Cortico-cortical Coherence Research Articles

State-Related Changes in MEG Functional Connectivity Reveal the Task-Positive Sensorimotor Network

Functional connectivity measures applied to magnetoencephalography (MEG) data have the capacity to elucidate neuronal networks. However, the task-related modulation of these measures is essential to identifying the functional relevance of the identified network. In this study, we provide evidence for the efficacy of measuring “state-related” (i.e., task vs. rest) changes in MEG functional connectivity for revealing a sensorimotor network. We investigate changes in functional connectivity, measured as cortico-cortical coherence (CCC), between rest blocks and the performance of a visually directed motor task in a healthy cohort. Task-positive changes in CCC were interpreted in the context of any concomitant modulations in spectral power. Task-related increases in whole-head CCC relative to the resting state were identified between areas established as part of the sensorimotor network as well as frontal eye fields and prefrontal cortices, predominantly in the beta and gamma frequency bands. This study provides evidence for the use of MEG to identify task-specific functionally connected sensorimotor networks in a non-invasive, patient friendly manner.

Cortical representation of different motor rhythms during bimanual movements

The cortical control of bimanual and unimanual movements involves complex facilitatory and inhibitory interhemispheric interactions. We analysed the part of the cortical network directly related to the motor output by corticomuscular (64 channel EEG-EMG) and cortico-cortical (EEG-EEG) coherence and delays at the frequency of a voluntarily maintained unimanual and bimanual rhythm and in the 15-30-Hz band during isometric contractions. Voluntary rhythms of each hand showed coherence with lateral cortical areas in both hemispheres and occasionally in the frontal midline region (60-80% of the recordings and 10-30%, respectively). They were always coherent between both hands, and this coherence was positively correlated with the interhemispheric coherence (p<0.01). Unilateral movements were represented mainly in the contralateral cortex (60-80 vs. 10-30% ipsilateral, p<0.01). Ipsilateral coherence was more common in left-hand movements, paralleled by more left-right muscle coherence. Partial corticomuscular coherence most often disappeared (p<0.05) when the contralateral cortex was the predictor, indicating a mainly indirect connection of ipsilateral/frontomesial representations with the muscle via contralateral cortex. Interhemispheric delays had a bimodal distribution (1-10 and 15-30ms) indicating direct and subcortical routes. Corticomuscular delays (mainly 12-25ms) indicated fast corticospinal projections and musculocortical feedback. The 15-30-Hz corticomuscular coherence during isometric contractions (60-70% of recordings) was strictly contralaterally represented without any peripheral left-right coherence. Thus, bilateral cortical areas generate voluntary unimanual and bimanual rhythmic movements. Interhemispheric interactions as detected by EEG-EEG coherence contribute to bimanual synchronization. This is distinct from the unilateral cortical representation of the 15-30-Hz motor rhythm during isometric movements.

Subcortical substrates of TMS induced modulation of the cortico-cortical connectivity

BackgroundTranscranial magnetic stimulation (TMS) can modulate transiently the physiological brain oscillations, e.g. the alpha rhythm. It has been hypothesized that this effect is not limited to the stimulated region but involves subcortical and distant cortical areas. MethodsWe applied single pulse TMS to the primary motor cortex (M1) of healthy subjects to interfere the cortical oscillatory activity recorded by simultaneous EEG and calculated the cortico-cortical coherence and power in the alpha and beta band. To study the structural substrate of the functional connectivity we performed diffusion tensor imaging and fractional anisotropy analysis (FA). To capture the pathways involved we applied probabilistic tractography to reconstruct the entire network. ResultsSuprathreshold TMS of M1 induced a consistent enhancement of interhemispheric cortico-cortical alpha band coherence that lasted ca. 175 ms. after the pulse has been applied. The changes were confined to the interhemispheric central EEG electrodes (i.e. C3-C4). There were no consistent changes in the beta band. Power analysis revealed a longer lasting increase in the beta band after TMS pulses. A cluster in the contralateral thalamus showed a linear relationship between regional FA and TMS induced change in alpha band coherence. Probabilistic tractography presents the transcallosal and the contralateral thalamocortical pathways as essential for the observed oscillatory synchronisation. ConclusionTMS induces an enhancement of oscillatory interaction between corresponding central regions of both hemispheres in the alpha band. The contralateral thalamus, transcallosal fibres and the contralateral thalamocortical pathways may constitute critical brain structures mediating the TMS induced change in oscillatory coupling.

