The involvement of endogenous brain rhythms in speech processing

Endogenous brain rhythms are at the core of neurobiological speech theories. These brain rhythms have been proposed to play a role for speech segmentation, attention allocation and temporal and spectral processes during speech perception. However, despite the strong theoretical foundations, direct empirical evidence for the involvement of endogenous brain rhythms in speech processing might be sparse, partly because of the inherent challenge of distinguishing these endogenous rhythms from responses to external stimuli. We here review popular oscillation-based theories (dynamic attending, active sensing, asymmetry sampling in time, and segmentation theories) and the empirical evidence for them, from studies using electrophysiological and brain stimulation approaches. Despite the popularity of current theoretical accounts, we conclude that there is little experimental research to date that provides unambiguous evidence for the involvement of endogenous brain rhythms as predicted by those oscillation-based accounts. Finally, we suggest potential directions for future research that could strengthen the evidence base for the role of endogenous rhythms in speech processing.