Estimation of time delay by coherence analysis

Using coherence analysis (which is an extensively used method to study the correlations in frequency domain, between two simultaneously measured signals) we estimate the time delay between two signals. This method is suitable for time delay estimation of narrow band coherence signals for which the conventional methods cannot be reliably applied. We show by analysing coupled R\"ossler attractors with a known delay, that the method yields satisfactory results. Then, we apply this method to human pathologic tremor. The delay between simultaneously measured traces of Electroencephalogram (EEG) and Electromyogram (EMG) data of subjects with essential hand tremor is calculated. We find that there is a delay of 11-27 milli-seconds ($ms$) between the tremor correlated parts (cortex) of the brain (EEG) and the trembling hand (EMG) which is in agreement with the experimentally observed delay value of 15 $ms$ for the cortico-muscular conduction time. By surrogate analysis we calculate error-bars of the estimated delay.Comment: 21 pages, 8 figures, elstart.cls file included. Accepted for publication in Physica

Relation between post-movement-beta-synchronisation and corticomuscular coherence [Abstract]

Objective: To analyse post-movement-beta-synchronisation in the EEG and EEG-EMG coherence simultaneously. Background: The mechanisms and function of EEG synchronistion in the beta-band after the end of a short movement is not clear. The corticomusucular coupling during isometric muscle contractions occurs in the same beta-band. It is unclear however, if these two features of cortical motor physiology are related. Methods: 64-channel EEG was measured simultaneously with surface EMG of the right FDI-muscle in 11 healthy volunteers. Subjects kept a constant medium strength contraction of the FDI-muscle during the entire experiment. Superimposed on this they performed repetitive self-paced brisk short contractions. Time-frequency analysis including coherence over time was performed with respect to the onset of the brisk movements and averaged for 40 contrcations in each subject. Results: Post-movement-beta synchronisation (PMBS) was found in the contralateral electrodes C1, C3 and C5 with a maximum 1-2.5sec. after the brisk movements in the frequency range between 16 and 27 Hz for all the subjects. In 9 of the subjects there was coherence between the EEG recorded from these electrodes and the FDI in the same frequency range as the PMBS and with the maximum occuring at the same time. The other two subjects did not show any corticomuscular coherence. Conclusions: Post-movement-beta-synchronisation coincides with corticomuscular coherence in the same frequency band. Thus PMBS is not merely a cortical phenomen but seems to involve the whole corticomuscular system, possibly reflecting recalibration after brisk movements

Cortical representation of voluntary and non-voluntary motor rhythms [Abstract]

Background: Coupled bilateral cortical activity seems to be the basis for intermanual coordination, but its direct relation to the peripheral bimanual movements is still unclear. Methods: We analyzed corticomuscular coherence between 64-channel EEG and bilateral hand/finger extensor and flexor EMG and intermuscular coherence between left and right muscle activity in 18 healthy subjects during unilateral and bilateral fast rhythmic hand/finger movements and isometric contractions on both sides. Results: Partial coherence between two separated coherent areas and muscle and corticomusuclar/cortico-cortical delays were calculated. Bilateral voluntary rhythms of each hand showed coherence with lateral cortical areas on both sides in 60-80% of the recordings and occasionally with the frontal midline region (10-30%). They were always coherent between both hands. Unilateral rhythmic movements were represented in the ipsilateral cortex in only 20%-30% of the recordings tending to be more frequent with the left hand, paralleled by more frequent left-right muscle coherence. Partial corticomuscular coherence was most often abolished (p� 0.05) when the cortical signal contralateral to the coherent muscle was used as the predictor indicating that the ipsilateral and occasional frontomesial connection with the muscle was mainly indirect via the contralateral cortex. Cortico-cortical delays showed mainly bidirectional interaction at the movement frequency and were bimodally distributed ranging between 1-10 ms and 15-30 ms indicating direct cortical and subcortical routes. Corticomuscular delays ranged mainly between 12-25 ms indicating fast corticospinal projections, and musculocortical feedback showed similar delays. These corticomuscular delays were not significantly different for the 15-30 Hz coherence encountered in 60-70% of the recordings during isometric contractions. However this involuntary corticomuscular rhythm was strictly unilaterally represented and did not show coherence between left and right muscles. Conclusions: We conclude that there is a fundamental difference between the complex bilateral cortical network representing and controlling a voluntary motor rhythm and the cortical representation of non-voluntary 15-30 Hz rhythm as well as pathological non-voluntary rhythms likeorganic tremors

