The effects of alterations in conditioning stimulus intensity on short interval intracortical inhibition

Physiological Biomarkers of Upper Motor Neuron Dysfunction in ALS

Upper motor neuron (UMN) dysfunction is an important feature of amyotrophic lateral sclerosis (ALS) for the diagnosis and understanding of pathogenesis. The identification of UMN signs forms the basis of ALS diagnosis, although may be difficult to discern, especially in the setting of severe muscle weakness. Transcranial magnetic stimulation (TMS) techniques have yielded objective physiological biomarkers of UMN dysfunction in ALS, enabling the interrogation of cortical and subcortical neuronal networks with diagnostic, pathophysiological, and prognostic implications. Transcranial magnetic stimulation techniques have provided pertinent pathogenic insights and yielded novel diagnostic and prognostic biomarkers. Cortical hyperexcitability, as heralded by a reduction in short interval intracortical inhibition (SICI) and an increase in short interval intracortical facilitation (SICF), has been associated with lower motor neuron degeneration, patterns of disease evolution, as well as the development of specific ALS clinical features including the split hand phenomenon. Reduction in SICI has also emerged as a potential diagnostic aid in ALS. More recently, physiological distinct inhibitory and facilitatory cortical interneuronal circuits have been identified, which have been shown to contribute to ALS pathogenesis. The triple stimulation technique (TST) was shown to enhance the diagnostic utility of conventional TMS measures in detecting UMN dysfunction. Resting-state EEG is a novel neurophysiological technique developed for directly interrogating cortical neuronal networks in ALS, that have yielded potentially useful physiological biomarkers of UMN dysfunction. The present review discusses physiological biomarkers of UMN dysfunction in ALS, encompassing conventional and novel TMS techniques developed to interrogate the functional integrity of the corticomotoneuronal system, focusing on pathogenic, diagnostic, and prognostic utility.

Novel approaches to assessing upper motor neuron dysfunction in motor neuron disease/amyotrophic lateral sclerosis: IFCN handbook chapter

Identifying upper motor neuron (UMN) dysfunction is fundamental to the diagnosis and understanding of disease pathogenesis in motor neuron disease (MND). The clinical assessment of UMN dysfunction may be difficult, particularly in the setting of severe muscle weakness. From a physiological perspective, transcranial magnetic stimulation (TMS) techniques provide objective biomarkers of UMN dysfunction in MND and may also be useful to interrogate cortical and network function. Single, paired- and triple pulse TMS techniques have yielded novel diagnostic and prognostic biomarkers in MND, and have provided important pathogenic insights, particularly pertaining to site of disease onset. Cortical hyperexcitability, as heralded by reduced short interval intracortical inhibition (SICI) and increased short interval intracortical facilitation, has been associated with the onset of lower motor neuron degeneration, along with patterns of disease spread, development of specific clinical features such as the split hand phenomenon, and may provide an indication about the rate of disease progression. Additionally, reduction of SICI has emerged as a potential diagnostic aid in MND. The triple stimulation technique (TST) was shown to enhance the diagnostic utility of conventional TMS measures in detecting UMN dysfunction in MND. Separately, sophisticated brain imaging techniques have uncovered novel biomarkers of neurodegeneration that have bene associated with progression. The present review will discuss the utility of TMS and brain neuroimaging derived biomarkers of UMN dysfunction in MND, focusing on recently developed TMS techniques and advanced neuroimaging modalities that interrogate structural and functional integrity of the corticomotoneuronal system, with an emphasis on pathogenic, diagnostic, and prognostic utility.

Contribution of neurophysiology to the diagnosis and monitoring of ALS

This chapter describes the role of neurophysiological techniques in diagnosing and monitoring amyotrophic lateral sclerosis (ALS). Despite many advances, electromyography (EMG) remains a keystone investigation from which to build support for a diagnosis of ALS, demonstrating the pathophysiological processes of motor unit hyperexcitability, denervation and reinnervation. We consider development of the different diagnostic criteria and the role of EMG therein. While not formally recognised by established diagnostic criteria, we discuss the pioneering studies that have demonstrated the diagnostic potential of transcranial magnetic stimulation (TMS) of the motor cortex and highlight the growing evidence for TMS in the diagnostic process. Finally, accurately monitoring disease progression is crucial for the successful implementation of clinical trials. Neurophysiological measures of disease state have been incorporated into clinical trials for over 20 years and we review prominent techniques for assessing disease progression.

When the heart inhibits the brain: Cardiac phases modulate short-interval intracortical inhibition

The phasic cardiovascular activity influences the central nervous system through the systolic baroreceptor inputs, inducing widespread inhibitory effects on behavior. Through transcranial magnetic stimulation (TMS) delivered during resting-state over the left primary motor cortex and across the different cardiac phases, we measured corticospinal excitability (CSE) and distinct indices of intracortical motor inhibition: short (SICI) and long (LICI) interval, corresponding to GABAA and GABAB neurotransmission, respectively. We found a significant effect of the cardiac phase on short-intracortical inhibition, without any influence on LICI. Specifically, SICI was stronger at systole compared to diastole. These results show a tight relationship between the cardiac cycle and the inhibitory neurotransmission within M1, and in particular with GABAA-ergic-mediated motor inhibition. We hypothesize that this process requires greater motor control via the gating mechanism and that this, in turn, needs to be recalibrated through the modulation of intracortical inhibition.

Considerable effects of lateralization and aging in intracortical excitation and inhibition

Introduction

Findings based on the use of transcranial magnetic stimulation and electromyography (TMS-EMG) to determine the effects of motor lateralization and aging on intracortical excitation and inhibition in the primary motor cortex (M1) are inconsistent in the literature. TMS and electroencephalography (TMS-EEG) measures the excitability of excitatory and inhibitory circuits in the brain cortex without contamination from the spine and muscles. This study aimed to investigate the effects of motor lateralization (dominant and non-dominant hemispheres) and aging (young and older) and their interaction effects on intracortical excitation and inhibition within the M1 in healthy adults, measured using TMS-EMG and TMS-EEG.

Methods

This study included 21 young (mean age = 28.1 ± 3.2 years) and 21 older healthy adults (mean age = 62.8 ± 4.2 years). A battery of TMS-EMG measurements and single-pulse TMS-EEG were recorded for the bilateral M1.

Results

Two-way repeated-measures analysis of variance was used to investigate lateralization and aging and the lateralization-by-aging interaction effect on neurophysiological outcomes. The non-dominant M1 presented a longer cortical silent period and larger amplitudes of P60, N100, and P180. Corticospinal excitability in older participants was significantly reduced, as supported by a larger resting motor threshold and lower motor-evoked potential amplitudes. N100 amplitudes were significantly reduced in older participants, and the N100 and P180 latencies were significantly later than those in young participants. There was no significant lateralization-by-aging interaction effect in any outcome.

