Modulation of corticospinal excitability by paired associative stimulation: Reproducibility of effects and intraindividual reliability

Cerebellar Modulation of Sensorimotor Associative Plasticity Is Impaired in Cervical Dystonia

BACKGROUND

In recent years, cervical dystonia (CD) has been recognized as a network disorder that involves not only the basal ganglia but other brain regions, such as the primary motor and somatosensory cortex, brainstem, and cerebellum. So far, the role of the cerebellum in the pathophysiology of dystonia is only poorly understood.

OBJECTIVE

The objective of this study was to investigate the role of the cerebellum on sensorimotor associative plasticity in patients with CD.

METHODS

Sixteen patients with CD and 13 healthy subjects received cerebellar transcranial direct current stimulation (ctDCS) followed by a paired associative stimulation (PAS) protocol based on transcranial magnetic stimulation that induces sensorimotor associative plasticity. Across three sessions the participants received excitatory anodal, inhibitory cathodal, and sham ctDCS in a double-blind crossover design. Before and after the intervention, motor cortical excitability and motor symptom severity were assessed.

RESULTS

PAS induced an increase in motor cortical excitability in both healthy control subjects and patients with CD. In healthy subjects this effect was attenuated by both anodal and cathodal ctDCS with a stronger effect of cathodal stimulation. In patients with CD, anodal stimulation suppressed the PAS effect, whereas cathodal stimulation had no influence on PAS. Motor symptom severity was unchanged after the intervention.

CONCLUSIONS

Cerebellar modulation with cathodal ctDCS had no effect on sensorimotor associative plasticity in patients with CD, in contrast with the net inhibitory effect in healthy subjects. This is further evidence that the cerebello-thalamo-cortical network plays a role in the pathophysiology of dystonia. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Retest reliability of repetitive transcranial magnetic stimulation over the healthy human motor cortex: a systematic review and meta-analysis

Introduction

Repetitive transcranial magnetic stimulation (rTMS) is used to induce long-lasting changes (aftereffects) in cortical excitability, which are often measured via single-pulse TMS (spTMS) over the motor cortex eliciting motor-evoked potentials (MEPs). rTMS includes various protocols, such as theta-burst stimulation (TBS), paired associative stimulation (PAS), and continuous rTMS with a fixed frequency. Nevertheless, subsequent aftereffects of rTMS are variable and seem to fail repeatability. We aimed to summarize standard rTMS procedures regarding their test-retest reliability. Hereby, we considered influencing factors such as the methodological quality of experiments and publication bias.

Methods

We conducted a literature search via PubMed in March 2023. The inclusion criteria were the application of rTMS, TBS, or PAS at least twice over the motor cortex of healthy subjects with measurements of MEPs via spTMS as a dependent variable. The exclusion criteria were measurements derived from the non-stimulated hemisphere, of non-hand muscles, and by electroencephalography only. We extracted test-retest reliability measures and aftereffects from the eligible studies. With the Rosenthal fail-safe N, funnel plot, and asymmetry test, we examined the publication bias and accounted for influential factors such as the methodological quality of experiments measured with a standardized checklist.

Results

A total of 15 studies that investigated test-retest reliability of rTMS protocols in a total of 291 subjects were identified. Reliability measures, i.e., Pearson's r and intraclass correlation coefficient (ICC) applicable from nine studies, were mainly in the small to moderate range with two experiments indicating good reliability of 20 Hz rTMS (r = 0.543) and iTBS (r = 0.55). The aftereffects of rTMS procedures seem to follow the heuristics of respective inhibition or facilitation, depending on the protocols' frequency, and application pattern. There was no indication of publication bias and the influence of methodological quality or other factors on the reliability of rTMS.

Conclusion

The reliability of rTMS appears to be in the small to moderate range overall. Due to a limited number of studies reporting test-retest reliability values and heterogeneity of dependent measures, we could not provide generalizable results. We could not identify any protocol as superior to the others.

Physiology of dystonia: Human studies

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

Intraclass Correlation in Paired Associative Stimulation and Metaplasticity

Paired associative stimulation (PAS) is a widely used noninvasive brain stimulation protocol to assess neural plasticity. Its reproducibility, however, has been rarely tested and with mixed results. With two consecutive studies, we aimed to provide further tests and a more systematic assessment of PAS reproducibility. We measured intraclass correlation coefficients (ICCs)—a widely used tool to assess whether groups of measurements resemble each other—in two PAS studies on healthy volunteers. The first study included five PAS sessions recording 10 MEPS every 10 min for an hour post-PAS. The second study included two PAS sessions recording 50 MEPS at 20 and 50 min post-PAS, based on analyses from the first study. In both studies PAS sessions were spaced one week apart. Within sessions ICC was fair to excellent for both studies, yet between sessions ICC was poor for both studies. We suggest that long term meta-plasticity effects (longer than one week) may interfere with between sessions reproducibility.

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

Background

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

Objective

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

Methods

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

Results

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

Conclusion

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

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.

A New Paired Associative Stimulation Protocol with High-Frequency Peripheral Component and High-Intensity 20 Hz Repetitive Transcranial Magnetic Stimulation-A Pilot Study

