title: 'I-wave' Recruitment Determines Response to tDCS in the Upper Limb, but Only So Far.
abstract: BACKGROUND
  Anodal transcranial direct current stimulation (a-tDCS) can facilitate primary motor cortex (M1), but the modulation of motor evoked potentials (MEPs) by a-tDCS varies between participants, and may depend on the balance between early versus late I-wave recruitment, as assessed by the difference in MEP latency between latero-medial and anterior-posterior cortical currents induced by transcranial magnetic stimulation (TMS).
  
  
  OBJECTIVE
  To date, the dependence of tDCS after-effects on I-wave recruitment has only been investigated in intrinsic hand muscles. In order to better understand the effects of tDCS across the upper limb, the present study examined I-wave recruitment and MEP modulation by a-tDCS or dual-hemisphere tDCS in muscles of the forearm (Extensor Carpi Radialis; ECR) and proximal upper limb (Biceps Brachii; BB).
  
  
  METHODS
  We conducted a randomized double-blind study with 18 healthy adults. Each received anodal, dual-hemisphere, or sham tDCS over M1 in separate sessions (tDCS, 1 mA for 15 min).
  
  
  RESULTS
  Linear regression analyzes showed a-tDCS modulated MEP size dependent on the latency difference in the ECR (P = 0.01) but not BB (P = 0.28). Individuals with small MEP latency differences showed the expected facilitation of ECR MEPs after a-tDCS, whereas those with large MEP latency differences had suppressed ECR MEPs after a-tDCS. This relationship was not present after dual-hemisphere or sham tDCS in either muscle (all P > 0.32).
  
  
  CONCLUSIONS
  I-wave recruitment can predict the after-effects of a-tDCS in the distal but not proximal upper limb. These findings provide further insight into the variability of tDCS after-effects, and the relationship between I-wave recruitment and putative mechanisms of tDCS.
authors: Movement Neuroscience Laboratory, Department of Sport and Exercise Science, and Centre for Brain Research, The University of Auckland, Auckland, New Zealand.
authors: Alana B
authors: AB
authors: McCambridge
authors: Movement Neuroscience Laboratory, Department of Sport and Exercise Science, and Centre for Brain Research, The University of Auckland, Auckland, New Zealand.
authors: James W
authors: JW
authors: Stinear
authors: Movement Neuroscience Laboratory, Department of Sport and Exercise Science, and Centre for Brain Research, The University of Auckland, Auckland, New Zealand. Electronic address: w.byblow@auckland.ac.nz.
authors: Winston D
authors: WD
authors: Byblow
mesh_terms: D000328:Adult; D004311:Double-Blind Method; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D065908:Transcranial Direct Current Stimulation; D050781:Transcranial Magnetic Stimulation; D034941:Upper Extremity / Q000502:physiology*; D055815:Young Adult
keywords: Electrode montage; I-waves; MEP; Proximal; TMS; tDCS