Metaplasticity in human cortex

Transcranial Direct Current Stimulation to Ameliorate Post-Stroke Cognitive Impairment

Post-stroke cognitive impairment is a common and disabling condition with few effective therapeutic options. After stroke, neural reorganization and other neuroplastic processes occur in response to ischemic injury, which can result in clinical improvement through spontaneous recovery. Neuromodulation through transcranial direct current stimulation (tDCS) is a promising intervention to augment underlying neuroplasticity in order to improve cognitive function. This form of neuromodulation leverages mechanisms of neuroplasticity post-stroke to optimize neural reorganization and improve function. In this review, we summarize the current state of cognitive neurorehabilitation post-stroke, the practical features of tDCS, its uses in stroke-related cognitive impairment across cognitive domains, and special considerations for the use of tDCS in the post-stroke patient population.

Acute effects of aerobic exercise on corticomotor plasticity in individuals with and without depression

BACKGROUND

Although complex in nature, the pathophysiology of depression involves reduced or impaired neuroplastic capabilities. Restoring or enhancing neuroplasticity may serve as a treatment target for developing therapies for depression. Aerobic exercise (AEx) has antidepressant benefits and may enhance neuroplasticity in depression although the latter has yet to be substantiated. Therefore, we sought to examine the acute effect of AEx on neuroplasticity in depression.

METHODS

Sixteen individuals with (DEP; 13 female; age = 28.5 ± 7.3; Montgomery-Äsberg Depression Rating Scale [MADRS] = 21.3 ± 5.2) and without depression (HC; 13 female; age 27.2 ± 7.5; MADRS = 0.8 ± 1.2) completed three experimental visits consisting of 15 min of low intensity AEx (LO) at 35% heart rate reserve (HRR), high intensity AEx (HI) at 70% HRR, or sitting (CON). Following AEx, excitatory paired associative stimulation (PAS25ms) was employed to probe neuroplasticity. Motor evoked potentials (MEP) were assessed via transcranial magnetic stimulation before and after PAS25ms to indicate acute changes in neuroplasticity.

RESULTS

PAS25ms primed with HI AEx led to significant increases in MEP amplitude compared to LO and CON. HI AEx elicited enhanced PAS25ms-induced neuroplasticity for up to 1-h post-PAS. There were no significant between-group differences.

CONCLUSION

HI AEx enhances PAS measured neuroplasticity in individuals with and without depression. HI AEx may have a potent influence on the brain and serve as an effective primer, or adjunct, to therapies that seek to harness neuroplasticity.

Single-Pore Nanofluidic Logic Memristor with Reconfigurable Synaptic Functions and Designable Combinations

The biological neural network is a highly efficient in-memory computing system that integrates memory and logical computing functions within synapses. Moreover, reconfiguration by environmental chemical signals endows biological neural networks with dynamic multifunctions and enhanced efficiency. Nanofluidic memristors have emerged as promising candidates for mimicking synaptic functions, owing to their similarity to synapses in the underlying mechanisms of ion signaling in ion channels. However, realizing chemical signal-modulated logic functions in nanofluidic memristors, which is the basis for brain-like computing applications, remains unachieved. Here, we report a single-pore nanofluidic logic memristor with reconfigurable logic functions. Based on the different degrees of protonation and deprotonation of functional groups on the inner surface of the single pore, the modulation of the memristors and the reconfiguration of logic functions are realized. More noteworthy, this single-pore nanofluidic memristor can not only avoid the average effects in multipore but also act as a fundamental component in constructing complex neural networks through series and parallel circuits, which lays the groundwork for future artificial nanofluidic neural networks. The implementation of dynamic synaptic functions, modulation of logic gates by chemical signals, and diverse combinations in single-pore nanofluidic memristors opens up new possibilities for their applications in brain-inspired computing.

Intrinsic Plasticity Mechanisms of Repetitive Transcranial Magnetic Stimulation

Repetitive transcranial magnetic stimulation (rTMS) has become an increasingly popular tool to modulate neural excitability and induce neural plasticity in clinical and preclinical models; however, the physiological mechanisms in which it exerts these effects remain largely unknown. To date, studies have primarily focused on characterizing rTMS-induced changes occurring at the synapse, with little attention given to changes in intrinsic membrane properties. However, accumulating evidence suggests that rTMS may induce its effects, in part, via intrinsic plasticity mechanisms, suggesting a new and potentially complementary understanding of how rTMS alters neural excitability and neural plasticity. In this review, we provide an overview of several intrinsic plasticity mechanisms before reviewing the evidence for rTMS-induced intrinsic plasticity. In addition, we discuss a select number of neurological conditions where rTMS-induced intrinsic plasticity has therapeutic potential before speculating on the temporal relationship between rTMS-induced intrinsic and synaptic plasticity.

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.

High-Definition Transcranial Direct Current Stimulation-Primed Intermittent Theta Burst Stimulation in Treatment-Resistant Depression: A Controlled Study

OBJECTIVES

The aim of this study was to evaluate whether high-definition transcranial direct current stimulation (HDtDCS) priming improves the efficacy of intermittent theta burst stimulation (iTBS) in improving TRD.

METHODOLOGY

A prospective hospital-based, randomized control study where the participants were divided into active or sham HDtDCS-primed iTBS stimulation groups for a total of 10 sessions and were assessed on clinical parameters at baseline, end of week 1, and end of week 2 was done. Primary outcome of the study was the difference in Hamilton Depression Rating Scale (HDRS) scores over 2 weeks of HDtDCS-primed iTBS.

RESULT

A significant effect of time was seen over HDRS scores in both active and sham groups with a large effect size. Significant effect of time was also found over the Clinical Global Impressions-Severity Scale scores of patients with a large effect size. The difference in the improvement in depressive severity as measured using HDRS and Clinical Global Impressions-Severity Scale scores between active and sham groups was also found to be significant with large effect sizes.

CONCLUSION

High-definition tDCS-primed iTBS is superior to normal iTBS in patients with depression who have failed a trial of 2 antidepressants, whereas both mechanisms are of benefit to the patients.

Priming transcranial direct current stimulation for improving hemiparetic upper limb in patients with subacute stroke: study protocol for a randomised controlled trial

INTRODUCTION

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modulates brain states by applying a weak electrical current to the brain cortex. Several studies have shown that anodal stimulation of the ipsilesional primary motor cortex (M1) may promote motor recovery of the affected upper limb in patients with stroke; however, a high-level clinical recommendation cannot be drawn in view of inconsistent findings. A priming brain stimulation protocol has been proposed to induce stable modulatory effects, in which an inhibitory stimulation is applied prior to excitatory stimulation to a brain area. Our recent work showed that priming theta burst magnetic stimulation demonstrated superior effects in improving upper limb motor function and neurophysiological outcomes. However, it remains unknown whether pairing a session of cathodal tDCS with a session of anodal tDCS will also capitalise on its therapeutic effects.

METHODS AND ANALYSIS

This will be a two-arm double-blind randomised controlled trial involving 134 patients 1-6 months after stroke onset. Eligible participants will be randomly allocated to receive 10 sessions of priming tDCS+robotic training, or 10 sessions of non-priming tDCS+robotic training for 2 weeks. The primary outcome is the Fugl-Meyer Assessment-upper extremity, and the secondary outcomes are the Wolf Motor Function Test and Modified Barthel Index. The motor-evoked potentials, regional oxyhaemoglobin level and resting-state functional connectivity between the bilateral M1 will be acquired and analysed to investigate the effects of priming tDCS on neuroplasticity.