Identifying seizure onset zone and epileptic networks with the dynamic imaging of coherent sources [Abstract]

Purpose: In the presurgical evaluation of children with refractory focalepilepsy the main difficulty is to locate the exact point of seizure onset.The aim of this study was to characterize the areas of seizure onset as wellas the epileptic network involved in seizure propagation usingDynamicimaging of coherent sources (DICS)of ictal EEGs. Method: DICS is an inverse solution in the frequency domain whichdescribes neuronal networks and coherence of oscillatory brain activityby applying a spatial filter (Gross et al. PNAS 2001; 98:694–699). In 15children with refractory focal epilepsy, typical seizures were selectedfrom the EEGs recorded during the presurgical evaluation. For every sei-zure, two data sets of 10 s duration were extracted: one EEG segmentcontained the seizure onset and the other segment included the middlepart of the seizure. For both segments, the frequency range was definedand analyzed with DICS. The brain area with the strongest power in thecorresponding frequency range was defined as a reference region and itscoherence with the entire brain was computed using DICS. The result ofthe reference region was compared with the electroclinical localizationof seizure onset as well as with the postoperative resection site to deter-mine concordance. Results: For the beginning of the seizure, a good concordance betweenresults of the DICS localization and postoperative outcome was achievedin all 15 patients. The analysis of seizure propagation revealed an epilep-tic network which resembled reverberation of epileptic activity betweendifferent brain areas. Conclusion: DICS may be a useful tool to define the seizure onset zoneand study epileptic networks

Network for parallel gamma synchronizations during upper limb movement [Abstract]

Objective: Our aim was to identify the sources of parallel gamma synchronizations (GS) and analyze the direction of information flow in their network, at the beginning of simple and combined upper limb movements. Background: GS at the onset of movements may promote the processing between functionally related cortico-subcortical neural populations. Methods: We measured 64-channel EEG in 11 healthy volunteers; surface EMG detected the movements of the dominant hand. In Task1 subjects kept a constant medium-strength contraction of the first dorsal interosseus muscle and superimposed on this they performed a repetitive voluntary self-paced brisk squeeze of an object. They executed brisk contraction in Task2 and constant contraction in Task3. Time-frequency analysis of the EEG signal was performed with multitaper method. GS sources were identified in five frequency bands (30-49Hz, 51-75Hz, 76-100Hz, 101-125Hz and 126-150Hz) with the beamformer inverse solution dynamic imaging of coherent sources by taking the EMG as the reference signal. The direction of information flow between the sources was estimated by renormalized partial directed coherence for each frequency band. To identify significant connections, the data driven surrogate test and the time reversal technique was performed. Results: The first three sources in consecutive order in each movement task, in every frequency band, were as follows: contralateral primary sensorimotor cortex (S1M1), dorsolateral prefrontal cortex (dPFC) and supplementary motor cortex (SMA). Gamma activity was detected in narrower low- and high-frequency bands in the contralateral thalamus (TH) and ipsilateral cerebellum (C), in all three tasks. In the combined Task1 additional low gamma activity appeared in the contralateral posterior parietal cortex (PPC). In every task, S1M1 had efferent information flow to the SMA and the dPFC; the latter had no afferent relation to the network. S1M1 and SMA had a bidirectional connection with the TH, and the C. Afferent information flow was detected from the PPC to the SMA and bidirectional flow between PPC and the TH, in the combined Task1. Conclusions: The same network could be identified for the parallel gamma synchronizations in the tasks; it was complemented by the PPC in the combined Task1. S1M1 drove the other cortical sources and had afferent activity from the TH and the C, which activated in variable frequency bands in the tasks

Post-movement beta synchronization in Wilson's disease [Abstract]

