Convergence of human brain mapping tools: neuronavigated TMS parameters and fMRI activity in the hand motor area

Rewiring the disordered connectome with circuit-based paired stimulation after stroke-a randomized, double-blind and controlled Phase II trial

The cortico-cortical paired associative stimulation, a combined stimulation based on two brain regions, may be an effective strategy for stroke rehabilitation. Our aim was to confirm that the cortico-cortical paired associative stimulation strengthens the connection between brain regions in the motor circuit and promotes improvements in motor function. This was a randomized double-blind, controlled Phase II trial. 44 Stroke patients were treated in a rehabilitation hospital from October 2020 to January 2021 and were randomly assigned to the sham stimulation group and the cortico-cortical paired associative stimulation group. Patients in both groups received 12 days of rehabilitation therapy. Cortico-cortical paired associative stimulation group received one treatment of cortico-cortical paired associative stimulation invention. Both groups received behavioural assessments such as the Fugl-Meyer upper-extremity scale and resting-state functional MRI scans prior to the intervention and on Day 14. 40 patients completed the intervention session. The results of Fugl-Meyer upper-extremity scale showed a more significant improvement in motor function in the cortico-cortical paired associative stimulation group (6.33 ± 1.29) than in the sham stimulation group (3.16 ± 1.38) (P < 0.001). The functional connectivity showed that cortico-cortical paired associative stimulation strengthens connections between brain regions. Correlation analysis confirmed that the enhancement of functional connectivity was positively correlated with the recovery of Fugl-Meyer upper-extremity scale (r2 = 0.146, P = 0.034; r2 = 0.211, P = 0.0093). The results of functional connectivity suggest that cortico-cortical paired associative stimulation strengthens connections between brain regions. It is expected that this study will provide a positive viewpoint for the neurorehabilitation of stroke patients based on the circuit-level plasticity. (Chinese Clinical Trial Registry: ChiCTR2000036685).

Mapping Brain Motor Functions Using Transcranial Magnetic Stimulation with a Volume Conductor Model and Electrophysiological Experiments

Transcranial magnetic stimulation (TMS) activates brain cells in a noninvasive manner and can be used for mapping brain motor functions. However, the complexity of the brain anatomy prevents the determination of the exact location of the stimulated sites, resulting in the limitation of the spatial resolution of multiple targets. The aim of this study is to map two neighboring muscles in cortical motor areas accurately and quickly. Multiple stimuli were applied to the subject using a TMS stimulator to measure the motor-evoked potentials (MEPs) in the corresponding muscles. For each stimulation condition (coil location and angle), the induced electric field (EF) in the brain was computed using a volume conductor model for an individualized head model of the subject constructed from magnetic resonance images. A post-processing method was implemented to determine a TMS hotspot using EF corresponding to multiple stimuli, considering the amplitude of the measured MEPs. The dependence of the computationally estimated hotspot distribution on two target muscles was evaluated (n = 11). The center of gravity of the first dorsal interosseous cortical representation was lateral to the abductor digiti minimi by a minimum of 2 mm. The localizations were consistent with the putative sites obtained from previous EF-based studies and fMRI studies. The simultaneous cortical mapping of two finger muscles was achieved with only several stimuli, which is one or two orders of magnitude smaller than that in previous studies. Our proposal would be useful in the preoperative mapping of motor or speech areas to plan brain surgery interventions.

Dissociating Language and Thought in Human Reasoning

What is the relationship between language and complex thought? In the context of deductive reasoning there are two main views. Under the first, which we label here the language-centric view, language is central to the syntax-like combinatorial operations of complex reasoning. Under the second, which we label here the language-independent view, these operations are dissociable from the mechanisms of natural language. We applied continuous theta burst stimulation (cTBS), a form of noninvasive neuromodulation, to healthy adult participants to transiently inhibit a subregion of Broca's area (left BA44) associated in prior work with parsing the syntactic relations of natural language. We similarly inhibited a subregion of dorsomedial frontal cortex (left medial BA8) which has been associated with core features of logical reasoning. There was a significant interaction between task and stimulation site. Post hoc tests revealed that performance on a linguistic reasoning task, but not deductive reasoning task, was significantly impaired after inhibition of left BA44, and performance on a deductive reasoning task, but not linguistic reasoning task, was decreased after inhibition of left medial BA8 (however not significantly). Subsequent linear contrasts supported this pattern. These novel results suggest that deductive reasoning may be dissociable from linguistic processes in the adult human brain, consistent with the language-independent view.

Active versus resting neuro-navigated robotic transcranial magnetic stimulation motor mapping

Transcranial magnetic stimulation (TMS) motor mapping is a safe, non-invasive method that can be used to study corticomotor organization. Motor maps are typically acquired at rest, and comparisons to maps obtained during muscle activation have been both limited and contradictory. Understanding the relationship between functional activation of the corticomotor system as recorded by motor mapping is crucial for their use clinically and in research. The present study utilized robotic TMS paired with personalized neuro-navigation to examine the relationship between resting and active motor map measures and their relationship with motor performance. Twenty healthy right-handed participants underwent resting and active robotic TMS motor mapping of the first dorsal interosseous to 10% maximum voluntary contraction. Motor map parameters including map area, volume, and measures of map centrality were compared between techniques using paired sample tests of difference and Bland-Altman plots and analysis. Map area, volume, and hotspot magnitude were larger in the active motor maps, while map center of gravity and hotspot locations remained consistent between both maps. No associations were observed between motor maps and motor performance as measured by the Purdue Pegboard Test. Our findings support previous suggestions that maps scale with muscle contraction. Differences in mapping outcomes suggest rest and active motor maps may reflect functionally different corticomotor representations. Advanced analysis methods may better characterize the underlying neurophysiology of both types of motor mapping.

Neural mechanisms mediating cross education: With additional considerations for the ageing brain

CALVERT, G.H.M., and CARSON, R.G. Neural mechanisms mediating cross education: With additional considerations for the ageing brain. NEUROSCI BIOBEHAV REV 21(1) XXX-XXX, 2021. - Cross education (CE) is the process whereby a regimen of unilateral limb training engenders bilateral improvements in motor function. The contralateral gains thus derived may impart therapeutic benefits for patients with unilateral deficits arising from orthopaedic injury or stroke. Despite this prospective therapeutic utility, there is little consensus concerning its mechanistic basis. The precise means through which the neuroanatomical structures and cellular processes that mediate CE may be influenced by age-related neurodegeneration are also almost entirely unknown. Notwithstanding the increased incidence of unilateral impairment in later life, age-related variations in the expression of CE have been examined only infrequently. In this narrative review, we consider several mechanisms which may mediate the expression of CE with specific reference to the ageing CNS. We focus on the adaptive potential of cellular processes that are subserved by a specific set of neuroanatomical pathways including: the corticospinal tract, corticoreticulospinal projections, transcallosal fibres, and thalamocortical radiations. This analysis may inform the development of interventions that exploit the therapeutic utility of CE training in older persons.

