title: Variability in response to transcranial direct current stimulation of the motor cortex.
abstract: BACKGROUND
  Responses to a number of different plasticity-inducing brain stimulation protocols are highly variable. However there is little data available on the variability of response to transcranial direct current stimulation (TDCS).
  
  
  OBJECTIVE
  We tested the effects of TDCS over the motor cortex on corticospinal excitability. We also examined whether an individual's response could be predicted from measurements of onset latency of motor evoked potential (MEP) following stimulation with different orientations of monophasic transcranial magnetic stimulation (TMS).
  
  
  METHODS
  Fifty-three healthy subjects participated in a crossover-design. Baseline latency measurements with different coil orientations and MEPs were recorded from the first dorsal interosseous muscle prior to the application of 10 min of 2 mA TDCS (0.057 mA/cm2). Thirty MEPs were measured every 5 min for up to half an hour after the intervention to assess after-effects on corticospinal excitability.
  
  
  RESULTS
  Anodal TDCS at 2 mA facilitated MEPs whereas there was no significant effect of 2 mA cathodal TDCS. A two-step cluster analysis suggested that approximately 50% individuals had only a minor, or no response to TDCS whereas the remainder had a facilitatory effect to both forms of stimulation. There was a significant correlation between the latency difference of MEPs (anterior-posterior stimulation minus latero-medial stimulation) and the response to anodal, but not cathodal TDCS.
  
  
  CONCLUSIONS
  The large variability in response to these TDCS protocols is in line with similar studies using other forms of non-invasive brain stimulation. The effects highlight the need to develop more robust protocols, and understand the individual factors that determine responsiveness.
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK.
authors.forename: Sarah
authors.initials: S
authors.lastname: Wiethoff
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK; Department of Neurology, Graduate School of Medicine, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. Electronic address: mhamada-tky@umin.net.
authors.forename: Masashi
authors.initials: M
authors.lastname: Hamada
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, Queen Square, London WC1N 3BG, UK.
authors.forename: John C
authors.initials: JC
authors.lastname: Rothwell
mesh_terms: D000293:Adolescent; D000328:Adult; D018592:Cross-Over Studies; D004558:Electric Stimulation / Q000379:methods*; D004566:Electrodes; D004867:Equipment Design; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000473:pathology / Q000502:physiology*; D009473:Neuronal Plasticity; D015203:Reproducibility of Results; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Facilitation; I-waves; Motor cortex; Plasticity; Transcranial direct current stimulation (TDCS)