Comparison of online and offline applications of dual-site transcranial alternatingcurrent stimulation (tACS) on functional connectivity between pre-supplementarymotor area (preSMA) and right inferior frontal gyrus (rIFG) for improving responseinhibition

BackgroundThe efficacy of transcranial alternating current stimulation (tACS) is thought to be brain state-dependent, such that tACS during task performance would be hypothesised to offer greater potential for inducing beneficial electrophysiological changes in the brain and associated behavioural improvement compared to tACS at rest. However, to date, no empirical study has directly tested this postulation. ObjectiveHere we compared the effects of tACS applied during a stop signal task (online) to the effects of the same tACS protocol applied prior to the task (offline) and a sham control stimulation. MethodsA total of 53 young, healthy adults (32 female; 18-35 yrs) received dual-site beta tACS over the right inferior frontal gyrus (rIFG) and pre-supplementary motor area (preSMA), which are thought to play critical roles in action cancellation, with phase-synchronised stimulation for 15 min with the aim of increasing functional connectivity. ResultsEEG connectivity analysis revealed significantly increased task-related functional connectivity following online but not offline tACS. Correlation analyses suggested that an increase in functional connectivity in the beta band at rest following online tACS was associated with an improvement in response inhibition. Interestingly, despite the lack of changes in functional connectivity at the target frequency range following offline tACS, significant improvements in response inhibition were still observed, suggesting offline tACS may still be efficacious in inducing behavioural changes, likely via a post-stimulation early plasticity mechanism. ConclusionOverall, the results indicate that online and offline dual-site beta tACS are beneficial in improving inhibitory control via distinct underlying mechanisms.