Efficient high-resolution TMS mapping of the human motor cortex by nonlinear regression

Transcranial magnetic stimulation (TMS) is a powerful tool to investigate causal structure-function relationships in the human brain. However, a precise delineation of the effectively stimulated neuronal populations is notoriously impeded by the widespread and complex distribution of the induced electric field. Here, we propose a method that allows rapid and feasible cortical localization at the individual subject level. The functional relationship between electric field and behavioral effect is quantified by combining experimental data with numerically modeled fields to identify the cortical origin of the modulated effect. Motor evoked potentials (MEPs) from three finger muscles were recorded for a set of random stimulations around the primary motor area. All induced electric fields were nonlinearly regressed against the elicited MEPs to identify their cortical origin. We could distinguish cortical muscle representation with high spatial resolution and localized them primarily on the crowns and rims of the precentral gyrus. A post-hoc analysis revealed exponential convergence of the method with the number of stimulations, yielding a minimum of about 180 random stimulations to obtain stable results. Establishing a functional link between the modulated effect and the underlying mode of action, the induced electric field, is a fundamental step to fully exploit the potential of TMS. In contrast to previous approaches, the presented protocol is particularly easy to implement, fast to apply, and very robust due to the random coil positioning and therefore is suitable for practical and clinical applications.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

SIGNIFICANCE

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

The Myelin Content of the Human Precentral Hand Knob Reflects Interindividual Differences in Manual Motor Control at the Physiological and Behavioral Level

The primary motor cortex hand area (M1_HAND) and adjacent dorsal premotor cortex (PMd) form the so-called motor hand knob in the precentral gyrus. M1_HAND and PMd are critical for dexterous hand use and are densely interconnected via corticocortical axons, lacking a sharp demarcating border. In 24 young right-handed volunteers, we performed multimodal mapping to delineate the relationship between structure and function in the right motor hand knob. Quantitative structural magnetic resonance imaging (MRI) at 3 tesla yielded regional R1 maps as a proxy of cortical myelin content. Participants also underwent functional MRI (fMRI). We mapped task-related activation and temporal precision, while they performed a visuomotor synchronization task requiring visually cued abduction movements with the left index or little finger. We also performed sulcus-aligned transcranial magnetic stimulation of the motor hand knob to localize the optimal site (hotspot) for evoking a motor evoked potential (MEP) in two intrinsic hand muscles. Individual motor hotspot locations varied along the rostrocaudal axis. The more rostral the motor hotspot location in the precentral crown, the longer were corticomotor MEP latencies. “Hotspot rostrality” was associated with the regional myelin content in the precentral hand knob. Cortical myelin content also correlated positively with task-related activation of the precentral crown and temporal precision during the visuomotor synchronization task. Together, our results suggest a link among cortical myelination, the spatial cortical representation, and temporal precision of finger movements. We hypothesize that the myelination of cortical axons facilitates neuronal integration in PMd and M1_HAND and, hereby, promotes the precise timing of movements.

SIGNIFICANCE STATEMENT Here we used magnetic resonance imaging and transcranial magnetic stimulation of the precentral motor hand knob to test for a link among cortical myelin content, functional corticomotor representations, and manual motor control. A higher myelin content of the precentral motor hand knob was associated with more rostral corticomotor presentations, with stronger task-related activation and a higher precision of movement timing during a visuomotor synchronization task. We propose that a high precentral myelin content enables fast and precise neuronal integration in M1 (primary motor cortex) and dorsal premotor cortex, resulting in higher temporal precision during dexterous hand use. Our results identify the degree of myelination as an important structural feature of the neocortex that is tightly linked to the function and behavior supported by the cortical area.

A generalized workflow for conducting electric field-optimized, fMRI-guided, transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive method to stimulate the cerebral cortex that has applications in psychiatry, such as in the treatment of depression and anxiety. Although many TMS targeting methods that use figure-8 coils exist, many do not account for individual differences in anatomy or are not generalizable across target sites. This protocol combines functional magnetic resonance imaging (fMRI) and iterative electric-field (E-field) modeling in a generalized approach to subject-specific TMS targeting that is capable of optimizing the stimulation site and TMS coil orientation. To apply this protocol, the user should (i) operationally define a region of interest (ROI), (ii) generate the head model from the structural MRI data, (iii) preprocess the functional MRI data, (iv) identify the single-subject stimulation site within the ROI, and (iv) conduct E-field modeling to identify the optimal coil orientation. In comparison with standard targeting methods, this approach demonstrates (i) reduced variability in the stimulation site across subjects, (ii) reduced scalp-to-cortical-target distance, and (iii) reduced variability in optimal coil orientation. Execution of this protocol requires intermediate-level skills in structural and functional MRI processing. This protocol takes ~24 h to complete and demonstrates how constrained fMRI targeting combined with iterative E-field modeling can be used as a general method to optimize both the TMS coil site and its orientation.

Motor cortical excitability and plasticity in patients with neurofibromatosis type 1

OBJECTIVE

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder that is associated with cognitive disabilities. Based on studies involving animals, the hypothesized cause of these disabilities results from increased activity of inhibitory interneurons that decreases synaptic plasticity. We obtained transcranial magnetic stimulation (TMS)-based measures of cortical inhibition, excitability and plasticity in individuals with NF1.

METHODS

We included 32 NF1 adults and 32 neurotypical controls. Cortical inhibition was measured with short-interval intracortical inhibition (SICI) and cortical silent period (CSP). Excitability and plasticity were studied with intermittent theta burst stimulation (iTBS).

RESULTS

The SICI and CSP response did not differ between NF1 adults and controls. The response upon iTBS induction was significantly increased in controls (70%) and in NF1 adults (83%). This potentiation lasted longer in controls than in individuals with NF1. Overall, the TMS response was significantly lower in NF1 patients (F(1, 41) = 7.552, p = 0.009).

CONCLUSIONS

Individuals with NF1 may have reduced excitability and plasticity, as indicated by their lower TMS response and attenuation of the initial potentiated response upon iTBS induction. However, our findings did not provide evidence for increased inhibition in NF1 patients.

SIGNIFICANCE

These findings have potential utility as neurophysiological outcome measures for intervention studies to treat cognitive deficits associated with NF1.

Altered activation in sensorimotor network after applying rTMS over the primary motor cortex at different frequencies

INTRODUCTION

Repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) can modulate brain activity both in the stimulated site and remote brain areas of the sensorimotor network. However, the modulatory effects of rTMS at different frequencies remain unclear. Here, we employed finger-tapping task-based fMRI to investigate alterations in activation of the sensorimotor network after the application of rTMS over the left M1 at different frequencies.

