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  • Journal of Phonetics (2002) 30, 261–279 doi:10.1006/jpho.2002.0173 Available online at http://www.idealibrary.com on

    Compensatory articulation during bilabial fricative production by regulating muscle stiffness

    Hiroaki Gomi* and Masaaki Honda NTT Communication Science Laboratories, NTT Corporation, Japan and CREST, Japan Science and Technology Corporation, Japan

    Takayuki Ito CREST, Japan Science and Technology Corporation, Japan

    Emi Z. Murano CREST, Japan Science and Technology Corporation, Japan and The University of Tokyo, Japan

    Received 9th November 2001, and accepted 3rd January 2002

    The cooperative mechanisms in articulatory movements were examined by using mechanical perturbations during bilabial phonemic tasks. The first experiment compares the differences in compensatory responses during sustained productions of the bilabial fricative /F/ for which lip constriction is required, and /a/, for which the lips and jaw are relatively relaxed. In the second experiment, we perturbed jaw movement with different load-onsets in the sentence ‘‘kono /aFaFa/ mitai’’. In both experiments, labial distances were recovered partly or fully by the downward shifts of the upper lip. The upper lip response was frequently prior to the EMG response observed in the sustained task. Additionally, initial downward displacement of the upper lip was frequently larger when the load was supplied during /F/ than when it was supplied during /a/ in the sustained and sentence tasks, respectively. The stiffness variation estimated by using a muscle linkage model indicates that the stiffness increases for the bilabial phonemic task in order to robustly configure a labial constriction. The results suggest that the change in passive stiffness regulated by the muscle activation level is important in generating quick cooperative articulation. r 2002 Elsevier Science Ltd. All rights reserved.

    *Address correspondence to H. Gomi, NTT Communication Science Laboratories, Wakamiya 3-1, Morinosato, Atsugi, Kanagawa 243-0198, Japan. E-mail: gomi@idea.brl.ntt.co.jp

    0095–4470/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

  • 262 H. Gomi et al.

    1. Introduction

    Although articulatory gestures vary depending on the speech context or constraining conditions, such as smoking a pipe or supporting the jaw with the arm, the articulatory organs cooperate to achieve the desired utterances. Cooperating articulatory movements can be seen not only under such planned or anticipated conditions, but also for unpredictable disturbances. When an unanticipated mechanical perturbation is inflicted on the lower lip during the bilabial explosive consonant /p/ or /b/, the closure between the upper and lower lips is accomplished by a downward shift of the upper lip (Abbs & Gracco, 1983, 1984; Gracco & Abbs, 1985). Similar compensatory movements were observed in bilabial phonemic tasks by applying an electrical perturbation to the lower lip (Folkins & Zimmermann, 1982) and in bilabial or linguo-dental phonemic tasks by applying a mechanical perturbation to the jaw (Folkins & Abbs, 1975; Kelso, Tuller, Bateson & Fowler, 1984; Shaiman, 1989).

    These studies have also demonstrated functional changes in the coordination of compensatory movements for different kinds of unanticipated perturbations. For example, Gracco & Abbs (1985) studied the changes in the coordination of upper and lower lips for different onsets of perturbations: delayed onset of perturbation caused a large nonautogenic compensation of the upper lip because there was insufficient time for autogenic compensation of the lower lip. Rapid changes in the electromyogram (EMG) (Abbs & Gracco, 1984; Kelso et al., 1984; Gracco & Abbs, 1985) and their task-dependencies imply a quick regulation mechanism of neural- linkage by suprabulbar pathways.

    In spite of these remarkable findings, compensatory responses without any change in the EMG have been reported (Kelso et al., 1984). Due to mechanical linkages between the lips, jaw, and tongue, as mentioned in Kelso et al. (1984), some passive dynamics (inertia of organs and/or muscle stiffness) could contribute to the cooperative (or task-conformable) behavior of these articulators. It has been suggested, on the other hand, that regulating the passive dynamics (mechanical impedance) is important when the arm is in interaction with manipulated objects or with external environments (Hogan, 1984; Mussa-Ivaldi Hogan & Bizzi, 1985), and that the mechanical impedance of the arm is governed by arm-kinematics and coordination of multiple-muscle activation (Mussa-Ivaldi et al., 1985; Gomi, 1998; Gomi & Osu, 1998).

