Synergetic Organization in Speech Rhythm

The Speech Cycling Task (SCT) is a novel experimental paradigm developed together with Robert Port and Keiichi Tajima at Indiana University. In a SCT, subjects repeat a phrase containing multiple prominent, or stressed, syllables in time with an external timer, which can be simple or complex. A phase-based collective variable is deened in the acoustic signal. This paper reports on two experiments using the SCT which together reveal many of the hallmarks of hierarchically coupled oscillatory processes. The rst experiment requires subjects to place the nal stressed syllable of a small phrase at speciied phases within the overall Phrase Repetition Cycle (PRC). It is clearly demonstrated that only three patterns, characterized by phases around 1 3 , 1 2 or 2 3 are reliably produced, and these points are attractors for other target phases. The system is thus multistable, and the attractors correspond to stable couplings between the metrical foot and the PRC. A second experiment examines the behavior of these attractors at increased rates. Faster rates lead to mode jumps between attractors. Previous experiments have also illustrated hysteresis as the system moves from one mode to the next. The dynamical organization is particularly interesting from a modeling point of view, as there is no single part of the speech production system which cycles at the level of either the metrical foot or the phrase repetition cycle. That is, there is no continuous kinematic observable in the system. Nonetheless, there is strong evidence that the macroscopic behavior of the entire production system is correctly described as hierarchically coupled oscillators. There are many parallels between this organization and the forms of inter-limb coupling observed in locomotion and rhythmic manual tasks.