Transcranial electrical stimulation motor threshold can estimate individualized tDCS dosage from reverse-calculation electric-field modeling

Authors

Kevin A. Caulfield, Medical University of South Carolina
Bashar W. Badran, Medical University of South Carolina
William H. DeVries, Medical University of South Carolina
Philipp M. Summers, Medical University of South Carolina
Emma Kofmehl, Medical University of South Carolina
Xingbao Li, Medical University of South Carolina
Jeffrey J. Borckardt, Medical University of South Carolina
Marom Bikson, CUNY City CollegeFollow
Mark S. George, Medical University of South Carolina

Document Type

Article

Publication Date

4-21-2020

Abstract

Background

Unique amongst brain stimulation tools, transcranial direct current stimulation (tDCS) currently lacks an easy or widely implemented method for individualizing dosage.

Objective

We developed a method of reverse-calculating electric-field (E-field) models based on Magnetic Resonance Imaging (MRI) scans that can estimate individualized tDCS dose. We also evaluated an MRI-free method of individualizing tDCS dose by measuring transcranial magnetic stimulation (TMS) motor threshold (MT) and single pulse, suprathreshold transcranial electrical stimulation (TES) MT and regressing it against E-field modeling. Key assumptions of reverse-calculation E-field modeling, including the size of region of interest (ROI) analysis and the linearity of multiple E-field models were also tested.

Methods

In 29 healthy adults, we acquired TMS MT, TES MT, and anatomical T1-weighted MPRAGE MRI scans with a fiducial marking the motor hotspot. We then computed a “reverse-calculated tDCS dose” of tDCS applied at the scalp needed to cause a 1.00 V/m E-field at the cortex. Finally, we examined whether the predicted E-field values correlated with each participant’s measured TMS MT or TES MT.

Results

We were able to determine a reverse-calculated tDCS dose for each participant using a 5 × 5 x 5 voxel grid region of interest (ROI) approach (average = 6.03 mA, SD = 1.44 mA, range = 3.75–9.74 mA). The Transcranial Electrical Stimulation MT, but not the Transcranial Magnetic Stimulation MT, significantly correlated with the ROI-based reverse-calculated tDCS dose determined by E-field modeling (R²= 0.45, p < 0.001).

Conclusions

Reverse-calculation E-field modeling, alone or regressed against TES MT, shows promise as a method to individualize tDCS dose. The large range of the reverse-calculated tDCS doses between subjects underscores the likely need to individualize tDCS dose. Future research should further examine the use of TES MT to individually dose tDCS as an MRI-free method of dosing tDCS.

Comments

This is the authors' manuscript of a work originally published in Brain Stimulation, available at https://doi.org/10.1016/j.brs.2020.04.007.

This is an open access work under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).