Consensus: Motor cortex plasticity protocols

Transcranial magnetic stimulation reveals diminished homoeostatic metaplasticity in cognitively impaired adults

Homoeostatic metaplasticity is a neuroprotective physiological feature that counterbalances Hebbian forms of plasticity to prevent network destabilization and hyperexcitability. Recent animal models highlight dysfunctional homoeostatic metaplasticity in the pathogenesis of Alzheimer's disease. However, the association between homoeostatic metaplasticity and cognitive status has not been systematically characterized in either demented or non-demented human populations, and the potential value of homoeostatic metaplasticity as an early biomarker of cognitive impairment has not been explored in humans. Here, we report that, through pre-conditioning the synaptic activity prior to non-invasive brain stimulation, the association between homoeostatic metaplasticity and cognitive status could be established in a population of non-demented human subjects (older adults across cognitive spectrums; all within the non-demented range). All participants (n = 40; age range, 65-74, 47.5% female) underwent a standardized neuropsychological battery, magnetic resonance imaging and a transcranial magnetic stimulation protocol. Specifically, we sampled motor-evoked potentials with an input/output curve immediately before and after repetitive transcranial magnetic stimulation to assess neural plasticity with two experimental paradigms: one with voluntary muscle contraction (i.e. modulated synaptic activity history) to deliberately introduce homoeostatic interference, and one without to serve as a control condition. From comparing neuroplastic responses across these experimental paradigms and across cohorts grouped by cognitive status, we found that (i) homoeostatic metaplasticity is diminished in our cohort of cognitively impaired older adults and (ii) this neuroprotective feature remains intact in cognitively normal participants. This novel finding suggests that (i) future studies should expand their scope beyond just Hebbian forms of plasticity that are traditionally assessed when using non-invasive brain stimulation to investigate cognitive ageing and (ii) the potential value of homoeostatic metaplasticity in serving as a biomarker for cognitive impairment should be further explored.

Impaired LTD-like motor cortical plasticity in female patients with major depression disorder

BACKGROUNDS

Long-term depression (LTD) is a form of physiologic plasticity that is important for reversal learning and may be linked to major depression. Few studies have examined LTP-like plasticity in patients with depression. It is unclear if continuous theta burst stimulation (cTBS) induced LTD is altered in depression patients.

METHODS

Here we recruited 29 healthy control subjects and 31 female patients with depression. We performed cTBS protocol on left motor cortex and employed motor evoked potentials (MEPs) response to measure LTD-like plasticity. Peripheral molecules were measured for correlation analyses to cortical plasticity.

RESULTS

Our results revealed persistent LTD-like plasticity deficits in female patients with depression. LTD-like plasticity was impaired in patients with depression despite the fact that peripheral concentrations of BDNF were comparable to that of healthy subjects.

CONCLUSIONS

Our findings provide evidence for impaired LTD-like plasticity in patients with depression which may be an important mechanism linked to the pathophysiology and treatment of this disorder.

Device-Based Modulation of Neurocircuits as a Therapeutic for Psychiatric Disorders

Device-based neuromodulation of brain circuits is emerging as a promising new approach in the study and treatment of psychiatric disorders. This work presents recent advances in the development of tools for identifying neurocircuits as therapeutic targets and in tools for modulating neurocircuits. We review clinical evidence for the therapeutic efficacy of circuit modulation with a range of brain stimulation approaches, including subthreshold, subconvulsive, convulsive, and neurosurgical techniques. We further discuss strategies for enhancing the precision and efficacy of neuromodulatory techniques. Finally, we survey cutting-edge research in therapeutic circuit modulation using novel paradigms and next-generation devices.

The Immediate Effects of Transcranial Direct Current Stimulation on Quadriceps Muscle Function in Individuals With a History of Anterior Cruciate Ligament Reconstruction: A Preliminary Investigation

CONTEXT

Altered quadriceps activation is common following anterior cruciate ligament reconstruction (ACLR), and can persist for years after surgery. These neural deficits are due, in part, to chronic central nervous system alterations. Transcranial direct current stimulation (tDCS) is a noninvasive modality, that is, believed to immediately increase motor neuron activity by stimulating the primary motor cortex, making it a promising modality to use improve outcomes in the ACLR population.

OBJECTIVE

To determine if a single treatment of tDCS would result in increased quadriceps activity and decreased levels of self-reported pain and dysfunction during exercise.

DESIGN

Randomized crossover design.

SETTING

Controlled laboratory.

PATIENTS

Ten participants with a history of ACLR (5 males/5 females, 22.9 [4.23] y, 176.57 [12.01] cm, 80.87 [16.86] kg, 68.1 [39.37] mo since ACLR).

INTERVENTIONS

Active tDCS and Sham tDCS.

MAIN OUTCOME MEASURES

Percentage of maximum electromyographic data of vastus medialis and lateralis, voluntary isometric strength, percentage of voluntary activation, and self-reported pain and symptom scores were measured. The 2 × 2 repeated-measures analysis of variance by limb were performed to explain the differences between time points (pre and post) and condition (tDCS and sham).

RESULTS

There was a significant time main effect for quadriceps percentage of maximum electromyographic of vastus medialis (F9,1 = 11.931, P = .01) and vastus lateralis (F9,1 = 9.132, P = .01), isometric strength (F9,1 = 5.343, P = .046), and subjective scores for pain (F9,1 = 15.499, P = .04) and symptoms (F9,1 = 15.499, P = .04). Quadriceps percentage of maximum electromyographic, isometric strength, and voluntary activation showed an immediate decline from pre to post regardless of tDCS condition. Subjective scores improved slightly after each condition.

CONCLUSIONS

One session of active tDCS did not have an immediate effect on quadriceps activity and subjective scores of pain and symptoms. To determine if tDCS is a valid modality for this patient population, a larger scale investigation with multiple treatments of active tDCS is warranted.

Efficacy of non-invasive brain stimulation on cognitive functioning in brain disorders: a meta-analysis

BACKGROUND

Cognition is commonly affected in brain disorders. Non-invasive brain stimulation (NIBS) may have procognitive effects, with high tolerability. This meta-analysis evaluates the efficacy of transcranial magnetic stimulation (TMS) and transcranial Direct Current Stimulation (tDCS) in improving cognition, in schizophrenia, depression, dementia, Parkinson's disease, stroke, traumatic brain injury, and multiple sclerosis.

METHODS

A PRISMA systematic search was conducted for randomized controlled trials. Hedges' g was used to quantify effect sizes (ES) for changes in cognition after TMS/tDCS v. sham. As different cognitive functions may have unequal susceptibility to TMS/tDCS, we separately evaluated the effects on: attention/vigilance, working memory, executive functioning, processing speed, verbal fluency, verbal learning, and social cognition.

RESULTS

We included 82 studies (n = 2784). For working memory, both TMS (ES = 0.17, p = 0.015) and tDCS (ES = 0.17, p = 0.021) showed small but significant effects. Age positively moderated the effect of TMS. TDCS was superior to sham for attention/vigilance (ES = 0.20, p = 0.020). These significant effects did not differ across the type of brain disorder. Results were not significant for the other five cognitive domains.

