body, presumably at the level of the motor cortex.

Motor cortex plasticity induced by theta burst stimulation is impaired in patients with obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is a respiratory condition occurring during sleep characterised by repeated collapse of the upper airway. Patients with OSA show altered brain structure and function that may manifest as impaired neuroplasticity. We assessed this hypothesis in 13 patients with moderate-to-severe OSA and 11 healthy control subjects. Transcranial magnetic stimulation was used to induce and measure neuroplastic changes in the motor cortex by assessing changes in motor-evoked potentials (MEPs) in a hand muscle. Baseline measurements of cortical excitability included active (AMT) and resting motor thresholds (RMT), and the maximal stimulator output producing a 1-mV MEP. Intracortical inhibition (ICI) was investigated with short- and long-interval ICI paradigms (SICI and LICI, respectively), and neuroplastic changes were induced using continuous theta burst stimulation (cTBS). At baseline, differences were found between groups for RMT (9.5% maximal stimulator output higher in OSA) and 1-mV MEPs (10.3% maximal stimulator output higher in OSA), but not AMT. No differences were found between groups for SICI or LICI. The response to cTBS was different between groups, with control subjects showing an expected reduction in MEP amplitude after cTBS, whereas the MEPs in patients with OSA did not change. The lack of response to cTBS suggests impaired long-term depression-like neuroplasticity in patients with OSA, which may be a consequence of sleep fragmentation or chronic blood gas disturbance in sleep. This reduced neuroplastic capacity may have implications for the learning, retention or consolidation of motor skills in patients with OSA.

Effect of repetitive afferent electrical stimulation of the lower limb on corticomotor excitability and implications for rehabilitation

Despite the extensive work published on the effects of electrical stimulation on corticomotor excitability, very few studies have focused on lower limb muscles. The aim of the present study was to determine the effect of high-frequency afferent electrical stimulation of the anterior thigh area on the corticomotor excitability of lower limb muscles. Twenty-two healthy subjects (mean age 23±7years) participated in the study. Electrical stimulation was applied for 60minutes on the anterior thigh area (frequency 100Hz, pulse duration 60μs, intensity below motor threshold). Motor-evoked potentials (MEP) of the rectus femoris (RF) and biceps femoris (BF) muscles were recorded before, and after, the electrical stimulation paradigm with a Magstim 200 stimulator. Analyses revealed a significant modulation in MEP amplitude for the RF but not for the BF muscle. Specifically, there was a significant reduction in MEP amplitude for the RF muscle immediately, 15minutes and 30minutes after the end of electrical stimulation when compared with baseline. Our results indicate that a 60-minute high-frequency electrical stimulation protocol applied on the anterior thigh area decreases the corticomotor excitability of the RF muscle. Although the exact duration remains unknown, the pattern of modulation observed indicates that the inhibitory effect lasts for more than 30minutes after the end of stimulation, giving enough time for clinicians to work on the desired motor task during rehabilitation.

Modulations of input-output properties of corticospinal tract neurons by repetitive dynamic index finger abductions

The goal of this study was to investigate how corticospinal tract neurons (CTNs) are modulated after repetitive dynamic muscle contractions. To address this question, changes of motor evoked potentials (MEPs) to transcranial magnetic stimulation and background EMG (B.EMG) activities were examined. Subjects were instructed to perform an isometric dynamic index finger abduction as accurately as possible under the target-force-matching tasks (10% or 30% MVC), while MEPs of a first dorsal interosseous (FDI) were elicited during performance of the task. After repetitive dynamic FDI contractions (100 trials), the following remarkable phenomena were observed: (1) both B.EMG activities and MEP amplitudes decreased in proportion to the number of trials, (2) these phenomena were most commonly observed in different conditions, i.e., different force levels and hands (preferred or non-preferred hands), and (3) after repetition of the tasks, the MEP amplitude/B.EMG (MEP/B.EMG) ratio became smaller. Decreases of B.EMG activities with reduction of MEP amplitudes and diminishing MEP/B.EMG ratio might suggest the occurrence of reorganization of input-output properties in CTNs for an efficient performance as a function of motor adaptation. Thus, we conclude that motor adaptation after repetitive dynamic muscle contractions probably occurs less specifically and due to susceptible modulations of spinal motoneurons reflected in the integrative functions of CTNs.

EFFECT OF THIOPENTONE ON MOTOR EVOKED POTENTIALS INDUCED BY TRANSCRANIAL MAGNETIC STIMULATION IN HUMANS

We have studied the effect of repeated doses of thiopentone on motor evoked potentials (MEP) after transcranial magnetic stimulation in 13 patients. Thiopentone was administered i.v. in an initial dose of 2 mg kg-1, followed by repeated doses (1 mg kg-1 every 3 min) until the appearance of burst suppression on the EEG. The total dose administered was mean 10.7 (SD 2.6) mg kg-1. The magnetic coil was placed over the MEP scalp stimulation region and evoked electromyographic responses were recorded from the contralateral abductor pollicis brevis. After an initial dose of thiopentone, reproducible MEP responses were recorded in all patients, but amplitudes were reduced to 42.8% of baseline values. Further administration of repeated doses of thiopentone produced a dose-dependent decrease in success rate of MEP recordings and a significant reduction in MEP amplitude (P < 0.01). Latency did not change significantly, although there was a tendency to increase. During burst suppression on the EEG, MEP was not recorded successfully in all patients. We conclude that MEP recording during the administration of thiopentone is feasible only at a minimum dose, with a marked reduction in MEP amplitude.

Influence of anesthetics--nitrous oxide in particular--on electromyographic response evoked by transcranial electrical stimulation of the cortex.

The influence of anesthetics usually used for neuroleptic anesthesia--nitrous oxide, fetanyl, flunitrazepam, and thiopental sodium--on motor evoked potentials (MEP) was examined in 15 patients during neurosurgical operations on the spinal cord, in 16 patients in traumatic coma, and in 6 healthy volunteers. MEP were recorded from the contralateral thenar and anterior tibial muscles in response to single transcranial electrical stimuli on the motor cortex. Intraoperatively, during neuroleptic anesthesia we found the amplitudes to be reduced to an average of 11% of the preoperative baselines for the thenar potentials, and to 7% of the preoperative baselines for the anterior tibial muscle potentials, despite a maximum stimulus strength of 750 V. A similar reduction of MEP amplitudes was observed in 6 volunteers during breathing of an oxygen/nitrous oxide mixture (34%/66%), whereas fentanyl, flunitrazepam, and thiopental had only a minor effect on MEP. We conclude that with respect to anesthesia-related suppression of amplitudes, an average of 5 to 15 electromyographic responses should be evaluated for intraoperative monitoring of MEP using the technique described here.