System-Wide Hierarchical Coupling of Brain and Body Rhythms Under State and Latent Stress

The present study assessed harmonic locking, the coordination of frequency that enables cross-frequency phase synchrony, between the EEG, heart rate, and breathing frequency. In two state-based experiments, harmonic locking emerged as a system-wide property of brain-body coordination, associated with cognitive demand, task-related physiological arousal, and negative affect valence. Cardiorespiratory rhythms coupled to low-frequency, network-level brain rhythms, which in turn coupled to higher-frequency brain activity, enabled by a coordinated, oppositional modulation of low- and high-frequency rhythms. The converging effects of cognitive demand, physiological arousal, and negatively-valenced affect suggest a conceptual and functional overlap, potentially unified as constituent elements of state stress. A third, meta-analytical experiment found individual differences in perceived stress and interoceptive sensitivity to physiological arousal predicted latent expression of brain-body coupling. Finally, distinct elements of the stress response engaged distinct modes of synchrony, contingent on arousal activation: a purely cognitive stress promoted theta-anchored intra-brain coupling; stress accompanied by cardiorespiratory activation promoted delta-anchored brain-body coupling. Together, this study identified harmonic locking as a stress-responsive, hierarchically-organized substrate of brain-body coordination.