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tDCS changes in motor excitability are specific to orientation of current flow

Vishal Rawiji, Matteo Ciocca, André Zacharia, David Soares, Dennis Truong, Marom Bikson, John Rothwell, Sven Bestmann
doi: https://doi.org/10.1101/149633
Vishal Rawiji
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
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Matteo Ciocca
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
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André Zacharia
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
3 Department of Neurology, University Hospitals of Geneva, Switzerland
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David Soares
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
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Dennis Truong
2 Department of Biomedical Engineering, The City College of The City University of New York, New York City, USA
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Marom Bikson
2 Department of Biomedical Engineering, The City College of The City University of New York, New York City, USA
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John Rothwell
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
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Sven Bestmann
1 Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London, London, UK
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Abstract

Measurements and models of current flow in the brain during transcranial Direct Current Stimulation (tDCS) indicate stimulation of regions in-between electrodes. Moreover, the cephalic cortex result in local fluctuations in current flow intensity and direction, and animal studies suggest current flow direction relative to cortical columns determines response to tDCS. Here we test this idea by measuring changes in cortico-spinal excitability by Transcranial Magnetic Stimulation Motor Evoked Potentials (TMS-MEP), following tDCS applied with electrodes aligned orthogonal (across) or parallel to M1 in the central sulcus. Current flow models predicted that the orthogonal electrode montage produces consistently oriented current across the hand region of M1 that flows along cortical columns, while the parallel electrode montage produces none-uniform current directions across the M1 cortical surface. We find that orthogonal, but not parallel, orientated tDCS modulates TMS-MEPs. We also show modulation is sensitive to the orientation of the TMS coil (PA or AP), which is through to select different afferent pathways to M1. Our results are consistent with tDCS producing directionally specific neuromodulation in brain regions in-between electrodes, but shows nuanced changes in excitability that are presumably current direction relative to column and axon pathway specific. We suggest that the direction of current flow through cortical target regions should be considered for targeting and dose-control of tDCS.

Highlights

  • Direction of current flow is important for tDCS after-effects.

  • tDCS modulates excitability between two electrodes.

  • tDCS differentially modulates PA and AP inputs into M1.

PA
postero-anterior
AP
antero-posterior
ML
medio-lateral
tDCS
transcranial direct current stimulation
MEP
motor evoked potential
M1
primary motor cortex
TMS
transcranial magnetic stimulation;
AP-TMS-MEPs
motor evoked potentials elicited with anterior-posterior directed TMS;
PA-TMS-MEPs
motor evoked potentials elicited with posterior-anterior directed TMS

Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Copyright 
The copyright holder for this preprint is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission.
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Posted June 13, 2017.
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tDCS changes in motor excitability are specific to orientation of current flow
Vishal Rawiji, Matteo Ciocca, André Zacharia, David Soares, Dennis Truong, Marom Bikson, John Rothwell, Sven Bestmann
bioRxiv 149633; doi: https://doi.org/10.1101/149633
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tDCS changes in motor excitability are specific to orientation of current flow
Vishal Rawiji, Matteo Ciocca, André Zacharia, David Soares, Dennis Truong, Marom Bikson, John Rothwell, Sven Bestmann
bioRxiv 149633; doi: https://doi.org/10.1101/149633

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