A sham stimulation-controlled trial of rTMS of the unaffected hemisphere in stroke patients

Neuronavigation increases the physiologic and behavioral effects of low-frequency rTMS of primary motor cortex in healthy subjects

Low-frequency repetitive transcranial magnetic stimulation (rTMS) can exert local and inter-hemispheric neuromodulatory effects on cortical excitability. These physiologic effects can translate into changes in motor behavior, and may offer valuable therapeutic interventions in recovery from stroke. Neuronavigated TMS can maximize accurate and consistent targeting of a given cortical region, but is a lot more involved that conventional TMS. We aimed to assess whether neuronavigation enhances the physiologic and behavioral effects of low-frequency rTMS. Ten healthy subjects underwent two experimental sessions during which they received 1600 pulses of either navigated or non-navigated 1 Hz rTMS at 90% of the resting motor threshold (RMT) intensity over the motor cortical representation for left first dorsal interosseous (FDI) muscle. We compared the effects of navigated and non-navigated rTMS on motor-evoked potentials (MEPs) to single-pulse TMS, intracortical inhibition (ICI) and intracortical facilitation (ICF) by paired-pulse TMS, and performance in various behavioral tasks (index finger tapping, simple reaction time and grip strength tasks). Following navigated rTMS, the amplitude of MEPs elicited from the contralateral (unstimulated) motor cortex was significantly increased, and was associated with an increase in ICF and a trend to decrease in ICI. In contrast, non-navigated rTMS elicited nonsignificant changes, most prominently ipsilateral to rTMS. Behaviorally, navigated rTMS significantly improved reaction time RT and pinch force with the hand ipsilateral to stimulation. Non-navigated rTMS lead to similar behavioral trends, although the effects did not reach significance. In summary, navigated rTMS leads to more robust modulation of the contralateral (unstimulated) hemisphere resulting in physiologic and behavioral effects. Our findings highlight the spatial specificity of inter-hemispheric TMS effects, illustrate the superiority of navigated rTMS for certain applications, and have implications for therapeutic applications of rTMS.

Bihemispheric brain stimulation facilitates motor recovery in chronic stroke patients

OBJECTIVE

Motor recovery after stroke depends on the integrity of ipsilesional motor circuits and interactions between the ipsilesional and contralesional hemispheres. In this sham-controlled randomized trial, we investigated whether noninvasive modulation of regional excitability of bilateral motor cortices in combination with physical and occupational therapy improves motor outcome after stroke.

METHODS

Twenty chronic stroke patients were randomly assigned to receive 5 consecutive sessions of either 1) bihemispheric transcranial direct current stimulation (tDCS) (anodal tDCS to upregulate excitability of ipsilesional motor cortex and cathodal tDCS to downregulate excitability of contralesional motor cortex) with simultaneous physical/occupational therapy or 2) sham stimulation with simultaneous physical/occupational therapy. Changes in motor impairment (Upper Extremity Fugl-Meyer) and motor activity (Wolf Motor Function Test) assessments were outcome measures while functional imaging parameters were used to identify neural correlates of motor improvement.

RESULTS

The improvement of motor function was significantly greater in the real stimulation group (20.7% in Fugl-Meyer and 19.1% in Wolf Motor Function Test scores) when compared to the sham group (3.2% in Fugl-Meyer and 6.0% in Wolf Motor Function Test scores). The effects outlasted the stimulation by at least 1 week. In the real-stimulation group, stronger activation of intact ipsilesional motor regions during paced movements of the affected limb were found postintervention whereas no significant activation changes were seen in the control group.

CONCLUSIONS

The combination of bihemispheric tDCS and peripheral sensorimotor activities improved motor functions in chronic stroke patients that outlasted the intervention period. This novel approach may potentiate cerebral adaptive processes that facilitate motor recovery after stroke.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that for adult patients with ischemic stroke treated at least 5 months after their first and only stroke, bihemispheric tDCS and simultaneous physical/occupational therapy given over 5 consecutive sessions significantly improves motor function as measured by the Upper Extremity Fugl-Meyer assessment (raw change treated 6.1 ± 3.4, sham 1.2 ± 1.0).

Focal and bi-directional modulation of lower limb motor cortex using anodal transcranial direct current stimulation

BACKGROUND

Because we are interested in non-invasive transcranial brain stimulation as an adjuvant to post-stroke walking therapy, we applied direct current stimulation (tDCS) preferentially to either the left or right lower limb motor cortex (M1) in two separate sessions and assessed the resulting modulation in both cortices.

OBJECTIVE/HYPOTHESIS

We hypothesized that tDCS applied preferentially to one lower limb M1 of healthy subjects would induce between-hemisphere opposite sign modulation.

METHODS

Transcranial magnetic stimulation (TMS) with the coil offset 2 cm either side of vertex was used to assess the percent change in rectified motor evoked potential (MEP) area recorded bilaterally from vastus lateralis (VL) and tibialis anterior (TA) of 10 subjects during weak tonic contraction.

RESULTS

ANOVA revealed an up-regulation of the target cortex and a down-regulation of the non-target cortex (p = 0.001) and no effects of hemisphere (left, right) or muscle (TA, VL). Significant modulation was evident in 78% of VL and TA muscles (all p < 0.05). Excitability increased in 60%, but decreased in 18%. For 43% when excitability increased, a simultaneous decrease in excitability was evident in homologous muscle responses providing support for our hypothesis.

