title: Intra-Subject Consistency and Reliability of Response Following 2 mA Transcranial Direct Current Stimulation.
abstract: Transcranial direct current stimulation (tDCS) is a popular non-invasive brain stimulation technique that has been shown to influence cortical excitability. While polarity specific effects have often been reported, this is not always the case, and variability in both the magnitude and direction of the effects have been observed.
  
  
  
  We aimed to explore the consistency and reliability of the effects of tDCS by investigating changes in cortical excitability across multiple testing sessions in the same individuals. A within subjects design was used to investigate the effects of anodal and cathodal tDCS applied to the motor cortex. Four experimental sessions were tested for each polarity in addition to two sham sessions.
  
  
  
  Transcranial magnetic stimulation (TMS) was used to measure cortical excitability (TMS recruitment curves). Changes in excitability were measured by comparing baseline measures and those taken immediately following 20 minutes of 2 mA stimulation or sham stimulation.
  
  
  
  Anodal tDCS significantly increased cortical excitability at a group level, whereas cathodal tDCS failed to have any significant effects. The sham condition also failed to show any significant changes. Analysis of intra-subject responses to anodal stimulation across four sessions suggest that the amount of change in excitability across sessions was only weakly associated, and was found to have poor reliability across sessions (ICC = 0.276). The effects of cathodal stimulation show even poorer reliability across sessions (ICC = 0.137). In contrast ICC analysis for the two sessions of sham stimulation reflect a moderate level of reliability (ICC = .424).
  
  
  
  Our findings indicate that although 2 mA anodal tDCS is effective at increasing cortical excitability at group level, the effects are unreliable across repeated testing sessions within individual participants. Our results suggest that 2 mA cathodal tDCS does not significantly alter cortical excitability immediately following stimulation and that there is poor reliability of the effect within the same individual across different testing sessions.
authors.affiliation: School of Psychology, University of Nottingham, Nottingham NG7 2RD, UK.
authors.forename: Katherine
authors.initials: K
authors.lastname: Dyke
authors.affiliation: School of Psychology, University of Nottingham, Nottingham NG7 2RD, UK.
authors.forename: Soyoung
authors.initials: S
authors.lastname: Kim
authors.affiliation: School of Medicine, Division of Psychiatry, Institute of Mental Health, University of Nottingham, Nottingham NG7 2TU, UK.
authors.forename: Georgina M
authors.initials: GM
authors.lastname: Jackson
authors.affiliation: School of Psychology, University of Nottingham, Nottingham NG7 2RD, UK. Electronic address: Stephen.jackson@nottingham.ac.uk.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Jackson
mesh_terms: D000328:Adult; D000071080:Cortical Excitability / Q000502:physiology*; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D015203:Reproducibility of Results; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Cortical excitability; Motor cortex; Transcranial direct current stimulation (tDCS); Transcranial magnetic stimulation (TMS)