Determinants of the induction of cortical plasticity by non-invasive brain stimulation in healthy subjects

Transcranial alternating current stimulation barely enhances working memory in healthy adults: A meta-analysis

Working memory (WM) is a pivotal neural mechanism for cognitive function and ability. Transcranial alternating current stimulation (tACS) was used to improve WM by entraining key brain rhythms. We submitted to meta-analysis 143 effects of tACS on WM performance, found in 42 reports published between 2014 and 2023, encompassing a total of 1386 healthy adults stimulated. The overall effect size of 134 interventions intended to improve WM equaled Hedges' g = 0.076 [0.039, 0.113]. However, after correcting for a significant publication bias this effect size dropped to zero. By contrast, 9 interventions distorting the brain synchronization using antiphase tACS reliably decreased WM performance, with Hedges' g = -0.266, [-0.458, -0.074]. Individuating the targeted frequency band was the only reliable moderator. The disparity between our null outcome and moderately positive tACS effects estimated by previous meta-analyses resulted from our inclusion of the most recent studies mostly reporting negligible effects. Our results suggest that current tACS protocols barely enhance WM in healthy adults. More research is needed to develop effective methods for WM stimulation.

Modulation of dorsal premotor cortex differentially influences visuomotor adaptation in young and older adults

The communication between dorsal premotor cortex (PMd) and primary motor cortex (M1) is important for visuomotor adaptation, but it is unclear how this relationship changes with advancing age. The present study recruited 21 young and 23 older participants for two experimental sessions during which intermittent theta burst stimulation (iTBS) or sham was applied over PMd. We assessed the effects of PMd iTBS on M1 excitability using motor evoked potentials (MEP) recorded from right first dorsal interosseous when single-pulse transcranial magnetic stimulation (TMS) was applied with posterior-anterior (PA) or anterior-posterior (AP) currents; and adaptation by quantifying error recorded during a visuomotor adaptation task (VAT). PMd iTBS potentiated PA (P < 0.0001) and AP (P < 0.0001) MEP amplitude in both young and older adults. PMd iTBS increased error in young adults during adaptation (P = 0.026), but had no effect in older adults (P = 0.388). Although PMd iTBS potentiated M1 excitability in both young and older adults, the intervention attenuated visuomotor adaptation specifically in young adults.

Metaplastic neuromodulation via transcranial direct current stimulation has no effect on corticospinal excitability and neuromuscular fatigue

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation tool with potential for managing neuromuscular fatigue, possibly due to alterations in corticospinal excitability. However, inconsistencies in intra- and inter- individual variability responsiveness to tDCS limit its clinical use. Emerging evidence suggests harnessing homeostatic metaplasticity induced via tDCS may reduce variability and boost its outcomes, yet little is known regarding its influence on neuromuscular fatigue in healthy adults. We explored whether cathodal tDCS (ctDCS) prior to exercise combined with anodal tDCS (atDCS) could augment corticospinal excitability and attenuate neuromuscular fatigue. 15 young healthy adults (6 males, 22 ± 4 years) participated in four pseudo-randomised neuromodulation sessions: sham stimulation prior and during exercise, sham stimulation prior and atDCS during exercise, ctDCS prior and atDCS during exercise, ctDCS prior and sham stimulation during exercise. The exercise constituted an intermittent maximal voluntary contraction (MVC) of the right first dorsal interosseous (FDI) for 10 min. Neuromuscular fatigue was quantified as an attenuation in MVC force, while motor evoked potential (MEP) amplitude provided an assessment of corticospinal excitability. MEP amplitude increased during the fatiguing exercise, whilst across time, force decreased. There were no differences in MEP amplitudes or force between neuromodulation sessions. These outcomes highlight the ambiguity of harnessing metaplasticity to ameliorate neuromuscular fatigue in young healthy individuals.

Age-related changes in responsiveness to non-invasive brain stimulation neuroplasticity paradigms: A systematic review with meta-analysis

OBJECTIVES

We aimed to summarise and critically appraise the available evidence for the effect of age on responsiveness to non-invasive brain stimulation (NBS) paradigms delivered to the primary motor cortex.

METHODS

Four databases (Medline, Embase, PsycINFO and Scopus) were searched from inception to February 7, 2023. Studies investigating age group comparisons and associations between age and neuroplasticity induction from NBS paradigms were included. Only studies delivering neuroplasticity paradigms to the primary motor cortex and responses measured via motor-evoked potentials (MEPs) in healthy adults were considered.

RESULTS

39 studies, encompassing 40 experiments and eight NBS paradigms were included: paired associative stimulation (PAS; n = 12), repetitive transcranial magnetic stimulation (rTMS; n = 2), intermittent theta burst stimulation (iTBS; n = 8), continuous theta burst stimulation (cTBS; n = 7), transcranial direct and alternating current stimulation ((tDCS; n = 7; tACS; n = 2)), quadripulse stimulation (QPS; n = 1) and i-wave periodic transcranial magnetic stimulation (iTMS; n = 1). Pooled findings from PAS paradigms suggested older adults have reduced post-paradigm responses, although there was considerable heterogeneity. Mixed results were observed across all other NBS paradigms and post-paradigm timepoints.

CONCLUSIONS/SIGNIFICANCE

Whilst age-dependent reduction in corticospinal excitability is possible, there is extensive inter- and intra-individual variability both within and between studies, making it difficult to draw meaningful conclusions from pooled analyses.

Instantaneous effects of prefrontal transcranial magnetic stimulation on brain oxygenation: A systematic review

This systematic review investigates how prefrontal transcranial magnetic stimulation (TMS) immediately influences neuronal excitability based on oxygenation changes measured by functional magnetic resonance imaging (fMRI) or functional near-infrared spectroscopy (fNIRS). A thorough understanding of TMS-induced excitability changes may enable clinicians to adjust TMS parameters and optimize treatment plans proactively. Five databases were searched for human studies evaluating brain excitability using concurrent TMS/fMRI or TMS/fNIRS. Thirty-seven studies (13 concurrent TMS/fNIRS studies, 24 concurrent TMS/fMRI studies) were included in a qualitative synthesis. Despite methodological inconsistencies, a distinct pattern of activated nodes in the frontoparietal central executive network, the cingulo-opercular salience network, and the default-mode network emerged. The activated nodes included the prefrontal cortex (particularly dorsolateral prefrontal cortex), insula cortex, striatal regions (especially caudate, putamen), anterior cingulate cortex, and thalamus. High-frequency repetitive TMS most consistently induced expected facilitatory effects in these brain regions. However, varied stimulation parameters (e.g., intensity, coil orientation, target sites) and the inter- and intra-individual variability of brain state contribute to the observed heterogeneity of target excitability and co-activated regions. Given the considerable methodological and individual variability across the limited evidence, conclusions should be drawn with caution.

Exercise-induced cortical disinhibition mediates the relationship between fitness and memory in older adults

Regular exercise benefits learning and memory in older adults, but the neural mechanisms mediating these effects remain unclear. Evidence in young adults indicates that acute exercise creates a favourable environment for synaptic plasticity by enhancing cortical disinhibition. As such, we investigated whether plasticity-related disinhibition mediated the relationship between cardiorespiratory fitness and memory function in healthy older adults (n = 16, mean age = 66.06). Participants completed a graded maximal exercise test and assessments of visual and verbal memory, followed by two counterbalanced sessions involving 20 min of either high-intensity interval training exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. In line with our hypotheses, we observed a positive correlation between cardiorespiratory fitness and verbal memory, which was mediated by plasticity-related cortical disinhibition. Our novel finding implicates cortical disinhibition as a mechanism through which the effects of acute bouts of exercise may translate to improved memory in older adults. This finding extends current understanding of the physiological mechanisms underlying the positive influence of cardiorespiratory fitness for memory function in older adults, and further highlights the importance of promoting exercise engagement to maintain cognitive health in later life. KEY POINTS: There are well established benefits of regular exercise for memory function in older adults, but the mechanisms are unclear. Cortical disinhibition is important for laying down new memories, and is enhanced following acute exercise in young adults, suggesting it is a potential mechanism underlying these benefits in ageing. Older adults completed a fitness test and assessments of memory, followed by two sessions involving either 20 min of exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. Cardiorespiratory fitness was positively associated with memory performance. Higher fitness was associated with enhanced cortical disinhibition following acute exercise. Cortical disinhibition completely mediated the relationship between fitness and memory. This novel finding provides a mechanistic account for the positive influence of cardiorespiratory fitness on memory in later life, and emphasises the importance of regular exercise for cognitive health in older populations.

