Optimization

This chapter introduces a theoretical framework, Optimal Control Theory, which will enable a phonology-extrinsic-timing-based, three-component model to determine values of controlled variables, and to model the influence of multiple factors on these parameter values. Key features of Optimal Control Theory models are discussed, as well as evidence for types of movement costs (including the cost of time) used in the models, and predictions of the models for the coordination of multiple effectors and hierarchical control. Finally, the chapter reviews Optimal Control Theory models currently used to account for timing phenomena in speech.

The prosodic governance of surface phonetic variation: Support for an alternative approach III

Effects of prosodic structure on surface phonetics are modeled in AP/TD in two ways: 1) via a set of PI and MuT adjustment mechanisms used to model lengthening effects at boundaries and on prominent syllables, and 2) via a hierarchy of coupled syllable, cross-word foot, and phrase oscillators, used to model poly-subconstituent shortening effects, and to control overall speech rate. These mechanisms are challenged by 1) findings presented in previous chapters that suggest that longer durations associated with boundaries and prominences are due to longer surface duration specifications, 2) findings presented here that show that polysyllabic shortening does not affect all words in an utterance, inconsistent with an oscillator-based mechanism that controls all aspects of any produced utterance, and 3) findings relating to speech rate presented in previous chapters which suggest that speech rate specifications relate to surface durations, rather than to planning oscillator frequencies. Patterns of speech timing presented in this chapter thus suggest that there are reasons to be uncertain whether periodicity is a major factor in speech motor control in typical speaking circumstances, and therefore call into question the use of suprasegmental oscillators.

Evidence motivating consideration of an alternative approach

This chapter begins to motivate the development of an alternative approach to speech production by pointing out three potential difficulties with the highly-successful Articulatory Phonology/Task Dynamics approach. First, it discusses the extensive nature of modifications to AP/TD default specifications required to account for the wide variety of surface phonetic forms. The need for a large number of adjustments in AP/TD raises questions about the appropriateness of the AP/TD default-adjustment approach, which would have been more appropriate if the default, non-prominent, phrase-medial, normal-speech-rate specifications could be used most of the time. Second, it discusses the lack of a principled explanation for behaviors described by Fitts’ law. While the theory can accommodate some aspects of Fitts’ law, others are not explained or accommodated. Finally, it suggests that AP/TD’s gestural score architecture raises the risk of spatial interference among overlapping, independent gestures. These three challenges taken together set the stage for the discussion of additional challenges in Chapter 4, which further motivate consideration of phonology-extrinsic-timing-based approaches to speech motor control.

Articulatory Phonology/Task Dynamics

This chapter summarizes the basic mechanisms of the Articulatory Phonology model, currently the most thoroughly worked-out model in the literature, with a focus on its system-intrinsic mechanisms used to account for systematic variation in speech timing. Key features of the model are reviewed, and oscillator-based mechanisms are described for timing control for articulatory gestures, control of inter-gestural coordination, prosodic timing control, and the control of overall speech rate. Strengths of the AP/TD approach are discussed, which include facts that are well-accounted-for within this model, such as the predominance of CV syllables within the world’s languages, as well as characteristics of processing within the model that are assumed to be advantageous, such as the avoidance of the need to explicitly plan the details of articulatory movement when planning an utterance. This presentation forms the basis of the evaluation presented in subsequent chapters.

Summary and conclusion

This volume compares two very different approaches to modeling speech planning: Articulatory Phonology, with quantitative phonological representations and a set of phonology-intrinsic timing mechanisms, and XT/3C, an alternative model with non-quantitative symbolic phonological representations and general-purpose phonology-extrinsic timing mechanisms. It argues that the motor-control literature for both speech and non-speech supports the XT/3C approach, which expands on earlier models based on Generative Phonology to include a Phonological Planning Component to set the symbolic goals for an utterance, a separate Phonetic Planning Component to provide the quantitative target specifications for the utterance, and a Motor-Sensory Implementation Component to track and adjust the movements required to reach those targets on time. It preserves the insights of a symbol-based phonology, while also providing a comprehensive account of systematic phonetic variation, including timing.