Paired associative stimulation (PAS) is a stimulation technique combining transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS) that can induce plastic changes in the human motor system. A PAS protocol consisting of a high-intensity single TMS pulse given at 100% of stimulator output (SO) and high-frequency 100-Hz PNS train, or "the high-PAS" was designed to promote corticomotoneuronal synapses. Such PAS, applied as a long-term intervention, has demonstrated therapeutic efficacy in spinal cord injury (SCI) patients. Adding a second TMS pulse, however, rendered this protocol inhibitory. The current study sought for more effective PAS parameters. Here, we added a third TMS pulse, i.e., a 20-Hz rTMS (three pulses at 96% SO) combined with high-frequency PNS (six pulses at 100 Hz). We examined the ability of the proposed stimulation paradigm to induce the potentiation of motor-evoked potentials (MEPs) in five human subjects and described the safety and tolerability of the new protocol in these subjects. In this study, rTMS alone was used as a control. In addition, we compared the efficacy of the new protocol in five subjects with two PAS protocols consisting of PNS trains of six pulses at 100 Hz combined with (a) single 100% SO TMS pulses (high-PAS) and (b) a 20-Hz rTMS at a lower intensity (three pulses at 120% RMT). The MEPs were measured immediately after, and 30 and 60 min after the stimulation. Although at 0 and 30 min there was no significant difference in the induced MEP potentiation between the new PAS protocol and the rTMS control, the MEP potentiation remained significantly higher at 60 min after the new PAS than after rTMS alone. At 60 min, the new protocol was also more effective than the two other PAS protocols. The new protocol caused strong involuntary twitches in three subjects and, therefore, its further characterization is needed before introducing it for clinical research. Additionally, its mechanism plausibly differs from PAS with high-frequency PNS that has been used in SCI patients.

The Relationship Between Corticomotor Reorganization and Acute Pain Severity: A Randomized, Controlled Study Using Rapid Transcranial Magnetic Stimulation Mapping

OBJECTIVE

Although acute pain has been shown to reduce corticomotor excitability, it remains unknown whether this response resolves over time or is related to symptom severity. Furthermore, acute pain research has relied upon data acquired from the cranial "hotspot," which do not provide valuable information regarding reorganization, such as changes to the distribution of a painful muscle's representation within M1. Using a novel, rapid transcranial magnetic stimulation (TMS) mapping method, this study aimed to 1) explore the temporal profile and variability of corticomotor reorganization in response to acute pain and 2) determine whether individual patterns of corticomotor reorganization are associated with differences in pain, sensitivity, and somatosensory organization.

METHODS

Corticomotor (TMS maps), pain processing (pain intensity, pressure pain thresholds), and somatosensory (two-point discrimination, two-point estimation) outcomes were taken at baseline, immediately after injection (hypertonic [n = 20] or isotonic saline [n = 20]), and at pain resolution. Follow-up measures were recorded every 15 minutes until 90 minutes after injection.

RESULTS

Corticomotor reorganization persisted at least 90 minutes after pain resolution. Corticomotor depression was associated with lower pain intensity than was corticomotor facilitation (r = 0.47 [P = 0.04]). These effects were not related to somatosensory reorganization or peripheral sensitization mechanisms.

CONCLUSIONS

Individual patterns of corticomotor reorganization during acute pain appear to be related to symptom severity, with early corticomotor depression possibly reflecting a protective response. These findings hold important implications for the management and potential prevention of pain chronicity. However, further research is required to determine whether these adaptations relate to long-term outcomes in clinical populations.

The effect of breaking up prolonged sitting on paired associative stimulation-induced plasticity

Paired associative stimulation (PAS) can induce plasticity in the motor cortex, as measured by changes in corticospinal excitability (CSE). This effect is attenuated in older and less active individuals. Although a single bout of exercise enhances PAS-induced plasticity in young, physically inactive adults, it is not yet known if physical activity interventions affect PAS-induced neuroplasticity in middle-aged inactive individuals. Sixteen inactive middle-aged office workers participated in a randomized cross-over design investigating how CSE and short-interval intracortical inhibition (SICI) were affected by PAS preceded by 3 h of sitting (SIT), 3 h of sitting interrupted every 30 min by 3 min of frequent short bouts of physical activity (FPA) and 2.5 h of sitting followed by 25 min of moderate-intensity exercise (EXE). Transcranial magnetic stimulation was applied over the primary motor cortex (M1) of the dominant abductor pollicis brevis to induce recruitment curves before and 5 min and 30 min post-PAS. Linear mixed models were used to compare changes in CSE using time and condition as fixed effects and subjects as random effects. There was a main effect of time on CSE and planned within-condition comparisons showed that CSE was significantly increased from baseline to 5 min and 30 min post-PAS, in the FPA condition, with no significant changes in the SIT or EXE conditions. SICI decreased from baseline to 5 min post-PAS, but this was not related to changes in CSE. Our findings suggest that in middle-aged inactive adults, FPAs may promote corticospinal neuroplasticity. Possible mechanisms are discussed.

Corticomotor reorganization during short-term visuomotor training in the lower back: A randomized controlled study

INTRODUCTION

Accumulating evidence suggests that motor skill training is associated with structural and functional reorganization of the primary motor cortex. However, previous studies have focussed primarily upon the upper limb, and it is unclear whether comparable reorganization occurs following training of other regions, such as the lower back. Although this holds important implications for rehabilitation, no studies have examined corticomotor adaptations following short-term motor training in the lower back.

METHOD

The aims of this study were to (a) determine whether a short-term lumbopelvic tilt visuomotor task induced reorganization of the corticomotor representations of lower back muscles, (b) quantify the variability of corticomotor responses to motor training, and (c) determine whether any improvements in task performance were correlated with corticomotor reorganization. Participants were allocated randomly to perform a lumbopelvic tilt motor training task (n = 15) or a finger abduction control task involving no lumbopelvic movement (n = 15). Transcranial magnetic stimulation was used to map corticomotor representations of the lumbar erector spinae before, during, and after repeated performance of the allocated task.

RESULTS

No relationship between corticomotor reorganization and improved task performance was identified. Substantial variability was observed in terms of corticomotor responses to motor training, with approximately 50% of participants showing no corticomotor reorganization despite significant improvements in task performance.

CONCLUSION

These findings suggest that short-term improvements in lower back visuomotor task performance may be driven by changes in remote subcortical and/or spinal networks rather than adaptations in corticomotor pathways. However, further research using tasks of varying complexities and durations is required to confirm this hypothesis.