ETHICS AND DISSEMINATION

The study has been approved by the Research Ethics Committee of the Shanghai Yangzhi Rehabilitation Center (reference number: Yangzhi2023-022) and will be conducted in accordance with the Declaration of Helsinki of 1964, as revised in 2013.

TRIAL REGISTRATION NUMBER

ChiCTR2300074681.

Impact of Chronic Pain on Use-Dependent Plasticity: Corticomotor Excitability and Motor Representation in Musicians With and Without Pain

Long-term musical training induces adaptive changes in the functional representation of the motor cortex. It is unknown if the maladaptive plasticity associated with chronic pain, frequently affecting trained musicians, may alter the use-dependent plasticity in the motor cortex. This study investigated the interaction between adaptive and maladaptive plasticity in the motor pathways, in particular how chronic pain influences long-term use-dependent plasticity. Using transcranial magnetic stimulation (TMS), corticospinal excitability was assessed by measuring the amplitude of the motor-evoked potential (MEP), area of the motor map, volume, and center of gravity of the first dorsal interosseous muscle in 19 pain-free musicians, 17 upper limb/neck pain chronic pain musicians, and 19 pain-free non-musicians as controls. Motor map volume and MEP amplitude were smaller for both pain-free and chronic pain musicians compared to pain-free controls (P < 0.011). No significant differences were found between musicians with and without chronic pain. These findings confirm that long-term musical training can lead to focalized and specialized functional organization of the primary motor cortex. Moreover, the adaptive use-dependent plasticity acquired through fine-motor skill acquisition is not significantly compromised by the maladaptive plasticity typically associated with chronic pain, highlighting the potential of long-term sensorimotor training to counteract the effects of chronic pain in the motor system.

Combined multi-session transcranial alternating current stimulation (tACS) and language skills training improves individual gamma band activity and literacy skills in developmental dyslexia

Developmental dyslexia is characterized by the pathologically diminished ability to acquire reading and spelling skills. Accurate processing of acoustic information at the phonemic scale is crucial for successful sound-to-letter-mapping which, in turn, is elemental in reading and spelling. Altered activation patterns in the auditory cortex are thought to provide the neurophysiological basis for the inaccurate phonemic perception. Recently, transcranial electrical stimulation has been shown to be an effective method to ameliorate cortical activation patterns in the auditory cortex. In a sample of children and adolescents with dyslexia, we investigated the effect of multi-session transcranial alternating current stimulation delivered concurrently with a phonological training and in combination with a behavioral literacy skills training. Over a 5-week period the participants received 10 training sessions while gamma-tACS was administered over bilateral auditory cortex. We found that gamma-tACS shifted the peak frequency of auditory gamma oscillations reflecting a more fine-grained processing of time-critical acoustic information. This amelioration was accompanied by increased phonemic processing skills. Moreover, individuals who received gamma-tACS showed significant improvements in their spelling skills four months after the intervention. Our results demonstrate that multi-session gamma-tACS enhances the effects of a behavioral intervention and induces long-term improvement on literacy skills in dyslexia.

Excitatory-inhibitory modulation of transcranial focus ultrasound stimulation on human motor cortex

AIMS

Transcranial focus ultrasound stimulation (tFUS) is a promising non-invasive neuromodulation technology. This study aimed to evaluate the modulatory effects of tFUS on human motor cortex (M1) excitability and explore the mechanism of neurotransmitter-related intracortical circuitry and plasticity.

METHODS

Single pulse transcranial magnetic stimulation (TMS)-eliciting motor-evoked potentials (MEPs) were used to assessed M1 excitability in 10 subjects. Paired-pulse TMS was used to measure the effects of tFUS on GABA- and glutamate-related intracortical excitability and 1 H-MRS was used to assess the effects of repetitive tFUS on GABA and Glx (glutamine + glutamate) neurometabolic concentrations in the targeting region in nine subjects.

RESULTS

The etFUS significantly increased M1 excitability, decreased short interval intracortical inhibition (SICI) and long interval intracortical inhibition (LICI). The itFUS significantly suppressed M1 excitability, increased SICI, LICI, and decreased intracortical facilitation (ICF). Seven times of etFUS decreased the GABA concentration (6.32%), increased the Glx concentration (12.40%), and decreased the GABA/Glx ratio measured by MRS, while itFUS increased the GABA concentration (18.59%), decreased Glx concentration (0.35%), and significantly increased GABA/Glx ratio.

CONCLUSION

The findings support that tFUS with different parameters can exert excitatory and inhibitory neuromodulatory effects on the human motor cortex. We provide novel insights that tFUS change cortical excitability and plasticity by regulating excitatory-inhibition balance related to the GABAergic and glutamatergic receptor function and neurotransmitter metabolic level.

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.

Lasting enhancements in neural efficiency by multi-session transcranial direct current stimulation during working memory training

The neural basis for long-term behavioral improvements resulting from multi-session transcranial direct current stimulation (tDCS) combined with working memory training (WMT) remains unclear. In this study, we used task-related electroencephalography (EEG) measures to investigate the lasting neurophysiological effects of anodal high-definition (HD)-tDCS applied over the left dorsolateral prefrontal cortex (dlPFC) during a challenging WMT. Thirty-four healthy young adults were randomized to sham or active tDCS groups and underwent ten 30-minute training sessions over ten consecutive days, preceded by a pre-test and followed by post-tests performed one day and three weeks after the last session, respectively, by performing high-load WM tasks along with EEG recording. Multi-session HD-tDCS significantly enhanced the behavioral benefits of WMT. Compared to the sham group, the active group showed facilitated increases in theta, alpha, beta, and gamma task-related oscillations at the end of training and significantly increased P300 response 3 weeks post-training. Our findings suggest that applying anodal tDCS over the left dlPFC during multi-session WMT can enhance the behavioral benefits of WMT and facilitate sustained improvements in WM-related neural efficiency.

Repetitive paired-pulse TMS increases motor cortex excitability and visuomotor skill acquisition in young and older adults

Transcranial magnetic stimulation (TMS) over primary motor cortex (M1) recruits indirect (I) waves that can be modulated by repetitive paired-pulse TMS (rppTMS). The purpose of this study was to examine the effect of rppTMS on M1 excitability and visuomotor skill acquisition in young and older adults. A total of 37 healthy adults (22 young, 18-32 yr; 15 older, 60-79 yr) participated in a study that involved rppTMS at early (1.4 ms) and late (4.5 ms) interstimulus intervals (ISIs), followed by the performance of a visuomotor training task. M1 excitability was examined with motor-evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. We found that rppTMS increased M1 excitability in young and old adults, with the greatest effects for PA TMS at the late ISI (4.5 ms). Motor skill acquisition was improved by rppTMS at an early (1.4 ms) but not late (4.5 ms) ISI in young and older adults. An additional study using a non-I-wave interval (3.5 ms) also showed increased M1 excitability and visuomotor skill acquisition. These findings show that rppTMS at both I-wave and non-I-wave intervals can alter M1 excitability and improve visuomotor skill acquisition in young and older adults.

