Abdul Rauf Anwar

Christian-Albrechts-Universität zu Kiel, Kiel, Schleswig-Holstein, Germany

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Publications (14)25.3 Total impact

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    ABSTRACT: Background / Purpose: The aim of this study was to identify if cross frequency coupling is present in Parkinson's disease (PD) patients during the ON state of deep brain stimulator (DBS). Also, to understand the influence of the DBS frequency on other frequency oscillations present in the brain of the PD patients. The source analysis method used here is the dynamic imaging of coherent sources (DICS) ( Gross et al . 2001 ) with a realistic boundary element method forward model ( Fuchs et al. 2002 ). Finally, to describe the cross frequency coupling in the identified sources using the measure frequency to frequency coupling ( Jirsa et al. 2013 ). Main conclusion: The source analysis revealed the networks influenced during the deep brain simulation in the PD patients. The existing frequency-frequency coupling revealed that this could be one reason in solving a larger puzzle of why deep brain stimulation affects only the involuntary and not the voluntary actions in the PD patients.
    20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014; 08/2014
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    ABSTRACT: Background / Purpose: Burst suppression (B-S): an electroencephalogram (EEG) pattern characterized by the quasiperiodic alternant phases of a high voltage activity (burst) and electrical silence (suppression), is considered as a global state of profound brain inactivation. Burst suppression can occur during different conditions such as early-onset epileptic encephalopathies, hypothermia, general anaesthesia and coma. Biophysical mechanisms underlying this broad range of inactivated brain states are poorly understood. Main conclusion: Delta activity during the burst phases was associated with sources in the thalamus and brainstem, as well as bilateral sources in the cortical regions mainly frontal and parietal. Whereas, the suppression phases were associated with coherent sources only in the cortical regions. Results of the RPDC analyses showed an ascending informational flow from the brainstem towards the thalamus and from the thalamus to cortical regions, which was absent during the suppression phases. Our findings support the notion that “cortical deafferentation” between the cortex and subcortical structures and desynchronizations exists especially in suppression phases of burst suppression EEG.
    20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014; 08/2014
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    ABSTRACT: Owing to the recent advances in multi-modal data analysis, the aim of the present study was to analyze the functional network of the brain which remained the same during the eyes-open (EO) and eyes-closed (EC) resting task. The simultaneously recorded electroencephalogram (EEG) and magnetoencephalogram (MEG) were used for this study, recorded from five distinct cortical regions of the brain. We focused on the `alpha' functional network, corresponding to the individual peak frequency in the alpha band. The total data set of 120 seconds was divided into three segments of 18 seconds each, taken from start, middle, and end of the recording. This segmentation allowed us to analyze the evolution of the underlying functional network. The method of time-resolved partial directed coherence (tPDC) was used to assess the causality. This method allowed us to focus on the individual peak frequency in the `alpha' band (7-13 Hz). Because of the significantly higher power in the recorded EEG in comparison to MEG, at the individual peak frequency of the alpha band, results rely only on EEG. The MEG was used only for comparison. Our results show that different regions of the brain start to `disconnect' from one another over the course of time. The driving signals, along with the feedback signals between different cortical regions start to recede over time. This shows that, with the course of rest, brain regions reduce communication with each another.
    08/2014; 2014:2825-8.
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    ABSTRACT: The most well-known non-invasive electric and magnetic field measurement modalities are the electroencephalography (EEG) and magnetoencephalography (MEG). The first aim of the study was to implement the recently developed realistic head model which uses an integrative approach for both the modalities. The second aim of this study was to find the network of coherent sources and the modes of interactions within this network during isometric contraction (ISC) at (15-30 Hz) in healthy subjects. The third aim was to test the effective connectivity revealed by both the modalities analyzing them separately and combined. The Welch periodogram method was used to estimate the coherence spectrum between the EEG and the electromyography (EMG) signals followed by the realistic head modelling and source analysis method dynamic imaging of coherent sources (DICS) to find the network of coherent sources at the individual peak frequency within the beta band in healthy subjects. The last step was to identify the effective connectivity between the identified sources using the renormalized partial directed coherence method. The cortical and sub-cortical network comprised of the primary sensory motor cortex (PSMC), secondary motor area (SMA), and the cerebellum (C). The cortical and sub-cortical network responsible for the isometric contraction was similar in both the modalities when analysing them separately and combined. The SNR was not significantly different between the two modalities separately and combined. However, the coherence values were significantly higher in the combined modality in comparison to each of the modality separately. The effective connectivity analysis revealed plausible additional connections in the combined modality analysis.