Coordination: Support for an alternative approach II

Two key features of the current AP/TD coupled-oscillator approach to movement coordination are that 1) coordination among gestures is treated as relative timing control, accomplished via planning-oscillator phase relationships, rather than coordination based on spatial information or absolute timing, and 2) coordination is based on the (relative) timing of movement onsets, rather than the timing of target achievement. Evidence bearing on both of these issues suggests that 1) patterns of relative timing do not necessarily require implementation via oscillator phase relationships, and 2) coordination is often based on the part of movement most closely related to the goal (often the endpoint), rather than on movement onsets (as proposed in recent versions of AP/TD). This chapter includes a discussion of alternative, i.e. non-oscillator-based, mechanisms that can model both the coordination of movements for synchronous target achievement, and the planning of movement timing when targets are sequential.

A sketch of a Phonology-Extrinsic-Timing-Based Three-Component model of speech production

This chapter presents the outline of a model of speech-production planning, based on symbolic phonology and the specification of surface-timing patterns using general-purpose timekeeping mechanisms. This phonology-extrinsic-timing-based, three-component (XT/3C) model includes a Phonological Planning Component, to set and prioritize the goals for an utterance; a Phonetic Planning Component, to quantitatively specify the acoustic targets and the movements to achieve them; and a Motor-Sensory Implementation component, to track the planned movements and adjust them to ensure that the targets are reached on time. This approach addresses some of the gaps in earlier speech-production models based on abstract symbolic phonology, by proposing a mechanism for the specification of context-appropriate surface phonetic variation, including timing. In this way it provides an alternative to the Task-Dynamics-based approach embodied in Articulatory Phonology.

How do timing mechanisms work?

This chapter addresses the nature of the general-purpose timekeeping mechanisms that are assumed in phonology-extrinsic-timing models of speech production. The first part of the chapter discusses some current questions about the nature of these mechanisms. The second part of the chapter presents Lee’s General Tau theory (Lee 1998, 2009), a theory of the temporal guidance of action in voluntary movement. This theory provides a crucial component for our phonology-extrinsic-timing-based, three-component model of speech production because its tau-coupling mechanism provides a way to plan movements with appropriate velocity profiles, as well as endpoint-based movement coordination. In doing so, it provides a general-purpose, phonology-extrinsic alternative to AP/TD’s use of oscillators for the control of the time-course of articulatory movement and coordination.

Evidence for an alternative approach to speech production, with three model components

Evidence presented in previous chapters suggests consideration of an alternative to the coupled-oscillator approach to modeling human speech planning and production processes. One alternative approach is based on the symbolic phonemic representations of Generative Phonology. This approach requires a separate mechanism to translate these symbolic representations into the quantitative context-appropriate specifications that can produce the surface form of an utterance. To date models of this type have not included comprehensive versions of components that complete this symbol-to-quantification translation process. This chapter reviews the evidence that supports a three-component model of this process, with an abstract symbolic Phonological Planning Component to specify the goals of the utterance at the level of contrastive feature cues; a Phonetic Planning Component to translate these goals into acoustic targets with their articulatory realizations; and a Motor-Sensory Implementation Component to track and adjust the articulatory movements which create the vocal-tract configurations that realize these goals. Development of such a three-component model would allow direct comparison with performance of the oscillator-based AP/TD model.

Phonology-extrinsic timing: Support for an alternative approach I

This chapter presents evidence that challenges models in which phonological representations are temporal in nature, and where timing mechanisms are phonology-specific and intrinsic to the phonology. For example, evidence for separate representations of 1) movement targets vs. other parts of movement, and 2) spatial vs. temporal aspects, is difficult to account for in phonology-intrinsic timing approaches, where all parts of movement are determined by the same, spatiotemporal phonological representation. In the AP/TD phonology-intrinsic-timing approach, surface time is not represented, specified, or tracked, but instead emerges from the phonological system. Evidence that speakers do in fact represent surface time motivates the consideration of a phonology-extrinsic-timing-based approach. This evidence comes from actors’ interactions with perceived events, and also from speech: 1) constraints on the amounts of structure-related lengthening to maintain phonological length contrasts in quantity languages, and 2) different articulatory strategies for producing a given duration pattern. Finally, evidence for general-purpose timekeeping mechanisms in speech and non-speech movements is discussed.