Interhemispheric inhibition between dorsal premotor and primary motor cortices is released during preparation of unimanual but not bimanual movements

Previous research applying transcranial magnetic stimulation during unimanual reaction time tasks indicates a transient change in the inhibitory influence of the dorsal premotor cortex over the contralateral primary motor cortex shortly after the presentation of an imperative stimulus. The degree of interhemispheric inhibition from the dorsal premotor cortex to the contralateral primary motor cortex shifts depending on whether the targeted effector representation in the primary motor cortex is selected for movement. Further, the timing of changes in inhibition covaries with the selection demands of the reaction time task. Less is known about modulation of dorsal premotor to primary motor cortex interhemispheric inhibition during the preparation of bimanual movements. In this study, we used a dual coil transcranial magnetic stimulation to measure dorsal premotor to primary motor cortex interhemispheric inhibition between both hemispheres during unimanual and bimanual simple reaction time trials. Interhemispheric inhibition was measured early and late in the 'pre-movement period' (defined as the period immediately after the onset of the imperative stimulus and before the beginning of voluntary muscle activity). We discovered that interhemispheric inhibition was more facilitatory early in the pre-movement period compared with late in the pre-movement period during unimanual reaction time trials. In contrast, interhemispheric inhibition was unchanged throughout the pre-movement period during symmetrical bimanual reaction time trials. These results suggest that there is greater interaction between the dorsal premotor cortex and contralateral primary motor cortex during the preparation of unimanual actions compared to bimanual actions.

Probing intrahemispheric interactions with a novel dual-site TMS setup

OBJECTIVE

Using dual-site transcranial magnetic stimulation (dsTMS), the effective connectivity between the primary motor cortex (M1) and adjacent brain areas such as the dorsal premotor cortex (PMd) can be investigated. However, stimulating two brain regions in close proximity (e.g., ±2.3 cm for intrahemispheric PMd-M1) is subject to considerable spatial restrictions that potentially can be overcome by combining two standard figure-of-eight coils in a novel dsTMS setup.

METHODS

After a technical evaluation of its induced electric fields, the dsTMS setup was tested in vivo (n = 23) by applying a short-interval intracortical inhibition (SICI) protocol. Additionally, the intrahemispheric PMd-M1 interaction was probed. E-field modelling was performed using SimNIBS.

RESULTS

The technical evaluation yielded no major alterations of the induced electric fields due to coil overlap. In vivo, the setup reliably elicited SICI. Investigating intrahemispheric PMd-M1 interactions was feasible (inter-stimulus interval 6 ms), resulting in modulation of M1 output.

CONCLUSIONS

The presented dsTMS setup provides a novel way to stimulate two adjacent brain regions with fewer technical and spatial limitations than previous attempts.

SIGNIFICANCE

This dsTMS setup enables more accurate and repeatable targeting of brain regions in close proximity and can facilitate innovation in the field of effective connectivity.

Enhancing Prediction of Forelimb Movement Trajectory through a Calibrating-Feedback Paradigm Incorporating RAT Primary Motor and Agranular Cortical Ensemble Activity in the Goal-Directed Reaching Task

Complete reaching movements involve target sensing, motor planning, and arm movement execution, and this process requires the integration and communication of various brain regions. Previously, reaching movements have been decoded successfully from the motor cortex (M1) and applied to prosthetic control. However, most studies attempted to decode neural activities from a single brain region, resulting in reduced decoding accuracy during visually guided reaching motions. To enhance the decoding accuracy of visually guided forelimb reaching movements, we propose a parallel computing neural network using both M1 and medial agranular cortex (AGm) neural activities of rats to predict forelimb-reaching movements. The proposed network decodes M1 neural activities into the primary components of the forelimb movement and decodes AGm neural activities into internal feedforward information to calibrate the forelimb movement in a goal-reaching movement. We demonstrate that using AGm neural activity to calibrate M1 predicted forelimb movement can improve decoding performance significantly compared to neural decoders without calibration. We also show that the M1 and AGm neural activities contribute to controlling forelimb movement during goal-reaching movements, and we report an increase in the power of the local field potential (LFP) in beta and gamma bands over AGm in response to a change in the target distance, which may involve sensorimotor transformation and communication between the visual cortex and AGm when preparing for an upcoming reaching movement. The proposed parallel computing neural network with the internal feedback model improves prediction accuracy for goal-reaching movements.

PMd and action preparation: bridging insights between TMS and single neuron research

Transcranial magnetic stimulation (TMS) research has furthered understanding of human dorsal premotor cortex (PMd) function due to its unrivalled ability to measure the inhibitory and facilitatory influences of PMd over the primary motor cortex (M1) in a temporally precise manner. TMS research indicates that PMd transiently modulates inhibitory output to effector representations within M1 during motor preparation, with the direction of modulation depending on which effectors are selected for response, and the timing of modulations co-varying with task selection demands. In this review, we critically assess this literature in the context of a dynamical systems approach used to model nonhuman primate (NHP) PMd/M1 single-neuron recordings during action preparation. Through this process, we identify gaps in the literature and propose future experiments.

Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

Impact of interhemispheric inhibition on bimanual movement control in young and old

Interhemispheric interactions demonstrate a crucial role for directing bimanual movement control. In humans, a well-established paired-pulse transcranial magnetic stimulation paradigm enables to assess these interactions by means of interhemispheric inhibition (IHI). Previous studies have examined changes in IHI from the active to the resting primary motor cortex during unilateral muscle contractions; however, behavioral relevance of such changes is still inconclusive. In the present study, we evaluated two bimanual tasks, i.e., mirror activity and bimanual anti-phase tapping, to examine behavioral relevance of IHI for bimanual movement control within this behavioral framework. Two age groups (young and older) were evaluated as bimanual movement control demonstrates evident behavioral decline in older adults. Two types of IHI with differential underlying mechanisms were measured; IHI was tested at rest and during a motor task from the active to the resting primary motor cortex. Results demonstrate an association between behavior and short-latency IHI in the young group: larger short-latency IHI correlated with better bimanual movement control (i.e., less mirror activity and better bimanual anti-phase tapping). These results support the view that short-latency IHI represents a neurophysiological marker for the ability to suppress activity of the contralateral side, likely contributing to efficient bimanual movement control. This association was not observed in the older group, suggesting age-related functional changes of IHI. To determine underlying mechanisms of impaired bimanual movement control due to neurological disorders, it is crucial to have an in-depth understanding of age-related mechanisms to disentangle disorder-related mechanisms of impaired bimanual movement control from age-related ones.

