Melanie L. Matthies

Electromagnetic midsagittal articulometer systems for transducing speech articulatory movements

This paper describes two electromagnetic midsagittal articulometer (EMMA) systems that were developed for transducing articulatory movements during speech production. Alternating magnetic fields are generated by transmitter coils that are mounted in an assembly that fits on the head of a speaker. The fields induce alternating voltages in a number of small transducer coils that are attached to articulators in the midline plane, inside and outside the vocal tract. The transducers are connected by fine lead wires to receiver electronics whose output voltages are processed to yield measures of transducer locations as a function of time. Measurement error can arise with this method, because as the articulators move and change shape, the transducers can undergo a varying amount of rotational misalignment with respect to the transmitter axes; both systems are designed to correct for transducer misalignment. For this purpose, one system uses two transmitters and biaxial transducers; the other uses three transmitters and single-axis transducers. The systems have been compared with one another in terms of their performance, human subjects compatibility, and ease of use. Both systems can produce useful midsagittal-plane data on articular movement, and each one has a specific set of advantages and limitations. (Two commercially available systems are also described briefly for comparison purposes). If appropriate experimental controls are used, the three-transmitter system is preferable for practical reasons.

Speech motor control: acoustic goals, saturation effects, auditory feedback and internal models

Abstract A theoretical overview and supporting data are presented about the control of the segmental component of speech production. Findings of “motor-equivalent” trading relations between the contributions of two constrictions to the same acoustic transfer function provide preliminary support for the idea that segmental control is based on acoustic or auditory-perceptual goals. The goals are determined partly by non-linear, quantal relations (called “saturation effects”) between motor commands and articulatory movements and between articulation and sound. Since processing times would be too long to allow the use of auditory feedback for closed-loop error correction in achieving acoustic goals, the control mechanism must use a robust “internal model” of the relation between articulation and the sound output that is learned during speech acquisition. Studies of the speech of cochlear implant and bilateral acoustic neuroma patients provide evidence supporting two roles for auditory feedback in adults: maintenance of the internal model, and monitoring the acoustic environment to help assure intelligibility by guiding relatively rapid adjustments in “postural” parameters underlying average sound level, speaking rate and the amount of prosodically-based inflection of F0 and SPL.

Articulatory tradeoffs reduce acoustic variability during American English /r/ production

The American English phoneme /r/ has long been associated with large amounts of articulatory variability during production. This paper investigates the hypothesis that the articulatory variations used by a speaker to produce /r/ in different contexts exhibit systematic tradeoffs, or articulatory trading relations, that act to maintain a relatively stable acoustic signal despite the large variations in vocal tract shape. Acoustic and articulatory recordings were collected from seven speakers producing /r/ in five phonetic contexts. For every speaker, the different articulator configurations used to produce /r/ in the different phonetic contexts showed systematic tradeoffs, as evidenced by significant correlations between the positions of transducers mounted on the tongue. Analysis of acoustic and articulatory variabilities revealed that these tradeoffs act to reduce acoustic variability, thus allowing relatively large contextual variations in vocal tract shape for /r/ without seriously degrading the primary acoustic cue. Furthermore, some subjects appeared to use completely different articulatory gestures to produce /r/ in different phonetic contexts. When viewed in light of current models of speech movement control, these results appear to favor models that utilize an acoustic or auditory target for each phoneme over models that utilize a vocal tract shape target for each phoneme.

Trading relations between tongue-body raising and lip rounding in production of the vowel/u/ : a pilot motor equivalence study

Articulatory and acoustic data were used to explore the following hypothesis for the vowel /u/: The objective of articulatory movements is an acoustic goal; varying and reciprocal contributions of different articulators may help to constrain acoustic variation in achieving the goal. Previous articulatory studies of similar hypotheses, expressed entirely in articulatory terms, have been confounded by interdependencies of the variables being studied (e.g., lip and mandible displacements). One case in which this problem may be minimized is that of lip rounding and tongue-body raising (formation of a velo-palatal constriction) for the vowel /u/. Lip rounding and tongue-body raising should have similar acoustic effects for /u/, mainly to lower F2. In multiple repetitions, reciprocal contributions of lip rounding and tongue-body raising could help limit F2 variability for /u/; thus this experiment looked for complementary covariation (negative correlations) in measures of these two parameters. An electro-magnetic midsagittal articulometer (EMMA) was used to track movements of midsagittal points on the tongue body, upper and lower lips, and mandible for large numbers of repetitions of utterances containing /u/. (Interpretation of the data was aided by results from area-function-to-formant modeling.) Three of four subjects showed weak negative correlations, tentatively supporting the hypothesis; a fourth showed the opposite pattern: positive correlations of lip rounding and tongue raising. The results are discussed with respect to ideas about motor equivalence, the nature of speech motor programming, and potential improvements to the paradigm.

Goal-based speech motor control: A theoretical framework and some preliminary data

Abstract A theoretical framework for the segmental component of speech production is outlined and some preliminary supporting data are reviewed. According to the framework, articulatory movements are programmed to achieve sequences of goals that are defined in terms of articulatory and acoustic parameters. The goals are correlates of distinctive features. Some feature correlates are determined by quantal (non-linear) relations between articulation and sound. Goals may also be influenced by other principles, such as a compromise between sufficient perceptual contrast and economy of articulatory effort, which leads to the prediction that the goal definitions correspond to regions (as opposed to points) in acoustic and articulatory space. Thus the goals are characterized by some parameter variation, which is possible partly because listeners can understand variable speech. Before utterances are produced, goal specifications are modified by prosodic influences and reduction. The sequence of modified goal specifications is converted to smooth, appropriately-timed articulatory movements by the speech motor control system. This control and the resulting kinematics are constrained in part by the biomechanical properties of the articulators. To help keep acoustic variability within perceptually-acceptable limits, speech motor control mechanisms may include a strategy by which different parts of the vocal-tract area function are adjusted in a complementary (“motor equivalent”) manner. The strategy takes advantage of the fact that for some sounds, a similar acoustic transfer function can be achieved with somewhat different area functions. The existence of such a strategy and the idea that speech motor programming is based in part on acoustic goals are supported by data that show trading relations between lip rounding and tongue-body raising in production of the vowel /u/.

