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