After-effects of repetitive transcranial magnetic stimulation with parameter dependence on long-term potentiation-like plasticity and object recognition memory in rats

Objective

To investigate the after-effects of 25-Hz repetitive transcranial magnetic stimulation (rTMS) at 60, 100, and 120% resting motor threshold (rMT) on long-term potentiation (LTP) in the rat hippocampus, to clarify the intensity dependence of rTMS, and to determine whether it simultaneously affects learning and memory ability.

Methods

Five rats were randomly selected from 70 male Wistar rats, and evoked rMT potentials were recorded in response to magnetic stimulation. The remaining 65 rats were randomly assigned to five groups (n = 13), including sham rTMS, 1 Hz 100% rMT, and 25 Hz rTMS groups with 3 subgroups of 60% rMT, 100% rMT, and 120% rMT. Five rats in each group were anesthetized and induced by a priming TMS-test design for population spike (PS) response of the perforant path-dentate gyrus in the hippocampus; the remaining eight rats in each group were evaluated for object recognition memory in the novel object recognition (NOR) task after the different rTMS protocols.

Results

Forty-five percent (approximately 1.03 T) of the magnetic stimulator output was confirmed as rMT in the biceps femoris muscle. The PS ratio was ranked as follows: 25 Hz 100% rMT (267.78 ± 25.71%) > sham rTMS (182 ± 9.4%) >1 Hz 100% rMT (102.69 ± 6.64%) > 25 Hz 120% rMT (98 ± 11.3%) > 25 Hz 60% rMT (36 ± 8.5%). Significant differences were observed between the groups, except for the difference between the 25 Hz 120% rMT and the 1 Hz 100% rMT groups (p = 0.446). LTP was successfully induced over the 60-min recording period only in the sham rTMS and 25 Hz 100% rMT groups. Moreover, these two groups spent more time exploring a novel object than a familiar object during the NOR task (p < 0.001), suggesting long-term recognition memory retention. In the between-group analysis of the discrimination index, the following ranking was observed: 25 Hz 100% rMT (0.812 ± 0.158) > sham rTMS (0.653 ± 0.111) > 25 Hz 120% rMT (0.583 ± 0.216) >1 Hz 100% rMT (0.581 ± 0.145) > 25 Hz 60% rMT (0.532 ± 0.220).

Conclusion

The after-effect of 25-Hz rTMS was dependent on stimulus intensity and provided an inverted (V-shaped) bidirectional modulation on hippocampal plasticity that involved two forms of metaplasticity. Furthermore, the effects on the recognition memory ability were positively correlated with those on LTP induction in the hippocampus in vivo.

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.

The effects of D-Cycloserine on corticospinal excitability after repeated spaced intermittent theta-burst transcranial magnetic stimulation: A randomized controlled trial in healthy individuals

Repeated spaced TMS protocols, also termed accelerated TMS protocols, are of increasing therapeutic interest. The long-term potentiation (LTP)-like effects of repeated spaced intermittent theta-burst transcranial magnetic stimulation (iTBS) are presumed to be N-Methyl-D-Aspartate receptor (NMDA-R) dependent; however, this has not been tested. We tested whether the LTP-like effects of repeated spaced iTBS are influenced by low-dose D-Cycloserine (100 mg), an NMDA-R partial-agonist. We conducted a randomized, double-blind, placebo-controlled crossover trial in 20 healthy adults from August 2021-Feb 2022. Participants received repeated spaced iTBS, consisting of two iTBS sessions 60 minutes apart, to the primary motor cortex. The peak-to-peak amplitude of the motor evoked potentials (MEP) at 120% resting motor threshold (RMT) was measured after each iTBS. The TMS stimulus-response (TMS-SR; 100-150% RMT) was measured at baseline, +30 min, and +60 min after each iTBS. We found evidence for a significant Drug*iTBS effect in MEP amplitude, revealing that D-Cycloserine enhanced MEP amplitudes relative to the placebo. When examining TMS-SR, pairing iTBS with D-Cycloserine increased the TMS-SR slope relative to placebo after both iTBS tetani, and this was due to an increase in the upper bound of the TMS-SR. This indicates that LTP-like and metaplastic effects of repeated-spaced iTBS involve NMDA-R, as revealed by two measures of corticospinal excitability, and that low-dose D-Cycloserine facilitates the physiological effects of repeated spaced iTBS. However, extension of these findings to clinical populations and therapeutic protocols targeting non-motor regions of cortex requires empirical validation.

Stimulating human prefrontal cortex increases reward learning

Work in computational psychiatry suggests that mood disorders may stem from aberrant reinforcement learning processes. Specifically, it has been proposed that depressed individuals believe that negative events are more informative than positive events, resulting in higher learning rates from negative outcomes (Pulcu and Browning, 2019). In this proof-of-concept study, we investigated whether transcranial direct current stimulation (tDCS) applied to dorsolateral prefrontal cortex, as commonly used in depression treatment trials, might change learning rates for affective outcomes. Healthy adults completed an established reinforcement learning task (Pulcu and Browning, 2017) in which the information content of reward and loss outcomes was manipulated by varying the volatility of stimulus-outcome associations. Learning rates on the tasks were quantified using computational models. Stimulation over dorsolateral prefrontal cortex (DLPFC) but not motor cortex (M1) increased learning rates specifically for reward outcomes. The effects of prefrontal tDCS were cognitive state-dependent: tDCS applied during task performance increased learning rates for wins; tDCS applied before task performance decreased both win and loss learning rates. A replication study confirmed the key finding that tDCS to DLPFC during task performance increased learning rates specifically for rewards. Taken together, these findings demonstrate the potential of tDCS for modulating computational parameters of reinforcement learning that are relevant to mood disorders.

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.

Adaptive control of synaptic plasticity integrates micro- and macroscopic network function

Synaptic plasticity configures interactions between neurons and is therefore likely to be a primary driver of behavioral learning and development. How this microscopic-macroscopic interaction occurs is poorly understood, as researchers frequently examine models within particular ranges of abstraction and scale. Computational neuroscience and machine learning models offer theoretically powerful analyses of plasticity in neural networks, but results are often siloed and only coarsely linked to biology. In this review, we examine connections between these areas, asking how network computations change as a function of diverse features of plasticity and vice versa. We review how plasticity can be controlled at synapses by calcium dynamics and neuromodulatory signals, the manifestation of these changes in networks, and their impacts in specialized circuits. We conclude that metaplasticity-defined broadly as the adaptive control of plasticity-forges connections across scales by governing what groups of synapses can and can't learn about, when, and to what ends. The metaplasticity we discuss acts by co-opting Hebbian mechanisms, shifting network properties, and routing activity within and across brain systems. Asking how these operations can go awry should also be useful for understanding pathology, which we address in the context of autism, schizophrenia and Parkinson's disease.

Bibliometric and visual analysis of transcranial direct current stimulation in the web of science database from 2000 to 2022 via CiteSpace

Objective

This study aimed to evaluate the current research hotspots and development tendency of Transcranial Direct Current Stimulation (tDCS) in the field of neurobiology from a bibliometric perspective by providing visualized information to scientists and clinicians.