Changes in sound pressure and fundamental frequency contours following changes in hearing status.

Sound-pressure level (SPL) and fundamental frequency (F0) contours were obtained from four postlingually deafened adults who received cochlear implants and from a subject with Neurofibromatosis-2 (NF2) who had her hearing severely reduced following surgery to remove an auditory-nerve tumor and to implant an auditory brainstem implant. SPL and F0 contours for each phrase in passages read before and after changes in hearing were averaged over repeated readings and then normalized with respect to the highest SPL or F0 value in the contour. The regularity of each average contour was measured by calculating differences between successive syllable means and averaging the absolute values of these differences. With auditory feedback made available, the cochlear implant user with the least contour variation preimplant showed no change but all of the remaining speakers produced less variable F0 contours and three also produced less variable SPL contours. In complementary fashion, when the NF2 speaker had her auditory feedback severely reduced, she produced more variable F0 and SPL contours. The results are interpreted as supporting a dual-process theory of the role of auditory feedback in speech production, according to which one role of self-hearing is to monitor transmission conditions, leading the speaker to make changes in speech postures aimed at maintaining intelligibility.

A preliminary study of the effects of cochlear implants on the production of sibilants

The potential influence of auditory information in the production of /s/ and /integral of/ was explored for postlingually deafened adults with four-channel Ineraid cochlear implants. Analyses of the spectra of the sibilant sounds were compared for speech obtained prior to implant activation, after early implant use and after 6 months of use. In addition, the output of the Ineraid device (measured at each of the four electrodes) was analyzed with pre- and postactivation speech samples to explore whether the speech production changes were potentially audible to the cochlear-implant user. Results indicated that subjects who showed abnormally low or incorrect contrast between /s/ and /integral of/ preactivation, and who received significant auditory benefit from their implants were able to increase the distinctiveness of their productions of the two speech sounds.

Tongue surface displacement during bilabial stops

The goals of this study were to characterize tongue surface displacement during production of bilabial stops and to refine current estimates of vocal-tract wall impedance using direct measurements of displacement in the vocal tract during closure. In addition, evidence was obtained to test the competing claims of passive and active enlargement of the vocal tract during voicing. Tongue displacement was measured and tongue compliance was estimated in four subjects during production of /aba/ and /apa/. All subjects showed more tongue displacement during /aba/ than during /apa/, even though peak intraoral pressure is lower for /aba/. In consequence, compliance estimates were much higher for /aba/, ranging from 5.1 to 8.5 x 10(-5) cm3/dyn. Compliance values for /apa/ ranged from 0.8 to 2.3 x 10(-5) cm3/dyn for the tongue body, and 0.52 x 10(-5) for the single tongue tip point that was measured. From combined analyses of tongue displacement and intraoral pressure waveforms for one subject, it was concluded that smaller tongue displacements for /p/ than for /b/ may be due to active stiffening of the tongue during /p/, or to intentional relaxation of tongue muscles during /b/ (in conjunction with active tongue displacement during /b/).

Changes in speech intelligibility of postlingually deaf adults after cochlear implantation.

Objective This study examines changes in the intelligibility of CVC words spoken by postlingually deafened adults after they have had 6 to 12 mo of experience with a cochlear implant. The hypothesis guiding the research is that the intelligibility of these speakers will improve after extended use of a cochlear implant. The paper also describes changes in CVC word intelligibility analyzed by phoneme class and by features. Design The speech of eight postlingually deaf adults was recorded before activation of the speech processors of their cochlear implants and at 6 mo and 1 yr after activation. Seventeen listeners with no known impairment of hearing completed a word identification task while listening to each implant user’s speech in noise. The percent information transmitted by the speakers in their pre- and postactivation recordings was measured for 11 English consonants and eight vowels separately. Results An overall improvement in word intelligibility was observed: seven of the eight speakers showed improvement in vowel intelligibility and six speakers showed improvement in consonant intelligibility. However, the intelligibility of specific consonant and vowel features varied greatly across speakers. Conclusions Extended use of a cochlear implant by postlingually deafened adults tends to enhance their intelligibility.

Gestural timing effects in the ‘‘perfect memory’’ sequence observed under three rates by electromagnetometry

In a well‐known example due to Browman and Goldstein [Lab. Phon. I, 341–376 (1990)] the /ktm/ sequence in the phrase ‘‘perfect memory’’ is contrasted between careful (list) and fluent production conditions. X‐ray microbeam data were used to show that although in the fluent case coproduction of the /m/ can mask the acoustic releases of the /k/ and /t/, both stops are nonetheless articulated. The current work uses EMMA data to examine this sequence in greater detail: 8 subjects produced the phrase in a carrier context under normal, fast, and clear rate conditions. Results confirm that tongue dorsum and tip movements toward velar and apical closure occur regardless of rate and observable acoustic effect. In addition, while movement amplitudes decreased somewhat as rate increased, little variation in the durations associated with the consonant gestures was observed. Instead, changes in rate primarily effected V to V duration and the relative phasing of the velar and apical closing gestures: the tongue tip max...