CONCLUSIONS

The results indicate a modest effectiveness and focality of anodal tDCS when applied to lower limb M1, suggesting in a human model that the strength and depth of polarizing cortical currents induced by tDCS likely depend on inter-individual differences in the electrical properties of superficial brain structures.

The role of the unaffected hemisphere in motor recovery after stroke

The contribution of the ipsilateral (nonaffected) hemisphere to recovery of motor function after stroke is controversial. Under the assumption that functionally relevant areas within the ipsilateral motor system should be tightly coupled to the demand we used fMRI and acoustically paced movements of the right index finger at six different frequencies to define the role of these regions for recovery after stroke. Eight well-recovered patients with a chronic striatocapsular infarction of the left hemisphere were compared with eight age-matched participants. As expected the hemodynamic response increased linearly with the frequency of the finger movements at the level of the left supplementary motor cortex (SMA) and the left primary sensorimotor cortex (SMC) in both groups. In contrast, a linear increase of the hemodynamic response with higher tapping frequencies in the right premotor cortex (PMC) and the right SMC was only seen in the patient group. These results support the model of an enhanced bihemispheric recruitment of preexisting motor representations in patients after subcortical stroke. Since all patients had excellent motor recovery contralesional SMC activation appears to be efficient and resembles the widespread, bilateral activation observed in healthy participants performing complex movements, instead of reflecting maladaptive plasticity.

The role of noninvasive techniques in stroke therapy

Noninvasive techniques such as functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) have provided insight into understanding how neural connections are altered in consequence to cerebrovascular injury. The first part of this review will briefly survey some of the methodological issues and limitations related to noninvasive poststroke motor recovery studies. The second section will investigate some of the different neural mechanisms that underlie neurorehabilitation in stroke patients. The third part will explore our current understanding of motor memory processing, describe the neural structures that subserve motor memory consolidation, and discuss the current literature related to memory reconsolidation in healthy adults. Lastly, this paper will suggest the potential therapeutic applications of integrating noninvasive tools with memory consolidation and reconsolidation theories to enhance motor recovery. The overall objective of this work is to demonstrate how noninvasive technologies have been utilized in the multidisciplinary field of clinical behavioral neuroscience and to highlight their potential to be employed as clinical tools to promote individualized motor recovery in stroke patients.

[Cerebral plasticity: from bench to bedside in stroke treatment]

It has long been believed that cerebral lesions were irreversible in the adult human brain. However, the spontaneous improvement in functional outcome observed in the following weeks after cerebral ischemia suggests plasticity phenomenons involving postischemic neuronal network reorganization. Regarding the large prevalence of stroke in industrialized countries, and the few available treatments, the understanding of cerebral plasticity has become an important issue but also a potential source of new therapeutic approaches in stroke. Thus, "constraint induced therapy" and repetitive transcranial magnetic stimulation (rTMS) are based on the concept of local but also remote consequences of the ischemic focal lesion. Cell-therapy is based on the capacity of stem cells to respond to hypoxic signals and adapt their phenotype to the host organ, but above all to release cytokines locally and boost endogeneous repair mechanisms. We could consider to perform in the future a sequential treatment with fibrinolysis, stem cell therapy, repetitive transcranial magnetic stimulation and constraint-induced therapy in the same patient.

Improved discrimination of visual stimuli following repetitive transcranial magnetic stimulation

Background

Repetitive transcranial magnetic stimulation (rTMS) at certain frequencies increases thresholds for motor-evoked potentials and phosphenes following stimulation of cortex. Consequently rTMS is often assumed to introduce a “virtual lesion” in stimulated brain regions, with correspondingly diminished behavioral performance.

Methodology/Principal Findings

Here we investigated the effects of rTMS to visual cortex on subjects' ability to perform visual psychophysical tasks. Contrary to expectations of a visual deficit, we find that rTMS often improves the discrimination of visual features. For coarse orientation tasks, discrimination of a static stimulus improved consistently following theta-burst stimulation of the occipital lobe. Using a reaction-time task, we found that these improvements occurred throughout the visual field and lasted beyond one hour post-rTMS. Low-frequency (1 Hz) stimulation yielded similar improvements. In contrast, we did not find consistent effects of rTMS on performance in a fine orientation discrimination task.

Conclusions/Significance

Overall our results suggest that rTMS generally improves or has no effect on visual acuity, with the nature of the effect depending on the type of stimulation and the task. We interpret our results in the context of an ideal-observer model of visual perception.

Repetitive transcranial magnetic stimulation at 1Hz and 5Hz produces sustained improvement in motor function and disability after ischaemic stroke

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is a simple and non-invasive method of augmenting motor recovery after stroke, probably mediated by restoring inter-hemispheric activation balance. This placebo-controlled pilot study examined the possible benefit of stimulating the lesioned hemisphere (5-Hz rTMS) or inhibiting the contra-lesional hemisphere (1-Hz rTMS) on clinical recovery of motor function in patients with ischaemic stroke and assessed the sustainability of the response.

METHODS

Sixty patients with ischaemic stroke (>1 month from onset) with mild-to-moderate hemiparesis were randomized to receive 10 daily sessions of either sham rTMS, 5-Hz ipsi-lesional rTMS or 1-Hz contra-lesional rTMS, in addition to a standard physical therapy protocol. Serial assessments were made over a period of 12 weeks by the thumb-index finger tapping test (FT), Activity Index (AI) score and the modified Rankin Scale (mRS).

