Transcranial Direct Current Stimulation (tDCS)/Transcranial Alternating Current Stimulation (tACS)Original ArticlePerception of Comfort During Active and Sham Transcranial Direct Current Stimulation: A Double Blind Study
Introduction
In the last decade non-invasive techniques to modulate cortical excitability (e.g. transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS)) have been used in clinical rehabilitation as well as in the laboratory to influence cognitive, perceptual, motor and mood states [1]. It is therefore important to know the extent to which the use of these techniques may have adverse effects or cause discomfort to participants undergoing neurostimulation. To this aim, this study primarily assessed the level of discomfort and the subjective symptoms associated to neurostimulation delivered using tDCS. In addition, since inadequate blinding may be a cause of concern in the evaluation of the therapeutic effect of tDCS in clinical interventions, we assessed the extent to which participants could discriminate, in a double-blind administration of neurostimulation, whether tDCS operated in either a sham (placebo) or active mode.
While several studies have reported that there are no serious adverse health effects when participants undergo tDCS neurostimulation [1], some studies have reported some degree of discomfort and the presence of minor side effects such as tingling, itching and burning to a greater extent in the active than in the sham condition (e.g. Refs. [2], [3], [4]).
The present study aimed to extend the knowledge on the above issue in several distinct ways. Firstly, the length of the stimulation used was longer (i.e. 30 min) and associated with a higher current strength (i.e. 2 mA) than in most previously reported studies (e.g. Refs. [2], [4], [5]), where exposures lasted 20 min at the most, with a maximum current strength of 1.5 mA. Secondly, we used a double-blind delivery of the stimulation, while in previous reports this was not consistently applied (e.g. Ref. [4]). Thirdly, unlike studies using questionnaires completed by participants following the termination of the stimulation session (e.g. Ref. [4]), we asked participants to provide ratings on their levels of comfort during the stimulation period, thus reducing the risk of memory biases influencing the answers provided in the questionnaires.
Finally, Kessler et al. raised some concerns on the effectiveness of the blinding procedure, possibly due to the limited ramp-up time usually applied in the sham condition (between 10 and 15 s). Hence, we increased the ramp-up time for both active and sham conditions to 30 s to reduce the risk of compromising the blinding. Moreover, to ascertain the robustness of the double-blinding procedure, we collected data on the ability of participants to discriminate whether the stimulation was either on or off.
The data reported here were collected in two studies primarily aimed to assess the impact of tDCS neurostimulation on memory, attention and decision-making tasks. The results from these two studies will be independently described. One-hundred and ninety-five sessions were run in three different stimulation conditions (left anodal and right cathodal; right anodal and left cathodal; sham). Independent samples were tested in the different conditions.
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Participants
Participants taking part in the two studies were mostly university students aged between 18 and 34 years. In Study 1 there were 61 female and 38 male participants (mean age 20.7 and 21.1, respectively; t < 1), while 57 females and 39 males took part in Study 2 (mean age 21.2 and 21.9, respectively; t < 1). Thirty-three participants took part in both Study 1 and Study 2. The University of Essex Faculty of Science and Engineering Ethics Committee approved these studies. All participants were
Results
The mean levels of comfort experienced by participants in Study 1 and Study 2 are reported in Table 1. The only significant result obtained in the mixed factorial ANOVA on the data from Study 1 was main effect of time of measurement, indicating that levels of comfort increased from the beginning to the end of the exposure period (5.22 vs 7.35 vs 7.77), F (2, 194) = 83.7, Mean Square Error (MSE) = 1.96. The main effect of stimulation conditions and the interaction between stimulation conditions
Discussion
The present study provides useful and important addenda to the growing literature on safety aspects of the use of tDCS as a non-invasive neurostimulation tool. Summarizing the results obtained, firstly, no serious side effects were reported among a sample of one-hundred and twenty-nine participants undertaking bilateral active stimulation for a period of 30 min with a current strength of 2 mA. This being a longer stimulation period with a higher current level than those previously used in
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