    08/2014; 2014:6365-8.
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    ABSTRACT: Background / Purpose: The aim of this study was to investigate the dynamics of neuronal networks during focal seizures using dynamic imaging of coherent sources (DICS) ( Gross et al. 2001 ) and renormalized partial directed coherence (RPDC) ( Schelter et al. 2009 ). Ictal EEG recordings from a patient with drug resistant focal epilepsy, due to a focal cortical dysplasia (FCD) in the left parieto-occipital region, (shown by a high resolution 3-T MRI) were analyzed. Main conclusion: DICS revealed the neuronal networks concomitant with the location of the FCD, shown by a high resolution 3-T MRI and areas of decreased metabolism shown by functional neuroimaging methods. The sources identified during the seizure onset and propagation phases were similar. Only the causality was different, showing that the strongest source, located in the occipito-temporal region, is most probably a pacemaker/seizure onset zone of the ictal neuronal networks in this case. The DICS analyses of pre-seizure phase showed the sources in the DMN areas of the brain. We can conclude that analyses of multiple habitual seizures of the same patients by the methods of DICS and RPDC gives us valuable information regarding the seizure onset zone and ictal networks. It can be a useful additive tool during the pre-epilepsy surgical investigations of the patients with drug resistant focal epilepsies.
    20th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2014; 07/2014
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    ABSTRACT: Electroencephalography (EEG) and magnetoencephalography (MEG) are the two modalities for measuring neuronal dynamics at a millisecond temporal resolution. Different source analysis methods, to locate the dipoles in the brain from which these dynamics originate, have been readily applied to both modalities alone. However, direct comparisons and possible advantages of combining both modalities have rarely been assessed during voluntary movements using coherent source analysis. In the present study, the cortical and sub-cortical network of coherent sources at the finger tapping task frequency (2-4 Hz) and the modes of interaction within this network were analysed in 15 healthy subjects using a beamformer approach called the dynamic imaging of coherent sources (DICS) with subsequent source signal reconstruction and renormalized partial directed coherence analysis (RPDC). MEG and EEG data were recorded simultaneously allowing the comparison of each of the modalities separately to that of the combined approach. We found the identified network of coherent sources for the finger tapping task as described in earlier studies when using only the MEG or combined MEG+EEG whereas the EEG data alone failed to detect single sub-cortical sources. The signal-to-noise ratio (SNR) level of the coherent rhythmic activity at the tapping frequency in MEG and combined MEG+EEG data was significantly higher than EEG alone. The functional connectivity analysis revealed that the combined approach had more active connections compared to either of the modalities during the finger tapping (FT) task. These results indicate that MEG is superior in the detection of deep coherent sources and that the SNR seems to be more vital than the sensitivity to theoretical dipole orientation and the volume conduction effect in the case of EEG.
    PLoS ONE 03/2014; 9(3):e91441. · 3.53 Impact Factor
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    ABSTRACT: Brain activity can be measured using different modalities. Since most of the modalities tend to complement each other, it seems promising to measure them simultaneously. In to be presented research, the data recorded from Functional Magnetic Resonance Imaging (fMRI) and Near Infrared Spectroscopy (NIRS), simultaneously, are subjected to causality analysis using time-resolved partial directed coherence (tPDC). Time-resolved partial directed coherence uses the principle of state space modelling to estimate Multivariate Autoregressive (MVAR) coefficients. This method is useful to visualize both frequency and time dynamics of causality between the time series. Afterwards, causality results from different modalities are compared by estimating the Spearman correlation. In to be presented study, we used directionality vectors to analyze correlation, rather than actual signal vectors. Results show that causality analysis of the fMRI correlates more closely to causality results of oxy-NIRS as compared to deoxy-NIRS in case of a finger sequencing task. However, in case of simple finger tapping, no clear difference between oxy-fMRI and deoxy-fMRI correlation is identified.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:2628-2631.