Response bias reveals the role of interhemispheric inhibitory networks in movement preparation and execution

Human movement is influenced by various cognitive processes, such as bias, that dynamically shape competing movement representations. However, the neurophysiological mechanisms underlying the effects of bias on movement selection across the lifespan remains poorly understood. Healthy young (n = 21) and older (n = 20) adults completed a choice reaction-time task necessitating left- or right-hand responses to imperative stimuli (IS). Response bias was manipulated via a cue that informed participants a particular response was 70% likely (i.e., the IS was either congruent, or incongruent, with the cue); biasing was either fixed for blocks of trials (block-wise bias) or varied from trial-to-trial (trial-wise bias). As well as assessing the behavioural manifestations of bias, we used transcranial magnetic stimulation to determine changes in corticospinal excitability (CSE) and short- and long-interval interhemispheric inhibition (SIHI, LIHI) during movement preparation and execution. Participants responded more quickly, and accurately, in congruent compared to incongruent trials. CSE decreases occurred in both hands following the cue, consistent with the 'inhibition for impulse control' hypothesis of preparatory inhibition. In contrast, IHI modulations occurred in a hand-specific manner. Greater SIHI was observed during movement preparation in the hand biased away from, compared to the hand biased towards, the cue; furthermore, greater SIHI was observed during movement execution in the hand biased towards the cue when it was not required to respond (i.e., incongruent trial) compared to when it was required to respond (congruent trial). Additionally, during the movement preparation period, the LIHI ratio of the hand biased towards, compared to the hand biased away from, the cue was greatest when the cue varied trial-by-trial. Overall, the IHI results provide support for the 'inhibition for competition resolution' hypothesis, with hand specific modulation of inhibition during movement preparation and execution.

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

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

Neurophysiological modulations in the (pre)motor-motor network underlying age-related increases in reaction time and the role of GABA levels - a bimodal TMS-MRS study

It has been argued that age-related changes in the neurochemical and neurophysiological properties of the GABAergic system may underlie increases in reaction time (RT) in older adults. However, the role of GABA levels within the sensorimotor cortices (SMC) in mediating interhemispheric interactions (IHi) during the processing stage of a fast motor response, as well as how both properties explain interindividual differences in RT, are not yet fully understood. In this study, edited magnetic resonance spectroscopy (MRS) was combined with dual-site transcranial magnetic stimulation (dsTMS) for probing GABA+ levels in bilateral SMC and task-related neurophysiological modulations in corticospinal excitability (CSE), and primary motor cortex (M1)-M1 and dorsal premotor cortex (PMd)-M1 IHi, respectively. Both CSE and IHi were assessed during the preparatory and premotor period of a delayed choice RT task. Data were collected from 25 young (aged 18-33 years) and 28 older (aged 60-74 years) healthy adults. Our results demonstrated that older as compared to younger adults exhibited a reduced bilateral CSE suppression, as well as a reduced magnitude of long latency M1-M1 and PMd-M1 disinhibition during the preparatory period, irrespective of the direction of the IHi. Importantly, in older adults, the GABA+ levels in bilateral SMC partially accounted for task-related neurophysiological modulations as well as individual differences in RT. In contrast, in young adults, neither task-related neurophysiological modulations, nor individual differences in RT were associated with SMC GABA+ levels. In conclusion, this study contributes to a comprehensive initial understanding of how age-related differences in neurochemical properties and neurophysiological processes are related to increases in RT.

Exploring cortico-cortical interactions during action preparation by means of dual-coil transcranial magnetic stimulation: A systematic review

Action preparation is characterized by a set of complex and distributed processes that occur in multiple brain areas. Interestingly, dual-coil transcranial magnetic stimulation (TMS) is a relevant technique to probe effective connectivity between cortical areas, with a high temporal resolution. In the current systematic review, we aimed at providing a detailed picture of the cortico-cortical interactions underlying action preparation focusing on dual-coil TMS studies. We considered four theoretical processes (impulse control, action selection, movement initiation and action reprogramming) and one task modulator (movement complexity). The main findings highlight 1) the interplay between primary motor cortex (M1) and premotor, prefrontal and parietal cortices during action preparation, 2) the varying (facilitatory or inhibitory) cortico-cortical influence depending on the theoretical processes and the TMS timing, and 3) the key role of the supplementary motor area-M1 interactions that shape the preparation of simple and complex movements. These findings are of particular interest for clinical perspectives, with a need to better characterize functional connectivity deficiency in clinical population with altered action preparation.

Tuning the Corticospinal System: How Distributed Brain Circuits Shape Human Actions

Interactive behaviors rely on the operation of several processes allowing the control of actions, including their selection, withholding, and cancellation. The corticospinal system provides a unique route through which multiple brain circuits can exert control over bodily motor acts. In humans, the influence of these modulatory circuits on the corticospinal system can be probed using various transcranial magnetic stimulation (TMS) protocols. Here, we review neural data from TMS studies at the basis of our current understanding of how diverse pathways—including intra-cortical, trans-cortical, and subcortico-cortical circuits—contribute to action control by tuning the activity of the corticospinal system. Critically, when doing so, we point out important caveats in the field that arise from the fact that these circuits, and their impact on the corticospinal system, have not been considered equivalently for action selection, withholding, and cancellation. This has led to the misleading view that some circuits or regions are specialized in specific control processes and that they produce particular modulatory changes in corticospinal excitability (e.g., generic vs. specific modulation of corticospinal excitability). Hence, we point to the need for more transversal research approaches in the field of action control.