Internalized Timing of Isochronous Sounds Is Represented in Neuromagnetic Beta Oscillations

Moving in synchrony with an auditory rhythm requires predictive action based on neurodynamic representation of temporal information. Although it is known that a regular auditory rhythm can facilitate rhythmic movement, the neural mechanisms underlying this phenomenon remain poorly understood. In this experiment using human magnetoencephalography, 12 young healthy adults listened passively to an isochronous auditory rhythm without producing rhythmic movement. We hypothesized that the dynamics of neuromagnetic beta-band oscillations (~20 Hz)-which are known to reflect changes in an active status of sensorimotor functions-would show modulations in both power and phase-coherence related to the rate of the auditory rhythm across both auditory and motor systems. Despite the absence of an intention to move, modulation of beta amplitude as well as changes in cortico-cortical coherence followed the tempo of sound stimulation in auditory cortices and motor-related areas including the sensorimotor cortex, inferior-frontal gyrus, supplementary motor area, and the cerebellum. The time course of beta decrease after stimulus onset was consistent regardless of the rate or regularity of the stimulus, but the time course of the following beta rebound depended on the stimulus rate only in the regular stimulus conditions such that the beta amplitude reached its maximum just before the occurrence of the next sound. Our results suggest that the time course of beta modulation provides a mechanism for maintaining predictive timing, that beta oscillations reflect functional coordination between auditory and motor systems, and that coherence in beta oscillations dynamically configure the sensorimotor networks for auditory-motor coupling.

Alpha coherence predicts accuracy during a visuomotor tracking task

It has been shown that synchrony of neuronal oscillations plays a critical role in effective communication between functionally distinct brain areas involving motor-sensory integration. However, the patterns of cortico–cortical coupling and their relation to behavioural success are widely unknown. Here, we analysed changes in cortico–cortical coherence during an unimanual visuomotor task and their correlation with performance. A 28-channel-EEG was attained in 27 healthy subjects during the tracking of an irregularly fluctuating target on a screen by manipulating a force sensor with the right index finger and thumb. For oscillatory power in the alpha (8–12Hz) and the lower beta-band (beta1, 13–20Hz), we found a decrease in central and occipital areas during performance. Interregional coherence between contralateral frontal and central areas was enhanced in the alpha band. In beta1, we observed a marked increase of coherence in centroparietal regions of both hemispheres extending to occipital and frontal regions in beta2 (21–30Hz). Most prominently, correlation analysis between alpha coherence and performance accuracy indicated that higher occipitocentral (i.e. visuomotor) coherence is associated with better visuomotor performance whereas high tracking error is associated with enhanced frontocentral coupling, suggesting additional activation of a frontoparietal control network. These results provide further evidence that coherent brain oscillations in alpha and beta bands significantly contribute to effective functional integration of visual and motor areas.

09 Frontotemporal dementia: reduced auditory MisMatch Negativity response and abnormal cortico-cortical MEG coherence

BackgroundBehavioural variant Frontotemporal dementia (bv-FTD) causes disabling behaviour and personality changes. A systems level understanding of neurocognitive dysfunction remains underdeveloped, relative to the molecular biology and neuropsychological. We examined responses...

Stability-dependent behavioural and electro-cortical reorganizations during intentional switching between bimanual tapping modes

This study investigated behavioural and electro-cortical reorganizations accompanying intentional switching between two distinct bimanual coordination tapping modes (In-phase and Anti-phase) that differ in stability when produced at the same movement rate. We expected that switching to a less stable tapping mode (In-to-Anti switching) would lead to larger behavioural perturbations and require supplementary neural resources than switching to a more stable tapping mode (Anti-to-In switching). Behavioural results confirmed that the In-to-Anti switching lasted longer than the Anti-to-In switching. A general increase in attention-related neural activity was found at the moment of switching for both conditions. Additionally, two condition-dependent EEG reorganizations were observed. First, a specific increase in cortico-cortical coherence appeared exclusively during the In-to-Anti switching. This result may reflect a strengthening in inter-regional communication in order to engage in the subsequent, less stable, tapping mode. Second, a decrease in motor-related neural activity (increased beta spectral power) was found for the Anti-to-In switching only. The latter effect may reflect the interruption of the previous, less stable, tapping mode. Given that previous results on spontaneous Anti-to-In switching revealing an inverse pattern of EEG reorganization (decreased beta spectral power), present findings give new insight on the stability-dependent neural correlates of intentional motor switching.

