title: The variability of intracortical inhibition and facilitation.
abstract: OBJECTIVE
  To assess the variability of transcranial magnetic stimulation paired pulse measurements of cortical excitability between subjects, between sessions and within subjects within sessions.
  
  
  METHODS
  In experiment 1, intracortical inhibition and facilitation were assessed with a fixed conditioning stimulus intensity (CSI) of 80% of active motor threshold (AMT) whereas in experiment 2, the effect of different CSIs (60-110% of AMT) was investigated.
  
  
  RESULTS
  Experiment 1 revealed that subjects differed significantly in the degree of inhibition and facilitation. Between sessions the variability was substantial as predicted by high within session variability. Experiment 2 allowed determination of individual thresholds for inhibition and facilitation. These thresholds were the best predictor of the amount of inhibition or facilitation at a given CSI. Across subjects we observed a high correlation of the threshold for inhibition (expressed in terms of maximum stimulator output) with AMT (r=0.93). Results for facilitation were more variable.
  
  
  CONCLUSIONS
  The variability was high if a single CSI was used to compare the percent intracortical inhibition or facilitation between subjects, or between sessions. Much less variable was the threshold for intracortical inhibition/facilitation, which was highly correlated to AMT. We suggest that the ratio of CSI:AMT is a robust and useful additional measure of the integrity of neuronal circuits underlying intracortical inhibition/facilitation.
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, The National Hospital for Neurology and Neurosurgery, Institute of Neurology, Box 77, Royal Free and University College Medical School, Queen Square, London WC1N 3BG, UK. m.orth@ion.ucl.ac.uk
authors.forename: M
authors.initials: M
authors.lastname: Orth
authors.forename: A H
authors.initials: AH
authors.lastname: Snijders
authors.forename: J C
authors.initials: JC
authors.lastname: Rothwell
mesh_terms: D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology; D004558:Electric Stimulation; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology*