Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control

Genetic influences on neural plasticity

Neural plasticity refers to the capability of the brain to alter function or structure in response to a range of events and is a crucial component of both functional recovery after injury and skill learning in healthy individuals. A number of factors influence neural plasticity and recovery of function after brain injury. The current review considers the impact of genetic factors. Polymorphisms in the human genes coding for brain-derived neurotrophic factor and apolipoprotein E have been studied in the context of plasticity and stroke recovery and are discussed here in detail. Several processes involved in plasticity and stroke recovery, such as depression or pharmacotherapy effects, are modulated by other genetic polymorphisms and are also discussed. Finally, new genetic polymorphisms that have not been studied in the context of stroke are proposed as new directions for study. A better understanding of genetic influences on recovery and response to therapy might allow improved treatment after a number of forms of central nervous system injury.

Interhemispheric interactions between the human primary somatosensory cortices

In the somatosensory domain it is still unclear at which processing stage information reaches the opposite hemispheres. Due to dense transcallosal connections, the secondary somatosensory cortex (S2) has been proposed to be the key candidate for interhemispheric information transfer. However, recent animal studies showed that the primary somatosensory cortex (S1) might as well account for interhemispheric information transfer. Using paired median nerve somatosensory evoked potential recordings in humans we tested the hypothesis that interhemispheric inhibitory interactions in the somatosensory system occur already in an early cortical processing stage such as S1. Conditioning right S1 by electrical median nerve (MN) stimulation of the left MN (CS) resulted in a significant reduction of the N20 response in the target (left) S1 relative to a test stimulus (TS) to the right MN alone when the interstimulus interval between CS and TS was between 20 and 25 ms. No such changes were observed for later cortical components such as the N20/P25, N30, P40 and N60 amplitude. Additionally, the subcortically generated P14 response in left S1 was also not affected. These results document the existence of interhemispheric inhibitory interactions between S1 in human subjects in the critical time interval of 20–25 ms after median nerve stimulation.

Interhemispheric asymmetry of corticomotor excitability after chronic cerebellar infarcts

Early after stroke, there is loss of intracortical facilitation (ICF) and increase in short-interval intracortical inhibition (SICI) in the primary motor cortex (M1) contralateral to a cerebellar infarct. Our goal was to investigate intracortical M1 function in the chronic stage following cerebellar infarcts (>4 months). We measured resting motor threshold (rMT), SICI, ICF, and ratios between motor-evoked potential amplitudes (MEP) and supramaximal M response amplitudes (MEP/M; %), after transcranial magnetic stimulation was applied to the M1 contralateral (M1(contralesional)) and ipsilateral (M1(ipsilesional)) to the cerebellar infarct in patients and to both M1s of healthy age-matched volunteers. SICI was decreased in M1(contralesional) compared to M1(ipsilesional) in the patient group in the absence of side-to-side differences in controls. There were no significant interhemispheric or between-group differences in rMT, ICF, or MEP/M (%). Our results document disinhibition of M1(contralesional) in the chronic phase after cerebellar stroke.

Human locomotor adaptive learning is proportional to depression of cerebellar excitability

Human locomotor adaptive learning is thought to involve the cerebellum, but the neurophysiological mechanisms underlying this process are not known. While animal research has pointed to depressive modulation of cerebellar outputs, a direct correlation between adaptive learning and cerebellar depression has never been demonstrated. Here, we used transcranial magnetic stimulation to assess excitability changes occurring in the cerebellum and primary motor cortex (M1) after individuals learned a new locomotor pattern on a split-belt treadmill. To control for potential changes associated to task performance complexity, the same group of subjects was also assessed after performing 2 other locomotor tasks that did not elicit learning. We found that only adaptive learning resulted in reduction of cerebellar inhibition. This effect was strongly correlated with the magnitude of learning (r = 0.78). In contrast, M1 excitability changes were not specific to learning but rather occurred in association with task complexity performance. Our results demonstrate that locomotor adaptive learning in humans is proportional to cerebellar excitability depression. This finding supports the theory that adaptive learning is mediated, at least in part, by long-term depression in Purkinje cells. This knowledge opens the opportunity to target cerebellar processes with noninvasive brain stimulation to enhance motor learning.

The dynamic regulation of cortical excitability is altered in episodic ataxia type 2

Episodic ataxia type 2 and familial hemiplegic migraine are two rare hereditary disorders that are linked to dysfunctional ion channels and are characterized clinically by paroxysmal neurological symptoms. Impaired regulation of cerebral excitability is thought to play a role in the occurrence of these paroxysms, but the underlying mechanisms are poorly understood. Normal ion channels are crucial for coordinating neuronal firing in response to facilitatory input. Thus, we hypothesized that channel dysfunction in episodic ataxia type 2 and familial hemiplegic migraine may impair the ability to adjust cerebral excitability after facilitatory events. We tested this hypothesis in patients with episodic ataxia type 2 (n = 6), patients with familial hemiplegic migraine (n = 7) and healthy controls (n = 13). All subjects received a high-frequency burst (10 pulses at 20 Hz) of transcranial magnetic stimulation to transiently increase the excitability of the motor cortex. Acute burst-induced excitability changes were probed at 50, 250, 500 and 1000 ms after the end of the burst. This was done using single-pulse transcranial magnetic stimulation to assess corticospinal excitability, and paired-pulse transcranial magnetic stimulation at an interstimulus interval of 2 and 10 ms to assess intracortical inhibition and facilitation, respectively. The time course of burst-induced excitability changes differed between groups. Healthy controls showed a short-lived increase in excitability that was only present 50 ms after the burst. In contrast, patients with episodic ataxia type 2 showed an abnormally prolonged increase in corticospinal excitability that was still present 250 ms after the transcranial magnetic stimulation burst. Furthermore, while controls showed a decrease in intracortical facilitation during the 1 s period following the transcranial magnetic stimulation burst, patients with episodic ataxia type 2 had increased intracortical facilitation 1000 ms after the burst. Intracortical inhibition was unaltered between groups. Patients with familial hemiplegic migraine were not significantly different from either controls or patients with episodic ataxia type 2. Together, these findings indicate that patients with episodic ataxia type 2 have an excessive increase in motor cortex excitability following a strong facilitatory input. We argue that this deficient control of cortical excitability may set the stage for the emergence of paroxysmal neural dysfunction in this disorder.