Study the neuronal mechanisms of mild traumatic brain injury using an integrated multi-modal neuroimaging approach with MEG and DTI

Event Abstract Back to Event Study the neuronal mechanisms of mild traumatic brain injury using an integrated multi-modal neuroimaging approach with MEG and DTI Mingxiong Huang1*, Rebecca Theilmann1, Ashley Robb2, Annemarie Angeles2, Sharon Nichols1, Mithun Diwakar1, Angela Drake3, Michael Levy4, Tao Song1, Li Cui1, Dewleen Baker1 and Roland Lee1 1 University of California, United States 2 VA San Diego Healthcare System, United States 3 Naval Medical Center, United States 4 Children's Hospital , United States Traumatic brain injury (TBI) is a leading cause of sustained impairment in military and civilian populations. However, there is currently no “gold standard” for diagnosing mild TBI. This is because that mild (and some moderate) TBI can be difficult to diagnose due to lack of visible lesions on conventional acute MRI or CT, and due to limited understanding of pathophysiology for mild TBI. Injured brain tissues in TBI patients generate pathological low-frequency neuronal magnetic signal (delta waves: 1-4 Hz) that can be measured and localized by MEG. One key issue is that the neurophysiological cause of the MEG slow-waves observed in mild TBI patients is not clearly understood. Another key issue is that the relationship between the generation of MEG slow-wave and potential reduction of brain functional connectivity (also measured by MEG using cortico-cortical coherence) in the mild TBI patients has not been established. The lack of understanding of these observed abnormalities motivates us in the present study to examine the neuronal basis of mTBI using multiple modalities, including MEG and diffusion tensor imaging (DTI). The results show: 1) The integrated approach with MEG and DTI is substantially more sensitive than conventional CT and MRI in detecting subtle neuronal injury in mild TBI; 2) MEG slow-waves in mild TBI patients originate from cortical gray-matter areas that experience de-afferentation due to axonal injuries in the white-matter fibers with reduced fractional anisotropy; 3) Brain areas generating MEG slow-waves also show reduction of functional connectivity (in beta and gama frequency bands) with other brain areas as measured by MEG using cortico-cortical coherence; 4) Findings from the integrated imaging approach are consistent with post-concussive symptoms; 5) In some cases, abnormal MEG delta-waves were observed in subjects without obvious DTI abnormality, indicating that MEG may be more sensitive than DTI in diagnosing mild TBI. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Oral Presentation Topic: MEG: Clinical applications Citation: Huang M, Theilmann R, Robb A, Angeles A, Nichols S, Diwakar M, Drake A, Levy M, Song T, Cui L, Baker D and Lee R (2010). Study the neuronal mechanisms of mild traumatic brain injury using an integrated multi-modal neuroimaging approach with MEG and DTI. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00264 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 01 Apr 2010; Published Online: 01 Apr 2010. * Correspondence: Mingxiong Huang, University of California, San Diego, United States, mxhuang@ucsd.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Mingxiong Huang Rebecca Theilmann Ashley Robb Annemarie Angeles Sharon Nichols Mithun Diwakar Angela Drake Michael Levy Tao Song Li Cui Dewleen Baker Roland Lee Google Mingxiong Huang Rebecca Theilmann Ashley Robb Annemarie Angeles Sharon Nichols Mithun Diwakar Angela Drake Michael Levy Tao Song Li Cui Dewleen Baker Roland Lee Google Scholar Mingxiong Huang Rebecca Theilmann Ashley Robb Annemarie Angeles Sharon Nichols Mithun Diwakar Angela Drake Michael Levy Tao Song Li Cui Dewleen Baker Roland Lee PubMed Mingxiong Huang Rebecca Theilmann Ashley Robb Annemarie Angeles Sharon Nichols Mithun Diwakar Angela Drake Michael Levy Tao Song Li Cui Dewleen Baker Roland Lee Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Enhancement of long‐range EEG coherence by synchronous bifocal transcranial magnetic stimulation