Neuroanatomical Predictors of Transcranial Direct Current Stimulation (tDCS)-Induced Modifications in Neurocognitive Task Performance in Typically Developing Individuals

Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation technique gaining more attention in neurodevelopmental disorders (NDDs). Due to the phenotypic heterogeneity of NDDs, tDCS is unlikely to be equally effective in all individuals. The present study aimed to establish neuroanatomical markers in typically developing (TD) individuals that may be used for the prediction of individual responses to tDCS. Fifty-seven male and female children received 2 mA anodal and sham tDCS, targeting the left dorsolateral prefrontal cortex (DLPFCleft), right inferior frontal gyrus, and bilateral temporoparietal junction. Response to tDCS was assessed based on task performance differences between anodal and sham tDCS in different neurocognitive tasks (N-back, flanker, Mooney faces detection, attentional emotional recognition task). Measures of cortical thickness (CT) and surface area (SA) were derived from 3 Tesla structural MRI scans. Associations between neuroanatomy and task performance were assessed using general linear models (GLM). Machine learning (ML) algorithms were employed to predict responses to tDCS. Vertex-wise estimates of SA were more closely linked to differences in task performance than measures of CT. Across ML algorithms, highest accuracies were observed for the prediction of N-back task performance differences following stimulation of the DLPFCleft, where 65% of behavioral variance was explained by variability in SA. Lower accuracies were observed for all other tasks and stimulated regions. This suggests that it may be possible to predict individual responses to tDCS for some behavioral measures and target regions. In the future, these models might be extended to predict treatment outcome in individuals with NDDs.

Neurochemical Predictors of Generalized Learning Induced by Brain Stimulation and Training

Methods of cognitive enhancement for humans are most impactful when they generalize across tasks. However, the extent to which such "transfer" is possible via interventions is widely debated. In addition, the contribution of excitatory and inhibitory processes to such transfer is unknown. Here, in a large-scale neuroimaging individual differences study with humans (both sexes), we paired multitasking training and noninvasive brain stimulation (transcranial direct current stimulation, tDCS) over multiple days and assessed performance across a range of paradigms. In addition, we varied tDCS dosage (1.0 and 2.0 mA), electrode montage (left or right prefrontal regions), and training task (multitasking vs a control task) and assessed GABA and glutamate concentrations via ultrahigh field 7T magnetic resonance spectroscopy. Generalized benefits were observed in spatial attention, indexed by visual search performance, when multitasking training was combined with 1.0 mA stimulation targeting either the left or right prefrontal cortex (PFC). This transfer effect persisted for ∼30 d post intervention. Critically, the transferred benefits associated with right prefrontal tDCS were predicted by pretraining concentrations of glutamate in the PFC. Thus, the effects of this combined stimulation and training protocol appear to be linked predominantly to excitatory brain processes.

The effects of intermittent theta burst stimulation over dorsal premotor cortex on primary motor cortex plasticity in young and older adults

Previous transcranial magnetic stimulation (TMS) research suggests that the dorsal premotor cortex (PMd) influences neuroplasticity within the primary motor cortex (M1) through indirect (I) wave interneuronal circuits. However, it is unclear how the influence of PMd on the plasticity of M1 I-waves changes with advancing age. This study therefore investigated the neuroplastic effects of intermittent theta burst stimulation (iTBS) to M1 early and late I-wave circuits when preceded by iTBS (PMd iTBS-M1 iTBS) or sham stimulation (PMd sham-M1 iTBS) to PMd in 15 young and 16 older adults. M1 excitability was assessed with motor evoked potentials (MEP) recorded from the right first dorsal interosseous using posterior-anterior (PA) and anterior-posterior (AP) current TMS at standard stimulation intensities (PA1mV, AP1mV) and reduced stimulation intensities (PA0.5mV, early I-waves; AP0.5mV, late I-waves). PMd iTBS-M1 iTBS lowered the expected facilitation of PA0.5mV (to M1 iTBS) in young and older adults (P = 0.009), whereas the intervention had no effect on AP0.5mV facilitation in either group (P = 0.305). The modulation of PA0.5mV following PMd iTBS-M1 iTBS may reflect a specific influence of PMd on different I-wave circuits that are involved in M1 plasticity within young and older adults.

Exploring new horizons: Emerging therapeutic strategies for pediatric stroke

Pediatric stroke presents unique challenges, and optimizing treatment strategies is essential for improving outcomes in this vulnerable population. This review aims to provide an overview of new, innovative, and potential treatments for pediatric stroke, with a primary objective to stimulate further research in this field. Our review highlights several promising approaches in the realm of pediatric stroke management, including but not limited to stem cell therapy and robotic rehabilitation. These innovative interventions offer new avenues for enhancing functional recovery, reducing long-term disability, and tailoring treatments to individual patient needs. The findings of this review underscore the importance of ongoing research and development of innovative treatments in pediatric stroke. These advancements hold significant clinical relevance, offering the potential to improve the lives of children affected by stroke by enhancing the precision, efficacy, and accessibility of therapeutic interventions. Embracing these innovations is essential in our pursuit of better outcomes and a brighter future for pediatric stroke care.

Probing Our Built-in Calculator: A Systematic Narrative Review of Noninvasive Brain Stimulation Studies on Arithmetic Operation-Related Brain Areas

This systematic review presented a comprehensive survey of studies that applied transcranial magnetic stimulation and transcranial electrical stimulation to parietal and nonparietal areas to examine the neural basis of symbolic arithmetic processing. All findings were compiled with regard to the three assumptions of the triple-code model (TCM) of number processing. Thirty-seven eligible manuscripts were identified for review (33 with healthy participants and 4 with patients). Their results are broadly consistent with the first assumption of the TCM that intraparietal sulcus both hold a magnitude code and engage in operations requiring numerical manipulations such as subtraction. However, largely heterogeneous results conflicted with the second assumption of the TCM that the left angular gyrus subserves arithmetic fact retrieval, such as the retrieval of rote-learned multiplication results. Support is also limited for the third assumption of the TCM, namely, that the posterior superior parietal lobule engages in spatial operations on the mental number line. Furthermore, results from the stimulation of brain areas outside of those postulated by the TCM show that the bilateral supramarginal gyrus is involved in online calculation and retrieval, the left temporal cortex in retrieval, and the bilateral dorsolateral prefrontal cortex and cerebellum in online calculation of cognitively demanding arithmetic problems. The overall results indicate that multiple cortical areas subserve arithmetic skills.

Effects of pain on cortical homeostatic plasticity in humans: a systematic review

Homeostatic plasticity (HP) is a negative feedback mechanism that prevents excessive facilitation or depression of cortical excitability (CE). Cortical HP responses in humans have been investigated by using 2 blocks of noninvasive brain stimulation with a no-stimulation block in between. A healthy HP response is characterized by reduced CE after 2 excitatory stimulation blocks and increased CE when using inhibitory stimulation. Conversely, impaired HP responses have been demonstrated in experimental and chronic pain conditions. Therefore, this systematic review aimed to provide an overview of the effect of pain on cortical HP in humans. Scopus, Embase, and PubMed were searched from inception until November 20, 2023. The included studies (1) compared experimental or clinical pain conditions with healthy controls, (2) induced HP using 2 blocks of stimulation with a no-stimulation interval, and (3) evaluated CE measures such as motor-evoked potentials. Four studies were included, consisting of 5 experiments and 146 participants, of whom 63 were patients with chronic pain and 48 were subjected to an experimental pain model. This systematic review found support for an HP impairment in pain compared with that in pain-free states, reflected by a lack of CE reduction after excitatory-excitatory HP induction over the primary motor cortex. Inhibitory-inhibitory HP induction did not produce a consistent HP response across studies, independent of pain or pain-free states. Standardization of HP induction protocols and outcome calculations is needed to ensure reproducibility and study comparison. Future HP studies may consider investigating sensory domains including nociception, which would further our understanding of abnormal HP regulation in pain conditions.

Bilateral Corticomotor Reorganization and Symptom Development in Response to Acute Unilateral Hamstring Pain: A Randomized, Controlled Study

Accumulating evidence demonstrates that pain induces adaptations in the corticomotor representations of affected muscles. However, previous work has primarily investigated the upper limb, with few studies examining corticomotor reorganization in response to lower limb pain. This is important to consider, given the significant functional, anatomical, and neurophysiological differences between upper and lower limb musculature. Previous work has also focused on unilateral corticomotor changes in response to muscle pain, despite an abundance of literature demonstrating that unilateral pain conditions are commonly associated with bilateral motor dysfunction. For the first time, this study investigated the effect of unilateral acute hamstring pain on bilateral corticomotor organization using transcranial magnetic stimulation (TMS) mapping. Corticomotor outcomes (TMS maps), pain, mechanical sensitivity (pressure pain thresholds), and function (maximal voluntary contractions) were recorded from 28 healthy participants at baseline. An injection of pain-inducing hypertonic (n = 14) or pain-free isotonic (n = 14) saline was then administered to the right hamstring muscle, and pain ratings were collected every 30 seconds until pain resolution. Follow-up measures were taken immediately following pain resolution and at 25, 50, and 75 minutes post-pain resolution. Unilateral acute hamstring pain induced bilateral symptom development and changes in corticomotor reorganization. Two patterns of reorganization were observed-corticomotor facilitation and corticomotor depression. Corticomotor facilitation was associated with increased mechanical sensitivity and decreased function bilaterally (all P < .05). These effects persisted for at least 75 minutes after pain resolution. PERSPECTIVE: These findings suggest that individual patterns of corticomotor reorganization may contribute to ongoing functional deficits of either limb following acute unilateral lower limb pain. Further research is required to assess these adaptations and the possible long-term implications for rehabilitation and reinjury risk in cohorts with acute hamstring injury.

