Neuromodulating the rhythms of cognition

Rhythmic non-invasive brain stimulation (rh-NIBS) allows to modulate neural oscillations and study the functional role of these brain rhythms for cognition. We hope to draw attention to often neglected aspects of this field that limit the interpretations of the findings and their translational potential. We here review current rh-NIBS trends and propose to conceptually differentiate oscillatory synchronization, aimed at enhancing an intrinsic oscillatory amplitude, from frequency-shifting, designed to speed-up or slow-down a given oscillatory rhythm. At the same time, we offer a precise mechanistic account of these two rh-NIBS protocols that accounts for inter-individual differences in stimulation outcomes. Finally, we gap the bridge between entrainment, understood as an online manipulation of neural oscillations via rh-NIBS, versus plasticity, defined as the aftereffects of the TMS offline protocols. Specifically, we bring forward a promising possibility that the aftereffects of rh-NIBS protocols, preferably tuned to the dominant oscillatory frequency, might produce the desired outcome through a successful online oscillatory tuning, understood as a prerequisite for the generation of synaptic plasticity reflecting enduring aftereffects. This conceptual and mechanistic framework aims to provide a deeper theoretical understanding of recommended rh-NIBS best practices for noninvasively studying dynamic oscillation-cognition relationships in cognitive and clinical research.