title: Inter- and Intra-individual Variability in Response to Transcranial Direct Current Stimulation (tDCS) at Varying Current Intensities.
abstract: BACKGROUND
  Translation of transcranial direct current stimulation (tDCS) from research to clinical practice is hindered by a lack of consensus on optimal stimulation parameters, significant inter-individual variability in response, and in sufficient intra-individual reliability data.
  
  
  OBJECTIVE
  Inter-individual differences in response to anodal tDCS at a range of current intensities were explored. Intra-individual reliability in response to anodal tDCS across two identical sessions was also investigated.
  
  
  METHODS
  Twenty-nine subjects participated in a crossover study. Anodal-tDCS using four different current intensities (0.2, 0.5, 1 and 2 mA), with an anode size of 16 cm2, was tested. The 0.5 mA condition was repeated to assess intra-individual variability. TMS was used to elicit 40 motor-evoked potentials (MEPs) before 10 min of tDCS, and 20 MEPs at four time-points over 30 min following tDCS.
  
  
  RESULTS
  ANOVA revealed no main effect of TIME for all conditions except the first 0.5 mA condition, and no differences in response between the four current intensities. Cluster analysis identified two clusters for the 0.2 and 2 mA conditions only. Frequency distributions based on individual subject responses (excitatory, inhibitory or no response) to each condition indicate possible differential responses between individuals to different current intensities. Test-retest reliability was negligible (ICC(2,1) = -0.50).
  
  
  CONCLUSIONS
  Significant inter-individual variability in response to tDCS across a range of current intensities was found. 2 mA and 0.2 mA tDCS were most effective at inducing a distinct response. Significant intra-individual variability in response to tDCS was also found. This has implications for interpreting results of single-session tDCS experiments.
authors: School of Psychiatry, University of New South Wales, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia; Black Dog Institute, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia.
authors: Taariq
authors: T
authors: Chew
authors: School of Psychiatry, University of New South Wales, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia; Black Dog Institute, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia.
authors: Kerrie-Anne
authors: KA
authors: Ho
authors: School of Psychiatry, University of New South Wales, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia; Black Dog Institute, Prince of Wales Hospital, Hospital Road, Randwick, NSW 2031, Australia; St George Hospital, South Eastern Sydney Health, Level 2, James Laws House, Gray St., Kogarah, NSW 2217, Australia. Electronic address: colleen.loo@unsw.edu.au.
authors: Colleen K
authors: CK
authors: Loo
mesh_terms: D000328:Adult; D018592:Cross-Over Studies; D004311:Double-Blind Method; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D015203:Reproducibility of Results; D013997:Time Factors; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: Current intensity; Inter-individual variability; Intra-individual variability; Motor cortex; Reliability; Transcranial direct current stimulation