Goal-directed visuomotor skill learning: off-line enhancement and the importance of the primary motor cortex

PURPOSE

The time course and neural substrates of motor skill learning are not well-understood in healthy or neurologic patient populations. Certain motor skills undergo off-line skill enhancement following training and the primary motor cortex (M1) may be involved. It is unknown if goal-directed visuomotor skill undergoes off-line enhancement or if M1 is associated with that enhancement.

METHODS

32 right-handed, healthy subjects were randomly assigned to two groups: real repetitive transcranial magnetic stimulation (rTMS) or sham rTMS applied to the contralateral M1 immediately following one 20-minute finger tracking training session. Tracking performance and cortical excitability were assessed before and after training, following rTMS and 24 hours post-training.

RESULTS

Results demonstrate that skill performance continues to develop for at least 30 minutes after training completion, is maintained for 24 hours post-training, and is not affected by inhibitory rTMS applied to M1. Level of skill improvement was associated with the degree of intracortical inhibition increase.

CONCLUSIONS

These results suggest dispersed information processing for goal-directed visuomotor skill learning following training and a relationship between cortical excitability and skill development in healthy individuals. These findings invite further investigation of the neural mechanisms underlying motor skill learning and may have rehabilitation implications for patients with neurologic injury.

Recovery of motor function after stroke

The human brain possesses a remarkable ability to adapt in response to changing anatomical (e.g., aging) or environmental modifications. This form of neuroplasticity is important at all stages of life but is critical in neurological disorders such as amblyopia and stroke. This review focuses upon our new understanding of possible mechanisms underlying functional deficits evidenced after adult-onset stroke. We review the functional interactions between different brain regions that may contribute to motor disability after stroke and, based on this information, possible interventional approaches to motor stroke disability. New information now points to the involvement of non-primary motor areas and their interaction with the primary motor cortex as areas of interest. The emergence of this new information is likely to impact new efforts to develop more effective neurorehabilitative interventions using transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) that may be relevant to other neurological disorders such as amblyopia.

Novel methods for quantifying neurophysiologic properties of the human lumbar paraspinal muscles

Our understanding the neurophysiologic characteristics of the human paraspinal muscles has historically been hindered by the lack of experimental techniques to examine these muscles function in vivo. In this article we describe a paired-pulse transcranial magnetic stimulation (TMS) protocol to quantify intracortical facilitation (ICF) and short-interval intracortical inhibition (SICI) of the lumbar paraspinal muscles, and an electromechanical tapping protocol to measure the amplitude of the short-latency stretch reflex. Test-retest reliability of these protocols was examined across two sessions separated by 30-min in healthy adults. We assessed relative reliability by calculating the intraclass correlation coefficient (ICC), and absolute reliability was assessed via coefficient of variation (CV). ICF and SICI in the lumbar paraspinal muscles exhibited the classical facilitatory and inhibitory responses observed in appendicular skeletal muscles (∼30% facilitation and inhibition, respectively). The motor evoked potential amplitude (MEP), ICF, SICI, and stretch reflex amplitude measurements did not significantly differ between the two testing sessions (p>0.05). The MEP amplitude, ICF and stretch reflex amplitude exhibited the highest relative and absolute reliability (ICC=0.89-0.91, CV=10.6-11.1%); whereas the SICI measure exhibited somewhat lower reliability (ICC=0.75, CV=20.1%). The stretch reflex protocol performed in the first testing session did not influence the TMS outcome measures in the second testing session (p>0.05). These innovative methods may be useful in studying basic physiology, the pathology of low back pain, as well as the mechanisms of action of treatment interventions.

Impaired inter-hemispheric facilitatory connectivity in schizophrenia

OBJECTIVES

To investigate the inter-hemispheric connections between the dorsal premotor cortex (dPM) and contralateral primary motor cortex (M1) in schizophrenia.

METHODS

Sixteen medicated, nine unmedicated schizophrenia patients and 20 healthy age-matched subjects were studied by twin-coil Transcranial Magnetic Stimulation. To activate distinct facilitatory and inhibitory transcallosal pathways between dPM and the contralateral M1, the intensity of dPM stimulation was adjusted to be either suprathreshold (110% of resting motor threshold) or subthreshold (80% of active motor threshold). Interstimulus intervals between conditioning stimulus and test stimulus were 6, 8 and 15 ms.