Sensorimotor cortex neurometabolite levels as correlate of motor performance in normal aging: evidence from a 1H-MRS study

Aging is associated with gradual alterations in the neurochemical characteristics of the brain, which can be assessed in-vivo with proton-magnetic resonance spectroscopy (¹H-MRS). However, the impact of these age-related neurochemical changes on functional motor behavior is still poorly understood. Here, we address this knowledge gap and specifically focus on the neurochemical integrity of the left sensorimotor cortex (SM1) and the occipital lobe (OCC), as both regions are main nodes of the visuomotor network underlying bimanual control. ¹H-MRS data and performance on a set of bimanual tasks were collected from a lifespan (20-75 years) sample of 86 healthy adults. Results indicated that aging was accompanied by decreased levels of N-acetylaspartate (NAA), glutamate-glutamine (Glx), creatine ​+ ​phosphocreatine (Cr) and myo-inositol (mI) in both regions, and decreased Choline (Cho) in the OCC region. Lower NAA and Glx levels in the SM1 and lower NAA levels in the OCC were related to poorer performance on a visuomotor bimanual coordination task, suggesting that NAA could serve as a potential biomarker for the integrity of the motor system supporting bimanual control. In addition, lower NAA, Glx, and mI levels in the SM1 were found to be correlates of poorer dexterous performance on a bimanual dexterity task. These findings highlight the role for ¹H-MRS to study neurochemical correlates of motor performance across the adult lifespan.

Disorders of fine motor skills after a stroke: the processes of neuroplasticity and sensorimotor integration

Background. Impairment of fine motor skills in the hand is one of the most frequent causes of the persistent loss of professional skills, social maladjustment, and the impossibility of self-care in patients after a stroke, which ultimately leads to a significant reduction in the quality of their life. The article discusses the features of the fine motor skills’ impairment in the hand in patients after a stroke, in the context of a lateralized hemispheric lesion. Methods. We have studied 26 patients after a primary ischemic stroke in the pool of middle cerebral artery of the right (n=12) or left (n=14) brain hemisphere. The average age of patients was 55.7±7.3 years. Patients with a right-sided ischemic stroke were comparable to those with a left-sided stroke in their age, disease duration, size of the lesion and the gender ratio. Results. All the patients after an ischemic stroke had motor impairment in the form of a hemiparesis of a mild or moderate degree. Discussion. We suggest the existence of differentiated mechanisms for the development of fine and highly coordinated voluntary movements in the hand of patients after an ischemic stroke, depending on the lateralization of the supratentorial lesion: diffuse deficit of the afferent support in a right-sided ischemic stroke vs. bilateral efferent deficit for a left hemisphere lesion. Conclusion. The obtained data on the differentiated mechanisms for the development of fine and highly coordinated voluntary movements in the hand of patients after an ischemic stroke warrant the necessity of a further, more targeted research on those disorders in the post-stroke period, on order to optimize the existing rehabilitation approaches and improve the functional potential and quality of life of such patients.

Brain networks and their relevance for stroke rehabilitation

Stroke has long been regarded as focal disease with circumscribed damage leading to neurological deficits. However, advances in methods for assessing the human brain and in statistics have enabled new tools for the examination of the consequences of stroke on brain structure and function. Thereby, it has become evident that stroke has impact on the entire brain and its network properties and can therefore be considered as a network disease. The present review first gives an overview of current methodological opportunities and pitfalls for assessing stroke-induced changes and reorganization in the human brain. We then summarize principles of plasticity after stroke that have emerged from the assessment of networks. Thereby, it is shown that neurological deficits do not only arise from focal tissue damage but also from local and remote changes in white-matter tracts and in neural interactions among wide-spread networks. Similarly, plasticity and clinical improvements are associated with specific compensatory structural and functional patterns of neural network interactions. Innovative treatment approaches have started to target such network patterns to enhance recovery. Network assessments to predict treatment response and to individualize rehabilitation is a promising way to enhance specific treatment effects and overall outcome after stroke.

The supplementary motor area modulates interhemispheric interactions during movement preparation

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

GABA Levels in Left and Right Sensorimotor Cortex Correlate across Individuals

Differences in γ-aminobutyric acid (GABA) levels measured with Magnetic Resonance Spectroscopy have been shown to correlate with behavioral performance over a number of tasks and cortical regions. These correlations appear to be regionally and functionally specific. In this study, we test the hypothesis that GABA levels will be correlated within individuals for functionally related regions-the left and right sensorimotor cortex. In addition, we investigate whether this is driven by bulk tissue composition. GABA measurements using edited MRS data were acquired from the left and right sensorimotor cortex in 24 participants. T1-weighted MR images were also acquired and segmented to determine the tissue composition of the voxel. GABA level is shown to correlate significantly between the left and right regions (r = 0.64, p < 0.03). Tissue composition is highly correlated between sides, but does not explain significant variance in the bilateral correlation. In conclusion, individual differences in GABA level, which have previously been described as functionally and regionally specific, are correlated between homologous sensorimotor regions. This correlation is not driven by bulk differences in voxel tissue composition.

GABA levels and measures of intracortical and interhemispheric excitability in healthy young and older adults: an MRS-TMS study

Edited magnetic resonance spectroscopy (MRS) and transcranial magnetic stimulation (TMS) have often been used to study the integrity of the GABAergic neurotransmission system in healthy aging. To investigate whether the measurement outcomes obtained with these 2 techniques are associated with each other in older human adults, gamma-aminobutyric acid (GABA) levels in the left sensorimotor cortex were assessed with edited MRS in 28 older (63-74 years) and 28 young adults (19-34 years). TMS at rest was then used to measure intracortical inhibition (short-interval intracortical inhibition/long-interval intracortical inhibition), intracortical facilitation, interhemispheric inhibition from left to right primary motor cortex (M1) and recruitment curves of left and right M1. Our observations showed that short-interval intracortical inhibition and long-interval intracortical inhibition in the left M1 were reduced in older adults, while GABA levels did not significantly differ between age groups. Furthermore, MRS-assessed GABA within left sensorimotor cortex was not correlated with TMS-assessed cortical excitability or inhibition. These observations suggest that healthy aging gives rise to altered inhibition at the postsynaptic receptor level, which does not seem to be associated with MRS-assessed GABA+ levels.

