Perception of comfort during transcranial DC stimulation: Effect of NaCl solution concentration applied to sponge electrodes
Introduction
Transcranial direct current stimulation (tDCS) applied via surface electrodes targeting sensorimotor cortex can induce long-lasting changes in cortical excitability (Nitsche and Paulus, 2000, Nitsche and Paulus, 2001, Nitsche et al., 2003a, Nitsche et al., 2003b, Nitsche et al., 2003c, Uy and Ridding, 2003) and have functional effects such as increasing tactile discrimination (Rogalewski et al., 2004). TDCS employs the passage of a constant direct current between an anodal and cathodal electrode placed over the scalp. The application of direct current to human skin has been associated with the production of a range of pruritic sensations, from mild tingling through itching to sharp prickling (Ledger, 1992), and current-evoked pruritis has been reported during tDCS (Siebner et al., 2004, Fregni et al., 2005, Nitsche et al., 2003c, Nitsche et al., 2004a, Nitsche et al., 2004b, Nitsche et al., 2004c, Nitsche et al., 2005), transcutaneous electrical stimulation (Edwards et al., 1976, Tuckett, 1982, Ikoma et al., 2005), and iontophoresis (Low and Reed, 1993, Prausnitz, 1996). Furthermore, iontophoresis of increasing concentrations of chemical pruritogen, histamine, increases itch intensity in a concentration dependent manner (Darsow et al., 2000, Drzezga et al., 2001, Mochizuki et al., 2003) and participants undergoing iontophoresis of saline (140 mM) at 1 mA report a perception of itch sensation similar to that induced by iontophoresis of low concentration histamine (Mochizuki, pers. comm. April 2006). Modulation of ionic concentration of a solution also changes the conductivity of the solution, which impacts upon the voltage required to drive an electrical current, and it has previously been reported that voltage can modulate perception of sensation during tDCS, whereby, voltages above 10 V induce skin sensation whereas those below 10 V are not usually associated with perception of current (Lang et al., 2005).
To assist the conduction of current tDCS studies have used a number of aqueous solutions to dampen the sponge electrodes, including tap water (Rosenkranz et al., 2000, Liebetanz et al., 2002, Antal et al., 2003a, Antal et al., 2003b, Antal et al., 2004a, Antal et al., 2004b, Fregni et al., 2005), electrode conductance gel (Baudewig et al., 2001), and, most frequently, saltwater (NaCl) (Nitsche and Paulus, 2000, Nitsche and Paulus, 2001, Nitsche et al., 2003b, Nitsche et al., 2003c, Nitsche et al., 2004a, Nitsche et al., 2004b, Nitsche et al., 2004c, Nitsche et al., 2005, Lang et al., 2004, Lang et al., 2005, Hummel et al., 2005, Gandiga et al., 2006). However, the concentrations of NaCl solutions are not normally reported, and there has been no study of the optimal concentration that minimizes current-induced pruritis and stimulation voltage. In the present study we have evaluated how the sensation of current-evoked pruritis during 1 mA tDCS depends on the concentration of NaCl in solution and the applied voltage.
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Participants
Fourteen healthy and tDCS-naïve volunteers (7 male and 7 female; 20–60 years), recruited from university staff and students, participated in this study. This study was approved by Human Ethics Committee of University of Western Australia and was performed in accordance with the Declaration of Helsinki. All participants gave written informed consent.
Electrolyte solutions
Three concentrations of NaCl were determined relative to biological levels of sodium in the human body: low NaCl concentration was equivalent to the
Perception of comfort
Stimulation with all solutions was tolerable and no participant elected to withdraw from the study. Descriptions of sensation experienced during tDCS, in participants’ own words, likened the sensation of stimulation to itching rather than pain, particularly under the anodal electrode. All subjects were able to detect the current and commented that the itching sensation was most pronounced beneath the centre of the electrode on the forehead (anodal electrode). Irrespective of electrolyte
Discussion
Our findings demonstrate a relationship between the electrolytic concentration of NaCl and the perception of comfort, and the voltage required to carry 1 mA, during tDCS. Our findings also demonstrate that tDCS is a comfortable technique, with participants describing the feeling of stimulation as a pruritic sensation of itch rather than pain. For all solutions current was perceptible 1 min into tDCS.
The perception of sensations (i.e. current detection thresholds and itch intensity) has a complex
Conclusions
TDCS is a comfortable method of brain stimulation, however, minimization of tDCS-evoked pruritis is desirable not only for maximizing comfort, but for the effectiveness of sham stimulation protocols. Based on our findings we recommend the use of solutions with relatively low NaCl concentration, in the range 15 mM (comparable to intracellular NaCl concentration) to 140 mM (normal saline, plasma NaCl concentration), as tDCS at these concentrations is more likely to be perceived as comfortable,
Acknowledgements
We thank Dr. Hideki Mochizuki from the National Centre of Neurology and Psychiatry, Japan, for assistance with understanding of itch physiology and interpretation of scales in relation to electrically evoked itch with saline and histamine. Jane Dundas was supported by an Australian Postgraduate Award.
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