Rethinking clinical trials of transcranial direct current stimulation: participant and assessor blinding is inadequate at intensities of 2mA

PLoS One. 2012;7(10):e47514. doi: 10.1371/journal.pone.0047514. Epub 2012 Oct 17.


Background: Many double-blind clinical trials of transcranial direct current stimulation (tDCS) use stimulus intensities of 2 mA despite the fact that blinding has not been formally validated under these conditions. The aim of this study was to test the assumption that sham 2 mA tDCS achieves effective blinding.

Methods: A randomised double blind crossover trial. 100 tDCS-naïve healthy volunteers were incorrectly advised that they there were taking part in a trial of tDCS on word memory. Participants attended for two separate sessions. In each session, they completed a word memory task, then received active or sham tDCS (order randomised) at 2 mA stimulation intensity for 20 minutes and then repeated the word memory task. They then judged whether they believed they had received active stimulation and rated their confidence in that judgement. The blinded assessor noted when red marks were observed at the electrode sites post-stimulation.

Results: tDCS at 2 mA was not effectively blinded. That is, participants correctly judged the stimulation condition greater than would be expected to by chance at both the first session (kappa level of agreement (κ) 0.28, 95% confidence interval (CI) 0.09 to 0.47 p=0.005) and the second session (κ=0.77, 95%CI 0.64 to 0.90), p=<0.001) indicating inadequate participant blinding. Redness at the reference electrode site was noticeable following active stimulation more than sham stimulation (session one, κ=0.512, 95%CI 0.363 to 0.66, p<0.001; session two, κ=0.677, 95%CI 0.534 to 0.82) indicating inadequate assessor blinding.

Conclusions: Our results suggest that blinding in studies using tDCS at intensities of 2 mA is inadequate. Positive results from such studies should be interpreted with caution.

Publication types

  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Double-Blind Method
  • Electric Stimulation Therapy / adverse effects
  • Electric Stimulation Therapy / methods*
  • Electricity*
  • Female
  • Humans
  • Judgment
  • Male
  • Memory / physiology
  • Middle Aged
  • Skin
  • Task Performance and Analysis
  • Young Adult

Grant support

GLM is supported by the National Health & Medical Research Council of Australia ID 571090. This funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No corporate funding or sponsorship was received for the conduct of this study. No government or institutional funding was received for the conduct of this study.