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    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 05/2013; · 3.12 Impact Factor
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    ABSTRACT: Several studies demonstrated that resting-state EEG power differs tremendously between school-aged children and adults. Low-frequency oscillations (delta and theta, < 7 Hz) are dominant in children but become less prominent in the adult brain, where higher-frequency alpha oscillations (8-12 Hz) dominate the mature brain rhythm. However, this assessment of developmental effects with EEG power mapping is restricted to the scalp level and blind to the information flow between brain regions, thus limiting insights about brain development. In contrast dynamic source synchronization provides a tool to study inter-regional directionality on the cortical and sub-cortical source level. In this study we investigated functional and directed connectivity (information flow) with renormalized partial directed coherence during resting state EEG (eyes open and eyes closed) recordings in 17 school-aged children and 17 young adults. First, we found higher spectral mean source power in children relative to adults, irrespective of the examined frequency band and resting state. We further found that coherence values were stronger in adults compared to children in all frequency bands. The directed within-group coherence analysis indicated information flow from frontal to parietal sources in children, while information flow from parietal to frontal was observed in adults. In addition, significant thalamocortical connectivity was unidirectional (i.e., outflow to cortical regions) in adults, but bidirectional in children. Group comparison confirmed results of the single subject analyses for both functional and directed connectivity. Our results suggest that both functional and directed connectivity are sensitive to brain maturation as the distribution and directionality of functional connections differs between the developing and adult brain.
    NeuroImage 04/2013; · 6.13 Impact Factor
  • Basal Ganglia. 03/2013; 3(1):70–71.
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    ABSTRACT: The concept of focal epilepsies includes a seizure origin in brain regions with hyper synchronous activity (epileptogenic zone and seizure onset zone) and a complex epileptic network of different brain areas involved in the generation, propagation, and modulation of seizures. The purpose of this work was to study functional and effective connectivity between regions involved in networks of epileptic seizures. The beginning and middle part of focal seizures from ictal surface EEG data were analyzed using dynamic imaging of coherent sources (DICS), an inverse solution in the frequency domain which describes neuronal networks and coherences of oscillatory brain activities. The information flow (effective connectivity) between coherent sources was investigated using the renormalized partial directed coherence (RPDC) method. In 8/11 patients, the first and second source of epileptic activity as found by DICS were concordant with the operative resection site; these patients became seizure free after epilepsy surgery. In the remaining 3 patients, the results of DICS / RPDC calculations and the resection site were discordant; these patients had a poorer post-operative outcome. The first sources as found by DICS were located predominantly in cortical structures; subsequent sources included some subcortical structures: thalamus, Nucl. Subthalamicus and cerebellum. DICS seems to be a powerful tool to define the seizure onset zone and the epileptic networks involved. Seizure generation seems to be related to the propagation of epileptic activity from the primary source in the seizure onset zone, and maintenance of seizures is attributed to the perpetuation of epileptic activity between nodes in the epileptic network. Despite of these promising results, this proof of principle study needs further confirmation prior to the use of the described methods in the clinical praxis.
