title: Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities.
abstract: BACKGROUND
  Recently we have shown that transcranial random noise (tRNS) and 140 Hz transcranial alternating current stimulations (tACS), applied over the primary motor cortex (M1) and using 10 min stimulation duration and 1 mA intensity, significantly increases cortical excitability as measured by motor evoked potentials at rest before and after stimulation.
  
  
  OBJECTIVE
  Here, by decreasing the stimulation intensity in 0.2 mA steps from 1.0 mA, we investigate to what extent intensity depends on the induced after-effects.
  
  
  METHODS
  All twenty-five subjects participated in two different experimental sessions each. They received tACS using 140 Hz frequency and full spectrum tRNS at five different intensities on separate days. Sham stimulation was used as a control.
  
  
  RESULTS
  Instead of receiving a simple threshold, unexpectedly, in these two independent data sets at threshold intensities of 0.4 mA we found a switch of the already known excitation achieved with an intensity of 1 mA to inhibition. The intermediate intensity ranges of 0.6 and 0.8 mA had no effect at all. Interestingly, the inhibition produced by 140 Hz tACS was stronger than that induced by tRNS.
  
  
  CONCLUSIONS
  In summary, we have shown here the possibility of selectively controlling the enhancement or reduction of M1 excitability by applying different intensities of high frequency transcranial electrical stimulation.
authors.affiliation: Department of Child and Adolescent Psychiatry, Psychosomatics and Psychotherapy, Goethe-University of Frankfurt am Main Deutschordenstraße 50 (House 92) D-60528 Frankfurt am Main, Germany. Vera.Moliadze@kgu.de
authors.forename: Vera
authors.initials: V
authors.lastname: Moliadze
authors.forename: Deniz
authors.initials: D
authors.lastname: Atalay
authors.forename: Andrea
authors.initials: A
authors.lastname: Antal
authors.forename: Walter
authors.initials: W
authors.lastname: Paulus
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology*; D004558:Electric Stimulation / 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