MATERIALS AND METHODS

Forty-five right-handed healthy participants were randomly divided into three groups by rTMS frequency (HF, high-frequency, 3 Hz; LF, low-frequency, 1 Hz; and SHAM) and underwent two task-fMRI sessions (RH, finger-tapping with right index finger; LH, finger-tapping with left index finger) before and after applying rTMS over the left M1. We defined regions of interest (ROIs) in the sensorimotor network based on group-level activation maps (pre-rTMS) from RH and LH tasks and calculated the percentage signal change (PSC) for each ROI. We then assessed the differences of PSC within HF or LF groups and between groups.

RESULTS

Application of rTMS at different frequencies resulted in a change in activation of several areas of the sensorimotor network. We observed the increased PSC in M1 after high-frequency stimulation, while we detected the reduced PSC in the primary sensory cortex (S1), ventral premotor cortex (PMv), supplementary motor cortex (SMA), and putamen after low-frequency stimulation. Moreover, the PSC in the SMA, dorsal premotor cortex (PMd), and putamen in the HF group was higher than in the LF group after stimulation.

CONCLUSION

Our findings suggested that activation alterations within sensorimotor network are dependent on the frequency of rTMS. Therefore, our findings contribute to understanding the effects of rTMS on brain activation in healthy individuals and ultimately may further help to suggest mechanisms of how rTMS could be employed as a therapeutic tool.

Targeting brain functions from the scalp: Transcranial brain atlas based on large-scale fMRI data synthesis

Transcranial brain mapping techniques, such as functional near-infrared spectroscopy (fNIRS) and transcranial magnetic stimulation (TMS), have been playing an increasingly important role in studies of human brain functions. Given a brain function of interest, fNIRS probes and TMS coils should be properly placed on the scalp to ensure that the function is effectively measured or modulated. However, since brain activity is inside the skull and invisible to the researcher during placement, this blind targeting may cause the device to partially or completely miss the functional target, resulting in inconsistent experimental results and divergent clinical outcomes, especially when participants' structural MRI data are not available. To address this issue, we propose here a framework for targeting a designated function directly from the scalp. First, a functional brain atlas for the targeted brain function is constructed via a meta-analysis of large-scale functional magnetic resonance imaging datasets. Second, the functional brain atlas is presented on the scalp surface by using a transcranial mapping previously established from an structural MRI dataset (n ​= ​114), resulting in a novel functional transcranial brain atlas (fTBA). Finally, a low-cost, portable scalp-navigation system is used to localize the transcranial device on the individual's scalp with the guidance of the fTBA. To demonstrate the feasibility of the targeting framework, both fNIRS and TMS mapping experiments were conducted. The results show that fTBA-guided fNIRS positioning can detect functional activity with high sensitivity and specificity for working memory and motor systems; Moreover, compared with traditional TMS targeting approaches (e.g. the International 10-20 System and the conventional 5-cm rule), the fTBA suggested motor stimulation site is closesr to both the motor hotspot and the center of gravity of motor evoked potentials (MEP-COG). In summary, the proposed method unblinds the transcranial function targeting process using prior information, providing an effective and straightforward approach to transcranial brain mapping studies, especially those without participants' structural MRI data.

Elucidating the Structural and Functional Correlates of Upper-Limb Poststroke Motor Impairment

Background and Purpose—

Many studies have attempted to bring to light the neural correlates of poststroke motor impairment, but few have used multimodal approach to explain it. The aim of this study was to elucidate neural structural and functional correlates of upper limb motor impairment by combining electrophysiological, anatomic, and functional neuroimaging data.

Methods—

Forty ischemic stroke patients (median [min–max] age: 63 [33–82] years, time poststroke: 3.5 [1.1–58] months) with unilateral upper limb weakness were included. The upper limb motor impairment was defined by a motor composite score. Simple linear analysis followed by multiple linear regression analysis were performed to identify which variables (corticospinal excitability, laterality indices within the primary motor cortex or corticospinal [CST], and corpus callosum tracts integrity) were the best explaining factors of upper limb motor impairment.

Results—

There was a significant correlation between the resting motor threshold ratio and CST damage ( r = −0.50 [95% CI, −0.70 to −0.22]; P <0.001) as well as the motor-evoked potentials amplitude ( r = −0.73 [95% CI, −0.85 to −0.54]; P <0.001). Only the resting motor threshold ratio was retained by the multiple regression model and explained half of the variance (49%; P <0.001) of the upper limb motor impairment after stroke.

Conclusions—

The implementation of quantitative neurophysiological measurements such as the resting motor threshold as a surrogate marker of impairment could be considered in neurorehabilitation trials.

The supplementary motor area modulates interhemispheric interactions during movement preparation

The execution of coordinated hand movements requires complex interactions between premotor and primary motor areas in the two hemispheres. The supplementary motor area (SMA) is involved in movement preparation and bimanual coordination. How the SMA controls bimanual coordination remains unclear, although there is evidence suggesting that the SMA could modulate interhemispheric interactions. With a delayed-response task, we investigated interhemispheric interactions underlying normal movement preparation and the role of the SMA in these interactions during the delay period of unimanual or bimanual hand movements. We used functional MRI and transcranial magnetic stimulation in 22 healthy volunteers (HVs), and then in two models of SMA dysfunction: (a) in the same group of HVs after transient disruption of the right SMA proper by continuous transcranial magnetic theta-burst stimulation; (b) in a group of 22 patients with congenital mirror movements (CMM), whose inability to produce asymmetric hand movements is associated with SMA dysfunction. In HVs, interhemispheric connectivity during the delay period was modulated according to whether or not hand coordination was required for the forthcoming movement. In HVs following SMA disruption and in CMM patients, interhemispheric connectivity was modified during the delay period and the interhemispheric inhibition was decreased. Using two models of SMA dysfunction, we showed that the SMA modulates interhemispheric interactions during movement preparation. This unveils a new role for the SMA and highlights its importance in coordinated movement preparation.

Does Resting Motor Threshold Predict Motor Hand Recovery After Stroke?

Background: Resting Motor threshold (rMT) is one of the measurement obtained by Transcranial Magnetic Stimulation (TMS) that reflects corticospinal excitability. As a functional marker of the corticospinal pathway, the question arises whether rMT is a suitable biomarker for predicting post-stroke upper limb function. To that aim, we conducted a systematic review of relevant studies that investigated the clinical significance of rMT in stroke survivors by using correlations between upper limb motor scores and rMT. Methods: Studies that reported correlations between upper limb motor function and rMT as a measure of corticospinal excitability in distal arm muscle were identified via a literature search in stroke patients. Two authors extracted the data using a home-made specific form. Subgroup analyses were carried out with patients classified with respect to time post-stroke onset (early vs. chronic stage) and stroke location (cortical, subcortical, or cortico-subcortical). Methodological quality of the study was also evaluated by a published checklist. Results: Eighteen studies with 22 groups (n = 508 stroke patients) were included in this systematic review. Mean methodological quality score was 14.75/24. rMT was often correlated with motor function or hand dexterity (n = 15/22, 68%), explaining on average 31% of the variance of the motor score. Moreover, the results did not seem impacted if patients were examined at the early or chronic stages of stroke. Two findings could not be properly interpreted: (i) the fact that the rMT is an independent predictor of motor function as several confounding factors are well-established, and, (ii) whether the stroke location impacts this prediction. Conclusion: Most of the studies found a correlation between rMT and upper limb motor function after stroke. However, it is still unclear if rMT is an independent predictor of upper limb motor function when taking into account for age, time post stroke onset and level of corticospinal tract damage as confounding factors. Clear-cut conclusions could not be drawn at that time but our results suggest that rMT could be a suitable candidate although future investigations are needed. Systematic Review Registration Number: (https://www.crd.york.ac.uk/prospero/): ID 114317.

