Phase locking

Phase locking is the condition in which a self-oscillating system maintains a fixed phase relationship with an external periodic signal or with another oscillator in a coupled network. Unlike mere frequency entrainment, which only requires equality of frequencies, phase locking implies a constant phase difference — often zero or π — that is stable against perturbations. The phenomenon is the fundamental mechanism underlying synchronization in neural ensembles, power grids, and cardiac tissue.

Phase locking is not always desirable. In the auditory system, the breakdown of phase locking at high frequencies explains the upper limit of pitch perception. In epilepsy, pathological phase locking across cortical regions correlates with seizure onset. The transition from phase-locked to drifting behavior is governed by the same bifurcation structure that produces Arnold tongues in driven oscillators.