Modulation of the Direction and Magnitude of Hebbian Plasticity in Human Motor Cortex by Stimulus Intensity and Concurrent Inhibition

The Effect of Cerebellar rTMS on Modulating Motor Dysfunction in Neurological Disorders: a Systematic Review

The effectiveness of cerebellar repetitive transcranial magnetic stimulation (rTMS) on motor dysfunction in patients with neurological disorders has received increasing attention because of its potential for neuromodulation. However, studies on the neuromodulatory effects, parameters, and safety of rTMS implementation in the cerebellum to alleviate motor dysfunction are limited. This systematic review aimed to evaluate the effectiveness and safety of cerebellar rTMS treatment for motor dysfunction caused by neurological disorders and to review popular stimulation parameters. Five electronic databases-Medline, Web of Science, Scopus, Cochrane Library, and Embase-were searched for relevant research published from inception to July 2022. All randomized controlled trials (RCTs) that reported the effects of cerebellar rTMS combined with behavioral rating scales on motor dysfunction were eligible for enrollment. Additionally, reference lists of the enrolled studies were manually checked. Among 1156 articles screened, 21 RCTs with 666 subjects were included. rTMS conducted on the cerebellum showed an improvement in stroke (spasticity, balance, and gait), cervical dystonia, Parkinson's disease (tremor), cerebellar ataxia, and essential tremor but not in multiple sclerosis. The 8-shaped coil with a diameter of 70 mm was determined as the most common therapeutic choice. None of the studies reported severe adverse events except mild side effects in three. Therefore, rTMS appears to be a promising and safe technique for the treatment of motor dysfunction, targeting the cerebellum to induce motor behavioral improvement. Further rigorous RCTs, including more samples and longer follow-up periods, are required to precisely explore the effective stimulation parameters and possible mechanisms.

Utility of transcranial magnetic stimulation in the assessment of spinal cord injury: Current status and future directions

Comprehensive assessment following traumatic spinal cord injury (SCI) is needed to improve prognostication, advance the understanding of the neurophysiology and better targeting of clinical interventions. The International Standards for Neurological Classification of Spinal Cord Injury is the most common clinical examination recommended for use after a SCI. In addition, there are over 30 clinical assessment tools spanning across different domains of the International Classification of Functioning, Disability, and Health that have been validated and recommended for use in SCI. Most of these tools are subjective in nature, have limited value in predicting neurologic recovery, and do not provide insights into neurophysiological mechanisms. Transcranial magnetic stimulation (TMS) is a non-invasive neurophysiology technique that can supplement the clinical assessment in the domain of body structure and function during acute and chronic stages of SCI. TMS offers a better insight into neurophysiology and help in better detection of residual corticomotor connectivity following SCI compared to clinical assessment alone. TMS-based motor evoked potential and silent period duration allow study of excitatory and inhibitory mechanisms following SCI. Changes in muscle representations in form of displacement of TMS-based motor map center of gravity or changes in the map area can capture neuroplastic changes resulting from SCI or following rehabilitation. Paired-pulse TMS measures help understand the compensatory reorganization of the cortical circuits following SCI. In combination with peripheral stimulation, TMS can be used to study central motor conduction time and modulation of spinal reflexes, which can be used for advanced diagnostic and treatment purposes. To strengthen the utility of TMS in SCI assessment, future studies will need to standardize the assessment protocols, address population-specific concerns, and establish the psychometric properties of TMS-based measurements in the SCI population.

Effects of Skin Stimulation on Sensory-Motor Networks Excitability: Possible Implications for Physical Training in Amyotrophic Lateral Sclerosis

Background

Many different trials were assessed for rehabilitation of patients with amyotrophic lateral sclerosis (ALS), with non-unique results. Beside the effects on muscle trophism, some of the encouraging results of physical training could be ascribed to the modulation of cortical excitability, which was found hyperexcited in ALS.

Objective

The effects of tactile skin stimulation in the modulation of the sensory-motor integrative networks in healthy subjects were assayed through the paired associative stimulation (PAS) protocol.