RESULTS

Schizophrenia patients had comparable efficacy of the inhibitory pathway. On the other hand, medicated patients showed less facilitation of contralateral M1 following dPM stimulation at 80% of active motor threshold, at interstimulus interval=8 ms. The individual amount of facilitation induced by dPM conditioning at 80% of active motor threshold at interstimulus interval=8 ms correlated negatively with negative symptoms.

CONCLUSIONS

Inter-hemispheric facilitatory dPM-M1 connectivity is selectively altered in schizophrenia.

SIGNIFICANCE

This study produced evidence that dPM-M1 connectivity is dysfunctional and that correlates with negative symptoms. These results converge with previous studies which strongly hypothesize that inter- and intra-hemispheric connectivity disturbances may play a major role in schizophrenia.

Cast immobilization increases long-interval intracortical inhibition

Immobilization reduces muscle performance, and despite these performance losses being associated with neural impairments little is known regarding adaptations in cortical properties. We utilized transcranial magnetic stimulation to assess changes in flexor carpi radialis (FCR) intracortical facilitation (ICF), and short- and long-interval intracortical inhibition (SICI and LICI) in healthy humans undergoing 3 weeks of immobilization. Measurements were obtained at rest and during contraction (15% intensity). Central activation and the Hoffman reflex (H-reflex) were also assessed. Strength decreased 43.2% +/- 6.1% following immobilization, and central activation also decreased (97.5% +/- 2.4% to 73.2% +/- 8.3%). No changes in ICF, SICI, or LICI were observed at rest; however, LICI was increased during contraction (67.5% +/- 6.9% to 53.1% +/- 6.7% of unconditioned response). The increase in LICI correlated with the loss of strength (r = -0.63). The H-reflex increased following immobilization. These findings suggest that immobilization increases intracortical inhibition during contraction, and this increase is primarily mediated by GABA(B) receptors.

Differential patterns of interhemispheric functional disconnection in mild and advanced multiple sclerosis

BACKGROUND

Patients with multiple sclerosis may present altered patterns of connectivity between the two brain hemispheres. To date, only transcallosal connectivity between the two primary motor cortices (M1) has been investigated functionally in patients with multiple sclerosis.

OBJECTIVES

The aim of this study was to investigate whether connectivity between the dorsal premotor cortex and the contralateral M1 was altered in patients with multiple sclerosis, and to see whether clinical progression is accompanied by exacerbated dorsal premotor cortex-M1 disconnectivity.

METHODS

A twin-coil transcranial magnetic stimulation approach was used to investigate both excitatory and inhibitory interhemispheric connections between the left dorsal premotor cortex and the contralateral M1 in 18 multiple sclerosis patients without disability, in 18 multiple sclerosis patients with advanced disease and in 12 age-matched healthy subjects. To activate distinct inhibitory and facilitatory transcallosal pathways, the intensity of dorsal premotor cortex stimulation was adjusted to be either suprathreshold (110% of resting motor threshold) or subthreshold (80% of active motor threshold).

RESULTS

Our sample of patients with multiple sclerosis showed altered patterns of interhemispheric dorsal premotor cortex-M1 functional connectivity even in the absence of clinical deficits. Facilitatory connections originating from dorsal premotor cortex were reduced in multiple sclerosis patients with or without disability, while inhibitory dorsal premotor cortex-M1 connections were altered only in disabled patients.

CONCLUSIONS

The current study demonstrates that functional excitatory connectivity originating from non-primary motor areas is compromised in multiple sclerosis patients even in the absence of clinical disability. Clinical disease progression leads to an impairment of both excitatory and inhibitory transcallosal connections.

The use of transcranial magnetic stimulation in cognitive neuroscience: a new synthesis of methodological issues

Transcranial magnetic stimulation (TMS) has become a mainstay of cognitive neuroscience, thus facing new challenges due to its widespread application on behaviorally silent areas. In this review we will summarize the main technical and methodological considerations that are necessary when using TMS in cognitive neuroscience, based on a corpus of studies and technical improvements that has become available in most recent years. Although TMS has been applied only relatively recently on a large scale to the study of higher functions, a range of protocols that elucidate how this technique can be used to investigate a variety of issues is already available, such as single pulse, paired pulse, dual-site, repetitive and theta burst TMS. Finally, we will touch on recent promising approaches that provide powerful new insights about causal interactions among brain regions (i.e., TMS with other neuroimaging techniques) and will enable researchers to enhance the functional resolution of TMS (i.e., state-dependent TMS). We will end by briefly summarizing and discussing the implications of the newest safety guidelines.

Intracortical excitability after repetitive hand movements is differentially affected in cortical versus subcortical strokes

Repetitive training of isolated movements induces reorganization of motor cortical representations. To elucidate the mechanisms of practice-dependent cortical plasticity within the lesioned central motor system at the time of the application of a therapeutic intervention, we examined the effect of repetitive movements on intracortical facilitation (ICF) and inhibition of agonist (extensor carpi radialis [ECR]) and antagonist (flexor carpi ulnaris) muscles of the hand shortly after the movements, by the paired-pulse technique in patients with cortical (n = 9) and subcortical strokes (n = 11). Short intracortical inhibition and ICF were studied by using interstimulus intervals of 2 and 8 milliseconds, respectively, and their interaction with active or passive movement. The active movement produced significantly larger motor-evoked potentials in the ECR muscle in both patient groups. Short intracortical inhibition was particularly decreased after cortical stroke, whereas it was still significant after subcortical stroke. ICF increased significantly after movements compared with rest in the ECR for subcortical stroke patients only. We conclude that repetitive active movements increase the excitability of the motor cortex representing the agonist muscle and interact with intracortical facilitatory circuits in the subcortical stroke group but not in the cortical stroke group. This interaction of circuitry has been reported previously in control subjects and seems to still operate after subcortical stroke during active movement. Alternative networks may be recruited for active movement after cortical stroke. This finding proposes lesion-specific mechanisms of reorganization during the same rehabilitative intervention. Distinct rehabilitative strategies may be required to optimize the activation of the physiologic motor network for different lesions.

