Bimanual recoupling by visual cueing in callosal disconnection

Interhemispheric interplay between the left and right premotor cortex during grasping as assessed by dynamic causal modelling

Research on the contribution of the ipsilateral hemisphere to unilateral movements, and how it is mediated by transcallosal connections, has so far provided contradictory findings. By using dynamic causal modelling (DCM) and Parametric Empirical Bayes analyses applied to fMRI data, we sought to describe effective connectivity during pantomimed and imagined right-hand grasping within the grasping network, namely the anterior intraparietal sulcus, ventral and dorsal (PMd) premotor cortex, supplementary motor area and primary motor cortex (M1). The two-fold aim of the present work was to explore a) whether right and left parieto-frontal areas show similar connectivity couplings, and b) the interhemispheric dynamics between these regions across the two hemispheres. We detected a network architecture comparable across hemispheres during executed but not imagined grasping movements. Furthermore, during pantomimed grasping the interhemispheric crosstalk was mainly driven by premotor areas: we found an inhibitory influence from the right PMd toward the left premotor and motor areas and excitatory couplings between homologous ventral premotor and supplementary motor regions. Overall, our results support the view that dissociable components of unilateral grasping execution are encoded by a non-lateralized set of brain areas complexly intertwined by interhemispheric dynamics, whereas motor imagery obeys different principles.

Altered Topological Organization in the Sensorimotor Network After Application of Different Frequency rTMS

The application of repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex (M1) could influence the intrinsic brain activity in the sensorimotor network (SMN). However, how rTMS modulates the topological organization of the SMN remains unclear. In this study, we employed resting-state fMRI to investigate the topological alterations in the functional SMN after application of different frequency rTMS over the left M1. To accomplish this, we collected MRI data from 45 healthy participants who were randomly divided into three groups based on rTMS frequency (HF, high-frequency 3 Hz; LF, low-frequency 1 Hz; and SHAM). Individual large-scale functional SMN was constructed by correlating the mean time series among 29 regions of interest (ROI) in the SMN and was fed into graph-based network analyses at multiple levels of global organization and nodal centrality. Our results showed that compared with the network metrics before rTMS stimulation, the left paracentral lobule (PCL) exhibited reduced nodal degree and betweenness centrality in the LF group after rTMS, while the right supplementary motor area (SMA) exhibited reduced nodal betweenness centrality in the HF group after rTMS. Moreover, rTMS-related alterations in nodal metrics might have been attributable to the changes in connectivity patterns and local activity of the affected nodes. These findings reflected the potential of using rTMS over M1 as an effective intervention to promote motor function rehabilitation.

Functional Neuroimaging in Stroke Recovery and Neurorehabilitation: Conceptual Issues and Perspectives

Background

In stroke, functional neuroimaging has become a potent diagnostic tool; opened new insights into the pathophysiology of ischaemic damage in the human brain; and made possible the assessment of functional–structural relationships in postlesion recovery.

Summary of review

Here, we give a critical account on the potential and limitation of functional neuroimaging and discuss concepts related to the use of neuroimaging for exploring the neurobiological and neuroanatomical mechanisms of poststroke recovery and neurorehabilitation. We identify and provide evidence for five hypotheses that functional neuroimaging can provide new insights into: adaptation occurs at the level of functional brain systems; the brain–behaviour relationship varies with recovery and over time; functional neuroimaging can improve our ability to predict recovery and select individuals for rehabilitation; mechanisms of recovery reflect different pathophysiological phases; and brain adaptation may be modulated by experience and specific rehabilitation. The significance and application of this new evidence is discussed, and recommendations made for investigations in the field.

Conclusion

Functional neuroimaging is an important tool to explore the mechanisms underlying brain plasticity and, thereby, to guide clinical research in neurorehabilitation.

