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The speed-curvature power law is a celebrated law of motor control expressing a relation between the kinematic property of speed and the geometric property of curvature. We aimed to assess whether speech movements obey this law just as movements from other domains do. We describe a metronome-driven speech elicitation paradigm designed to cover a wide range of speeds. We recorded via electromagnetic articulometry speech movements in sequences of the form /CV…/ from nine speakers (five German, four English) speaking at eight distinct rates. First, we demonstrate that the paradigm of metronome-driven manipulations results in speech movement data consistent with earlier reports on the kinematics of speech production. Second, analysis of our data in their full three-dimensions and using advanced numerical differentiation methods offers stronger evidence for the law than that reported in previous studies devoted to its assessment. Finally, we demonstrate the presence of a clear rate dependency of the power law’s parameters. The robustness of the speed-curvature relation in our datasets lends further support to the hypothesis that the power law is a general feature of human movement. We place our results in the context of other work in movement control and consider implications for models of speech production.
The present work is a compilation of three original research articles submitted (or already published) in international peer-reviewed venues of the field of speech science. These three articles address the topics of fundamental motor laws in speech and dynamics of corresponding speech movements:
1. Kuberski, Stephan R. and Adamantios I. Gafos (2019). "The speed-curvature power law in tongue movements of repetitive speech". PLOS ONE 14(3). Public Library of Science. doi: 10.1371/journal.pone.0213851.
2. Kuberski, Stephan R. and Adamantios I. Gafos (In press). "Fitts' law in tongue movements of repetitive speech". Phonetica: International Journal of Phonetic Science. Karger Publishers. doi: 10.1159/000501644
3. Kuberski, Stephan R. and Adamantios I. Gafos (submitted). "Distinct phase space topologies of identical phonemic sequences". Language. Linguistic Society of America.
The present work introduces a metronome-driven speech elicitation paradigm in which participants were asked to utter repetitive sequences of elementary consonant-vowel syllables. This paradigm, explicitly designed to cover speech rates from a substantially wider range than has been explored so far in previous work, is demonstrated to satisfy the important prerequisites for assessing so far difficult to access aspects of speech. Specifically, the paradigm's extensive speech rate manipulation enabled elicitation of a great range of movement speeds as well as movement durations and excursions of the relevant effectors. The presence of such variation is a prerequisite to assessing whether invariant relations between these and other parameters exist and thus provides the foundation for a rigorous evaluation of the two laws examined in the first two contributions of this work.
In the data resulting from this paradigm, it is shown that speech movements obey the same fundamental laws as movements from other domains of motor control do. In particular, it is demonstrated that speech strongly adheres to the power law relation between speed and curvature of movement with a clear speech rate dependency of the power law's exponent. The often-sought or reported exponent of one third in the statement of the law is unique to a subclass of movements which corresponds to the range of faster rates under which a particular utterance is produced. For slower rates, significantly larger values than one third are observed. Furthermore, for the first time in speech this work uncovers evidence for the presence of Fitts' law. It is shown that, beyond a speaker-specific speech rate, speech movements of the tongue clearly obey Fitts' law by emergence of its characteristic linear relation between movement time and index of difficulty. For slower speech rates (when temporal pressure is small), no such relation is observed. The methods and datasets obtained in the two assessment above provide a rigorous foundation both for addressing implications for theories and models of speech as well as for better understanding the status of speech movements in the context of human movements in general.
All modern theories of language rely on a fundamental segmental hypothesis according to which the phonological message of an utterance is represented by a sequence of segments or phonemes. It is commonly assumed that each of these phonemes can be mapped to some unit of speech motor action, a so-called speech gesture.
For the first time here, it is demonstrated that the relation between the phonological description of simple utterances and the corresponding speech motor action is non-unique. Specifically, by the extensive speech rate manipulation in the herein used experimental paradigm it is demonstrated that speech exhibits clearly distinct dynamical organizations underlying the production of simple utterances. At slower speech rates, the dynamical organization underlying the repetitive production of elementary /CV/ syllables can be described by successive concatenations of closing and opening gestures, each with its own equilibrium point. As speech rate increases, the equilibria of opening and closing gestures are not equally stable yielding qualitatively different modes of organization with either a single equilibrium point of a combined opening-closing gesture or a periodic attractor unleashed by the disappearance of both equilibria. This observation, the non-uniqueness of the dynamical organization underlying what on the surface appear to be identical phonemic sequences, is an entirely new result in the domain of speech. Beyond that, the demonstration of periodic attractors in speech reveals that dynamical equilibrium point models do not account for all possible modes of speech motor behavior.
Fitts' law, perhaps the most celebrated law of human motor control, expresses a relation between the kinematic property of speed and the non-kinematic, task-specific property of accuracy. We aimed to assess whether speech movements obey this law using a metronome-driven speech elicitation paradigm with a systematic speech rate control. Specifically, using the paradigm of repetitive speech, we recorded via electromagnetic articulometry speech movement data in sequences of the form /CV.../ from 6 adult speakers. These sequences were spoken at 8 distinct rates ranging from extremely slow to extremely fast. Our results demonstrate, first, that the present paradigm of extensive metronome-driven manipulations satisfies the crucial prerequisites for evaluating Fitts' law in a subset of our elicited rates. Second, we uncover for the first time in speech evidence for Fitts' law at the faster rates and specifically beyond a participant-specific critical rate. We find no evidence for Fitts' law at the slowest metronome rates. Finally, we discuss implications of these results for models of speech.