Perfect Timing: Effects of Auditory Stimulation on Alpha Oscillations During Wakefulness and the Transition to Sleep are Phase-dependent in Humans

The waking brain's ubiquitous alpha oscillations are thought to play an important role in managing the brain's resources, inhibiting neural activity as a function of their phase and amplitude. In accordance with this physiological excitability, perceptual and cognitive processes fluctuate with alpha oscillations. Here we demonstrate that the alpha rhythm can be manipulated with sound in a phase-dependent manner, showing that repeated phase-locked sounds alter the frequency of these oscillations in a spatially localised manner. We draw on oscillator theory to explore the origin of this frequency change, using phase-locked auditory evoked potentials to show a putative phase-reset mechanism, which is dependent on the amplitude of the endogenous alpha oscillations. Finally, we demonstrate the functional relevance of this approach by showing that we can modulate the transition to sleep, using sound, in a phase-dependent manner. Overall, we conclude that the phase of alpha oscillations can be exploited in real time, and highlight alpha phase-locked auditory stimulation as a powerful method by which to both selectively augment and investigate the brain's pertinent oscillations. ### Competing Interest Statement N.G. is part in a patent application on the ecHT technology, assigned to MIT, and is a founder in a company that utilises it.