CONCLUSIONS

Our results revealed that both TMS and tDCS elicit a small trans-diagnostic effect on working memory, tDCS also improved attention/vigilance across diagnoses. Effects on the other domains were not significant. Observed ES were small, yet even slight cognitive improvements may facilitate daily functioning. While NIBS can be a well-tolerated treatment, its effects appear domain specific and should be applied only for realistic indications (i.e. to induce a small improvement in working memory or attention).

Convergent Associative Motor Cortical Plasticity Induced by Conditional Somatosensory and Motor Reaction Afferents

Objective: Associative motor cortical plasticity can be non-invasively induced by paired median nerve electric stimulation and transcranial magnetic stimulation (TMS) of the primary motor cortex (M1). This study investigates whether a simultaneous motor reaction of the other hand advances the associative plasticity in M1.

Methods: Twenty-four right-handed subjects received conventional paired associative stimulation (PAS) and PAS with simultaneous motor reaction (PASmr) with at least a 1-week interval. The PASmr protocol additionally included left abductor pollicis brevis muscle movement responding to a digital sound. The motor reaction time was individually measured. The M1 excitability was examined by the motor evoked potential (MEP), short-interval intracortical inhibition (SICI), and intracortical facilitation (ICF) before and after the PAS protocols.

Results: The conventional PAS protocol significantly facilitated MEP and suppressed SICI. A negative correlation between the reaction time and the MEP change, and a positive correlation between the reaction time and the ICF change were found in the PASmr protocol. By subgrouping analysis, we further found significant facilitation of MEP and a reduction of ICF in the subjects with fast reaction times but not in those with slow reaction times.

Conclusion: Synchronized motor reaction ipsilateral to the stimulated M1 induces associative M1 motor plasticity through the spike-timing dependent principle. MEP and ICF change could represent this kind of plasticity. The current findings provide a novel insight into designing rehabilitation programs concerning motor function.

Probing rapid network reorganization of motor and language functions via neuromodulation and neuroimaging

Motor and cognitive functions are organized in large-scale networks in the human brain that interact to enable flexible adaptation of information exchange to ever-changing environmental conditions. In this review, we discuss the unique potential of the consecutive combination of repetitive transcranial magnetic stimulation (rTMS) and functional neuroimaging to probe network organization and reorganization in the healthy and lesioned brain. First, we summarize findings highlighting the flexible (re-)distribution and short-term reorganization in motor and cognitive networks in the healthy brain. Plastic after-effects of rTMS result in large-scale changes on the network level affecting both local and remote activity within the stimulated network as well as interactions between the stimulated and distinct functional networks. While the number of combined rTMS-fMRI studies in patients with brain lesions remains scarce, preliminary evidence suggests that the lesioned brain flexibly (re-)distributes its computational capacities to functionally reorganize impaired brain functions, using a similar set of mechanisms to achieve adaptive network plasticity compared to short-term reorganization observed in the healthy brain after rTMS. In general, both short-term reorganization in the healthy brain and stroke-induced reorganization seem to rely on three general mechanisms of adaptive network plasticity that allow to maintain and recover function: i) interhemispheric changes, including increased contribution of homologous regions in the contralateral hemisphere and increased interhemispheric connectivity, ii) increased interactions between differentially specialized networks and iii) increased contributions of domain-general networks after disruption of more specific functions. These mechanisms may allow for computational flexibility of large-scale neural networks underlying motor and cognitive functions. Future studies should use complementary approaches to address the functional relevance of adaptive network plasticity and further delineate how these general mechanisms interact to enable network flexibility. Besides furthering our neurophysiological insights into brain network interactions, identifying approaches to support and enhance adaptive network plasticity may result in clinically relevant diagnostic and treatment approaches.

Brain stimulation in zero gravity: transcranial magnetic stimulation (TMS) motor threshold decreases during zero gravity induced by parabolic flight

We are just beginning to understand how spaceflight may impact brain function. As NASA proceeds with plans to send astronauts to the Moon and commercial space travel interest increases, it is critical to understand how the human brain and peripheral nervous system respond to zero gravity. Here, we developed and refined head-worn transcranial magnetic stimulation (TMS) systems capable of reliably and quickly determining the amount of electromagnetism each individual needs to detect electromyographic (EMG) threshold levels in the thumb (called the resting motor threshold (rMT)). We then collected rMTs in 10 healthy adult participants in the laboratory at baseline, and subsequently at three time points onboard an airplane: (T1) pre-flight at Earth gravity, (T2) during zero gravity periods induced by parabolic flight and (T3) post-flight at Earth gravity. Overall, the subjects required 12.6% less electromagnetism applied to the brain to cause thumb muscle activation during weightlessness compared to Earth gravity, suggesting neurophysiological changes occur during brief periods of zero gravity. We discuss several candidate explanations for this finding, including upward shift of the brain within the skull, acute increases in cortical excitability, changes in intracranial pressure, and diffuse spinal or neuromuscular system effects. All of these possible explanations warrant further study. In summary, we documented neurophysiological changes during brief episodes of zero gravity and thus highlighting the need for further studies of human brain function in altered gravity conditions to optimally prepare for prolonged microgravity exposure during spaceflight.

Induction of LTD-like corticospinal plasticity by low-frequency rTMS depends on pre-stimulus phase of sensorimotor μ-rhythm

Background

Neural oscillations reflect rapidly changing brain excitability states. We have demonstrated previously with EEG-triggered transcranial magnetic stimulation (TMS) of human motor cortex that the positive vs. negative peak of the sensorimotor μ-oscillation reflect corticospinal low-vs. high-excitability states. In vitro experiments showed that induction of long-term depression (LTD) by low-frequency stimulation depends on the postsynaptic excitability state.

Objective/Hypothesis: We tested the hypothesis that induction of LTD-like corticospinal plasticity in humans by 1 Hz repetitive TMS (rTMS) is enhanced when rTMS is synchronized with the low-excitability state, but decreased or even shifted towards long-term (LTP)-like plasticity when synchronized with the high-excitability state.

Methods

We applied real-time EEG-triggered 1-Hz-rTMS (900 pulses) to the hand area of motor cortex in healthy subjects. In a randomized double-blind three-condition crossover design, pulses were synchronized to either the positive or negative peak of the sensorimotor μ-oscillation, or were applied at random phase (control). The amplitude of motor evoked potentials was recorded as an index of corticospinal excitability before and after 1-Hz-rTMS.

Results

1-Hz-rTMS at random phase resulted in a trend towards LTD-like corticospinal plasticity. RTMS in the positive peak condition (i.e., the low-excitability state) induced significant LTD-like plasticity. RTMS in the negative peak condition (i.e., the high-excitability state) showed a trend towards LTP-like plasticity, which was significantly different from the other two conditions.

Conclusion

The level of corticospinal depolarization reflected by phase of the μ-oscillation determines the degree of corticospinal plasticity induced by low-frequency rTMS, a finding that may guide future personalized therapeutic stimulation.

