title: Effects of Electrode Drift in Transcranial Direct Current Stimulation.
abstract: BACKGROUND
  Conventional transcranial direct current stimulation (tDCS) methods involve application of weak electrical current through electrodes encased in saline-soaked sponges affixed to the head using elastic straps. In the absence of careful preparation, electrodes can drift from their original location over the course of a tDCS session.
  
  
  OBJECTIVE
  The current paper investigates the influence of electrode drift on distribution of electric fields generated by conventional tDCS.
  
  
  METHODS
  MRI-derived finite element models of electric fields produced by tDCS were used to investigate the influence of incremental drift in electrodes for two of the most common electrode montages used in the literature: M1/SO (motor to contralateral supraorbital) and F3/F4 (bilateral frontal). Based on these models, we extracted predicted current intensity from 20 representative structures in the brain.
  
  
  RESULTS
  Results from separate RM-ANOVAs for M1/SO and F3/F4 montages demonstrated that 5% incremental drift in electrode position significantly changed the distribution of current delivered by tDCS to the human brain (F's > 8.6, P's < 0.001). Pairwise comparisons demonstrated that as little as 5% drift was able to produce significant differences in current intensity in structures distributed across the brain (P's < 0.03).
  
  
  CONCLUSIONS
  Drift in electrode position during a session of tDCS produces significant alteration in the intensity of stimulation delivered to the brain. Elimination of this source of variability will facilitate replication and interpretation of tDCS findings. Furthermore, measurement and statistically accounting for drift may prove important for better characterizing the effects of tDCS on the human brain and behavior.
authors.affiliation: Cognitive Aging and Memory Clinical Translational Research Program, Institute on Aging, Department of Aging and Geriatric Research, University of Florida, USA. Electronic address: ajwoods@ufl.edu.
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Woods
authors.affiliation: Cognitive Aging and Memory Clinical Translational Research Program, Institute on Aging, Department of Aging and Geriatric Research, University of Florida, USA.
authors.forename: Vaughn
authors.initials: V
authors.lastname: Bryant
authors.affiliation: Center for Cognitive Neuroscience, Laboratory for Cognition and Neural Stimulation, Department of Neurology, University of Pennsylvania, USA.
authors.forename: Daniela
authors.initials: D
authors.lastname: Sacchetti
authors.affiliation: Center for Cognitive Neuroscience, Laboratory for Cognition and Neural Stimulation, Department of Neurology, University of Pennsylvania, USA.
authors.forename: Felix
authors.initials: F
authors.lastname: Gervits
authors.affiliation: Center for Cognitive Neuroscience, Laboratory for Cognition and Neural Stimulation, Department of Neurology, University of Pennsylvania, USA.
authors.forename: Roy
authors.initials: R
authors.lastname: Hamilton
mesh_terms: D001921:Brain / Q000502:physiology*; D004566:Electrodes; D020342:Finite Element Analysis; D006801:Humans; D008279:Magnetic Resonance Imaging; D008959:Models, Neurological; D015203:Reproducibility of Results; D065908:Transcranial Direct Current Stimulation / Q000379:methods*
keywords: Electrode drift; Electrode placement; MRI-Derived finite element models; Transcranial direct current stimulation; tDCS reproducibility