Interhemispheric Imbalance Research Articles

SY1.6. Therapeutic application of transcranial magnetic stimulation in stroke

Hundreds of studies have investigated the possible benefits of non-invasive brain stimulation techniques for promoting recovery and outcomes after stroke. Most of these have applied suppressive forms of transcranial magnetic and direct current stimulation to contralesional motor cortex, based on the interhemispheric imbalance model. This model posits that the contralesional primary motor cortex is hyperexcitable after stroke and excessively inhibits the ipsilesional primary motor cortex via transcallosal projections. The assumption is that reorganisation and recovery in the ipsilesional hemisphere can be promoted by reducing the excitability of contralesional hemisphere, to reduce its transcallosal inhibition of the ipsilesional hemisphere. However, this general strategy doesn’t reliably improve motor performance, and non-invasive brain stimulation techniques have not translated to routine clinical care. This presentation will identify the limitations of previous work, and draw from more recent studies that prompt a re-think of the interhemispheric imbalance model, and the role of interhemispheric inhibition in motor control. Deepening our understanding of interhemispheric interactions after stroke may enable more personalised therapeutic applications of transcranial stimulation to promote recovery and improve outcomes.

Altered interhemispheric signal propagation in schizophrenia and depression

ObjectiveAltered interhemispheric connectivity is implicated in the pathophysiology of schizophrenia (SCZ) and major depressive disorder (MDD) and may account for deficits in lateralized cognitive processes. We measured transcranial magnetic stimulation evoked interhemispheric signal propagation (ISP), a non-invasive measure of transcallosal connectivity, and hypothesized that the SCZ and MDD groups will have increased ISP compared to healthy controls. MethodsWe evaluated ISP over the dorsolateral prefrontal cortex in 34 patients with SCZ and 34 patients with MDD compared to 32 age and sex-matched healthy controls. ResultsISP was significantly increased in patients with SCZ and patients with MDD compared to healthy controls but did not differ between patient groups. There were no effects of antidepressant, antipsychotic, and benzodiazepine medications on ISP and our results remained unchanged after re-analysis with a region of interest method. ConclusionAltered ISP was found in both SCZ and MDD patient groups. This indicates that disruptions of interhemispheric signaling processes can be indexed with ISP across psychiatric populations. SignificanceThese findings enhance our knowledge of the physiological mechanisms of interhemispheric imbalances in SCZ and MDD, which may serve as potential treatment targets in future patients.

Evidence for interhemispheric imbalance in stroke patients as revealed by combining transcranial magnetic stimulation and electroencephalography.

Interhemispheric interactions in stroke patients are frequently characterized by abnormalities, in terms of balance and inhibition. Previous results showed an impressive variability, mostly given to the instability of motor‐evoked potentials when evoked from the affected hemisphere. We aim to find reliable interhemispheric measures in stroke patients with a not‐evocable motor‐evoked potential from the affected hemisphere, by combining transcranial magnetic stimulation (TMS) and electroencephalography. Ninteen stroke patients (seven females; 61.26 ± 9.8 years) were studied for 6 months after a first‐ever stroke in the middle cerebral artery territory. Patients underwent four evaluations: clinical, cortical, corticospinal, and structural. To test the reliability of our measures, the evaluations were repeated after 3 weeks. To test the sensitivity, 14 age‐matched healthy controls were compared to stroke patients. In stroke patients, stimulation of the affected hemisphere did not result in any inhibition onto the unaffected. The stimulation of the unaffected hemisphere revealed a preservation of the inhibition mechanism onto the affected. This resulted in a remarkable interhemispheric imbalance, whereas this mechanism was steadily symmetric in healthy controls. This result was stable when cortical evaluation was repeated after 3 weeks. Importantly, patients with a better recovery of the affected hand strength were the ones with a more stable interhemispheric balance. Finally, we found an association between microstructural integrity of callosal fibers, suppression of interhemispheric TMS‐evoked activity and interhemispheric connectivity. We provide direct and sensitive cortical measures of interhemispheric imbalance in stroke patients. These measures offer a reliable means of distinguishing healthy and pathological interhemispheric dynamics.