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Positive vs. negative phase of μ-rhythm are states of low vs. high excitability.

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1-Hz-rTMS coupled to positive but not negative phase results in LTD-like plasticity.

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Phase of μ-rhythm determines effect size of 1-Hz-rTMS induced plasticity.

Ca2+ channel dynamics explain the nonlinear neuroplasticity induction by cathodal transcranial direct current stimulation over the primary motor cortex

Transcranial direct current stimulation (tDCS) induces polarity-dependent neuroplasticity: with conventional protocols, anodal tDCS results in excitability enhancement while cathodal stimulation reduces excitability. However, partially non-linear responses are observed with increased stimulation intensity and/or duration. Cathodal tDCS with 2 mA for 20 min reverses the excitability-diminishing plasticity induced by stimulation with 1 mA into excitation, while cathodal tDCS with 3 mA again results in excitability diminution. Since tDCS generates NMDA receptor-dependent neuroplasticity, such non-linearity could be explained by different levels of calcium concentration changes, which have been demonstrated in animal models to control for the directionality of plasticity. In this study, we tested the calcium dependency of non-linear cortical plasticity induced by cathodal tDCS in human subjects in a placebo controlled, double-blind and randomized design. The calcium channel blocker flunarizine was applied in low (2.5 mg), medium (5 mg) or high (10 mg) dosages before 20 min cathodal motor cortex tDCS with 3 mA in 12 young healthy subjects. After-effects of stimulation were monitored with TMS-induced motor evoked potentials (MEPs) until 2 h after stimulation. The results show that motor cortical excitability-diminishing after-effects of stimulation were unchanged, diminished, or converted to excitability enhancement with low, medium and high dosages of flunarizine. These results suggest a calcium-dependency of the directionality of tDCS-induced neuroplasticity, which may have relevant implications for future basic and clinical research.

Low-intensity repetitive paired associative stimulation targeting the motor hand area at theta frequency causes a lasting reduction in corticospinal excitability

OBJECTIVE

Sub-motor threshold 5 Hz repetitive paired associative stimulation (5 Hz-rPAS_25ms) produces a long-lasting increase in corticospinal excitability. Assuming a spike-timing dependent plasticity-like (STDP-like) mechanism, we hypothesized that 5 Hz-rPAS at a shorter inter-stimulus interval (ISI) of 15 ms (5 Hz-rPAS_15ms) would exert a lasting inhibitory effect on corticospinal excitability.

METHODS

20 healthy volunteers received two minutes of 5 Hz-rPAS_15ms. Transcranial magnetic stimulation (TMS) was applied over the motor hotspot of the right abductor pollicis brevis muscle at 90% active motor threshold. Sub-motor threshold peripheral electrical stimulation was given to the left median nerve 15 ms before each TMS pulse. We assessed changes in mean amplitude of the unconditioned motor evoked potential (MEP), short-latency intracortical inhibition (SICI), intracortical facilitation (ICF), short-latency afferent inhibition (SAI), long-latency afferent inhibition (LAI), and cortical silent period (CSP) before and for 60 minutes after 5-Hz rPAS_15ms.

RESULTS

Subthreshold 5-Hz rPAS_15ms produced a 20-40% decrease in mean MEP amplitude along with an attenuation in SAI, lasting at least 60 minutes. A follow-up experiment revealed that MEP facilitation was spatially restricted to the target muscle.

CONCLUSIONS

Subthreshold 5-Hz rPAS_15ms effectively suppresses corticospinal excitability. Together with the facilitatory effects of subthreshold 5-Hz rPAS_25ms (Quartarone et al., J Physiol 2006;575:657-670), the results show that sub-motor threshold 5-Hz rPAS induces STDP-like bidirectional plasticity in the motor cortex.

SIGNIFICANCE

The results of the present study provide a new short-time paradigm of long term depression (LTD) induction in human sensory-motor cortex.

The integrated model of glutamate and dopamine hypothesis for schizophrenia: Prediction and personalized medicine for prevent potential treatment-resistant patients

Treatment-resistant schizophrenia (TRS) is one of the subgroups of schizophrenia of which little is known with regard to its optimal mechanism. Treatment response, either as full remission of symptoms or prediction by biomarker, is important in psychiatry. We have proposed a model that integrates dopaminergic and glutamatergic systems with the biological interactions of TRS patients. We hypothesize that the subgroups of schizophrenia may be determined by glutamatergic and dopaminergic concentrations prior to medical treatment. This hypothesis implies that higher glutamatergic concentration in the brain with normalized or decreased dopamine synthesis capacity may explain aspects of TRS as observed in clinical medical practice, neuroimaging measurements, and brain stimulations. According to this hypothesis, the ability to prescribe a proper medication combination, to predict the outcome in first-episode psychosis, and personalized medicine for chronic schizophrenia patients can be applied into practice. This represents an initial step in explaining psychosis due to the valence of two neurotransmitters. Future studies are needed to examine the validity of this mechanism.

The Effects of 1 mA tACS and tRNS on Children/Adolescents and Adults: Investigating Age and Sensitivity to Sham Stimulation

The aim of this study was to investigate the effect of transcranial random noise (tRNS) and transcranial alternating current (tACS) stimulation on motor cortex excitability in healthy children and adolescents. Additionally, based on our recent results on the individual response to sham in adults, we explored this effect in the pediatric population. We included 15 children and adolescents (10-16 years) and 28 adults (20-30 years). Participants were stimulated four times with 20 Hz and 140 Hz tACS, tRNS, and sham stimulation (1 mA) for 10 minutes over the left M1_HAND. Single-pulse MEPs (motor evoked potential), short-interval intracortical inhibition, and facilitation were measured by TMS before and after stimulation (baseline, 0, 30, 60 minutes). We also investigated aspects of tolerability. According to the individual MEPs response immediately after sham stimulation compared to baseline (Wilcoxon signed-rank test), subjects were regarded as responders or nonresponders to sham. We did not find a significant age effect. Regardless of age, 140 Hz tACS led to increased excitability. Incidence and intensity of side effects did not differ between age groups or type of stimulation. Analyses on responders and nonresponders to sham stimulation showed effects of 140 Hz, 20 Hz tACS, and tRNS on single-pulse MEPs only for nonresponders. In this study, children and adolescents responded to 1 mA tRNS and tACS comparably to adults regarding the modulation of motor cortex excitability. This study contributes to the findings that noninvasive brain stimulation is well tolerated in children and adolescents including tACS, which has not been studied before. Finally, our study supports a modulating role of sensitivity to sham stimulation on responsiveness to a broader stimulation and age range.