Methods

In total, 15 healthy subjects were enrolled. In the standard PAS session, the average amplitude of the motor evoked potential (MEP) after 10 stimuli of transcranial magnetic stimulation (TMS) was measured at the baseline and after the PAS protocol (0, 10, 20, 30, and 60 min). In the skin stimulation session, the average amplitude of the MEP was measured before and after 10 min of skin stimulation over the hand. Subsequently, each subject underwent the PAS stimulation and the measure of the average amplitude of the MEP (0, 10, 20, 30, and 60 min).

Results

The tactile skin stimulation on healthy subjects increases the PAS-induced sensory-motor network hyperexcitability in healthy subjects.

Conclusion

Skin stimulation should be avoided in the physiotherapeutic approaches for patients with ALS, given the possible hyperexciting effects on the already upmodulated sensory-motor networks. They can be taken into account for diseases characterized by downregulation of cortical and transcortical networks.

Using Brain Imaging to Improve Spatial Targeting of Transcranial Magnetic Stimulation for Depression

Transcranial magnetic stimulation (TMS) is an effective treatment for depression but is limited in that the optimal therapeutic target remains unknown. Early TMS trials lacked a focal target and thus positioned the TMS coil over the prefrontal cortex using scalp measurements. Over time, it became clear that this method leads to variation in the stimulation site and that this could contribute to heterogeneity in antidepressant response. Newer methods allow for precise positioning of the TMS coil over a specific brain location, but leveraging these precise methods requires a more precise therapeutic target. We review how neuroimaging is being used to identify a more focal therapeutic target for depression. We highlight recent studies showing that more effective TMS targets in the frontal cortex are functionally connected to deep limbic regions such as the subgenual cingulate cortex. We review how connectivity might be used to identify an optimal TMS target for use in all patients and potentially even a personalized target for each individual patient. We address the clinical implications of this emerging field and highlight critical questions for future research.

High-intensity, low-frequency repetitive transcranial magnetic stimulation enhances excitability of the human corticospinal pathway

Paired corticospinal-motoneuronal stimulation (PCMS) is the repeated pairing of transcranial magnetic stimulation (TMS) with peripheral nerve stimulation to modify corticospinal synapses; however, it has yet to be determined whether PCMS modulates cortical excitability in a manner similar to paired-associative stimulation protocols. In this study, we first examined the effects of PCMS on adductor pollicis motor evoked potentials (MEPs). In experiment 1, on 2 separate days PCMS (repetitive, high-intensity TMS and ulnar nerve stimulation pairs; 1.5-ms interstimulus interval; 0.1 Hz) was compared with control conditioning of repetitive high-intensity TMS-only stimuli (0.1 Hz). Before and after conditioning, adductor pollicis MEPs were elicited using low-intensity TMS in three different coil orientations to preferentially activate corticospinal axons directly (thus bypassing cortical effects) or indirectly (cortical effects present). Unexpectedly, similar MEP increases were seen for all orientations on both PCMS (129 to 136% of baseline) and control days (108 to 129% of baseline). Given the common factor between conditioning protocols was repeated, high-intensity TMS, further experiments were performed to characterize this repetitive TMS (rTMS) protocol. In experiment 2, an intensity dependence of the rTMS protocol was revealed by a lack of change in MEPs elicited after repetitive low-intensity TMS (0.1 Hz; P = 0.37). In experiment 3, MEP recruitment curve and paired pulse analyses showed that the high-intensity rTMS protocol increased MEPs over a range of stimulus intensities but that effects were not accompanied by changes in intracortical inhibition or facilitation (P > 0.12). These experiments reveal a novel high-intensity, low-frequency rTMS protocol for enhancing corticospinal excitability.NEW & NOTEWORTHY In this study, we present a novel, intensity-dependent repetitive transcranial magnetic stimulation (rTMS) protocol that induces lasting, plastic changes within the corticospinal tract. High-intensity rTMS at a frequency of 0.1 Hz induces facilitation of motor evoked potentials (MEPs) lasting at least 35 min. Additionally, these changes are not limited only to small MEPs but occur throughout the recruitment curve. Finally, facilitation of MEPs following high-intensity rTMS does not appear to be due to changes in intracortical inhibition or facilitation.