P.296 Interhemispheric imbalance of motor cortex excitability in affective disorders – a systematic review and meta-analysis

Tuberculosis (TB) is of one the most infectious disease caused by Mycobacterium tuberculosis which remains a serious public health problem. Emergence of multi-drug resistant strains of M. tuberculosis led to development of new and more potent anti-tuberculosis agents. The aim of this study was to correlate the chemical structures of the inhibitory compounds with their experimental activities. In this study, analogs of 2,4-disubstituted quinoline derivatives as potent anti-tubercular agents was subjected to quantitative structure–activity relationship (QSAR) analysis in order to build a QSAR model for predicting the activities of these compounds. In order to build the regression model, Genetic Function Approximation (GFA) and Multi-linear Regression approach were used to predict the activities of inhibitory compounds. Based on the prediction, the best validation model was found to have squared correlation coefficient (R2) of 0.9367, adjusted squared correlation coefficient (R2 adj) value of 0.9223 and cross validation coefficient (Qcv2) value of 0.8752. The chosen model was subjected to external validations and the model was found to have (R2 test) of 0.8215 and Y-randomization Coefficient (cRp2) of 0.6633. The proposed QSAR model provides a valuable approach for designing more potent anti-tubercular agents.

Promotion of Poststroke Motor-Function Recovery with Repetitive Transcranial Magnetic Stimulation by Regulating the Interhemispheric Imbalance.

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain-stimulation technique that transiently modulates cerebral cortex excitability, achieving overall positive results in poststroke motor-function recovery. Excessive inhibition of the ipsilesional-affected hemisphere by the contralesional-unaffected hemisphere has seriously hindered poststroke motor-function recovery. Hence, intracortical disinhibition can be used as an approach to managing poststroke brain injury. This technique promotes neural plasticity for faster motor-function recovery. rTMS relieves unilateral inhibition of the brain function by regulatinga interhemispheric-imbalanced inhibition. This paper summarized 12 studies from 2016 to date, focusing on rTMS on motor function after acute and chronic stroke by regulating the interhemispheric imbalance of inhibitory inputs. Although rTMS studies have shown promising outcomes on recovery of motor functions in stroke patients, different intervention methods may lead to discrepancies in results. A uniform optimal stimulus model cannot routinely be used, mainly due to the stimulus schemes, stroke types and outcome-measuring differences among studies. Thus, the effect of rTMS on poststroke motor-function recovery should be investigated further to standardize the rTMS program for optimal poststroke motor-function recovery. More randomized, placebo-controlled clinical trials with standardized rTMS protocols are needed to ensure the effectiveness of the treatment.

Neurophysiological signatures of hand motor response to dual-transcranial direct current stimulation in subacute stroke: a TMS and MEG study

BackgroundDual transcranial direct current stimulation (tDCS) to the bilateral primary motor cortices (M1s) has potential benefits in chronic stroke, but its effects in subacute stroke, when behavioural effects might be expected to be greater, have been relatively unexplored. Here, we examined the neurophysiological effects and the factors influencing responsiveness of dual-tDCS in subacute stroke survivors.MethodsWe conducted a randomized sham-controlled crossover study in 18 survivors with first-ever, unilateral subcortical ischaemic stroke 2–4 weeks after stroke onset and 14 matched healthy controls. Participants had real dual-tDCS (with an ipsilesional [right for controls] M1 anode and a contralesional M1 [left for controls] cathode; 2 mA for 20mins) and sham dual-tDCS on separate days, with concurrent paretic [left for controls] hand exercise. Using transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG), we recorded motor evoked potentials (MEPs), the ipsilateral silent period (iSP), short-interval intracortical inhibition, and finger movement-related cortical oscillations before and immediately after tDCS.ResultsStroke survivors had decreased excitability in ipsilesional M1 with a relatively excessive transcallosal inhibition from the contralesional to ipsilesional hemisphere at baseline compared with controls, as quantified by decreased MEPs and increased iSP duration. Dual-tDCS led to increased MEPs and decreased iSP duration in ipsilesional M1. The magnitude of the tDCS-induced MEP increase in stroke survivors was predicted by baseline contralesional-to-ipsilesional transcallosal inhibition (iSP) ratio. Baseline post-movement synchronization in α-band activity in ipsilesional M1 was decreased after stroke compared with controls, and its tDCS-induced increase correlated with upper limb score in stroke survivors. No significant adverse effects were observed during or after dual-tDCS.ConclusionsTask-concurrent dual-tDCS in subacute stroke can safely and effectively modulate bilateral M1 excitability and inter-hemispheric imbalance and also movement-related α-activity.