Theta burst stimulation in humans: a need for better understanding effects of brain stimulation in health and disease

Repetitive transcranial stimulation (rTMS) paradigms have been used to induce lasting changes in brain activity and excitability. Previous methods of stimulation were long, often ineffective and produced short-lived and variable results. A new non-invasive brain stimulation technique was developed in John Rothwell's laboratory in the early 2000s, which was named 'theta burst stimulation' (TBS). This used rTMS applied in burst patterns of newly acquired 50 Hz rTMS machines, which emulated long-term potentiation/depression-like effects in brain slices. This stimulation paradigm created long-lasting changes in brain excitability, using efficient, very rapid stimulation, which would affect behaviour, with the aim to influence neurological diseases in humans. We describe the development of this technique, including findings and limitations identified since then. We discuss how pitfalls facing TBS reflect those involving both older and newer, non-invasive stimulation techniques, with suggestions of how to overcome these, using personalised, 'closed loop' stimulation methods. The challenge in most non-invasive stimulation techniques remains in identifying their exact mechanisms of action in the context of neurological disease models. The development of TBS provides the backdrop for describing John's contribution to the field, inspiring our own scientific endeavour thanks to his unconditional support, and unfailing kindness.

Altered response to repetitive transcranial magnetic stimulation in patients with chronic primary insomnia

BACKGROUND

We aimed at evaluating the amplitude changes of the motor evoked potentials (MEPs) induced by of low-frequency (LF) repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) in10 patients with primary insomnia (PI) and in 10 age-matched healthy controls.

METHODS

Median peak-to-peak MEP amplitudes were assessed in all subjects at three times: at baseline (T₀), after the first train of a single rTMS session (T₁), and after the whole rTMS procedure (T₂). This consists of 20 trains of 1 Hz stimulation with 50 stimuli per train and an intertrain interval of 30 s.

RESULTS

Resting motor threshold (RMT) and MEPs amplitude did not differ between the two groups at T₀. A reduction of MEP size was observed at both T₁ and T₂ in all subjects, but this was significantly less pronounced in patients than in control subjects.

CONCLUSIONS

The lack of MEP inhibition reflects an altered response to LF rTMS in patients with PI. These rTMS findings are indicative of an altered cortical plasticity in inhibitory circuits within M1 in PI. Subjects with PI exhibited an impairment of the LTD-like mechanisms induced by inhibitory rTMS, thus providing further support to the involvement of GABA neurotransmission in the pathophysiology of PI.

Contribution of transcranial magnetic stimulation in restless legs syndrome: pathophysiological insights and therapeutical approaches

Transcranial magnetic stimulation (TMS) may offer a reliable means to characterize significant pathophysiologic and neurochemical aspects of restless legs syndrome (RLS). Namely, TMS has revealed specific patterns of changes in cortical excitability and plasticity, in particular dysfunctional inhibitory mechanisms and sensorimotor integration, which are thought to be part of the pathophysiological mechanisms of RLS rather than reflect a non-specific consequence of sleep architecture alteration. If delivered repetitively, TMS is able to transiently modulate the neural activity of the stimulated and connected areas. Some studies have begun to therapeutically use repetitive TMS (rTMS) to improve sensory and motor disturbances in RLS. High-frequency rTMS applied over the primary motor cortex or the supplementary motor cortex, as well as low-frequency rTMS over the primary somatosensory cortex, seem to have transient beneficial effects. However, further studies with larger patient samples, repeated sessions, an optimized rTMS setup, and clinical follow-up are needed in order to corroborate preliminary results. Thus, we performed a systematic search of all the studies that have used TMS and rTMS techniques in patients with RLS.

Non-invasive High Frequency Repetitive Transcranial Magnetic Stimulation (hfrTMS) Robustly Activates Molecular Pathways Implicated in Neuronal Growth and Synaptic Plasticity in Select Populations of Neurons

Patterns of neuronal activity that induce synaptic plasticity and memory storage activate kinase cascades in neurons that are thought to be part of the mechanism for synaptic modification. One such cascade involves induction of phosphorylation of ribosomal protein S6 in neurons due to synaptic activation of AKT/mTOR and via a different pathway, activation of MAP kinase/ERK1/2. Here, we show that phosphorylation of ribosomal protein S6 can also be strongly activated by high frequency repetitive transcranial magnetic stimulation (hfrTMS). HfrTMS was delivered to lightly anesthetized rats using a stimulation protocol that is a standard for inducing LTP in the perforant path in vivo (trains of 8 pulses at 400 Hz repeated at intervals of 1/10 s). Stimulation produced stimulus-locked motor responses but did not elicit behavioral seizures either during or after stimulation. After as little as 10 min of hfrTMS, immunostaining using phospho-specific antibodies for the phosphorylated form of ribosomal protein S6 (rpS6) revealed robust induction of rpS6 phosphorylation in large numbers of neurons in the cortex, especially the piriform cortex, and also in thalamic relay nuclei. Quantification revealed that the extent of the increased immunostaining depended on the number of trains and stimulus intensity. Of note, immunostaining for the immediate early genes Arc and c-fos revealed strong induction of IEG expression in many of the same populations of neurons throughout the cortex, but not the thalamus. These results indicate that hfrTMS can robustly activate molecular pathways critical for plasticity, which may contribute to the beneficial effects of TMS on recovery following brain and spinal cord injury and symptom amelioration in human psychiatric disorders. These molecular processes may be a useful surrogate marker to allow optimization of TMS parameters for maximal therapeutic benefit.

Inferring Causality from Noninvasive Brain Stimulation in Cognitive Neuroscience

Noninvasive brain stimulation (NIBS) techniques, such as transcranial magnetic stimulation or transcranial direct and alternating current stimulation, are advocated as measures to enable causal inference in cognitive neuroscience experiments. Transcending the limitations of purely correlative neuroimaging measures and experimental sensory stimulation, they allow to experimentally manipulate brain activity and study its consequences for perception, cognition, and eventually, behavior. Although this is true in principle, particular caution is advised when interpreting brain stimulation experiments in a causal manner. Research hypotheses are often oversimplified, disregarding the underlying (implicitly assumed) complex chain of causation, namely, that the stimulation technique has to generate an electric field in the brain tissue, which then evokes or modulates neuronal activity both locally in the target region and in connected remote sites of the network, which in consequence affects the cognitive function of interest and eventually results in a change of the behavioral measure. Importantly, every link in this causal chain of effects can be confounded by several factors that have to be experimentally eliminated or controlled to attribute the observed results to their assumed cause. This is complicated by the fact that many of the mediating and confounding variables are not directly observable and dose-response relationships are often nonlinear. We will walk the reader through the chain of causation for a generic cognitive neuroscience NIBS study, discuss possible confounds, and advise appropriate control conditions. If crucial assumptions are explicitly tested (where possible) and confounds are experimentally well controlled, NIBS can indeed reveal cause-effect relationships in cognitive neuroscience studies.

Focal transcranial magnetic stimulation in awake rats: Enhanced glucose uptake in deep cortical layers

BACKGROUND

Transcranial magnetic stimulation (TMS) is an emerging neuromodulation tool. However, preclinical models of TMS are limited.

OBJECTIVE

To develop a method for performing TMS in awake rats and to characterize neuronal response to TMS by mapping glucose uptake following TMS administration.

METHODS

A headpost was implanted into rat skull serving as a refence to guide TMS target. Motor threshold measurement was used as the metric to assess the consistency in TMS delivery across animals and across sessions. Using a fluorescent glucose analogue (2-NBDG) as a marker of neuronal activity, we mapped glucose uptake in response to TMS of the rat motor cortex.