Interregional coupling of distant brain regions can be measured by electroencephalographic (EEG) coherence reflecting the spatial-temporal correlation between two oscillatory signals. It has been suggested that this coherence in activity is a signature of functional integration of multimodal neuronal networks. Repetitive transcranial magnetic stimulation (rTMS) is a well-established technique for non-invasive cortical stimulation. Its modulating effects outlast the train of stimulation and affect behavior. In the present study, we tested the hypothesis that cortico-cortical coherence between distant brain areas can be selectively enhanced by synchronous bifocal rTMS. Cortico-cortical coherence was assessed in 16 healthy human subjects before and after three trains of synchronous high-frequency (10 Hz) rTMS to the left primary motor cortex and the visual cortex at the occipital pole simultaneously. Stimulation of the left M1 alone served as the control condition. Coherence and spectral power were measured between these areas on the stimulated and the homologue contralateral side. Synchronous bifocal rTMS induced an increase of interregional coupling in the alpha and lower beta band on the stimulated side without effects on spectral power. These data indicate that synchronous bifocal rTMS is a feasible technique for selective modulation of interregional EEG coherence. Furthermore, they raise the hypothesis that interventional enhancement of long-range coherence may effectively modulate interregional integration with behavioral consequences.

Imaging the human motor system’s beta-band synchronization during isometric contraction

Rhythmic synchronization likely subserves interactions among neuronal groups. One of the best studied rhythmic synchronization phenomena in the human nervous system is the beta-band (15–30 Hz) synchronization in the motor system. In this study, we imaged structures across the human brain that are synchronized to the motor system’s beta rhythm. We recorded whole-head magnetoencephalograms (MEG) and electromyograms (EMG) of left/right extensor carpi radialis muscle during left/right wrist extension. We analyzed coherence, on the one hand between the EMG and neuronal sources in the brain, and on the other hand between different brain sources, using a spatial filtering approach. Cortico-muscular coherence analysis revealed a spatial maximum of coherence to the muscle in motor cortex contralateral to the muscle in accordance with earlier findings. Moreover, by applying a two-dipole source model, we unveiled significantly coherent clusters of voxels in the ipsilateral cerebellar hemisphere and ipsilateral cerebral motor regions. The spatial pattern of coherence to the right and left arm EMG was roughly mirror reversed across the midline, in agreement with known physiology. Subsequently, we analyzed the brain-wide pattern of beta-band coherence to the motor cortex contralateral to the contracting muscle. This analysis did not reveal any convincing pattern. Because the prior cortico-muscular analysis had demonstrated the expected pattern in our data, this negative finding demonstrates a current limitation of the applied method for cortico-cortical coherence analysis. We conclude that during an isometric muscle contraction, several distributed brain regions form a brain-wide beta-band network for motor control.

Multimodal imaging of brain reorganization in motor areas of the contralesional hemisphere of well recovered patients after capsular stroke

Clinical recovery after stroke can be significant and has been attributed to plastic reorganization and recruitment of novel areas previously not engaged in a given task. As equivocal results have been reported in studies using single imaging or electrophysiological methods, here we applied an integrative multimodal approach to a group of well-recovered chronic stroke patients (n = 11; aged 50-81 years) with left capsular lesions. Focal activation during recovered hand movements was assessed with EEG spectral analysis and H2(15)O-PET with EMG monitoring, cortico-cortical connectivity with EEG coherence analysis (cortico-cortical coherence) and corticospinal connectivity with transcranial magnetic stimulation (TMS). As seen from comparisons with age-matched controls, our patients showed enhanced recruitment of the lateral premotor cortex of the lesioned hemisphere [Brodmann area (BA) 6], lateral premotor and to a lesser extent primary sensorimotor and parietal cortex of the contralesional hemisphere (CON-H; BA 4 and superior parietal lobule) and left cerebellum (patients versus controls, Z > 3.09). EEG coherence analysis showed that after stroke cortico-cortical connections were reduced in the stroke hemisphere but relatively increased in the CON-H (ANOVA, contrast analysis, P < 0.05), suggesting a shift of functional connectivity towards the CON-H. Nevertheless, fast conducting corticospinal transmission originated exclusively from the lesioned hemisphere. No direct ipsilateral motor evoked potentials (MEPs) could be elicited with TMS over the contralesional primary motor cortex (iM1) in stroke patients. We conclude that (i) effective recovery is based on enhanced utilization of ipsi- and contralesional resources, (ii) basic corticospinal commands arise from the lesioned hemisphere without recruitment of ('latent') uncrossed corticospinal tract fibres and (iii) increased contralesional activity probably facilitates control of recovered motor function by operating at a higher-order processing level, similar to but not identical with the extended network concerned with complex movements in healthy subjects.

