title: Comparing cortical plasticity induced by conventional and high-definition 4 × 1 ring tDCS: a neurophysiological study.
abstract: BACKGROUND
  Transcranial direct current stimulation (tDCS) induces long-lasting NMDA receptor-dependent cortical plasticity via persistent subthreshold polarization of neuronal membranes. Conventional bipolar tDCS is applied with two large (35 cm(2)) rectangular electrodes, resulting in directional modulation of neuronal excitability. Recently a newly designed 4 × 1 high-definition (HD) tDCS protocol was proposed for more focal stimulation according to the results of computational modeling. HD tDCS utilizes small disc electrodes deployed in 4 × 1 ring configuration whereby the physiological effects of the induced electric field are thought to be grossly constrained to the cortical area circumscribed by the ring.
  
  
  OBJECTIVE
  We aim to compare the physiological effects of both tDCS electrode arrangements on motor cortex excitability.
  
  
  METHODS
  tDCS was applied with 2 mA for 10 min. Fourteen healthy subjects participated, and motor cortex excitability was monitored by transcranial magnetic stimulation (TMS) before and after tDCS.
  
  
  RESULTS
  Excitability enhancement following anodal and a respective reduction after cathodal stimulation occurred in both, conventional and HD tDCS. However, the plastic changes showed a more delayed peak at 30 min and longer lasting after-effects for more than 2 h after HD tDCS for both polarities, as compared to conventional tDCS.
  
  
  CONCLUSIONS
  The results show that this new electrode arrangement is efficient for the induction of neuroplasticity in the primary motor cortex. The pattern of aftereffects might be compatible with the concept of GABA-mediated surround inhibition, which should be explored in future studies directly.
authors.affiliation: Department of Clinical Neurophysiology, Georg-August-University Göttingen, Göttingen, Germany.
authors.forename: Hsiao-I
authors.initials: HI
authors.lastname: Kuo
authors.forename: Marom
authors.initials: M
authors.lastname: Bikson
authors.forename: Abhishek
authors.initials: A
authors.lastname: Datta
authors.forename: Preet
authors.initials: P
authors.lastname: Minhas
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
authors.forename: Min-Fang
authors.initials: MF
authors.lastname: Kuo
authors.forename: Michael A
authors.initials: MA
authors.lastname: Nitsche
mesh_terms: D000328:Adult; D001931:Brain Mapping; D004558:Electric Stimulation / Q000379:methods*; D004566:Electrodes; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009473:Neuronal Plasticity / Q000502:physiology*; D009474:Neurons / Q000502:physiology*