Plasticity and dystonia: a hypothesis shrouded in variability

Studying plasticity mechanisms with Professor John Rothwell was a shared highlight of our careers. In this article, we discuss non-invasive brain stimulation techniques which aim to induce and quantify plasticity, the mechanisms and nature of their inherent variability and use such observations to review the idea that excessive and abnormal plasticity is a pathophysiological substrate of dystonia. We have tried to define the tone of our review by a couple of Professor John Rothwell's many inspiring characteristics; his endless curiosity to refine knowledge and disease models by scientific exploration and his wise yet humble readiness to revise scientific doctrines when the evidence is supportive. We conclude that high variability of response to non-invasive brain stimulation plasticity protocols significantly clouds the interpretation of historical findings in dystonia research. There is an opportunity to wipe the slate clean of assumptions and armed with an informative literature in health, re-evaluate whether excessive plasticity has a causal role in the pathophysiology of dystonia.

Rewiring the Lesioned Brain: Electrical Stimulation for Post-Stroke Motor Restoration

Electrical stimulation has been extensively applied in post-stroke motor restoration, but its treatment mechanisms are not fully understood. Stimulation of neuromotor control system at multiple levels manipulates the corresponding neuronal circuits and results in neuroplasticity changes of stroke survivors. This rewires the lesioned brain and advances functional improvement. This review addresses the therapeutic mechanisms of different stimulation modalities, such as noninvasive brain stimulation, peripheral electrical stimulation, and other emerging techniques. The existing applications, the latest progress, and future directions are discussed. The use of electrical stimulation to facilitate post-stroke motor recovery presents great opportunities in terms of targeted intervention and easy applicability. Further technical improvements and clinical studies are required to reveal the neuromodulatory mechanisms and to enhance rehabilitation therapy efficiency in stroke survivors and people with other movement disorders.

Priming Effects of Water Immersion on Paired Associative Stimulation-Induced Neural Plasticity in the Primary Motor Cortex

We aimed to verify whether indirect-wave (I-wave) recruitment and cortical inhibition can regulate or predict the plastic response to paired associative stimulation with an inter-stimulus interval of 25 ms (PAS25), and also whether water immersion (WI) can facilitate the subsequent PAS25-induced plasticity. To address the first question, we applied transcranial magnetic stimulation (TMS) to the M1 hand area, while alternating the direction of the induced current between posterior-to-anterior and anterior-to-posterior to activate two independent synaptic inputs to the corticospinal neurons. Moreover, we used a paired stimulation paradigm to evaluate the short-latency afferent inhibition (SAI) and short-interval intracortical inhibition (SICI). To address the second question, we examined the motor evoked potential (MEP) amplitudes before and after PAS25, with and without WI, and used the SAI, SICI, and MEP recruitment curves to determine the mechanism underlying priming by WI on PAS25. We demonstrated that SAI, with an inter-stimulus interval of 25 ms, might serve as a predictor of the response to PAS25, whereas I-wave recruitment evaluated by the MEP latency difference was not predictive of the PAS25 response, and found that 15 min WI prior to PAS25 facilitated long-term potentiation (LTP)-like plasticity due to a homeostatic increase in cholinergic activity.

Sex differences in Parkinson's disease: A transcranial magnetic stimulation study

BACKGROUND

Demographic and clinical studies imply that female sex may be protective for PD, but pathophysiological evidence to support these observations is missing. In early PD, functional changes may be detected in primary motor cortex using transcranial magnetic stimulation.

OBJECTIVE

We hypothesised that if pathophysiology differs between sexes in PD, this will be reflected in differences of motor cortex measurements.

METHODS

Forty-one newly diagnosed PD patients (22 males, 19 females) were clinically assessed using MDS-UPDRS part III, and various measures of cortical excitability and sensorimotor cortex plasticity were measured over both hemispheres, corresponding to the less and more affected side, using transcranial magnetic stimulation. Twenty-three healthy (10 men, 13 women) participants were studied for comparison.

RESULTS

Among patients, no significant differences between sexes were found in age, age of diagnosis, symptom duration, and total or lateralized motor score. However, male patients had disturbed interhemispheric balance of motor thresholds, caused by decreased resting and active motor thresholds in the more affected hemisphere. Short interval intracortical inhibition was more effective in female compared to male patients in both hemispheres. Female patients had a preserved physiological focal response to sensorimotor plasticity protocol, whereas male patients showed an abnormal spread of the protocol effect.

CONCLUSION

The study provides one of the first neurophysiological evidences of sex differences in early PD. Female patients have a more favorable profile of transcranial magnetic stimulation measures, possibly reflecting a more successful cortical compensation or delayed maladaptive changes in the sensorimotor cortex. © 2019 International Parkinson and Movement Disorder Society.

A Systematic Review of Paired Associative Stimulation (PAS) to Modulate Lower Limb Corticomotor Excitability: Implications for Stimulation Parameter Selection and Experimental Design

Non-invasive neuromodulatory interventions have the potential to influence neural plasticity and augment motor rehabilitation in people with stroke. Paired associative stimulation (PAS) involves the repeated pairing of single pulses of electrical stimulation to a peripheral nerve and single pulses of transcranial magnetic stimulation over the contralateral primary motor cortex. Efficacy of PAS in the lower limb of healthy and stroke populations has not been systematically appraised. Optimal protocols including stimulation parameter settings have yet to be determined. This systematic review (a) examines the efficacy of PAS on lower limb corticomotor excitability in healthy and stroke populations and (b) evaluates the stimulation parameters employed. Five databases were searched for randomized, non-randomized, and pre-post experimental studies evaluating lower limb PAS in healthy and stroke populations. Two independent reviewers identified eligible studies and assessed methodological quality using a modified Downs and Blacks Tool and the TMS Checklist. Intervention stimulation parameters and TMS measurement details were also extracted and compared. Twelve articles, comprising 24 experiments, met the inclusion criteria. Four articles evaluated PAS in people with stroke. Following a single session of PAS, 21 experiments reported modulation of corticomotor excitability, lasting up to 60 min; however, the research lacked methodological rigor. Intervention stimulation parameters were highly variable across experiments, and whilst these appeared to influence efficacy, variations in the intervention and outcome assessment methods hindered the ability to draw conclusions about optimal parameters. Lower limb PAS research requires further investigation before considering its translation into clinical practice. Eight key recommendations serve as guide for enhancing future research in the field.

