Systematic evaluation of the impact of stimulation intensity on neuroplastic after‐effects induced by transcranial direct current stimulation

Abstract

Contemporary non-invasive neuromodulatory techniques, such as transcranial direct current stimulation (tDCS), have shown promising potential in both restituting impairments in cortical physiology in clinical settings, as well as modulating cognitive abilities in the healthy population. However, neuroplastic after-effects of tDCS are highly dependent on stimulation parameters, relatively short lasting, and not expectedly uniform between individuals. The present study systematically investigates the full range of current intensity between 0.5 and 2.0 mA on left primary motor cortex (M1) plasticity, as well as the impact of individual-level covariates on explaining inter-individual variability. Thirty-eight healthy subjects were divided into groups of anodal and cathodal tDCS. Five DC intensities (sham, 0.5, 1.0, 1.5 and 2.0 mA) were investigated in separate sessions. Using transcranial magnetic stimulation (TMS), 25 motor-evoked potentials (MEPs) were recorded before, and 10 time points up to 2 h following 15 min of tDCS. Repeated-measures ANOVAs indicated a main effect of intensity for both anodal and cathodal tDCS. With anodal tDCS, all active intensities resulted in equivalent facilitatory effects relative to sham while for cathodal tDCS, only 1.0 mA resulted in sustained excitability diminution. An additional experiment conducted to assess intra-individual variability revealed generally good reliability of 1.0 mA anodal tDCS (ICC(2,1) = 0.74 over the first 30 min). A post hoc analysis to discern sources of inter-individual variability confirmed a previous finding in which individual TMS SI (stimulus intensity for 1 mV MEP amplitude) sensitivity correlated negatively with 1.0 mA anodal tDCS effects on excitability. Our study thus provides further insights on the extent of non-linear intensity-dependent neuroplastic after-effects of anodal and cathodal tDCS.

Response variability of different anodal transcranial direct current stimulation intensities across multiple sessions

“…We think this is the reason why four recent studies found different results when they assessed tDCS test-retest reliability using Grand Mean values. Particularly, two studies tested the intra-individual consistency of M1-atDCS for repeated sessions showing a fair to good degree of reliability (ICC = 0.565 [29]; ICC = 0.738 [35]) of tDCS effects; in other words individual effects were consistent on different days. However, fairly worse results of intra-individual reliability have been reported by other investigations indicating a lack of reproducibility (ICC = −0.500 [27]; ICC = 0.062 [33]) for repeated tDCS sessions.…”

“…Surprisingly, only few studies exploring tDCS effects based on MEP measures have employed a control condition (see for review [23]), and those who performed this control generally do not present individual data [5, 35, 50–53]. To date, only two studies that reported reliability of tDCS effects across multiple sessions included a sham condition and presented as well individual responses [33, 34].…”

“…With this in mind, another key question is whether individuals respond in a consistent and predictable manner to repeated tDCS sessions. To date, several studies have measured the reliability by intra-individual assessment of tDCS-responses showing inconsistent results across studies [27, 29, 33–35]. Besides other methodological differences, these inconsistencies could be due to the lack of control conditions, which cannot rule out the variability introduced by the measuring tool (TMS), or testing different tDCS intensities.…”

Can Transcranial Direct Current Stimulation Improve Muscle Power in Individuals With Advanced Weight-Training Experience?

“…Current intensity may be critical for a-tDCS motor response and recent findings suggest that excitability after-effects do not linearly correlate with stimulation intensity, as lower intensities (0.5 and 1 mA) display equal or great effects in comparison to higher intensities (21). Future studies should explore the intensity-response relationship between a-tDCS and explosive strength performance.…”

Extending the parameter range for tDCS: Safety and tolerability of 4 mA stimulation

“…Previous studies have shown that prolongation of stimulation duration can lead to a switch of directionality of effects [2], and a similar switch from long-term depression to potentiation-like plasticity has been demonstrated for higher stimulation intensity [3]. Moreover, dependent on baseline excitability, different stimulation intensities seem to have maximum efficacy [4]. Thus systematic exploration of safety and effects of extended tDCS protocols is critical for identification of maximally efficient stimulation approaches.…”