RESULTS

In contrast to control patients, those receiving active rTMS as ipsi-lesional 5-Hz stimulation or 1-Hz contra-lesional stimulation showed statistically significant improvement on the FT test, AI scores and mRS score at 2 weeks, and the effect was sustained over the 12-week observation period. No significant adverse events were observed during treatment in either group.

CONCLUSIONS

Repetitive TMS has beneficial effects on motor recovery that can be translated to clinically meaningful improvement in disability in patients with post-stroke hemiparesis, with a well-sustained effect. The similarity of inhibitory and stimulatory rTMS in producing these effects supports the inter-hemispheric balance hypothesis and encourages further research into their use in long-term neurorehabilitation programmes of patients with stroke.

Stimulating conversation: enhancement of elicited propositional speech in a patient with chronic non-fluent aphasia following transcranial magnetic stimulation

Although evidence suggests that patients with left hemisphere strokes and non-fluent aphasia who receive 1Hz repetitive transcranial magnetic stimulation (rTMS) over the intact right inferior frontal gyrus experience persistent benefits in naming, it remains unclear whether the effects of rTMS in these patients generalize to other language abilities. We report a subject with chronic non-fluent aphasia who showed stable deficits of elicited propositional speech over the course of 5 years, and received 1200 pulses of 1Hz rTMS daily for 10 days at a site identified as being optimally responsive to rTMS in this patient. Consistent with prior studies there was improvement in object naming, with a statistically significant improvement in action naming. Improvement was also demonstrated in picture description at 2, 6, and 10 months after rTMS with respect to the number of narrative words and nouns, sentence length, and use of closed class words. Compared to his baseline performance, the patient showed significant improvement on the Western Aphasia Battery (WAB) subscale for spontaneous speech. These findings suggest that manipulation of the intact contralesional cortex in patients with non-fluent aphasia may result in language benefits that generalize beyond naming to include other aspects of language production.

Minimal forced use without constraint stimulates spontaneous use of the impaired upper extremity following motor cortex injury

The purpose of this study was to determine if recovery of neurologically impaired hand function following isolated motor cortex injury would occur without constraint of the non-impaired limb, and without daily forced use of the impaired limb. Nine monkeys (Macaca mulatta) received neurosurgical lesions of various extents to arm representations of motor cortex in the hemisphere contralateral to the preferred hand. After the lesion, no physical constraints were placed on the ipsilesional arm/hand and motor testing was carried out weekly with a maximum of 40 attempts in two fine motor tasks that required use of the contralesional hand for successful food acquisition. These motor tests were the only "forced use" of the contralesional hand. We also tested regularly for spontaneous use of the contralesional hand in a fine motor task in which either hand could be used for successful performance. This minimal intervention was sufficient to induce recovery of the contralesional hand to such a functional level that eight of the monkeys chose to use that hand on some trials when either hand could be used. Percentage use of the contralesional hand (in the task when either hand could be used) varied considerably among monkeys and was not related to lesion volume or recovery of motor skill. These data demonstrate a remarkable capacity for recovery of spontaneous use of the impaired hand following localized frontal lobe lesions. Clinically, these observations underscore the importance of therapeutic intervention to inhibit the induction of the learned nonuse phenomenon after neurological injury.

Modulating cortical connectivity in stroke patients by rTMS assessed with fMRI and dynamic causal modeling

Data derived from transcranial magnetic stimulation (TMS) studies suggest that transcallosal inhibition mechanisms between the primary motor cortex of both hemispheres may contribute to the reduced motor performance of stroke patients. We here investigated the potential of modulating pathological interactions between cortical motor areas by means of repetitive TMS using functional magnetic resonance imaging (fMRI) and dynamic causal modeling (DCM). Eleven subacute stroke patients were scanned 1-3 months after symptom onset while performing whole hand fist closure movements. After a baseline scan, patients were stimulated with inhibitory 1-Hz rTMS applied over two different locations: (i) vertex (control stimulation) and (ii) primary motor cortex (M1) of the unaffected (contralesional) hemisphere. Changes in the endogenous and task-dependent effective connectivity were assessed by DCM of a bilateral network comprising M1, lateral premotor cortex, and the supplementary motor area (SMA). The results showed that rTMS applied over contralesional M1 significantly improved the motor performance of the paretic hand. The connectivity analysis revealed that the behavioral improvements were significantly correlated with a reduction of the negative influences originating from contralesional M1 during paretic hand movements. Concurrently, endogenous coupling between ipsilesional SMA and M1 was significantly enhanced only after rTMS applied over contralesional M1. Therefore, rTMS applied over contralesional M1 may be used to transiently remodel the disturbed functional network architecture of the motor system. The connectivity analyses suggest that both a reduction of pathological transcallosal influences (originating from contralesional M1) and a restitution of ipsilesional effective connectivity between SMA and M1 underlie improved motor performance.

