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      Effects of Cathode Location and the Size of Anode on Anodal Transcranial Direct Current Stimulation Over the Leg Motor Area in Healthy Humans

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          Abstract

          Objective: Non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) involves passing low currents through the brain and is a promising tool for the modulation of cortical excitability. In this study, we investigated the effects of cathode location and the size of anode for anodal tDCS of the right-leg area of the motor cortex, which is challenging due to its depth and orientation in the inter-hemispheric fissure.

          Methods: We first computationally investigated the effects of cathode location and the size of the anode to find the best montage for specificity of stimulation effects for the targeted leg motor area using finite element analysis (FEA). We then compared the best electrode montage found from FEA with the conventional montage (contralateral supraorbital cathode) via neurophysiological testing of both, the targeted as well as the contralateral leg motor area.

          Results: The conventional anodal tDCS electrode montage for leg motor cortex stimulation using a large-anode (5 cm × 7 cm, current strength 2 mA) affected the contralateral side more strongly in both the FEA and the neurophysiological testing when compared to other electrode montages. A small-anode (3.5 cm × 1 cm at 0.2 mA) with the same current density at the electrode surface and identical contralateral supraorbital cathode placement improved specificity. The best cathode location for the small-anode in terms of specificity for anodal tDCS of the right-leg motor area was T7 (10–10 EEG system).

          Conclusion: A small-anode (3.5 cm × 1 cm) with the same current density at the electrode surface as a large-anode (5 cm × 7 cm) resulted in similar cortical excitability alterations of the targeted leg motor cortex respresentation. In relation to the other stimulation conditions, the small-anode montage with the cathode positioned at T7 resulted in the best specificity.

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          10/20, 10/10, and 10/5 systems revisited: their validity as relative head-surface-based positioning systems.

          Valer Jurcak, Daisuke Tsuzuki, Ippeita Dan (2007)
          With the advent of multi-channel EEG hardware systems and the concurrent development of topographic and tomographic signal source localization methods, the international 10/20 system, a standard system for electrode positioning with 21 electrodes, was extended to higher density electrode settings such as 10/10 and 10/5 systems, allowing more than 300 electrode positions. However, their effectiveness as relative head-surface-based positioning systems has not been examined. We previously developed a virtual 10/20 measurement algorithm that can analyze any structural MR head and brain image. Extending this method to the virtual 10/10 and 10/5 measurement algorithms, we analyzed the MR images of 17 healthy subjects. The acquired scalp positions of the 10/10 and 10/5 systems were normalized to the Montreal Neurological Institute (MNI) stereotactic coordinates and their spatial variability was assessed. We described and examined the effects of spatial variability due to the selection of positioning systems and landmark placement strategies. As long as a detailed rule for a particular system was provided, it yielded precise landmark positions on the scalp. Moreover, we evaluated the effective spatial resolution of 329 scalp landmark positions of the 10/5 system for multi-subject studies. As long as a detailed rule for landmark setting was provided, 241 scalp positions could be set effectively when there was no overlapping of two neighboring positions. Importantly, 10/10 positions could be well separated on a scalp without overlapping. This study presents a referential framework for establishing the effective spatial resolutions of 10/20, 10/10, and 10/5 systems as relative head-surface-based positioning systems.
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            Design and construction of a realistic digital brain phantom.

            D.L. Collins, A.P. Zijdenbos, V Kollokian … (1998)
            After conception and implementation of any new medical image processing algorithm, validation is an important step to ensure that the procedure fulfills all requirements set forth at the initial design stage. Although the algorithm must be evaluated on real data, a comprehensive validation requires the additional use of simulated data since it is impossible to establish ground truth with in vivo data. Experiments with simulated data permit controlled evaluation over a wide range of conditions (e.g., different levels of noise, contrast, intensity artefacts, or geometric distortion). Such considerations have become increasingly important with the rapid growth of neuroimaging, i.e., computational analysis of brain structure and function using brain scanning methods such as positron emission tomography and magnetic resonance imaging. Since simple objects such as ellipsoids or parallelepipedes do not reflect the complexity of natural brain anatomy, we present the design and creation of a realistic, high-resolution, digital, volumetric phantom of the human brain. This three-dimensional digital brain phantom is made up of ten volumetric data sets that define the spatial distribution for different tissues (e.g., grey matter, white matter, muscle, skin, etc.), where voxel intensity is proportional to the fraction of tissue within the voxel. The digital brain phantom can be used to simulate tomographic images of the head. Since the contribution of each tissue type to each voxel in the brain phantom is known, it can be used as the gold standard to test analysis algorithms such as classification procedures which seek to identify the tissue "type" of each image voxel. Furthermore, since the same anatomical phantom may be used to drive simulators for different modalities, it is the ideal tool to test intermodality registration algorithms. The brain phantom and simulated MR images have been made publicly available on the Internet (http://www.bic.mni.mcgill.ca/brainweb).
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              Modulating parameters of excitability during and after transcranial direct current stimulation of the human motor cortex.