Neuronavigated 1 Hz rTMS of the left angular gyrus combined with visuospatial therapy in post-stroke neglect.

Visuospatial neglect (VSN) may be caused by an inter-hemispheric imbalance of neural activity after brain injury. Repetitive transcranial magnetic stimulation (rTMS) allows rebalancing restoration to a certain degree, relieving neglect symptoms. This study investigates the therapeutic effect of 1 Hz rTMS applied over the left angular gyrus combined with visual scanning training in patients with left VSN in the subacute stroke phase. Twenty-eight patients with VSN were randomly assigned to either experimental (fifteen sessions of rTMS consisted of 1800 magnetic pulses delivered to the left angular gyrus with a neuronavigation control), or control group (fifteen sessions of sham stimulation), followed by visual scanning training. VSN severity was assessed both before and after treatment with a 3-month follow up employing the Behavioural Inattention Test and functional measures. No statistically significant differences were detected in outcome measures between the rTMS and sham groups after completion of 3-week therapy and at 3-month follow up. The magnitude of stimulation effects was not associated either with lesion volume, its location, or baseline motor threshold. Our study did not confirm efficacy of 1 Hz rTMS over the angular gyrus as an adjuvant method to visual scanning training in patients with VSN in the subacute stroke.

Linking ITCZ Migrations to the AMOC and North Atlantic/Pacific SST Decadal Variability

AbstractWe examine the link between migrations in the intertropical convergence zone (ITCZ) and changes in the Atlantic meridional overturning circulation (AMOC), Atlantic multidecadal variability (AMV), and Pacific decadal oscillation (PDO). We use a coupled climate model that allows us to integrate over climate noise and assess underlying mechanisms. We use an ensemble of ten 300-yr-long simulations forced by a 50-yr oscillatory North Atlantic Oscillation (NAO)-derived surface heat flux anomaly in the North Atlantic, and a 4000-yr-long preindustrial control simulation performed with GFDL CM2.1. In both setups, an AMV phase change induced by a change in the AMOC’s cross-equatorial heat transport forces an atmospheric interhemispheric energy imbalance that is compensated by a change in the cross-equatorial atmospheric heat transport due to a meridional ITCZ shift. Such linkages occur on decadal time scales in the ensemble driven by the imposed forcing, and internally on multicentennial time scales in the control. Regional precipitation anomalies differ between the ensemble and the control for a zonally averaged ITCZ shift of similar magnitude, which suggests a dependence on time scale. Our study supports observational evidence of an AMV–ITCZ link in the twentieth century and further links it to the AMOC, whose long-time-scale variability can influence the phasing of ITCZ migrations. In contrast to the AMV, our calculations suggest that the PDO does not drive ITCZ migrations, because the PDO does not modulate the interhemispheric energy balance.