Synchronization of parietal and premotor areas during preparation and execution of praxis hand movements

Objective We sought to determine temporal patterns of functional connectivity between the parietal, premotor, and motor cortices during preparation and execution of praxis hand movements. Methods Normal subjects were instructed to perform six transitive (tool use) and intransitive (communicative gesture) self-paced pantomimes with the right hand while recording 64-channel electroencephalography (EEG) and electromyography (EMG) from right thumb and forearm flexors. Focusing on corticocortical coherence, we explored the time-course of synchronously active parietal and premotor circuits involved in these motor tasks. Trials were marked for EMG onset and averaged across subjects to determine changes in coherence relative to baseline between parietal, premotor, and motor areas. Results Coherence of homologous electrode pairs was similar when comparing transitive and intransitive movements. During preparation, beta band (18–22 Hz) coherence was maximal between electrodes over the left parietal lobe and left premotor electrodes. Additionally during preparation, the premotor area showed high coherence to the motor hand area and the parietal cortex. Electrodes over the supplementary motor area also showed coherence to the motor and parietal, but not the premotor area. Before and during execution, a second peak of high coherence increase was present in each area that demonstrated coherence increases during preparation. There was no coherence increase between parietal and motor areas. Coherence rapidly diminished 1.5–2.0 s after movement onset. Conclusions Patterns of increased corticocortical coupling within a parietal, premotor, and motor network are present during preparation and execution of praxis movements. Significance This study adds to evidence that parietofrontal networks may be critical for integrating preparatory and motor-related activity for praxis movements.

Corticocortical coupling in chronic stroke: its relevance to recovery.

The mechanisms behind motor recovery from stroke are not clearly understood. Functional imaging studies have demonstrated task-related brain activation in several motor areas, but few studies have attempted to correlate this with stroke outcome. Moreover, these studies have focused on how motor areas may individually contribute to compensation. Here, the authors investigate whether different cortical areas interact to form dynamic assemblies that may then compensate for disability. The authors investigated corticocortical coherence in 16 healthy subjects and 25 patients with chronic stroke involving one cerebral hemisphere and having varying degrees of motor recovery. Scalp EEG was recorded at rest and while right-handed subjects performed a unimanual grip task. The degree of functional recovery after stroke was assessed using a range of outcome measures. Compared with healthy subjects, hand-related asymmetries in task-related EEG-EEG coherence were increased between mesial and lateral frontal regions of the affected hemisphere, over mesial frontal regions, and over lateral frontal areas of the unaffected hemisphere when patients with stroke gripped with their affected hand. Mesial hand-related asymmetries in task-related power and coherence were negatively correlated with recovery. Increases in task-related coupling between cortical areas may dynamically compensate for brain damage after stroke. Some of this increased coupling, particularly that over mesial frontal areas, decreases as patients make a functional recovery.

