Discrete sampling in perception via neuronal oscillations-Evidence from rhythmic, non-invasive brain stimulation

A variety of perceptual phenomena suggest that, in contrast to our everyday experience, our perception may be discrete rather than continuous. The possibility of such discrete sampling processes inevitably prompts the question of how such discretization is implemented in the brain. Evidence from neurophysiological measurements suggest that neural oscillations, particularly in the lower frequencies, may provide a mechanism by which such discretization can be implemented. It is hypothesized that cortical excitability is rhythmically enhanced or reduced along the positive and negative half-cycle of such oscillations. In recent years, rhythmic non-invasive brain stimulation approaches such as rhythmic transcranial magnetic stimulation (rTMS) and transcranial alternating current stimulation (tACS) are increasingly used to test this hypothesis. Both methods are thought to entrain endogenous brain oscillations, allowing them to alter their power, frequency, and phase in order to study their roles in perception. After a brief introduction to the core mechanisms of both methods, we will provide an overview of rTMS and tACS studies probing the role of brain oscillations for discretized perception in different domains and will contrast these results with unsuccessful attempts. Further, we will discuss methodological pitfalls and challenges associated with the methods.