Posterior brain lesions selectively alter alpha oscillatory activity and predict visual performance in hemianopic patients

Alpha oscillatory frequency and amplitude have been linked to visual processing and to the excitability of the visual cortex at rest. Therefore, posterior brain lesions, which damage the neural circuits of the visual system might induce alterations in the alpha oscillatory activity. To investigate this hypothesis, EEG activity was recorded during eyes-closed resting state in patients with hemianopia with posterior brain lesions, patients without hemianopia with anterior brain lesions and age-matched healthy controls. Patients with posterior lesions revealed a selective slowdown of individual alpha frequency in both the intact and the lesioned hemisphere and a reduction of alpha amplitude in the lesioned hemisphere, resulting in an interhemispheric imbalanced oscillatory alpha activity, while no significant alterations in the alpha range were found in patients with anterior lesions. This suggests a crucial role of posterior cortices in coordinating alpha oscillations in the visual system. Moreover, right posterior lesions had a more severe reduction of individual alpha frequency and altering of the interhemispheric distribution of the alpha amplitude, in line with the notion of the prominence of the right posterior cortices in balancing the interhemispheric functioning. Crucially, the duration of the in individual alpha frequency and the interhemispheric imbalance in alpha amplitude were directly linked to visuo-spatial performance across all participants and to impaired visual detection abilities in hemianopics, therefore supporting a functional role of alpha oscillations in visual processing and suggesting that activity in this frequency range at rest represents a neurophysiological marker reliably reflecting the integrity and the functionality of the visual system in humans.

Enhanced Effective Connectivity From Ipsilesional to Contralesional M1 in Well-Recovered Subcortical Stroke Patients.

Background and Purpose: Interhemispheric imbalance may provide a framework for developing new strategies to facilitate post-stroke motor recovery especially for patients in chronic stage. Using effective connectivity analysis, we aimed to investigate interactions between the bilateral primary motor cortices (M1) and their correlations with motor function and M1-related structural and functional changes in well-recovered patients with chronic subcortical ischemic stroke.Methods: Twenty subcortical stroke patients and 20 normal controls underwent multimodal magnetic resonance imaging (MRI) examinations. During the movement of the affected hand, functional MRI was used to calculate the M1 activation and M1-M1 effective connectivity. Diffusion tensor imaging was used to compute the fractional anisotropy (FA) of the affected corticospinal tract (CST) and M1-M1 anatomical connection. After intergroup comparisons, we tested whether the altered M1-M1 effective connectivity was correlated with the motor function, M1 activation and FA of the affected CST and M1-M1 anatomical connection in patients.Results: Compared to normal controls, stroke patients exhibited increased excitatory effective connectivity from ipsilesional to contralesional M1 and increased ipsilesional M1 activation; however, they showed reduced FA values in the affected CST and M1-M1 anatomical connection. The increased effective connectivity was positively correlated with motor score and the FA of the M1-M1 anatomical connection, but not with the M1 activation or the FA of the affected CST in these patients.Conclusions: These findings suggest that the enhancement of M1-M1 effective connectivity from ipsilesional to contralesional hemisphere depends on the integrity of the underlying M1-M1 anatomical connection (i.e., less deficits of the M1-M1 anatomical connection, greater enhancement of the corresponding effective connectivity), and such M1-M1 effective connectivity enhancement plays a supportive role in motor function in chronic subcortical stroke.

Association between imbalance of cortical brain activity and successful motor recovery in sub-acute stroke patients with upper limb hemiparesis: a functional near-infrared spectroscopy study.

This study was designed to determine the association between motor functional recovery and interhemispheric imbalance in cortical brain activity in sub-cortical stroke patients with moderate-to-severe upper limb hemiparesis admitted to the convalescent rehabilitation ward. The study included first-ever stroke patients with moderate-to-severe upper limb hemiparesis who received multidisciplinary rehabilitation therapy in the rehabilitation ward. Motor function of the affected upper extremity was evaluated by the Fugl-Meyer assessment and action research arm test at 1 (T1) and 3 months (T2) after stroke onset. We also conducted serial functional near-infrared spectroscopy at the same time points and calculated the laterality index, which is based on changes in oxyhaemoglobin in primary sensorimotor cortex (Brodmann Area 4), pre-motor cortex and supplementary motor cortex (PMC + SMA, BA6). The study included eight patients (seven females, mean age: 68.8). Both the Fugl-Meyer assessment and action research arm test scores improved significantly during the study. Laterality index did not change significantly from T1 to T2. There was a no significant correlation between changes in laterality index in each region and improvement in Fugl-Meyer assessment score. In contrast, a significant and negative correlation was noted between ΔLI in Brodmann Area 4 and improvement in action research arm test score. Our results suggested that activation of the non-lesional hemisphere in sub-acute stroke associated with motor recovery in moderate-to-severe upper limb hemiparesis. A multidisciplinary rehabilitation of stroke patients with moderate-to-severe upper limb hemiparesis might enhance the compensatory movements and pre-existing motor network from the non-lesional motor cortex.