6-Hz primed low-frequency rTMS to contralesional M1 in two cases with middle cerebral artery stroke

This case study contrasted two subjects with stroke who received 6-Hz primed low-frequency repetitive transcranial magnetic stimulation (rTMS) to the contralesional primary motor area (M1) to disinhibit ipsilesional M1. Functional magnetic resonance imaging (fMRI) showed that the intervention disrupted cortical activation at contralesional M1. Subject 1 showed decreased intracortical inhibition and increased intracortical facilitation following intervention during paired-pulse TMS testing of ipsilesional M1. Subject 2, whose precentral knob was totally obliterated and who did not show an ipsilesional motor evoked potential at pretest, still did not show any at posttest; however, her fMRI did show a large increase in peri-infarct zone cortical activation. Behavioral results were mixed, indicating the need for accompanying behavioral training to capitalize on the brain organization changes induced with rTMS.

Characterizing the application of transcranial direct current stimulation in human pharyngeal motor cortex

Transcranial direct current stimulation (tDCS) is a novel intervention that can modulate brain excitability in health and disease; however, little is known about its effects on bilaterally innervated systems such as pharyngeal motor cortex. Here, we assess the effects of differing doses of tDCS on the physiology of healthy human pharyngeal motor cortex as a prelude to designing a therapeutic intervention in dysphagic patients. Healthy subjects (n = 17) underwent seven regimens of tDCS (anodal 10 min 1 mA, cathodal 10 min 1 mA, anodal 10 min 1.5 mA, cathodal 10 min 1.5 mA, anodal 20 min 1 mA, cathodal 20 min 1 mA, Sham) on separate days, in a double blind randomized order. Bihemispheric motor evoked potential (MEP) responses to single-pulse transcranial magnetic stimulation (TMS) as well as intracortical facilitation (ICF) and inhibition (ICI) were recorded using a swallowed pharyngeal catheter before and up to 60 min following the tDCS. Compared with sham, both 10 min 1.5 mA and 20 min 1 mA anodal stimulation induced increases in cortical excitability in the stimulated hemisphere (+44 +/- 17% and +59 +/- 16%, respectively; P < 0.005) whereas only 10 min 1.5 mA cathodal stimulation induced inhibition (-26 +/- 4%, P = 0.02). There were neither contralateral hemisphere changes nor any evidence for ICI or ICF in driving the ipsilateral effects. In conclusion, anodal tDCS can alter pharyngeal motor cortex excitability in an intensity-dependent manner, with little evidence for transcallosal spread. Anodal stimulation may therefore provide a useful means of stimulating pharyngeal cortex and promoting recovery in dysphagic patients.

Differential effects of high-frequency repetitive transcranial magnetic stimulation over ipsilesional primary motor cortex in cortical and subcortical middle cerebral artery stroke

OBJECTIVE

Facilitation of cortical excitability of the ipsilesional primary motor cortex (M1) may improve dexterity of the affected hand after stroke. The effects of 10 Hz repetitive transcranial magnetic stimulation (rTMS) over ipsilesional M1 on movement kinematics and neural activity were examined in patients with subcortical or cortical stroke.

METHODS

Twenty-nine patients with impaired dexterity after stroke (16 subcortical middle cerebral artery [MCA] strokes, 13 MCA strokes involving subcortical tissue and primary or secondary cortical sensorimotor areas) received 1 session of 10 Hz rTMS (5-second stimulation, 25-second break, 1,000 pulses, 80% of the resting motor threshold) applied over: 1) ipsilesional M1 and 2) vertex (control stimulation). For behavioral testing, 29 patients performed index finger and hand tapping movements with the affected and unaffected hand prior to and following each rTMS application. For functional magnetic resonance imaging, 18 patients performed index finger tapping movements with the affected and unaffected hand before and after each rTMS application.

RESULTS

Ten-Hz rTMS over ipsilesional M1, but not over vertex, improved movement kinematics in 14 of 16 patients with subcortical stroke, but not in patients with additional cortical stroke. Ten-Hz rTMS slightly deteriorated dexterity of the affected hand in 7 of 13 cortical stroke patients. At a neural level, rTMS over ipsilesional M1 reduced neural activity of the contralesional M1 in 11 patients with subcortical stroke, but caused a widespread bilateral recruitment of primary and secondary motor areas in 7 patients with cortical stroke. Activity in ipsilesional M1 at baseline correlated with improvement of index finger tapping frequency induced by rTMS.

INTERPRETATION

The beneficial effects of 10 Hz rTMS over ipsilesional M1 on motor function of the affected hand depend on the extension of MCA stroke. Neural activity in ipsilesional M1 may serve as a surrogate marker for the effectiveness of facilitatory rTMS.

Motor cortex plasticity predicts recovery in acute stroke

Repetitive transcranial magnetic stimulation of the brain given as intermittent theta burst stimulation (iTBS) can induce long-term potentiation (LTP)-like changes in the stimulated hemisphere and long-term depression (LTD)-like changes in the opposite hemisphere. We evaluated whether LTP- and LTD-like changes produced by iTBS in acute stroke correlate with outcome at 6 months. We evaluated the excitability of affected hemisphere (AH) and unaffected hemisphere (UH) by measuring motor threshold and motor-evoked potential (MEP) amplitude under baseline conditions and after iTBS of AH in 17 patients with acute ischemic stroke. Baseline amplitude of MEPs elicited from AH was significantly smaller than that of MEPs elicited from UH, and baseline motor threshold was higher for the AH. Higher baseline MEP values in UH correlated with poor prognosis. iTBS produced a significant increase in MEP amplitude for AH that was significantly correlated with recovery. A nonsignificant decrease in MEP amplitude was observed for the UH. When the decrease in the amplitude of UH MEPs was added to the regression model, the correlation was even higher. Functional recovery is directly correlated with LTP-like changes in AH and LTD-like changes in UH and inversely correlated with the baseline excitability of UH.