Cellular mechanisms underlying carry-over effects after magnetic stimulation

Magnetic fields are widely used for neuromodulation in clinical settings. The intended effect of magnetic stimulation is that neural activity resumes its pre-stimulation state right after stimulation. Many theoretical and experimental works have focused on the cellular and molecular basis of the acute neural response to magnetic field. However, effects of magnetic stimulation can still last after the termination of the magnetic stimulation (named "carry-over effects"), which could generate profound effects to the outcome of the stimulation. However, the cellular and molecular mechanisms of carry-over effects are largely unknown, which renders the neural modulation practice using magnetic stimulation unpredictable. Here, we investigated carry-over effects at the cellular level, using the combination of micro-magnetic stimulation (µMS), electrophysiology, and computation modeling. We found that high frequency magnetic stimulation could lead to immediate neural inhibition in ganglion neurons from Aplysia californica, as well as persistent, carry-over inhibition after withdrawing the magnetic stimulus. Carry-over effects were found in the neurons that fired action potentials under a variety of conditions. The carry-over effects were also observed in the neurons when the magnetic field was applied across the ganglion sheath. The state of the neuron, specifically synaptic input and membrane potential fluctuation, plays a significant role in generating the carry-over effects after magnetic stimulation. To elucidate the cellular mechanisms of such carry-over effects under magnetic stimulation, we simulated a single neuron under magnetic stimulation with multi-compartment modeling. The model successfully replicated the carry-over effects in the neuron, and revealed that the carry-over effect was due to the dysfunction of the ion channel dynamics that were responsible for the initiation and sustaining of membrane excitability. A virtual voltage-clamp experiment revealed a compromised Na conductance and enhanced K conductance post magnetic stimulation, rendering the neurons incapable of generating action potentials and, therefore, leading to the carry over effects. Finally, both simulation and experimental results demonstrated that the carry-over effects could be controlled by disturbing the membrane potential during the post-stimulus inhibition period. Delineating the cellular and ion channel mechanisms underlying carry-over effects could provide insights to the clinical outcomes in brain stimulation using TMS and other modalities. This research incentivizes the development of novel neural engineering or pharmacological approaches to better control the carry-over effects for optimized clinical outcomes.

Preventive exercise and physical rehabilitation promote long-term potentiation-like plasticity expression in patients with multiple sclerosis

BACKGROUND AND PURPOSE

Loss of long-term potentiation (LTP) expression has been associated with a worse disease course in relapsing-remitting multiple sclerosis (RR-MS) and represents a pathophysiological hallmark of progressive multiple sclerosis (PMS). Exercise and physical rehabilitation are the most prominent therapeutic approaches to promote synaptic plasticity. We aimed to explore whether physical exercise is able to improve the expression of LTP-like plasticity in patients with multiple sclerosis (MS).

METHODS

In 46 newly diagnosed RR-MS patients, we explored the impact of preventive exercise on LTP-like plasticity as assessed by intermittent theta-burst stimulation. Patients were divided into sedentary or active, based on physical activity performed during the 6 months prior to diagnosis. Furthermore, in 18 patients with PMS, we evaluated the impact of an 8-week inpatient neurorehabilitation program on clinical scores and LTP-like plasticity explored using paired associative stimulation (PAS). Synaptic plasticity expression was compared in patients and healthy subjects.

RESULTS

Reduced LTP expression was found in RR-MS patients compared with controls. Exercising RR-MS patients showed a greater amount of LTP expression compared with sedentary patients. In PMS patients, LTP expression was reduced compared with controls and increased after 8 weeks of rehabilitation. In this group of patients, LTP magnitude at baseline predicted the improvement in hand dexterity.

CONCLUSIONS

Both preventive exercise and physical rehabilitation may enhance the expression of LTP-like synaptic plasticity in MS, with potential beneficial effects on disability accumulation.

Personalized depth-specific neuromodulation of the human primary motor cortex via ultrasound

Non-invasive brain stimulation has the potential to boost neuronal plasticity in the primary motor cortex (M1), but it remains unclear whether the stimulation of both superficial and deep layers of the human motor cortex can effectively promote M1 plasticity. Here, we leveraged transcranial ultrasound stimulation (TUS) to precisely target M1 circuits at depths of approximately 5 mm and 16 mm from the cortical surface. Initially, we generated computed tomography images from each participant's individual anatomical magnetic resonance images (MRI), which allowed for the generation of accurate acoustic simulations. This process ensured that personalized TUS was administered exactly to the targeted depths within M1 for each participant. Using long-term depression and long-term potentiation (LTD/LTP) theta-burst stimulation paradigms, we examined whether TUS over distinct depths of M1 could induce LTD/LTP plasticity. Our findings indicated that continuous theta-burst TUS-induced LTD-like plasticity with both superficial and deep M1 stimulation, persisting for at least 30 min. In comparison, sham TUS did not significantly alter M1 excitability. Moreover, intermittent theta-burst TUS did not result in the induction of LTP- or LTD-like plasticity with either superficial or deep M1 stimulation. These findings suggest that the induction of M1 plasticity can be achieved with ultrasound stimulation targeting distinct depths of M1, which is contingent on the characteristics of TUS. KEY POINTS: The study integrated personalized transcranial ultrasound stimulation (TUS) with electrophysiology to determine whether TUS targeting superficial and deep layers of the human motor cortex (M1) could elicit long-term depression (LTD) or long-term potentiation (LTP) plastic changes. Utilizing acoustic simulations derived from individualized pseudo-computed tomography scans, we ensured the precision of TUS delivery to the intended M1 depths for each participant. Continuous theta-burst TUS targeting both the superficial and deep layers of M1 resulted in the emergence of LTD-like plasticity, lasting for at least 30 min. Administering intermittent theta-burst TUS to both the superficial and deep layers of M1 did not lead to the induction of LTP- or LTD-like plastic changes. We suggest that theta-burst TUS targeting distinct depths of M1 can induce plasticity, but this effect is dependent on specific TUS parameters.

Nonmedication Devices in Development for the Treatment of Alzheimer's Disease

 Huge investments continue to be made in treatment for Alzheimer's disease (AD), with more than one hundred drugs currently in development. Pharmacological approaches and drug development, particularly those targeting amyloid-β, have dominated the therapeutic landscape. At the same time, there is also a growing interest in devices for treating AD. This review aimed to identify and describe devices under development for AD treatment. In this review, we queried the devices that are in development for the treatment of AD. PubMed was searched through the end of 2021 using the terms "device," "therapeutics," and "Alzheimer's" for articles that report on devices to treat AD. Ten devices with 31 references were identified as actively being developed for the treatment of AD. Many of these devices are far along in development. Device-based therapies are often overlooked when evaluating treatment approaches to AD. However, many devices for treating AD are in development and some show promising results.

Different Effect Sizes of Motor Skill Training Combined with Repetitive Transcranial versus Trans-Spinal Magnetic Stimulation in Healthy Subjects

Repetitive transcranial magnetic stimulation (rTMS) is used to enhance motor training (MT) performance. The use of rTMS is limited under certain conditions, such as after a stroke with severe damage to the corticospinal tract. This raises the question as to whether repetitive trans-spinal magnetic stimulation (rSMS) can also be used to improve MT. A direct comparison of the effect size between rTMS and rSMS on the same MT is still lacking. Before conducting the study in patients, we determined the effect sizes of different stimulation approaches combined with the same motor training in healthy subjects. Two experiments (E1 and E2) with 96 subjects investigated the effect size of combining magnetic stimulation with the same MT. In E1, high-frequency rTMS, rSMS, and spinal sham stimulation (sham-spinal) were applied once in combination with MT, while one group only received the same MT (without stimulation). In E2, rTMS, rSMS, and sham-spinal were applied in combination with MT over several days. In all subjects, motor tests and motor-evoked potentials were evaluated before and after the intervention period. rTMS had the greatest effect on MT, followed by rSMS and then sham-spinal. Daily stimulation resulted in additional training gains. This study suggests that rSMS increases excitability and also enhances MT performance. This current study provides a basis for further research to discover whether patients who cannot be treated effectively with rTMS would benefit from rSMS.

