Modulation of the Alpha Oscillation Associated with Semantic and Somatic Self-referential Processing with Non-invasive Brain Stimulation
Abstract
Who am I? What am I? While philosophers have pondered existential questions such as these for centuries, neuroscientists are beginning to reveal neural correlates that may partly underlie our semantic (verbal, psychological) and somatic (non-verbal, physical) senses of ourselves. Non-invasive brain stimulation (NIBS) can then introduce exogenous electric fields over brain regions to modulate a person’s self-referential processing (SRP). Effective NIBS protocols can help establish causal connections between brain and self-experience, helping develop clinical interventions for mental health problems.
In this dissertation, I present a series of studies investigating the neuro-electrophysiology partially mediating SRP by combining electroencephalography (EEG) with NIBS. First, I present a systematic review of NIBS research seeking to modulate SRP as background for the current research. Then, two studies are presented that validate a novel experimental task for measuring the subjective and EEG responses during both semantic and somatic SRP in comparison with resting state and a simple external attention task; we demonstrated the significant role of alpha oscillations in distinguishing semantic and somatic SRP, both from each other and from internal (resting state) and external attention more generally. Next, three studies aimed to modulate alpha activity associated with SRP with either transcranial direct or alternating current stimulation (tDCS/tACS) over the medial prefrontal cortex (MPFC) or tACS over the bilateral inferior parietal lobule (IPL). Results showed that, compared to sham NIBS, significant changes in scalp-level alpha power occurred in all three experiments without any changes in self-reported mood or attention. Further, mPFC-tDCS and IPL-tACS modulated source-level alpha power and phase synchrony but not mPFC-tACS during somatic and semantic SRP, while only mPFC-tDCS modulated functional connectivity between posterior cingulate cortex and left IPL. Finally, we found that the effects of NIBS varied based on the order in which real vs. sham NIBS was conducted across two experimental sessions. Our findings provide insights into SRP's neuroscience and how NIBS may modify it.