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.

Low frequency (0.5Hz) rTMS over the right (non-dominant) motor cortex does not affect ipsilateral hand performance in healthy humans

Reduction of excitability of the dominant primary motor cortex (M1) improves ipsilateral hand function in healthy subjects. In analogy, inhibition of non-dominant M1 should also improve ipsilateral performance. In order to investigate this hypothesis, we have used slow repetitive transcranial magnetic stimulation (rTMS) and the Purdue Pegboard test. Twenty-eight volunteers underwent 10 minutes of either 0.5Hz rTMS over right M1 or sham rTMS (coil perpendicular to scalp). The motor task was performed before, immediately after, and 20 minutes after rTMS. In both groups, motor performance improved significantly throughout the sessions. rTMS inhibition of the non-dominant M1 had no significant influence over ipsilateral or contralateral manual dexterity, even though the results were limited by unequal performance between groups at baseline. This is in contrast to an improvement in left hand function previously described following slow rTMS over left M1, and suggests a less prominent physiological transcallosal inhibition from right to left M1.

Interhemispheric competition after stroke: brain stimulation to enhance recovery of function of the affected hand

BACKGROUND AND PURPOSE

Within the concept of interhemispheric competition, technical modulation of the excitability of motor areas in the contralesional and ipsilesional hemisphere has been applied in an attempt to enhance recovery of hand function following stroke. This review critically summarizes the data supporting the use of novel electrophysiological concepts in the rehabilitation of hand function after stroke.

SUMMARY OF REVIEW

Repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are powerful tools to inhibit or facilitate cortical excitability. Modulation of cortical excitability may instantaneously induce plastic changes within the cortical network of sensorimotor areas, thereby improving motor function of the affected hand after stroke. No significant adverse effects have been noted when applying brain stimulation in stroke patients. To date, however, the clinical effects are small to moderate and short lived. Future work should elucidate whether repetitive administration of rTMS or tDCS over several days and the combination of these techniques with behavioral training (ie, physiotherapy) could result in an enhanced effectiveness.

CONCLUSION

Brain stimulation is a safe and promising tool to induce plastic changes in the cortical sensorimotor network to improve motor behavior after stroke. However, several methodological issues remain to be answered to further improve the effectiveness of these new approaches.

Recovery of function in humans: cortical stimulation and pharmacological treatments after stroke

In this contribution, we first provide an overview of general principles of reorganisation in the human brain, and point out possible biomarkers of recovery. Subsequently, we expand on possibilities of adjuvant therapy in human rehabilitation using cortical stimulation and pharmacological treatments. Finally, we suggest future directions for research in this field.

Modulation of effects of intermittent theta burst stimulation applied over primary motor cortex (M1) by conditioning stimulation of the opposite M1

The excitability of the human primary motor cortex (M1) as tested with transcranial magnetic stimulation (TMS) depends on its previous history of neural activity. Homeostatic plasticity might be one important physiological mechanism for the regulation of corticospinal excitability and synaptic plasticity. Although homeostatic plasticity has been demonstrated locally within M1, it is not known whether priming M1 could result in similar homeostatic effects in the homologous M1 of the opposite hemisphere. Here, we sought to determine whether down-regulating excitability (priming) in the right (R) M1 with 1-Hz repetitive transcranial magnetic stimulation (rTMS) changes the excitability-enhancing effect of intermittent theta burst stimulation (iTBS) applied over the homologous left (L) M1. Subjects were randomly allocated to one of four experimental groups in a sham-controlled parallel design with real or sham R M1 1-Hz TMS stimulation always preceding L M1 iTBS or sham by about 10 min. The primary outcome measure was corticospinal excitability in the L M1, as measured by recruitment curves (RCs). Secondary outcome measures included pinch force, simple reaction time, and tapping speed assessed in the right hand. The main finding of this study was that preconditioning R M1 with 1-Hz rTMS significantly decreased the excitability-enhancing effects of subsequent L M1 iTBS on RCs. Application of 1-Hz rTMS over R M1 alone and iTBS over L M1 alone resulted in increased RC in L M1 relative to sham interventions. The present findings are consistent with the hypothesis that homeostatic mechanisms operating across hemispheric boundaries contribute to regulate motor cortical function in the primary motor cortex.

Cortical Activity in Relation to Velocity Dependent Movement Resistance in the Flexor Muscles of the Hand After Stroke

Background. The role of spinal networks in spasticity is well investigated, but little is known about possible cortical contributions to hypertonicity across a joint. Objective. The authors hypothesized that there are cortical activation correlates to spasticity in stroke patients with increased muscle tone of the wrist flexors. Methods. Stroke patients and controls were scanned using event-related functional magnetic resonance imaging (fMRI) during slow and fast passive movements of the hand with simultaneous recording of passive movement resistance (PMR). Results. Control participants had velocity-dependent activity (greater for slow than fast movements) of 2 types, in areas that were also more active in passive movement than rest (eg, relative increase in activation in contralateral S1 and M1 was greater for slow than fast) and in areas that were also more active in rest than passive movement (eg, relative decrease in activation in occipital areas and ipsilateral precentral gyrus was greater for fast than slow). In the patient group, with large interindividual variation of spasticity, we found an association between PMR and the velocity-dependent activity in ipsilateral S1 (area 3b) extending into M1 (area 4a), contralateral cingulate cortex, supplementary motor area (SMA), Brodmann Area 45 (BA 45), and cerebellum. Post hoc testing also revealed a similar correlation in S1 and M1 bilaterally in controls and showed that patients activated ipsilateral S1 and M1 more than controls in the velocity-dependent condition. Conclusions. The findings suggest the possibility of ipsilateral sensory and motor cortical involvement in spasticity after stroke, which warrant further investigation.