Motor learning enhanced by combined motor imagery and noninvasive brain stimulation is associated with reduced short-interval intracortical inhibition

BACKGROUND

Motor imagery (MI) improves motor skill learning, which is further enhanced when MI is paired with primary motor cortex transcranial brain stimulation or with electrical stimulation of the peripheral median nerve. Applying both stimulation types (here with 25 ms intervals) is called paired associative stimulation (PAS25). The final primary motor cortex output is determined by combined excitatory and intracortical inhibitory circuits, and reducing the latter is associated with enhanced synaptic transmission and efficacy. Indeed, short-interval intracortical inhibition (SICI) inhibits motor evoked potentials (MEPs), and motor learning has been associated with decreased SICI and increased cortical excitability. Here, we investigated whether cortical excitability and SICI are altered by PAS25 applied after MI-induced modulation of motor learning.

METHODS

Peak acceleration of a hand-grasping movement and MEPs and SICI were measured before and after MI alone, PAS25 alone, and MI followed by PAS25 in 16 healthy participants to evaluate changes in motor learning, corticospinal excitability, and intracortical inhibition.

RESULTS

After PAS25 alone, MEP amplitude increased while peak acceleration was unchanged. However, PAS25 applied following MI not only significantly enhanced both peak acceleration (p = 0.011) and MEP amplitude (p = 0.004) but also decreased SICI (p = 0.011). Moreover, we found that this decrease in SICI was significantly correlated with both the peak acceleration (r = 0.49, p = 0.029) and the MEP amplitude (r = 0.56, p = 0.013).

CONCLUSIONS

These results indicate that brain function altered by PAS25 of the motor cortex enhances MI-induced motor learning and corticospinal excitability and decreases SICI, suggesting that SICI underlies, at least in part, PAS25 modulation of motor learning.

Somatosensory and transcranial direct current stimulation effects on manual dexterity and motor cortex function: A metaplasticity study

BACKGROUND

Non-invasive neuromodulation may provide treatment strategies for neurological deficits affecting movement, such as stroke. For example, weak electrical stimulation applied to the hand by wearing a "mesh glove" (MGS) can transiently increase primary motor cortex (M1) excitability. Conversely, transcranial direct current stimulation with the cathode over M1 (c-tDCS) can decrease corticomotor excitability.

OBJECTIVE/HYPOTHESIS

We applied M1 c-tDCS as a priming adjuvant to MGS and hypothesised metaplastic effects would be apparent in improved motor performance and modulation of M1 inhibitory and facilitatory circuits.

METHODS

Sixteen right-handed neurologically healthy individuals participated in a repeated measures cross-over study; nine minutes of sham- or c-tDCS followed by 30 min of suprasensory threshold MGS. Dexterity of the non-dominant (left) hand was assessed using the grooved pegboard task, and measures of corticomotor excitability, intracortical facilitation, short-latency afferent inhibition (SAI), short-interval intracortical inhibition (SICI), and SAI in the presence of SICI (SAIxSICI), were obtained at baseline, post-tDCS, and 0, 30 and 60 min post-MGS.

RESULTS

There was a greater improvement in grooved pegboard completion times with c-tDCS primed MGS than sham + MGS. There was also more pronounced disinhibition of SAI. However, disinhibition of SAI in the presence of SICI was less and rest motor threshold higher compared to sham + MGS.

CONCLUSIONS

The results indicate a metaplastic modulation of corticomotor excitability with c-tDCS primed MGS. Further studies are warranted to determine how various stimulation approaches can induce metaplastic effects on M1 neuronal circuits to boost functional gains obtained with motor practice.

Assessment of neuroplasticity in late-life depression with transcranial magnetic stimulation

BACKGROUND

Studies using Transcranial Magnetic Stimulation (TMS), a non-invasive method of brain stimulation, have implicated impaired neuroplasticity in the pathophysiology of depression in younger adults. The role of neuroplasticity in late-life depression (LLD) has not yet been explored using TMS.

