Abstract
Alpha oscillations may play a role in managing the brain’s resources, inhibiting neural activity as a function of their phase and amplitude, and are changed in many brain disorders. Developing minimally intrusive tools to modulate alpha activity and identifying the parameters that determine its response to exogenous modulators, is essential for the implementation of focussed interventions. I introduce Alpha Closed-Loop Auditory Stimulation (αCLAS) as an EEG-based method to augment and investigate these brain rhythms in humans with specificity and selectivity, using targeted auditory stimulation. In this thesis, I extensively characterise this new technique, demonstrate proof of principle, explore its putative mechanisms, and provide functional applications. Across four independent studies, I demonstrate (1) that αCLAS alters alpha power, frequency, and connectivity in a phase, amplitude and topography-dependent manner (2 experiments, n=20, n=28). (2) Using a single-pulse-αCLAS evoked potentials approach I show that the effects of auditory stimuli on alpha oscillations and resulting evoked potentials can be explained within the theoretical framework of oscillator theory and a phase-reset mechanism (2 experiments, n=8, n=7). I then demonstrate the functional relevance of the approach by showing that (3) αCLAS modulates sleep onset dynamics in an alpha phase-dependent manner (n=16), and (4) applying the technique in the context of working memory retention (2 experiments, n=19, n =24).This thesis represents an initial exploration of sounds phase-locked to alpha rhythms, confirming a clear, and hitherto unknown, alpha phase dependent effect of sound on the EEG and on the sleep onset process. These foundational investigations provide insights into the interactions between the EEG and sound, and how the instantaneous features of the EEG may be used to optimise neuromodulation. Many questions remain on the manipulation, and indeed the meaning, of alpha oscillations. αCLAS may provide a new avenue through which to ask these questions.