Towards assessing corticospinal excitability bilaterally: Validation of a double-coil TMS method

BACKGROUND

For several decades, Transcranial magnetic stimulation (TMS) has been used to monitor corticospinal excitability (CSE) changes in various contexts. Habitually, single-coil TMS is applied over one primary motor cortex (M1), eliciting motor-evoked potentials (MEPs) in a contralateral limb muscle, usually a hand effector. However, in many situations, it would be useful to obtain MEPs in both hands simultaneously, to track CSE bilaterally. Such an approach requires stimulating both M1 concurrently while avoiding interference between the two descending stimuli.

NEW METHOD

We examined MEPs obtained at rest using a double-coil TMS approach where the two M1 are stimulated with a 1ms inter-pulse interval (double-coil_1ms). MEPs were acquired using double-coil_1ms (MEP_double) or single-coil (MEP_single) TMS, at five different intensities of stimulation (100, 115, 130, 145 or 160% of the resting motor threshold, rMT). Given the 1ms inter-pulse interval in double-coil_1ms trials, MEP_double were either evoked by a 1st (MEP_double-1) or a 2nd (MEP_double-2) TMS pulse.

RESULTS

All MEP_TYPE (MEP_TYPE=MEP_single, MEP_double-1 and MEP_double-2) were equivalent, regardless of the hand within which they were elicited, the intensity of stimulation or the pulse order.

COMPARISON WITH EXISTING METHOD

This method allows one to observe state-related CSE changes for the two hands simultaneously on a trial-by-trial basis.

CONCLUSION

These results infer the absence of any neural interactions between the two cortico-spinal volleys with double-coil_1ms TMS. Hence, this technique can be reliably used to assess CSE bilaterally, opening new research perspectives for scientists interested in physiological markers of activity in the motor output system.

Contrasting Modulatory Effects from the Dorsal and Ventral Premotor Cortex on Primary Motor Cortex Outputs

The dorsal and ventral premotor cortices (PMd and PMv, respectively) each take part in unique aspects for the planning and execution of hand movements. These premotor areas are components of complex anatomical networks that include the primary motor cortex (M1) of both hemispheres. One way that PMd and PMv could play distinct roles in hand movements is by modulating the outputs of M1 differently. However, patterns of effects from PMd and PMv on the outputs of M1 have not been compared systematically. Our goals were to study how PMd within the same (i.e., ipsilateral or iPMd) and in the opposite hemisphere (i.e., contralateral or cPMd) can shape M1 outputs and then compare these effects with those induced by PMv. We used paired-pulse protocols with intracortical microstimulation techniques in sedated female cebus monkeys while recording EMG signals from intrinsic hand and forearm muscles. A conditioning stimulus was delivered in iPMd or cPMd concurrently or before a test stimulus in M1. The patterns of modulatory effects from PMd were compared with those from PMv collected in the same animals. Striking differences were revealed. Conditioning stimulation in iPMd induced more frequent and powerful inhibitory effects on M1 outputs compared with iPMv. In the opposite hemisphere, cPMd conditioning induced more frequent and powerful facilitatory effects than cPMv. These contrasting patterns of modulatory effects could allow PMd and PMv to play distinct functions for the control of hand movements and predispose them to undertake different, perhaps somewhat opposite, roles in motor recovery after brain injury.SIGNIFICANCE STATEMENT The dorsal and ventral premotor cortices (PMd and PMv, respectively) are two specialized areas involved in the control of hand movements in primates. One way that PMd and PMv could participate in hand movements is by modulating or shaping the primary motor cortex (M1) outputs to hand muscles. Here, we studied the patterns of modulation from PMd within the same and in the opposite hemisphere on the outputs of M1 and compared them with those from PMv. We found that PMd and PMv have strikingly different effects on M1 outputs. These contrasting patterns of modulation provide a substrate that may allow PMd and PMv to carry distinct functions for the preparation and execution of hand movements and for recovery after brain injury.

Altered microstructure rather than morphology in the corpus callosum after lower limb amputation

The corpus callosum (CC) has been implicated in the reorganization of the brain following amputation. However, it is unclear which regions of the CC are involved in this process. In this study, we explored the morphometric and microstructural changes in CC subregions in patients with unilateral lower limb amputation. Thirty-eight patients and 38 age- and gender-matched normal controls were included. The CC was divided into five regions, and the area, thickness and diffusion parameters of each region were investigated. While morphometric analysis showed no significant differences between the two groups, amputees showed significant higher values in axial diffusivity, radial diffusivity and mean diffusivity in region II of the CC, which connects the bilateral premotor and supplementary motor areas. In contrast, the mean fractional anisotropy value of the fibers generated by these cortical areas, as measured by tractography, was significantly smaller in amputees. These results demonstrate that the interhemispheric pathways contributing to motor coordination and imagery are reorganized in lower limb amputees.

Modulatory Effects of the Ipsi and Contralateral Ventral Premotor Cortex (PMv) on the Primary Motor Cortex (M1) Outputs to Intrinsic Hand and Forearm Muscles in Cebus apella

The ventral premotor cortex (PMv) is a key node in the neural network involved in grasping. One way PMv can carry out this function is by modulating the outputs of the primary motor cortex (M1) to intrinsic hand and forearm muscles. As many PMv neurons discharge when grasping with either arm, both PMv within the same hemisphere (ipsilateral; iPMv) and in the opposite hemisphere (contralateral; cPMv) could modulate M1 outputs. Our objective was to compare modulatory effects of iPMv and cPMv on M1 outputs to intrinsic hand and forearm muscles. We used paired-pulse protocols with intracortical microstimulations in capuchin monkeys. A conditioning stimulus was applied in either iPMv or cPMv simultaneously or prior to a test stimulus in M1 and the effects quantified in electromyographic signals. Modulatory effects from iPMv were predominantly facilitatory, and facilitation was much more common and powerful on intrinsic hand than forearm muscles. In contrast, while the conditioning of cPMv could elicit facilitatory effects, in particular to intrinsic hand muscles, it was much more likely to inhibit M1 outputs. These data show that iPMv and cPMv have very different modulatory effects on the outputs of M1 to intrinsic hand and forearm muscles.