Paired Associative Stimulation as a Tool to Assess Plasticity Enhancers in Chronic Stroke

Background and Purpose

The potential for adaptive plasticity in the post-stroke brain is difficult to estimate, as is the demonstration of central nervous system (CNS) target engagement of drugs that show promise in facilitating stroke recovery. We set out to determine if paired associative stimulation (PAS) can be used (a) as an assay of CNS plasticity in patients with chronic stroke, and (b) to demonstrate CNS engagement by memantine, a drug which has potential plasticity-modulating effects for use in motor recovery following stroke.

Methods

We examined the effect of PAS in fourteen participants with chronic hemiparetic stroke at five time-points in a within-subjects repeated measures design study: baseline off-drug, and following a week of orally administered memantine at doses of 5, 10, 15, and 20 mg, comprising a total of seventy sessions. Each week, MEP amplitude pre and post-PAS was assessed in the contralesional hemisphere as a marker of enhanced or diminished plasticity. Strength and dexterity were recorded each week to monitor motor-specific clinical status across the study period.

Results

We found that MEP amplitude was significantly larger after PAS in baseline sessions off-drug, and responsiveness to PAS in these sessions was associated with increased clinical severity. There was no observed increase in MEP amplitude after PAS with memantine at any dose. Motor threshold (MT), strength, and dexterity remained unchanged during the study.

Conclusion

Paired associative stimulation successfully induced corticospinal excitability enhancement in chronic stroke subjects at the group level. However, this response did not occur in all participants, and was associated with increased clinical severity. This could be an important way to stratify patients for future PAS-drug studies. PAS was suppressed by memantine at all doses, regardless of responsiveness to PAS off-drug, indicating CNS engagement.

Effect of Paired Associative Stimulation on Motor Cortex Excitability in Rats

Paired associative stimulation (PAS), combining transcranial magnetic stimulation (TMS) with electrical peripheral nerve stimulation (PNS) in pairs with an optimal interstimulus interval (ISI) in between, has been shown to influence the excitability of the motor cortex (MC) in humans. However, the underlying mechanisms remain unclear. This study was designed to explore an optimal protocol of PAS, which can modulate the excitability of MC in rats, and to investigate the underlying mechanisms. The resting motor thresholds (RMTs) of TMS-elicited motor evoked potentials (MEPs) recorded from the gastrocnemius muscle and the latency of P1 component of somatosensory evoked potentials (SEPs) induced by electrical tibial nerve stimulation were determined in male Sprague-Dawley rats (n=10). Sixty rats were then randomly divided into 3 groups: a PAS group (further divided into 10 subgroups at various ISIs calculated by using the latency of P1, n=5, respectively), a TMS (only) group (n=5) and a PNS (only) group (n=5). Ninety repetitions of PAS, TMS and PNS were administered to the rats in the 3 groups, respectively, at the frequency of 0.05 Hz and the intensity of TMS at 120% RMT and that of PNS at 6 mA. RMTs and motor evoked potentials' amplitude (MEPamp) were recorded before and immediately after the interventions. It was found that the MEPamp significantly decreased after PAS at ISI of 5 ms (P<0.05), while the MEPamp significantly increased after PAS at ISI of 15 ms, as compared with those before the intervention (P<0.05). However, the RMT did not change significantly after PAS at ISI of 5 ms or 15 ms (P>0.05). PAS at other ISIs as well as the sole use of TMS and PNS induced no remarkable changes in MEPamp and RMT. In conclusion, PAS can influence motor cortex excitability in rats. Neither TMS alone nor PNS alone shows significant effect.

Physical Activity, Nutrition, Cognition, Neurophysiology, and Short-Time Synaptic Plasticity in Healthy Older Adults: A Cross-Sectional Study

The aging brain undergoes remodeling processes because of biological and environmental factors. To counteract brain aging, neuronal plasticity should be preserved. The aim of this study was to test if the capacity of generating short-time synaptic plasticity in older adults may be related to either physical activity, nutritional status, cognition, or neurophysiological activity. Thirty-six participants (mean age 73.3 ± 5.9 years) received transcranial magnetic stimulation in combination with peripheral nerve stimulation to experimentally induce short-time synaptic plasticity by paired associative stimulation (PAS). Adaptations in neuronal excitability were assessed by motor-evoked potential (MEP) in the right m. tibialis anterior before and after PAS. The Physical Activity Questionnaire 50+ and the StepWatchTM captured physical activity levels. Nutritional status was assessed by the Mini Nutritional Assessment. Cognition was assessed by reaction time for a divided attention test and with the Montreal Cognitive Assessment. Neurophysiological activity was assessed by electroencephalography during the divided attention test. MEPs of the highest stimulation intensity resulted significantly different comparing before, 5 min, or 30 min after PAS (p < 0.05). Data-driven automatic hierarchical classification of the individual recruitment curve slopes over the three-time points indicated four different response types, however, response groups did not significantly differ based on physical activity, nutritional status, cognition, or neurophysiological activity. In a second-level analysis, participants having an increased slope showed a significant higher energy expenditure (z = -2.165, p = 0.030, r = 0.36) and revealed a significant higher power activity in the alpha frequency band (z = -2.008, p = 0.046, r = 0.37) at the prefrontal-located EEG electrodes, compared to the participants having a decreased slope. This study hints toward older adults differing in their neuronal excitability which is strongly associated to their short-time synaptic plasticity levels. Furthermore, a physically active lifestyle and higher EEG power in the alpha frequency band seem to be connected to the capacity of generating long-term potentiation-like synaptic plasticity in older adults. Future studies should consider more sensitive assessments and bigger sample sizes to get a broad scope of the older adults' population.