Response Hand and Motor Set Differentially Modulate the Connectivity of Brain Pathways During Simple Uni-manual Motor Behavior

We investigated the flexible modulation of undirected functional connectivity (uFC) of brain pathways during simple uni-manual responding. Two questions were central to our interests: (1) does response hand (dominant vs. non-dominant) differentially modulate connectivity and (2) are these effects related to responding under varying motor sets. fMRI data were acquired in twenty right-handed volunteers who responded with their right (dominant) or left (non-dominant) hand (blocked across acquisitions). Within acquisitions, the task oscillated between periodic responses (promoting the emergence of motor sets) or randomly induced responses (disrupting the emergence of motor sets). Conjunction analyses revealed eight shared nodes across response hand and condition, time series from which were analyzed. For right hand responses connectivity of the M1 ←→ Thalamus and SMA ←→ Parietal pathways was more significantly modulated during periodic responding. By comparison, for left hand responses, connectivity between five network pairs (including M1 and SMA, insula, basal ganglia, premotor cortex, parietal cortex, thalamus) was more significantly modulated during random responding. uFC analyses were complemented by directed FC based on multivariate autoregressive models of times series from the nodes. These results were complementary and highlighted significant modulation of dFC for SMA → Thalamus, SMA → M1, basal ganglia → Insula and basal ganglia → Thalamus. The results demonstrate complex effects of motor organization and task demand and response hand on different connectivity classes of fMRI data. The brain's sub-networks are flexibly modulated by factors related to motor organization and/or task demand, and our results have implications for assessment of medical conditions associated with motor dysfunction.

Altered functional network connectivity in preterm infants: antecedents of cognitive and motor impairments?

Very preterm infants (≤ 31 weeks gestational age) are at high risk for brain injury and delayed development. Applying functional connectivity and graph theory methods to resting state MRI data (fcMRI), we tested the hypothesis that preterm infants would demonstrate alterations in connectivity measures both globally and in specific networks related to motor, language and cognitive function, even when there is no anatomical imaging evidence of injury. Fifty-one healthy full-term controls and 24 very preterm infants without significant neonatal brain injury, were evaluated at term-equivalent age with fcMRI. Preterm subjects showed lower functional connectivity from regions associated with motor, cognitive, language and executive function, than term controls. Examining brain networks using graph theory measures of functional connectivity, very preterm infants also exhibited lower rich-club coefficient and assortativity but higher small-worldness and no significant difference in modularity when compared to term infants. The findings provide evidence that functional connectivity exhibits deficits soon after birth in very preterm infants in key brain networks responsible for motor, language and executive functions, even in the absence of anatomical lesions. These functional network measures could serve as prognostic biomarkers for later developmental disabilities and guide decisions about early interventions.

Anatomical and functional correlates of cortical motor threshold of the dominant hand

BACKGROUND

Resting Motor threshold (rMT) provides information about cortical motor excitability. Interestingly, the influences of the structural or functional variability of the motor system on the rMT inter-individual variability have been poorly investigated.

OBJECTIVE/HYPOTHESIS

To investigate relationships between rMT and measures of brain structures and function of the motor system. The hypothesis is that cortical excitability not only depends on the primary motor cortex (M1) but also on the integration of information originating from its vicinity such as premotor (PMd and SMA) and post-central (S1) cortices.

METHODS

We measured brain structures, including grey and white matter properties (cortical volume and fiber coherence respectively), and functional interaction (resting-state functional connectivity-FC) in areas contributing to the corticospinal tract axons, i. e, M1, S1, SMA and PMd in the dominant hemisphere of 21 healthy subjects.

RESULTS

The rMT was inversely correlated with the FC between PMd and M1 (r = -0.496, 95%CI: -0.764; -0.081; p = 0.02) and the grey matter volume of the dominant hemisphere (r = -0.463, 95%CI: -0.746; -0.039; p = 0.03). The multiple regression analysis model retained the FC between M1 and PMd (coefficient: -25 ± 9) as well as the grey matter volume of the dominant hemisphere (coefficient: -0.15 ± 0.06) explaining 44% of the variance of the rMT (p: 0.005). When adding age and coil-to-cortex distance, two factors known to influence rMT, the model reached a R2 of 75% (p: 0.0001).

CONCLUSIONS

These results underline the major role of the PMd and the cortico-cortical connections toward M1 in the excitation of the corticospinal fibers likely through trans-synaptic pathways.

Advancing non-invasive neuromodulation clinical trials in children: Lessons from perinatal stroke

Applications of non-invasive brain stimulation including therapeutic neuromodulation are expanding at an alarming rate. Increasingly established scientific principles, including directional modulation of well-informed cortical targets, are advancing clinical trial development. However, high levels of disease burden coupled with zealous enthusiasm may be getting ahead of rational research and evidence. Experience is limited in the developing brain where additional issues must be considered. Properly designed and meticulously executed clinical trials are essential and required to advance and optimize the potential of non-invasive neuromodulation without risking the well-being of children and families. Perinatal stroke causes most hemiplegic cerebral palsy and, as a focal injury of defined timing in an otherwise healthy brain, is an ideal human model of developmental plasticity. Advanced models of how the motor systems of young brains develop following early stroke are affording novel windows of opportunity for neuromodulation clinical trials, possibly directing neuroplasticity toward better outcomes. Reviewing the principles of clinical trial design relevant to neuromodulation and using perinatal stroke as a model, this article reviews the current and future issues of advancing such trials in children.