Materials and methods

Publications related to tDCS published between 2000 and 2022 were retrieved from the Web of Science Core Collection (WOSCC) on May 5, 2022. Bibliometric features including the number of publications and citations, citation frequency, H-index, journal impact factors, and journal citation reports were summarized using Microsoft Office Excel. Co-authorship, citation, co-citation, and co-occurrence analyses among countries, institutions, authors, co-authors, journals, publications, references, and keywords were analyzed and visualized using CiteSpace (version 6.1.R3).

Results

A total of 4,756 publications on tDCS fulfilled the criteria we designed and then were extracted from the WOSCC. The United States (1,190 publications, 25.02%) and Harvard University (185 publications, 3.89%) were the leading contributors among all the countries and institutions, respectively. NITSCHE MA and FREGNI F, two key researchers, have made great achievements in tDCS. Brain Stimulation (306 publications) had the highest number of publications relevant to tDCS and the highest number of citations (4,042 times). In terms of potential hotspots, we observed through reference co-citation analysis timeline viewer related to tDCS that "depression"#0, "Sensorimotor network"#10, "working memory"#11, and "Transcranial magnetic stimulation"#9 might be the future research hotspots, while keywords with the strong burst and still ongoing were "intensity" (2018-2022), "impairment" (2020-2022), "efficacy" (2020-2022), and "guideline" (2020-2022).

Conclusion

This was the first-ever study of peer-reviewed publications relative to tDCS using several scientometric and visual analytic methods to quantitatively and qualitatively reveal the current research status and trends in the field of tDCS. Through the bibliometric method, we gained an in-depth understanding of the current research status and development trend on tDCS. Our research and analysis results might provide some practical sources for academic scholars and clinicians.

Autonomy, procedural and substantive: a discussion of the ethics of cognitive enhancement

As cognitive enhancement research advances, important ethical questions regarding individual autonomy and freedom are raised. Advocates of cognitive enhancement frequently adopt a procedural approach to autonomy, arguing that enhancers improve an individual's reasoning capabilities, which are quintessential to being an autonomous agent. On the other hand, critics adopt a more nuanced approach by considering matters of authenticity and self-identity, which go beyond the mere assessment of one's reasoning capacities. Both positions, nevertheless, require further philosophical scrutiny. In this paper, we investigate the ethics of cognitive enhancement through the lenses of political and philosophical arguments about autonomy and freedom. In so doing, we contend that a substantive, relational account of individual autonomy offers a more holistic understanding of the ethical concerns of cognitive enhancement.

Neuromodulation as an Augmenting Strategy for Behavioral Therapies for Anxiety and PTSD: a Narrative Review

PURPOSE OF REVIEW

Post-traumatic stress disorder (PTSD) is a prevalent problem. Despite current treatments, symptoms may persist, and neuromodulation therapies show great potential. A growing body of research suggests that transcranial magnetic stimulation (TMS) is effective as a standalone treatment for PTSD, with recent research demonstrating promising use when combined synergistically with behavioral treatments. In this review, we survey this literature including data suggesting mechanisms involved in anxiety and PTSD that may be targeted by neurostimulation.

RECENT FINDINGS

Evidence suggests the mechanism of action for TMS that contributes to behavioral change may be enhanced neural plasticity via increased functionality of prefrontal and subcortical/limbic structures and associated networks. Some research has demonstrated a behavioral change in PTSD and anxiety due to enhanced extinction learning or improved ability to think flexibly and reduce ruminative tendencies. Growing evidence suggests TMS may be best used as a therapeutic adjunct, at least acutely, for extinction-based exposure therapies in patients by accelerating therapy response.

SUMMARY

While TMS has shown promise as a standalone intervention, augmentation with psychotherapy is one avenue of interest. Non-responders to current EBPs might particularly benefit from this sort of targeted approach, and it may shorten treatment length, which would help the successful completion of a course of therapy.

The Effect of Repetitive Transcranial Magnetic Stimulation in Synchronization with Effortful Swallowing on Post-stroke Dysphagia

Either effortful swallowing exercise or repetitive transcranial magnetic stimulation (rTMS) is considered as an effective method to treat dysphagia after stroke. Thus, synchronizing these two interventions may improve the efficiency of treatment. This trial intended to explore the effects of rTMS and effortful swallowing exercise on the recovery of swallowing function in patients after stroke. A total of 56 patients with post-stroke dysphagia who were able to actively cooperate with the training were analyzed in this study. We experimented with different intervention effects of rTMS synchronization with effortful swallowing training (group 1), rTMS (group 2), and traditional swallowing training alone (group 3). Every patient completed conventional swallowing training 5 days a week for 2 weeks. Patients in group 1 and group 2 underwent 10 consecutive sessions of 5 Hz rTMS over the affected mylohyoid cortical region. Fiberoptic endoscopic dysphagia severity scale (FEDSS), penetration/aspiration scale (PAS), standardized swallowing assessment (SSA), and functional oral intake scale (FOIS) were assessed and compared across the groups. No significant difference in FEDSS, PAS, SSA, or FOIS scores was found at baseline among the three groups. The mean change values of the FEDSS score, PAS score, SSA score, and FOIS score between baseline and post-intervention of the three groups (H = 16.05, P < 0.001; H = 21.70, P < 0.001; F (2, 53) = 9.68, P < 0.001; H = 18.26, P < 0.001; respectively) were statistically significant. In addition, the mean change values of FEDSS, PAS, SSA, and FOIS scores in participants in group 1 (all P < 0.001) and group 2 (P = 0.046; P = 0.045; P = 0.028; P = 0.032; respectively) were significantly higher than in group 3. Similarly, the mean change values of FEDSS, PAS, SSA, and FOIS scores were significantly higher in participants in group 1 than in group 2 (P = 0.046; P = 0.038; P = 0.042; P = 0.044; respectively). The results revealed that the conjunction of rTMS and effortful swallowing training was an effective method to facilitate the recovery of swallowing function in stroke patients. The present clinical trial provided a new treatment method for the functional restoration of swallowing in stroke patients, which may further facilitate the recovery of swallowing function in stroke patients with swallowing disorders.

Exploration on neurobiological mechanisms of the central–peripheral–central closed-loop rehabilitation

Central and peripheral interventions for brain injury rehabilitation have been widely employed. However, as patients’ requirements and expectations for stroke rehabilitation have gradually increased, the limitations of simple central intervention or peripheral intervention in the rehabilitation application of stroke patients’ function have gradually emerged. Studies have suggested that central intervention promotes the activation of functional brain regions and improves neural plasticity, whereas peripheral intervention enhances the positive feedback and input of sensory and motor control modes to the central nervous system, thereby promoting the remodeling of brain function. Based on the model of a central–peripheral–central (CPC) closed loop, the integration of center and peripheral interventions was effectively completed to form “closed-loop” information feedback, which could be applied to specific brain areas or function-related brain regions of patients. Notably, the closed loop can also be extended to central and peripheral immune systems as well as central and peripheral organs such as the brain–gut axis and lung–brain axis. In this review article, the model of CPC closed-loop rehabilitation and the potential neuroimmunological mechanisms of a closed-loop approach will be discussed. Further, we highlight critical questions about the neuroimmunological aspects of the closed-loop technique that merit future research attention.

