Perception of comfort during transcranial DC stimulation: Effect of NaCl solution concentration applied to sponge electrodes

https://doi.org/10.1016/j.clinph.2007.01.010Get rights and content

Abstract

Objective

To investigate the relationship between perception of comfort and electrolyte concentration and applied voltage during transcranial direct current stimulation (tDCS).

Methods

NaCl solutions (15, 140 and 220 mM NaCl) or deionised water were used as electrolytes to dampen tDCS sponge electrodes. Subjects (14, 7M, 20–60 years of age) rated comfort on an 11-point scale during 2 min of tDCS (1 mA).

Results

Overall participants rated tDCS as comfortable. Perception of comfort was negatively correlated with NaCl concentration (Spearman’s ρ = −0.88; p < 0.05), and a logarithmic relationship was found between applied voltage and ionic strength of electrolytes (Pearson’s r = −0.635; p < 0.01). There was no relationship between applied voltage and perception of comfort.

Conclusions

The application of NaCl solutions between 15 and 140 mM to sponge electrodes is more likely to be perceived as comfortable during tDCS.

Significance

The reporting of solution concentration and ratings of perception would be useful adjuncts to tDCS studies.

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.

Section snippets

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.

References (48)

  • M.A. Nitsche et al.

    Level of action of cathodal DC polarisation induced inhibition of the human motor cortex

    Clin Neurophysiol

    (2003)
  • A.K. Nugroho et al.

    Transdermal iontophoresis of the dopamine agonist 5-OH-DPAT in human skin in vitro

    J Control Release

    (2005)
  • W. Paulus

    Transcranial direct current stimulation (tDCS)

    Suppl Clin Neurophysiol

    (2003)
  • M.R. Prausnitz

    The effects of electrical current applied to skin: a review for transdermal drug delivery

    Adv Drug Deliv Rev

    (1996)
  • K. Rosenkranz et al.

    Diminution of training-induced transient motor cortex plasticity by weak transcranial direct current stimulation in the human

    Neurosci Lett

    (2000)
  • G. Stagni et al.

    Iontophoretic current and intradermal microdialysis recovery in humans

    J Pharmacol Toxicol

    (1999)
  • R.P. Tuckett

    Itch evoked by electrical stimulation of the skin

    J Invest Dermatol

    (1982)
  • J. Uy et al.

    Increased cortical excitability induced by transcranial DC and peripheral nerve stimulation

    J Neurosci Methods

    (2003)
  • A. Antal et al.

    Excitability changes induced in the primary visual cortex by transcranial direct current stimulation: direct electrophysiological evidence

    Invest Opthalmol Vis Sci

    (2004)
  • A. Antal et al.

    Manipulation of phosphene thresholds by transcranial direct current stimulation in man

    Exp Brain Res

    (2003)
  • Antal A, Kincses TZ, Nitsche MA, Paulus W. Modulation of moving phosphene thresholds by transcranial direct current...
  • A. Antal et al.

    Facilitation of visuo-motor learning by transcranial direct current stimulation of the motor and extrastriate visual areas in humans

    Eur J Neurosci

    (2004)
  • G. Ardolino et al.

    Non-synaptic mechanisms underlie the after-effects of cathodal transcutaneous direct current stimulation of the human brain

    J Physiol

    (2005)
  • J.N. Baraniuk et al.

    Hypertonic saline nasal provocation stimulates nociceptive nerves, substance P release, and glandular mucous exocytosis in normal humans

    Am J Respir Crit Care Med

    (1999)
  • Cited by (0)

    View full text