title: Evidence that transcranial direct current stimulation (tDCS) generates little-to-no reliable neurophysiologic effect beyond MEP amplitude modulation in healthy human subjects: A systematic review.
abstract: BACKGROUND
  Transcranial direct current stimulation (tDCS) is a form of neuromodulation that is increasingly being utilized to examine and modify a number of cognitive and behavioral measures. The theoretical mechanisms by which tDCS generates these changes are predicated upon a rather large neurophysiological literature. However, a robust systematic review of this neurophysiological data has not yet been undertaken.
  
  
  METHODS
  tDCS data in healthy adults (18-50) from every neurophysiological outcome measure reported by at least two different research groups in the literature was collected. When possible, data was pooled and quantitatively analyzed to assess significance. When pooling was not possible, data was qualitatively compared to assess reliability.
  
  
  RESULTS
  Of the 30 neurophysiological outcome measures reported by at least two different research groups, tDCS was found to have a reliable effect on only one: MEP amplitude. Interestingly, the magnitude of this effect has been significantly decreasing over the last 14 years.
  
  
  CONCLUSIONS
  Our systematic review does not support the idea that tDCS has a reliable neurophysiological effect beyond MEP amplitude modulation - though important limitations of this review (and conclusion) are discussed. This work raises questions concerning the mechanistic foundations and general efficacy of this device - the implications of which extend to the steadily increasing tDCS psychological literature.
authors.affiliation: University of Melbourne, School of Psychological Sciences, Melbourne, VIC, Australia. Electronic address: jared.cooney.horvath@gmail.com.
authors.forename: Jared Cooney
authors.initials: JC
authors.lastname: Horvath
authors.affiliation: University of Melbourne, School of Psychological Sciences, Melbourne, VIC, Australia.
authors.forename: Jason D
authors.initials: JD
authors.lastname: Forte
authors.affiliation: University of Melbourne, School of Psychological Sciences, Melbourne, VIC, Australia.
authors.forename: Olivia
authors.initials: O
authors.lastname: Carter
mesh_terms: D000293:Adolescent; D000328:Adult; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping / Q000379:methods; D004569:Electroencephalography; D005071:Evoked Potentials; D019054:Evoked Potentials, Motor; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008875:Middle Aged; D015203:Reproducibility of Results; D065908:Transcranial Direct Current Stimulation / Q000379:methods*; D050781:Transcranial Magnetic Stimulation / Q000379:methods; D055815:Young Adult
keywords: Electroencephalography (EEG); Event related potential (ERP); Functional magnetic resonance imaging (fMRI); Neurophysiology; Systematic review; Transcranial direct current stimulation (tDCS); Transcranial magnetic stimulation (TMS)