Conclusion

Lateralization and aging have independent and significant effects on intracortical excitation and inhibition in healthy adults. The functional decline of excitatory and inhibitory circuits in the M1 is associated with aging.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Cortical inhibition and facilitation are mediated by distinct physiological processes

A complex interaction of inhibitory and facilitatory interneuronal processes may underlie development of cortical excitability in the human motor cortex. To determine whether distinct interneuronal processes mediated cortical excitability, threshold tracking transcranial magnetic stimulation was utilised to assess cortical excitability, with figure-of-eight coil oriented in posterior-anterior (PA), anterior-posterior (AP) and latero-medial (LM) directions. Motor evoked potential (MEP) responses were recorded over the contralateral abductor pollicis brevis. Resting motor threshold (RMT), short interval intracortical inhibition (SICI), short interval intracortical facilitation (SICF) and intracortical facilitation were recorded. Significant effects of coil orientation were evident on SICI (F = 8.560, P = 0.002) and SICF (F = 7.132, P = 0.003). SICI was greater when recorded with PA (9.7 ± 10.9%, P = 0.029) and AP (13.1 ± 7.0%, P = 0.003) compared to LM (5.2 ± 7.3%) directed currents. SICF was significantly greater with PA (-14.7 ± 8.1%, P = 0.016) and LM (-14.7 ± 8.8%, P = 0.005) compared to AP (-9.1 ± 7.2%) coil orientations. SICI recorded with LM and PA coil orientations were correlated (R = 0.7, P = 0.002), as was SICF recorded with AP vs LM (R = 0.60, P = 0.019) and LM vs PA (R = 0.69, P = 0.002) coil orientations. RMT was significantly smaller with PA compared to AP (P < 0.001) and LM (P = 0.018) stimulation. Recruitment of distinct interneuronal processes with variable cortical orientation and thresholds underlies short interval intracortical inhibition and facilitation.

The recruitment of indirect waves within primary motor cortex during motor imagery: A directional transcranial magnetic stimulation study

Motor imagery (MI) refers to the mental simulation of an action without overt movement. While numerous transcranial magnetic stimulation (TMS) studies provided evidence for a modulation of corticospinal excitability and intracortical inhibition during MI, the neural signature within the primary motor cortex is not clearly established. In the current study, we used directional TMS to probe the modulation of the excitability of early and late indirect waves (I-waves) generating pathways during MI. Corticospinal responses evoked by TMS with posterior-anterior (PA) and anterior-posterior (AP) current flow within the primary motor cortex evoke preferentially early and late I-waves, respectively. Seventeen participants were instructed to stay at rest or to imagine maximal isometric contractions of the right flexor carpi radialis. We demonstrated that the increase of corticospinal excitability during MI is greater with PA than AP orientation. By using paired-pulse stimulations, we confirmed that short-interval intracortical inhibition (SICI) increased during MI in comparison to rest with PA orientation, whereas we found that it decreased with AP orientation. Overall, these results indicate that the pathways recruited by PA and AP orientations that generate early and late I-waves are differentially modulated by MI.

Paired pulse transcranial magnetic stimulation in the assessment of biceps voluntary activation in individuals with tetraplegia

After spinal cord injury (SCI), motoneuron death occurs at and around the level of injury which induces changes in function and organization throughout the nervous system, including cortical changes. Muscle affected by SCI may consist of both innervated (accessible to voluntary drive) and denervated (inaccessible to voluntary drive) muscle fibers. Voluntary activation measured with transcranial magnetic stimulation (VATMS) can quantify voluntary cortical/subcortical drive to muscle but is limited by technical challenges including suboptimal stimulation of target muscle relative to its antagonist. The motor evoked potential (MEP) in the biceps compared to the triceps (i.e., MEP ratio) may be a key parameter in the measurement of biceps VATMS after SCI. We used paired pulse TMS, which can inhibit or facilitate MEPs, to determine whether the MEP ratio affects VATMS in individuals with tetraplegia. Ten individuals with tetraplegia following cervical SCI and ten non-impaired individuals completed single pulse and paired pulse VATMS protocols. Paired pulse stimulation was delivered at 1.5, 10, and 30 ms inter-stimulus intervals (ISI). In both the SCI and non-impaired groups, the main effect of the stimulation pulse (paired pulse compared to single pulse) on VATMS was not significant in the linear mixed-effects models. In both groups for the stimulation parameters we tested, the MEP ratio was not modulated across all effort levels and did not affect VATMS. Linearity of the voluntary moment and superimposed twitch moment relation was lower in SCI participants compared to non-impaired. Poor linearity in the SCI group limits interpretation of VATMS. Future work is needed to address methodological issues that limit clinical application of VATMS.

Stimulation with acoustic white noise enhances motor excitability and sensorimotor integration

Auditory white noise (WN) is widely used in neuroscience to mask unwanted environmental noise and cues, e.g. TMS clicks. However, to date there is no research on the influence of WN on corticospinal excitability and potentially associated sensorimotor integration itself. Here we tested the hypothesis, if WN induces M1 excitability changes and improves sensorimotor performance. M1 excitability (spTMS, SICI, ICF, I/O curve) and sensorimotor reaction-time performance were quantified before, during and after WN stimulation in a set of experiments performed in a cohort of 61 healthy subjects. WN enhanced M1 corticospinal excitability, not just during exposure, but also during silence periods intermingled with WN, and up to several minutes after the end of exposure. Two independent behavioural experiments highlighted that WN improved multimodal sensorimotor performance. The enduring excitability modulation combined with the effects on behaviour suggest that WN might induce neural plasticity. WN is thus a relevant modulator of corticospinal function; its neurobiological effects should not be neglected and could in fact be exploited in research applications.

Modulation of spontaneous motor unit potentials by a new motor cortical magnetic stimulation method in amyotrophic lateral sclerosis

BACKGROUND

Patients with amyotrophic lateral sclerosis (ALS) show altered cortical excitability. In this study, we measure modulation of spontaneous motor unit potentials (sMUPs) in hand muscles by multifocal cortical stimulation with a newly developed wearable transcranial rotating permanent magnet stimulator (TRPMS).

METHODS

We conducted cross-sectional and longitudinal electromyographic assessments in 40 and 20 ALS patients, respectively, of the stimulation-induced peak increase in the count of sMUPs in two hand muscles modulated by unilateral TRPMS stimulation of the primary motor cortex. We measured peak sMUP counts during several short sessions consisting of 10 stimuli over 60 s and 30 s post-stimulation periods. The longitudinal component involved an initial assessment at an early stage of the disease and up to five follow-up assessments at least 3 months apart.

RESULTS

TRPMS stimulation produced no device-related adverse effects. It showed an inverted V-shaped modulation of the peak sMUP counts as a function of ALS functional rating scale revised scores. The ratios of ALS subjects showing peak sMUP count increases between early and intermediate stages (χ2 = 4.086, df = 1, p = 0.043) and intermediate and late stages (χ2 = 4.29, df = 1, p = 0.038) in cross-sectional data were significantly different. Longitudinal assessment also produced a significant (z = 2.31, p = 0.021) result, with all subjects showing a post-initial visit increase in peak sMUP counts.