RESULTS

The average motor threshold (n = 41) was 34.6 ± 6.3 % of maximum stimulator output (MSO). The variability of motor threshold across animals was similar to what has been reported in human studies. Furthermore, there was no significant difference in motor threshold measured across 3 separate days. Enhancement in fluorescent signals were TMS dose (power)-dependent, which centered around the motor cortex, covering an area medial-laterally 2 mm, rostral-caudally 4 mm at 55 % MSO, and 3 mm at 35 % MSO. The count of total cells with significant fluorescent signal was: 107 ± 23 (55 % MSO), 73 ± 11 (35 % MSO) and 42 ± 11 (sham, 5% MSO).

CONCLUSIONS

Our method allows for consistent motor threshold assessment for longitudinal studies. Notably, cells with fluorescent signal enhancement were consistently aggregated in deep cortical layers, with minimal enhancement in superficial layers COMPARISONS WITH EXISTING METHOD(S): To our knowledge, this is the first study of focal TMS in awake rodents.

Establishing a Causal Role for Medial Prefrontal Cortex in Reality Monitoring

Reality monitoring is defined as the ability to distinguish internally self-generated information from externally-derived information. Functional imaging studies have consistently found that the medial prefrontal cortex (mPFC) is a key brain region subserving reality monitoring. This study aimed to determine a causal role for mPFC in reality monitoring using navigated repetitive transcranial magnetic stimulation (nrTMS). In a subject-blinded sham-controlled crossover design, healthy individuals received either active or sham nrTMS targeting mPFC. Active modulation of mPFC using nrTMS at a frequency of 10 Hz, significantly improved identification of both self-generated and externally-derived information during reality monitoring, when compared to sham or baseline. Targeted excitatory modulation of mPFC also improved positive mood, reduced negative mood, and increased overall alertness/arousal. These results establish optimal nrTMS dosing parameters that maximized tolerability/comfort and induced significant neuromodulatory effects in the mPFC target. Importantly, this is a proof-of-concept study that establishes the mPFC as a novel brain target that can be stimulated with nrTMS to causally impact both higher-order reality monitoring and mood.

Evolving concepts on bradykinesia

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease and other parkinsonisms. The various clinical aspects related to bradykinesia and the pathophysiological mechanisms underlying bradykinesia are, however, still unclear. In this article, we review clinical and experimental studies on bradykinesia performed in patients with Parkinson's disease and atypical parkinsonism. We also review studies on animal experiments dealing with pathophysiological aspects of the parkinsonian state. In Parkinson's disease, bradykinesia is characterized by slowness, the reduced amplitude of movement, and sequence effect. These features are also present in atypical parkinsonisms, but the sequence effect is not common. Levodopa therapy improves bradykinesia, but treatment variably affects the bradykinesia features and does not significantly modify the sequence effect. Findings from animal and patients demonstrate the role of the basal ganglia and other interconnected structures, such as the primary motor cortex and cerebellum, as well as the contribution of abnormal sensorimotor processing. Bradykinesia should be interpreted as arising from network dysfunction. A better understanding of bradykinesia pathophysiology will serve as the new starting point for clinical and experimental purposes.

Trends of Repetitive Transcranial Magnetic Stimulation From 2009 to 2018: A Bibliometric Analysis

Repetitive transcranial magnetic stimulation (rTMS) technology, which is amongst the most used non-invasive brain stimulation techniques currently available, has developed rapidly from 2009 to 2018. However, reports on the trends of rTMS using bibliometric analysis are rare. The goal of the present bibliometric analysis is to analyze and visualize the trends of rTMS, including general (publication patterns) and emerging trends (research frontiers), over the last 10 years by using the visual analytic tool CiteSpace V. Publications related to rTMS from 2009 to 2018 were retrieved from the Web of Science (WoS) database, including 2,986 peer-reviewed articles/reviews. Active authors, journals, institutions, and countries were identified by WoS and visualized by CiteSpace V, which could also detect burst changes to identify emerging trends. GraphPad Prism 8 was used to analyze the time trend of annual publication outputs. The USA ranked first in this field. Pascual-Leone A (author A), Fitzgerald PB (author B), George MS (author C), Lefaucheur JP (author D), and Fregni F (author E) made great contributions to this field of study. The most prolific institution to publish rTMS-related publications in the last decade was the University of Toronto. The journal Brain Stimulation published most papers. Lefaucheur et al.'s paper in 2014, and the keyword "sham controlled trial" showed the strongest citation bursts by the end of 2018, which indicates increased attention to the underlying work, thereby indicating the research frontiers. This study reveals the publication patterns and emerging trends of rTMS based on the records published from 2009 to 2018. The insights obtained have reference values for the future research and application of rTMS.

Probing EEG activity in the targeted cortex after focal transcranial electrical stimulation

BACKGROUND

Recording electroencephalography (EEG) from the targeted cortex immediately before and after focal transcranial electrical stimulation (TES) remains a challenge.

METHODS

We introduce a hybrid stimulation-recording approach where a single EEG electrode is inserted into the inner electrode of a double-ring montage for focal TES. The new combined electrode was placed at the C3 position of the EEG 10-20 system. Neuronal activity was recorded in two volunteers before and after 20 Hz alternating-current TES at peak-to-peak intensities of 1 and 2 mA. TES-induced electric field distributions were simulated with SIMNIBS software.

RESULTS

Using the hybrid stimulation-recording set-up, EEG activity was successfully recorded directly before and after TES. Simulations revealed comparable electrical fields in the stimulated cortex for the pseudomonopolar montage with and without embedded EEG electrode.

CONCLUSION

The hybrid TES-EEG approach can be used to probe after-effects of focal TES on neuronal activity in the targeted cortex.

Theta Burst Transcranial Magnetic Stimulation of Fronto-Parietal Networks: Modulation by Mental State

Transcranial magnetic stimulation (TMS) treats neuropsychiatric disorders, but effects of stimulation are highly state-dependent and in most therapeutic applications, mental state is not controlled. This exploratory proposal will test the broad hypothesis that when TMS, specifically intermittent theta burst stimulation (iTBS), is applied during a controlled mental state, network changes will be facilitated, compared to stimulation when mental state is uncontrolled. We will focus on the dorsolateral prefrontal cortex (dlPFC) and the associated fronto-parietal network (FPN), which subserves cognitive control, an important neural and behavioral target of therapeutic TMS. After a baseline functional magnetic resonance imaging (fMRI) session, iTBS will be administered to 40 healthy subjects in three sessions over three days in a within-subjects, cross-over design: (1) dlPFC stimulation by iTBS alone, (2) dlPFC stimulation by iTBS while simultaneously performing a cognitive task, and (3) vertex (control) iTBS stimulation. Immediately after each iTBS session, we will measure blood oxygenation level-dependent (BOLD) activation during a cognitive control task (“n-back” task) and during the resting state, using BOLD connectivity and arterial spin labeling (ASL). We will test hypotheses that persisting neural changes and performance enhancement induced by iTBS to the dlPFC, compared to iTBS to the vertex, will affect the FPN, and these effects will be modulated by whether or not subjects receive iTBS when they are engaged in a cognitive control task. Demonstrating this interaction between iTBS and mental state will lay critical groundwork for future studies to show how controlling mental state during TMS can improve therapeutic effects.