Functional connectivity and information flow of the respiratory neural network in chronic obstructive pulmonary disease

Breathing involves a complex interplay between the brainstem automatic network and cortical voluntary command. How these brain regions communicate at rest or during inspiratory loading is unknown. This issue is crucial for several reasons: (i) increased respiratory loading is a major feature of several respiratory diseases, (ii) failure of the voluntary motor and cortical sensory processing drives is among the mechanisms that precede acute respiratory failure, (iii) several cerebral structures involved in responding to inspiratory loading participate in the perception of dyspnea, a distressing symptom in many disease. We studied functional connectivity and Granger causality of the respiratory network in controls and patients with chronic obstructive pulmonary disease (COPD), at rest and during inspiratory loading. Compared with those of controls, the motor cortex area of patients exhibited decreased connectivity with their contralateral counterparts and no connectivity with the brainstem. In the patients, the information flow was reversed at rest with the source of the network shifted from the medulla towards the motor cortex. During inspiratory loading, the system was overwhelmed and the motor cortex became the sink of the network. This major finding may help to understand why some patients with COPD are prone to acute respiratory failure. Network connectivity and causality were related to lung function and illness severity. We validated our connectivity and causality results with a mathematical model of neural network. Our findings suggest a new therapeutic strategy involving the modulation of brain activity to increase motor cortex functional connectivity and improve respiratory muscles performance in patients. Hum Brain Mapp 37:2736–2754, 2016. © 2016 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc.

A comprehensive study of sensorimotor cortex excitability in chronic cocaine users: Integrating TMS and functional MRI data

BACKGROUND

Disruptions in motor control are often overlooked features of chronic cocaine users. During a simple sensorimotor integration task, for example, cocaine users activate a larger area of cortex than controls but have lower functional connectivity between the cortex and dorsal striatum, which is further correlated with poor performance. The purpose of this study was to determine whether abnormal cortical excitability in cocaine users was related to disrupted inhibitory or excitatory mechanisms, as measured by transcranial magnetic stimulation (TMS).

METHODS

A battery of TMS measures were acquired from 87 individuals (50 cocaine dependent, 37 controls). Functional MRI data were acquired from a subset of 28 individuals who performed a block-design finger tapping task.

RESULTS

TMS measures revealed that cocaine users had significantly higher resting motor thresholds and higher intracortical cortical facilitation (ICF) than controls. There was no between-group difference in either measure of cortical inhibition. Task-evoked BOLD signal in the motor cortex was significantly correlated with ICF in the cocaine users. There was no significant difference in brain-skull distance between groups.

CONCLUSION

These data demonstrated that cocaine users have disrupted cortical facilitation (as measured with TMS), which is related to elevated BOLD signal. Cortical inhibition, however, is largely intact. Given the relationship between ICF and glutamatergic agents, this may be a potentially fruitful and treatable target in addiction. Finally, among controls the distance from the scalp to the cortex was correlated with the motor threshold which may be a useful parameter to integrate into therapeutic TMS protocols in the future.

Inter-individual variability in cortical excitability and motor network connectivity following multiple blocks of rTMS

The responsiveness to non-invasive neuromodulation protocols shows high inter-individual variability, the reasons of which remain poorly understood. We here tested whether the response to intermittent theta-burst stimulation (iTBS) - an effective repetitive transcranial magnetic stimulation (rTMS) protocol for increasing cortical excitability - depends on network properties of the cortical motor system. We furthermore investigated whether the responsiveness to iTBS is dose-dependent. To this end, we used a sham-stimulation controlled, single-blinded within-subject design testing for the relationship between iTBS aftereffects and (i) motor-evoked potentials (MEPs) as well as (ii) resting-state functional connectivity (rsFC) in 16 healthy subjects. In each session, three blocks of iTBS were applied, separated by 15min. We found that non-responders (subjects not showing an MEP increase of ≥10% after one iTBS block) featured stronger rsFC between the stimulated primary motor cortex (M1) and premotor areas before stimulation compared to responders. However, only the group of responders showed increases in rsFC and MEPs, while most non-responders remained close to baseline levels after all three blocks of iTBS. Importantly, there was still a large amount of variability in both groups. Our data suggest that responsiveness to iTBS at the local level (i.e., M1 excitability) depends upon the pre-interventional network connectivity of the stimulated region. Of note, increasing iTBS dose did not turn non-responders into responders. The finding that higher levels of pre-interventional connectivity precluded a response to iTBS could reflect a ceiling effect underlying non-responsiveness to iTBS at the systems level.

Neurorehabilitation: motor recovery after stroke as an example

The field of neurorehabilitation aims to translate neuroscience research toward the goal of maximizing functional recovery after neurological injury. A growing body of research indicates that the fundamental principles of neurological rehabilitation are applicable to a broad range of congenital, degenerative, and acquired neurological disorders. In this perspective, we will focus on motor recovery after acquired brain injuries such as stroke. Over the past few decades, a large body of basic and clinical research has created an experimental and theoretical foundation for approaches to neurorehabilitation. Recent randomized clinical trials all emphasize the requirement for intense progressive rehabilitation programs to optimally enhance recovery. Moreover, advances in multimodal assessment of patients with neuroimaging and neurophysiological tools suggest the possibility of individualized treatment plans based on recovery potential. There are also promising indications for medical as well as noninvasive brain stimulation paradigms to facilitate recovery. Ongoing or planned clinical studies should provide more definitive evidence. We also highlight unmet needs and potential areas of research. Continued research built upon a robust experimental and theoretical foundation should help to develop novel treatments to improve recovery after neurological injury.

Assessment of inter-hemispheric imbalance using imaging and noninvasive brain stimulation in patients with chronic stroke

OBJECTIVE

To determine how interhemispheric balance in stroke, measured using transcranial magnetic stimulation (TMS), relates to balance defined using neuroimaging (functional magnetic resonance [fMRI], diffusion-tensor imaging [DTI]) and how these metrics of balance are associated with clinical measures of upper-limb function and disability.

DESIGN

Cross sectional.

SETTING

Laboratory.

PARTICIPANTS

Patients with chronic stroke (N = 10; age, 63 ± 9 y) in a population-based sample with unilateral upper-limb paresis.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Interhemispheric balance was measured with TMS, fMRI, and DTI. TMS defined interhemispheric differences in the recruitment of corticospinal output, size of the corticomotor output maps, and degree of mutual transcallosal inhibition that they exerted on one another. fMRI studied whether cortical activation during the movement of the paretic hand was lateralized to the ipsilesional or to the contralesional primary motor cortex (M1), premotor cortex (PMC), and supplementary motor cortex (SMA). DTI was used to define interhemispheric differences in the integrity of the corticospinal tracts projecting from the M1. Clinical outcomes tested function (upper extremity Fugl-Meyer [UEFM]) and perceived disability in the use of the paretic hand (Motor Activity Log [MAL] amount score).

RESULTS

Interhemispheric balance assessed with TMS relates differently to fMRI and DTI. Patients with high fMRI lateralization to the ipsilesional hemisphere possessed stronger ipsilesional corticomotor output maps (M1: r = .831, P = .006; PMC: r = .797, P = .01) and better balance of mutual transcallosal inhibition (r = .810, P = .015). Conversely, we found that patients with less integrity of the corticospinal tracts in the ipsilesional hemisphere show greater corticospinal output of homologous tracts in the contralesional hemisphere (r = .850, P = .004). However, an imbalance in integrity and output do not relate to transcallosal inhibition. Clinically, although patients with less integrity of corticospinal tracts from the ipsilesional hemisphere showed worse impairments (UEFM) (r = -.768, P = .016), those with low fMRI lateralization to the ipsilesional hemisphere had greater perception of disability (MAL amount score) (M1: r = .883, P = .006; PMC: r = .817, P = .007; SMA: r = .633, P = .062).