CONCLUSIONS

These results are consistent with delayed onset of upper motor neuronal dysfunction with respect to onset of clinical features. However, the above results need to be confirmed in a larger sample of patients and with multiple lines of evidence.

Test-Retest Reliability of Short-Interval Intracortical Inhibition Assessed by Threshold-Tracking and Automated Conventional Techniques

Two novel short-interval intracortical inhibition (SICI) protocols, assessing SICI across a range of interstimulus intervals (ISIs) using either parallel threshold-tracking transcranial magnetic stimulation (TT-TMS) or automated conventional TMS (cTMS), were recently introduced. However, the test-retest reliability of these protocols has not been investigated, which is important if they are to be introduced in the clinic. SICI was recorded in 18 healthy subjects using TT-TMS (T-SICI) and cTMS (A-SICI). All subjects were examined at four identical sessions, i.e., morning and afternoon sessions on 2 d, 5-7 d apart. Both SICI protocols were performed twice at each session by the same observer. In one of the sessions, another observer performed additional examinations. Neither intraobserver nor interobserver measures of SICI differed significantly between examinations, except for T-SICI at ISI 3 ms (p = 0.00035) and A-SICI at ISI 2.5 ms (p = 0.0103). Intraday reliability was poor-to-good for A-SICI and moderate-to-good for T-SICI. Interday and interobserver reliabilities of T-SICI and A-SICI were moderate-to-good. Although between-subject variation constituted most of the total variation, SICI repeatability in an individual subject was poor. The two SICI protocols showed no considerable systematic bias across sessions and had a comparable test-retest reliability profile. Findings from the present study suggest that both SICI protocols may be reliably and reproducibly employed in research studies, but should be used with caution for individual decision-making in clinical settings. Studies exploring reliability in patient cohorts are warranted to investigate the clinical utility of these two SICI protocols.

Influence of Voluntary Contraction Level, Test Stimulus Intensity and Normalization Procedures on the Evaluation of Short-Interval Intracortical Inhibition

Short-interval intracortical inhibition (SICI) represents an inhibitory phenomenon acting at the cortical level. However, SICI estimation is based on the amplitude of a motor-evoked potential (MEP), which depends on the discharge of spinal motoneurones and the generation of compound muscle action potential (M-wave). In this study, we underpin the importance of taking into account the proportion of spinal motoneurones that are activated or not when investigating the SICI of the right flexor carpi radialis (normalization with maximal M-wave (Mmax) and MEP_test, respectively), in 15 healthy subjects. We probed SICI changes according to various MEP_test amplitudes that were modulated actively (four levels of muscle contraction: rest, 10%, 20% and 30% of maximal voluntary contraction (MVC)) and passively (two intensities of test transcranial magnetic stimulation (TMS): 120 and 130% of motor thresholds). When normalized to MEP_test, SICI remained unchanged by stimulation intensity and only decreased at 30% of MVC when compared with rest. However, when normalized to Mmax, we provided the first evidence of a strong individual relationship between SICI and MEP_test, which was ultimately independent from experimental conditions (muscle states and TMS intensities). Under similar experimental conditions, it is thus possible to predict SICI individually from a specific level of corticospinal excitability in healthy subjects.

Effect of paired-pulse stimulus parameters on the two phases of short interval intracortical inhibition in the quadriceps muscle group

BACKGROUND

Short interval intracortical inhibition (SICI) is commonly used to assess inhibition in the motor cortex and is known to be affected by the paired-pulse stimulus parameters (i.e., interstimulus interval [ISI], conditioning stimulus intensity [CSI] and test stimulus intensity [TSI]) used during testing. While the effects of stimulus parameters are well-studied in the upper-extremity, evidence in the lower-extremity is lacking.

OBJECTIVE

To comprehensively examine the effects of alterations in paired-pulse stimulus parameters on the two phases of SICI in the quadriceps muscle group.

METHODS

Seventeen adults (8 males, 9 females) volunteered to participate in this study. SICI was examined over a range of CSIs (70-90% active motor threshold [AMT]), TSIs (100-140% AMT), and ISIs (1.0-3.0 ms) using both EMG and torque responses elicited by transcranial magnetic stimulation (TMS).

RESULTS

The results indicated that SICI at 1.0 ms ISI was best revealed with a CSI of 70% and TSI ≥110% AMT, whereas SICI at 2.5 ms ISI was best revealed with a CSI of 80-90% and a TSI of ≥130% AMT. Unlike upper-extremity muscles, evaluating SICI with a CSI of 70% AMT and an ISI of 1.0 ms produced the greatest inhibition for all TSIs. In general, inhibitory effects were contaminated by facilitatory effects when using a TSI of 100% AMT.

CONCLUSIONS

The amount of inhibition was dependent on the stimulation parameters used during testing. A CSI of 70% AMT, ISI of 1.0 ms, and TSI of ≥110% AMT appear to be optimal for measuring SICI in the quadriceps muscle; however, other parameters can be used if careful consideration is given to the described interaction between the parameters.

Proton Magnetic Resonance Spectroscopy of the motor cortex reveals long term GABA change following anodal Transcranial Direct Current Stimulation

Anodal transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) has been reported to increase the firing rates of neurons and to modulate the gamma-aminobutyric acid (GABA) concentration. To date, knowledge about the nature and duration of these tDCS induced effects is incomplete. We aimed to investigate long-term effects of anodal tDCS over M1 on GABA dynamics in humans. Repeated magnetic resonance spectroscopy (MRS) was employed to measure relative GABA concentration in M1 for approximately 64 minutes after stimulation. The study was performed on 32 healthy subjects. Either anodal or sham tDCS were applied for 10 minutes with the active electrode over the left M1 and the reference electrode over the right supra-orbital region. Pre and post-tDCS MRS scans were performed to acquire GABA-edited spectra using 3 T Prisma Siemens scanner. GABA signals showed no change over time in the sham tDCS group, whereas anodal tDCS resulted in a significant early decrease within 25 minutes after tDCS and then significant late decrease after 66 minutes which continued until the last test measurements. The late changes in GABA concentration might be related to long-term plasticity mechanism. These results contribute to a better understanding of the neurochemical mechanism underlying long-term cortical plasticity following anodal tDCS.