Adaptability and reproducibility of a memory disruption rTMS protocol in the PharmaCog IMI European project

Transcranial magnetic stimulation (TMS) can interfere with cognitive processes, such as transiently impairing memory. As part of a multi-center European project, we investigated the adaptability and reproducibility of a previously published TMS memory interfering protocol in two centers using EEG or fMRI scenarios. Participants were invited to attend three experimental sessions on different days, with sham repetitive TMS (rTMS) applied on day 1 and real rTMS on days 2 and 3. Sixty-eight healthy young men were included. On each experimental day, volunteers were instructed to remember visual pictures while receiving neuronavigated rTMS trains (20 Hz, 900 ms) during picture encoding at the left dorsolateral prefrontal cortex (L-DLPFC) and the vertex. Mixed ANOVA model analyses were performed. rTMS to the L-DLPFC significantly disrupted recognition memory on experimental day 2. No differences were found between centers or between fMRI and EEG recordings. Subjects with lower baseline memory performances were more susceptible to TMS disruption. No stability of TMS-induced memory interference could be demonstrated on day 3. Our data suggests that adapted cognitive rTMS protocols can be implemented in multi-center studies incorporating standardized experimental procedures. However, our center and modality effects analyses lacked sufficient statistical power, hence highlighting the need to conduct further studies with larger samples. In addition, inter and intra-subject variability in response to TMS might limit its application in crossover or longitudinal studies.

Longitudinal and cross-sectional investigations of long-term potentiation-like cortical plasticity in bipolar disorder type II and healthy individuals

Visual evoked potential (VEP) plasticity is a promising assay for noninvasive examination of long-term potentiation (LTP)-like synaptic processes in the cerebral cortex. We conducted longitudinal and cross-sectional investigations of VEP plasticity in controls and individuals with bipolar disorder (BD) type II. VEP plasticity was assessed at baseline, as described previously (Elvsåshagen et al. Biol Psychiatry 2012), and 2.2 years later, at follow-up. The longitudinal sample with VEP data from both time points comprised 29 controls and 16 patients. VEP data were available from 13 additional patients at follow-up (total n = 58). VEPs were evoked by checkerboard reversals in two premodulation blocks before and six blocks after a plasticity-inducing block of prolonged (10 min) visual stimulation. VEP plasticity was computed by subtracting premodulation VEP amplitudes from postmodulation amplitudes. Saliva samples for cortisol analysis were collected immediately after awakening in the morning, 30 min later, and at 12:30 PM, at follow-up. We found reduced VEP plasticity in BD type II, that impaired plasticity was present in the euthymic phases of the illness, and that VEP plasticity correlated negatively with depression severity. There was a positive association between VEP plasticity and saliva cortisol in controls, possibly reflecting an inverted U-shaped relationship between cortisol and synaptic plasticity. VEP plasticity exhibited moderate temporal stability over a period of 2.2 years. The present study provides additional evidence for impaired LTP-like cortical plasticity in BD type II. VEP plasticity is an accessible method, which may help elucidate the pathophysiological and clinical significance of synaptic dysfunction in psychiatric disorders.

Abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions in cervical dystonia

The cerebellum can influence the responsiveness of the primary motor cortex (M1) to undergo spike timing-dependent plastic changes through a complex mechanism involving multiple relays in the cerebello-thalamo-cortical pathway. Previous TMS studies showed that cerebellar cortex excitation can block the increase in M1 excitability induced by a paired-associative stimulation (PAS), while cerebellar cortex inhibition would enhance it. Since cerebellum is known to be affected in many types of dystonia, this bidirectional modulation was assessed in 22 patients with cervical dystonia and 23 healthy controls. Exactly opposite effects were found in patients: cerebellar inhibition suppressed the effects of PAS, while cerebellar excitation enhanced them. Another experiment comparing healthy subjects maintaining the head straight with subjects maintaining the head turned as the patients found that turning the head is enough to invert the cerebellar modulation of M1 plasticity. A third control experiment in healthy subjects showed that proprioceptive perturbation of the sterno-cleido-mastoid muscle had the same effects as turning the head. We discuss these finding in the light of the recent model of a mesencephalic head integrator. We also suggest that abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions of the integrator in cervical dystonia.

Corticospinal Plasticity in Bilateral Primary Motor Cortices Induced by Paired Associative Stimulation to the Dominant Hemisphere Does Not Differ between Young and Older Adults

Older adults have been shown to exhibit a reduction in the lateralization of neural activity. Although neuroplasticity induced by noninvasive brain stimulation has been reported to be attenuated in the targeted motor cortex of older adults, it remains possible that the plasticity effects may instead manifest in a more distributed (bilateral) network. Furthermore, attention, which modulates neuroplasticity in young adults, may influence these effects. To address these questions, plasticity was induced in young (19–32 years) and older (65–78 years) adults using transcranial magnetic stimulation (TMS) paired with peripheral nerve stimulation. The plasticity effects induced by this paired associative stimulation (PAS) protocol in the targeted and nontargeted hemispheres were probed using TMS-induced motor-evoked potentials (MEPs) recorded from the abductor pollicis brevis (APB) muscle of each hand. PAS-induced effects were highly variable across individuals, with only half of the participants in each group demonstrating the expected increase in MEP amplitude. Contrary to predictions, however, PAS-induced corticospinal plasticity manifests predominately in the targeted hemisphere for both young and older adults. Attention to the target hand did not enhance corticospinal plasticity. The results suggest that plasticity does not manifest differently across bilateral corticospinal pathways between young and older adults.