CONCLUSIONS

In patients with chronic motor deficits of the upper limb, fMRI may serve to mark perceived disability and transcallosal influence between hemispheres. DTI-based integrity of the corticospinal tracts, however, may be useful in categorizing the range of functional impairments of the upper limb. Further, in patients with extensive corticospinal damage, DTI may help infer the role of the contralesional hemisphere in recovery.

Dose-dependent effects of theta burst rTMS on cortical excitability and resting-state connectivity of the human motor system

Theta burst stimulation (TBS), a specific protocol of repetitive transcranial magnetic stimulation (rTMS), induces changes in cortical excitability that last beyond stimulation. TBS-induced aftereffects, however, vary between subjects, and the mechanisms underlying these aftereffects to date remain poorly understood. Therefore, the purpose of this study was to investigate whether increasing the number of pulses of intermittent TBS (iTBS) (1) increases cortical excitability as measured by motor-evoked potentials (MEPs) and (2) alters functional connectivity measured using resting-state fMRI, in a dose-dependent manner. Sixteen healthy, human subjects received three serially applied iTBS blocks of 600 pulses over the primary motor cortex (M1 stimulation) and the parieto-occipital vertex (sham stimulation) to test for dose-dependent iTBS effects on cortical excitability and functional connectivity (four sessions in total). iTBS over M1 increased MEP amplitudes compared with sham stimulation after each stimulation block. Although the increase in MEP amplitudes did not differ between the first and second block of M1 stimulation, we observed a significant increase after three blocks (1800 pulses). Furthermore, iTBS enhanced resting-state functional connectivity between the stimulated M1 and premotor regions in both hemispheres. Functional connectivity between M1 and ipsilateral dorsal premotor cortex further increased dose-dependently after 1800 pulses of iTBS over M1. However, no correlation between changes in MEP amplitudes and functional connectivity was detected. In summary, our data show that increasing the number of iTBS stimulation blocks results in dose-dependent effects at the local level (cortical excitability) as well as at a systems level (functional connectivity) with a dose-dependent enhancement of dorsal premotor cortex-M1 connectivity.

Connectivity-based approaches in stroke and recovery of function

After focal damage, cerebral networks reorganise their structural and functional anatomy to compensate for both the lesion itself and remote effects. Novel developments in the analysis of functional neuroimaging data enable us to assess in vivo the specific contributions of individual brain areas to recovery of function and the effect of treatment on cortical reorganisation. Connectivity analyses can be used to investigate the effect of stroke on cerebral networks, and help us to understand why some patients make a better recovery than others. This systems-level view also provides insights into how neuromodulatory interventions might target pathological network configurations associated with incomplete recovery. In the future, such analyses of connectivity could help to optimise treatment regimens based on the individual network pathology underlying a particular neurological deficit, thereby opening the way for stratification of patients based on the possible response to an intervention.

Effects of aerobic fitness on aging-related changes of interhemispheric inhibition and motor performance

Physical fitness has been long associated with maintenance and improvement of motor performance as we age. In particular, measures of psychomotor speed and motor dexterity tend to be higher in physically fit aging adults as compared to their sedentary counterparts. Using functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS), we explored the patterns of neural activity that may, in part, account for differences between individuals of varying physical fitness levels. In this study, we enrolled both sedentary and physically fit middle age (40–60) and younger (18–30) adults and measured upper extremity motor performance during behavioral testing. In a follow-up session, we employed TMS and fMRI to assess levels of interhemispheric communication during unimanual tasks. Results show that increased physical fitness is associated with better upper extremity motor performance on distal dexterity assessments and increased levels of interhemispheric inhibition in middle age adults. Further, the functional correlates of changes of ipsilateral activity appears to be restricted to the aging process as younger adults of varying fitness levels do not differ in hemispheric patterns of activity or motor performance. We conclude that sedentary aging confers a loss of interhemispheric inhibition that is deleterious to some aspects of motor function, as early as midlife, but these changes can be mediated by chronic engagement in aerobic exercise.

Reappraisal of the anatomical landmarks of motor and premotor cortical regions for image-guided brain navigation in TMS practice

Image-guided navigation systems dedicated to transcranial magnetic stimulation (TMS) have been recently developed and offer the possibility to visualize directly the anatomical structure to be stimulated. Performing navigated TMS requires a perfect knowledge of cortical anatomy, which is very variable between subjects. This study aimed at providing a detailed description of sulcal and gyral anatomy of motor cortical regions with special interest to the inter-individual variability of sulci. We attempted to identify the most stable structures, which can serve as anatomical landmarks for motor cortex mapping in navigated TMS practice. We analyzed the 3D reconstruction of 50 consecutive healthy adult brains (100 hemispheres). Different variants were identified regarding sulcal morphology, but several anatomical structures were found to be remarkably stable (four on dorsoventral axis and five on rostrocaudal axis). These landmarks were used to define a grid of 12 squares, which covered motor cortical regions. This grid was used to perform motor cortical mapping with navigated TMS in 12 healthy subjects from our cohort. The stereotactic coordinates (x-y-z) of the center of each of the 12 squares of the mapping grid were expressed into the standard Talairach space to determine the corresponding functional areas. We found that the regions whose stimulation produced almost constantly motor evoked potentials mainly correspond to the primary motor cortex, with rostral extension to premotor cortex and caudal extension to posterior parietal cortex. Our anatomy-based approach should facilitate the expression and the comparison of the results obtained in motor mapping studies using navigated TMS.

The EEG correlates of the TMS-induced EMG silent period in humans

Application of magnetic or electrical stimulation to the motor cortex can result in a period of electromyography (EMG) silence in a tonically active peripheral muscle. This period of EMG silence is referred to as the silent period (SP). The duration of SP shows intersubject variability and reflects the integrity of cortical and corticospinal pathways. A non-invasive technique for assessing the duration of SP is the combination of Transcranial Magnetic Stimulation (TMS) with EMG. Utilizing TMS-EMG, several studies have reported on the shortening or lengthening of SP in neuropsychiatric disorders such as schizophrenia, bipolar disorder, depression, obsessive compulsive disorder, epilepsy, Parkinson's disease, and stroke. However, cortical, corticospinal and peripheral components are difficult to disentangle from EMG alone. Here, we use the multimodal neuroimaging technique of TMS-EMG combined with concurrent electroencephalography (EEG) recording to further examine the cortical origin of SP and the cortical oscillatory activity that underlies SP genesis. We demonstrate that the duration of SP is related to the temporal characteristics of the cortical reactivity and the power of delta to alpha oscillations in both local and remote areas ipsilateral and contralateral to the stimulation site, and beta oscillations locally. We illustrate that, compared to EMG, the EEG indices of the SP provide additional information about the brain dynamics and propose that the EEG measures of SP may be used in future clinical and research investigations to more precisely delineate the mechanisms underlying inhibitory impairments.