Utility of threshold tracking transcranial magnetic stimulation in ALS

Upper motor neuron [UMN] and lower motor neuron [LMN] dysfunction, in the absence of sensory features, is a pathognomonic feature of amyotrophic lateral sclerosis [ALS]. Although the precise mechanisms have yet to be elucidated, one leading hypothesis is that UMN precede LMN dysfunction, which is induced by anterograde glutamatergic excitotoxicity. Transcranial magnetic stimulation (TMS) is a neurophysiological tool that provides a non-invasive and painless assessment of cortical function. Threshold tracking methodologies have been recently adopted for TMS, whereby changes in threshold rather than motor evoked potential (MEP) amplitude serve as outcome measures. This technique is reliable and provides a rapid assessment of cortical function in ALS. Utilisng the threshold tracking TMS technique, cortical hyperexcitability was demonstrated as an early feature in sporadic ALS preceding the onset of LMN dysfunction and possibly contributing to disease spread. Separately, cortical hyperexcitability was reported to precede the clinical onset of familial ALS. Of further relevance, the threshold tracking TMS technique was proven to reliably distinguish ALS from mimicking disorders, even in the presence of a comparable degree of LMN dysfunction, suggesting a diagnostic utility of TMS. Taken in total, threshold tracking TMS has provided support for a cortical involvement at the earliest detectable stages of ALS, underscoring the utility of the technique for probing the underlying pathophysiology. The present review will discuss the physiological processes underlying TMS parameters, while further evaluating the pathophysiological and diagnostic utility of threshold tracking TMS in ALS.

An optimal protocol for measurement of corticospinal excitability, short intracortical inhibition and intracortical facilitation in the rectus femoris

The study aimed to determine the optimal application of single- and paired-pulse transcranial magnetic stimulation (TMS) in the rectus femoris. Twenty-nine male adults participated in the study, which involved 5 separate experiments. Experiments 1 to 3 assessed the effect of conditioning stimulus (CS) intensity (60, 70, 80 and 90% active motor threshold, AMT), contraction strength (5, 10, 20 and 50% maximum voluntary contraction, MVC), and inter-stimulus interval (ISI, 2-5 ms for short-interval intracortical inhibition, SICI and 10-15 ms for intracortical facilitation, ICF) on SICI and ICF. In Experiment 4, 30 measurements of corticospinal excitability (CSE), SICI and ICF were recorded, with the minimum number of consecutive measurements required as a probability of falling within the 95% CI determined. In Experiment 5, within- and between-day reliability of CSE, SICI and ICF was assessed. The results suggest that for SICI, a CS of 70% AMT, ISI of 2 ms, and contraction strength of 5 or 10% MVC induces the greatest level of inhibition. Negligible differences in ICF were seen across stimulus variables. The minimum number of measurements required to obtain an accurate estimate of CSE, SICI and ICF was 21, 18 and 17, respectively. Using the optimal stimulus variables and number of measurements, CSE, SICI and ICF can be measured reliably both within- and between-days (intraclass correlation coefficient, ICC ≥ 0.87, ≥0.74, and ≥0.61, respectively). The current findings can be used to guide future investigations using single- and paired-pulse TMS to elicit responses in the rectus femoris.

Test-retest reliability of short-interval intracortical inhibition and intracortical facilitation in patients with schizophrenia

Impaired short-interval intracortical inhibition (SICI) elicited by paired-pulse TMS (ppTMS) has emerged as one of the most consistent TMS findings in patients with schizophrenia. Reduction of SICI has been reproducibly found, suggesting that SICI may be a new biomarker indexing the inhibitory dysfunction in schizophrenia. This study evaluated whether SICI has the test-retest reliability suitable for clinical trial and research applications. SICIs, intracortical facilitation (ICF), and other ppTMS effects were obtained using inter-stimulus intervals (ISIs) from 1 to 500 ms, on 2 occasions about 3-4 weeks apart in patients with schizophrenia and healthy individuals. Acceptable test-retest reliabilities were found for SICI (at 1 and 3 ms ISIs) in both patients and controls (all intraclass correlation coefficients r > 0.6). However, test-retest reliability in longer ISIs and in ICF were modest or poor. To our knowledge, this is the first study demonstrated the acceptable reliability of SICI measure in patients with schizophrenia. The data support SICI as a reliable biomarker for schizophrenia.

Short-interval intracortical inhibition: Comparison between conventional and threshold-tracking techniques

Background

Short-interval intracortical inhibition (SICI) is conventionally measured as the relative amplitude reduction of motor evoked potentials (MEPs) by subthreshold conditioning stimuli. In threshold-tracking SICI (T-SICI), stimulus intensity is instead adjusted repeatedly to maintain a constant MEP and inhibition is measured as the relative threshold increase. T-SICI is emerging as a useful diagnostic test, but its relationship to conventional amplitude SICI (A-SICI) is unclear.

Objective

To compare T-SICI and its reliability with conventional A-SICI measurements.

Methods

In twelve healthy volunteers (6 men, median age 30 years), conventional and T-SICI were recorded at conditioning stimuli (CS) of 50–80% resting motor threshold (RMT) and interstimulus interval of 2.5 ms. Measurements were repeated on the same day and at least a week later by a single operator.

Results

Across the CS range, mean group T-SICI showed a strong linear relationship to the mean group values measured by conventional technique (y = 29.7–0.3x, R² = 0.99), but there was considerable interindividual variability. At CS 60–80% RMT, T-SICI had excellent intraday (intraclass correlation coefficient, ICC, 0.81–0.92) and adequate-to-excellent interday (ICC 0.61–0.88) reproducibility. Conventional SICI took longer to complete (median of 5.8 vs 3.8 min, p < 0.001) and tended to have poorer reproducibility (ICC 0.17–0.42 intraday, 0.37–0.51 interday). With T-SICI, smaller sample sizes were calculated for equally powered interventional studies.

Conclusion

The close relationship between conventional and T-SICI suggests that both techniques reflect similar cortical inhibitory mechanisms. Threshold-tracking measurements of SICI may be able to improve reproducibility, to shorten acquisition time and to reduce sample sizes for interventional studies compared with the conventional technique.

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Threshold-tracking SICI correlates well with conventional measures.

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It shows a trend towards better reproducibility on a group level.

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Threshold-tracking SICI is potentially quicker to obtain.

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It may require a smaller sample size in interventional studies.

GABA and primary motor cortex inhibition in young and older adults: a multimodal reliability study

The effects of healthy aging on γ-aminobutyric acid (GABA) within primary motor cortex (M1) remain poorly understood. Studies have reported contrasting results, potentially due to limitations with the common assessment technique. The aim of the present study was to investigate the effect of healthy aging on M1 GABA concentration and neurotransmission using a multimodal approach. Fifteen young and sixteen older adults participated in this study. Magnetic resonance spectroscopy (MRS) was used to measure M1 GABA concentration. Single-pulse and threshold-tracking paired-pulse transcranial magnetic stimulation (TMS) protocols were used to examine cortical silent period duration, short- and long-interval intracortical inhibition (SICI and LICI), and late cortical disinhibition (LCD). The reliability of TMS measures was examined with intraclass correlation coefficient analyses. SICI at 1 ms was reduced in older adults (15.13 ± 2.59%) compared with young (25.66 ± 1.44%; P = 0.002). However, there was no age-related effect for cortical silent period duration, SICI at 3 ms, LICI, or LCD (all P > 0.66). The intersession reliability of threshold-tracking measures was good to excellent for both young (range 0.75-0.96) and older adults (range 0.88-0.93). Our findings indicate that extrasynaptic inhibition may be reduced with advancing age, whereas GABA concentration and synaptic inhibition are maintained. Furthermore, MRS and threshold-tracking TMS provide valid and reliable assessment of M1 GABA concentration and neurotransmission, respectively, in young and older adults.NEW & NOTEWORTHY γ-Aminobutyric acid (GABA) in primary motor cortex was assessed in young and older adults using magnetic resonance spectroscopy and threshold-tracking paired-pulse transcranial magnetic stimulation. Older adults exhibited reduced extrasynaptic inhibition (short-interval intracortical inhibition at 1 ms) compared with young, whereas GABA concentration and synaptic inhibition were similar between age groups. We demonstrate that magnetic resonance spectroscopy and threshold-tracking provide valid and reliable assessments of primary motor cortex GABA concentration and neurotransmission, respectively.