OBJECTIVE

This study aimed at evaluating motor cortical neuroplasticity using paired associative stimulation (PAS). Single-pulse TMS was used to induce motor-evoked potentials (MEP) in the contralateral hand muscle before and after PAS. The potentiation of MEP amplitudes after PAS was used as an indirect index of associative plasticity and long-term potentiation (LTP) (i.e. PAS-LTP).

RESULTS

48 older adults with depression and 34 age-matched healthy controls (HC) were compared. PAS- LTP was successfully induced in 68.8% of older adults with depression and 47.1% of HC. At the group level, older adults with depression failed to show statistically significant induction of neuroplasticity, which was observed in HC. However, no significant differences were observed between the two groups for PAS-LTP.

CONCLUSION

Our results suggest that associative plasticity does not differ substantially between older adults with depression and age-matched HC. Continued research is needed to more comprehensively understand the role of neuroplasticity in the pathophysiology of LLD.

The effects of individualised intermittent theta burst stimulation in the prefrontal cortex: A TMS-EEG study

Recent studies have highlighted variability in response to theta burst stimulation (TBS) in humans. TBS paradigm was originally developed in rodents to mimic gamma bursts coupled with theta rhythms, and was shown to elicit long-term potentiation. The protocol was subsequently adapted for humans using standardised frequencies of stimulation. However, each individual has different rhythmic firing pattern. The present study sought to explore whether individualised intermittent TBS (Ind iTBS) could outperform the effects of two other iTBS variants. Twenty healthy volunteers received iTBS over left prefrontal cortex using 30 Hz at 6 Hz, 50 Hz at 5 Hz, or individualised frequency in separate sessions. Ind iTBS was determined using theta-gamma coupling during the 3-back task. Concurrent use of transcranial magnetic stimulation and electroencephalography (TMS-EEG) was used to track changes in cortical plasticity. We also utilised mood ratings using a visual analogue scale and assessed working memory via the 3-back task before and after stimulation. No group-level effect was observed following either 30 or 50 Hz iTBS in TMS-EEG. Ind iTBS significantly increased the amplitude of the TMS-evoked P60, and decreased N100 and P200 amplitudes. A significant positive correlation between neurophysiological change and change in mood rating was also observed. Improved accuracy in the 3-back task was observed following both 50 Hz and Ind iTBS conditions. These findings highlight the critical importance of frequency in the parameter space of iTBS. Tailored stimulation parameters appear more efficacious than standard paradigms in neurophysiological and mood changes. This novel approach presents a promising option and benefits may extend to clinical applications.

Involvement of different neuronal components in the induction of cortical plasticity with associative stimulation

BACKGROUND

Paired associative stimulation (PAS), with stimulus interval of 21.5 or 25 ms, using transcranial magnetic stimulation in the posterior-anterior (PA) current direction, produces a long-term-potentiation-like effect. Stimulation with PA directed current generates both early and late indirect (I)-waves while that in anterior-posterior (AP) current predominantly elicits late I-waves. Short interval intracortical inhibition (SICI) inhibits late I-waves but not early I-waves.

OBJECTIVE

To investigate how cortical inhibition modulates the effects of PAS.

METHODS

PAS at stimulus interval of 21.5 ms conditioned by SICI (SICI-PAS) was compared to PAS alone with both PA and AP directed currents.

RESULTS

PAS with both current directions increased cortical excitability. SICI-PAS increased cortical excitability in the PA but not the AP current direction.

CONCLUSIONS

Both early and late I-waves circuits can mediate cortical PAS plasticity under different conditions. Plasticity induction with the late but not the early I-wave circuits is blocked by SICI.

