Individual-specific effects of transcranial electrical stimulation on 40-Hz auditory steady-state responses

Although transcranial electrical stimulation (tES) has shown positive behavioral outcomes in neuropsychiatric disorders, evidence has been mixed thus far. Therefore, the possibility of tracking brain activity changes following tES via neurophysiological markers would benefit a better understanding of tES effects and the future development of tES protocols. One promising marker is the auditory steady-state response (ASSR), an externally controlled oscillatory brain activity, typically at 40 Hz, evoked by a periodic auditory stimulus. In the present study, we aimed to test the offline effects of different types of tES on 40-Hz ASSR. Participants underwent four conditions of tES which was applied over the left posterior parietal cortex: transcranial direct current (tDCS), transcranial alternating current (tACS), oscillatory transcranial direct current (otDCS) and sham stimulation. Individually determined theta frequency was delivered in the tACS and otDCS protocols. Immediately following the tES application, electroencephalogram (EEG) was recorded during 40-Hz auditory click stimulation. The analysis of time-frequency data from 29 participants showed that phase-locking and amplitude of 40-Hz ASSR did not differ among tES conditions. However, baseline ASSR (during sham stimulation) had a significant negative correlation with change in ASSR relative to sham (tES – sham) as well as a positive correlation with the magnitude of change (|tES – sham|) following each tES protocol. The results suggest that tES effects on 40-Hz ASSR may be individual-specific and likely depend on the baseline neuronal state. The findings encourage future research to focus on individual factors that may contribute to tES outcomes.