Multiple motor systems spontaneously track speech structures at distinct levels with individual variability.

Abstract

Tracking the hierarchical structure of connected speech is a central component of speech perception, yet the timescales at which motor systems contribute to this process remain poorly understood, especially given strong individual differences in auditory-motor coupling strength. Using a modified speech-to-speech synchronization (SSS) paradigm with hierarchically structured speech, we recorded participants' articulatory vocalization, eye movements, and cardiac activity while they synchronized their speech to the syllable rhythm and then continued the rhythm after stimulus offset. Syllable-rate synchronization performance exhibited the characteristic bimodal distribution that separated high and low synchronizers, allowing us to examine how individual differences in auditory-motor coupling shape the contributions of multiple motor systems to speech tracking. We found that vocalization showed spontaneous word-level tracking and timing shifts consistent with temporal prediction based on high-level structures, and these signatures were markedly stronger in high synchronizers. Meanwhile, covert oculomotor activity exhibited spontaneous syllable-rate tracking, occurring in high synchronizers only, with temporal dynamics that diverged from overt production. Finally, high synchronizers showed subtle structure-dependent modulation of autonomic cardiac activity. Together, these findings demonstrate that articulatory and oculomotor activity provide complementary contributions to speech tracking, and that the presence and organization of these contributions vary systematically across individuals. This multisystem approach offers behavioral and physiological markers for the embodied mechanisms of hierarchical speech processing.