Magnetoencephalographic study of hand and foot sensorimotor organization in 325 consecutive patients evaluated for tumor or epilepsy surgery

Objectives

The presence of intracranial lesions or epilepsy may lead to functional reorganization and hemispheric lateralization. We applied a clinical magnetoencephalography (MEG) protocol for the localization of the contralateral and ipsilateral S1 and M1 of the foot and hand in patients with non-lesional epilepsy, stroke, developmental brain injury, traumatic brain injury and brain tumors. We investigated whether differences in activation patterns could be related to underlying pathology.

Methods

Using dipole fitting, we localized the sources underlying sensory and motor evoked magnetic fields (SEFs and MEFs) of both hands and feet following unilateral stimulation of the median nerve (MN) and posterior tibial nerve (PTN) in 325 consecutive patients. The primary motor cortex was localized using beamforming following a self-paced repetitive motor task for each hand and foot.

Results

The success rate for motor and sensory localization for the feet was significantly lower than for the hands (motor_hand 94.6% versus motor_feet 81.8%, p < 0.001; sensory_hand 95.3% versus sensory_feet 76.0%, p < 0.001). MN and PTN stimulation activated 86.6% in the contralateral S1, with ipsilateral activation < 0.5%. Motor cortex activation localized contralaterally in 76.1% (5.2% ipsilateral, 7.6% bilateral and 11.1% failures) of all motor MEG recordings. The ipsilateral motor responses were found in 43 (14%) out of 308 patients with motor recordings (range: 8.3–50%, depending on the underlying pathology), and had a higher occurrence in the foot than in the hand (motor_foot 44.8% versus motor_hand 29.6%, p = 0.031). Ipsilateral motor responses tended to be more frequent in patients with a history of stroke, traumatic brain injury (TBI) or developmental brain lesions (p = 0.063).

Conclusions

MEG localization of sensorimotor cortex activation was more successful for the hand compared to the foot. In patients with neural lesions, there were signs of brain reorganization as measured by more frequent ipsilateral motor cortical activation of the foot in addition to the traditional sensory and motor activation patterns in the contralateral hemisphere. The presence of ipsilateral neural reorganization, especially around the foot motor area, suggests that careful mapping of the hand and foot in both contralateral and ipsilateral hemispheres prior to surgery might minimize postoperative deficits.

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Using MEG, S1 and M1 responses of the hand and foot were mapped in patients with brain tumors or epilepsy.

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Localization of the hand was more successful than of the foot.

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Ipsilateral S1 responses were rarely seen but ipsilateral M1 responses differed by underlying pathology and limb.

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Results indicate that differential sensorimotor re-organization can occur in the presence of pathology.

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Ipsilateral and contralateral mapping of the hand and foot should be done to minimize postsurgical dysfunction.

The Plasticity of Brain Gray Matter and White Matter following Lower Limb Amputation

Accumulating evidence has indicated that amputation induces functional reorganization in the sensory and motor cortices. However, the extent of structural changes after lower limb amputation in patients without phantom pain remains uncertain. We studied 17 adult patients with right lower limb amputation and 18 healthy control subjects using T1-weighted magnetic resonance imaging and diffusion tensor imaging. Cortical thickness and fractional anisotropy (FA) of white matter (WM) were investigated. In amputees, a thinning trend was seen in the left premotor cortex (PMC). Smaller clusters were also noted in the visual-to-motor regions. In addition, the amputees also exhibited a decreased FA in the right superior corona radiata and WM regions underlying the right temporal lobe and left PMC. Fiber tractography from these WM regions showed microstructural changes in the commissural fibers connecting the bilateral premotor cortices, compatible with the hypothesis that amputation can lead to a change in interhemispheric interactions. Finally, the lower limb amputees also displayed significant FA reduction in the right inferior frontooccipital fasciculus, which is negatively correlated with the time since amputation. In conclusion, our findings indicate that the amputation of lower limb could induce changes in the cortical representation of the missing limb and the underlying WM connections.

Functional resting-state connectivity of the human motor network: differences between right- and left-handers

Handedness is associated with differences in activation levels in various motor tasks performed with the dominant or non-dominant hand. Here we tested whether handedness is reflected in the functional architecture of the motor system even in the absence of an overt motor task. Using resting-state functional magnetic resonance imaging we investigated 18 right- and 18 left-handers. Whole-brain functional connectivity maps of the primary motor cortex (M1), supplementary motor area (SMA), dorsolateral premotor cortex (PMd), pre-SMA, inferior frontal junction and motor putamen were compared between right- and left-handers. We further used a multivariate linear support vector machine (SVM) classifier to reveal the specificity of brain regions for classifying handedness based on individual resting-state maps. Using left M1 as seed region, functional connectivity analysis revealed stronger interhemispheric functional connectivity between left M1 and right PMd in right-handers as compared to left-handers. This connectivity cluster contributed to the individual classification of right- and left-handers with 86.2% accuracy. Consistently, also seeding from right PMd yielded a similar handedness-dependent effect in left M1, albeit with lower classification accuracy (78.1%). Control analyses of the other resting-state networks including the speech and the visual network revealed no significant differences in functional connectivity related to handedness. In conclusion, our data revealed an intrinsically higher functional connectivity in right-handers. These results may help to explain that hand preference is more lateralized in right-handers than in left-handers. Furthermore, enhanced functional connectivity between left M1 and right PMd may serve as an individual marker of handedness.

Aging and motor inhibition: a converging perspective provided by brain stimulation and imaging approaches

The ability to inhibit actions, one of the hallmarks of human motor control, appears to decline with advancing age. Evidence for a link between changes in inhibitory functions and poor motor performance in healthy older adults has recently become available with transcranial magnetic stimulation (TMS). Overall, these studies indicate that the capacity to modulate intracortical (ICI) and interhemispheric (IHI) inhibition is preserved in high-performing older individuals. In contrast, older individuals exhibiting motor slowing and a declined ability to coordinate movement appear to show a reduced capability to modulate GABA-mediated inhibitory processes. As a decline in the integrity of the GABA-ergic inhibitory processes may emerge due to age-related loss of white and gray matter, a promising direction for future research would be to correlate individual differences in structural and/or functional integrity of principal brain networks with observed changes in inhibitory processes within cortico-cortical, interhemispheric, and/or corticospinal pathways. Finally, we underscore the possible links between reduced inhibitory functions and age-related changes in brain activation patterns.