Can noninvasive brain stimulation measure and modulate developmental plasticity to improve function in stroke-induced cerebral palsy?

The permanent nature of motor deficits is a consistent cornerstone of cerebral palsy definitions. Such pessimism is disheartening to children, families, and researchers alike and may no longer be appropriate for it ignores the fantastic plastic potential of the developing brain. Perinatal stroke is presented as the ideal human model of developmental neuroplasticity following distinct, well-defined, focal perinatal brain injury. Elegant animal models are merging with human applied technology methods, including noninvasive brain stimulation for increasingly sophisticated models of plastic motor development following perinatal stroke. In this article, how potential central therapeutic targets are identified and potentially modulated to enhance motor function within these models is discussed. Also, future directions and emerging clinical trials are reviewed.

Interregional compensatory mechanisms of motor functioning in progressing preclinical neurodegeneration

Understanding brain reserve in preclinical stages of neurodegenerative disorders allows determination of which brain regions contribute to normal functioning despite accelerated neuronal loss. Besides the recruitment of additional regions, a reorganisation and shift of relevance between normally engaged regions are a suggested key mechanism. Thus, network analysis methods seem critical for investigation of changes in directed causal interactions between such candidate brain regions. To identify core compensatory regions, fifteen preclinical patients carrying the genetic mutation leading to Huntington's disease and twelve controls underwent fMRI scanning. They accomplished an auditory paced finger sequence tapping task, which challenged cognitive as well as executive aspects of motor functioning by varying speed and complexity of movements. To investigate causal interactions among brain regions a single Dynamic Causal Model (DCM) was constructed and fitted to the data from each subject. The DCM parameters were analysed using statistical methods to assess group differences in connectivity, and the relationship between connectivity patterns and predicted years to clinical onset was assessed in gene carriers.

In preclinical patients, we found indications for neural reserve mechanisms predominantly driven by bilateral dorsal premotor cortex, which increasingly activated superior parietal cortices the closer individuals were to estimated clinical onset. This compensatory mechanism was restricted to complex movements characterised by high cognitive demand. Additionally, we identified task-induced connectivity changes in both groups of subjects towards pre- and caudal supplementary motor areas, which were linked to either faster or more complex task conditions. Interestingly, coupling of dorsal premotor cortex and supplementary motor area was more negative in controls compared to gene mutation carriers. Furthermore, changes in the connectivity pattern of gene carriers allowed prediction of the years to estimated disease onset in individuals.

Our study characterises the connectivity pattern of core cortical regions maintaining motor function in relation to varying task demand. We identified connections of bilateral dorsal premotor cortex as critical for compensation as well as task-dependent recruitment of pre- and caudal supplementary motor area. The latter finding nicely mirrors a previously published general linear model-based analysis of the same data. Such knowledge about disease specific inter-regional effective connectivity may help identify foci for interventions based on transcranial magnetic stimulation designed to stimulate functioning and also to predict their impact on other regions in motor-associated networks.

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Connectivity of a motor network is altered in preclinical neurodegeneration.

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Dynamic Causal Modelling reveals task-dependent recruitment of pre- and caudal SMA.

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Connectivity of the dorsal premotor cortex reveals compensatory mechanisms.

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DCM allows prediction of years to clinical onset in preclinical patients.

Modeling developmental plasticity after perinatal stroke: defining central therapeutic targets in cerebral palsy

Perinatal stroke is presented as the ideal human model of developmental neuroplasticity. The precise timing, mechanisms, and locations of specific perinatal stroke diseases provide common examples of well defined, focal, perinatal brain injuries. Motor disability (hemiparetic cerebral palsy) constitutes the primary adverse outcome and the focus of models explaining how motor systems develop in health and after early injury. Combining basic science animal work with human applied technology (functional magnetic resonance imaging, diffusion tensor imaging, and transcranial magnetic stimulation), a model of plastic motor development after perinatal stroke is presented. Potential central therapeutic targets are revealed. The means to measure and modulate these targets, including evidence-based rehabilitation therapies and noninvasive brain stimulation, are suggested. Implications for clinical trials and future directions are discussed.

Changes in callosal motor fiber integrity after subcortical stroke of the pyramidal tract

In the healthy brain, there are close correlations between task-related activation of the primary motor cortex (M1), the magnitude of interhemispheric inhibition, and microstructural properties of transcallosal fiber tracts. After subcortical stroke affecting the pyramidal tract (PT), an abnormal pattern of bilateral activity develops in M1. With this prospective longitudinal study, we aimed to determine whether a morphological correlate of poststroke disinhibition could be measured within 20 days and 6 months of PT stroke. Using diffusion tensor imaging with tractography, we delineated transcallosal motor fibers (CMF) in nine PT stroke patients, six patients with subcortical infarct not affecting the PT (NonPT) and six transient ischemic attack patients. We compared changes in CMF fractional anisotropy ratios (rFA) with rFA in a distinct bundle of callosal occipital fibers (COF). At the initial time point, there were no significant differences in rFA between groups and fiber bundles. At follow-up, PT-group rFA(CMF) was significantly lower than PT-group rFA(COF) and NonPT-group rFA(CMF). PT-group rFA(CMF) decreased over time and correlated with rFA of the PT (rFA(PT)) retrograde to the infarct at 6 months. Our data suggest a progressive degenerative transsynaptic effect of PT stroke on CMF, which could be a morphological correlate of transcallosal disinhibition.

Mechanical flutter stimulation induces a lasting response in the sensorimotor cortex as revealed with BOLD fMRI

It has been recently shown that 20 min of mechanical flutter stimulation induces lasting motor cortical excitability changes, as assessed by transcranial magnetic stimulation in relaxed hand muscles. The present functional magnetic resonance imaging (fMRI) study aims to examine if such neuromodulatory changes are reflected in the BOLD signal during a motor test. Therefore, two groups were recruited: one group receiving whole-hand flutter stimulation with a frequency of 25 Hz (FSTIM group, n = 22) and a second group receiving no stimulation (NOSTIM group, n = 22). As motor test finger-to-thumb tapping was performed to activate a wide sensorimotor network during the fMRI measurements. Three fMRI measurements were obtained with this test: before stimulation (PRE), after stimulation (POST1), and 1 h after stimulation (POST2). Three regions of interest (ROIs) were defined: primary motor area (M1), primary somatosensory area (S1), and supplementary motor area. In the absence of baseline differences between both groups, the FSTIM group showed increased movement-related brain activations compared with the NOSTIM group, both at POST1 and POST2. ROI analysis revealed increased blood-oxygenation-level-dependent (BOLD) responses within contralateral S1 (+20%) and M1 (+25%) at POST1, which lasted until POST2. These poststimulatory effects within S1 and M1 obviously reflect neuroplastic changes associated with augmented cortical excitability. These findings are of high clinical relevance, for example, to improve the treatment of stroke patients.

