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Dive into the research topics where Alexander Hunold is active.

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Featured researches published by Alexander Hunold.


NeuroImage | 2014

Comparison of three-shell and simplified volume conductor models in magnetoencephalography☆

Matti Stenroos; Alexander Hunold; Jens Haueisen

Experimental MEG source imaging studies have typically been carried out with either a spherically symmetric head model or a single-shell boundary-element (BEM) model that is shaped according to the inner skull surface. The concepts and comparisons behind these simplified models have led to misunderstandings regarding the role of skull and scalp in MEG. In this work, we assess the forward-model errors due to different skull/scalp approximations and due to differences and errors in model geometries. We built five anatomical models of a volunteer using a set of T1-weighted MR scans and three common toolboxes. Three of the models represented typical models in experimental MEG, one was manually constructed, and one contained a major segmentation error at the skull base. For these anatomical models, we built forward models using four simplified approaches and a three-shell BEM approach that has been used as reference in previous studies. Our reference model contained in addition the skull fine-structure (spongy bone). We computed signal topographies for cortically constrained sources in the left hemisphere and compared the topographies using relative error and correlation metrics. The results show that the spongy bone has a minimal effect on MEG topographies, and thus the skull approximation of the three-shell model is justified. The three-shell model performed best, followed by the corrected-sphere and single-shell models, whereas the local-spheres and single-sphere models were clearly worse. The three-shell model was the most robust against the introduced segmentation error. In contrast to earlier claims, there was no noteworthy difference in the computation times between the realistically-shaped and sphere-based models, and the manual effort of building a three-shell model and a simplified model is comparable. We thus recommend the realistically-shaped three-shell model for experimental MEG work. In cases where this is not possible, we recommend a realistically-shaped corrected-sphere or single-shell model.


NeuroImage | 2016

Transcranial direct current stimulation changes resting state functional connectivity: A large-scale brain network modeling study

Tim Kunze; Alexander Hunold; Jens Haueisen; Viktor K. Jirsa; Andreas Spiegler

Transcranial direct current stimulation (tDCS) is a noninvasive technique for affecting brain dynamics with promising application in the clinical therapy of neurological and psychiatric disorders such as Parkinsons disease, Alzheimers disease, depression, and schizophrenia. Resting state dynamics increasingly play a role in the assessment of connectivity-based pathologies such as Alzheimers and schizophrenia. We systematically applied tDCS in a large-scale network model of 74 cerebral areas, investigating the spatiotemporal changes in dynamic states as a function of structural connectivity changes. Structural connectivity was defined by the human connectome. The main findings of this study are fourfold: Firstly, we found a tDCS-induced increase in functional connectivity among cerebral areas and among EEG sensors, where the latter reproduced empirical findings of other researchers. Secondly, the analysis of the network dynamics suggested synchronization to be the main mechanism of the observed effects. Thirdly, we found that tDCS sharpens and shifts the frequency distribution of scalp EEG sensors slightly towards higher frequencies. Fourthly, new dynamic states emerged through interacting areas in the network compared to the dynamics of an isolated area. The findings propose synchronization as a key mechanism underlying the changes in the spatiotemporal pattern formation due to tDCS. Our work supports the notion that noninvasive brain stimulation is able to bias brain dynamics by affecting the competitive interplay of functional subnetworks.


Frontiers in Human Neuroscience | 2014

Localization of the Epileptogenic Foci in Tuberous Sclerosis Complex: A Pediatric Case Report

Alexander Hunold; Jens Haueisen; Banu Ahtam; Chiran Doshi; Chellamani Harini; Susana Camposano; Simon K. Warfield; Patricia Ellen Grant; Yoshio Okada; Christos Papadelis

Tuberous sclerosis complex (TSC) is a rare disorder of tissue growth and differentiation, characterized by benign hamartomas in the brain and other organs. Up to 90% of TSC patients develop epilepsy and 50% become medically intractable requiring resective surgery. The surgical outcome of TSC patients depends on the accurate identification of the epileptogenic zone consisting of tubers and the surrounding epileptogenic tissue. There is conflicting evidence whether the epileptogenic zone is in the tuber itself or in abnormally developed surrounding cortex. Here, we report the localization of the epileptiform activity among the many cortical tubers in a 4-year-old patient with TSC-related refractory epilepsy undergoing magnetoencephalography (MEG), electroencephalography (EEG), and diffusion tensor imaging (DTI). For MEG, we used a prototype system that offers higher spatial resolution and sensitivity compared to the conventional adult systems. The generators of interictal activity were localized using both EEG and MEG with equivalent current dipole (ECD) and minimum norm estimation (MNE) methods according to the current clinical standards. For DTI, we calculated four diffusion scalar parameters for the fibers passing through four ROIs defined: (i) at a large cortical tuber identified at the right quadrant, (ii) at the normal appearing tissue contralateral to the tuber, (iii) at the cluster formed by ECDs fitted at the peak of interictal spikes, and (iv) at the normal appearing tissue contralateral to the cluster. ECDs were consistently clustered at the vicinity of the large calcified cortical tuber. MNE and ECDs indicated epileptiform activity in the same areas. DTI analysis showed differences between the scalar values of the tracks passing through the tuber and the ECD cluster. In this illustrative case, we provide evidence from different neuroimaging modalities, which support the view that epileptiform activity may derive from abnormally developed tissue surrounding the tuber rather than the tuber itself.


