Simultaneous Triple Modes of Cross-Frequency Coupling in Brainstem Nonlinear Oscillator Networks: Cooperative Rhythms of Respiration, Heartbeat, and Brainwaves

Cross-frequency coupling (CFC) has been proposed as a fundamental mechanism mediating communication between neuronal assemblies through rhythmic interactions across multiple frequency bands, including delta, theta, alpha, beta, and gamma oscillations. Recent findings suggest that slow harmonics in the delta and theta ranges within the brainstem underlie cardiorespiratory rhythms through phase-phase CFC (Kawai, 2023). In contrast, higher-frequency gamma oscillations (>30 Hz) convey information-rich signals via phase-amplitude CFC mechanisms. To date, triple CFC modes have not been characterized in any brain region. Notably, simultaneous delta-theta-gamma coupling--encompassing both phase-phase and phase-amplitude interactions--appears to operate cooperatively, suggesting functional integration through emergent synchrony within the brainstem. Multiple recordings from the nucleus tractus solitarius (NTS) demonstrate that the power and coherence of these synchronized oscillations exhibit distinct spatiotemporal patterns along the dorsoventral axis, reflecting differentiation among large-scale efferent systems and cytoarchitectural domains (Kawai, 2018a; Negishi and Kawai, 2011). Robust gamma activity, phase-coupled with delta and theta oscillations generated by resilient harmonic oscillators within the NTS and the broader brainstem network, may constitute a cooperative mechanism for large-scale homeostatic regulation. The dynamic balance of signal power between slow (delta/theta) and fast (gamma) components could nonlinearly modulate oscillator network dynamics and widespread projection systems throughout the brain. Such integrative neural dynamics likely support adaptive, whole-body responses to fluctuations in the interoceptive environment.