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Advanced methods for noninvasive brain stimulation
Matti Stenroos1, Jens Haueisen2, Axel Thielscher3,4, Christoph Zrenner5,6
1Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Finland; 2Institute of Biomedical Engineering and Informatics, Ilmenau University of Technology, Germany; 3Department of Electrical Engineering, Technical University of Denmark, Denmark; 4Danish Research Center for Magnetic Resonance, Copenhagen University Hospital, Denmark; 5Department of Neurology & Stroke, University of Tübingen, Germany; 6Hertie-Institute for Clinical Brain Research, University of Tübingen, Germany
Organizer: Matti Stenroos, Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, Finland
To understand and treat the human brain, we need to understand its dynamical activity. Brain oscillations, for example, have been associated with perceptive and cognitive functions, but the mechanism of coupling between the oscillations and brain function is not well understood yet. To study this coupling or the interaction between nodes in a functional brain network, we need to be able to modulate the brain in a controlled way and measure the effects of this modulation online. Non-invasive electric or magnetic brain stimulation (NIBS) offers a way for modulating the brain activity directly at electrophysiological level, and the effects of stimulation can be measured using electro- or magnetoencephalography (EEG, MEG).
To reliably measure the effects of electric stimulation during the stimulation and to ensure the repeatability of the experiment, we need dedicated hardware. To plan a stimulation study, to accurately navigate the stimulation coil, and to interpret the measured data, we greatly benefit from modeling of the stimulation and measurement. To optimally manipulate brain dynamics, we need to time the stimulation accurately with brain oscillations that need to be extracted from EEG data in near real time. In this symposium, we discuss these key methodological issues of NIBS and combined NIBS+MEG/EEG and present our latest results. The symposium brings together renowned experts in the field of methodological developments for NIBS & EEG/MEG.
The symposium is complementary to Gregor Thut's keynote lecture, taking a more methodological perspective.
9:20am - 9:40am
Dry-electrode cap for simultaneous electroencephalography and transcranial electrical stimulation
TU Ilmenau, Germany
We introduce a bifunctional flexible textile cap for simultaneous TES-EEG applications with novel electrode materials, textile stimulation electrodes and dry EEG electrodes. We verified the functionality of this cap in a study on ten volunteers, analyzing the stimulation effect of TES on visual evoked potentials (VEPs). In accordance to previously reported stimulation effects, the amplitude of the N75 component was modulated post stimulation. Further, we report for the first time a significant reduction of the P100 component in VEPs measured simultaneously during TES. The novel bifunctional cap overcomes limitations of conventional equipment for simultaneous TES-EEG studies.
9:40am - 10:00am
Modeling of electric and magnetic brain stimulation: what to model & how to validate the model?
Danish Research Center for Magnetic Resonance, Copenhagen University Hospital Hvidovre, Denmark
I will provide evidence that field modeling informed by individual structural MRI combined with electromyographical recordings of peripheral muscle responses to TMS can help to pinpoint the stimulated brain region and reveal the stimulation depth of TMS. I will further show that realistic field modeling can help to determine the origin of interindividual differences in the physiological TMS effects. Finally, I will report on recent advances in Magnetic Resonance Current Density Imaging (MRCDI) to measure the current flow pattern caused by the weak currents injected by electric brain stimulation in the human brain.
10:00am - 10:20am
University of Tübingen, Germany
Using custom-built state-dependent millisecond-accurate electroencephalography-triggered transcranial magnetic stimulation (EEG-TMS) of human motor cortex, we demonstrate that phases of EEG peak negativity versus EEG peak positivity of the endogenous sensorimotor µ-alpha rhythm reflect high- vs. low-excitability states of corticospinal neurons. Moreover, otherwise identical repetitive TMS, triggered consistently at these high-excitability vs. low-excitability states, leads to LTP- vs. LTD-like change in corticospinal excitability. This raises the intriguing possibility that real-time information of instantaneous brain state can be utilized to control direction of plasticity in humans, a prospective with clear potential for therapeutic modulation of brain networks in psychiatric and neurological disease.
10:20am - 10:40am
Real-time realistic E-field computation for TMS navigation and dosing
Aalto University, Finland
In navigated transcranial magnetic stimulation (nTMS), stimulation is focused and dosed using a model of TMS-induced electric field in the brain. While off-line TMS simulation studies have applied highly realistic, computationally complex models, field models in real-time nTMS still assume the head spherically symmetric. In this study, we quantify errors associated with spherical models and suggest a computationally efficient surface-based model for real-time nTMS. Our optimized modeling approach can be coupled with either a boundary- or a finite-element field solver. The first results suggest that a realistic single-shell model is a good compromise between speed and accuracy.