Perspective on Nanofluidic Memristors: from Mechanism to Application

Nanofluidic memristors are memory resistors based on nanoconfined fluidic systems exhibiting history-dependent ion conductivity. Toward establishing powerful computing systems beyond the Harvard architecture, these ion-based neuromorphic devices attracted enormous research attention owing to the unique characteristics of ion-based conductors. However, the design of nanofluidic memristor is still at a primary state and a systematic guidance on the rational design of nanofluidic system is desperately required for the development of nanofluidic-based neuromorphic devices. Herein, we proposed a systematic review on the history, main mechanism and potential application of nanofluidic memristors in order to give a prospective view on the design principle of memristors based on nanofluidic systems. Furthermore, based on the present status of these devices, some fundamental challenges for this promising area were further discussed to show the possible application of these ion-based devices.

Adding a Second iTBS Block in 15 or 60 Min Time Interval Does Not Increase iTBS Effects on Motor Cortex Excitability and the Responder Rates

The use of metaplasticity-based intermittent theta-burst stimulation (iTBS) protocols including several stimulation blocks could be a possible approach to increasing stimulation effectiveness. Our aim was to investigate the neurophysiological effects of two protocols with a short and a long interval between blocks. Seventeen healthy volunteers received four protocols in a pseudorandomized order: iTBS 0-15 (two blocks of active iTBS of primary motor cortex (M1) separated by 15 min and a control stimulation block of the vertex in 60 min from the first block); iTBS 0-60 (active iTBS, a control block in 15 min, and an active block in 60 min); iTBS 0 (active iTBS and two control blocks with the same intervals); and Control (three control blocks). The motor evoked potentials (MEPs) were measured before the first and after the second and third blocks. We have shown no significant differences between the effects of the protocols on both the motor cortex excitability and the responder rates. No significant changes of MEPs were observed after all the protocols. The reliability for the responsiveness to a single block between two sessions was insignificant. Our data confirm low reproducibility of the response to iTBS and suggest that the use of repeated protocols does not increase the responder rates or neurophysiological effects of iTBS.

Repeated motor cortex theta-burst stimulation produces persistent strengthening of corticospinal motor output and durable spinal cord structural changes in the rat

BACKGROUND

The strength of connections between motor cortex (MCX) and muscle can be augmented with a variety of stimulation protocols. Augmenting MCX-to-muscle connection strength by neuromodulation may be a way to enhance the intact motor system's capacity for acquiring motor skills and promote function after injury to strengthen spared connections. But this enhancement must be maintained for functional improvements.

OBJECTIVE

We determined if brief MCX muscle evoked potential (MEP) enhancement produced by intermittent theta burst stimulation (iTBS) can be converted into a longer and structurally durable form of response enhancement with repeated daily and longer-term application.

METHODS

Electrical iTBS was delivered through an implanted MCX epidural electrode and MEPs were recorded using implanted EMG electrodes in awake naïve rats. MCX activity was modulated further using chemogenetic (DREADDs) excitation and inhibition. Corticospinal tract (CST) axons were traced and immunochemistry used to measure CST synapses.

RESULTS

A single MCX iTBS block (600 pulses) produced MEP LTP lasting ∼30-45 min. Concatenating five iTBS blocks within a 30-min session produced MEP LTP lasting 24-48 h, which could be strengthened or weakened by bidirectional MCX activity modulation. Effect duration was not changed. Finally, daily induction of this persistent MEP LTP with daily iTBS for 10-days produced MEP enhancement outlasting the stimulation period by at least 10 days, and accompanied by CST axonal outgrowth and structural changes at the CST-spinal interneuron synapse.

CONCLUSION

Our findings inform the mechanisms of iTBS and provide a framework for designing neuromodulatory strategies to promote durable enhancement of cortical motor actions.

Repetitive transcranial magnetic stimulation for post-traumatic stress disorder: Lights and shadows

We have read with interest the publication that describes the available data related to the use of neuromodulation strategies for the treatment of post-traumatic stress disorder (PTSD). Despite treatment advances, however, a substantial proportion of PTSD patients receiving psychological and/or pharmacological treatment do not reach an adequate clinical response. In their paper, the authors draw attention to the current understanding of the use of repetitive transcranial magnetic stimulation (rTMS) as a potential treatment for PTSD. Most of the previous studies indeed applied both inhibitory (1 Hz) and excitatory (> 1 Hz, up to 20 Hz) rTMS to the right and/or left dorsolateral prefrontal cortex. Despite larger therapeutic effects observed when high-frequency stimulation was applied, the question of which side and frequency of stimulation is the most successful is still debated. The authors also reported on the after-effect of rTMS related to neuroplasticity and identified the intermittent theta burst stimulation as a technique of particular interest because of it showed the most effective improvement on PTSD symptoms. However, although numerous studies have highlighted the possible beneficial use of rTMS protocols for PTSD, the exact mechanism of action remains unclear. In their conclusions, the authors stated that rTMS has been demonstrated to be effective for the treatment of PTSD symptoms. Nevertheless, we believe that further research with homogeneous samples, standardized protocols, and objective outcome measures is needed to identify specific therapeutic targets and to better define significant changes when active and sham stimulation procedures are compared.

Using non-invasive transcranial direct current stimulation for neglect and associated attentional deficits following stroke

Neglect is a disabling neuropsychological syndrome that is frequently observed following right-hemispheric stroke. Affected individuals often present with multiple attentional deficits, ranging from reduced orienting towards contralesional space to a generalized impairment in maintaining attention over time. Although a degree of spontaneous recovery occurs in most patients, in some individuals this condition can be treatment-resistant with prominent ongoing non-spatial deficits. Further, there is a large inter-individual variability in response to different therapeutic approaches. Given its potential to alter neuronal excitability and affect neuroplasticity, non-invasive brain stimulation is a promising tool that could potentially be utilized to facilitate recovery. However, there are many outstanding questions regarding its implementation in this heterogeneous patient group. Here we provide a critical overview of the available evidence on the use of non-invasive electrical brain stimulation, focussing on transcranial direct current stimulation (tDCS), to improve neglect and associated attentional deficits after right-hemispheric stroke. At present, there is insufficient robust evidence supporting the clinical use of tDCS to alleviate symptoms of neglect. Future research would benefit from careful study design, enhanced precision of electrical montages, multi-modal approaches exploring predictors of response, tailored dose-control applications and increased efforts to evaluate standalone tDCS versus its incorporation into combination therapy.

Exploring the role of the left DLPFC in fatigue during unresisted rhythmic movements

Understanding central fatigue during motor activities is important in neuroscience and different medical fields. The central mechanisms of motor fatigue are known in depth for isometric muscle contractions; however, current knowledge about rhythmic movements and central fatigue is rather scarce. In this study, we explored the role of an executive area (left dorsolateral prefrontal cortex [DLPFC]) in fatigue development during rhythmic movement execution, finger tapping (FT) at the maximal rate, and fatigue after effects on the stability of rhythmic patterns. Participants (n = 19) performed six sets of unresisted FT (with a 3 min rest in‐between). Each set included four interleaved 30 s repetitions of self‐selected (two repetitions) and maximal rate FT (two repetitions) without rest in‐between. Left DLPFC involvement in the task was perturbed by transcranial static magnetic stimulation (tSMS) in two sessions (one real and one sham). Moreover, half of the self‐selected FT repetitions were performed concurrently with a demanding cognitive task, the Stroop test. Compared with sham stimulation, real tSMS stimulation prevented waning in tapping frequency at the maximal rate without affecting perceived levels of fatigue. Participants' engagement in the Stroop test just prior to maximal FT reduced the movement amplitude during this mode of execution. Movement variability at self‐selected rates increased during Stroop execution, especially under fatigue previously induced by maximal FT. Our results indicate cognitive‐motor interactions and a prominent role of the prefrontal cortex in fatigue and the motor control of simple repetitive movement patterns. We suggest the need to approach motor fatigue including cognitive perspectives.