Modulation of brain activity with transcranial direct current stimulation: Targeting regions implicated in impaired illness awareness in schizophrenia

BackgroundImpaired illness awareness or insight into illness (IIA) is a common feature of schizophrenia that contributes to medication nonadherence and poor clinical outcomes. Neuroimaging studies suggest IIA may arise from interhemispheric imbalance in frontoparietal regions, particularly in the posterior parietal area (PPA) and the dorsolateral prefrontal cortex (dlPFC). In this pilot study, we examined the effects of transcranial direct current stimulation (tDCS) on brain regions implicated in IIA. MethodsEleven patients with schizophrenia with IIA (≥3 PANSS G12) and 10 healthy controls were included. A crossover design was employed where all participants received single-session bi-frontal, bi-parietal, and sham stimulation in random order. For each condition, we measured (i) blood oxygen level-dependent (BOLD) response to an illness awareness task pre- and post-stimulation, (ii) regional cerebral blood-flow (rCBF) prior to and during stimulation, and (iii) changes in illness awareness. ResultsAt baseline, patients with schizophrenia showed higher BOLD-response to an illness awareness task in the left-PPA compared to healthy controls. Bi-parietal stimulation reduced the interhemispheric imbalance in the PPA compared to sham stimulation. Relatedly, bi-parietal stimulation increased rCBF beneath the anode (21% increase in the right-PPA), but not beneath the cathode (5.6% increase in the left-PPA). Bi-frontal stimulation did not induce changes in rCBF. We found no changes in illness awareness. ConclusionAlthough single-session tDCS did not improve illness awareness, this pilot study provides mechanistic justification for future investigations to determine if multi-session bi-parietal tDCS can induce sustained changes in brain activity in the PPA in association with improved illness awareness.

Dopaminergic modulation of motor network compensatory mechanisms in Parkinson's disease.

The dopaminergic system has a unique gating function in the initiation and execution of movements. When the interhemispheric imbalance of dopamine inherent to the healthy brain is disrupted, as in Parkinson's disease (PD), compensatory mechanisms act to stave off behavioral changes. It has been proposed that two such compensatory mechanisms may be (a) a decrease in motor lateralization, observed in drug-naïve PD patients and (b) reduced inhibition - increased facilitation. Seeking to investigate the differential effect of dopamine depletion and subsequent substitution on compensatory mechanisms in non-drug-naïve PD, we studied 10 PD patients and 16 healthy controls, with patients undergoing two test sessions - "ON" and "OFF" medication. Using a simple visually-cued motor response task and fMRI, we investigated cortical motor activation - in terms of laterality, contra- and ipsilateral percent BOLD signal change and effective connectivity in the parametric empirical Bayes framework. We found that decreased motor lateralization persists in non-drug-naïve PD and is concurrent with decreased contralateral activation in the cortical motor network. Normal lateralization is not reinstated by dopamine substitution. In terms of effective connectivity, disease-related changes primarily affect ipsilaterally-lateralized homotopic cortical motor connections, while medication-related changes affect contralaterally-lateralized homotopic connections. Our findings suggest that, in non-drug-naïve PD, decreased lateralization is no longer an adaptive cortical mechanism, but rather the result of maladaptive changes, related to disease progression and long-term dopamine replacement. These findings highlight the need for the development of noninvasive therapies, which would promote the adaptive mechanisms of the PD brain.

Reduced Interhemispheric Functional Connectivity in Obsessive-Compulsive Disorder Patients.