Inconsistent Effects of Parietal α-tACS on Pseudoneglect across Two Experiments: A Failed Internal Replication

Transcranial electrical stimulation (tES) is being investigated as an experimental and clinical interventional technique in human participants. While promising, important limitations have been identified, including weak effect sizes and high inter- and intra-individual variability of outcomes. Here, we compared two "inhibitory" tES-techniques with supposedly different mechanisms of action as to their effects on performance in a visuospatial attention task, and report on a direct replication attempt. In two experiments, 2 × 20 healthy participants underwent tES in three separate sessions testing different protocols (10 min stimulation each) with a montage targeting right parietal cortex (right parietal-left frontal, electrode-sizes: 3cm × 3cm-7 cm × 5 cm), while performing a perceptual line bisection (landmark) task. The tES-protocols were compared as to their ability to modulate pseudoneglect (thought to be under right hemispheric control). In experiment 1, sham-tES was compared to transcranial alternating current stimulation at alpha frequency (10 Hz; α-tACS) (expected to entrain "inhibitory" alpha oscillations) and to cathodal transcranial direct current stimulation (c-tDCS) (shown to suppress neuronal spiking activity). In experiment 2, we attempted to replicate the findings of experiment 1, and establish frequency-specificity by adding a 45 Hz-tACS condition to α-tACS and sham. In experiment 1, right parietal α-tACS led to the expected changes in spatial attention bias, namely a rightward shift in subjective midpoint estimation (relative to sham). However, this was not confirmed in experiment 2 and in the complete sample. Right parietal c-tDCS and 45 Hz-tACS had no effect. These results highlight the importance of replication studies, adequate statistical power and optimizing tES-interventions for establishing the robustness and reliability of electrical stimulation effects, and best practice.

Paired Associative Stimulation Targeting the Tibialis Anterior Muscle using either Mono or Biphasic Transcranial Magnetic Stimulation

Paired associative stimulation (PAS) protocols induce plastic changes within the motor cortex. The objectives of this study were to investigate PAS effects targeting the tibialis anterior (TA) muscle using a biphasic transcranial magnetic stimulation (TMS) pulse form and, to determine whether a reduced intensity of this pulse would lead to significant changes as has been reported for hand muscles using a monophasic TMS pulse. Three interventions were investigated: (1) suprathreshold PA_bi-PAS (n = 11); (2) suprathreshold PA_mono-PAS (n = 11) where PAS was applied using a biphasic or monophasic pulse form at 120% resting motor threshold (RMT); (3) subthreshold PA_bi-PAS (n = 10) where PAS was applied as for (1) at 95% active motor threshold (AMT). The peak-to-peak motor evoked potentials (MEPs) were quantified prior to, immediately following, and 30 min after the cessation of the intervention. TA MEP size increased significantly for all interventions immediately post (61% for suprathreshold PA_bi-PAS, 83% for suprathreshold PA_mono-PAS, 55% for subthreshold PA_bi-PAS) and 30 min after the cessation of the intervention (123% for suprathreshold PA_bi-PAS, 105% for suprathreshold PA_mono-PAS, 80% for subthreshold PA_bi-PAS. PAS using a biphasic pulse form at subthreshold intensities induces similar effects to conventional PAS.

Variability in Response to Quadripulse Stimulation of the Motor Cortex

BACKGROUND

Responses to plasticity-inducing brain stimulation protocols are highly variable. However, no data are available concerning the variability of responses to quadripulse stimulation (QPS).

OBJECTIVE

We assessed the QPS parameters of motor cortical plasticity induction in a systematic manner, and later investigated the variability of QPS using optimal parameters.

METHODS

First, two different interburst intervals (IBI) with the same total number of pulses were compared. Next we investigated three different IBIs with a different total number of pulses but with same duration of intervention. We also compared the after-effects of monophasic and biphasic QPS. Finally, variability of QPS was tested in 35 healthy subjects. Twenty motor evoked potentials (MEPs) were measured every 5-10 min for up to one hour after intervention.

RESULTS

QPS at an IBI of 5 s produced MEPs changes that are dependent on the interstimulus interval of the four magnetic pulses, consistent with previous reports. Unexpectedly, QPS at an IBI of 2.5 s did not induce any plasticity, even with the same total number of pulses, that is, 1440. QPS at an IBI of 7.5 s produced a variable response but was likely to be comparable to conventional QPS. Biphasic QPS had shorter lasting after-effects compared with monophasic QPS. Finally, the after-effects of QPS were relatively consistent across subjects: more than 80% of subjects responded as expected in the excitatory QPS at an IBI of 5 s.

CONCLUSIONS

The IBI, total duration of the procedure and pulse waveform strongly affected the magnitude or duration of the plasticity induced by QPS. In this cohort, 80% of subjects responded to excitatory QPS as expected.

Intra-Subject Consistency and Reliability of Response Following 2 mA Transcranial Direct Current Stimulation

BACKGROUND

Transcranial direct current stimulation (tDCS) is a popular non-invasive brain stimulation technique that has been shown to influence cortical excitability. While polarity specific effects have often been reported, this is not always the case, and variability in both the magnitude and direction of the effects have been observed.

OBJECTIVE/HYPOTHESIS

We aimed to explore the consistency and reliability of the effects of tDCS by investigating changes in cortical excitability across multiple testing sessions in the same individuals. A within subjects design was used to investigate the effects of anodal and cathodal tDCS applied to the motor cortex. Four experimental sessions were tested for each polarity in addition to two sham sessions.

METHODS

Transcranial magnetic stimulation (TMS) was used to measure cortical excitability (TMS recruitment curves). Changes in excitability were measured by comparing baseline measures and those taken immediately following 20 minutes of 2 mA stimulation or sham stimulation.

RESULTS

Anodal tDCS significantly increased cortical excitability at a group level, whereas cathodal tDCS failed to have any significant effects. The sham condition also failed to show any significant changes. Analysis of intra-subject responses to anodal stimulation across four sessions suggest that the amount of change in excitability across sessions was only weakly associated, and was found to have poor reliability across sessions (ICC = 0.276). The effects of cathodal stimulation show even poorer reliability across sessions (ICC = 0.137). In contrast ICC analysis for the two sessions of sham stimulation reflect a moderate level of reliability (ICC = .424).

CONCLUSIONS

Our findings indicate that although 2 mA anodal tDCS is effective at increasing cortical excitability at group level, the effects are unreliable across repeated testing sessions within individual participants. Our results suggest that 2 mA cathodal tDCS does not significantly alter cortical excitability immediately following stimulation and that there is poor reliability of the effect within the same individual across different testing sessions.