Bilateral Motor Cortex Plasticity in Individuals With Chronic Stroke, Induced by Paired Associative Stimulation

Background: In the chronic phase after stroke, cortical excitability differs between the cerebral hemispheres; the magnitude of this asymmetry depends on degree of motor impairment. It is unclear whether these asymmetries also affect capacity for plasticity in corticospinal tract excitability or whether hemispheric differences in plasticity are related to chronic sensorimotor impairment. Methods: Response to paired associative stimulation (PAS) was assessed bilaterally in 22 individuals with chronic hemiparesis. Corticospinal excitability was measured as the area under the motor-evoked potential (MEP) recruitment curve (AUC) at baseline, 5 minutes, and 30 minutes post-PAS. Percentage change in contralesional AUC was calculated and correlated with paretic motor and somatosensory impairment scores. Results: PAS induced a significant increase in AUC in the contralesional hemisphere ( P = .041); in the ipsilesional hemisphere, there was no significant effect of PAS ( P = .073). Contralesional AUC showed significantly greater change in individuals without an ipsilesional MEP ( P = .029). Percentage change in contralesional AUC between baseline and 5 m post-PAS correlated significantly with FM score ( r = −0.443; P = .039) and monofilament thresholds ( r = 0.444, P = .044). Discussion: There are differential responses to PAS within each cerebral hemisphere. Contralesional plasticity was increased in individuals with more severe hemiparesis, indicated by both the absence of an ipsilesional MEP and a greater degree of motor and somatosensory impairment. These data support a body of research showing compensatory changes in the contralesional hemisphere after stroke; new therapies for individuals with chronic stroke could exploit contralesional plasticity to help restore function.

Impaired neuroplasticity in the prefrontal cortex in depression indexed through paired associative stimulation

BACKGROUND

Dysfunctional neuroplasticity may be one of the pathophysiological mechanisms underlying major depression. We have previously established methods to assess neuroplasticity from the dorsolateral prefrontal cortex (DLPFC) using a paired associative stimulation (PAS) paradigm, which pairs a preceding peripheral nerve stimulation with subsequent transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). We aimed to investigate neuroplasticity through the PAS paradigm in the DLPFC in patients with depression compared to healthy subjects.

METHODS

Twenty-nine patients with depression and 28 healthy controls participated in this study. There were no significant age or sex differences between the two groups. All participants received PAS paradigm in the DLPFC. We analyzed PAS induced potentiation from the DLPFC in both groups calculating the power of TMS-evoked potentials (TEP). A two-way ANOVA with PAS effect as a within-subject factor and diagnostic group as a between-subject factor was performed to examine the group differences in the PAS paradigm.

RESULTS

DLPFC-PAS induced a significant potentiation at the stimulation site in both patients and healthy subjects (mean ± SD: 1.24 ± 0.33 [μV] vs. 1.48 ± 0.28 [μV]). However, when we compared PAS potentiation between patients and healthy subjects, there were significant main effects of PAS (F_1,53  = 68.63, p < 0.0001) and PAS-by-diagnostic group interaction (F_1,53  = 25.05, p < 0.0001). Post hoc analysis demonstrated that patients had a significantly lower PAS potentiation compared to healthy subjects (t₅₅  = 3.128, p = 0.003).

CONCLUSTIONS

Our findings provide evidence for impaired neuroplasticity in DLPFC in patients with depression compared to healthy subjects. Such findings may ultimately help us understand the pathophysiology of MDD and mechanisms involved in its treatment.

PDGF Modulates Synaptic Excitability and Short-Latency Afferent Inhibition in Multiple Sclerosis

Maintenance of synaptic plasticity reserve is crucial to contrast clinical deterioration in MS and PDGF plays a key role in this phenomenon. Indeed, higher cerebrospinal fluid PDGF concentration correlates with improved clinical recovery after a relapse, and the amplitude of LTP-like cortical plasticity in relapsing-remitting MS patients. However, LTP-like cortical plasticity varies depending on the individual level of inhibitory cortical circuits. Aim of this study was to explore whether PDGF-CSF concentration correlates with inhibitory cortical circuits explored by means of transcranial magnetic stimulation in patients affected by relapsing-remitting MS. We further performed electrophysiological experiments evaluating GABAergic transmission in the experimental autoimmune encephalomyelitis (EAE) hippocampus. Our results reveal that increased CSF PDGF concentration correlates with decreased short afferent inhibition in the motor cortex in MS patients and decreased GABAergic activity in EAE. These findings show that PDGF affects GABAergic activity both in MS patients and in EAE hippocampus.