Current intensity- and polarity-specific online and aftereffects of transcranial direct current stimulation: An fMRI study

Transcranial direct current stimulation (tDCS) induces polarity‐ and dose‐dependent neuroplastic aftereffects on cortical excitability and cortical activity, as demonstrated by transcranial magnetic stimulation (TMS) and functional imaging (fMRI) studies. However, lacking systematic comparative studies between stimulation‐induced changes in cortical excitability obtained from TMS, and cortical neurovascular activity obtained from fMRI, prevent the extrapolation of respective physiological and mechanistic bases. We investigated polarity‐ and intensity‐dependent effects of tDCS on cerebral blood flow (CBF) using resting‐state arterial spin labeling (ASL‐MRI), and compared the respective changes to TMS‐induced cortical excitability (amplitudes of motor evoked potentials, MEP) in separate sessions within the same subjects (n = 29). Fifteen minutes of sham, 0.5, 1.0, 1.5, and 2.0‐mA anodal or cathodal tDCS was applied over the left primary motor cortex (M1) in a randomized repeated‐measure design. Time‐course changes were measured before, during and intermittently up to 120‐min after stimulation. ROI analyses indicated linear intensity‐ and polarity‐dependent tDCS after‐effects: all anodal‐M1 intensities increased CBF under the M1 electrode, with 2.0‐mA increasing CBF the greatest (15.3%) compared to sham, while all cathodal‐M1 intensities decreased left M1 CBF from baseline, with 2.0‐mA decreasing the greatest (−9.3%) from sham after 120‐min. The spatial distribution of perfusion changes correlated with the predicted electric field, as simulated with finite element modeling. Moreover, tDCS‐induced excitability changes correlated more strongly with perfusion changes in the left sensorimotor region compared to the targeted hand‐knob region. Our findings reveal lasting tDCS‐induced alterations in cerebral perfusion, which are dose‐dependent with tDCS parameters, but only partially account for excitability changes.

Virtual Reality and Noninvasive Brain Stimulation in Stroke: How Effective Is Their Combination for Upper Limb Motor Improvement?-A Meta-Analysis

BACKGROUND

Efforts to augment post-stroke upper limb (UL) motor improvement include the use of newer interventions such as noninvasive brain stimulation (NIBS) and task practice in virtual reality environments (VEs). Despite increasing interest in using a combination of these 2 interventions, the effectiveness of this combination to enhance UL motor improvement outcomes has not been examined.

OBJECTIVE

To evaluate the effectiveness of a combination of NIBS and task practice in a VE to augment post-stroke UL motor improvement.

METHODS

We conducted a systematic search of the published literature using standard methodology. The Down and Black checklist and the Physiotherapy Evidence Database Research Organization Scale were used to assess study quality. We compared changes in UL impairment and activity levels between active stimulation and sham or other interventions using standardized mean differences and derived a summary effect size.

RESULTS

We retrieved 5 studies that examined the role of a combination of NIBS and task practice in a VE to optimize UL motor improvement. These 5 studies included 3 randomized controlled trials, 1 cross-sectional study, and 1 crossover study. There was level 1a evidence that the combination was beneficial in subacute stroke. There was level 1b evidence that provision of real stimulation was not superior to sham stimulation in chronic stroke. Effect sizes favoring the combination were moderate for improvements in UL impairment and small for activity levels.

CONCLUSIONS

Preliminary evidence supports the effectiveness of this combination in subacute stroke. Emergent questions need to be addressed to derive maximum benefit of this combination to augment post-stroke UL motor improvement.

LEVEL OF EVIDENCE

I.

Activity in the dorsal ACC causes deterioration of sequential motor performance due to anxiety

Performance anxiety can profoundly affect motor performance, even in experts such as professional athletes and musicians. Previously, the neural mechanisms underlying anxiety-induced performance deterioration have predominantly been investigated for individual one-shot actions. Sports and music, however, are characterized by action sequences, where many individual actions are assembled to develop a performance. Here, utilizing a novel differential sequential motor learning paradigm, we first show that performance at the junctions between pre-learnt action sequences is particularly prone to anxiety. Next, utilizing functional magnetic resonance imaging (fMRI), we reveal that performance deterioration at the junctions is parametrically correlated with activity in the dorsal anterior cingulate cortex (dACC). Finally, we show that 1 Hz repetitive transcranial magnetic stimulation of the dACC attenuates the performance deterioration at the junctions. These results demonstrate causality between dACC activity and impairment of sequential motor performance due to anxiety, and suggest new intervention techniques against the deterioration.

Efficacy of tRNS and 140 Hz tACS on motor cortex excitability seemingly dependent on sensitivity to sham stimulation

This study investigates the effect of corticospinal excitability during sham stimulation on the individual response to transcranial non-invasive brain stimulation (tNIBS). Thirty healthy young adults aged 24.2 ± 2.8 S.D. participated in the study. Sham, as well as 1 mA of tRNS and 140 Hz tACS stimulation were applied for 10 min each at different sessions. The effect of each stimulation type was quantified by recording TMS-induced, motor evoked potentials (MEPs) before (baseline) and at fixed time points after stimulation (T0, T30, T60 min.). According to the individual response to sham stimulation at T0 in comparison to baseline MEPs, subjects were regarded as responder or non-responder to sham. Following, MEPs at T0, T30 and T60 after verum or sham stimulation were assessed with a repeated measures ANOVA with the within-subject factor stimulation (sham, tRNS, 140 Hz tACS) and the between-subjects factor group (responder vs non-responder). We found that individuals who did not show immediately changes in excitability in sham stimulation sessions were the ones who responded to active stimulation conditions. On the other hand, individuals who responded to sham condition, by either increases or decreases in MEPS, did not respond to active verum stimulation. This result suggests that the presence or lack of responses to sham stimulation can provide a marker for how individuals will respond to tRNS/tACS and thus provide an explanation for the variability in interindividual response. The results of this study draw attention to the general reactivity of the brain, which can be taken into account when planning future studies using tNIBS.

Evaluation of brain SERT with 4-[18F]-ADAM/micro-PET and hearing protective effects of dextromethorphan in hearing loss rat model

This study investigated the protective effects of dextromethorphan (DXM) on noise-induced hearing loss (NIHL) in rats. This study aimed to improve the auditory threshold and to understand the protective effects of DXM against N-methyl-d-aspartate (NMDA)-induced neurite degeneration of serotonergic neurons. The animals were exposed to 8-kHz narrowband noise at a 118-dB sound pressure level for 3.5 h. The hearing thresholds were determined by measuring the auditory brainstem response to click stimuli. Serotonin transporter (SERT) expression was determined through micro-positron emission tomography (PET) using N,N-dimethyl-2-(2-amino-4-¹⁸F-fluorophenylthio)benzylamine (4-[¹⁸F]-ADAM). We also investigated the effects of DXM on NMDA-induced morphological changes in the primary cultures of rat serotonergic neurons. NIHL significantly improved after prophylactic treatment with DXM (p < .05). SERT density in DXM-treated rats was significantly higher than that in non-DXM-treated rats. Because prophylactic medication restored the NMDA-inhibited neurite length of serotonergic neurons and presented SERT density, DXM could be a potential agent in alleviating NIHL.