Nonparetic arm force does not overinhibit the paretic arm in chronic poststroke hemiparesis

OBJECTIVE

To determine whether nonparetic arm force overinhibits the paretic arm in patients with chronic unilateral poststroke hemiparesis.

DESIGN

Case-control neurophysiological and behavioral study of patients with chronic stroke.

SETTING

Research institution.

PARTICIPANTS

Eighty-six referred patients were screened to enroll 9 participants (N=9) with a >6 month history of 1 unilateral ischemic infarct that resulted in arm hemiparesis with residual ability to produce 1Nm of wrist flexion torque and without contraindication to transcranial magnetic stimulation. Eight age- and handedness-matched healthy volunteers without neurologic diagnosis were studied for comparison.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURE

Change in interhemispheric inhibition targeting the ipsilesional primary motor cortex (M1) during nonparetic arm force. We hypothesized that interhemispheric inhibition would increase more in healthy controls than in patients with hemiparesis.

RESULTS

Healthy age-matched controls had significantly greater increases in inhibition from their active to resting M1 than patients with stroke from their active contralesional to resting ipsilesional M1 in the same scenario (20%±7% vs -1%±4%, F1,12=6.61, P=.025). Patients with greater increases in contralesional to ipsilesional inhibition were better performers on the 9-hole peg test of paretic arm function.

CONCLUSIONS

Our findings reveal that producing force with the nonparetic arm does not necessarily overinhibit the paretic arm. Though our study is limited in generalizability by the small sample size, we found that greater active contralesional to resting ipsilesional M1 inhibition was related with better recovery in this subset of patients with chronic poststroke.

White matter integrity of motor connections related to training gains in healthy aging

Impaired motor skill acquisition is a feature of older age. Acquisition of new motor skills requires the interplay between different cortical motor areas. Using diffusion tensor imaging we reconstructed cortico-cortical connections between the primary motor cortex (M1) and secondary motor areas in 11 older and 11 young participants who took part in a motor skill acquisition paradigm with the nondominant left hand. Examining the extent to which tract-related integrity correlated with training gains we found that white matter integrity of fibers connecting contralateral M1 with both contralateral (r = 0.85) and ipsilateral supplementary motor areas (r = 0.92) were positively associated in old participants. Also, fibers connecting contralateral M1 with ipsilateral dorsal premotor (r = 0.82) and fibers connecting ipsilateral dorsal premotor and supplementary motor area (r = 0.88) were positively related to skill acquisition (all p < 0.05). A similar structure-behavior relationship was not present in the young control subjects suggesting a critical role of brain structural integrity for motor learning in healthy aging.

Speed-dependent contribution of callosal pathways to ipsilateral movements

Transcallosal inhibitory interactions between primary motor cortices are important to suppress unintended movements in a resting limb during voluntary activation of the contralateral limb. The functional contribution of transcallosal inhibition targeting the voluntary active limb remains unknown. Using transcranial magnetic stimulation, we examined transcallosal inhibition [by measuring interhemispheric inhibition (IHI) and the ipsilateral silent period (iSP)] in the preparatory and execution phases of isotonic slower self-paced and ballistic movements performed by the ipsilateral index finger into abduction and the elbow into flexion in intact humans. We demonstrate decreased IHI in the preparatory phase of self-paced and ballistic index finger and elbow movements compared to rest; the decrease in IHI was larger during ballistic than self-paced movements. In contrast, in the execution phase, IHI and the iSP increased during ballistic compared to self-paced movements. Transcallosal inhibition was negatively correlated with reaction times in the preparatory phase and positively correlated with movement amplitude in the execution phase. Together, our results demonstrate a widespread contribution of transcallosal inhibition to ipsilateral movements of different speeds with a functional role during rapid movements; at faster speeds, decreased transcallosal inhibition in the preparatory phase may contribute to start movements rapidly, while the increase in the execution phase may contribute to stop the movement. We argue that transcallosal pathways enable signaling of the time of discrete behavioral events during ipsilateral movements, which is amplified by the speed of a movement.

Modulation of transcallosal inhibition by bilateral activation of agonist and antagonist proximal arm muscles

Transcallosal inhibitory interactions between proximal representations in the primary motor cortex remain poorly understood. In this study, we used transcranial magnetic stimulation to examine the ipsilateral silent period (iSP; a measure of transcallosal inhibition) in the biceps and triceps brachii during unilateral and bilateral isometric voluntary contractions. Healthy volunteers performed 10% of maximal isometric voluntary elbow flexion or extension with one arm while the contralateral arm remained at rest or performed 30% of maximal isometric voluntary elbow flexion or extension. The iSP was measured in the arm performing 10% contractions, and electromyographic (EMG) recordings were comparable across conditions. The iSP onset and duration in the biceps and triceps brachii were comparable. In both muscles, the iSP depth and area were increased during bilateral contractions of homologous agonist muscles (extension-extension and flexion-flexion) compared with a unilateral contraction, whereas during bilateral contractions of nonhomologous antagonist muscles (extension-flexion and flexion-extension), the iSP depth and area were decreased compared with a unilateral contraction, and sometimes facilitation of EMG was seen. This effect was never observed during bilateral activation of homologous muscles. The size of responses evoked by cervicomedullary electrical stimulation in the arm that made 10% contractions remained unchanged across conditions. Thus transcallosal inhibition targeting triceps and biceps brachii is upregulated by voluntary contraction of the contralateral agonist muscle and downregulated by voluntary contraction of the contralateral antagonist muscle. We speculate that these reciprocal task-dependent interactions between bilateral flexor and extensor arm regions of the motor cortex may contribute to coupling between the arms during motor behavior.

Corticospinal modulation induced by sounds depends on action preparedness

A loud acoustic stimulus (LAS) presented during movement preparation can induce an early release of the prepared action. Because loud sound has been found to have an inhibitory effect on motor cortex excitability, it is possible that the motor cortex plays little role in the early release of prepared responses. We sought to shed new light on this suggestion by probing changes in corticospinal excitability after LAS presentation during preparation for an anticipatory action. Unexpectedly, we show that the changes in corticospinal excitability after LAS presentation are not fixed. Based on the magnitude of motor-evoked potentials elicited by transcranial magnetic and electric stimulation of the motor cortex, we demonstrate that the effects of auditory stimuli on corticospinal excitability depend on the level of readiness for action: inhibition in early preparation and facilitation close to movement onset. We also show that auditory stimuli can regulate intracortical excitability by increasing intracortical facilitation and reducing short-interval intracortical inhibition. Together, these findings indicate that, at least in part, the early release of motor responses by auditory stimuli involves the motor cortex.