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Back to table of contents Previous article Next article LetterFull AccessIn ReplyPaul B. Rosenberg, M.D., and Marshall Balish, M.D., Ph.D., Paul B. RosenbergSearch for more papers by this author, M.D., and Marshall BalishSearch for more papers by this author, M.D., Ph.D., Neurology Service, Department of Veterans Affairs Medical Center, Washington, DCPublished Online:1 May 2003AboutSectionsView EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail SIR: Schutter et al. present an intriguing and testable hypothesis regarding how and why slow rate (1 Hz) rTMS may be effective for treatment of depression. They suggest that a cortical circuit involving left frontal and right parietal regions is important in depression and that decreased functional connectivity between these regions occurs in depression. They suggest that slow rate magnetic stimulation rather than improving local left frontal cortical function enhances the connectivity of this region with the opposite parietal cortex.Our finding of a therapeutic effect of slow left frontal stimulation is only “counterintuitive” if left frontal hypofunction is a marker of depression and slow rTMS further causes neural inhibition. In fact, results of functional imaging in depressed patients are not highly replicable and may differentiate subgroups of patients. For instance, a study using single photon emission computed tomography in refractory depressed patients showed hypoperfusion in the inferior frontal, anterior temporal, and anterior cingulate.1 How does slow rTMS to the dorsolateral frontal cortex modulate this network of altered function? Our patients were suffering not only from refractory depression but also from long-standing posttraumatic stress disorder (PTSD); which networks (cortical-cortical, cortical-subcortical, cortical-limbic) have altered function? How do these networks interact with the dorsolateral frontal region? How does rTMS at slow or fast rates alter function in the stimulated region and in networks connected to the stimulated region?As Schutter et al. suggest, data are just beginning to accrue on how rTMS may alter functional connectivity. Jing and Takigawa2 showed that directed coherence between cortical regions increased after rTMS, particularly in the direction from frontal to parietal; however, they studied only normal subjects after limited rapid (10 Hz) rTMS, and their findings were larger within the hemisphere than between hemispheres. It remains a stretch from this observation to enhancement of left frontal to right parietal functional connectivity. In a more recent study, Strens et al.3 showed that subthreshold slow (1 Hz) rTMS over the motor cortex increased ipsilateral cortico-cortical coherence and increased coherence between left and right motor cortices. The effects lasted up to 25 minutes after a single session of 1,500 stimuli. However, there are no published data on changes in coherence after a typical course of rTMS treatment of depression (usually 10 treatments over 2 weeks).We thank Schutter et al. for their hypothesis, which moves the concepts behind rTMS treatment beyond simply altering neuronal excitability in brain regions near the stimulator and moves them into the realm of altering cortical connectivity. We would merely point out the many questions that remain to be answered both with respect to the underlying pathophysiology of depression and PTSD and the mechanisms of brain function modulation by rTMS.References1 Mayberg HS, Lewis PJ, Regenold W, et al: Paralimbic hypoperfusion in unipolar depression. J Nucl Med 1994; 35:929-934Medline, Google Scholar2 Jing H, Takigawa M: Observation of EEG coherence after repetitive transcranial magnetic stimulation. Clin Neurophysiol 2000; 111:1620-1631Crossref, Medline, Google Scholar3 Strens LH, Oliviero A, Bloem BR, et al: The effects of subthreshold 1 Hz repetitive TMS on cortico-cortical and interhemispheric coherence. Clin Neurophysiol 2002 113:1279-1285Google Scholar FiguresReferencesCited byDetailsCited ByNone Volume 15Issue 2 May 2003Pages 244-245 Metrics History Published online 1 May 2003 Published in print 1 May 2003

Persistent effects of high frequency repetitive TMS on the coupling between motor areas in the human.

Repetitive transcranial magnetic stimulation (rTMS) shows promise as a treatment for various movement and psychiatric disorders. How rTMS may have persistent effects on cortical function remains unclear. We hypothesised that it may act by modulating cortico-cortical connectivity. To this end we assessed cortico-cortical coherence before and after high frequency rTMS of the motor cortex. Sixteen healthy subjects received a single train (5 Hz, active motor threshold, 50 stimuli) of rTMS to the left motor hand area. Spectral power and coherence estimates were calculated between different EEG signals at rest and while muscles of the distal upper limb were tonically contracted. Repetitive TMS over the left motor hand area caused a significant decrease in the intrahemispheric EEG-EEG coherence between motor and premotor cortex in the 10.7-13.6 Hz (upper alpha band) lasting a few minutes after stimulation. There was no significant change in interhemispheric EEG-EEG coherence between motor areas. Thus, high frequency rTMS of the motor cortex decreases ipsilateral cortico-cortical intrahemispheric in the upper alpha band.

Task-related EEG-EEG coherence depends on dopaminergic activity in Parkinson's disease.

We investigated whether functional coupling between different cortical areas is impaired in Parkinson's disease, using corticocortical coherence as a surrogate measure of coupling. We recorded scalp EEG from different sites in seven parkinsonian patients while they tracked a visual target using their wrist, or copied the same movement from memory. Differences in EEG-EEG coherence between the tracking and copying tasks and their respective controls, visual tracking alone and fixation of a stationary target, were determined on and off levodopa. After levodopa we found extensive task-specific and broad band cortico-cortical coherence. Off levodopa cortico-cortical coherence was much reduced. Ascending dopaminergic projections from the ventral mesencephalon may therefore be important in determining the pattern and extent of corticocortical coupling during executive tasks.