Efficacy and safety of transcranial direct current stimulation over the left dorsolateral prefrontal cortex in children and adolescents with attention-deficit/hyperactivity disorder: a randomized, triple-blinded, sham-controlled, crossover trial

Introduction

Although pharmacological treatment for Attention-Deficit/Hyperactivity Disorder (ADHD) has demonstrated efficacy, several individuals persist in experiencing social and academic impairment. Additionally, the occurrence of significant side effects may render the use of psychotropic medications untenable. However, Transcranial Direct Current Stimulation (tDCS), a non-invasive brain stimulation technique, shows promising results in treating ADHD.

Objectives

To investigate the efficacy and safety of tDCS on the performance of children and adolescents with ADHD in neuropsychological tests involving visual attention, visual and verbal working memory, and inhibitory control.

Methodology

This study was a triple-blind, randomized, sham-controlled, crossover clinical trial. The intervention consisted of a daily session of tDCS (2 mA) or sham targeting the left dorsolateral prefrontal cortex (L-DLPFC), for 30 min, on five consecutive days. The primary outcome was change in the Visual Attention Test, Fourth Edition (TAVIS-4) before and after each intervention. Subjects were also evaluated pre and post-tDCS using the Digit Span subtest of the Wechsler Intelligence Scale for Children, Fifth Edition (WISC-V), the Developmental Neuropsychological Assessment, Second Edition (NEPSY-II) Inhibiting Response (IR) subtest, and the Corsi Block-Tapping Task.

Results

Fifteen individuals were included, and no statistically significant difference was observed when comparing the results of the TAVIS-4, the IR of NEPSY-II, and the intragroup Digit Span subtest of WISC-V undertaken before and after the procedure. Adverse events were mainly self-limiting and transient. The participants did not perceive any benefit from tDCS when measured on the Patient Global Impression of Improvement (PGI-I) Scale.

Conclusion

This study did not meet its primary endpoint and found no performance enhancement in any investigated neuropsychological outcomes relating to the intervention group.

The effect of time of day on Special Judo Fitness Test in active judokas: Evaluation in terms of chronotype

The present cross-sectional study aims to examine the effect of time of day on the judo-specific performance in judokas, considering their chronotypes. Twenty-four male judokas participated in the study where the Morningness-Eveningness Questionnaire was administered, and on it, they were divided into morning-type (MT:12-judokas) and evening-type groups (ET:12-judokas). Afterwards, the Special Judo Fitness Test (SJFT) was applied to both groups at three different times (morning: 09:00 h, afternoon: 13:00 h and evening: 17:00 h) with body temperature measured before and after every SJFT performance. As a result, the Group*Test Time interaction significantly affected overall throwing performance during SJFT [F(2,44) = 29.437, p = 0.001, η2p: .572]. Furthermore, a significant time effect was found for the SJFT index [F(2,44) = 5.118, p = 0.010, η2p: .189] and for the Group*Test Time interaction with the mean value of the index [F(2,44) = 24.424, p < 0.001, η2p: .526]. Furthermore, body temperature had a significant time effect [F(2,44) = 301.454, p < 0.001, η2p: .932] and the Group*Test Time interaction [F(2,44) = 5.802, p = 0.006, η2p: .209]. In conclusion, coaches and exercise experts should consider judo athletes' chronotype when planning special training programs to improve judo-specific anaerobic capacity. Furthermore, to minimize the impact of time of day and chronotype on athletes' performance in competitions, it is recommended that MT athletes develop their judo-specific anaerobic capacity in the evening hours, when their performance is lower, while ET athletes should do so in the morning hours when their performance is lower via randori training.

Semi-automated motor hotspot search (SAMHS): a framework toward an optimised approach for motor hotspot identification

Background

Motor hotspot identification represents the first step in the determination of the motor threshold and is the basis for the specification of stimulation intensity used for various Transcranial Magnetic Stimulation (TMS) applications. The level of experimenters' experience and the methodology of motor hotspot identification differ between laboratories. The need for an optimized and time-efficient technique for motor hotspot identification is therefore substantial.

Objective

With the current work, we present a framework for an optimized and time-efficient semi-automated motor hotspot search (SAMHS) technique utilizing a neuronavigated robot-assisted TMS system (TMS-cobot). Furthermore, we aim to test its practicality and accuracy by a comparison with a manual motor hotspot identification method.

Method

A total of 32 participants took part in this dual-center study. At both study centers, participants underwent manual hotspot search (MHS) with an experienced TMS researcher, and the novel SAMHS procedure with a TMS-cobot (hereafter, called cobot hotspot search, CHS) in a randomized order. Resting motor threshold (RMT), and stimulus intensity to produce 1 mV (SI1mV) peak-to-peak of motor-evoked potential (MEP), as well as MEPs with 120% RMT and SI1mV were recorded as outcome measures for comparison.

Results

Compared to the MHS method, the CHS produced lower RMT, lower SI1mV and a trend-wise higher peak-to-peak MEP amplitude in stimulations with SI1mV. The duration of the CHS procedure was longer than that of the MHS (15.60 vs. 2.43 min on average). However, accuracy of the hotspot was higher for the CHS compared to the MHS.

Conclusions

The SAMHS procedure introduces an optimized motor hotspot determination system that is easy to use, and strikes a fairly good balance between accuracy and speed. This new procedure can thus be deplored by experienced as well as beginner-level TMS researchers.

Limited evidence for reliability of low and high frequency rTMS over the motor cortex

OBJECTIVE

The aim of this study was to investigate the reliability of low-frequency and high-frequency repetitive transcranial magnetic stimulation (rTMS) on healthy individuals over the motor cortex. A secondary outcome was the assessment if low-frequency rTMS results in inhibition and high-frequency rTMS results in facilitation.

METHODS

In this experiment, 30 healthy participants received on four consecutive days one session each with application of 1 Hz or 20 Hz rTMS over the left motor cortex. 1 Hz and 20 Hz were applied in alternating order, whereby the starting frequency was randomized. Motor evoked potentials (MEPs) were measured before and after each session. Reliability measures were intraclass and Pearson's correlation coefficient (ICC and r).

RESULTS

ICCs and r values were low to moderate. Notably, within subgroups of less confounded measures, we found good r values for 20 Hz rTMS. The group-level analysis did not demonstrate a clear low-frequency inhibition and high-frequency facilitation pattern. At the single-subject level, only one participant exhibited significant changes consistent with the expected pattern, with concurrent decreases in MEPs following 1 Hz sessions and increases following 20 Hz sessions.

CONCLUSION

The investigated neuromodulatory protocols show low to moderate reliability. Results are questioning the low-frequency inhibition and high-frequency facilitation pattern.

SIGNIFICANCE

Methodological improvements for the usage of rTMS are necessary to increase validity and reliability of non-invasive brain stimulation.

Ageing attenuates exercise-enhanced motor cortical plasticity

Cardiorespiratory exercise is known to modulate motor cortical plasticity in young adults, but the influence of ageing on this relationship is unknown. Here, we compared the effects of a single session of cardiorespiratory exercise on motor cortical plasticity in young and older adults. We acquired measures of cortical excitatory and inhibitory activity of the primary motor cortex using transcranial magnetic stimulation (TMS) from 20 young (mean ± SD = 25.30 ± 4.00 years, 14 females) and 20 older (mean ± SD = 64.10 ± 6.50 years, 11 females) healthy adults. Single- and paired-pulse TMS measurements were collected before and after a 20 min bout of high-intensity interval cycling exercise or an equivalent period of rest, and again after intermittent theta burst stimulation (iTBS). In both young (P = 0.027, Cohen's d = 0.87) and older adults (P = 0.006, Cohen's d = 0.85), there was an increase in glutamatergic excitation and a reduction in GABAergic inhibition from pre- to postexercise. However, in contrast to younger adults, older adults showed an attenuated plasticity response to iTBS following exercise (P = 0.011, Cohen's d = 0.85). These results demonstrate an age-dependent decline in cortical plasticity and indicate that a preceding bout of high-intensity interval exercise might be less effective for enhancing primary motor cortex plasticity in older adults. Our findings align with the hypothesis that the capacity for cortical plasticity is altered in older age. KEY POINTS: Exercise enhances motor cortical plasticity in young adults, but how ageing influences this effect is unknown. Here, we compared primary motor cortical plasticity responses in young and older adults before and after a bout of high-intensity interval exercise and again after a plasticity-inducing protocol, intermittent theta burst stimulation. In both young and older adults, exercise led to an increase in glutamatergic excitation and a reduction in GABAergic inhibition. Our key result was that older adults showed an attenuated plasticity response to theta burst stimulation following exercise, relative to younger adults. Our findings demonstrate an age-dependent decline in exercise-enhanced cortical plasticity and indicate that a preceding bout of high-intensity interval exercise might be less effective for enhancing primary motor cortex plasticity in older adults.

