Elsevier

Clinical Neurophysiology

Volume 121, Issue 12, December 2010, Pages 2165-2171
Clinical Neurophysiology

Electrode-distance dependent after-effects of transcranial direct and random noise stimulation with extracephalic reference electrodes

https://doi.org/10.1016/j.clinph.2010.04.033Get rights and content

Abstract

Objective

To evaluate the importance of the distance between stimulation electrodes, in various montages, on the ability to induce sustained cortical excitability changes using transcranial direct and random noise stimulation.

Methods

Twelve healthy subjects participated in four different experimental conditions. The stimulation electrode was always placed over the primary motor cortex; the reference electrode was placed at the contralateral orbit or at the ipsilateral/contralateral arm. MEPs were recorded in order to measure changes in cortical excitability over time.

Results

The distance between the two electrodes correlates negatively with the duration and magnitude of induced after-effects.

Conclusions

In particular when using extracephalic reference electrodes with transcranial electric stimulation techniques, the stimulation intensity has to be adapted to account for interelectrode distance.

Significance

Electrode distance plays a critical role in the induction for stimulation after-effects in tDCS and tRNS studies, and must be taken into account in future studies and also when making comparisons with the published literature.

Introduction

Soon after the invention of the voltaic pile, direct current stimulation was frequently used to treat neurological disorders (Hellwag and Jacobi, 1802). However, the method of applying transcranial direct current stimulation (tDCS) could recently be established on a firmer scientific level by using transcranial magnetic stimulation (TMS) of the motor cortex (M1) (Nitsche et al., 2007, Nitsche and Paulus, 2000, Priori et al., 1998) and the visual cortex (Antal et al., 2001) as evaluation methods for brain stimulation effects. tDCS is able to modify the excitability of motor, visual, somatosensory, and prefrontal cortices and has been shown to be functionally relevant in different visuomotor and cognitive tasks (Antal et al., 2003, Antal et al., 2004a, Antal et al., 2004b, Fregni et al., 2005, Kincses et al., 2004, Matsunaga et al., 2004, Nitsche et al., 2003).

More recently Terney et al. (2008) showed that another type of transcranial electrical stimulation, alternating current stimulation with random amplitude and frequency variation – transcranial random noise stimulation (tRNS) –, was able to produce a significant increase in regional motor-cortical excitability in healthy subjects. Advantages of this new technique compared to tDCS include it’s insensitivity to electrode polarity and further reduction of skin-sensations under the electrodes during stimulation.

When targeting the M1 with tDCS, and by using a cephalic reference electrode, only the M1-contralateral forehead arrangement resulted in significant excitability changes post-stimulation (Nitsche and Paulus, 2001, Nitsche and Paulus, 2000). However, this bipolar montage with equally sized electrodes has the disadvantage of an unwanted excitability change under the reference electrode. In principle this problem can be circumvented by increasing the size of the reference electrode in order to reduce local current density (Nitsche et al., 2007). A potential alternative that could be implemented to avoid unwanted effects in the brain is an extracephalic reference electrode, e.g. using a collar bone reference (Elbert et al., 1981) or using arm montages (Cogiamanian et al., 2007, Priori et al., 2008) or even the leg (Lippold and Redfearn, 1964, Meyer-Schwickerath and Magun, 1951). However, in these studies induced effects were evaluated immediately after the stimulation, long lasting after-effects were not studied.

Here we compared the efficacy of various montages using cranial and extracranial reference electrode positions for tDCS and tRNS. In addition we tested whether the distance between electrodes could influence the duration and magnitude of tDCS/tRNS-induced after-effects.

Section snippets

Subjects

Altogether 12 subjects participated in the experiments. All subjects were right handed according to the Edinburgh Handedness Inventory (Oldfield, 1971). The investigation was approved by the ethics committee of the University of Göttingen and we conformed to the Declaration of Helsinki.

tDCSanodal and tRNS

tDCSanodal and tRN stimulation were delivered by a battery-driven stimulator (NeuroConn GmbH, Ilmenau, Germany) through conductive-rubber electrodes. For tRNS in the stimulation mode “noise” there is a random

Left M1-contralateral forehead vs. Left M1- contralateral upper arm

tDCSanodal with 1 mA intensity and 10 min duration using the Left M1 – contralateral forehead montage showed a classical behaviour (overview in (Nitsche et al., 2008): the increase of excitability went up to 44% above baseline. In contrast, using the Left M1 – contralateral upper arm montage the MEP amplitudes increased clearly less with only one time point significantly higher. When compared with Left M1 – contralateral forehead montage, repeated measures of ANOVA revealed significant main

Discussion

These experiments were designed to compare the efficacy of cephalic with extracephalic reference electrodes when applying either tDCS or tRNS to the motor cortex. Originally we expected differences being caused by different current flow directions. Two unexpected findings occurred. First, tDCS or tRNS after-effects are comparable, at least when intensity is corrected for. Second, when increasing the interelectrode distance, which can easily be achieved at the upper extremity, we need higher

Conflict of interest

The authors have no financial or personal conflicts of interest.

Acknowledgments

Supported by the German Ministry of Health (BMBF 01GQ 0782 (VM)) and the Rose Foundation (T298/14375/2004 and T298/14376/2004).

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