Motor control and neural plasticity through interhemispheric interactions

The corpus callosum, which is the largest white matter structure in the human brain, connects the 2 cerebral hemispheres. It plays a crucial role in maintaining the independent processing of the hemispheres and in integrating information between both hemispheres. The functional integrity of interhemispheric interactions can be tested electrophysiologically in humans by using transcranial magnetic stimulation, electroencephalography, and functional magnetic resonance imaging. As a brain structural imaging, diffusion tensor imaging has revealed the microstructural connectivity underlying interhemispheric interactions. Sex, age, and motor training in addition to the size of the corpus callosum influence interhemispheric interactions. Several neurological disorders change hemispheric asymmetry directly by impairing the corpus callosum. Moreover, stroke lesions and unilateral peripheral impairments such as amputation alter interhemispheric interactions indirectly. Noninvasive brain stimulation changes the interhemispheric interactions between both motor cortices. Recently, these brain stimulation techniques were applied in the clinical rehabilitation of patients with stroke by ameliorating the deteriorated modulation of interhemispheric interactions. Here, we review the interhemispheric interactions and mechanisms underlying the pathogenesis of these interactions and propose rehabilitative approaches for appropriate cortical reorganization.

Premotor-motor interhemispheric inhibition is released during movement initiation in older but not young adults

Neural interactions between contralateral motor regions are thought to be instrumental in the successful preparation, and execution, of volitional movements. Here we investigated whether healthy ageing is associated with a change in functional connectivity, as indicated by the ability to modulate interhemispheric interactions during movement preparation in a manner that assists rapid movement responses. Thirteen young (mean age 22.2 years) and thirteen older (68.5 years) adults rapidly abducted their left index finger as soon as possible in response to a visual imperative signal, presented 500 ms after a visual warning signal.

Interactions between left dorsal premotor cortex (LPMd) and right primary motor cortex (RM1) and between left primary motor cortex (LM1) and RM1 were investigated at six time points between the warning signal and the volitional response using paired-pulse transcranial magnetic stimulation. Relative to the inhibitory interactions measured at rest, both young and older adults released LM1-RM1 inhibition beginning 250 ms after the warning signal, with no significant differences between groups. LPMd-RM1 interactions became facilitatory (from the onset of the imperative signal onwards) in the older, but not the young, group. Regression analyses revealed that for the older adults, modulation of LPMd-RM1 interactions early in the preparation period was associated with faster responses, suggesting that specifically timed modulation of these pathways may be a compensatory mechanism to offset, at least in part, slowing of motor responses. The results suggest a greater reliance on premotor regions during the preparation of simple motor actions with advancing age.

Modulation of interhemispheric interactions across symmetric and asymmetric bimanual force regulations

The corpus callosum is essential for neural communication between the left and right hemispheres. Although spatiotemporal coordination of bimanual movements is mediated by the activity of the transcallosal circuit, it remains to be addressed how transcallosal neural activity is involved in the dynamic control of bimanual force execution in human. To address this issue, we investigated transcallosal inhibition (TCI) elicited by single-pulse transcranial magnetic stimulation (TMS) in association with the coordination condition of bimanual force regulation. During a visually-guided bimanual force tracking task, both thumbs were abducted either in-phase (symmetric condition) or 180° out-of-phase (asymmetric condition). TMS was applied to the left primary motor cortex to elicit the disturbance of ipsilateral left force tracking due to TCI. The tracking accuracy was equivalent between the two conditions, but the synchrony of the left and right tracking trajectories was higher in the symmetric condition than in the asymmetric condition. The magnitude of force disturbance and TCI were larger during the symmetric condition than during the asymmetric condition. Right unimanual force tracking influenced neither the force disturbance nor TCI during tonic left thumb abduction. Additionally, these TMS-induced ipsilateral motor disturbances only appeared when the TMS intensity was strong enough to excite the transcallosal circuit, irrespective of whether the crossed corticospinal tract was activated. These findings support the hypotheses that interhemispheric interactions between the motor cortices play an important role in modulating bimanual force coordination tasks, and that TCI is finely tuned depending on the coordination condition of bimanual force regulation.

Similar cerebral motor plans for real and virtual actions

A simple movement, such as pressing a button, can acquire different meanings by producing different consequences, such as starting an elevator or switching a TV channel. We evaluated whether the brain activity preceding a simple action is modulated by the expected consequences of the action itself. To further this aim, the motor-related cortical potentials were compared during two key-press actions that were identical from the kinematics point of view but different in both meaning and consequences. In one case (virtual grasp), the key-press started a video clip showing a hand moving toward a cup and grasping it; in the other case, the key-press did not produce any consequence (key-press). A third condition (real grasp) was also compared, in which subjects actually grasped the cup, producing the same action presented in the video clip. Data were collected from fifteen subjects. The results showed that motor preparation for virtual grasp (starting 3 s before the movement onset) was different from that of the key-press and similar to the real grasp preparation–as if subjects had to grasp the cup in person. In particular, both virtual and real grasp presented a posterior parietal negativity preceding activity in motor and pre-motor areas. In summary, this finding supports the hypothesis that motor preparation is affected by the meaning of the action, even when the action is only virtual.