Revisiting the excitation/inhibition imbalance hypothesis of ASD through a clinical lens

Autism spectrum disorder (ASD) currently affects 1 in 59 children, although the aetiology of this disorder remains unknown. Faced with multiple seemingly disparate and noncontiguous neurobiological alterations, Rubenstein and Merzenich hypothesized that imbalances between excitatory and inhibitory neurosignaling (E/I imbalance) underlie ASD. Since this initial statement, there has been a major focus examining this exact topic spanning both clinical and preclinical realms. The purpose of this article is to review the clinical neuroimaging literature surrounding E/I imbalance as an aetiology of ASD. Evidence for E/I imbalance is presented from several complementary clinical techniques including magnetic resonance spectroscopy, magnetoencephalography and transcranial magnetic stimulation. Additionally, two GABAergic potential interventions for ASD, which explicitly attempt to remediate E/I imbalance, are reviewed. The current literature suggests E/I imbalance as a useful framework for discussing the neurobiological etiology of ASD in at least a subset of affected individuals. While not constituting a completely unifying aetiology, E/I imbalance may be relevant as one of several underlying neuropathophysiologies that differentially affect individuals with ASD. Such statements do not diminish the value of the E/I imbalance concept-instead they suggest a possible role for the characterization of E/I imbalance, as well as other underlying neuropathophysiologies, in the biologically-based subtyping of individuals with ASD for potential applications including clinical trial enrichment as well as treatment triage.

Effect of aerobic exercise prior to modified constraint-induced movement therapy outcomes in individuals with chronic hemiparesis: a study protocol for a randomized clinical trial

BACKGROUND

Recovery of upper limb function in individuals after a stroke remains challenging. Modified constraint-induced movement therapy (m-CIMT) has strong evidence for increasing the use and recovery of sensorimotor function of the paretic upper limb. Recent studies have shown that priming with aerobic exercise prior to task-specific training potentiates upper limb recovery in individuals with stroke. This protocol describes a randomized clinical trial designed to determine whether priming with moderate-high intensity aerobic exercise prior to m-CIMT will improve the manual dexterity of the paretic upper limb in individuals with chronic hemiparesis.

METHODS

Sixty-two individuals with chronic hemiparesis will be randomized into two groups: Aerobic exercise + m-CIMT or Stretching + m-CIMT. m-CIMT includes 1) restraint of the nonparetic upper limb for 90% of waking hours, 2) intensive task-oriented training of the paretic upper limb for 3 h/day for 10 days and 3) behavior interventions for improving treatment adherence. Aerobic exercise will be conducted on a stationary bicycle at intervals of moderate to high intensity. Participants will be evaluated at baseline, 3, 30, and 90 days postintervention by the following instruments: Motor Activity Log, Nottingham Sensory Assessment, Wolf Motor Function Test, Box and Block Test, Nine-Hole Peg Test, Stroke Specific Quality of Life Scale and three-dimensional kinematics. The data will be tested for normality and homogeneity. Parametric data will be analyzed by two-way ANOVA with repeated measures and Bonferroni's adjustment. For nonparametric data, the Friedman test followed by the Wilcoxon test with Bonferroni's adjustment will be used to compare the ratings for each group. To compare the groups in each assessment, the Mann-Whitney test will be used.

DISCUSSION

This study will provide valuable information about the effect of motor priming for fine upper limb skill improvement in people with chronic poststroke hemiparesis, bringing new evidence about the association of two therapies commonly used in clinical practice.

TRIAL REGISTRATION

This trial was retrospectively registered (registration number RBR-83pwm3 ) on 07 May 2018.

Determinants of Inter-Individual Variability in Corticomotor Excitability Induced by Paired Associative Stimulation

Transcranial magnetic stimulation (TMS) is a well-established tool in probing cortical plasticity in vivo. Changes in corticomotor excitability can be induced using paired associative stimulation (PAS) protocol, in which TMS over the primary motor cortex is conditioned with an electrical peripheral nerve stimulation of the contralateral hand. PAS with an inter-stimulus interval of 25 ms induces long-term potentiation (LTP)-like effects in cortical excitability. However, the response to a PAS protocol tends to vary substantially across individuals. In this study, we used univariate and multivariate data-driven methods to investigate various previously proposed determinants of inter-individual variability in PAS efficacy, such as demographic, cognitive, clinical, neurophysiological, and neuroimaging measures. Forty-one right-handed participants, comprising 22 patients with amnestic mild cognitive impairment (MCI) and 19 healthy controls (HC), underwent the PAS protocol. Prior to stimulation, demographic, genetic, clinical, as well as structural and resting-state functional MRI data were acquired. The two groups did not differ in any of the variables, except by global cognitive status. Univariate analysis showed that only 61% of all participants were classified as PAS responders, irrespective of group membership. Higher PAS response was associated with lower TMS intensity and with higher resting-state connectivity within the sensorimotor network, but only in responders, as opposed to non-responders. We also found an overall positive correlation between PAS response and structural connectivity within the corticospinal tract, which did not differ between groups. A multivariate random forest (RF) model identified age, gender, education, IQ, global cognitive status, sleep quality, alertness, TMS intensity, genetic factors, and neuroimaging measures (functional and structural connectivity, gray matter (GM) volume, and cortical thickness as poor predictors of PAS response. The model resulted in low accuracy of the RF classifier (58%; 95% CI: 42 - 74%), with a higher relative importance of brain connectivity measures compared to the other variables. We conclude that PAS variability in our sample was not well explained by factors known to influence PAS efficacy, emphasizing the need for future replication studies.

Priming the Motor Cortex With Anodal Transcranial Direct Current Stimulation Affects the Acute Inhibitory Corticospinal Responses to Strength Training

Frazer, AK, Howatson, G, Ahtiainen, JP, Avela, J, Rantalainen, T, and Kidgell, DJ. Priming the motor cortex with anodal transcranial direct current stimulation affects the acute inhibitory corticospinal responses to strength training. J Strength Cond Res 33(2): 307-317, 2019-Synaptic plasticity in the motor cortex (M1) is associated with strength training (ST) and can be modified by transcranial direct current stimulation (tDCS). The M1 responses to ST increase when anodal tDCS is applied during training due to gating. An additional approach to improve the M1 responses to ST, which has not been explored, is to use anodal tDCS to prime the M1 before a bout of ST. We examined the priming effects of anodal tDCS of M1 on the acute corticospinal responses to ST. In a randomized double-blinded cross-over design, changes in isometric strength, corticospinal excitability, and inhibition (assessed as area under the recruitment curve [AURC] using transcranial magnetic stimulation) were analyzed in 13 adults exposed to 20 minutes of anodal tDCS and sham tDCS followed by a ST session of the right elbow flexors. We observed a significant decrease in isometric elbow-flexor strength immediately after training (11-12%; p < 0.05), which was not different between anodal tDCS and sham tDCS. Transcranial magnetic stimulation revealed a 24% increase in AURC for corticospinal excitability after anodal tDCS and ST; this increase was not different between conditions. However, there was a 14% reduction in AURC for corticospinal inhibition when anodal tDCS was applied before ST when compared with sham tDCS and ST (all p < 0.05). Priming anodal tDCS had a limited effect in facilitating corticospinal excitability after an acute bout of ST. Interestingly, the interaction of anodal tDCS and ST seems to affect the excitability of intracortical inhibitory circuits of the M1 through nonhomeostatic mechanisms.

