Synergistic entrainment: a multiscale neurodynamic framework for closed-loop cross-modal neuromodulation

The brain's higher cognitive functions rely on the dynamic integration of multimodal information, supported by coordinated neural oscillations. However, current neuromodulation approaches often fail to reliably control these dynamics due to high inter-individual variability and limited mechanistic grounding. To address this challenge, we introduce the Synergistic Entrainment Framework, which conceptualizes the brain as a dynamical system in which neural oscillations create periodic windows of excitability that regulate when information can be effectively processed. Within this framework, rhythmic inputs from different physical modalities (e.g., sensory streams and electromagnetic stimulation) can interact nonlinearly, leading to amplification or stabilization of neural responses, when their timing is aligned with ongoing cortical phase dynamics. We propose that this interaction operates across three coupled scales: (i) microscopic constructive interference that amplifies local neural responses, (ii) mesoscopic phase resetting that aligns regional excitability, and (iii) macroscopic network synchronization constrained by the brain's structural connectome. By explicitly linking these multiscale mechanisms, we propose the framework as a theoretical perspective and operational scaffold for formulating testable hypotheses about state-dependent, closed-loop neuromodulation. Rather than claiming empirical validation, it aims to organize operational criteria for future studies of how stimulation timing may be aligned with intrinsic brain dynamics.