One of the most fundamental problems in research on spoken language is to understand how the categorical, systemic knowledge that speakers have in the form of a phonological grammar maps onto the continuous, high-dimensional physical speech act that transmits the linguistic message. The invariant units of phonological analysis have no invariant analogue in the signal—any given phoneme can manifest itself in many possible variants, depending on context, speech rate, utterance position and the like, and the acoustic cues for a given phoneme are spread out over time across multiple linguistic units. Speakers and listeners are highly knowledgeable about the lawfully structured variation in the signal and they skillfully exploit articulatory and acoustic trading relations when speaking and perceiving. For the scientific description of spoken language understanding this association between abstract, discrete categories and continuous speech dynamics remains a formidable challenge. Articulatory Phonology and the associated Task Dynamic model present one particular proposal on how to step up to this challenge using the mathematics of dynamical systems with the central insight being that spoken language is fundamentally based on the production and perception of linguistically defined patterns of motion. In Articulatory Phonology, primitive units of phonological representation are called gestures. Gestures are defined based on linear second order differential equations, giving them inherent spatial and temporal specifications. Gestures control the vocal tract at a macroscopic level, harnessing the many degrees of freedom in the vocal tract into low-dimensional control units. Phonology, in this model, thus directly governs the spatial and temporal orchestration of vocal tract actions.