Invasive cortical stimulation to promote recovery of function after stroke: a critical appraisal

BACKGROUND AND PURPOSE

Residual motor deficits frequently linger after stroke. Search for newer effective strategies to promote functional recovery is ongoing. Brain stimulation, as a means of directing adaptive plasticity, is appealing. Animal studies and Phase I and II trials in humans have indicated safety, feasibility, and efficacy of combining rehabilitation and concurrent invasive cortical stimulation. However, a recent Phase III trial showed no advantage of the combination. We critically review results of various trials and discuss the factors that contributed to the distinctive result.

SUMMARY OF REVIEW

Regarding cortical stimulation, it is important to determine the (1) location of peri-infarct representations by integrating multiple neuroanatomical and physiological techniques; (2) role of other mechanisms of stroke recovery; (3) viability of peri-infarct tissue and descending pathways; (4) lesion geometry to ensure no alteration/displacement of current density; and (5) applicability of lessons generated from noninvasive brain stimulation studies in humans. In terms of combining stimulation with rehabilitation, we should understand (1) the principle of homeostatic plasticity; (2) the effect of ongoing cortical activity and phases of learning; and (3) that subject-specific intervention may be necessary.

CONCLUSIONS

Future cortical stimulation trials should consider the factors that may have contributed to the peculiar results of the Phase III trial and address those in future study designs.

Mechanisms and applications of theta-burst rTMS on the human motor cortex

Theta-burst Stimulation (TBS) is a novel form of repetitive transcranial magnetic stimulation (rTMS). Applied over the primary motor cortex it has been successfully used to induce changes in cortical excitability. The advantage of this stimulation paradigm is that it is able to induce strong and long lasting effects using a lower stimulation intensity and a shorter time of stimulation compared to conventional rTMS protocols. Since its first description, TBS has been used in both basic and clinical research in the last years and more recently it has been expanded to other domains than the motor system. Its capacity to induce synaptic plasticity could lead to therapeutic implications for neuropsychiatric disorders. The neurobiological mechanisms of TBS are not fully understood at present; they may involve long-term potentiation (LTP)- and depression (LTD)-like processes, as well as inhibitory mechanisms modulated by GABAergic activity. This article highlights current hypotheses regarding the mechanisms of action of TBS and some central factors which may influence cortical responses to TBS. Furthermore, previous and ongoing research performed in the field of TBS on the motor cortex is summarized.

Interaction Between Finger Opposition Movements and Aftereffects of 1Hz-rTMS on Ipsilateral Motor Cortex

One-hertz repetitive transcranial magnetic stimulation (1Hz-rTMS) over ipsilateral motor cortex is able to modify up to 30 min the motor performance of repetitive finger opposition movements paced with a metronome at 2 Hz. We investigated whether the long-lasting rTMS effect on motor behavior can be modulated by subsequent engagement of the contralateral sensorimotor system. Motor task was performed in different experimental conditions: immediately after rTMS, 30 min after rTMS, or when real rTMS was substituted with sham rTMS. Subjects performing the motor task immediately after rTMS showed modifications in motor behavior ≤30 min after rTMS. On the other hand, when real rTMS was substituted with sham stimulation or when subjects performed the motor task 30 min after the rTMS session, the effect was no longer present. These findings suggest that the combination of ipsilateral 1Hz-rTMS and voluntary movement is crucial to endure the effect of rTMS on the movement itself, probably acting on synaptic plasticity-like mechanism. This finding might provide some useful hints for neurorehabilitation protocols.

Modulating activity in the motor cortex affects performance for the two hands differently depending upon which hemisphere is stimulated

We modulated neural excitability in the human motor cortex to investigate behavioral effects for both hands. In a previous study, we showed that decreasing excitability in the dominant motor cortex led to a decline in performance for the contralateral hand and an improvement for the ipsilateral hand; increasing excitability produced the opposite effects. Research suggests that the ipsilateral effects were mediated by interhemispheric inhibition. Physiological evidence points to an asymmetry in interhemispheric inhibition between the primary motor cortices, with stronger inhibitory projections coming from the dominant motor cortex. In the present study, we examined whether there is a hemispheric asymmetry in the effects on performance when modulating excitability in the motor cortex. Anodal and cathodal transcranial direct current stimulation were applied to the motor cortex of 17 participants, targeting the non-dominant hemisphere on one day and the dominant hemisphere on another day, along with one sham session. Participants performed a finger-sequence coordination task with each hand before and after stimulation. The dependent variable was calculated as the percentage of change in the number of correct keystrokes. We found that the effects of transcranial direct current stimulation depended upon which hemisphere was stimulated; modulating excitability in the dominant motor cortex significantly affected performance for the contralateral and ipsilateral hands, whereas modulating excitability in the non-dominant motor cortex only had a significant impact for the contralateral hand. These results provide evidence for a hemispheric asymmetry in the ipsilateral effects of modulating excitability in the motor cortex and may be important for clinical research on motor recovery.