Comparing GABA-dependent physiological measures of inhibition with proton magnetic resonance spectroscopy measurement of GABA using ultra-high-field MRI

Imbalances in glutamatergic (excitatory) and GABA (inhibitory) signalling within key brain networks are thought to underlie many brain and mental health disorders, and for this reason there is considerable interest in investigating how individual variability in localised concentrations of these molecules relate to brain disorders. Magnetic resonance spectroscopy (MRS) provides a reliable means of measuring, in vivo, concentrations of neurometabolites such as GABA, glutamate and glutamine that can be correlated with brain function and dysfunction. However, an issue of much debate is whether the GABA observed and measured using MRS represents the entire pool of GABA available for measurement (i.e., metabolic, intracellular, and extracellular) or is instead limited to only some portion of it. GABA function can also be investigated indirectly in humans through the use of non-invasive transcranial magnetic stimulation (TMS) techniques that can be used to measure cortical excitability and GABA-mediated physiological inhibition. To investigate this issue further we collected in a single session both types of measurement, i.e., TMS measures of cortical excitability and physiological inhibition and ultra-high-field (7 T) MRS measures of GABA, glutamate and glutamine, from the left sensorimotor cortex of the same group of right-handed individuals. We found that TMS and MRS measures were largely uncorrelated with one another, save for the plateau of the TMS IO curve that was negatively correlated with MRS-Glutamate (Glu) and intra-cortical facilitation (10ms ISI) that was positively associated with MRS-Glutamate concentration. These findings are consistent with the view that the GABA concentrations measured using the MRS largely represent pools of GABA that are linked to tonic rather than phasic inhibition and thus contribute to the inhibitory tone of a brain area rather than GABAergic synaptic transmission.

Transcranial Magnetic Stimulation for the Assessment of Neurodegenerative Disease

Transcranial magnetic stimulation (TMS) is a noninvasive technique that has provided important information about cortical function across an array of neurodegenerative disorders, including Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and related extrapyramidal disorders. Application of TMS techniques in neurodegenerative diseases has provided important pathophysiological insights, leading to the development of pathogenic and diagnostic biomarkers that could be used in the clinical setting and therapeutic trials. Abnormalities of TMS outcome measures heralding cortical hyperexcitability, as evidenced by a reduction of short-interval intracortical inhibition and increased in motor-evoked potential amplitude, have been consistently identified as early and intrinsic features of amyotrophic lateral sclerosis (ALS), preceding and correlating with the ensuing neurodegeneration. Cortical hyperexcitability appears to form the pathogenic basis of ALS, mediated by trans-synaptic glutamate-mediated excitotoxic mechanisms. As a consequence of these research findings, TMS has been developed as a potential diagnostic biomarker, capable of identifying upper motor neuronal pathology, at earlier stages of the disease process, and thereby aiding in ALS diagnosis. Of further relevance, marked TMS abnormalities have been reported in other neurodegenerative diseases, which have varied from findings in ALS. With time and greater utilization by clinicians, TMS outcome measures may prove to be of utility in future therapeutic trial settings across the neurodegenerative disease spectrum, including the monitoring of neuroprotective, stem-cell, and genetic-based strategies, thereby enabling assessment of biological effectiveness at early stages of drug development.

The role of white matter microstructure in inhibitory deficits in patients with schizophrenia

BACKGROUND

Inhibitory-excitatory (I-E) imbalance has increasingly been proposed as a fundamental mechanism giving rise to many schizophrenia-related pathophysiology. The integrity of I-E functions should require precise and rapid electrical signal transmission.

OBJECTIVE/HYPOTHESIS

We hypothesized that part of the I-E abnormality in schizophrenia may originate from their known abnormal white matter connectivity that may interfere the I-E functions.

METHODS

We test this using short-interval intracortical inhibition (SICI) vs. intracortical facilitation (ICF) which is a non-invasive measurement of I-E signaling. SICI-ICF from left motor cortex and white matter microstructure were assessed in schizophrenia patients and healthy controls.

RESULTS

Schizophrenia patients showed significantly reduced SICI but not ICF. White matter microstructure as measured by fraction anisotropy (FA) in diffusion tensor imaging had a significant effect on SICI in patients, such that weaker SICI was associated with lower FA in several white matter tracts, most strongly with left corona radiata (r = -0.68, p = 0.0002) that contains the fibers connecting with left motor cortex. Left corticospinal tract, which carries the motor fibers to peripheral muscular output, also showed significant correlation with SICI (r = -0.54, p = 0.005). Mediation analysis revealed that much of the schizophrenia disease effect on SICI can be accounted for by mediation through left corona radiata. SICI was also significantly associated with the performance of processing speed in patients.

CONCLUSION

This study demonstrated the importance of structural circuitry integrity in inhibitory signaling in schizophrenia, and encouraged modeling the I-E dysfunction in schizophrenia from a circuitry perspective.

Unravelling motor networks in patients with chronic disorders of consciousness: A promising minimally invasive approach

Behavioral responsiveness and awareness levels correlate with the degree of functional connectivity within cortical-thalamocortical networks, whose breakdown accounts for chronic disorders of consciousness (DOC). Our study was aimed at assessing the role of the primary motor area (M1) and premotor-M1 circuitry dysfunction in motor output deterioration in minimally conscious state (MCS) and unresponsive wakefulness syndrome (UWS) patients. As a control group, we included a healthy subject (HC) sample in the study. We evaluated the effects of different types of transcranial magnetic stimuli over M1 by recording post-stimulus time histogram (PSTH), which includes a series of peaks of unit firing activity that match with D and I-waves, characterizing the descending corticospinal volleys evoked by transcranial magnetic stimuli. As compared to HC, DOC patients showed a dysfunction of intra-M1 and premotor-M1 circuits, which correlated with the Coma Recovery Scale-Revised scorings. Nonetheless, one UWS patient showed a partially preserved premotor-M1 circuitry, paralleled by a severe intra-M1 circuitry dysfunction. Our data suggest that motor unresponsiveness in some DOC patients may be due to a pure motor output failure, as in the functional locked-in syndrome (fLIS), rather than to a premotor-motor connectivity impairment, which instead characterizes MCS and UWS.