We show the fundamental role of executive areas in fatigue caused by very simple repetitive movements. Fatigue developed less during the maximal frequency of movement production, while the left DLPFC received magnetic stimulation (in right‐handers). The role of cognitive‐motor interaction in fine motor control was also clear when participants engaged in cognitive tasks. At the clinical level, our work reinforces the need to treat fatigue from a comprehensive perspective.

Single session transcranial direct current stimulation to the primary motor cortex fails to enhance early motor sequence learning in Parkinson's disease

INTRODUCTION

Explicit motor sequence learning is impaired in Parkinson's disease (PD). Transcranial direct current stimulation (tDCS) applied over the motor cortex in healthy can improve explicit motor learning, but comparative effects in PD are unknown. This exploratory study aims to examine the effect of single session tDCS on explicit motor sequence learning in PD.

METHODS

Thirty-three people with mild to moderate PD learnt a short and long finger tapping sequence with their right hand. Participants received either anodal, cathodal, or sham tDCS applied over the left primary motor cortex during task practice. Single- and dual-task finger tapping performance was assessed before and after task practice and functional near-infrared spectroscopy used to measure task related changes of oxygenated haemoglobin.

RESULTS

Finger tapping performance of short and long sequences under single-task conditions significantly improved following practice (p = 0.010 and p < 0.001, respectively). A condition-by-time interaction trend was observed for the long finger tapping sequence (p = 0.069) driven by improved performance in the cathodal (p = 0.001) and sham (p < 0.001) tDCS conditions, but not anodal tDCS (p = 0.198). The primary and premotor cortex and supplementary motor area were active in all tasks. No interaction or main effects were observed for task related changes of oxygenated haemoglobin.

CONCLUSIONS

PD patients retain the capacity to learn an explicit sequence of movements. Motor cortex tDCS does not improve explicit motor learning in PD and anodal tDCS may even suppress the rate of learning.

Motor cortex plasticity response to acute cardiorespiratory exercise and intermittent theta-burst stimulation is attenuated in premanifest and early Huntington’s disease

Huntington’s disease (HD) mouse models suggest that cardiovascular exercise may enhance neuroplasticity and delay disease signs, however, the effects of exercise on neuroplasticity in people with HD are unknown. Using a repeated-measures experimental design, we compared the effects of a single bout of high-intensity exercise, moderate-intensity exercise, or rest, on motor cortex synaptic plasticity in 14 HD CAG-expanded participants (9 premanifest and 5 early manifest) and 20 CAG-healthy control participants, using transcranial magnetic stimulation. Measures of cortico-motor excitability, short-interval intracortical inhibition and intracortical facilitation were obtained before and after a 20-min bout of either high-intensity interval exercise, moderate-intensity continuous exercise, or rest, and again after intermittent theta burst stimulation (iTBS). HD participants showed less inhibition at baseline compared to controls. Whereas the control group showed increased excitability and facilitation following high-intensity exercise and iTBS, the HD group showed no differences in neuroplasticity responses following either exercise intensity or rest, with follow-up Bayesian analyses providing consistent evidence that these effects were absent in the HD group. These findings indicate that exercise-induced synaptic plasticity mechanisms in response to acute exercise may be attenuated in HD, and demonstrate the need for future research to further investigate exercise and plasticity mechanisms in people with HD.

Transcranial magnetic stimulation as a tool to induce and explore plasticity in humans

Activity-dependent synaptic plasticity is the main theoretical framework to explain mechanisms of learning and memory. Synaptic plasticity can be explored experimentally in animals through various standardized protocols for eliciting long-term potentiation and long-term depression in hippocampal and cortical slices. In humans, several non-invasive protocols of repetitive transcranial magnetic stimulation and transcranial direct current stimulation have been designed and applied to probe synaptic plasticity in the primary motor cortex, as reflected by long-term changes in motor evoked potential amplitudes. These protocols mimic those normally used in animal studies for assessing long-term potentiation and long-term depression. In this chapter, we first discuss the physiologic basis of theta-burst stimulation, paired associative stimulation, and transcranial direct current stimulation. We describe the current biophysical and theoretical models underlying the molecular mechanisms of synaptic plasticity and metaplasticity, defined as activity-dependent changes in neural functions that modulate subsequent synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), in the human motor cortex including calcium-dependent plasticity, spike-timing-dependent plasticity, the role of N-methyl-d-aspartate-related transmission and gamma-aminobutyric-acid interneuronal activity. We also review the putative microcircuits responsible for synaptic plasticity in the human motor cortex. We critically readdress the issue of variability in studies investigating synaptic plasticity and propose available solutions. Finally, we speculate about the utility of future studies with more advanced experimental approaches.

The Normal Human Adult Hypothalamus Proteomic Landscape: Rise of Neuroproteomics in Biological Psychiatry and Systems Biology

The human hypothalamus is central to the regulation of neuroendocrine and neurovegetative systems, as well as modulation of chronobiology and behavioral aspects in human health and disease. Surprisingly, a deep proteomic analysis of the normal human hypothalamic proteome has been missing for such an important organ so far. In this study, we delineated the human hypothalamus proteome using a high-resolution mass spectrometry approach which resulted in the identification of 5349 proteins, while a multiple post-translational modification (PTM) search identified 191 additional proteins, which were missed in the first search. A proteogenomic analysis resulted in the discovery of multiple novel protein-coding regions as we identified proteins from noncoding regions (pseudogenes) and proteins translated from short open reading frames that can be missed using the traditional pipeline of prediction of protein-coding genes as a part of genome annotation. We also identified several PTMs of hypothalamic proteins that may be required for normal hypothalamic functions. Moreover, we observed an enrichment of proteins pertaining to autophagy and adult neurogenesis in the proteome data. We believe that the hypothalamic proteome reported herein would help to decipher the molecular basis for the diverse range of physiological functions attributed to it, as well as its role in neurological and psychiatric diseases. Extensive proteomic profiling of the hypothalamic nuclei would further elaborate on the role and functional characterization of several hypothalamus-specific proteins and pathways to inform future research and clinical discoveries in biological psychiatry, neurology, and system biology.