Transcranial direct current stimulation: a noninvasive tool to facilitate stroke recovery

Electrical brain stimulation, a technique developed many decades ago and then largely forgotten, has re-emerged recently as a promising tool for experimental neuroscientists, clinical neurologists and psychiatrists in their quest to causally probe cortical representations of sensorimotor and cognitive functions and to facilitate the treatment of various neuropsychiatric disorders. In this regard, a better understanding of adaptive and maladaptive plasticity in natural stroke recovery over the last decade and the idea that brain polarization may modulate neuroplasticity has led to the use of transcranial direct current stimulation (tDCS) as a potential enhancer of natural stroke recovery. We will review tDCS's successful utilization in pilot and proof-of-principle stroke recovery studies, the different modes of tDCS currently in use, and the potential mechanisms underlying the neural effects of tDCS.

Induction of Cortical Plastic Changes in Wrist Muscles by Paired Associative Stimulation in the Recovery Phase of Stroke Patients

Background. Paired associative stimulation (PAS) combining peripheral nerve and transcranial magnetic stimulation (TMS) have been proposed to induce long-term changes in excitability of the cerebral cortex and potentially optimize motor recovery in stroke patients. Objective. This pilot study examined whether short-lasting changes in cortical excitability could be induced by a single session of PAS within the first months after stroke. Methods. Six hemiparetic patients with a subcortical stroke were included. The single session PAS protocol was applied at 1, 5, and 12 months after stroke. During the follow-up, the clinical recovery of wrist function was assessed in parallel to the PAS study by the Fugl-Meyer motor scale and dynamometry of wrist extension. Results. The PAS protocol induced a significant extensor carpi radialis motor evoked potential facilitation (mean +78.5%) on the paretic side 5 months after stroke. The facilitation was still present 12 months after stroke but on average smaller (+30 %). Conclusions. These electrophysiological findings suggest that patients with subcortical infarcts may respond to PAS in an earlier than later period after stroke. If the clinical efficacy of interventions such as PAS is confirmed, it could be proposed early as add-on therapy to optimize training-induced plasticity processes.

Poststroke dysphagia rehabilitation by repetitive transcranial magnetic stimulation: a noncontrolled pilot study

Poststroke dysphagia is frequent and significantly increases patient mortality. In two thirds of cases there is a spontaneous improvement in a few weeks, but in the other third, oropharyngeal dysphagia persists. Repetitive transcranial magnetic stimulation (rTMS) is known to excite or inhibit cortical neurons, depending on stimulation frequency. The aim of this noncontrolled pilot study was to assess the feasibility and the effects of 1-Hz rTMS, known to have an inhibitory effect, on poststroke dysphagia. Seven patients (3 females, age = 65 +/- 10 years), with poststroke dysphagia due to hemispheric or subhemispheric stroke more than 6 months earlier (56 +/- 50 months) diagnosed by videofluoroscopy, participated in the study. rTMS at 1 Hz was applied for 20 min per day every day for 5 days to the healthy hemisphere to decrease transcallosal inhibition. The evaluation was performed using the dysphagia handicap index and videofluoroscopy. The dysphagia handicap index demonstrated that the patients had mild oropharyngeal dysphagia. Initially, the score was 43 +/- 9 of a possible 120 which decreased to 30 +/- 7 (p < 0.05) after rTMS. After rTMS, there was an improvement of swallowing coordination, with a decrease in swallow reaction time for liquids (p = 0.0506) and paste (p < 0.01), although oral transit time, pharyngeal transit time, and laryngeal closure duration were not modified. Aspiration score significantly decreased for liquids (p < 0.05) and residue score decreased for paste (p < 0.05). This pilot study demonstrated that rTMS is feasible in poststroke dysphagia and improves swallowing coordination. Our results now need to be confirmed by a randomized controlled study with a larger patient population.

Dual-hemisphere tDCS facilitates greater improvements for healthy subjects' non-dominant hand compared to uni-hemisphere stimulation

Background

Transcranial direct current stimulation (tDCS) is a non-invasive technique that has been found to modulate the excitability of neurons in the brain. The polarity of the current applied to the scalp determines the effects of tDCS on the underlying tissue: anodal tDCS increases excitability, whereas cathodal tDCS decreases excitability. Research has shown that applying anodal tDCS to the non-dominant motor cortex can improve motor performance for the non-dominant hand, presumably by means of changes in synaptic plasticity between neurons. Our previous studies also suggest that applying cathodal tDCS over the dominant motor cortex can improve performance for the non-dominant hand; this effect may result from modulating inhibitory projections (interhemispheric inhibition) between the motor cortices of the two hemispheres. We hypothesized that stimultaneously applying cathodal tDCS over the dominant motor cortex and anodal tDCS over the non-dominant motor cortex would have a greater effect on finger sequence performance for the non-dominant hand, compared to stimulating only the non-dominant motor cortex. Sixteen right-handed participants underwent three stimulation conditions: 1) dual-hemisphere – with anodal tDCS over the non-dominant motor cortex, and cathodal tDCS over the dominant motor cortex, 2) uni-hemisphere – with anodal tDCS over the non-dominant motor cortex, and 3) sham tDCS. Participants performed a finger-sequencing task with the non-dominant hand before and after each stimulation. The dependent variable was the percentage of change in performance, comparing pre- and post-tDCS scores.