Demand on skillfulness modulates interhemispheric inhibition of motor cortices

The role of primary motor cortex (M1) in the control of hand movements is still unclear. Functional magnetic resonance imaging (fMRI) studies of unimanual performance reported a relationship between level of precision of a motor task and additional ipsilateral M1 (iM1) activation. In the present study, we determined whether the demand on accuracy of a movement influences the magnitude of the inhibitory effect between primary motor cortices (IHI). We used transcranial magnetic stimulation (TMS) to measure active IHI (aIHI) of the iM1 on the contralateral M1 (cM1) in the premovement period of a left-hand motor task. Ten healthy participants manipulated a joystick to point to targets of two different sizes. For aIHI, the conditioning stimulus (CS) was applied to iM1, and the test stimulus (TS) to cM1, with an interstimulus interval of 10 ms. The amount of the inhibitory effect of the CS on the motor-evoked potential (MEP) of the subsequent TS was expressed as percentage of the mean MEP amplitude evoked by the single TS. Across different time points of aIHI measurements in the premovement period, there was a significant effect for target size on aIHI. Preparing to point to small targets was associated with weaker aIHI compared with pointing to large targets. The present findings suggest that, during the premovement period, aIHI from iM1 on cM1 is modulated by the demand on accuracy of the motor task. This is consistent with task fMRI findings showing bilateral M1 activation during high-precision movements but only unilateral M1 activity during low-precision movements.

Differential contribution of bilateral supplementary motor area to the effective connectivity networks induced by task conditions using dynamic causal modeling

Functional imaging studies have indicated hemispheric asymmetry of activation in bilateral supplementary motor area (SMA) during unimanual motor tasks. However, the hemispherically special roles of bilateral SMAs on primary motor cortex (M1) in the effective connectivity networks (ECN) during lateralized tasks remain unclear. Aiming to study the differential contribution of bilateral SMAs during the motor execution and motor imagery tasks, and the hemispherically asymmetric patterns of ECN among regions involved, the present study used dynamic causal modeling to analyze the functional magnetic resonance imaging data of the unimanual motor execution/imagery tasks in 12 right-handed subjects. Our results demonstrated that distributions of network parameters underlying motor execution and motor imagery were significantly different. The variation was mainly induced by task condition modulations of intrinsic coupling. Particularly, regardless of the performing hand, the task input modulations of intrinsic coupling from the contralateral SMA to contralateral M1 were positive during motor execution, while varied to be negative during motor imagery. The results suggested that the inhibitive modulation suppressed the overt movement during motor imagery. In addition, the left SMA also helped accomplishing left hand tasks through task input modulation of left SMA→right SMA connection, implying that hemispheric recruitment occurred when performing nondominant hand tasks. The results specified differential and altered contributions of bilateral SMAs to the ECN during unimanual motor execution and motor imagery, and highlighted the contributions induced by the task input of motor execution/imagery.

Primary motor cortex excitability is modulated with bimanual training

Bimanual visuomotor movement has been shown to enhance cortical motor activity in both hemispheres, especially when movements require simultaneous activation of homologous muscle groups (in-phase movement). It is currently unclear if these adaptations are specific to motor preparatory areas or if they also involve changes in primary motor cortex (M1). The present study investigated the representation of wrist muscles within motor cortex before and following bimanual movement training that was in-phase, anti-phase with or without motor preparation. Motor evoked potentials (MEPs) for the extensor carpi radialis muscle (ECR) cortical territory were acquired and analyzed before and following bimanual movement. The cortical representation was quantified and compared in terms of spatial extent and MEP amplitude, in two different experiments involving distinct movement training types. In Experiment 1, participants performed bimanual wrist flexion/extension movements to targets which involved in-phase movements, either following a 2s preparation period (In-phase preparation), or without the preparation period (In-phase no preparation). In Experiment 2, training involved antagonist muscle groups activated simultaneously (Anti-phase) with the addition of the 2s preparation period. In-phase bimanual movement enhanced the spatial representation of ECR in M1, and did not show a difference in MEP amplitude of the cortical area. It may be that simultaneous activation of homologous M1 representations in both hemispheres, in combination with activity from premotor areas, leads to a greater increase in plasticity in terms of increased M1 spatial extent of trained muscles.