Effects of transcranial direct current stimulation over human motor cortex on cognitive-motor and sensory-motor functions

The primary motor cortex (M1) is broadly acknowledged for its crucial role in executing voluntary movements. Yet, its contributions to cognitive and sensory functions remain largely unexplored. Transcranial direct current stimulation (tDCS) is a noninvasive neurostimulation method that can modify brain activity, thereby enabling the establishment of a causal link between M1 activity and behavior. This study aimed to investigate the online effects of tDCS over M1 on cognitive-motor and sensory-motor functions. Sixty-four healthy participants underwent either anodal or sham tDCS while concurrently performing a set of standardized robotic tasks. These tasks provided sensitive and objective assessments of brain functions, including action selection, inhibitory control, cognitive control of visuomotor skills, proprioceptive sense, and bimanual coordination. Our results revealed that anodal tDCS applied to M1 enhances decision-making capacity in selecting appropriate motor actions and avoiding distractors compared to sham stimulation, suggesting improved action selection and inhibitory control capabilities. Furthermore, anodal tDCS reduces the movement time required to accomplish bimanual movements, suggesting enhanced bimanual performance. However, we found no impact of anodal tDCS on cognitive control of visuomotor skills and proprioceptive sense. This study suggests that augmenting M1 activity via anodal tDCS influences cognitive-motor and sensory-motor functions in a task-dependent manner.

Rethinking Remapping: Circuit Mechanisms of Recovery after Stroke

Stroke is one of the most common causes of disability, and there are few treatments that can improve recovery after stroke. Therapeutic development has been hindered because of a lack of understanding of precisely how neural circuits are affected by stroke, and how these circuits change to mediate recovery. Indeed, some of the hypotheses for how the CNS changes to mediate recovery, including remapping, redundancy, and diaschisis, date to more than a century ago. Recent technological advances have enabled the interrogation of neural circuits with ever greater temporal and spatial resolution. These techniques are increasingly being applied across animal models of stroke and to human stroke survivors, and are shedding light on the molecular, structural, and functional changes that neural circuits undergo after stroke. Here we review these studies and highlight important mechanisms that underlie impairment and recovery after stroke. We begin by summarizing knowledge about changes in neural activity that occur in the peri-infarct cortex, specifically considering evidence for the functional remapping hypothesis of recovery. Next, we describe the importance of neural population dynamics, disruptions in these dynamics after stroke, and how allocation of neurons into spared circuits can restore functionality. On a more global scale, we then discuss how effects on long-range pathways, including interhemispheric interactions and corticospinal tract transmission, contribute to post-stroke impairments. Finally, we look forward and consider how a deeper understanding of neural circuit mechanisms of recovery may lead to novel treatments to reduce disability and improve recovery after stroke.

Effectiveness of transcranial direct current stimulation on balance and gait in patients with multiple sclerosis: systematic review and meta-analysis of randomized clinical trials

BACKGROUND

Motor impairments are very common in neurological diseases such as multiple sclerosis. Noninvasive brain stimulation could influence the motor function of patients.

OBJECTIVE

The aim of this meta-analysis was to evaluate the effectiveness of transcranial direct current stimulation (tDCS) on balance and gait ability in patients with multiple sclerosis. Additionally, a secondary aim was to compare the influence of the stimulation location of tDCS on current effectiveness.

METHODS

A search was conducted for randomized controlled trials published up to May 2023 comparing the application of tDCS versus a sham or control group. The primary outcome variables were balance and gait ability.

RESULTS

Eleven studies were included in the qualitative analysis, and ten were included in the quantitative analysis, which included 230 patients with multiple sclerosis. The average effect of tDCS on gait functionality was superior to that of the control group (SMD = -0.71; 95% CI, -1.05 to -0.37). However, the overall results of the tDCS vs. sham effect on static balance did not show significant differences between groups (MD = 1.26, 95% CI, -1.31 to 3.82). No significant differences were found when different locations of tDCS were compared.

CONCLUSIONS

These results reveal that tDCS is an effective treatment for improving gait ability with a low quality of evidence. However, the application of tDCS has no effect on static balance in patients with multiple sclerosis with very low quality of evidence. Similarly, there seems to be no difference regarding the stimulation area with tDCS.

Corticospinal excitability changes during muscle relaxation and contraction in motor imagery

To enhance smooth muscle contraction and relaxation during rehabilitation and sports activities, a comprehensive understanding of the motor control mechanisms within the central nervous system is necessary. However, current knowledge on these aspects is insufficient. Therefore, this study aimed to deepen our understanding of motor controls, by investigating the alterations in corticospinal excitability within cortical motor areas related to muscle contraction and relaxation using motor imagery with a reaction time task paradigm. Transcranial magnetic stimulation was used to measure the motor-evoked potentials in the first dorsal interosseous muscle of the right hand after the 'go' signal. Static weak muscle contraction (Experiment 1: 18 healthy participants) and resting state (Experiment 2: 16 healthy participants) were applied as background factors, and a trial without motor imagery was performed as a control. Muscle contraction was maintained in the background in the contraction motor imagery. A decrease in excitability in the relaxation motor imagery task occurred compared with the control. When the muscles were at rest, an increase in excitability in the contraction motor imagery and a transient increase in excitability in the relaxation motor imagery occurred compared with the control condition. Hence, the excitability of contraction and relaxation motor imagery is characterized by a continuous increase in excitability, transient increase and subsequent decrease in excitability, respectively. These results suggest that muscle contraction sensory information in the background condition may be necessary for muscle relaxation. Matching the background conditions may be crucial when utilizing motor imagery for rehabilitation or sports training.

Moderate intensity aerobic exercise may enhance neuroplasticity of the contralesional hemisphere after stroke: a randomised controlled study

Upregulation of neuroplasticity might help maximize stroke recovery. One intervention that appears worthy of investigation is aerobic exercise. This study aimed to determine whether a single bout of moderate intensity aerobic exercise can enhance neuroplasticity in people with stroke. Participants were randomly assigned (1:1) to a 20-min moderate intensity exercise intervention or remained sedentary (control). Transcranial magnetic stimulation measured corticospinal excitability of the contralesional hemisphere by recording motor evoked potentials (MEPs). Intermittent Theta Burst Stimulation (iTBS) was used to repetitively activate synapses in the contralesional primary motor cortex, initiating the early stages of neuroplasticity and increasing excitability. It was surmised that if exercise increased neuroplasticity, there would be a greater facilitation of MEPs following iTBS. Thirty-three people with stroke participated in this study (aged 63.87 ± 10.30 years, 20 male, 6.13 ± 4.33 years since stroke). There was an interaction between Time*Group on MEP amplitudes (P = 0.009). Participants allocated to aerobic exercise had a stronger increase in MEP amplitude following iTBS. A non-significant trend indicated time since stroke might moderate this interaction (P = 0.055). Exploratory analysis suggested participants who were 2-7.5 years post stroke had a strong MEP facilitation following iTBS (P < 0.001). There was no effect of age, sex, resting motor threshold, self-reported physical activity levels, lesion volume or weighted lesion load (all P > 0.208). Moderate intensity cycling may enhance neuroplasticity in people with stroke. This therapy adjuvant could provide opportunities to maximize stroke recovery.

Neuronal Plasticity and Age-Related Functional Decline in the Motor Cortex

Physiological aging causes a decline of motor function due to impairment of motor cortex function, losses of motor neurons and neuromuscular junctions, sarcopenia, and frailty. There is increasing evidence suggesting that the changes in motor function start earlier in the middle-aged stage. The mechanism underlining the middle-aged decline in motor function seems to relate to the central nervous system rather than the peripheral neuromuscular system. The motor cortex is one of the responsible central nervous systems for coordinating and learning motor functions. The neuronal circuits in the motor cortex show plasticity in response to motor learning, including LTP. This motor cortex plasticity seems important for the intervention method mechanisms that revert the age-related decline of motor function. This review will focus on recent findings on the role of plasticity in the motor cortex for motor function and age-related changes. The review will also introduce our recent identification of an age-related decline of neuronal activity in the primary motor cortex of middle-aged mice using electrophysiological recordings of brain slices.