Objective: To analyze the post-movement beta synchronization(PMBS) of the electroencephalogram (EEG) in Wilson’s disease. Background: Wilson’s disease is an autosomal recessive inherited disorder of copper metabolism. Its most common neurological symptoms (tremor, parkinsonism, dystonia, ataxia, chorea, dysarthria) are mainly related to dysfunction of the basal ganglia-thalamo-corticaland the cerebello-thalamo-cortical pathways. Post-movement beta synchronization is a transient power increase in the beta frequency band, which can be detected above the sensorimotor cortex 1-2 s after the termination of the movement. It is postulated that it reflects active inhibition and information processing. In essential tremor normal PMBS power but increased latency can be measured whereas in Parkinson’s disease PMBS latency is normal but its power is decreased. Methods: Ten patients with neurological manifestation of Wilson’s disease and ten controls performed self-paced movement with the dominant hand during EEG acquisition. Five electrodes located in the region of the sensorimotor cortical areas were selected for evaluation (C3, C1, Cz, C2, C4). The power and latency of post-movement beta synchronization were calculated after power spectral analysis with multi taper method. Results: PMBS power contralateral to the movement was significantly lower in patients with Wilson’s disease (1,9460,7%) than in controls (2,560,7%; p50,01). In all electrode position the latency of PMBS was significantly longer in the Wilson group (1,3460,45s) compared to controls (0,9360,44s; p50,005). The severity and type of neurological symptoms and the location and size of the MRI abnormalities were not correlated with the changes of PMBS. However, alterations of PMBS tended to be more pronounced in patients with more severe neurological symptoms. Conclusions: PMBS is affected in Wilson’s disease with neurological manifestation indicating altered information processing in the sensorimotor cortex. PMBS abnormalities are the combination of changes observed in Parkinson’s disease (decrease of power) and essential tremor (elongation of latencies). This may reflect the pathological changes in both the basal ganglia-thalamo-cortical circuit and cerebello-thalamo-cortical loop in Wilson’s disease

Effective use of microelectrode recording for the implantation of deep brain electrodes into the nucleus subthalamicus in advanced Parkinson's disease [Abstract]

Objective: For electrode implantation into the STN in Parkin-son’s disease most DBS centers use microelectrode recording (MER)with multiple trajectories. The aim of the study is to identify MERpatterns to predict optimal electrode position and to keep clinicaltesting as short as possible in order to reduce operation time andadverse events. Background: There is an ongoing debate whether MER is associ-ated with higher operation risks and time-consuming or leading to asignificant better clinical outcome. Here, we focus on correlationsbetween background activity and firing rate to predict optimal elec-trode placement. Methods: 10 patients with advanced PD underwent bilateral STNDBS operation. Recording was done simultaneously for 3–5 micro-electrodes using a ben-gun for multiple trajectories. MER data wererecorded and analyzed postoperatively using spike2VRand wave_clusVRfor segmentation and spike sorting. The overall mean amplitude wasdetermined for each patient separately for firing rate and backgroundactivity to define an individual threshold and the needles with the fir-ing rate and background activity above this threshold was selected. Results: In 8 out of 10 patients the permanent electrode wasimplanted in the trajectory of the microelectrode with the highestbackground activity compared to all other microelectrodes, in the other 2 patients a neighbouring trajectory was chosen because of lowthresholds for side effects. The postoperatively chosen active contactof the permanent electrode was in 70% in the place of the highestfiring rate. Conclusions: MER in implantation of electrodes into the STN isvery helpful to predict the optimum stimulation place analysingbackground activity and firing rate. Finding a good ratio of effectand threshold for side effects at that place further clinical testingmight be useless and the permanent electrode could be implanteddirectly. This helps to save operation time and might reduce adverseevents, e.g. haemorrhage und infections

Movement-related beta and gamma synchronization of the supplementary and primary motor cortex measured in epilepsy patients during longterm video EEG monitoring with subdural electrodes [Abstract]

Introduction: Exploration of sensorimotor integration processes during movement regulation is crucial to understand the pathophysiology of movement disorders and the effect of neuromodulation therapy. In Parkinson’s disease, dysfunction of supplementary motor cortex (SMA) has a primary role in evoking typical symptoms. In this study we detected post-movement beta (PMBS) and gamma synchronization of the SMA and primary motor cortex with electrocorticography (ECoG) in patients with epilepsy. PMBS is an electrophysiological indicator of sensorimotor integration, its parameters alter differently in several movement disorders. Methods: ECoG in 3 patients with epilepsy was recorded during invasive preoperative long-term video EEG monitoring through subdural strip and grid electrodes placed on the SMA and lobulus paracentralis and the representation field of the hand area in the primary motor cortex. Patients were requested in the interictal period to repeat short flexions of each thumb thirty times voluntarily; the trials were averaged with respect to the offset of the brisk movements. Time-frequency analysis of power was performed with multitaper method. Results: Post-movement synchronization could be detected mainly in the gamma frequency band above SMA and in the beta band above primary motor cortex. Latencies of post-movement synchronization varied in the SMA and primary motor cortex. Conclusions: Activity of the SMA in the two hemispheres cannot be detected with electroencephalography or magnetoencephalography. In our study we showed the first time that post-movement synchronization appears mainly in the gamma band in the SMA. Evaluation of the latencies supports the hypothesis that post-movement synchronization indicates a motor network activity