Mixed modeling for irregularly sampled and correlated functional data: Speech science applications.

The speech sciences often employ complex experimental designs requiring models with multiple covariates and crossed random effects. For curve-like data such as time-varying signals, single-time-point feature extraction is commonly used as data reduction technique to make the data amenable to statistical hypothesis testing, thereby discarding a wealth of information. The present paper discusses the application of functional linear mixed models, a functional analogue to linear mixed models. This type of model allows for the holistic evaluation of curve dynamics for data with complex correlation structures due to repeated measures on subjects and stimulus items. The nonparametric, spline-based estimation technique allows for correlated functional data to be observed irregularly, or even sparsely. This means that information on variation in the temporal domain is preserved. Functional principal component analysis is used for parsimonious data representation and variance decomposition. The basic functionality and usage of the model is illustrated based on several case studies with different data types and experimental designs. The statistical method is broadly applicable to any types of data that consist of groups of curves, whether they are articulatory or acoustic time series data, or generally any types of data suitably modeled based on penalized splines.

The difficulty of articulatory complexity.

In our commentary, we offer some support for the view that frequency rather than a language-independent definition of complexity is a main factor determining speech production in healthy adults. We further discuss the limits of defining articulatory complexity based on transcription data. If we want to gauge the impact of substantive constraints on speech production, context-specific production dynamics should be considered, as has been underscored by articulatory-acoustic work on speech errors.

Articulatory and Acoustic Characteristics of German Fricative Clusters.

BACKGROUND: We investigate the articulatory-acoustic relationship in German fricative sequences. We pursue the possibility that /f/#sibilant and /s#ʃ/ sequences are in principle subject to articulatory overlap in a similar fashion, yet due to independent articulators being involved, there is a significant difference in the acoustic consequences. We also investigate the role of vowel context and stress. METHODS: We recorded electropalatographic and acoustic data from 9 native speakers of German. RESULTS: Results are compatible with the hypothesis that the temporal organization of fricative clusters is globally independent of cluster type with differences between clusters appearing mainly in degree. Articulatory overlap may be obscured acoustically by a labiodental constriction, similarly to what has been reported for stops. CONCLUSION: Our data suggest that similar principles of articulatory coordination underlie German fricative clusters independently of their segmental composition. The general auditory-acoustic patterning of the fricative sequences can be predicted by taking into account that aerodynamicacoustic consequences of gestural overlap may vary as a function of the articulators involved. We discuss possible sources for differences in degrees of overlap and place our results in the context of previously reported asymmetries among the fricatives in regressive place assimilation.

Voice onset time in consonant cluster errors: can phonetic accommodation differentiate cognitive from motor errors?

PURPOSE: Phonetic accommodation in speech errors has traditionally been used to identify the processing level at which an error has occurred. Recent studies have challenged the view that noncanonical productions may solely be due to phonetic, not phonological, processing irregularities, as previously assumed. The authors of the present study investigated the relationship between phonological and phonetic planning processes on the basis of voice onset time (VOT) behavior in consonant cluster errors. METHOD: Acoustic data from 22 German speakers were recorded while eliciting errors on sibilant-stop clusters. Analyses consider VOT duration as well as intensity and spectral properties of the sibilant. RESULTS: Of all incorrect responses, 28% failed to show accommodation. Sibilant intensity and spectral properties differed from correct responses irrespective of whether VOT was accommodated. CONCLUSIONS: The data overall do not allow using (a lack of) accommodation as a diagnostic as to the processing level at which an error has occurred. The data support speech production models that allow for an integrated view of phonological and phonetic processing.

Temporal organization of complex onsets and codas in American English: testing the predictions of a gestural coupling model.

This study systematically investigates the temporal organization of American English onset and coda consonant clusters on the basis of kinematic data. Results from seven speakers suggest that consonants in complex onsets are organized globally with respect to the following vowel, while consonants in complex codas are organized locally relative to the preceding vowel. These results support the competitive coupling model hypothesized for complex onsets, a model according to which consonant gestures in onsets are each coupled in-phase to the vowel, and antiphase with each other. The results are overall also consistent with the noncompetitive coupling relations assumed for codas, by which only the first consonant in a cluster is coupled antiphase with the vowel, and any subsequent consonants are coupled antiphase to each other. However, our data also show that the segmental composition of the cluster affects the timing relationship in codas, particularly/lC/coda clusters pattern differently from other clusters and do not adhere to the predicted timing relations. The data contribute to our understanding of the interaction of linguistic structure and motor control of the articulators in speech production.

Acoustic consequences of articulatory variability during productions of /t/ and /k/ and its implications for speech error research.

An increasing number of studies has linked certain types of articulatory or acoustic variability with speech errors, but no study has yet examined the relationship between such articulatory variability and acoustics. The present study aims to evaluate the acoustic properties of articulatorily errorful /k/ and /t/ stimuli to determine whether these errors are consistently reflected in the acoustics. The most frequent error observed in the articulatory data is the production of /k/ and /t/ with simultaneous tongue tip and tongue dorsum constrictions. Spectral analysis of these stimuli's bursts shows that /k/ and /t/ are differently affected by such co-production errors: co-production of tongue tip and tongue dorsum during intended /k/ results in typical /k/ spectra (and hence in tokens robustly classified as /k/), while co-productions during intended /t/ result in spectra with roughly equal prominence at both the mid-frequency (/k/-like) and high-frequency (/t/-like) ranges (and hence in tokens ambiguous between /k/ and /t/). This outcome is not due to an articulatory timing difference, but to tongue dorsum constriction having an overall greater effect on the acoustic than a tongue tip constriction when the two are co-produced.