Effects of Different Analysis Strategies on Paired Associative Stimulation. A Pooled Data Analysis from Three Research Labs

Paired associative stimulation (PAS) is a widely used transcranial magnetic stimulation (TMS) paradigm to non-invasively induce synaptic plasticity in the human brain in vivo. Altered PAS-induced plasticity has been demonstrated for several diseases. However, researchers are faced with a high inter- and intra-subject variability of the PAS response. Here, we pooled original data from nine PAS studies from three centers and analyzed the combined dataset of 190 healthy subjects with regard to age dependency, the role of stimulation parameters and the effect of different statistical methods. We observed no main effect of the PAS intervention over all studies (F(2;362) = 0.44; p = 0.644). The rate of subjects showing the expected increase of motor evoked potential (MEP) amplitudes was 53%. The PAS effect differed significantly between studies as shown by a significant interaction effect (F(16;362) = 1.77; p = 0.034) but post-hoc testing did not reveal significant effects after correction for multiple tests. There was a trend toward increased variability of the PAS effect in older subjects. Acquisition parameters differed across studies but without systematically influencing changes in MEP-size. The use of post/baseline quotients systematically indicated stronger PAS effects than post/baseline difference or the logarithm of the post/baseline quotient. The non-significant PAS effects across studies and a wide range of responder rates between studies indicate a high variability of this method. We were thus not able to replicate findings from a previous meta-analysis showing robust effects of PAS. No pattern emerged regarding acquisition parameters that at this point could guide future studies to reduce variability and help increase response rate. For future studies, we propose to report the responder rate and recommend the use of the logarithmized post/baseline quotient for further analyses to better address the possibility that results are driven by few extreme cases.

Associative plasticity in the human motor cortex is enhanced by concurrently targeting separate muscle representations with excitatory and inhibitory protocols

Paired associative stimulation (PAS) induces changes in the excitability of human sensorimotor cortex that outlast the procedure. PAS typically involves repeatedly pairing stimulation of a peripheral nerve that innervates an intrinsic hand muscle with transcranial magnetic stimulation over the representation of that muscle in the primary motor cortex. Depending on the timing of the stimuli (interstimulus interval of 25 or 10 ms), PAS leads to either an increase (PAS25) or a decrease (PAS10) in excitability. Both protocols, however, have been associated with an increase in excitability of nearby muscle representations not specifically targeted by PAS. Based on these spillover effects, we hypothesized that an additive, excitability-enhancing effect of PAS25 applied to one muscle representation may be produced by simultaneously applying PAS25 or PAS10 to a nearby representation. In different experiments prototypical PAS25 targeting the left thumb representation [abductor pollicis brevis (APB)] was combined with either PAS25 or PAS10 applied to the left little finger representation [abductor digiti minimi (ADM)] or, in a control experiment, with PAS10 also targeting the APB. In an additional control experiment PAS10 targeted both representations. The plasticity effects were quantified by measuring the amplitude of motor evoked potentials (MEPs) recorded before and after PAS. As expected, prototypical PAS25 was associated with an increase in MEP amplitude in the APB muscle. This effect was enhanced when PAS also targeted the ADM representation but only when a different interstimulus timing (PAS10) was used. These results suggest that PAS-induced plasticity is modified by concurrently targeting separate motor cortical representations with excitatory and inhibitory protocols.

LTP-like plasticity in the visual system and in the motor system appear related in young and healthy subjects

LTP-like plasticity measured by visual evoked potentials (VEP) can be induced in the intact human brain by presenting checkerboard reversals. Also associated with LTP-like plasticity, around two third of participants respond to transcranial magnetic stimulation (TMS) with a paired-associate stimulation (PAS) protocol with a potentiation of their motor evoked potentials. LTP-like processes are also required for verbal and motor learning tasks. We compared effect sizes, responder rates and intercorrelations as well as the potential influence of attention between these four assessments in a group of 37 young and healthy volunteers. We observed a potentiation effect of the N75 and P100 VEP component which positively correlated with plasticity induced by PAS. Subjects with a better subjective alertness were more likely to show PAS and VEP potentiation. No correlation was found between the other assessments. Effect sizes and responder rates of VEP potentiation were higher compared to PAS. Our results indicate a high variability of LTP-like effects and no evidence for a system-specific nature. As a consequence, studies wishing to assess individual levels of LTP-like plasticity should employ a combination of multiple assessments.

Augmenting LTP-Like Plasticity in Human Motor Cortex by Spaced Paired Associative Stimulation

Paired associative stimulation (PAS_LTP) of the human primary motor cortex (M1) can induce LTP-like plasticity by increasing corticospinal excitability beyond the stimulation period. Previous studies showed that two consecutive PAS_LTP protocols interact by homeostatic metaplasticity, but animal experiments provided evidence that LTP can be augmented by repeated stimulation protocols spaced by ~30min. Here we tested in twelve healthy selected PAS_LTP responders the possibility that LTP-like plasticity can be augmented in the human M1 by systematically varying the interval between two consecutive PAS_LTP protocols. The first PAS_LTP protocol (PAS1) induced strong LTP-like plasticity lasting for 30-60min. The effect of a second identical PAS_LTP protocol (PAS₂) critically depended on the time between PAS₁ and PAS₂. At 10min, PAS₂ prolonged the PAS₁-induced LTP-like plasticity. At 30min, PAS₂ augmented the LTP-like plasticity induced by PAS₁, by increasing both magnitude and duration. At 60min and 180min, PAS₂ had no effect on corticospinal excitability. The cumulative LTP-like plasticity after PAS₁ and PAS₂ at 30min exceeded significantly the effect of PAS₁ alone, and the cumulative PAS₁ and PAS₂ effects at 60min and 180min. In summary, consecutive PAS_LTP protocols interact in human M1 in a time-dependent manner. If spaced by 30min, two consecutive PAS_LTP sessions can augment LTP-like plasticity in human M1. Findings may inspire further research on optimized therapeutic applications of non-invasive brain stimulation in neurological and psychiatric diseases.