Task-Dependent Modulation of Inputs to Proximal Upper Limb Following Transcranial Direct Current Stimulation of Primary Motor Cortex

Cathodal transcranial DC stimulation (c-tDCS) suppresses excitability of primary motor cortex (M1) controlling contralateral hand muscles. This study assessed whether c-tDCS would have similar effects on ipsi- and contralateral M1 projections to a proximal upper limb muscle. Transcranial magnetic stimulation (TMS) of left M1 was used to elicit motor evoked potentials (MEPs) in the left and right infraspinatus (INF) muscle immediately before and after c-tDCS of left M1, and at 20 and 40 min, post-c-tDCS. TMS was delivered as participants preactivated each INF in isolation (left, right) or both INF together (bilateral). After c-tDCS, ipsilateral MEPs in left INF and contralateral MEPs in right INF were suppressed in the left task but not in the bilateral or right tasks, indicative of task-dependent modulation. Ipsilateral silent period duration in the left INF was reduced after c-tDCS, indicative of altered transcallosal inhibition. These findings may have implications for the use of tDCS as an adjunct to therapy for the proximal upper limb after stroke.

Older adults exhibit more intracortical inhibition and less intracortical facilitation than young adults

BACKGROUND

Aging results in decreased neuromuscular function, which is likely associated with neurologic alterations. At present little is known regarding age-related changes in intracortical properties.

METHODS

In this study we used transcranial magnetic stimulation (TMS) to measure intracortical facilitation (ICF), short- and long-interval intracortical inhibition (SICI and LICI), motor evoked potential amplitude, and silent period duration in young and older adults (21.4+/-0.8years and 70.9+/-1.8years). These variables were assessed from the flexor carpi radialis muscle of the non-dominant arm under resting conditions, and during a submaximal contraction (intensity 15% maximum strength).

RESULTS

Older adults exhibited increased SICI and LICI in comparison to young adults (SICI: 29.0+/-9.2% vs. 46.2+/-4.8% of unconditioned pulse; LICI: 6.5+/-1.7% vs. 15.8+/-3.3% of unconditioned pulse; P=0.04), and less ICF under resting conditions (74.6+/-8.7% vs. 104.9+/-6.9% of unconditioned pulse; P=0.02). These age-related differences disappeared during contraction, although the older adults did exhibit a longer silent period during contraction (112.5+/-6.5 vs. 84.0+/-3.9ms; P<0.01).

CONCLUSIONS

Collectively, these findings suggest increased GABA mediated intracortical inhibition with age.

Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function

This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at 'very low' intensities which elicit only short-latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non-linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity-dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.

Alteration of cortical excitability in patients with fibromyalgia

We assessed cortical excitability and intracortical modulation systematically, by transcranial magnetic stimulation (TMS) of the motor cortex, in patients with fibromyalgia. In total 46 female patients with fibromyalgia and 21 normal female subjects, matched for age, were included in this study. TMS was applied to the hand motor area of both hemispheres and motor evoked potentials (MEPs) were recorded for the first interosseous muscle of the contralateral hand. Single-pulse stimulation was used for measurements of the rest motor threshold (RMT) and suprathreshold MEP. Paired-pulse stimulation was used to assess short intracortical inhibition (SICI) and intracortical facilitation (ICF). Putative correlations were sought between changes in electrophysiological parameters and major clinical features of fibromyalgia, such as pain, fatigue, anxiety, depression and catastrophizing. The RMT on both sides was significantly increased in patients with fibromyalgia and suprathreshold MEP was significantly decreased bilaterally. However, these alterations, suggesting a global decrease in corticospinal excitability, were not correlated with clinical features. Patients with fibromyalgia also had lower ICF and SICI on both sides, than controls, these lower values being correlated with fatigue, catastrophizing and depression. These neurophysiological alterations were not linked to medication, as similar changes were observed in patients with or without psychotropic treatment. In conclusion, fibromyalgia is associated with deficits in intracortical modulation involving both GABAergic and glutamatergic mechanisms, possibly related to certain aspects of the pathophysiology of this chronic pain syndrome. Our data add to the growing body of evidence for objective and quantifiable changes in brain function in fibromyalgia.

Effectiveness of Virtual Reality Exercises in STroke Rehabilitation (EVREST): rationale, design, and protocol of a pilot randomized clinical trial assessing the Wii gaming system

BACKGROUND

Evidence suggests that increasing intensity of rehabilitation results in better motor recovery. Limited evidence is available on the effectiveness of an interactive virtual reality gaming system for stroke rehabilitation. EVREST was designed to evaluate feasibility, safety and efficacy of using the Nintendo Wii gaming virtual reality (VRWii) technology to improve arm recovery in stroke patients.