Mirror symmetric bimanual movement priming can increase corticomotor excitability and enhance motor learning

Repetitive mirror symmetric bilateral upper limb may be a suitable priming technique for upper limb rehabilitation after stroke. Here we demonstrate neurophysiological and behavioural after-effects in healthy participants after priming with 20 minutes of repetitive active-passive bimanual wrist flexion and extension in a mirror symmetric pattern with respect to the body midline (MIR) compared to an control priming condition with alternating flexion-extension (ALT). Transcranial magnetic stimulation (TMS) indicated that corticomotor excitability (CME) of the passive hemisphere remained elevated compared to baseline for at least 30 minutes after MIR but not ALT, evidenced by an increase in the size of motor evoked potentials in ECR and FCR. Short and long-latency intracortical inhibition (SICI, LICI), short afferent inhibition (SAI) and interhemispheric inhibition (IHI) were also examined using pairs of stimuli. LICI differed between patterns, with less LICI after MIR compared with ALT, and an effect of pattern on IHI, with reduced IHI in passive FCR 15 minutes after MIR compared with ALT and baseline. There was no effect of pattern on SAI or FCR H-reflex. Similarly, SICI remained unchanged after 20 minutes of MIR. We then had participants complete a timed manual dexterity motor learning task with the passive hand during, immediately after, and 24 hours after MIR or control priming. The rate of task completion was faster with MIR priming compared to control conditions. Finally, ECR and FCR MEPs were examined within a pre-movement facilitation paradigm of wrist extension before and after MIR. ECR, but not FCR, MEPs were consistently facilitated before and after MIR, demonstrating no degradation of selective muscle activation. In summary, mirror symmetric active-passive bimanual movement increases CME and can enhance motor learning without degradation of muscle selectivity. These findings rationalise the use of mirror symmetric bimanual movement as a priming modality in post-stroke upper limb rehabilitation.

Vision modulates corticospinal suppression in a functionally specific manner during movement of the opposite limb

The effect of vision on the excitability of corticospinal projections to the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles of right human forearm was investigated before and during discrete movement of the opposite limb. An external force opposed the initial phase of the movement (wrist flexion) and assisted the reverse phase, so that recruitment of the wrist extensors was minimized. Three conditions were used as follows: viewing the inactive right limb (Vision), viewing the mirror image of the moving left limb (Mirror), and with vision of the right limb occluded (No Vision). Transcranial magnetic stimulation was delivered to the left motor cortex: before, at the onset of, or during the left limb movement to obtain motor evoked potentials (MEPs) in the muscles of the right forearm. At and following movement onset, MEPs obtained in the right FCR were smaller in the Vision condition than in the Mirror and No Vision conditions. A distinct pattern of variation was obtained for the ECR. In all conditions, MEPs in this muscle were elevated upon or following movement of the opposite limb. An additional analysis of ipsilateral silent periods indicated that interhemispheric inhibition plays a role in mediating these effects. Activity-dependent changes in corticospinal output to a resting limb during discrete actions of the opposite limb are thus directly contingent upon where one looks. Furthermore, the extent to which vision exerts an influence upon projections to specific muscles varies in accordance with the functional contribution of their homologs to the intended action.

Coordination of uncoupled bimanual movements by strictly timed interhemispheric connectivity

Independent use of both hands is characteristic of human action in daily life. By nature, however, in-phase bimanual movements, for example clapping, are easier to accomplish than anti-phase movements, for example playing the piano. It is commonly agreed that interhemispheric interactions play a central role in the coordination of bimanual movements. However, the spatial, temporal, and physiological properties of the interhemispheric signals that coordinate different modes of bimanual movements are still not completely understood. More precisely, do individual interhemispheric connectivity parameters have behavioral relevance for bimanual rapid anti-phase coordination? To address this question, we measured movement-related interhemispheric interactions, i.e., inhibition and facilitation, and correlated them with the performance during bimanual coordination. We found that movement-related facilitation from right premotor to left primary motor cortex (rPMd-lM1) predicted performance in anti-phase bimanual movements. It is of note that only fast facilitation during the preparatory period of a movement was associated with success in anti-phase movements. Modulation of right to left primary motor interaction (rM1-lM1) was not related to anti-phase but predicted bimanual in-phase and unimanual behavior. These data suggest that strictly timed modulation of interhemispheric rPMd-lM1 connectivity is essential for independent high-frequency use of both hands. The rM1-lM1 results indicate that adjustment of connectivity between homologous M1 may be important for the regulation of homologous muscle synergies.

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

Functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) are well-established tools for investigating the human motor system in-vivo. We here studied the relationship between movement-related fMRI signal changes in the primary motor cortex (M1) and electrophysiological properties of the hand motor area assessed with neuronavigated TMS in 17 healthy subjects. The voxel showing the highest task-related BOLD response in the left hand motor area during right hand movements was identified for each individual subject. This fMRI peak voxel in M1 served as spatial target for coil positioning during neuronavigated TMS. We performed correlation analyses between TMS parameters, BOLD signal estimates and effective connectivity parameters of M1 assessed with dynamic causal modeling (DCM). The results showed a negative correlation between the movement-related BOLD signal in left M1 and resting as well as active motor threshold (MT) obtained for left M1. The DCM analysis revealed that higher excitability of left M1 was associated with a stronger coupling between left supplementary motor area (SMA) and M1. Furthermore, BOLD activity in left M1 correlated with ipsilateral silent period (ISP), i.e. the stronger the task-related BOLD response in left M1, the higher interhemispheric inhibition effects targeting right M1. DCM analyses revealed a positive correlation between the coupling of left SMA with left M1 and the duration of ISP. The data show that TMS parameters assessed for the hand area of M1 do not only reflect the intrinsic properties at the stimulation site but also interactions with remote areas in the human motor system.

What have We Learned from "Perturbing" the Human Cortical Motor System with Transcranial Magnetic Stimulation?

The purpose of this paper is twofold. First, we will review different approaches that one can use with transcranial magnetic stimulation (TMS) to study both its effects on motor behavior and on neural connections in the human brain. Second, we will present evidence obtained in TMS-based studies showing that the dorsal premotor area (PMd), the ventral premotor area (PMv), the supplementary motor area (SMA), and the pre-supplementary motor area (pre-SMA) each have different roles to play in motor behavior. We highlight the importance of the PMd in response selection based on arbitrary cues and in the control of arm movements, the PMv in grasping and in the discrimination of bodily actions, the SMA in movement sequencing and in bimanual coordination, and the pre-SMA in cognitive control. We will also discuss ways in which TMS can be used to chart “true” cerebral reorganization in clinical populations and how TMS might be used as a therapeutic tool to facilitate motor recovery after stroke. We will end our review by discussing some of the methodological challenges and future directions for using this tool in basic and clinical neuroscience.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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