Background: Neuroimaging studies have shown that the high synchrony of spontaneous neural activity in the homotopic regions between hemispheres is an important functional structural feature of normal human brains, and this feature is abnormal in the patients with various mental disorders. However, little is known about this feature in obsessive–compulsive disorder (OCD). This study aimed to further analyze the underlying neural mechanisms of OCD and to explore whether clinical characteristics are correlated with the alerted homotopic connectivity in patients with OCD. Methods: Using voxel-mirrored homotopic connectivity (VMHC) during resting state, we compared 46 OCD patients and 46 healthy controls (HCs) matched for age, gender, and education level. A partial correlation analysis was used to investigate the relationship between altered VMHC and clinical characteristics in patients with OCD. Results: Patients with OCD showed lower VMHC than HCs in fusiform gyrus/inferior occipital gyrus, lingual gyrus, postcentral gyrus/precentral gyrus, putamen, and orbital frontal gyrus. A significant positive correlation was observed between altered VMHC in the angular gyrus/middle occipital gyrus and illness duration in patients. Conclusions: Interhemispheric functional imbalance may be an essential aspect of the pathophysiological mechanism of OCD, which is reflected not only in the cortico-striato-thalamo-cortical (CSTC) loop but also elsewhere in the brain.

Impaired illness awareness in schizophrenia and posterior corpus callosal white matter tract integrity

Impaired illness awareness (Imp-IA) in schizophrenia is associated with interhemispheric imbalance, resulting in left hemisphere dominance, primarily within the posterior parietal area (PPA). This may represent an interhemispheric “disconnection syndrome” between PPAs. To test this hypothesis, we aimed to determine if diffusion-based measures of white matter integrity were disrupted in the corpus callosal tracts linking PPAs (i.e., splenium) in patients with Imp-IA in schizophrenia. T1-weighted and diffusion-weighted scans were acquired on a 1.5T GE scanner for 100 participants with a DSM-IV-TR diagnosis of schizophrenia and 134 healthy controls aged 18 to 79 years. The corpus callosal white matter tracts were compared among patients with Imp-IA (n = 40), intact illness awareness (n = 60), and healthy controls. White matter disruption was measured with fractional anisotropy (FA) and mean diffusivity (MD). Group differences in FA were found in the splenium, with patients with Imp-IA having the lowest FA, which remained significant after controlling for sex, age, global cognition, and premorbid intelligence. No group differences in MD were observed. Splenial white matter tracts of the corpus callosum appear compromised in patients with Imp-IA. Transcallosal interhemispheric PPA white matter disruption may represent a “disconnection syndrome”, manifesting as Imp-IA in schizophrenia. Future studies are required to investigate the effects of noninvasive brain stimulation interventions, such as transcranial direct current or magnetic stimulation, on Imp-IA in association with white matter changes in patients with schizophrenia.

A Strong Role for the AMOC in Partitioning Global Energy Transport and Shifting ITCZ Position in Response to Latitudinally Discrete Solar Forcing in CESM1.2

Abstract Ocean circulation responses to interhemispheric radiative imbalance can damp north–south migrations of the intertropical convergence zone (ITCZ) by reducing the burden on atmospheric energy transport. The role of the Atlantic meridional overturning circulation (AMOC) in such dynamics has not received much attention. Here, we present coupled climate modeling results that suggest AMOC responses are of first-order importance to muting ITCZ shift magnitudes as a pair of hemispherically asymmetric solar forcing bands is moved from equatorial to polar latitudes. The cross-equatorial energy transport response to the same amount of interhemispheric forcing becomes systematically more ocean-centric when higher latitudes are perturbed in association with strengthening AMOC responses. In contrast, the responses of the Pacific subtropical cell are not monotonic and cannot predict this variance in the ITCZ’s equilibrium position. Overall, these results highlight the importance of the meridional distribution of interhemispheric radiative imbalance and the rich buffering of internal feedbacks that occurs in dynamic versus thermodynamic (slab) ocean modeling experiments. Mostly, the results imply that the problem of developing a theory of ITCZ migration is entangled with that of understanding the AMOC’s response to hemispherically asymmetric radiative forcing—a difficult topic deserving of focused analysis across more climate models.