The effect of psychotropic drugs on cortical excitability and plasticity measured with transcranial magnetic stimulation: Implications for psychiatric treatment

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) is an emerging treatment for neuropsychiatric disorders. Patients in rTMS treatment typically receive concomitant psychotropic medications, which affect neuronal excitability and plasticity and may interact to affect rTMS treatment outcomes. A greater understanding of these drug effects may have considerable implications for optimizing multi-modal treatment of psychiatric patients, and elucidating the mechanism(s) of action (MOA) of rTMS.

METHOD

We summarized the empirical literature that tests how psychotropic drugs affect cortical excitability and plasticity, using varied experimental TMS paradigms.

RESULTS

Glutamate antagonists robustly attenuate plasticity, largely without changes in excitability per se; antiepileptic drugs show the opposite pattern of effects, while calcium channel blockers attenuate plasticity. Benzodiazepines have moderate and variable effects on plasticity, and negligible effects on excitability. Antidepressants with potent 5HT transporter inhibition reduce both excitability and alter plasticity, while antidepressants with other MOAs generally lack either effect. Catecholaminergic drugs, cholinergic agents and lithium have minimal effects on excitability but exhibit robust and complex, non-linear effects in TMS plasticity paradigms.

LIMITATIONS

These effects remain largely untested in sustained treatment protocols, nor in clinical populations. In addition, how these medications impact clinical response to rTMS remains largely unknown.

CONCLUSIONS

Psychotropic medications exert robust and varied effects on cortical excitability and plasticity. We encourage the field to more directly and fully investigate clinical pharmaco-TMS studies to improve outcomes.

Modulation of Fronto-Striatal Functional Connectivity Using Transcranial Magnetic Stimulation

Background: The fronto-striatal network is involved in various motor, cognitive, and emotional processes, such as spatial attention, working memory, decision-making, and emotion regulation. Intermittent theta burst transcranial magnetic stimulation (iTBS) has been shown to modulate functional connectivity of brain networks. Long stimulation intervals, as well as high stimulation intensities are typically applied in transcranial magnetic stimulation (TMS) therapy for mood disorders. The role of stimulation intensity on network function and homeostasis has not been explored systematically yet. Objective: In this pilot study, we aimed to modulate fronto-striatal connectivity by applying iTBS at different intensities to the left dorso-lateral prefrontal cortex (DLPFC). We measured individual and group changes by comparing resting state functional magnetic resonance imaging (rsfMRI) both pre-iTBS and post-iTBS. Differential effects of individual sub- vs. supra-resting motor-threshold stimulation intensities were assessed. Methods: Sixteen healthy subjects underwent excitatory iTBS at two intensities [90% and 120% of individual resting motor threshold (rMT)] on separate days. Six-hundred pulses (2 s trains, 8 s pauses, duration of 3 min, 20 s) were applied over the left DLPFC. Directly before and 7 min after stimulation, task-free rsfMRI sessions, lasting 10 min each, were conducted. Individual seed-to-seed functional connectivity changes were calculated for 10 fronto-striatal and amygdala regions of interest with the SPM toolbox DPABI. Results: Sub-threshold-iTBS increased functional connectivity directly between the left DLPFC and the left and right caudate, respectively. Supra-threshold stimulation did not change fronto-striatal functional connectivity but increased functional connectivity between the right amygdala and the right caudate. Conclusion: A short iTBS protocol applied at sub-threshold intensities was not only sufficient, but favorable, in order to increase bilateral fronto-striatal functional connectivity, while minimizing side effects. The absence of an increase in functional connectivity after supra-threshold stimulation was possibly caused by network homeostatic effects.

Self-Paced Online vs. Cue-Based Offline Brain-Computer Interfaces for Inducing Neural Plasticity

: Brain-computer interfaces (BCIs), operated in a cue-based (offline) or self-paced (online) mode, can be used for inducing cortical plasticity for stroke rehabilitation by the pairing of movement-related brain activity with peripheral electrical stimulation. The aim of this study was to compare the difference in cortical plasticity induced by the two BCI modes. Fifteen healthy participants participated in two experimental sessions: cue-based BCI and self-paced BCI. In both sessions, imagined dorsiflexions were extracted from continuous electroencephalogram (EEG) and paired 50 times with the electrical stimulation of the common peroneal nerve. Before, immediately after, and 30 minutes after each intervention, the cortical excitability was measured through the motor-evoked potentials (MEPs) of tibialis anterior elicited through transcranial magnetic stimulation. Linear mixed regression models showed that the MEP amplitudes increased significantly (p < 0.05) from pre- to post- and 30-minutes post-intervention in terms of both the absolute and relative units, regardless of the intervention type. Compared to pre-interventions, the absolute MEP size increased by 79% in post- and 68% in 30-minutes post-intervention in the self-paced mode (with a true positive rate of ~75%), and by 37% in post- and 55% in 30-minutes post-intervention in the cue-based mode. The two modes were significantly different (p = 0.03) at post-intervention (relative units) but were similar at both post timepoints (absolute units). These findings suggest that immediate changes in cortical excitability may have implications for stroke rehabilitation, where it could be used as a priming protocol in conjunction with another intervention; however, the findings need to be validated in studies involving stroke patients.

Visuomotor task acquisition is reduced by priming paired associative stimulation in older adults

Transcranial magnetic stimulation may represent an effective means for improving motor function in the elderly. The aim of this study was therefore to investigate the effects of paired associative stimulation (PAS; a plasticity-inducing transcranial magnetic stimulation paradigm) on acquisition of a novel visuomotor task in young and older adults. Fourteen young (20.4 ± 0.6 years) and 13 older (69.0 ± 1.6 years) adults participated in 3 experimental sessions during which training was preceded (primed) by PAS. Within each session, the interstimulus interval used for PAS was set at either the N20 latency plus 5 ms (PAS_LTP), the N20 latency minus 10 ms (PAS_LTD), or a constant 100 ms (PAS_Control). After training, the level of motor skill was not different between PAS conditions in young subjects (all p-values > 0.2), but was reduced by both PAS_LTP (p = 0.02) and PAS_LTD (p = 0.0001) in older subjects. Consequently, priming PAS was detrimental to skill acquisition in older adults, possibly suggesting a need for interventions that are optimized for use in elderly populations.