title: Effects of a common transcranial direct current stimulation (tDCS) protocol on motor evoked potentials found to be highly variable within individuals over 9 testing sessions.
abstract: Transcranial direct current stimulation (tDCS) uses a weak electric current to modulate neuronal activity. A neurophysiologic outcome measure to demonstrate reliable tDCS modulation at the group level is transcranial magnetic stimulation engendered motor evoked potentials (MEPs). Here, we conduct a study testing the reliability of individual MEP response patterns following a common tDCS protocol. Fourteen participants (7m/7f) each underwent nine randomized sessions of 1 mA, 10 min tDCS (3 anode; 3 cathode; 3 sham) delivered using an M1/orbito-frontal electrode montage (sessions separated by an average of ~5.5 days). Fifteen MEPs were obtained prior to, immediately following and in 5 min intervals for 30 min following tDCS. TMS was delivered at 130 % resting motor threshold using neuronavigation to ensure consistent coil localization. A number of non-experimental variables were collected during each session. At the individual level, considerable variability was seen among different testing sessions. No participant demonstrated an excitatory response ≥20 % to all three anodal sessions, and no participant demonstrated an inhibitory response ≥20 % to all three cathodal sessions. Intra-class correlation revealed poor anodal and cathodal test-retest reliability [anode: ICC(2,1) = 0.062; cathode: ICC(2,1) = 0.055] and moderate sham test-retest reliability [ICC(2,1) = 0.433]. Results also revealed no significant effect of tDCS at the group level. Using this common protocol, we found the effects of tDCS on MEP amplitudes to be highly variable at the individual level. In addition, no significant effects of tDCS on MEP amplitude were found at the group level. Future studies should consider utilizing a more strict experimental protocol to potentially account for intra-individual response variations.
authors.affiliation: Melbourne School of Psychological Sciences, University of Melbourne, Redmond Barry Building, Melbourne, VIC, 3010, Australia. jared.cooney.horvath@gmail.com.|Departments of Medicine and Neurology, St. Vincent's Hospital, University of Melbourne, Melbourne, VIC, Australia. jared.cooney.horvath@gmail.com.|Melbourne Graduate School of Education, University of Melbourne, Melbourne, VIC, Australia. jared.cooney.horvath@gmail.com.
authors.forename: Jared Cooney
authors.initials: JC
authors.lastname: Horvath
authors.affiliation: Departments of Medicine and Neurology, St. Vincent's Hospital, University of Melbourne, Melbourne, VIC, Australia.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Vogrin
authors.affiliation: Melbourne School of Psychological Sciences, University of Melbourne, Redmond Barry Building, Melbourne, VIC, 3010, Australia.
authors.forename: Olivia
authors.initials: O
authors.lastname: Carter
authors.affiliation: Melbourne School of Psychological Sciences, University of Melbourne, Redmond Barry Building, Melbourne, VIC, 3010, Australia.|Departments of Medicine and Neurology, St. Vincent's Hospital, University of Melbourne, Melbourne, VIC, Australia.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Cook
authors.affiliation: Melbourne School of Psychological Sciences, University of Melbourne, Redmond Barry Building, Melbourne, VIC, 3010, Australia.
authors.forename: Jason D
authors.initials: JD
authors.lastname: Forte
mesh_terms: D000293:Adolescent; D000328:Adult; D004576:Electromyography / Q000379:methods; 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: Longitudinal; Motor evoked potentials (MEPs); Reliability; Transcranial direct current stimulation (tDCS); Transcranial magnetic stimulation (TMS)