Crossfrequency

Crossfrequency, in neuroscience and signal processing, denotes the interaction between neural oscillations at different frequency bands. It is commonly referred to as cross-frequency coupling (CFC). Researchers study how slow rhythms modulate the timing or amplitude of faster rhythms, and sometimes how faster activity influences slower processes, to understand coordinated activity across brain networks.

The most studied forms are phase-amplitude coupling, in which the phase of a low-frequency oscillation modulates the amplitude of a high-frequency oscillation; phase-phase coupling, where the phases of two bands align; and amplitude-amplitude coupling, where the envelopes of two bands co-vary. These interactions can occur within a cortical region or across distant areas, and may reflect communication between neural ensembles or hierarchical organization of processing.

Analysis typically uses band-pass filtering and the Hilbert transform to extract instantaneous phase and amplitude. Metrics include modulation index for PAC, coherence- or phase-locking-based measures for PPC, and correlation of envelopes for AAC. Comodulograms visualize coupling across pairs of frequency bands. Statistical validation often employs surrogate data and multiple comparison controls to assess significance.

Crossfrequency coupling has been observed in EEG, MEG, intracranial EEG, and local field potentials in animals. It is associated with cognitive processes such as memory formation, attention, perception, and motor control. In clinical contexts, altered CFC has been reported in epilepsy, schizophrenia, Parkinson’s disease, and aging, and it is explored for biomarker development and brain-computer interfaces.

Interpretation requires caution due to non-sinusoidal waveforms, non-stationarity, and potential artifacts from filtering or volume conduction. Robust analysis combines multiple metrics and appropriate controls to distinguish true interactions from spurious effects.