Alpha rhythm transcranial electrical stimulation to inferior parietal cortex increases alpha power and phase synchrony while attending to mind-body self-states.

Self-referential processing (SRP) refers to the human brain's response to semantic and somatic self-related information. Recent developments in modulating semantic and somatic SRP using non-invasive brain stimulation supported the efficacy of transcranial direct current stimulation in modulating alpha electroencephalography (alpha-EEG) during SRP. Meanwhile, although alpha transcranial alternating current stimulation (alpha-tACS) shows greater efficacy in modulating alpha-EEG, the efficacy of alpha-tACS for modulating alpha-EEG during SRP has not been evaluated. The current study investigates the effects of alpha-tACS compared to sham stimulation over the medial prefrontal cortex and the bilateral inferior parietal lobule on alpha-EEG during both semantic and somatic SRP in two separate experiments. Semantic SRP was provoked by introspection on life roles (e.g., "friend"), while somatic SRP was provoked by interoception upon sensations occurring in the exterior body (e.g., "shoulders") during the experimental task, and alpha-EEG responses during SRP were compared to those occurring during resting state and an external attention control condition. Results indicated that while alpha-tACS to the medial prefrontal cortex did not produce significant source-level alpha-EEG changes, alpha-tACS to inferior parietal cortex increased alpha-EEG source power and phase synchrony when participants received real stimulation during the first experimental session. An exploratory analysis also indicated that real stimulation reduced alpha-EEG power during semantic but not somatic SRP during the first session but not the second session. Our results demonstrate that while alpha-tACS can modulate alpha-EEG during SRP, the effects may be dependent on the ordering of real vs. sham stimulation sessions and stimulation sites.