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

Transcranial electrical stimulation (tES) has shown promise for modulating brain function and related behavioral performance, but evidence has been mixed thus far. 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. This study examined the offline effects of different types of tES on 40-Hz ASSR. Participants underwent four conditions of tES, which were 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. Following the tES application, electroencephalogram (EEG) was recorded during 40-Hz auditory click stimulation. Mixed-effects modeling revealed no significant group-level differences in phase locking or evoked amplitude between stimulation conditions. However, both baseline (sham) ASSR and the change in ASSR following tES had a substantial interindividual variability. Exploratory analysis showed that individuals with lower baseline ASSR had increased synchronization following tES. Furthermore, the increase in ASSR synchronization was linked to higher memory gain; however, the relationship was observed only in otDCS condition. The findings encourage future research focusing on individual factors that may contribute to tES outcomes.