title: Investigating TMS-EEG Indices of Long-Interval Intracortical Inhibition at Different Interstimulus Intervals.
abstract: Long-interval intracortical inhibition (LICI) is a transcranial magnetic stimulation (TMS) paradigm that uses paired magnetic stimuli separated by 100-200 ms to investigate the activity of cortical GABAergic interneurons. While commonly applied, the mechanisms contributing to LICI are not well understood, and growing evidence suggests that inhibition observed at different interstimulus intervals (ISI) may involve non-identical processes.
  
  
  
  This study aims to utilise combined TMS-EEG to more thoroughly characterise LICI at different ISIs, as the TMS-evoked EEG potential (TEP) can provide more direct insight into the cortical response to stimulation that is not subject to variations in spinal cord excitability that can confound the motor evoked potential (MEP).
  
  
  
  In 12 subjects (22.6 ± 0.9 years), LICI was applied using two ISIs of 100 ms (LICI100) and 150 ms (LICI150), while TEPs were recorded using simultaneous high-definition EEG.
  
  
  
  Analysis of EEG data within a region of interest (C3 electrode) showed that test alone stimulation produced three consistent TEP peaks (corresponding to P30, N100 and P180) that were all significantly inhibited following paired-pulse stimulation. However, for P30, inhibition varied between LICI conditions, with reduced amplitude following LICI100(P = 0.03) but not LICI150(P = 0.3). In contrast, the N100 and P180 were significantly reduced by LICI at both intervals (all P-values < 0.05). In addition, topographical analyses suggested that the global change in P30, N40 and P180 differed between LICI conditions.
  
  
  
  These findings suggest that LICI100and LICI150reflect complex measurements of cortical inhibition with differential contributions from comparable circuits.
authors: Discipline of Physiology, School of Medicine, The University of Adelaide, Adelaide, Australia. Electronic address: george.opie@adelaide.edu.au.
authors: George M
authors: GM
authors: Opie
authors: Brain and Mental Health Laboratory, School of Psychological Sciences and Monash Biomedical Imaging, Monash Institute of Cognitive and Clinical Neuroscience, Monash University, Australia.
authors: Nigel C
authors: NC
authors: Rogasch
authors: Robinson Research Institute, School of Medicine, The University of Adelaide, Adelaide, Australia; Discipline of Psychiatry, School of Medicine, The University of Adelaide, Adelaide, Australia.
authors: Mitchell R
authors: MR
authors: Goldsworthy
authors: Robinson Research Institute, School of Medicine, The University of Adelaide, Adelaide, Australia.
authors: Michael C
authors: MC
authors: Ridding
authors: Discipline of Physiology, School of Medicine, The University of Adelaide, Adelaide, Australia.
authors: John G
authors: JG
authors: Semmler
mesh_terms: D004569:Electroencephalography / Q000379:methods*; D004576:Electromyography / Q000379:methods; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: Electroencephalography; GABA; Interstimulus interval; Long-interval intracortical inhibition; TMS-evoked potential; Transcranial magnetic stimulation