Results

A repeated measures ANOVA yielded a significant effect of tDCS condition (F(2,30) = 4.468, p = .037). Post-hoc analyses revealed that dual-hemisphere stimulation improved performance significantly more than both uni-hemisphere (p = .021) and sham stimulation (p = .041).

Conclusion

We propose that simultaneously applying cathodal tDCS over the dominant motor cortex and anodal tDCS over the non-dominant motor cortex produced an additive effect, which facilitated motor performance in the non-dominant hand. These findings are relevant to motor skill learning and to research studies of motor recovery after stroke.

Levodopa improves procedural motor learning in chronic stroke patients

OBJECTIVE

To test the hypothesis that administration of dopamine precursor levodopa improves procedural motor learning (defined as the ability to acquire novel movement patterns gradually through practice) in patients with residual motor deficits in the chronic phase after stroke (> or =1 y after stroke).

DESIGN

A double-blind, placebo-controlled, randomized crossover design.

SETTING

Neurology department in a German university.

PARTICIPANTS

Eighteen patients with chronic motor dysfunction because of stroke (13 men, 5 women; age range, 53-78 y; mean time poststroke +/- SD, 3.3+/-2.1 y).

INTERVENTION

Patients received 3 doses of levodopa (100mg of levodopa plus 25mg of carbidopa) or placebo before 1 session of procedural motor learning.

MAIN OUTCOME MEASURES

Procedural motor learning performed by using the paretic hand assessed by using a modified version of the serial reaction time task with a probabilistic sequence. The primary outcome measure was the difference in reaction times between random and sequential elements.

RESULTS

Levodopa significantly improved our primary outcome measure, procedural motor learning, compared with placebo (P<.05). Reaction times to random elements, analysis of error rates, psychophysical assessments, and performance in a simple motor task were comparable between conditions, indicating that better learning under levodopa was not caused by differences in response styles, arousal, mood, or motor reaction times but that levodopa modulated learning.

CONCLUSIONS

Our results show that levodopa may improve procedural motor learning in patients with chronic stroke, in line with our hypothesis. These findings suggest that this interventional strategy in combination with customary rehabilitative treatments could significantly improve the outcome of neurorehabilitation in the chronic stage after stroke. (Clinicaltrials.gov identifier NCT00126087.)

Design for the Everest Randomized Trial of Cortical Stimulation and Rehabilitation for Arm Function Following Stroke

Background. Cortical stimulation (CS) combined with rehabilitation may improve upper limb motor function after stroke. Objective. Describe the study design for the Everest Clinical Trial, a randomized single-blinded pivotal device trial, testing safety and efficacy of epidural CS delivered during rehabilitation for upper limb motor function in patients with ischemic stroke. Method . A total of 174 participants from 21 centers with hemiplegia at least 4 months after acute ischemic stroke are randomized in a 2:1 ratio to investigational or control groups. Investigational patients undergo implantation of cortical electrode and pulse generator and receive 6 weeks of upper limb rehabilitation with subthreshold CS delivered only during therapy. Control group patients receive the same therapy without device implantation or stimulation. Primary outcome measures include the upper extremity Fugl-Meyer (UEFM) score and the arm motor ability test (AMAT) measured at baseline and 1, 4, 12, and 24 weeks following rehabilitation treatment with primary endpoint at 4 weeks following treatment. A successful outcome is defined as an improvement in UEFM of at least 4.5 points and in AMAT of at least 0.21 points from baseline to primary endpoint. A 20% better success rate between investigational and control groups will be considered clinically meaningful. Adverse events occurring during the study will be identified. Results. Not applicable. Conclusions . The Everest Clinical Trial is the first randomized pivotal trial on the safety and efficacy of direct CS delivered during rehabilitation for recovery of upper limb motor function in patients with ischemic stroke.

Impaired interhemispheric interactions in patients with major depression

Previous studies have shown that patients with major depression have an interhemispheric imbalance between right and left prefrontal and motor cortex. We aimed to investigate the interhemispheric interactions in patients with major depression using repetitive transcranial magnetic stimulation (rTMS). Thirteen patients with major depression and 14 age-matched healthy subjects participated in this study. Corticospinal excitability before and after 1 Hz rTMS (applied to the left primary motor cortex) was assessed in the left and right motor cortex and these results were compared with those in healthy subjects. There was a significant difference in the interhemispheric effects between patients with depression and healthy subjects. In healthy subjects, 1 Hz rTMS significantly decreased corticospinal excitability in the stimulated, left hemisphere and increased it in the contralateral, right hemisphere. In depressed subjects, 1 Hz rTMS also decreased corticospinal excitability in the left hemisphere; however, it induced no significant changes in corticospinal excitability in the contralateral, right hemisphere. In addition, there was a significant correlation between the degree of interhemispheric modulation and the severity of the depression as indexed by the Beck Depression Inventory scores. Our findings showing a decreased interhemispheric modulation in patients with major depression are consistent with the notion that mood disorders are associated with slow interhemispheric switching mechanisms.