Impaired long-term potentiation-like cortical plasticity in presymptomatic genetic frontotemporal dementia

Neurophysiological biomarkers were assessed using a transcranial magnetic stimulation multiparadigm approach in 13 presymptomatic (n = 13 Granulin) and 14 symptomatic (n = 11 Granulin, n = 3 C9orf72) subjects with a pathogenic mutation for frontotemporal dementia (FTD). Intracortical facilitation and long-term potentiation-like plasticity were impaired in presymptomatic carriers, compared to healthy controls, more than 15 years before expected symptom onset. In symptomatic carriers, a decrease in short-interval intracortical inhibition, compared to presymptomatic carriers, was found. In conclusion, these biomarkers could provide the footprints of specific physiopathological processes in the development of this disease and possibly support the diagnosis of autosomal-dominant FTD. Ann Neurol 2016;80:472-476.

Technical aspects of neurostimulation: Focus on equipment, electric field modeling, and stimulation protocols

Neuromodulation is a field of science, medicine, and bioengineering that encompasses implantable and non-implantable technologies for the purpose of improving quality of life and functioning of humans. Brain neuromodulation involves different neurostimulation techniques: transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), vagus nerve stimulation (VNS), and deep brain stimulation (DBS), which are being used both to study their effects on cognitive brain functions and to treat neuropsychiatric disorders. The mechanisms of action of neurostimulation remain incompletely understood. Insight into the technical basis of neurostimulation might be a first step towards a more profound understanding of these mechanisms, which might lead to improved clinical outcome and therapeutic potential. This review provides an overview of the technical basis of neurostimulation focusing on the equipment, the present understanding of induced electric fields, and the stimulation protocols. The review is written from a technical perspective aimed at supporting the use of neurostimulation in clinical practice.

Modulation of Cortical Inhibitory Circuits after Cathodal Transcranial Direct Current Stimulation over the Primary Motor Cortex

Here, we aimed to evaluate whether cathodal transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) and primary somatosensory cortex (S1) can modulate cortical inhibitory circuits. Sixteen healthy subjects participated in this study. Cathodal tDCS was positioned over the left M1 (M1 cathodal) or left S1 (S1 cathodal) with an intensity of 1 mA for 10 min. Sham tDCS was applied for 10 min over the left M1 (sham). Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) were recorded from the right abductor pollicis brevis (APB) muscle before the intervention (pre) and 10 and 30 min after the intervention (post 1 and post 2, respectively). Cortical inhibitory circuits were evaluated using short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI). M1 cathodal decreased single-pulse MEP amplitudes at post 1 and decreased SAI at post 1 and post 2; however, SICI did not exhibit any change. S1 cathodal and sham did not show any changes in MEP amplitudes at any of the three time points. These results demonstrated that cathodal tDCS over the M1 not only decreases the M1 excitability but also affects the cortical inhibitory circuits related to SAI.

Exploring the Evolution of Cortical Excitability Following Acute Stroke

BACKGROUND

Evolution of changes in intracortical excitability following stroke, particularly in the contralesional hemisphere, is being increasingly recognized in relation to maximizing the potential for functional recovery.

OBJECTIVE

The present study utilized a prospective longitudinal design over a 12-month period from stroke onset, to investigate the evolution of intracortical excitability involving both motor cortices and their relationship to recovery, and whether such changes were influenced by baseline stroke characteristics.

METHODS

Thirty-one patients with acute unilateral ischemic stroke were recruited from a tertiary hospital stroke unit. Comprehensive clinical assessments and cortical excitability were undertaken at stroke onset using a novel threshold-tracking paired-pulse transcranial magnetic stimulation technique, and repeated at 3-, 6-, and 12-month follow-up in 17 patients who completed the longitudinal assessment.

RESULTS

Shortly following stroke, short-interval intracortical inhibition (SICI) was significantly reduced in both lesioned and contralesional hemispheres that correlated with degree of recovery over the subsequent 3 months. Over the follow-up period, ipsilesional SICI remained reduced in all patient groups, while SICI over the contralesional hemisphere remained reduced only in the groups with cortical stroke or more baseline functional impairment.

CONCLUSIONS

The current study has demonstrated that evolution of intracortical excitability, particularly over the contralesional hemisphere, may vary between patients with differing baseline stroke and clinical characteristics, suggesting that ongoing contralesional network recruitment may be necessary for those patients who have significant disruptions to the integrity of ipsilesional motor pathways. Results from the present series have implications for the development of neuromodulatory brain stimulation protocols to harness and thereby facilitate stroke recovery.

A Single Session of Repetitive Transcranial Magnetic Stimulation Over the Dorsolateral Prefrontal Cortex in Patients With Unresponsive Wakefulness Syndrome

Background. The level of consciousness is regulated by the activation of the frontal parietal network since a disruption within this interregional cortical connectivity is associated with a consciousness breakdown. Objective. The primary aim of the present study was to examine the feasibility and the safety of a single session of repetitive transcranial magnetic stimulation (rTMS), delivered over the dorsolateral prefrontal cortex (DLPFC), in patients with unresponsive wakefulness syndrome (UWS). As a secondary aim, we investigated the efficacy of rTMS over DLPFC on cortico-cortical connectivity as tested with paired-pulse and dual-coil TMS techniques. Methods. We enrolled 10 healthy and 10 postanoxic UWS subjects. After clinical assessment, subjects underwent a single/paired pulse TMS paradigm evaluating the inhibitory and facilitatory intracortical circuits. In addition, several interregional interactions between primary motor, dorsal and ventral premotor, supplementary motor, and posterior parietal cortex areas were investigated by means of dual-coil TMS technique. These parameters were evaluated at baseline, immediately after, and 60 minutes after a protocol of 10-Hz rTMS delivered at the level of the DLPFC. Results. rTMS over DLPFC did not cause any adverse events. We showed that rTMS over DLPFC did not induce, at group level, any clinical improvement or intra-/intercortical connectivity changes. Interestingly, in 3 patients rTMS induced a significant, although transient, clinical improvement associated with a short-lasting reshaping of brain connectivity. Conclusions. We demonstrated that a single session of 10-Hz rTMS over the right DLPFC may transiently improve consciousness and partially restore the connectivity within several cortical areas in some patients with UWS.