Impact of different intensities of intermittent theta burst stimulation on the cortical properties during TMS-EEG and working memory performance

Intermittent theta burst stimulation (iTBS) is a noninvasive brain stimulation technique capable of increasing cortical excitability beyond the stimulation period. Due to the rapid induction of modulatory effects, prefrontal application of iTBS is gaining popularity as a therapeutic tool for psychiatric disorders such as depression. In an attempt to increase efficacy, higher than conventional intensities are currently being applied. The assumption that this increases neuromodulatory may be mechanistically false for iTBS. This study examined the influence of intensity on the neurophysiological and behavioural effects of iTBS in the prefrontal cortex. Sixteen healthy participants received iTBS over prefrontal cortex at either 50, 75 or 100% resting motor threshold in separate sessions. Single-pulse TMS and concurrent electroencephalography (EEG) was used to assess changes in cortical reactivity measured as TMS-evoked potentials and oscillations. The n-back task was used to assess changes in working memory performance. The data can be summarised as an inverse U-shape relationship between intensity and iTBS plastic effects, where 75% iTBS yielded the largest neurophysiological changes. Improvement in reaction time in the 3-back task was supported by the change in alpha power, however, comparison between conditions revealed no significant differences. The assumption that higher intensity results in greater neuromodulatory effects may be false, at least in healthy individuals, and should be carefully considered for clinical populations. Neurophysiological changes associated with working memory following iTBS suggest functional relevance. However, the effects of different intensities on behavioural performance remain elusive in the present healthy sample.

Spinal Excitability Changes after Transspinal and Transcortical Paired Associative Stimulation in Humans

Paired associative stimulation (PAS) produces enduring neuroplasticity based on Hebbian associative plasticity. This study established the changes in spinal motoneuronal excitability by pairing transcortical and transspinal stimulation. Transcortical stimulation was delivered after (transspinal-transcortical PAS) or before (transcortical-transspinal PAS) transspinal stimulation. Before and after 40 minutes of each PAS protocol, spinal neural excitability was assessed based on the amplitude of the transspinal-evoked potentials (TEPs) recorded from ankle muscles of both legs at different stimulation intensities (recruitment input-output curve). Changes in TEPs amplitude in response to low-frequency stimulation and paired transspinal stimuli were also established before and after each PAS protocol. TEP recruitment input-output curves revealed a generalized depression of TEPs in most ankle muscles of both legs after both PAS protocols that coincided with an increased gain only after transcortical-transspinal PAS. Transcortical-transspinal PAS increased and transspinal-transcortical PAS decreased the low-frequency-dependent TEP depression, whereas neither PAS protocol affected the TEP depression observed upon paired transspinal stimuli. These findings support the notion that transspinal and transcortical PAS has the ability to alter concomitantly cortical and spinal synaptic activity. Transspinal and transcortical PAS may contribute to the development of rehabilitation strategies in people with bilateral increased motoneuronal excitability due to cortical or spinal lesions.

A method to induce human cortical long-term potentiation by acoustic stimulation

OBJECT

Acoustic stimulation induced LTP in the human auditory cortex was successfully recorded for the first time by electroencephalography (EEG) using a stimulus of 1 kHz pure-tone in 2005. However, it was barely reproduced, given considerable challenges to reliably elicit and accurately record the enhanced potentials in vivo. The purpose of this paper was to explore whether acoustic stimuli other than 1 kHz pure-tone could generate LTP or not.

MEASURES

To answer this question, we proposed a tetanic-stimulation paradigm of pure-tones, narrow-band noises (NBNs) and white noise (WN) to elicit LTP in human subjects.

RESULTS

The results showed that pure-tones with different frequency could elicit LTP in human auditory cortex, and proved for the first time that NBNs and WN could also achieve the same goal. Interestingly, it was also shown that the noises with certain bandwidth induced the greatest LTP and the WN induced LTP had the least variation over time and across subjects in comparison with pure-tones and NBNs.

CONCLUSIONS

In light of the results, we suggested to use the paradigm for broader studies of human in vivo cortical plasticity.