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

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.

Electric Field Dynamics in the Brain During Multi-Electrode Transcranial Electric Stimulation

“…It is to be expected that in the 0° conditions medial-lateral components will be larger than for the 180° condition. This does not diminish the validity of our findings as the direction of the electric field with respect to neural elements is most important for TES physiological effects 35,36 . Thus, differences in one electric field component across stimulation conditions are important to document and control for.…”

Scalp‐to‐cortex distance of left primary motor cortex and its computational head model: Implications for personalized neuromodulation

“…Except for the same scale (ie, mm) SCD and CT had, the vector‐like properties of SCD, including: (a) direction: SCD is a line starting from the point on the scalp and ending at the point on cerebral cortex and (b) magnitude: the measure of SCD representing the length of the line, also have profound impact on the modeling of tDCS. For instance, recent evidence confirmed that the directionality (or orientation) of the current injection can critically influence the field potential of the targeted region during transcranial electrical stimulation . Therefore, region‐specific SCD not only reflects the individual morphometric features, but also provides a useful and dynamic parameter to optimize the therapeutic protocol.…”

“…For instance, recent evidence confirmed that the directionality (or orientation) of the current injection can critically influence the field potential of the targeted region during transcranial electrical stimulation. 34,35 Therefore, region-specific SCD not only reflects the individual morphometric features, but also provides a useful and dynamic parameter to optimize the therapeutic protocol.…”

Electric field dependent effects of motor cortical TDCS

“…Previously, Rawji et al (2018) studied the excitability changes using two electrode montages that produced EFs approximately in the PA and ML directions (left M1, 1 mA, 10 min, FDI muscle, N=15); the depth component was not reported. They found that the EF in the PA direction modulated the MEPs, decreasing the excitability, while the EF in the ML direction did not (Rawji et al, 2018). The results of experiment 2 are in line with these findings, except that the effective component was E D , not E PA .…”

“…We first performed an exploratory sham-controlled motor cortical TDCS study and individually calculated the EFs in all our subjects. As the effects of TDCS are sensitive to the field direction (Rawji et al, 2018), we analysed all three orthogonal components of the EF. To find which cortical sites are potentially affected by the EF, we decided to use partial least squares (PLS) regression (Geladi and Kowalski, 1986;Wold et al, 2001), which is an effective method for finding relationships between dependent variables (here: MEP amplitude) and a large number of collinear predictor variables (here: EF in the cortex).…”