Motor cortical function and the precision grip

While task‐dependent changes in motor cortical outputs have been previously reported, the issue of whether such changes are specific for complex hand tasks remains unresolved. The aim of the present study was to determine whether cortical inhibitory tone and cortical output were greater during precision grip and power grip. Motor cortex excitability was undertaken by using the transcranial magnetic stimulation threshold tracking technique in 15 healthy subjects. The motor‐evoked potential (MEP) responses were recorded over the abductor pollicis brevis (APB), with the hand in the following positions: (1) rest, (2) precision grip and (3) power grip. The MEP amplitude (MEP amplitude _REST 23.6 ± 3.3%; MEP amplitude _{PRECISIONGRIP} 35.2 ± 5.6%; MEP amplitude _POWERGRIP 19.6 ± 3.4%, F = 2.4, P < 0.001) and stimulus‐response gradient (SLOPE_REST 0.06 ± 0.01; SLOPE_PRCISIONGRIP 0.15 ± 0.04; SLOPE _POWERGRIP 0.07 ± 0.01, P < 0.05) were significantly increased during precision grip. Short interval intracortical inhibition (SICI) was significantly reduced during the precision grip (SICI _REST 15.0 ± 2.3%; SICI _{PRECISIONGRIP} 9.7 ± 1.5%, SICI _POWERGRIP 15.9 ± 2.7%, F = 2.6, P < 0.05). The present study suggests that changes in motor cortex excitability are specific for precision grip, with functional coupling of descending corticospinal pathways controlling thumb and finger movements potentially forming the basis of these cortical changes.

This manuscript establishes that specific cortical mechanisms underlie the maintenance of the precision grip. The mechanisms appear distinct to the processes maintaining the power grip.

Effects of short-latency afferent inhibition on short-interval intracortical inhibition

Peripheral nerve stimulation inhibits the motor cortex, and the process has been termed short-latency afferent inhibition (SAI) at interstimulus intervals (ISIs) of ∼20 ms. The objective of the present study was to test how SAI interacts with short-interval intracortical inhibition (SICI) under different stimulation conditions. We studied 20 healthy volunteers. Surface electromyogram was recorded from the first dorsal interosseous muscle. Using paired- and triple-pulse paradigms, we investigated how SAI interacts with SICI under these different conditions. The effects of different conditioning stimulus (CS) intensities (0.6-0.9 active motor threshold), SAI latencies (23 and 25 ms), and ISIs (2 and 3 ms) for SICI were examined in rest and active conditions. SAI had inhibitory interactions with SICI at different CS intensities for rest or active SICI, at SAI latencies of 23 and 25 ms. This interaction occurred at weak CS intensities for SICI when there was no inhibition, and SICI became facilitatory in the presence of SAI. This can be explained by SICI inhibiting SAI and not by saturation of inhibition. The interaction between SAI and SICI was greater for SICI at ISI of 3 ms than for ISI of 2 ms, suggesting that different circuits may be activated at these ISIs. We conclude that SAI and SICI have inhibitory interactions that are influenced by factors such as ISI and muscle activities, which should be considered in design and interpretation of cortical interaction studies.

Individualized brain inhibition and excitation profile in response to paired-pulse TMS

Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) are generated from paired-pulse transcranial magnetic stimulations (ppTMS) using certain interstimulus intervals (ISIs). ppTMS provides an accessible technique to evaluate inhibitory and facilitatory motor neural circuits. However, SICI and ICF are highly variable such that individual variability is not captured by any one static ISI. The authors hypothesized that individuals may have individualized and relatively stable pattern of SICI-ICF profiles. They tested SICI and ICF profiles using ISIs from 1 to 500 ms, on 2 occasions about 3 weeks apart, and the test-retest reliability, in 23 healthy controls. Moderate-to-good test-retest reliabilities were found at ppTMS with 1 and 3 ms ISIs (SICI) and with 12, 15, 18, and 21 ms ISIs (ICF), but not with other control ISIs. A similar pattern of results was obtained for men and women. Interestingly, the peak facilitation, peak inhibition, and maximum inhibition and facilitation ranges were individualized, such that they varied considerably across individuals but had high repeatability within individual (Cronbach's α = 0.76 to 0.85). Therefore, individuals appear to have unique inhibition-facilitation profiles that are relatively stable. Although the functional implications of individualized profiles are currently unknown, the relatively stable profiles may index underlying neural inhibition and excitation traits.

Transcranial magnetic stimulation and amyotrophic lateral sclerosis: pathophysiological insights

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disorder of the motor neurons in the motor cortex, brainstem and spinal cord. A combination of upper and lower motor neuron dysfunction comprises the clinical ALS phenotype. Although the ALS phenotype was first observed by Charcot over 100 years ago, the site of ALS onset and the pathophysiological mechanisms underlying the development of motor neuron degeneration remain to be elucidated. Transcranial magnetic stimulation (TMS) enables non-invasive assessment of the functional integrity of the motor cortex and its corticomotoneuronal projections. To date, TMS studies have established motor cortical and corticospinal dysfunction in ALS, with cortical hyperexcitability being an early feature in sporadic forms of ALS and preceding the clinical onset of familial ALS. Taken together, a central origin of ALS is supported by TMS studies, with an anterograde transsynaptic mechanism implicated in ALS pathogenesis. Of further relevance, TMS techniques reliably distinguish ALS from mimic disorders, despite a compatible peripheral disease burden, thereby suggesting a potential diagnostic utility of TMS in ALS. This review will focus on the mechanisms underlying the generation of TMS measures used in assessment of cortical excitability, the contribution of TMS in enhancing the understanding of ALS pathophysiology and the potential diagnostic utility of TMS techniques in ALS.

The aging motor system as a model for plastic changes of GABA-mediated intracortical inhibition and their behavioral relevance

Since GABAA-mediated intracortical inhibition has been shown to underlie plastic changes throughout the lifespan from development to aging, here, the aging motor system was used as a model to analyze the interdependence of plastic alterations within the inhibitory motorcortical network and level of behavioral performance. Double-pulse transcranial magnetic stimulation (dpTMS) was used to examine inhibition by means of short-interval intracortical inhibition (SICI) of the contralateral primary motor cortex in a sample of 64 healthy right-handed human subjects covering a wide range of the adult lifespan (age range 20-88 years, mean 47.6 ± 20.7, 34 female). SICI was evaluated during resting state and in an event-related condition during movement preparation in a visually triggered simple reaction time task. In a subgroup (N = 23), manual motor performance was tested with tasks of graded dexterous demand. Weak resting-state inhibition was associated with an overall lower manual motor performance. Better event-related modulation of inhibition correlated with better performance in more demanding tasks, in which fast alternating activation of cortical representations are necessary. Declining resting-state inhibition was associated with weakened event-related modulation of inhibition. Therefore, reduced resting-state inhibition might lead to a subsequent loss of modulatory capacity, possibly reflecting malfunctioning precision in GABAAergic neurotransmission; the consequence is an inevitable decline in motor function.