Physiological Measurement | 2016

EEG and MEG: Sensitivity to epileptic spike activity as function of source orientation and depth

Alexander Hunold; Michael Funke; R. Eichardt; Matti Stenroos; Jens Haueisen

Simultaneous electroencephalography (EEG) and magnetoencephalography (MEG) recordings of neuronal activity from epileptic patients reveal situations in which either EEG or MEG or both modalities show visible interictal spikes. While different signal-to-noise ratios (SNRs) of the spikes in EEG and MEG have been reported, a quantitative relation of spike source orientation and depth as well as the background brain activity to the SNR has not been established. We investigated this quantitative relationship for both dipole and patch sources in an anatomically realistic cortex model. Altogether, 5600 dipole and 3300 patch sources were distributed on the segmented cortical surfaces of two volunteers. The sources were classified according to their quantified depths and orientations, ranging from 20 mm to 60 mm below the skin surface and radial and tangential, respectively. The source time-courses mimicked an interictal spike, and the simulated background activity emulated resting activity. Simulations were conducted with individual three-compartment boundary element models. The SNR was evaluated for 128 EEG, 102 MEG magnetometer, and 204 MEG gradiometer channels. For superficial dipole and superficial patch sources, EEG showed higher SNRs for dominantly radial orientations, and MEG showed higher values for dominantly tangential orientations. Gradiometers provided higher SNR than magnetometers for superficial sources, particularly for those with dominantly tangential orientations. The orientation dependent difference in SNR in EEG and MEG gradually changed as the sources were located deeper, where the interictal spikes generated higher SNRs in EEG compared to those in MEG for all source orientations. With deep sources, the SNRs in gradiometers and magnetometers were of the same order. To better detect spikes, both EEG and MEG should be used.


international conference of the ieee engineering in medicine and biology society | 2016

Modular multipin electrodes for comfortable dry EEG

Patrique Fiedler; Daniel Strohmeier; Alexander Hunold; Stefan Griebel; Richard Mühle; Maria Schreiber; Paulo Pedrosa; Beatriz Vasconcelos; C. Fonseca; F. Vaz; Jens Haueisen

Electrode and cap concepts for continuous and ubiquitous monitoring of brain activity will open up new fields of application and contribute to increased use of electroencephalography (EEG) in clinical routine, neurosciences, brain-computer-interfacing and out-of-the-lab monitoring. However, mobile and unobtrusive applications are currently hindered by the lack of applicable convenient and reliable electrode and cap systems. We propose a novel modular electrode concept based on a flexible polymer substrate, coated with electrically conductive metallic films. The overall concept enables design adaptation to different head regions and cap designs. We describe the single modules of the system and investigate the influence of electrode pin number, coating material and adduction force on electrode-skin impedance and perceived wearing comfort. Our results contribute to rapid and comfortable multichannel dry EEG.Electrode and cap concepts for continuous and ubiquitous monitoring of brain activity will open up new fields of application and contribute to increased use of electroencephalography (EEG) in clinical routine, neurosciences, brain-computer-interfacing and out-of-the-lab monitoring. However, mobile and unobtrusive applications are currently hindered by the lack of applicable convenient and reliable electrode and cap systems. We propose a novel modular electrode concept based on a flexible polymer substrate, coated with electrically conductive metallic films. The overall concept enables design adaptation to different head regions and cap designs. We describe the single modules of the system and investigate the influence of electrode pin number, coating material and adduction force on electrode-skin impedance and perceived wearing comfort. Our results contribute to rapid and comfortable multichannel dry EEG.


Biomedizinische Technik | 2012

Comparison of three- and single-shell volume conductor models in magnetoencephalography

M. Stenroos; Alexander Hunold; Roland Eichardt; Jens Haueisen

M. Stenroos, Department of Biomedical Engineering and Computational Science, Aalto University, Espoo, Finland / MRC Cognition and Brain Sciences Unit, Cambridge, England, matti.stenroos@aalto.fi A. Hunold, Institute of Biomedical Engineering and Informatics, Technical University Ilmenau, Germany, alexander.hunold@tu-ilmenau.de R. Eichardt, Institute of Biomedical Engineering and Informatics, Technical University Ilmenau, Germany, roland.eichardt@tu-ilmenau.de J. Haueisen, Institute of Biomedical Engineering and Informatics, Technical University Ilmenau, Germany, jens.haueisen@tu-ilmenau.de


Scientific Reports | 2018

Novel bifunctional cap for simultaneous electroencephalography and transcranial electrical stimulation

Sophia Wunder; Alexander Hunold; Patrique Fiedler; Falk Schlegelmilch; Klaus Schellhorn; Jens Haueisen