              Michael Nitsche, Antje Seeber, Kai Frommann … (2005)
              Weak transcranial direct current stimulation (tDCS) of the human motor cortex results in excitability shifts which occur during and after stimulation. These excitability shifts are polarity-specific with anodal tDCS enhancing excitability, and cathodal reducing it. To explore the origin of this excitability modulation in more detail, we measured the input-output curve and motor thresholds as global parameters of cortico-spinal excitability, and determined intracortical inhibition and facilitation, as well as facilitatory indirect wave (I-wave) interactions. Measurements were performed during short-term tDCS, which elicits no after-effects, and during other tDCS protocols which do elicit short- and long-lasting after-effects. Resting and active motor thresholds remained stable during and after tDCS. The slope of the input-output curve was increased by anodal tDCS and decreased by cathodal tDCS. Anodal tDCS of the primary motor cortex reduced intracortical inhibition and enhanced facilitation after tDCS but not during tDCS. Cathodal tDCS reduced facilitation during, and additionally increased inhibition after its administration. During tDCS, I-wave facilitation was not influenced but, for the after-effects, anodal tDCS increased I-wave facilitation, while cathodal tDCS had only minor effects. These results suggest that the effect of tDCS on cortico-spinal excitability during a short period of stimulation (which does not induce after-effects) primarily depends on subthreshold resting membrane potential changes, which are able to modulate the input-output curve, but not motor thresholds. In contrast, the after-effects of tDCS are due to shifts in intracortical inhibition and facilitation, and at least partly also to facilitatory I-wave interaction, which is controlled by synaptic activity.
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                Author and article information

                Contributors
                Journal
                Journal ID (nlm-ta): Front Neurosci
                Journal ID (iso-abbrev): Front Neurosci
                Journal ID (publisher-id): Front. Neurosci.
                Title: Frontiers in Neuroscience
                Publisher: Frontiers Media S.A.
                ISSN (Print): 1662-4548
                ISSN (Electronic): 1662-453X
                Publication date (Electronic): 04 July 2018
                Publication date Collection: 2018
                Volume: 12
                Electronic Location Identifier: 443
                Affiliations
                [1] 1Department of Clinical Neurophysiology, Universitätsmedizin Göttingen, Georg-August-Universität Göttingen , Göttingen, Germany
                [2] 2IfADo – Leibniz Research Center for Working Environment and Human Factors , Dortmund, Germany
                [3] 3Department of Biomedical Engineering, University at Buffalo, The State University of New York (SUNY) , Buffalo, NY, United States
                [4] 4Department of Neurology, BG University Hospital Bergmannsheil , Bochum, Germany
                Author notes

                Edited by: Mikhail Lebedev, Duke University, United States

                Reviewed by: Marco Iacoboni, University of California, Los Angeles, United States; Vassiliy Tsytsarev, University of Maryland, College Park, United States; Mayank A. Jog, Keck School of Medicine of USC, United States

                This article was submitted to Neural Technology, a section of the journal Frontiers in Neuroscience

                Article
                DOI: 10.3389/fnins.2018.00443
                PMC ID: 6039564
                PubMed ID: 30022928
                SO-VID: 90a3f225-00d9-4371-853c-2568196924cb
                Copyright © Copyright © 2018 Foerster, Rezaee, Paulus, Nitsche and Dutta.
                License:

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                Date received : 27 October 2017
                Date accepted : 12 June 2018
                Page count
                Figures: 6, Tables: 2, Equations: 4, References: 50, Pages: 12, Words: 0
                Funding
                Funded by: Alexander von Humboldt-Stiftung 10.13039/501100001655
                Funded by: Deutscher Akademischer Austauschdienst 10.13039/501100001655
                Categories
                Subject: Neuroscience
                Subject: Original Research

                ScienceOpen disciplines: Neurosciences
                Keywords: lower limb motor cortex,stimulation parameters,motor cortex excitability,modeling,transcranial direct current stimulation (tdcs)
                Data availability:
                ScienceOpen disciplines: Neurosciences
                Keywords: lower limb motor cortex, stimulation parameters, motor cortex excitability, modeling, transcranial direct current stimulation (tdcs)

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