Effectiveness of TES and rTMS for the Treatment of Insomnia: Meta-Analysis and Meta-Regression of Randomized Sham-Controlled Trials

Objectives: Transcranial electric stimulation (TES) and repetitive transcranial magnetic stimulation (rTMS) have experienced significant development in treating insomnia. This review aims to examine the effectiveness of randomized sham-controlled trials of TES and rTMS in improving insomnia and examine potential moderators associated with the effect of the treatment. Methods: Nine electronic databases were searched for studies comparing the effects of TES/rTMS with sham group on insomnia from the inception of these databases to June 25, 2021, namely, Medline, Embase, PsycINFO, CINAHL, Cochrane Library, Web of Science, PubMed, ProQuest Dissertation and Thesis, and CNKI. Meta-analyses were conducted to examine the effect of TES and rTMS in treating insomnia. Univariate meta-regression was performed to explore potential treatment moderators that may influence the pooled results. Risk of bias was assessed by using the Cochrane Risk of Bias Tool. Results: A total of 16 TES studies and 27 rTMS studies were included in this review. The pooled results indicated that there was no significant difference between the TES group and the sham group in improving objective measures of sleep. rTMS was superior to its sham group in improving sleep efficiency, total sleep time, sleep onset latency, wake up after sleep onset, and number of awakenings (all p < 0.05). Both TES and rTMS were superior to their sham counterparts in improving sleep quality as measured by the Pittsburgh Sleep Quality Index at post-intervention. The weighted mean difference for TES and rTMS were -1.17 (95% CI: -1.98, -0.36) and -4.08 (95% CI: -4.86, -3.30), respectively. Gender, total treatment sessions, number of pulses per session, and length of treatment per session were associated with rTMS efficacy. No significant relationship was observed between TES efficacy and the stimulation parameters. Conclusions: It seems that TES and rTMS have a chance to play a decisive role in the therapy of insomnia. Possible dose-dependent and gender difference effects of rTMS are suggested.

Metaplasticity in the human swallowing system: clinical implications for dysphagia rehabilitation

Dysphagia is a common and devastating complication following brain damage. Over the last 2 decades, dysphagia treatments have shifted from compensatory to rehabilitative strategies that facilitate neuroplasticity, which is the reorganization of neural networks that is essential for functional recovery. Moreover, there is growing interest in the application of cortical and peripheral neurostimulation to promote such neuroplasticity. Despite some preliminary positive findings, the variability in responsiveness toward these treatments remains substantial. The purpose of this review is to summarize findings on the effects of neurostimulation in promoting neuroplasticity for dysphagia rehabilitation and highlight the need to develop more effective treatment strategies. We then discuss the role of metaplasticity, a homeostatic mechanism of the brain to regulate plasticity changes, in helping to drive neurorehabilitation. Finally, a hypothesis on how metaplasticity could be applied in dysphagia rehabilitation to enhance treatment outcomes is proposed.

The Effect of a Single Session of Non-Invasive Brain Stimulation on Balance in Healthy Individuals: A Systematic Review and Best Evidence Synthesis

Aim: To evaluate the effects of a single session of non-invasive brain stimulation (NIBS) on postural balance. Introduction: The NIBS has been used widely in improving balance. However, the effect of a single session of NIBS on balance in healthy individuals has not been systemically reviewed. Methods: A systematic literature review and best evidence synthesis were conducted, according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines, to determine the effects of different NIBS techniques on balance function in healthy individuals. The methodological quality of included articles was assessed by the risk of bias, and the Downs and Black tool. Data were analyzed by using the best evidence synthesis. Thirty-five articles were included that used the following NIBS techniques: anodal transcranial direct current stimulation (a-tDCS), cathodal transcranial direct current stimulation (c-tDCS), continuous theta burst stimulation (cTBS), and repetitive transcranial magnetic stimulation (rTMS) on primary motor cortex (M1), supplementary motor area (SMA), dorsolateral prefrontal cortex (DLPFC), and cerebellum on balance. Results: Strong evidence showed that a-tDCS of M1, SMA improve balance in healthy participants, and the a-tDCS of DLPFC induces improvement only in dual task balance indices. Also, the findings indicate that cerebellar a-tDCS might significantly improve balance, if at least 10 min cerebellar a-tDCS with an intensity of ≥1 mA, over or maximum 1.5 cm below the inion, is used. Strong evidence showed that c-tDCS, cTBS, and rTMS are not effective on the balance. Conclusion: According to the results, the a-tDCS may be a useful technique to improve balance in healthy adults.

Protocols for inducing homeostatic plasticity reflected in the corticospinal excitability in healthy human participants: A systematic review and meta-analysis

Homeostatic plasticity complements synaptic plasticity by stabilising neural activity within a physiological range. In humans, homeostatic plasticity is investigated using two blocks of non-invasive brain stimulation (NIBS) with an interval without stimulation between blocks. The aim of this systematic review and meta-analysis was to investigate the effect of homeostatic plasticity induction protocols on motor evoked potentials (MEP) in healthy participants. Four databases were searched (Medline, Scopus, Embase and Cochrane library). Studies describing the application of two blocks of NIBS of the primary motor cortex with an interval of no stimulation between blocks reporting changes in corticospinal excitability by MEP amplitude were included. Thirty-seven reports with 55 experiments (700 participants) were included. Study quality was considered poor overall, with heterogeneity in study size, sample and designs. Two blocks of excitatory stimulation at the primary motor cortex produced a homeostatic response (decreased MEP) between 0 and 30 min post-protocols, when compared with a single stimulation block. Two blocks of inhibitory stimulation at the primary motor cortex using interval duration of 10 min or less produced a homeostatic response (increased MEP) between 0 and 30 min post-protocols, when compared with a single stimulation block. There were no differences in MEPs when compared with baseline MEPs. In conclusion, homeostatic plasticity induction using two blocks of NIBS with an interval of 10 min or less without stimulation between blocks produces a homeostatic response up to 30 min post-protocol. Improvements in participant selection, sample sizes and protocols of NIBS techniques are needed.

Age-related changes in motor cortex plasticity assessed with non-invasive brain stimulation: an update and new perspectives

It is commonly accepted that the brains capacity to change, known as plasticity, declines into old age. Recent studies have used a variety of non-invasive brain stimulation (NIBS) techniques to examine this age-related decline in plasticity in the primary motor cortex (M1), but the effects seem inconsistent and difficult to unravel. The purpose of this review is to provide an update on studies that have used different NIBS techniques to assess M1 plasticity with advancing age and offer some new perspective on NIBS strategies to boost plasticity in the ageing brain. We find that early studies show clear differences in M1 plasticity between young and older adults, but many recent studies with motor training show no decline in use-dependent M1 plasticity with age. For NIBS-induced plasticity in M1, some protocols show more convincing differences with advancing age than others. Therefore, our view from the NIBS literature is that it should not be automatically assumed that M1 plasticity declines with age. Instead, the effects of age are likely to depend on how M1 plasticity is measured, and the characteristics of the elderly population tested. We also suggest that NIBS performed concurrently with motor training is likely to be most effective at producing improvements in M1 plasticity and motor skill learning in older adults. Proposed NIBS techniques for future studies include combining multiple NIBS protocols in a co-stimulation approach, or NIBS strategies to modulate intracortical inhibitory mechanisms, in an effort to more effectively boost M1 plasticity and improve motor skill learning in older adults.

Transcranial Direct Current Stimulation Targeting the Ventromedial Prefrontal Cortex Reduces Reactive Aggression and Modulates Electrophysiological Responses in a Forensic Population

BACKGROUND

Studies have shown that impairments in the ventromedial prefrontal cortex play a crucial role in violent behavior in forensic patients who also abuse cocaine and alcohol. Moreover, interventions that aimed to reduce violence risk in those patients are found not to be optimal. A promising intervention might be to modulate the ventromedial prefrontal cortex by high-definition (HD) transcranial direct current stimulation (tDCS). The current study aimed to examine HD-tDCS as an intervention to increase empathic abilities and reduce violent behavior in forensic substance dependent offenders. In addition, using electroencephalography, we examined the effects on the P3 and the late positive potential of the event-related potentials in reaction to situations that depict victims of aggression.

