P 95 Frequency-specific after-effects of transcranial alternating current stimulation (tACS) on motor learning

Background Motor learning is associated with changes in synchronized oscillatory activity at alpha (8–12 Hz) and beta (12–30 Hz) frequencies. Numerous studies demonstrated that tACS can entrain ongoing oscillatory activity ( Antal and Herrmann, 2016 ) and modulate motor behavior ( Pollok et al., 2015 , Wach et al., 2013 ). However, the neurophysiological mechanisms underlying tACS ( Antal and Paulus, 2013 ) and its modulatory effect on motor learning remain ambiguous. Thus, the purpose of the study was to investigate the effects of tACS on the learning of a bimanual visuomotor coordination task and the accompanied oscillatory activity. Methods Up to now, 19 right-handed healthy volunteers participated in the study and performed the task which is divided into three different levels of complexity (easy, moderate, difficult). Concurrent to motor practice, participants received either 10 Hz tACS, 20 Hz tACS or a sham stimulation over the parietal cortex (P3/P4) for 20 min via small gel electrodes (3,14 cm 2 Ag/AgCl, amperage = 1 mA). Before and after tACS (immediately, 30 min, 1 day, 4 days, 7 days), movement speed and accuracy of the visuomotor coordination were assessed. Oscillatory activity was measured by electroencephalography (EEG, Starstim-Neuroelectrics, P3/P4/Fpz/Fz/Cz/Pz/Oz/VEOG). Results Analyzing motor performance, a three-way ANOVA (GROUPxTIMExCOMPLEXITY) revealed a significant main effect for TIME ( F (5, 80) = 13.285; p η 2  = 0.454), a significant main effect for COMPLEXITY ( F (2, 32) = 195.087; p η 2 p  = 0.924) and a significant TIMExCOMPLEXITY interaction ( F 10, 160) = 7.095; p η 2 p  = 0.307). Bonferroni-corrected post hoc t-tests yielded a significant difference from pre to post-test immediately after tACS for all groups indicating an improvement in the task. The comparison of 10 Hz tACS and 20 Hz tACS revealed a significant difference in visuomotor coordination performance at the moderate level 30 min after stimulation ( p p Conclusions The present study suggests that 10 Hz tACS and 20 Hz tACS increase EEG alpha activity which is in line with the theory of entrainment where synchronization can also occur at harmonics or subharmonics ( Antal and Herrmann, 2016 ). Nevertheless, while both tACS frequencies entrain alpha activity they seem to have a different effect on motor behavior, 10 Hz tACS slows down while 20 Hz tACS speeds up the visuomotor execution.