State-dependent effectiveness of cathodal transcranial direct current stimulation on cortical excitability

The extensive use of transcranial direct current stimulation (tDCS) in experimental and clinical settings does not correspond to an in-depth understanding of its underlying neurophysiological mechanisms. In previous studies, we employed an integrated system of Transcranial Magnetic Stimulation and Electroencephalography (TMS-EEG) to track the effect of tDCS on cortical excitability. At rest, anodal tDCS (a-tDCS) over the right Posterior Parietal Cortex (rPPC) elicits a widespread increase in cortical excitability. In contrast, cathodal tDCS (c-tDCS) fails to modulate cortical excitability, being indistinguishable from sham stimulation. Here we investigated whether an endogenous task-induced activation during stimulation might change this pattern, improving c-tDCS effectiveness in modulating cortical excitability. In Experiment 1, we tested whether performance in a Visuospatial Working Memory Task (VWMT) and a modified Posner Cueing Task (mPCT), involving rPPC, could be modulated by c-tDCS. Thirty-eight participants were involved in a two-session experiment receiving either c-tDCS or sham during tasks execution. In Experiment 2, we recruited sixteen novel participants who performed the same paradigm but underwent TMS-EEG recordings pre- and 10 min post- sham stimulation and c-tDCS. Behavioral results showed that c-tDCS significantly modulated mPCT performance compared to sham. At a neurophysiological level, c-tDCS significantly reduced cortical excitability in a frontoparietal network likely involved in task execution. Taken together, our results provide evidence of the state dependence of c-tDCS in modulating cortical excitability effectively. The conceptual and applicative implications are discussed.

Evaluating the Therapeutic Application of Neuromodulation in the Human Swallowing System

In the last two decades, the focus of neurogenic dysphagia management has moved from passive compensatory strategies to evidence-based rehabilitative approaches. Advances in technology have enabled the development of novel treatment approaches such as neuromodulation techniques, which target the promotion of neurological reorganization for functional recovery of swallowing. Given the rapid pace of development in the field, this review aims to summarize the current findings on the effects of neuromodulation techniques on the human swallowing system and evaluate their therapeutic potential for neurogenic dysphagia. Implications for future clinical research and practical considerations for using neuromodulation in clinical practice will also be discussed.

Frontal two-electrode transcranial direct current stimulation protocols may not affect performance on a combined flanker Go/No-Go task

Transcranial direct current stimulation (tDCS) has been tested to modulate cognitive control or response inhibition using various electrode montages. However, electrode montages and current polarities have not been systematically compared when examining tDCS effects on cognitive control and response inhibition. In this randomized, sham-controlled study, 38 healthy volunteers were randomly grouped into receiving one session of sham, anodal, and cathodal each in an electrode montage that targeted either the dorsolateral prefrontal cortex (DLPFC) or the fronto-medial (FM) region. Participants performed a combined flanker Go/No-Go task during stimulation. No effect of tDCS was found in the DLPFC and FM groups neither using anodal nor cathodal stimulation. No major adverse effects of tDCS were identified using either montage or stimulation type and the two groups did not differ in terms of the reported sensations. The present study suggests that single-session tDCS delivered in two two-electrode montages might not affect cognitive control or response inhibition, despite using widely popular stimulation parameters. This is in line with the heterogeneous findings in the field and calls for further systematic research to exclude less reliable methods from those with more pronounced effects, identify the determinants of responsiveness, and develop optimal ways to utilize this technique.

Non-Invasive Brain Stimulation and Sex/Polypeptide Hormones in Reciprocal Interactions: A Systematic Review

A better understanding of interindividual differences and the development of targeted therapies is one of the major challenges of modern medicine. The sex of a person plays a crucial role in this regard. This systematic review aimed to summarise and analyse available evidence on the mutual interactions between non-invasive brain stimulation and sex/polypeptide hormones. The PubMed database was searched from its inception to 31 March 2023, for (i) studies that investigated the impact of sex and/or polypeptide hormones on the effects induced by non-invasive brain stimulation, or (ii) studies that investigated non-invasive brain stimulation in the modulation of sex and/or polypeptide hormones. Eighteen studies (319 healthy and 96 disabled participants) were included. Most studies focused on female sex hormone levels during the menstrual cycle. The later follicular phase is associated with a weak between hemispheric and intracortical inhibition, strong intracortical facilitation, and high stimulation-induced neural and behavioural changes. The opposite effects are observed during the luteal phase. In addition, the participant's sex, presence and/or absence of real ovulation and increase in oestradiol level by chorionic gonadotropin injection influence the stimulation-induced neurophysiological and behavioural effects. In Parkinson's disease and consciousness disorders, the repetitive application of non-invasive brain stimulation increases oestradiol and dehydroepiandrosterone levels and reduces disability. To date, male hormones have not been sufficiently included in these studies. Here, we show that the sex and/or polypeptide hormones and non-invasive brain stimulation methods are in reciprocal interactions. This may be used to create a more effective and individualised approach for healthy individuals and individuals with disabilities.

Train duration and inter-train interval determine the direction and intensity of high-frequency rTMS after-effects

Background and objective

It has been proved that repetitive transcranial magnetic stimulation (rTMS) triggers the modulation of homeostatic metaplasticity, which causes the effect of rTMS to disappear or even reverse, and a certain length of interval between rTMS trains might break the modulation of homeostatic metaplasticity. However, it remains unknown whether the effects of high-frequency rTMS can be modulated by homeostatic metaplasticity by lengthening the train duration and whether homeostatic metaplasticity can be broken by prolonging the inter-train interval.

Methods

In this study, 15 subjects participated in two experiments including different rTMS protocols targeting the motor cortex. In the first experiment, high-frequency rTMS protocols with different train durations (2 s and 5 s) and an inter-train interval of 25 s were adopted. In the second experiment, high-frequency rTMS protocols with a train duration of 5 s and different inter-train intervals (50 s and 100 s) were adopted. A sham protocol was also included. Changes of motor evoked potential amplitude acquired from electromyography, power spectral density, and intra-region and inter-region functional connectivity acquired from electroencephalography in the resting state before and after each rTMS protocol were evaluated.

Results

High-frequency rTMS with 2 s train duration and 25 s inter-train interval increased cortex excitability and the power spectral density of bilateral central regions in the alpha frequency band and enhanced the functional connectivity between central regions and other brain regions. When the train duration was prolonged to 5 s, the after-effects of high-frequency rTMS disappeared. The after-effects of rTMS with 5 s train duration and 100 s inter-train interval were the same as those of rTMS with 2 s train duration and 25 s inter-train interval.

Conclusion

Our results indicated that train duration and inter-train interval could induce the homeostatic metaplasticiy and determine the direction of intensity of rTMS after-effects, and should certainly be taken into account when performing rTMS in both research and clinical practice.

Effects of active and sham tDCS on the soleus H-reflex during standing

Weak transcranial direct current stimulation (tDCS) is known to affect corticospinal excitability and enhance motor skill acquisition, whereas its effects on spinal reflexes in actively contracting muscles are yet to be established. Thus, in this study, we examined the acute effects of Active and Sham tDCS on the soleus H-reflex during standing. In fourteen adults without known neurological conditions, the soleus H-reflex was repeatedly elicited at just above M-wave threshold throughout 30 min of Active (N = 7) or Sham (N = 7) 2-mA tDCS over the primary motor cortex in standing. The maximum H-reflex (Hmax) and M-wave (Mmax) were also measured before and immediately after 30 min of tDCS. The soleus H-reflex amplitudes became significantly larger (by 6%) ≈1 min into Active or Sham tDCS and gradually returned toward the pre-tDCS values, on average, within 15 min. With Active tDCS, the amplitude reduction from the initial increase appeared to occur more swiftly than with Sham tDCS. An acute temporary increase in the soleus H-reflex amplitude within the first minute of Active and Sham tDCS found in this study indicates a previously unreported effect of tDCS on the H-reflex excitability. The present study suggests that neurophysiological characterization of Sham tDCS effects is just as important as investigating Active tDCS effects in understanding and defining acute effects of tDCS on the excitability of spinal reflex pathways.

Physiology of dystonia: Human studies

In this chapter, we discuss neurophysiological techniques that have been used in the study of dystonia. We examine traditional disease models such as inhibition and excessive plasticity and review the evidence that these play a causal role in pathophysiology. We then review the evidence for sensory and peripheral influences within pathophysiology and look at an emergent literature that tries to probe how oscillatory brain activity may be linked to dystonia pathophysiology.

Hemispheric Differences of 1 Hz rTMS over Motor and Premotor Cortex in Modulation of Neural Processing and Hand Function

INTRODUCTION

Non-invasive brain stimulation can modulate both neural processing and behavioral performance. Its effects may be influenced by the stimulated area and hemisphere. In this study (EC no. 09083), repetitive transcranial magnetic stimulation (rTMS) was applied to the primary motor cortex (M1) or dorsal premotor cortex (dPMC) of either the right or left hemisphere, while evaluating cortical neurophysiology and hand function.