An evaluation of the Aurora system as a flesh-point tracking tool for speech production research.

PURPOSE: Northern Digital Instruments (NDI; Waterloo, Ontario, Canada) manufactures a commercially available magnetometer device called Aurora that features real-time display of sensor position tracked in 3 dimensions. To test its potential for speech production research, data were collected to assess the measurement accuracy and reliability of the system. METHOD: First, sensors affixed at a known distance on a rigid ruler were moved systematically through the measurement space. Second, sensors attached to the speech articulators of a human participant were tracked during various speech tasks. RESULTS: In the ruler task, results showed mean distance errors of less than 1 mm, with some sensitivity to location within the measurement field. In the speech tasks, Euclidean distance between jaw-mounted sensors showed comparable accuracy; however, a high incidence of missing samples was observed, positively correlated with sensor velocity. CONCLUSIONS: The real-time positional feedback provided by the system makes it potentially useful in speech therapy applications. The overall missing data rate observed during speech tasks makes use of the system in its current form problematic for the quantitative measurement of speech articulator movements; however, NDI is actively working to improve the Aurora system for use in this context.

The role of a coda consonant as error trigger in repetition tasks.

Speech errors are known to exhibit an intrusion bias in that segments are added rather than deleted; also, a shared final consonant can cause an interaction of the initial consonants. A principled connection between these two phenomena has been drawn in a gestural account of errors: Articulatory measures revealed a preponderance of errors in which both the target and intruding gesture are co-produced, instead of one replacing the other. This gestural intrusion bias has been interpreted as an errorful coupling of gestures in a dynamically stable coordination mode (1:1, in-phase), triggered by the presence of a shared coda consonant. Capturing tongue motion with ultrasound, the current paper investigates whether shared gestural composition other than a coda can trigger gestural co-production errors. Subjects repeated two-word phrases with alternating initial stop or fricative consonants in a coda condition (e.g., top cop), a nocoda condition (e.g., taa kaa) and a three-word phrase condition (e.g., taa kaa taa). The no-coda condition showed a lower error rate than the coda condition. The three-word phrase condition elicited an intermediate error rate for the stop consonants, but a high error rate for the fricative alternations. While all conditions exhibited both substitution and co-production errors, a gestural intrusion bias emerged mainly for the coda condition. The findings suggest that the proportion of different error types (substitutions, co-production errors) differs as a function of stimulus type: not all alternating stimulus patterns that trigger errors result in an intrusion bias.

Tongue kinematics during utterances elicited with the SLIP technique.

In the past years, there have been an increasing number of instrumental investigations as to the nature of speech production errors, prompted by the concern that decades of transcription-based speech error data may be tainted by perceptual biases. While all of these instrumental studies suggest that errors are not, as previously thought, necessarily a matter of all-or-none, it is unclear what implications these studies have for phonological encoding as a cognitive process. Due to their repetition-based design, the ill-formed errors obtained in these studies may be articulation errors rather than cognitive planning errors. The present study reports for the first time tongue movement data collected during an error elicitation study based on the SLIP technique, which has traditionally been hypothesized to elicit errors at the phonological planning level. Results indicate that tongue kinematics during errors in the present task are comparable to those found in errorful utterances in repetition tasks. The findings are interpreted within a dynamic model of speech production as errors in phasing between the interacting consonant gestures.

Dynamic action units slip in speech production errors.

In the past, the nature of the compositional units proposed for spoken language has largely diverged from the types of control units pursued in the domains of other skilled motor tasks. A classic source of evidence as to the units structuring speech has been patterns observed in speech errors--"slips of the tongue". The present study reports, for the first time, on kinematic data from tongue and lip movements during speech errors elicited in the laboratory using a repetition task. Our data are consistent with the hypothesis that speech production results from the assembly of dynamically defined action units--gestures--in a linguistically structured environment. The experimental results support both the presence of gestural units and the dynamical properties of these units and their coordination. This study of speech articulation shows that it is possible to develop a principled account of spoken language within a more general theory of action.

A re-evaluation of the nature of speech errors in normal and disordered speakers.

It is well known that speech errors in normal and aphasic speakers share certain key characteristics. Traditionally, many of these errors are regarded as serial misorderings of abstract phonological segments, which maintain the phonetic well-formedness of the utterance. The current paper brings together the results of several articulatory studies undertaken independently for both subject populations. These show that, in an error, instead of one segment substituting for another, two segments are often produced simultaneously even though only one segment may be heard. Such data pose problems for current models of speech production by suggesting that the commonly assumed dichotomous distinction between phonological and phonetic errors may not be tenable in the current form or may even be altogether redundant.

Vowel production and perception: hyperarticulation without a hyperspace effect.

The ability of speakers to exaggerate speech sounds ("hyperarticulation") has led to the theory that the targets themselves must be hyperspace hyperarticulated. Johnson, Flemming, and Wright (1993) found that perceptual "best exemplar" choices for vowels were more speech extreme than listeners' own productions. Our first experiment, using their procedure, only partially replicated their results. Low vowels vowel perception showed a higher F1, consistent with hyperspace. Front vowels also showed more frontness in F2, but back vowels were less extreme ("hypoarticulated") on F2. Our second experiment used an identification and rating of each stimulus, yielding similar results of a smaller magnitude. Our results indicate that the perceptual space is calibrated to a particular (synthetic) vowel space, which is not related straightforwardly to the speakers' spaces. The original hyperspace hypothesis can be attributed to the methodology which led to extreme judgments and of the fronting of back vowels in California English. The present results indicate that no such hypothesis is needed. Vowel targets are measurable from an individual's productions, and the individual's perception of other speakers (even synthetic ones) is based on information about the vocal tract and dialect of the speaker.