METHODS

Pilot randomized study comparing, VRWii versus recreational therapy (RT) in patients receiving standard rehabilitation within six months of stroke with a motor deficit of > or =3 on the Chedoke-McMaster Scale (arm). In this study we expect to randomize 20 patients. All participants (age 18-85) will receive customary rehabilitative treatment consistent of a standardized protocol (eight sessions, 60 min each, over a two-week period).

OUTCOME MEASURES

The primary feasibility outcome is the total time receiving the intervention. The primary safety outcome is the proportion of patients experiencing intervention-related adverse events during the study period. Efficacy, a secondary outcome measure, will be measured by the Wolf Motor Function Test, Box and Block Test, and Stroke Impact Scale at the four-week follow-up visit. From November, 2008 to September, 2009 21 patients were randomized to VRWii or RT. Mean age, 61 (range 41-83) years. Mean time from stroke onset 25 (range 10-56) days.

CONCLUSIONS

EVREST is the first randomized parallel controlled trial assessing the feasibility, safety, and efficacy of virtual reality using Wii gaming technology in stroke rehabilitation. The results of this study will serve as the basis for a larger multicentre trial. ClinicalTrials.gov registration# NTC692523.

High T2 signal in primary lateral sclerosis supports the topographic distribution of fibers in the corpus callosum: assessing disease in the primary motor segment

PLS is a disease of the UMN, distinguished from ALS in prognosis and absence of LMN signs. We present, to our knowledge, the first conventional MR imaging visualization of the callosal motor segment, a concept previously supported by primate models, electrophysiologic studies, and postmortem examinations. Modification of the Witelson topographic scheme of the CC is supported by MR tractography. On the basis of 2 cases of PLS, we present conventional imaging confirmation of the revised topographic scheme of fiber distribution across the CC.

Neural integration of reaching and posture: interhemispheric spike correlations in cat motor cortex

To study the interlimb coordination of reaching and postural movements, chronically implanted microelectrodes were used to record single unit activity from the primary motor cortex (MI) of cats during performance of a trained reaching task. Recordings were made from both cerebral hemispheres to record neurons that modulated their activity during reaching (reach-related neurons) and supportive (posture-related neurons) movements of either forelimb. Evidence of temporal associations in the activities of simultaneously recorded reach- and posture-related neurons was evaluated using shuffle-corrected cross correlograms. The spike activity of approximately 34% of reach-related neurons was temporally correlated with the spike activity of simultaneously recorded posture-related neurons in the opposite motor cortex. Significant associations in the spike activity of neurons recorded from homotopic representational areas of the motor cortex in opposite hemispheres have not previously been reported. These interactions may have an important role in the coordination of opposite forelimbs during reaching movements and postural actions.

Mechanisms and Dynamics of Cortical Motor Inhibition in the Stop-signal Paradigm: A TMS Study

The ability to stop ongoing motor responses in a split-second is a vital element of human cognitive control and flexibility that relies in large part on prefrontal cortex. We used the stop-signal paradigm to elucidate the engagement of primary motor cortex (M1) in inhibiting an ongoing voluntary motor response. The stop-signal paradigm taps the ability to flexibly countermand ongoing voluntary behavior upon presentation of a stop signal. We applied single-pulse TMS to M1 at several intervals following the stop signal to track the time course of excitability of the motor system related to generating and stopping a manual response. Electromyography recorded from the flexor pollicis brevis allowed quantification of the excitability of the corticospinal tract and the involvement of intracortical GABABergic circuits within M1, indexed respectively by the amplitude of the motor-evoked potential and the duration of the late part of the cortical silent period (SP). The results extend our knowledge of the neural basis of inhibitory control in three ways. First, the results revealed a dynamic interplay between response activation and stopping processes at M1 level during stop-signal inhibition of an ongoing response. Second, increased excitability of inhibitory interneurons that drives SP prolongation was evident as early as 134 msec following the instruction to stop. Third, this pattern was followed by a stop-related reduction of corticospinal excitability implemented around 180 after the stop signal. These findings point to the recruitment of GABABergic intracortical inhibitory circuits within M1 in stop-signal inhibition and support the notion of stopping as an active act of control.

The estimation of short intra-cortical inhibition depends on the proportion of spinal motoneurones activated by corticospinal inputs

OBJECTIVE

The high variability of SICI limits its utility and by extension that of TMS in clinical neurophysiology. Non-linear summation of descending volleys due to heterogeneous motoneurone properties, on which MEP size depends, has not previously been implicated as an issue in SICI evaluation.

METHODS

MEP size and SICI were normalised to the test MEP (mV), and as a percentage of M(max) to take account of the proportion of motoneurone pool activated by TMS. Two EMG systems, producing large and small MEPs, were used to determine how the normalisation affected MEPs of different amplitude.

RESULTS

M(max) normalisation (i) counteracted the influence of recording conditions on the MEP size, (ii) revealed a significant influence of the test size on SICI (between medium and large MEPs), and of test size on the conditioning intensity (the larger the MEP, the stronger the SICI), and (iii) decreased the variability.

CONCLUSIONS

Data normalised to M(max) better reflect the motoneurone recruitment after SICI. To enhance reproducibility, MEP should be normalised to M(max). This adjusts for some of the non-linear properties at the spinal, and possibly, at cortical levels.