    PLoS ONE 01/2013; 8(10):e78422. · 3.53 Impact Factor
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    ABSTRACT: West syndrome is a severe epileptic encephalopathy of infancy with a poor developmental outcome. This syndrome is associated with the pathognomonic EEG feature of hypsarrhythmia. The aim of the study was to describe neuronal networks underlying hypsarrhythmia using the source analysis method (dynamic imaging of coherent sources or DICS) which represents an inverse solution algorithm in the frequency domain. In order to investigate the interaction within the detected network, a renormalized partial directed coherence (RPDC) method was also applied as a measure of the directionality of information flow between the source signals. Both DICS and RPDC were performed for EEG delta activity (1-4 Hz) in eight patients with West syndrome and in eight patients with partial epilepsies (control group). The brain area with the strongest power in the given frequency range was defined as the reference region. The coherence between this reference region and the entire brain was computed using DICS. After that, the RPDC was applied to the source signals estimated by DICS. The results of electrical source imaging were compared to results of a previous EEG-fMRI study which had been carried out using the same cohort of patients. As revealed by DICS, delta activity in hypsarrhythmia was associated with coherent sources in the occipital cortex (main source) as well as the parietal cortex, putamen, caudate nucleus and brainstem. In patients with partial epilepsies, delta activity could be attributed to sources in the occipital, parietal and sensory-motor cortex. In West syndrome, RPDC showed the strongest and most significant direction of ascending information flow from the brainstem towards the putamen and cerebral cortex. The neuronal network underlying hypsarrhythmia in this study resembles the network which was described in previous EEG-fMRI and PET studies with involvement of the brainstem, putamen and cortical regions in the generation of hypsarrhythmia. The RPDC suggests that brainstem could have a key role in the pathogenesis of West syndrome. This study supports the theory that hypsarrhythmia results from ascending brainstem pathways that project widely to basal ganglia and cerebral cortex.
    Brain Topography 08/2012; · 2.52 Impact Factor
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    ABSTRACT: Directionality analysis of signals originating from different parts of brain during motor tasks has gained a lot of interest. Since brain activity can be recorded over time, methods of time series analysis can be applied to medical time series as well. Granger Causality is a method to find a causal relationship between time series. Such causality can be referred to as a directional connection and is not necessarily bidirectional. The aim of this study is to differentiate between different motor tasks on the basis of activation maps and also to understand the nature of connections present between different parts of the brain. In this paper, three different motor tasks (finger tapping, simple finger sequencing, and complex finger sequencing) are analyzed. Time series for each task were extracted from functional magnetic resonance imaging (fMRI) data, which have a very good spatial resolution and can look into the sub-cortical regions of the brain. Activation maps based on fMRI images show that, in case of complex finger sequencing, most parts of the brain are active, unlike finger tapping during which only limited regions show activity. Directionality analysis on time series extracted from contralateral motor cortex (CMC), supplementary motor area (SMA), and cerebellum (CER) show bidirectional connections between these parts of the brain. In case of simple finger sequencing and complex finger sequencing, the strongest connections originate from SMA and CMC, while connections originating from CER in either direction are the weakest ones in magnitude during all paradigms.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2012; 2012:2287-90.
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    ABSTRACT: Parkinsonian tremor (PD), essential tremor (ET) and voluntarily mimicked tremor represent fundamentally different motor phenomena, yet, magnetoencephalographic and imaging data suggest their origin in the same motor centers of the brain. Using EEG-EMG coherence and coherent source analysis we found a different pattern of corticomuscular delays, time courses and central representations for the basic and double tremor frequencies typical for PD suggesting a wider range defective oscillatory activity. For the basic tremor frequency similar central representations in primary sensorimotor, prefrontal/premotor and diencephalic (e.g. thalamic) areas were reproduced for all three tremors. But renormalized partial directed coherence of the spatially filtered (source) signals revealed a mainly unidirectional flow of information from the diencephalon to cortex in voluntary tremor, e.g. a thalamocortical relay, as opposed to a bidirectional subcortico-cortical flow in PD and ET promoting uncontrollable, e.g. thalamocortical, loop oscillations. Our results help to understand why pathological tremors although originating from the physiological motor network are not under voluntary control and they may contribute to the solution of the puzzle why high frequency thalamic stimulation has a selective effect on pathological tremor leaving voluntary movement performance almost unaltered.
    NeuroImage 01/2012; 60(2):1331-9. · 6.13 Impact Factor