Rethinking interhemispheric imbalance as a target for stroke neurorehabilitation.

Patients with chronic stroke have been shown to have failure to release interhemispheric inhibition (IHI) from the intact to the damaged hemisphere before movement execution (premovement IHI). This inhibitory imbalance was found to correlate with poor motor performance in the chronic stage after stroke and has since become a target for therapeutic interventions. The logic of this approach, however, implies that abnormal premovement IHI is causal to poor behavioral outcome and should therefore be present early after stroke when motor impairment is at its worst. To test this idea, in a longitudinal study, we investigated interhemispheric interactions by tracking patients' premovement IHI for one year following stroke. We assessed premovement IHI and motor behavior five times over a 1-year period after ischemic stroke in 22 patients and 11 healthy participants. We found that premovement IHI was normal during the acute/subacute period and only became abnormal at the chronic stage; specifically, release of IHI in movement preparation worsened as motor behavior improved. In addition, premovement IHI did not correlate with behavioral measures cross-sectionally, whereas the longitudinal emergence of abnormal premovement IHI from the acute to the chronic stage was inversely correlated with recovery of finger individuation. These results suggest that interhemispheric imbalance is not a cause of poor motor recovery, but instead might be the consequence of underlying recovery processes. These findings call into question the rehabilitation strategy of attempting to rebalance interhemispheric interactions in order to improve motor recovery after stroke. Ann Neurol 2019;85:502-513.

Can TMS based neurophysiology inform the delivery of rTMS after stroke?

Transcranial magnetic stimulation (TMS) can be used to assess and alter cortical excitability in healthy and clinical populations. Studies using TMS demonstrate altered brain excitability and connectivity in both hemispheres during after stroke. Interhemispheric imbalance is noted after stroke such that the ipsilesional hemisphere shows less corticospinal excitability with smaller, or even absent, motor evoked potentials (MEPs). In contrast, the contralesional hemisphere often demonstrates equal or abnormally increased corticospinal excitability. Along with this imbalance, there can be altered in connectivity between the two hemispheres, often referred to as interhemispheric inhibition (IHI). After stroke, asymmetry in the level of IHI between the two hemispheres can be associated with functional recovery, although stroke severity may alter these relationships. Repetitive TMS (rTMS) alters local cortical excitability and counter interhemispheric imbalance after stroke. Yet, response to this intervention is highly variable with some stroke patients showing benefit while others do not.

The Use of Noninvasive Brain Stimulation, Specifically Transcranial Direct Current Stimulation After Stroke.

The Use of Noninvasive Brain Stimulation, Specifically Transcranial Direct Current Stimulation After Stroke.

Asymmetry in cortical thickness and subcortical volume in treatment-naïve major depressive disorder.

BackgroundNumerous cognitive and emotional functions are executed asymmetrically between the left and right hemispheres. Right hemisphere hyperactivity/left hemisphere hypoactivity often appears to be a feature in neuroimaging studies of depression. However, few studies have evaluated abnormalities in structural asymmetry in untreated patients with major depressive disorder (MDD).MethodsIn this study, 3-dimensional high-resolution structural magnetic resonance images were acquired from 35 treatment-naïve patients with MDD (mean age = 28.9 years, 22 females) and 35 normal controls. The asymmetry index in cortical thickness and subcortical volume were calculated based on an automated surface-based technique.ResultsAbnormalities in structural asymmetry in patients with MDD were mainly located in the cortical-striatal-pallidal-thalamic circuit, including the superior frontal cortex, rostral middle frontal cortex, caudal middle frontal cortex, nucleus accumbens, pallidum and thalamus. No significant correlation was observed between symptom severity and asymmetric measurements.ConclusionThese findings provide further evidence for the altered morphological interhemispheric imbalances in depression and these alterations were independent of depressive symptom severity, suggesting that cerebral asymmetry could be an appropriate indicator of morphological variations in mental disease.