Neuromodulation induced by transcranial electric stimulation (TES) exhibited promising potential for clinical practice. However, the underlying mechanisms remain subject of research. The combination of TES and electroencephalography (EEG) offers great potential for investigating these mechanisms and brain function in general, especially when performed simultaneously. In conventional applications, the combination of EEG and TES suffers from limitations on the electrode level (gel for electrode-skin interface) and the usability level (preparation time, reproducibility of positioning). To overcome these limitations, we designed a bifunctional cap for simultaneous TES–EEG applications. We used novel electrode materials, namely textile stimulation electrodes and dry EEG electrodes integrated in a flexible textile cap. We verified the functionality of this cap by analysing the effect of TES on visual evoked potentials (VEPs). In accordance with previous reports using standard TES, the amplitude of the N75 component was significantly decreased post-stimulation, indicating the feasibility of using this novel flexible cap for simultaneous TES and EEG. Further, we found a significant reduction of the P100 component only during TES, indicating a different brain modulation effect during and after TES. In conclusion, the novel bifunctional cap offers a novel tool for simultaneous TES–EEG applications in clinical research, therapy monitoring and closed-loop stimulation.


Biomedizinische Technik | 2017

Head phantoms for electroencephalography and transcranial electric stimulation: a skull material study

Alexander Hunold; Daniel Strohmeier; Patrique Fiedler; Jens Haueisen

Abstract Physical head phantoms allow the assessment of source reconstruction procedures in electroencephalography and electrical stimulation profiles during transcranial electric stimulation. Volume conduction in the head is strongly influenced by the skull, which represents the main conductivity barrier. Realistic modeling of its characteristics is thus important for phantom development. In the present study, we proposed plastic clay as a material for modeling the skull in phantoms. We analyzed five clay types varying in granularity and fractions of fire clay, each with firing temperatures from 550°C to 950°C. We investigated the conductivity of standardized clay samples when immersed in a 0.9% sodium chloride solution with time-resolved four-point impedance measurements. To test the reusability of the clay model, these measurements were repeated after cleaning the samples by rinsing in deionized water for 5 h. We found time-dependent impedance changes for approximately 5 min after immersion in the solution. Thereafter, the conductivities stabilized between 0.0716 S/m and 0.0224 S/m depending on clay type and firing temperatures. The reproducibility of the measurement results proved the effectiveness of the rinsing procedure. Clay provides formability, is permeable to ions, can be adjusted in conductivity value and is thus suitable for the skull modeling in phantoms.


ursi international symposium on electromagnetic theory | 2016

Dipole forward simulation guides transcranial electric stimulation of the hand knob area

Alexander Hunold; Klaus Schellhorn; Jens Haueisen

Transcranial electric stimulation (TES) is a non-invasive technique driving small currents to the brain to modulate neuronal activity. Applications of TES in therapy of neurophysiological disorders require implementations of focused TES to specific target areas in the brain. We introduce a new technique based on the idea of reciprocity, where targeted TES is guided by resampling electric field features originating from an electric current dipole in the target area. We compute the current density in the brain and the electric scalar potential on the scalp surface of a volunteer produced by a dipole in the target area. The resulting minima of the current density distribution are used to guide a bipolar TES stimulation. The maximum and minimum of the potential on the scalp surface serve for polarity selection of the TES electrodes. We compare our results to those obtained with a distributed beam former TES simulation for the same target area. We found that the beam former based TES produced the maximum of the current density in a different brain area than the target area, while our new method produced the maximum at the target area. In the target area, the beam former based TES produced a factor 3 lower current densities than our new method. Our new approach provides a simple means for guiding targeted TES in clinical studies.


international conference of the ieee engineering in medicine and biology society | 2016

Development of a head-phantom and measurement setup for lightning effects

Rene Machts; Alexander Hunold; Carsten Leu; Jens Haueisen; Michael Rock

Direct lightning strikes to human heads lead to various effects ranging from Lichtenberg figures, over loss of consciousness to death. The evolution of the induced current distribution in the head is of great interest to understand the effect mechanisms. This work describes a technique to model a simplified head-phantom to investigate effects during direct lightning strike. The head-phantom geometry, conductive and dielectric parameters were chosen similar to that of a human head. Three layers (brain, skull, and scalp) were created for the phantom using agarose hydrogel doped with sodium chloride and carbon. The head-phantom was tested on two different impulse generators, which reproduce approximate lightning impulses. The effective current and the current distribution in each layer were analyzed. The biggest part of the current flowed through the brain layer, approx. 70 % in cases without external flashover. Approx. 23 % of the current flowed through skull layer and 6 % through the scalp layer. However, the current decreased within the head-phantom to almost zero after a complete flashover on the phantom occurred. The flashover formed faster with a higher impulse current level. Exposition time of current through the head decreases with a higher current level of the lightning impulse. This mechanism might explain the fact that people can survive a lightning strike. The experiments help to understand lightning effects on humans.

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Jens Haueisen

Technische Universität Ilmenau

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Stefanie Freitag

Technische Universität Ilmenau

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Daniel Strohmeier

Technische Universität Ilmenau

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Matthias Klemm

Technische Universität Ilmenau

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Chiran Doshi

Boston Children's Hospital

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