METHODS

Fifteen healthy subjects participated in this placebo-controlled crossover study. Four sessions of real 1 Hz rTMS (110% of rMT, 900 pulses) over (i) left M1, (ii) right M1, (iii) left dPMC, (iv) right dPMC, and one session of (v) placebo 1 Hz rTMS (0% of rMT, 900 pulses) over the left M1 were applied in randomized order. Motor function of both hands (Jebsen-Taylor Hand Function Test (JTHFT)) and neural processing within both hemispheres (motor evoked potentials (MEPs), cortical silent period (CSP), and ipsilateral silent period (ISP)) were evaluated prior and after each intervention session.

RESULTS

A lengthening of CSP and ISP durations within the right hemisphere was induced by 1 Hz rTMS over both areas and hemispheres. No such intervention-induced neurophysiological changes were detected within the left hemisphere. Regarding JTHFT and MEP, no intervention-induced changes ensued. Changes of hand function correlated with neurophysiological changes within both hemispheres, more often for the left than the right hand.

CONCLUSIONS

Effects of 1 Hz rTMS can be better captured by neurophysiological than behavioral measures. Hemispheric differences need to be considered for this intervention.

A systematic review of the neurobiological effects of theta-burst stimulation (TBS) as measured using functional magnetic resonance imaging (fMRI)

Theta burst stimulation (TBS) is associated with the modulation of a range of clinical, cognitive, and behavioural outcomes, but specific neurobiological effects remain somewhat unclear. This systematic literature review investigated resting-state and task-based functional magnetic resonance imaging (fMRI) outcomes post-TBS in healthy human adults. Fifty studies that applied either continuous-or intermittent-(c/i) TBS, and adopted a pretest-posttest or sham-controlled design, were included. For resting-state outcomes following stimulation applied to motor, temporal, parietal, occipital, or cerebellar regions, functional connectivity generally decreased in response to cTBS and increased in response to iTBS, though there were some exceptions to this pattern of response. These findings are mostly consistent with the assumed long-term depression (LTD)/long-term potentiation (LTP)-like plasticity effects of cTBS and iTBS, respectively. Task-related outcomes following TBS were more variable. TBS applied to the prefrontal cortex, irrespective of task or state, also produced more variable responses, with no consistent patterns emerging. Individual participant and methodological factors are likely to contribute to the variability in responses to TBS. Future studies assessing the effects of TBS via fMRI must account for factors known to affect the TBS outcomes, both at the level of individual participants and of research methodology.

Transcranial Direct Current Stimulation (tDCS) in Pediatric Populations—– Voices from Typically Developing Children and Adolescents and their Parents

Background

Transcranial direct current stimulation (tDCS) is a brain stimulation technique currently being researched as an alternative or complimentary treatment for various neurological disorders. There is little knowledge about experiences of the participants of tDCS clinical research, especially from pediatric studies.

Methods

An interview study with typically developing minors (n = 19, mean age 13,66 years) participating in a tDCS study, and their parents (n = 18) was conducted to explore their views and experiences and inform the ethical analysis.

Results

Children (10–13 years old) and adolescents (14–18 years old) reported good experiences with the stimulation. Next to financial incentives, main motives to participate in the study were curiosity and the possibility to help develop treatments for children affected by neurological disorders. They could also see a potential of using tDCS in a non-medical setting, especially regarding the provision of equal opportunity, e.g. in education. Parents also presented a positive attitude towards tDCS and their children participation in the basic research study. Nevertheless, their understanding of tDCS was rather poor. Even though many of them understood the techniques, they often did not see the link between the (current) lack of side effects and an absence of longitudinal studies. Parents were also cautious about using tDCS for non-medical/enhancement purposes.

Conclusions

The study findings show a need for more transparent information about the state of the art of tDCS, its function and what it might be able to offer, especially considering the good acceptability of tDCS.

The effects of age and biological sex on the association between I-wave recruitment and the response to cTBS: an exploratory study

The neuroplastic response to continuous theta burst stimulation (cTBS) is inherently variable. The measurement of I-wave latencies has been shown to strongly predict the magnitude and direction of the response to cTBS, whereby longer latencies are associated with stronger long-term depression-like responses. However, potential differences in this association relating to age and sex have not been explored. We performed cTBS and measured I-wave recruitment (via MEP latencies) in 66 participants (31 female) ranging in age from 11 to 78 years. The influence of age and sex on the association between I-wave recruitment and the response to cTBS was tested using linear regression models. In contrast to previous studies, there was not a significant association between I-wave latencies and cTBS response at the group level (p = 0.142, R² = 0.033). However, there were interactions between I-waves and both age and sex when predicting cTBS response. Subgroup analysis revealed that preferential late I-wave recruitment predicted cTBS response in adolescent females, but not in adolescent or adult males or adult females. These data suggest that the generalisability of I-wave measurement in predicting the response to cTBS may be lower than initially believed. Prediction models should include age and sex, rather than I-wave latencies alone, as our findings suggest that, while each factor alone is not a strong predictor, these factors interact to influence the response to cTBS.

Investigation of neurobiological responses to theta burst stimulation during recovery from mild traumatic brain injury (mTBI)

OBJECTIVE

The ability of the brain to recover following neurological insult is of interest for mild traumatic brain injury (mTBI) populations. Investigating whether non-invasive brain stimulation (NIBS) can modulate neurophysiology and cognition may lead to the development of therapeutic interventions post injury. The purpose of this study was to investigate neurobiological effects of one session of intermittent theta burst stimulation (iTBS) to the dorsolateral prefrontal cortex (DLPFC) in participants recovering from mTBI.

METHOD

Changes to neurophysiology were assessed with electroencephalography (EEG) and transcranial magnetic stimulation combined with EEG (TMS-EEG). Digit span working memory accuracy assessed cognitive performance. 30 patients were assessed within one-month of sustaining a mTBI and 26 demographically matched controls were assessed. Participants were also assessed at 3-months (mTBI: N = 21, control: N = 26) and 6-months (mTBI: N = 15, control: N = 24).

RESULTS

Analyses demonstrated iTBS did not reliably modulate neurophysiological activity, and no differences in cognitive performance were produced by iTBS at any assessment time-point.

CONCLUSIONS

Factors responsible for our null results are unclear. Possible limitations to our experimental design are discussed.

SIGNIFICANCE

Our findings suggest additional research is required to establish the effects of iTBS on plasticity following mTBI, prior to therapeutic application.

DATA AND CODE AVAILABILITY STATEMENT

We do not have ethical approval to make this data publicly available, as our approval predated our inclusion of such approvals (which we now do routinely).

Can a single session of noninvasive brain stimulation applied over the prefrontal cortex prevent stress-induced cortisol release?

INTRODUCTION

A better understanding of how the hypothalamic-pituitary-adrenal (HPA) axis can be externally regulated is of major importance, especially because hyperreactivity to stress has been proposed as a key factor in the onset and maintenance of many psychiatric conditions. Over the past decades, numerous studies have investigated whether non-invasive brain stimulation (NIBS) can regulate HPA axis reactivity in acute stress situation. As the current results did not allow us to draw clear conclusions, we decided to conduct a systematic review of the literature investigating the effect of a single NIBS session on stress-induced cortisol release.

METHODS

We searched MEDLINE and Web Of Science for articles indexed through December 2021. Among the 246 articles identified, 15 fulfilled our inclusion criteria with a quality estimated between 52 and 93%.

RESULTS

Of the different NIBS used and targeted brain regions, stimulating the left dorsolateral prefrontal cortex, with either high frequency repetitive transcranial magnetic stimulation or anodal transcranial direct current stimulation, seems to be the most appropriate for reducing cortisol release in acute stress situations.

CONCLUSIONS

Despite the heterogeneity of the stimulation parameters, the characteristics of participants, the modalities of cortisol collection, the timing of the NIBS session in relation to the stressor exposure, and methodological considerations, stimulating the left dorsolateral prefrontal cortex can be efficient to modulate stress-induced cortisol release.

Genetic profile for dopamine signaling predicts brain functional reactivity to repetitive transcranial magnetic stimulation

Research integrating molecular and imaging data provides important insights into how the genetic profile associated with dopamine signaling influences inter-individual differences in brain functions. However, the effects of genetic variations in dopamine signaling on the heterogeneity of brain changes induced by repetitive transcranial magnetic stimulation (rTMS) still remain unclear. The current study examined the composite effects of genetic variations in dopamine-related genes on rTMS-induced brain responses in terms of the functional network connectivity and working memory performance. Healthy individuals (n = 30) participated in a randomized, double-blind, sham-controlled study with a crossover design of five consecutive days where active rTMS or sham stimulation sessions were administered over the left dorsolateral prefrontal cortex (DLPFC) of the brain. Participants were mostly women (n = 29) and genotyped for polymorphisms in the catechol-O-methyltransferase and D2 dopamine receptor genes and categorized according to their genetic composite scores: high vs. low dopamine signaling groups. Pre- and post-intervention data of resting-state functional magnetic resonance imaging and working memory performance were obtained from 27 individuals with active rTMS and 30 with sham stimulation sessions. The mean functional connectivity within the resting-state networks centered on the DLPFC increased in the high dopamine signaling group. Working memory performance also improved with rTMS in the high dopamine signaling group compared to that in the low dopamine signaling group. The present results suggest that genetic predisposition to higher dopamine signaling may be a promising neurobiological predictor for rTMS effects on cognitive enhancement.Trial registration: ClinicalTrials.gov (NCT02932085).