METHODS

Fifty male forensic patients with a substance dependence were tested in a double-blind, placebo-controlled randomized study. The patients received HD-tDCS 2 times a day for 20 minutes for 5 consecutive days. Before and after the intervention, the patients completed self-reports and performed the Point Subtraction Aggression Paradigm, and electroencephalography was recorded while patients performed an empathy task.

RESULTS

Results showed a decrease in aggressive responses on the Point Subtraction Aggression Paradigm and in self-reported reactive aggression in the active tDCS group. Additionally, we found a general increase in late positive potential amplitude after active tDCS. No effects on trait empathy and the P3 were found.

CONCLUSIONS

Current findings are the first to find positive effects of HD-tDCS in reducing aggression and modulating electrophysiological responses in forensic patients, showing the potential of using tDCS as an intervention to reduce aggression in forensic mental health care.

Repetitive Transcranial Magnetic Stimulation on the Affected Hemisphere Enhances Hand Functional Recovery in Subacute Adult Stroke Patients: A Randomized Trial

Objectives: Either motor training or repetitive transcranial magnetic stimulation (rTMS) could modulate the neural plasticity after stroke. Therefore, synchronizing the two interventions may optimize the efficiency of recovery. In the present study, we aim to investigate the effect of rTMS along with hand grip training on the neurobehavioral and hand functional recovery in one cohort of subacute stroke patients. Methods: Thirty-nine stroke patients were enrolled in a single-center, single-blinded, randomized clinical trial. We tested different intervention effects of rTMS and hand grip training (group A), rTMS alone (group B), and hand grip training alone (group C). For the rTMS-treated groups, patients received 10 consecutive sessions of 5-Hz stimulation over the affected hemisphere with 750 pulses. Jebsen-Taylor Hand Function Test (JTHFT), Fugl-Meyer assessment of upper extremity (FMA-UE), grip strength, modified Barthel index (mBI), and ipsilesional motor evoked potential (iMEP) latency were assessed and compared across the groups. Results: We found that only rTMS along with hand grip training group all improved in JTHFT, FMA-UE, grip strength, and mBI (p ≤ 0.01) compared with the baseline among the three groups. Furthermore, this study demonstrated that rTMS plus hand grip training had much better results in improvement of neurobehavioral outcomes compared to the rTMS alone- and hand grip training alone-treated patients (p < 0.05). However, no significant differences were detected in neurophysiologic outcome between intra-groups and inter-groups (p > 0.05). Conclusion: These proof-of-concept results suggested that rTMS alone with hand grip training was a unique approach to promote hand functional recovery in stroke patients. It provided important information to design a large-scale multi-center clinical trial to further demonstrate the efficiency of the combination of central and peripheral stimulation. Clinical Trial Registration: http://www.chictr.org.cn (#ChiCTR1900023443).

Interactions Between Cerebellum and the Intracortical Excitatory Circuits of Motor Cortex: a Mini-Review

Interactions between cerebellum (CB) and primary motor cortex (M1) are critical for effective motor function. Although the associated neurophysiological processes are yet to be fully characterised, a growing body of work using non-invasive brain stimulation (NIBS) techniques has significantly progressed our current understanding. In particular, recent developments with both transcranial magnetic (TMS) and direct current (tDCS) stimulation suggest that CB modulates the activity of local excitatory interneuronal circuits within M1. These circuits are known to be important both physiologically and functionally, and understanding the nature of their connectivity with CB therefore has the potential to provide important insight for NIBS applications. Consequently, this mini-review provides an overview of the emerging literature that has investigated interactions between CB and the intracortical excitatory circuits of M1.

Assessing and stabilizing atypical plasticity in autism spectrum disorder using rTMS: Results from a proof-of-principle study

OBJECTIVES

Emerging evidence implicates atypical plasticity in the neurophysiology of autism spectrum disorder (ASD). Specifically, autistic people demonstrated hyperplasticity in response to theta-burst stimulation (TBS). We hypothesized that autistic adults would display hyperplasticity to TBS and that repetitive transcranial magnetic stimulation (rTMS) - which potentiates brain inhibitory mechanisms - would 'stabilize' hyperplasticity.

METHODS

Using a randomized, cross-over design, plasticity was assessed using TBS in the left motor cortex (M1) in 31 autistic adults and 30 sex-, intelligence quotient-, and age-matched controls. Autistic adults (n = 29) were further randomized (1:1) to receive a single session of active (n = 14) or sham (n = 15) rTMS (6000 pulses at 20 Hz) over left M1 and plasticity was reassessed on the next day following rTMS.

RESULTS

Both long-term potentiation (LTP) and long-term depression (LTD) were significantly increased in the ASD group, indicating hyperplasticity. Active, but not sham rTMS, attenuated LTD in autistic adults.

CONCLUSIONS

We provided further evidence for the presence of brain hyperplasticity in ASD. To our knowledge, this is the first study to show preliminary evidence that an excessive LTD in ASD can be 'stabilized' using rTMS. Such 'stabilizing' effect of rTMS on LTP was not observed, likely due to small sample size or a more specific 'attenuating' effect of rTMS on LTD, compared to LTP.

SIGNIFICANCE

These findings indicate atypical brain inhibitory mechanisms behind hyperplasticity in ASD. Utilizing a larger sample, future replication studies could investigate therapeutic opportunities of 'mechanism-driven' rTMS.

The intervention, the patient and the illness - Personalizing non-invasive brain stimulation in psychiatry

Current hypotheses on the therapeutic action of non-invasive brain stimulation (NIBS) in psychiatric disorders build on the abundant data from neuroimaging studies. This makes NIBS a very promising tool for developing personalized interventions within a precision medicine framework. NIBS methods fundamentally vary in their neurophysiological properties. They comprise repetitive transcranial magnetic stimulation (rTMS) and its variants (e.g. theta burst stimulation - TBS) as well as different types of transcranial electrical stimulation (tES), with the largest body of evidence for transcranial direct current stimulation (tDCS). In the last two decades, significant conceptual progress has been made in terms of NIBS targets, i.e. from single brain regions to neural circuits and to functional connectivity as well as their states, recently leading to brain state modulating closed-loop approaches. Regarding structural and functional brain anatomy, NIBS meets an individually unique constellation, which varies across normal and pathophysiological states. Thus, individual constitutions and signatures of disorders may be indistinguishable at a given time point, but can theoretically be parsed along course- and treatment-related trajectories. We address precision interventions on three levels: 1) the NIBS intervention, 2) the constitutional factors of a single patient, and 3) the phenotypes and pathophysiology of illness. With examples from research on depressive disorders, we propose solutions and discuss future perspectives, e.g. individual MRI-based electrical field strength as a proxy for NIBS dosage, and also symptoms, their clusters, or biotypes instead of disorder focused NIBS. In conclusion, we propose interleaved research on these three levels along a general track of reverse and forward translation including both clinically directed research in preclinical model systems, and biomarker guided controlled clinical trials. Besides driving the development of safe and efficacious interventions, this framework could also deepen our understanding of psychiatric disorders at their neurophysiological underpinnings.