SIGNIFICANCE

To reduce variability is important because TMS is becoming widely adopted and is being used in patients.

Late Cortical Disinhibition in Human Motor Cortex: A Triple-Pulse Transcranial Magnetic Stimulation Study

In human motor cortex transcranial magnetic stimulation (TMS) has been used to identify short-interval intracortical inhibition (SICI) corresponding to γ-aminobutyric acid type A (GABAA) effects and long-interval intracortical inhibition (LICI) and the cortical silent period (SP) corresponding to postsynaptic GABAB effects. Presynaptic GABAB effects, corresponding to disinhibition, can also be identified with TMS and have been shown to be acting during LICI by measuring SICI after a suprathreshold priming stimulus (PS). The duration of disinhibition is not certain and, guided by studies in experimental preparations, we hypothesized that it may be longer-lasting than postsynaptic inhibition, leading to a period of late cortical disinhibition and consequently a net increase in corticospinal excitability. We tested this first by measuring the motor-evoked potential (MEP) to a test stimulus (TS), delivered after a PS at interpulse intervals (IPIs) ≤300 ms that encompassed the period of PS-induced LICI and its aftermath. MEP amplitude was initially decreased, but then increased at IPIs of 190–210 ms, reaching 160 ± 17% of baseline 200 ms after PS ( P < 0.05). SP duration was 181 ± 5 ms. A second experiment established that the onset of the later period of increased excitability correlated with PS intensity ( r2 = 0.99) and with the duration of the SP ( r2 = 0.99). The third and main experiment demonstrated that SICI was significantly reduced in strength at all IPIs ≤220 ms after PS. We conclude that TMS-induced LICI is associated with a period of disinhibition that is at first masked by LICI, but that outlasts LICI and gives rise to a period during which disinhibition predominates and net excitability is raised. Identification of this late period of disinhibition in human motor cortex may provide an opportunity to explore or modulate the behavior of excitatory networks at a time when inhibitory effects are restrained.

Differential contribution of the supplementary motor area to stabilization of a procedural motor skill acquired through different practice schedules

Behavioral studies have suggested that the stabilization of motor memory varies depending on the practice schedule. The neural substrates underlying this schedule-dependent difference in memory stabilization are not known. Here, we evaluated the effects of 1-Hz repetitive transcranial magnetic stimulation (rTMS) applied to different cortical regions and sham after one session of training (Day 1) of sequential motor skills acquired through blocked (each sequence was completely trained before training the next)-practice schedules and random (random training of 3 sequences)-practice schedules. The recall of sequences learned on Day 1 by Day 2 was measured in different groups of healthy volunteers. The rTMS over the supplementary motor area (SMA) but not over control regions or over the primary motor cortex (M1) immediately after practice or over SMA 6 h later reduced recall relative to sham only in the blocked-practice group. In contrast, recall in the random-practice group was unaffected by rTMS. These results document a differential contribution of the SMA to the stabilization of motor memories acquired through different practice schedules. More generally, they indicate that the anatomical substrates underlying motor-memory stabilization (or their temporal operation) do differ depending on the practice schedule.

The new neuroscience frontier: promoting neuroplasticity and brain repair in traumatic brain injury

Increased awareness of traumatic brain injury (TBI) in the military, a persistent call for evidence-based treatment, and recent government funding have revealed new research opportunities in neuroscience. This paper describes a relatively new frontier for research: that of the facilitation or enhancement of neuroplasticity and brain repair in TBI using novel treatment protocols. Such protocols, algorithmically introduced, may be tailored to the individual through the matching of neuromarkers with specific interventions. Examples of neuromarkers and interventions employed for the purpose of neuromodulation are reported. Problems with lack of controlled studies and inferring causation in correlational research are noted. Healthy skepticism and open-minded creativity are needed so that we can think in unorthodox ways, create partnerships, harness available knowledge and expertise, and ultimately develop effective treatments.

Modulation of motorcortical excitability by methylphenidate in adult voluntary test persons performing a go/nogo task

This study investigated the interaction between motorcortical excitability (short interval cortical inhibition, intracortical facilitation and long interval cortical inhibition), different requirement conditions [choice reaction test (CRT), attention/go/nogo], and their pharmacological modulation by methylphenidate (MPH) in normal healthy adults (n = 31) using a transcranial magnetic stimulation paradigm. MPH was administered in a dosage of 1 mg/kg body weight, maximum 60 mg. Additionally, serum level and clearance of MPH were controlled. The statistical analysis of variance revealed a significant three-way interaction of 2 (MPH) x 3 (CRT) x 6 (ISI) predicting motor evoked potential amplitudes (P = 0.032, MPH none and full dose, n = 31). In order to compare effects of dosage an additional between-subjects factor (half vs. full MPH dose) was introduced. None of the interactions involving this between-subject factor reached statistical significance. Exploring interactions with MPH only, a 3 (MPH none, half and full dose) x 3 (CRT) x 6 (ISI) analysis of variance revealed significant two-way interactions for MPH x ISI (P = 0.040) and condition x ISI (P < 0.001, n = 18). Effects observed for MPH were strongest on facilitatory processes, weaker for intracortical inhibition. In sum, MPH seems to interact via striato-thalamo-cortical pathways with original motorcortical processes (ISI), to a lesser extent with task-dependent or behavioral parameters (CRT).