Different current intensities of anodal transcranial direct current stimulation do not differentially modulate motor cortex plasticity

Transcranial direct current stimulation (tDCS) is a noninvasive technique that modulates the excitability of neurons within the motor cortex (M1). Although the aftereffects of anodal tDCS on modulating cortical excitability have been described, there is limited data describing the outcomes of different tDCS intensities on intracortical circuits. To further elucidate the mechanisms underlying the aftereffects of M1 excitability following anodal tDCS, we used transcranial magnetic stimulation (TMS) to examine the effect of different intensities on cortical excitability and short-interval intracortical inhibition (SICI). Using a randomized, counterbalanced, crossover design, with a one-week wash-out period, 14 participants (6 females and 8 males, 22–45 years) were exposed to 10 minutes of anodal tDCS at 0.8, 1.0, and 1.2 mA. TMS was used to measure M1 excitability and SICI of the contralateral wrist extensor muscle at baseline, immediately after and 15 and 30 minutes following cessation of anodal tDCS. Cortical excitability increased, whilst SICI was reduced at all time points following anodal tDCS. Interestingly, there were no differences between the three intensities of anodal tDCS on modulating cortical excitability or SICI. These results suggest that the aftereffect of anodal tDCS on facilitating cortical excitability is due to the modulation of synaptic mechanisms associated with long-term potentiation and is not influenced by different tDCS intensities.

Cortical excitability differences between flexor pollicis longus and APB

Although abductor pollicis brevis (APB) and flexor pollicis longus (FPL) share a common peripheral nerve supply, these muscles subserve different functions and may be differently affected in neurodegenerative disease such as amyotrophic lateral sclerosis (ALS). As a consequence, differences in cortical excitability may potentially develop in relation to these functional differences. Cortical excitability was assessed using the threshold tracking transcranial magnetic stimulation (TMS) technique in 15 healthy controls with motor responses recorded over the APB and FPL using surface electrode recordings. Short-interval intracortical inhibition (SICI) was significantly reduced from the FPL compared to APB (SICIFPL 6.9±1.8%; SICIAPB 10.7±1.4%, P<0.01). In addition, the FPL motor evoked potential amplitude (MEPFPL 14.7±2.3%; MEPAPB 21.7±3.9%; P<0.01) and cortical silent period duration (CSPFPL 174.7±6.7ms; CSPAPB 205.4±3.9ms, P<0.01) were significantly smaller. The findings in the present study indicate that cortical inhibition and corticomotoneuronal output is reduced when recording over the FPL. The differences in cortical excitability may develop as a consequence of varied function and could potentially explain the dissociated muscle atrophy evident in ALS.

Utility of transcranial magnetic stimulation in delineating amyotrophic lateral sclerosis pathophysiology

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disorder of the motor neurons in the motor cortex, brainstem, and spinal cord. The clinical phenotype of ALS is underscored by a combination of upper and lower motor neuron dysfunction. Although this phenotype was observed over 100 years ago, the site of ALS onset and the pathophysiological mechanisms underlying the development of motor neuron degeneration remain to be elucidated. Transcranial magnetic stimulation (TMS) enables noninvasive assessment of the functional integrity of the motor cortex and its corticomotoneuronal projections. To date, TMS studies have established cortical dysfunction in ALS, with cortical hyperexcitability being an early feature in sporadic forms of ALS and preceding the clinical onset of familial ALS. Taken together, a central origin of ALS is supported by TMS studies, with an anterograde dying-forward mechanism implicated in ALS pathogenesis. Of further relevance, TMS techniques reliably distinguish ALS from mimic disorders, despite a compatible peripheral disease burden, thereby suggesting a potential diagnostic utility of TMS in ALS. This chapter reviews the mechanisms underlying the generation of TMS parameters utilized in assessment of cortical excitability, the contribution of TMS in enhancing the understanding of ALS pathophysiology, and the potential diagnostic utility of TMS techniques in ALS.

Strength training reduces intracortical inhibition

AIM

Paired-pulse transcranial magnetic stimulation was used to investigate the influence of 4 weeks of heavy load squat strength training on corticospinal excitability and short-interval intracortical inhibition (rectus femoris muscle).

METHODS

Participants (n = 12) were randomly allocated to a strength training or control group. The strength training group completed 4 weeks of heavy load squat strength training. Recruitment curves were constructed to determine values for the slope of the curve, V50 and peak height. Short-interval intracortical inhibition was assessed using a subthreshold (0.7 × active motor threshold) conditioning stimulus, followed 3 ms later by a supra-threshold (1.2 × active motor threshold) test stimulus. All motor evoked responses were taken during 10% of maximal voluntary isometric contraction (MVC) torque and normalized to the maximal M-wave.

RESULTS

The strength training group attained 87% increases in 1RM squat strength (P < 0.01), significant increases in measures of corticospinal excitability (1.2 × Motor threshold: 116%, P = 0.016; peak height of recruitment curve = 105%, P < 0.001), and a 32% reduction in short-interval intracortical inhibition (P < 0.01) following the 4-week intervention compared with control. There were no changes in any dependent variable (P > 0.05) detected in the control group.

CONCLUSION

Repeated high force voluntary muscle activation in the form of short-term strength training reduces short-interval intracortical inhibition. This is consistent with studies involving skilled/complex tasks or novel movement patterns and acute studies investigating acute voluntary contractions.

Cortical dysfunction underlies disability in multiple sclerosis

BACKGROUND

Gray matter atrophy has been implicated in the development of secondary progressive multiple sclerosis (SPMS). Cortical function may be assessed by transcranial magnetic stimulation (TMS). Determining whether cortical dysfunction was a feature of SPMS could be of pathophysiological significance.

OBJECTIVES

Consequently, novel paired-pulse threshold tracking TMS techniques were used to assess whether cortical dysfunction was a feature of SPMS.

METHODS

Cortical excitability studies were undertaken in 15 SPMS, 25 relapsing-remitting MS patients (RRMS) and 66 controls.

RESULTS

Short interval intracortical inhibition (SPMS 3.0 ± 2.1%; RRMS 12.8 ± 1.7%, p < 0.01; controls 10.5 ± 0.7%, p < 0.01) and motor evoked potential (MEP) amplitude (SPMS 11.5 ± 2.2%; RRMS 26.3 ± 3.6%, p <0.05; controls 24.7 ± 1.8%, p < 0.01) were reduced in SPMS, while intracortical facilitation (SPMS -5.2 ± 1.9%; RRMS -2.0 ± 1.4, p < 0.05; controls -0.9 ± 0.7, p < 0.01) and resting motor threshold were increased (SPMS 67.5 ± 4.5%; RRMS 56.0 ± 1.5%, p < 0.01; controls 59.0 ± 1.1%, p < 0.001). Further, central motor conduction time was prolonged in SPMS (9.1 ± 1.2 ms, p < 0.001) and RRMS (7.0 ± 0.9 ms, p < 0.05) patients compared with controls (5.5 ± 0.2 ms). The observed changes in cortical function correlated with the Expanded Disability Status Scale.

CONCLUSION

Together, these findings suggest that cortical dysfunction is associated with disability in MS, and documentation of such cortical dysfunction may serve to quantify disease severity in MS.