Increase in PAS-induced neuroplasticity after a treatment course of transcranial direct current stimulation for depression

BACKGROUND

Several lines of evidence suggest that neuroplasticity is impaired in depression and improves with effective treatment. However until now, this evidence has largely involved measures such as learning and memory which can be influenced by subject effort and motivation. This pilot study aimed to objectively measure neuroplasticity in the motor cortex using paired associative stimulation (PAS), which induces short term neuroplastic changes. It is hypothesized that neuroplasticity would improve after effective treatment for depression.

METHODS

Neuroplasticity was measured in 18 depressed subjects before and after a course of anodal transcranial direct current stimulation (tDCS), given as treatment for depression. The relationships between PAS results, mood state and brain-derived neurotrophic factor (BDNF) serum levels were examined.

RESULTS

Neuroplasticity (PAS-induced change) was increased after a course of tDCS (t(17)=-2.651, p=0.017). Treatment with tDCS also led to significant mood improvement, but this did not correlate with improved neuroplasticity. Serum BDNF levels did not change after tDCS, or correlate with change in neuroplasticity after tDCS treatment.

LIMITATIONS

While this study showed evidence of improved neuroplasticity in the motor cortex after effective treatment, we are unable to present evidence that this change is generalized in the depressed brain. Also, the presence of antidepressant medications and the small sample of patients (n=18) meant the study could not definitively resolve the relationship between neuroplasticity, mood and BDNF.

CONCLUSION

This novel preliminary study provides evidence that a treatment course of tDCS can improve neuroplasticity in depressed patients.

Efficacy and interindividual variability in motor-cortex plasticity following anodal tDCS and paired-associative stimulation

Interindividual response variability to various motor-cortex stimulation protocols has been recently reported. Comparative data of stimulation protocols with different modes of action is lacking. We aimed to compare the efficacy and response variability of two LTP-inducing stimulation protocols in the human motor cortex: anodal transcranial direct current stimulation (a-tDCS) and paired-associative stimulation (PAS25). In two experiments 30 subjects received 1mA a-tDCS and PAS25. Data analysis focused on motor-cortex excitability change and response defined as increase in MEP applying different cut-offs. Furthermore, the predictive pattern of baseline characteristics was explored. Both protocols induced a significant increase in motor-cortical excitability. In the PAS25 experiments the likelihood to develop a MEP response was higher compared to a-tDCS, whereas for intracortical facilitation (ICF) the likelihood for a response was higher in the a-tDCS experiments. Baseline ICF (12 ms) correlated positively with an increase in MEPs only following a-tDCS and responders had significantly higher ICF baseline values. Contrary to recent studies, we showed significant group-level efficacy following both stimulation protocols confirming older studies. However, we also observed a remarkable amount of nonresponders. Our findings highlight the need to define sufficient physiological read-outs for a given plasticity protocol and to develop predictive markers for targeted stimulation.

Transcranial magnetic stimulation follow-up study in early Parkinson's disease: A decline in compensation with disease progression?

BACKGROUND

A number of neurophysiological abnormalities have been described in patients with Parkinson's disease, but very few longitudinal studies of how these change with disease progression have been reported. We describe measures of motor cortex inhibition and plasticity at 6 and 12 mo in 12 patients that we previously reported at initial diagnosis. Given the well-known interindividual variation in these measures, we were particularly concerned with the within-subject changes over time.

METHODS

Patients were assessed clinically, and transcranial magnetic stimulation (TMS) was used to measure motor cortical excitability, inhibition (short interval intracortical inhibition, cortical silent period), and plasticity (response to excitatory paired associative stimulation protocol) in both hemispheres. All measurements were performed 6 mo and 12 mo after the baseline experiments.

RESULTS

Asymmetry in clinical motor symptoms was reflected in asymmetry of plasticity and inhibition. In the group as a whole, little change was seen in any of the parameters over 12 mo. However, analysis of within-individual data showed clear correlations between changes in clinical asymmetry and asymmetry of response to paired associative stimulation protocol and cortical silent period.

CONCLUSIONS

Longitudinal changes in cortical silent period and response to paired associative stimulation protocol in Parkinson's disease reflect dynamic effects on motor cortex that are related to progression of motor signs. They are useful objective markers of early disease progression that could be used to detect effects of disease-modifying therapies. The decline in heightened plasticity that was present at disease onset may reflect failure of compensatory mechanisms that maintained function in the preclinical state.

Tuning pathological brain oscillations with neurofeedback: a systems neuroscience framework

Neurofeedback (NFB) is emerging as a promising technique that enables self-regulation of ongoing brain oscillations. However, despite a rise in empirical evidence attesting to its clinical benefits, a solid theoretical basis is still lacking on the manner in which NFB is able to achieve these outcomes. The present work attempts to bring together various concepts from neurobiology, engineering, and dynamical systems so as to propose a contemporary theoretical framework for the mechanistic effects of NFB. The objective is to provide a firmly neurophysiological account of NFB, which goes beyond traditional behaviorist interpretations that attempt to explain psychological processes solely from a descriptive standpoint whilst treating the brain as a “black box”. To this end, we interlink evidence from experimental findings that encompass a broad range of intrinsic brain phenomena: starting from “bottom-up” mechanisms of neural synchronization, followed by “top-down” regulation of internal brain states, moving to dynamical systems plus control-theoretic principles, and concluding with activity-dependent as well as homeostatic forms of brain plasticity. In support of our framework, we examine the effects of NFB in several brain disorders, including attention-deficit hyperactivity (ADHD) and post-traumatic stress disorder (PTSD). In sum, it is argued that pathological oscillations emerge from an abnormal formation of brain-state attractor landscape(s). The central thesis put forward is that NFB tunes brain oscillations toward a homeostatic set-point which affords an optimal balance between network flexibility and stability (i.e., self-organised criticality (SOC)).