Cortical and behavioral adaptations in response to short-term inphase versus antiphase bimanual movement training

Bimanual movement training (BMT) may be an effective rehabilitative protocol for movement-related deficits following a stroke; however, it is unclear how varying types of BMT induce cortical adaptations in the healthy population. Moreover, we lack a methodology to measure cortical adaptations in response to modes of movement training. Therefore, the present study measured the cued movement-related potential (MRP) to investigate cortical adaptations during cued inphase versus antiphase BMT that transferred to a unimanual task and how cortical modulations related to behavior. Three specific hypotheses were investigated: (1) cued inphase BMT would induce cortical adaptations within regions subserving motor preparation and movement execution, (2) repetitive cued unimanual training would induce cortical activity modulations associated with motor execution, and (3) increased cortical activity would be associated with enhanced performance. On three separate days, EEG was recorded from 22 electrodes during three types of cued movement training: inphase BMT, antiphase BMT and repetitive unimanual movement, in addition to pre- and post-training unimanual movement trials involving cued right wrist flexion. The MRP was measured for each repetition during each trial. Results showed a significant training-related increase in preparatory activation correlated with a behavioral enhancement following cued inphase BMT. This effect was not attributable to a change in arousal. No significant training-related modulation occurred in response to cued antiphase BMT or repetitive unimanual movement training. These results suggest that cortical adaptations in relation to the preparation of a cued movement enhance in response to cued inphase BMT, and the MRP is an effective measurement tool to assess training-related adaptations in response to inphase BMT specifically.

Human motor associative plasticity induced by paired bihemispheric stimulation

Paired associative stimulation (PAS) is an effective non-invasive method to induce human motor plasticity by the repetitive pairing of peripheral nerve stimulation and transcranial magnetic stimulation (TMS) at the primary motor cortex (M1) with a specific time interval. Although the repetitive pairing of two types of afferent stimulation might be a biological basis of neural plasticity and memory, other types of paired stimulation of the human brain have rarely been studied. We hypothesized that the repetitive pairing of TMS and interhemispheric cortico-cortical projection or paired bihemispheric stimulation (PBS), in which the right and left M1 were serially stimulated with a time interval of 15 ms, would produce an associative long-term potentiation (LTP)-like effect. In this study, 23 right-handed healthy volunteers were subjected to a 0.1 Hz repetition of 180 pairings of bihemispheric TMS, and physiological and behavioural measures of the motor system were compared before, immediately after, 20 min after and 40 min after PBS intervention. The amplitude of the motor evoked potential (MEP) induced by the left M1 stimulation and its input-output function increased for up to approximately 20 min post-PBS. Fine finger movements were also facilitated by PBS. Spinal excitability measured by the H-reflex was insensitive to PBS, suggesting a cortical mechanism. The associative LTP-like effect induced by PBS was timing dependent, occurring only when the interstimulus interval was 5-25 ms. These findings demonstrate that using PBS in PAS can induce motor cortical plasticity, and this approach might be applicable to the rehabilitation of patients with motor disorders.

Perceptual influence on bimanual coordination: an fMRI study

In bimanual coordination subjects typically show a spontaneous preference for movement symmetry. While there is experimental evidence for the principle of muscle homology, recent evidence suggested that bimanual coordination may be mediated as perceptual goals (Mechsner et al., 2001). To explore this controversy we performed a fMRI study in 11 healthy, right-handed subjects using bimanual index finger abductions and adductions in a congruous condition, i.e. both palms down, and incongruous conditions with either the left or right palm up. Our fMRI data showed a widespread bihemispheric network mediating proprioceptive coordination of the two hands with significant differences mainly for a perceptual dissociation: in the incongruous conditions with the one palm up there was a BOLD signal increase in a bilateral frontoparietal network involving the motor and premotor cortical areas, particularly in the right palm-up condition. These results accord with the notion that perceptual cues play an important role in the control of bilateral hand movements.

