When brain rhythms aren't 'rhythmic': implication for their mechanisms and meaning

•Brain rhythms are often brief and intermittent in time lasting a few cycles or less.•High power in the average can reflect the accumulated density of transient events on single trials rather than amplitude increases.•Signals with vastly different waveforms can create equal spectral power and duration.•Circuit mechanisms underlying different waveforms can be vastly different.•Non-oscillatory waveform feature can create spurious high spectral power and cross-frequency coupling.

Rhythms are a prominent signature of brain activity. Their expression is correlated with numerous examples of healthy information processing and their fluctuations are a marker of disease states. Yet, their causal or epiphenomenal role in brain function is still highly debated. We review recent studies showing brain rhythms are not always 'rhythmic', by which we mean representative of repeated cycles of activity. Rather, high power and continuous rhythms in averaged signals can represent brief transient events on single trials whose density accumulates in the average. We also review evidence showing time-domain signals with vastly different waveforms can exhibit identical spectral-domain frequency and power. Further, non-oscillatory waveform feature can create spurious high spectral power. Knowledge of these possibilities is essential when interpreting rhythms and is easily missed without considering pre-processed data. Lastly, we discuss how these findings suggest new directions to pursue in our quest to discover the mechanism and meaning of brain rhythms.