Increased primary motor cortical excitability by a single-pulse transcranial magnetic stimulation over the supplementary motor area

The supplementary motor area (SMA) is a secondary motor area that is involved in various complex hand movements. In animal studies, short latency and probably direct excitatory inputs from SMA to the primary motor cortex (M1) have been established. Although human imaging studies revealed functional connectivity between SMA and M1, its electrophysiological nature has been less studied. This study explored the connection between SMA and M1 in humans using a single-pulse transcranial magnetic stimulation (TMS) over SMA. First, TMS over SMA did not alter the corticospinal tract excitability measured by the size of motor evoked potential elicited by single-pulse TMS over M1. Next, we measured short-interval intracortical facilitation (SICF), which reflects the function of a facilitatory circuit within M1, with or without a single-pulse TMS over SMA. When the intensity of the second pulse in the SICF paradigm (S2) was as weak as 1.0 active motor threshold for a hand muscle, SMA stimulation significantly enhanced the SICF. Furthermore, this enhancement by SMA stimulation was spatially confined and had a limited time window. On the other hand, SMA stimulation did not alter short-interval intracortical inhibition or contralateral silent period duration, which reflects the function of an inhibitory circuit mediated by gamma-aminobutyric acid A (GABA(A)) or GABA(B) receptors, respectively. We conclude that a single-pulse TMS over SMA modulates a facilitatory circuit within M1.

Motor map reliability and aging: a TMS/fMRI study

This study compared the reliability of motor maps over 3 sessions from both neuronavigated transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) data between younger and older adults. Seven younger (ages 19-31) and seven older (ages 64-76) adults participated in three joint TMS/fMRI assessment sessions separated by 7 or 14 days. Sessions involved mapping of the right first dorsal interosseous muscle using single-pulse TMS immediately followed by block-design fMRI scanning involving volitional right-hand index finger to thumb oppositional squeeze. Intersession reliability of map volume, evaluated by intraclass correlation and Jaccard Coefficient between testing sessions, was more consistent for younger adults in both fMRI and TMS. A positive correlation was evidenced between fMRI and TMS map volumes and Jaccard Coefficients indicating spatial consistency across sessions between the two measures. Comparisons of map reliability between age groups showed that younger adults have more stable motor maps in both fMRI and TMS. fMRI and TMS maps show consistency across modalities. Future interpretation of motor maps should attempt to account for potential increased variability of such mapping in older age groups. Despite these age group differences in reliability, fMRI and TMS appear to offer consistent and complementary information about cortical representation of the first dorsal interosseous muscle.

State-dependent and timing-dependent bidirectional associative plasticity in the human SMA-M1 network

The supplementary motor area (SMA-proper) plays a key role in the preparation and execution of voluntary movements. Anatomically, SMA-proper is densely reciprocally connected to primary motor cortex (M1), but neuronal coordination within the SMA-M1 network and its modification by external perturbation are not well understood. Here we modulated the SMA-M1 network using MR-navigated multicoil associative transcranial magnetic stimulation in healthy subjects. Changes in corticospinal excitability were assessed by recording motor evoked potential (MEP) amplitude bilaterally in a hand muscle. We found timing-dependent bidirectional Hebbian-like MEP changes during and for at least 30 min after paired associative SMA-M1 stimulation. MEP amplitude increased if SMA stimulation preceded M1 stimulation by 6 ms, but decreased if SMA stimulation lagged M1 stimulation by 15 ms. This associative plasticity in the SMA-M1 network was highly topographically specific because paired associative stimulation of pre-SMA and M1 did not result in any significant MEP change. Furthermore, associative plasticity in the SMA-M1 network was strongly state-dependent because it required priming by near-simultaneous M1 stimulation to occur. We conclude that timing-dependent bidirectional associative plasticity is demonstrated for the first time at the systems level of a human corticocortical neuronal network. The properties of this form of plasticity are fully compatible with spike-timing-dependent plasticity as defined at the cellular level. The necessity of priming may reflect the strong interhemispheric connectivity of the SMA-M1 network. Findings are relevant for better understanding reorganization and potentially therapeutic modification of neuronal coordination in the SMA-M1 network after cerebral lesions such as stroke.

Optimizing experimental design for comparing models of brain function

This article presents the first attempt to formalize the optimization of experimental design with the aim of comparing models of brain function based on neuroimaging data. We demonstrate our approach in the context of Dynamic Causal Modelling (DCM), which relates experimental manipulations to observed network dynamics (via hidden neuronal states) and provides an inference framework for selecting among candidate models. Here, we show how to optimize the sensitivity of model selection by choosing among experimental designs according to their respective model selection accuracy. Using Bayesian decision theory, we (i) derive the Laplace-Chernoff risk for model selection, (ii) disclose its relationship with classical design optimality criteria and (iii) assess its sensitivity to basic modelling assumptions. We then evaluate the approach when identifying brain networks using DCM. Monte-Carlo simulations and empirical analyses of fMRI data from a simple bimanual motor task in humans serve to demonstrate the relationship between network identification and the optimal experimental design. For example, we show that deciding whether there is a feedback connection requires shorter epoch durations, relative to asking whether there is experimentally induced change in a connection that is known to be present. Finally, we discuss limitations and potential extensions of this work.

Transcranial magnetic stimulation intensities in cognitive paradigms

Background

Transcranial magnetic stimulation (TMS) has become an important experimental tool for exploring the brain's functional anatomy. As TMS interferes with neural activity, the hypothetical function of the stimulated area can thus be tested. One unresolved methodological issue in TMS experiments is the question of how to adequately calibrate stimulation intensities. The motor threshold (MT) is often taken as a reference for individually adapted stimulation intensities in TMS experiments, even if they do not involve the motor system. The aim of the present study was to evaluate whether it is reasonable to adjust stimulation intensities in each subject to the individual MT if prefrontal regions are stimulated prior to the performance of a cognitive paradigm.

Methods and Findings

Repetitive TMS (rTMS) was applied prior to a working memory task, either at the ‘fixed’ intensity of 40% maximum stimulator output (MSO), or individually adapted at 90% of the subject's MT. Stimulation was applied to a target region in the left posterior middle frontal gyrus (pMFG), as indicated by a functional magnetic resonance imaging (fMRI) localizer acquired beforehand, or to a control site (vertex). Results show that MT predicted the effect size after stimulating subjects with the fixed intensity (i.e., subjects with a low MT showed a greater behavioral effect). Nevertheless, the individual adaptation of intensities did not lead to stable effects.

Conclusion

Therefore, we suggest assessing MT and account for it as a measure for general cortical TMS susceptibility, even if TMS is applied outside the motor domain.