The suppressive influence of SMA on M1 in motor imagery revealed by fMRI and dynamic causal modeling

Although motor imagery is widely used for motor learning in rehabilitation and sports training, the underlying mechanisms are still poorly understood. Based on fMRI data sets acquired with very high temporal resolution (300 ms) under motor execution and imagery conditions, we utilized Dynamic Causal Modeling (DCM) to determine effective connectivity measures between supplementary motor area (SMA) and primary motor cortex (M1). A set of 28 models was tested in a Bayesian framework and the by-far best-performing model revealed a strong suppressive influence of the motor imagery condition on the forward connection between SMA and M1. Our results clearly indicate that the lack of activation in M1 during motor imagery is caused by suppression from the SMA. These results highlight the importance of the SMA not only for the preparation and execution of intended movements, but also for suppressing movements that are represented in the motor system but not to be performed.

Recovery from ischemic stroke: a translational research perspective for neurology

Ischemic stroke is the most frequent neurological disease, characterized by an age-related incidence and chronic disability in the majority of patients. A great challenge in acute stroke is to predict the degree to which a patient will eventually recover. Magnetic resonance imaging has revealed that treatment-induced reperfusion limits the extent of ischemic brain damage, thereby enabling rapid and profound recovery. Nevertheless, patients may retain deficits in motor, sensory or cognitive functions due to the residual lesion. Functional neuroimaging and transcranial magnetic stimulation have shown that recovery is associated with abnormal activation in the perilesional vicinity and in brain areas remote from the lesion. This is likely related to altered functional properties or morphological changes in both cerebral hemispheres. Recent neurorehabilitative strategies, including forced use, mental imagery and peripheral nerve or cortex stimulation, aim at modulating these functional networks. Accordingly, translational research has provided new vistas on the neurobiological mechanisms of recovery and opened future avenues for science-based pharmacological and neurophysiological training strategies in stroke.

Intrahemispheric dysfunction in primary motor cortex without corpus callosum: a transcranial magnetic stimulation study

Background

The two human cerebral hemispheres are continuously interacting, through excitatory and inhibitory influences and one critical structure subserving this interhemispheric balance is the corpus callosum. Interhemispheric neurophysiological abnormalities and intrahemispheric behavioral impairments have been reported in individuals lacking the corpus callosum. The aim of this study was to examine intrahemispheric neurophysiological function in primary motor cortex devoid of callosal projections.

Methods

Intracortical excitatory and inhibitory systems were tested in three individuals with complete agenesis of the corpus callosum and sixteen healthy individuals. These systems were assessed using transcranial magnetic stimulation (TMS) protocols: motor threshold at rest, paired-pulse curve, and cortical silent period.

Results

TMS revealed no difference between the patient and control groups on the motor threshold measure, as well as intracortical facilitation and intracortical inhibition systems as tested by paired stimulation. However, intrahemispheric inhibitory function was found to be abnormal in participants without callosal projections, as the cortical silent period duration was significantly increased in the patient group.

Conclusion

These data suggest that in addition to previously reported impaired interhemispheric function, patients lacking the entire corpus callosum also display abnormal intrahemispheric excitability of the primary motor cortex.

Functional activation within the PI–DWI mismatch region in recovery from ischemic stroke: preliminary observations

In this study, we sought to investigate if brain tissue affected by ischemia can accommodate areas of activation related to restoration of brain function following ischemic stroke. In two patients perfusion imaging (PI) and diffusion weighted imaging (DWI) obtained in the acute phase after stroke was coregistered with BOLD imaging of brain functions acquired when profound recovery had occurred. Both patients suffered from thrombembolic brain infarction due to dissection of the internal carotid artery (ICA) characterized by a severe PI-DWI mismatch in the acute stage of stroke. Following ICA recanalization and clinical recovery BOLD imaging showed task-specific activation adjacent to the infarct lesion within the former PI-DWI mismatch area. The data in these two stroke patients provide evidence that brain tissue at risk of infarction as shown by the PI-DWI mismatch can survive and, thereby, constitute the major site underlying post-ischemic recovery.