Brain plasticity and genetic factors

Brain plasticity refers to changes in brain function and structure that arise in a number of contexts. One area in which brain plasticity is of considerable interest is recovery from stroke, both spontaneous and treatment-induced. A number of factors influence these poststroke brain events. The current review considers the impact of genetic factors. Polymorphisms in the human genes coding for brain-derived neurotrophic factor (BDNF) and apolipoprotein E (ApoE) have been studied in the context of plasticity and/or stroke recovery and are discussed here in detail. Several other genetic polymorphisms are indirectly involved in stroke recovery through their modulating influences on processes such as depression and pharmacotherapy effects. Finally, new genetic polymorphisms that have not been studied in the context of stroke are proposed as new directions for study. A better understanding of genetic influences on recovery and response to therapy might allow improved treatment after stroke.

The ipsilateral motor cortex contributes to cross-limb transfer of performance gains after ballistic motor practice

Although it has long been known that practicing a motor task with one limb can improve performance with the limb opposite, the mechanisms remain poorly understood. Here we tested the hypothesis that improved performance with the untrained limb on a fastest possible (i.e. ballistic) movement task depends partly on cortical circuits located ipsilateral to the trained limb. The idea that crossed effects, which are important for the learning process, might occur in the 'untrained' hemisphere following ballistic training is based on the observation that tasks requiring strong descending drive generate extensive bilateral cortical activity. Twenty-one volunteers practiced a ballistic index finger abduction task with their right hand, and corticospinal excitability was assessed in two hand muscles (first dorsal interosseus, FDI; adductor digiti minimi, ADM). Eight control subjects did not train. After training, repetitive transcranial magnetic stimulation (rTMS; 15 min at 1 Hz) was applied to the left (trained) or right (untrained) motor cortex to induce a 'virtual lesion'. A third training group received sham rTMS, and control subjects received rTMS to the right motor cortex. Performance and corticospinal excitability (for FDI) increased in both hands for training but not control subjects. rTMS of the left, trained motor cortex specifically reduced training-induced gains in motor performance for the right, trained hand, and rTMS of the right, untrained motor cortex specifically reduced performance gains for the left, untrained hand. Thus, cortical processes within the untrained hemisphere, ipsilateral to the trained hand, contribute to early retention of ballistic performance gains for the untrained limb.

Arm movement maps evoked by cortical magnetic stimulation in a robotic environment

Many neurological diseases result in a severe inability to reach for which there is no proven therapy. Promising new interventions to address reaching rehabilitation using robotic training devices are currently under investigation in clinical trials but the neural mechanisms that underlie these interventions are not understood. Transcranial magnetic stimulation (TMS) may be used to probe such mechanisms quickly and non-invasively, by mapping muscle and movement representations in the primary motor cortex (M1). Here we investigate movement maps in healthy young subjects at rest using TMS in the robotic environment, with the goal of determining the range of TMS accessible movements, as a starting point for the study of cortical plasticity in combination with robotic therapy. We systematically stimulated the left motor cortex of 14 normal volunteers while the right hand and forearm rested in the cradle of a two degree-of-freedom planar rehabilitation robot (IMT). Maps were created by applying 10 stimuli at each of nine locations (3x3 cm(2) grid) centered on the M1 movement hotspot for each subject, defined as the stimulation location that elicited robot cradle movements of the greatest distance. TMS-evoked movement kinematics were measured by the robotic encoders and ranged in magnitude from 0 to 3 cm. Movement maps varied by subject and by location within a subject. However, movements were very consistent within a single stimulation location for a given subject. Movement vectors remained relatively constant (limited to <90 degrees section of the planar field) within some subjects across the entire map, while others covered a wider range of directions. This may be due to individual differences in cortical physiology or anatomy, resulting in a practical limit to the areas that are TMS-accessible. This study provides a baseline inventory of possible TMS-evoked arm movements in the robotic reaching trainer, and thus may provide a real-time, non-invasive platform for neurophysiology based evaluation and therapy in motor rehabilitation settings.

Effects of somatosensory stimulation on motor function after subacute stroke

BACKGROUND

Previous works showed potentially beneficial effects of a single session of peripheral nerve sensory stimulation (PSS) on motor function of a paretic hand in patients with subacute and chronic stroke.

OBJECTIVE

To investigate the influence of the use of different stimulus intensities over multiple sessions (repetitive PSS [RPSS]) paired with motor training.

METHODS

To address this question, 22 patients were randomized within the second month after a single hemispheric stroke in a parallel design to application of 2-hour RPSS at 1 of 2 stimulus intensities immediately preceding motor training, 3 times a week, for 1 month. Jebsen-Taylor test (JTT, primary endpoint measure), pinch force, Functional Independence Measure (FIM), and corticomotor excitability to transcranial magnetic stimulation were measured before and after the end of the treatment month. JTT, FIM scores, and pinch force were reevaluated 2 to 3 months after the end of the treatment.

RESULTS

Baseline motor function tests were comparable across the 2 RPSS intensity groups. JTT improved significantly in the lower intensity RPSS group but not in the higher intensity RPSS group at month 1. This difference between the 2 groups reduced by months 2 to 3.