Short duration event related cerebellar TDCS enhances visuomotor adaptation

BACKGROUND

Transcranial direct current stimulation (TDCS) is typically applied before or during a task, for periods ranging from 5 to 30 min.

HYPOTHESIS

We hypothesise that briefer stimulation epochs synchronous with individual task actions may be more effective.

METHODS

In two separate experiments, we applied brief bursts of event-related anodal stimulation (erTDCS) to the cerebellum during a visuomotor adaptation task.

RESULTS

The first study demonstrated that 1 s duration erTDCS time-locked to the participants' reaching actions enhanced adaptation significantly better than sham. A close replication in the second study demonstrated 0.5 s erTDCS synchronous with the reaching actions again resulted in better adaptation than standard TDCS, significantly better than sham. Stimulation either during the inter-trial intervals between movements or after movement, during assessment of visual feedback, had no significant effect. Because short duration stimulation with rapid onset and offset is more readily perceived by the participants, we additionally show that a non-electrical vibrotactile stimulation of the scalp, presented with the same timing as the erTDCS, had no significant effect.

CONCLUSIONS

We conclude that short duration, event related, anodal TDCS targeting the cerebellum enhances motor adaptation compared to the standard model. We discuss possible mechanisms of action and speculate on neural learning processes that may be involved.

Revisiting the Rotational Field TMS Method for Neurostimulation

Transcranial magnetic stimulation (TMS) is a non-invasive technique that has shown high efficacy in the treatment of major depressive disorder (MDD) and is increasingly utilized for various neuropsychiatric disorders. However, conventional TMS is limited to activating only a small fraction of neurons that have components parallel to the induced electric field. This likely contributes to the significant variability observed in clinical outcomes. A novel method termed rotational field TMS (rfTMS or TMS 360°) enables the activation of a greater number of neurons by reducing the sensitivity to orientation. Recruitment of a larger number of neurons offers the potential to enhance efficacy and reduce variability in the treatment of clinical indications for which neuronal recruitment and organization may play a significant role, such as MDD and stroke. The potential of the method remains to be validated in clinical trials. Here, we revisit and describe in detail the rfTMS method, its principles, mode of operation, effects on the brain, and potential benefits for clinical TMS.

Sensory and motor cortical excitability changes induced by rTMS and sensory stimulation in stroke: A randomized clinical trial

Background

The ability to produce coordinated movement is dependent on dynamic interactions through transcallosal fibers between the two cerebral hemispheres of the brain. Although typically unilateral, stroke induces changes in functional and effective connectivity across hemispheres, which are related to sensorimotor impairment and stroke recovery. Previous studies have focused almost exclusively on interhemispheric interactions in the primary motor cortex (M1).

Objective

To identify the presence of interhemispheric asymmetry (ASY) of somatosensory cortex (S1) excitability and to investigate whether S1 repetitive transcranial magnetic stimulation (rTMS) combined with sensory stimulation (SS) changes excitability in S1 and M1, as well as S1 ASY, in individuals with subacute stroke.

Methods

A randomized clinical trial. Participants with a single episode of stroke, in the subacute phase, between 35 and 75 years old, were allocated, randomly and equally balanced, to four groups: rTMS/sham SS, sham rTMS/SS, rTMS/SS, and sham rTMS/Sham SS. Participants underwent 10 sessions of S1 rTMS of the lesioned hemisphere (10 Hz, 1,500 pulses) followed by SS. SS was applied to the paretic upper limb (UL) (active SS) or non-paretic UL (sham SS). TMS-induced motor evoked potentials (MEPs) of the paretic UL and somatosensory evoked potential (SSEP) of both ULs assessed M1 and S1 cortical excitability, respectively. The S1 ASY index was measured before and after intervention. Evaluator, participants and the statistician were blinded.

Results

Thirty-six participants divided equally into groups (nine participants per group). Seven patients were excluded from MEP analysis because of failure to produce consistent MEP. One participant was excluded in the SSEP analysis because no SSEP was detected. All somatosensory stimulation groups had decreased S1 ASY except for the sham rTMS/Sham SS group. When compared with baseline, M1 excitability increased only in the rTMS/SS group.

Conclusion

S1 rTMS and SS alone or in combination changed S1 excitability and decreased ASY, but it was only their combination that increased M1 excitability.

Clinical trial registration

clinicaltrials.gov, identifier (NCT03329807).

A Scalable Framework for Closed-Loop Neuromodulation with Deep Learning

Closed-loop neuromodulation measures dynamic neural or physiological activity to optimize interventions for clinical and nonclinical behavioral, cognitive, wellness, attentional, or general task performance enhancement. Conventional closed-loop stimulation approaches can contain biased biomarker detection (decoders and error-based triggering) and stimulation-type application. We present and verify a novel deep learning framework for designing and deploying flexible, data-driven, automated closed-loop neuromodulation that is scalable using diverse datasets, agnostic to stimulation technology (supporting multi-modal stimulation: tACS, tDCS, tFUS, TMS), and without the need for personalized ground-truth performance data. Our approach is based on identified periods of responsiveness - detected states that result in a change in performance when stimulation is applied compared to no stimulation. To demonstrate our framework, we acquire, analyze, and apply a data-driven approach to our open sourced GX dataset, which includes concurrent physiological (ECG, EOG) and neuronal (EEG) measures, paired with continuous vigilance/attention-fatigue tracking, and High-Definition transcranial electrical stimulation (HD-tES). Our framework's decision process for intervention application identified 88.26% of trials as correct applications, showed potential improvement with varying stimulation types, or missed opportunities to stimulate, whereas 11.25% of trials were predicted to stimulate at inopportune times. With emerging datasets and stimulation technologies, our unifying and integrative framework; leveraging deep learning (Convolutional Neural Networks - CNNs); demonstrates the adaptability and feasibility of automated multimodal neuromodulation for both clinical and nonclinical applications.

Combined neuromuscular electrical stimulation and transcutaneous spinal direct current stimulation increases motor cortical plasticity in healthy humans

Introduction

Neuromuscular electrical stimulation (NMES) induces neural plasticity of the central nervous system (CNS) and improves motor function in patients with CNS lesions. However, the extended stimulus duration of NMES reduces its clinical applicability. Transcutaneous spinal direct current stimulation (tsDCS), which increases afferent input, may enhance the effects and reduce the stimulus duration of NMES. This study investigated the excitability of the motor cortex, somatosensory cortex, and spinal motor neurons after the combined stimulation of NMES and tsDCS.

Methods

Among the 55 participants in this study, 24 were allocated to experiment 1, 15 to experiment 2, and 16 to experiment 3. They received intervention for 20 min on different days: (1) NMES combined with tsDCS (NMES + tsDCS), (2) NMES combined with sham tsDCS (NMES + sham tsDCS), and (3) sham NMES combined with tsDCS (sham NMES + tsDCS). NMES was delivered to the right common peroneal nerve at 25 Hz with the intensity at 120% of the motor threshold. For tsDCS, the cathodal electrode was positioned on the thoracic 10th–12th vertebral levels, and the anodal electrode was located on the right shoulder. The stimulus intensity was 2.5 mA. In experiment 1, motor evoked potentials (MEPs) and short-latency intracortical inhibition (SICI) were measured by transcranial magnetic stimulation up to 60 min after stimulation. The spinal motor neurons’ excitability was assessed by recording the posterior root muscle reflex (PRMR) induced via transcutaneous spinal cord stimulation in experiment 2, and the primary somatosensory cortex excitability was evaluated by recording the somatosensory evoked potentials (SEPs) in experiment 3 up to 15 min after stimulation.

Results

Compared to before the stimulation, NMES + tsDCS significantly increased MEP for 60 min or more, and significantly decreased SICI immediately after. Conversely contrast, the PRMR significantly decreased immediately after, and SEPs were unchanged.

Discussion

These results suggest that simultaneous afferent inputs from different stimulus positions critically induce primary motor cortex plasticity. The combined stimulation of NMES with tsDCS may facilitate the development of a new neurorehabilitation technique.