title: Inter- and intra-subject variability of motor cortex plasticity following continuous theta-burst stimulation.
abstract: BACKGROUND
  The potential of non-invasive brain stimulation (NIBS) for studying, and inducing, functionally relevant neuroplasticity is dependent on protocols that can induce lasting, robust and reliable effects. A current limiting factor is the large inter- and intra-subject variability in NIBS-induced neuroplastic responses. There has been some study of inter-subject response variability and factors that contribute to it; however, intra-subject response variability has, so far, received little investigation.
  
  
  OBJECTIVE
  By testing participants on multiple occasions we aimed to (1) compare inter- and intra-subject variability of neuroplastic responses induced by continuous theta-burst stimulation (cTBS); (2) determine whether the transcranial magnetic stimulation (TMS) intensity used to measure cTBS-induced neuroplastic responses contributes to response variability; (3) determine whether assessment of factors known to influence response variability can be used to explain some of the variability in cTBS-induced neuroplastic responses across experimental sessions.
  
  
  METHODS
  In three separate experimental sessions, motor-evoked potential (MEP) input-output (IO) curves were obtained before and after cTBS, and questionnaire-based assessments of physical activity and perceived stress were obtained.
  
  
  RESULTS
  cTBS-induced MEP suppression was greatest at the upper end of the IO curve (150-180% resting motor threshold; RMT) and most consistent across subjects and across experimental sessions when assessed with a TMS intensity of 150% RMT. The magnitude of cTBS-induced MEP suppression evoked at 150% RMT correlated with self-reported perceived stress, but not with self-reported physical activity.
  
  
  CONCLUSIONS
  The most reliable TMS intensity to probe cTBS-induced long-term depression (LTD)-like neuroplastic responses is 150% RMT. This is unlikely to simply be a ceiling effect and, we suggest, may be due to changes in the descending volley evoked at higher stimulus intensities. The perceived stress scale appears to be sufficiently sensitive to measure the influence of subject stress on LTD-like neuroplastic responses.
authors.affiliation: The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia; School of Psychology and Exercise Science, Murdoch University, Australia. Electronic address: a.vallence@murdoch.edu.au.
authors.forename: A-M
authors.initials: AM
authors.lastname: Vallence
authors.affiliation: The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia.
authors.forename: M R
authors.initials: MR
authors.lastname: Goldsworthy
authors.affiliation: The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia.
authors.forename: N A
authors.initials: NA
authors.lastname: Hodyl
authors.affiliation: Discipline of Physiology, School of Medical Sciences, University of Adelaide, Australia.
authors.forename: J G
authors.initials: JG
authors.lastname: Semmler
authors.affiliation: The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia.
authors.forename: J B
authors.initials: JB
authors.lastname: Pitcher
authors.affiliation: The Robinson Research Institute, School of Paediatrics and Reproductive Health, University of Adelaide, Australia.
authors.forename: M C
authors.initials: MC
authors.lastname: Ridding
mesh_terms: D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D009473:Neuronal Plasticity / Q000502:physiology*; D010465:Perception; D015203:Reproducibility of Results; D057566:Self Report; D013315:Stress, Psychological; D011795:Surveys and Questionnaires; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: continuous theta-burst stimulation; neuroplasticity; reproducibility; response variability; stress; transcranial magnetic stimulation