CONCLUSIONS

These results indicate that multiple sessions of RPSS could facilitate training effects on motor function after subacute stroke depending on the intensity of stimulation. It is proposed that careful dose-response studies are needed to optimize parameters of RPSS stimulation before designing costly, larger, double-blind, multicenter clinical trials.

The response to paired motor cortical stimuli is abolished at a spinal level during human muscle fatigue

During maximal exercise, supraspinal fatigue contributes significantly to the decline in muscle performance but little is known about intracortical inhibition during such contractions. Long-interval inhibition is produced by a conditioning motor cortical stimulus delivered via transcranial magnetic stimulation (TMS) 50-200 ms prior to a second test stimulus. We aimed to delineate changes in this inhibition during a sustained maximal voluntary contraction (MVC). Eight subjects performed a 2 min MVC of elbow flexors. Single test and paired (conditioning-test interval of 100 ms) stimuli were delivered via TMS over the motor cortex every 7-8 s throughout the effort and during intermittent MVCs in the recovery period. To determine the role of spinal mechanisms, the protocol was repeated but the TMS test stimulus was replaced by cervicomedullary stimulation which activates the corticospinal tract. TMS motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) were recorded from biceps brachii. Unconditioned MEPs increased progressively with fatigue, whereas CMEPs increased initially but returned to the control value in the final 40 s of contraction. In contrast, both conditioned MEPs and CMEPs decreased rapidly with fatigue and were virtually abolished within 30 s. In recovery, unconditioned responses required <30 s but conditioned MEPs and CMEPs required 90 s to return to control levels. Thus, long-interval inhibition increased markedly as fatigue progressed. Contrary to expectations, subcortically evoked CMEPs were inhibited as much as MEPs. This new phenomenon was also observed in the first dorsal interosseous muscle. Tested with a high intensity conditioning stimulus during a fatiguing maximal effort, long-interval inhibition of MEPs was increased primarily by spinal rather than motor cortical mechanisms. The spinal mechanisms exposed here may contribute to the development of central fatigue in human muscles.

Patterns of spatio-temporal correlations in the neural activity of the cat motor cortex during trained forelimb movements

In order to study how neurons in the primary motor cortex (MI) are dynamically linked together during skilled movement, we recorded simultaneously from many cortical neurons in cats trained to perform a reaching and retrieval task using their forelimbs. Analysis of task-related spike activity in the MI of the hemisphere contralateral to the reaching forelimb (in identified forelimb or hindlimb representations) recorded through chronically implanted microwires, was followed by pairwise evaluation of temporally correlated activity in these neurons during task performance using shuffle corrected cross-correlograms. Over many months of recording, a variety of task-related modulations of neural activities were observed in individual efferent zones. Positively correlated activity (mainly narrow peaks at zero or short latencies) was seen during task performance frequently between neurons recorded within the forelimb representation of MI, rarely within the hindlimb area of MI, and never between forelimb and hindlimb areas. Correlated activity was frequently observed between neurons with different patterns of task-related activity or preferential activity during different task elements (reaching, feeding, etc.), and located in efferent zones with dissimilar representation as defined by intracortical microstimulation. The observed synchronization of action potentials among selected but functionally varied groups of MI neurons possibly reflects dynamic recruitment of network connections between efferent zones during skilled movement.

Neural substrates of low-frequency repetitive transcranial magnetic stimulation during movement in healthy subjects and acute stroke patients. A PET study

The aim of the study was to investigate, with an rTMS/PET protocol, the after-effects induced by 1-Hz repetitive transcranial magnetic stimulation (rTMS) in the regional cerebral blood flow (rCBF) of the primary motor cortex (M1) contralateral to that stimulated during a movement. Eighteen healthy subjects underwent a baseline PET scan followed, in randomized order, by a session of Real/Sham low-frequency (1 Hz) subthreshold rTMS over the right M1 for 23 min. The site of stimulation was fMRI-guided. After each rTMS session (real or sham), subjects underwent behavioral hand motor tests and four PET scans. During the first two scans, ten subjects (RH group) moved the right hand ipsilateral to the stimulated site and eight subjects (LH group) moved the left contralateral hand. All remained still during the last two scans (rest). Two stroke patients underwent the same protocol with rTMS applied on contralesional M1. Compared with Sham-rTMS, Real-rTMS over the right M1 was followed by a significant increase of rCBF during right hand movement in left S1M1, without any significant change in motor performance. The effect lasted less than 1 h. The same rTMS-induced S1M1 overactivation was observed in the two stroke patients. Commissural connectivity between right dorsal premotor cortex and left M1 after real-rTMS was observed with a psychophysiological interaction analysis in healthy subjects. No major changes were found for the left hand. These results give further arguments in favor of a plastic commissural connectivity between M1 both in healthy subjects and in stroke patients, and reinforce the potential for therapeutic benefit of low-frequency rTMS in stroke rehabilitation.

Contribution of transcranial magnetic stimulation to the understanding of functional recovery mechanisms after stroke

Motor impairments are a major cause of morbidity and disability after stroke. This article reviews evidence obtained using transcranial magnetic stimulation (TMS) that provides new insight into mechanisms of impaired motor control and disability. They briefly discuss the use of TMS in the diagnosis, prognosis, and therapy of poststroke motor disability. Particular emphasis is placed on TMS as a tool to explore mechanisms of neuroplasticity during spontaneous and treatment-induced recovery of motor function to develop more rational and clinically useful interventions for stroke rehabilitation.