    For articulatory movements, which involve mechanical interaction and coupling among lips, jaw, palate, and tongue, however, few attempts have so far been made at regulating the passive dynamics of articulators by changing muscle activity. Thus, the effect of the passive dynamics has not been well examined. In addition to the functional neural-linkages previously mentioned, we focus, in this study, on compensatory responses to a perturbation of the jaw for bilabial fricative consonants to clarify how the passive dynamics vary and how passive characteristics are exploited during coordinated articulatory movements.

    We carried out two experiments. In the first, by using sustained productions of /F/ and /a/, we focused on the behavioral and EMG responses of the articulators associated with jaw perturbation to examine passive and active (or neuronally driven) components. In the second, we observed compensatory responses for jaw perturbations with different onsets during sentence productions. Additionally, by

  • Compensatory articulation by muscle stiffness 263

    using these observed behaviors and a mechanical linkage model, the stiffness variation of the linkage between the upper lip and jaw were characterized. A part of this work has been presented elsewhere (Ito, Gomi & Honda, 2000a).

    2. Method

    2.1. Experimental setup

    Fig. 1(a) shows the experimental setup. The subject sat on a chair with his shoulders fixed to the back of the chair by straps, and his head was strapped to a head- support device. The jaw of the subject was tightly held between a formed metal/ plastic teeth-splint and a chin-plate, which were connected to the bar beneath the jaw by a piano wire. This mechanism enables us to perturb jaw motion in the jaw open/close directions by moving a bar connected to a torque motor (Shinmaywa DD-B09 with 1296k pulse/rev encoder) placed at the side of the head (roughly coinciding with jaw rotational center) (see Fig. 1(b)). Using a wire-driven

    Figure 1. (a) Jaw perturbation system and experimental setup. (b) Side view of the configuration of experimental setup and the movement direction. A screw between a formed metal/plastic teeth-splint and a chin plate tightly fixes the jaw.

  • 264 H. Gomi et al.

    mechanism, we achieve low constraints in the roll, pitch, and yaw of the jaw with respect to the wire-connection point to the bar, and can easily fix the head to the headrest. Previous methods have delivered forces to the end of the jaw splint protruding from the mouth thus producing rotational forces that may dislodge the splint and/or strain the teeth (Folkins & Abbs, 1975; Kelso et al., 1984; Shaiman, 1989). Our cantilever system eliminates these forces by applying a largely translational force (relative to the teeth) just under the chin.

    The torque motor was controlled by a Digital Signal Processer (Texas Instruments TMS320C40). A force sensor (Nitta UFS-3012A15) was installed between the motor and the beam. As a result of active load compensations by the controller, the gravitational forces on the jaw attachment, the bar, and the beam were cancelled, and their inertial forces were reduced to ensure natural movements as much as possible, resulting in an inertial load of 101 g.

    The positions of the upper lip, lower lip, jaw (beam of teeth-splint), and two nose markers were measured with a 3D-optical position sensor (OPTOTRAK 3020) at 250 Hz. The nose markers were used for off-line calibration. We confirmed that head movement caused by the jaw perturbation was too small to affect the markers’ positions (and their derivatives) relative to the nose. The acoustic signal was simultaneously recorded at 16 kHz after low-pass filtering (8 kHz).

    In the perturbation experiments of the sustained and sentence production tasks (see the Experimental procedure section), the EMG of orbicularis oris superior (OOS) and inferior (OOI) were recorded using bipolar surface electrodes (Ag/AgCl; MEDICOTEST NEUROLINE700) placed 18–20 mm apart center to center. These signals were digitally recorded (2 kHz) after filtering (50 Hz–1.5 kHz band pass), and then rectified and smoothed (temporal averaging of 5 ms for the sustained phonemic task to maintain time accuracy and 10 ms for the sentence production task). The electrodes for EMG of OOS were placed just above the vermilion border of the upper lip on the right side, and those for OOI were placed just below the vermilion border of the lower lip on the same side.

    To investigate the activities of the perioral muscles, in the distinct experiment (see Fig. 8), the EMG of the upper lip elevation muscles (ULE) and the depressor anguli oris (DAO) were recorded using a pair of insulated hooked wires (Alloy P91, 0.05 mm diameter) with their tips disinsulated 0.5 mm and bent. To insert the electrodes in the levator labii superioris (one of ULE), the needle (30 gauge, 19 mm) was inserted 10 mm deep at a point about 13 mm lateral and 5 mm above the inferior border of the nasal alae, and the tip of the needle was directed 45