Speech Movement Variability in People Who Stutter: A Vocal Tract Magnetic Resonance Imaging Study

An open-source toolbox for measuring vocal tract shape from real-time magnetic resonance images

Real-time magnetic resonance imaging (rtMRI) is a technique that provides high-contrast videographic data of human anatomy in motion. Applied to the vocal tract, it is a powerful method for capturing the dynamics of speech and other vocal behaviours by imaging structures internal to the mouth and throat. These images provide a means of studying the physiological basis for speech, singing, expressions of emotion, and swallowing that are otherwise not accessible for external observation. However, taking quantitative measurements from these images is notoriously difficult. We introduce a signal processing pipeline that produces outlines of the vocal tract from the lips to the larynx as a quantification of the dynamic morphology of the vocal tract. Our approach performs simple tissue classification, but constrained to a researcher-specified region of interest. This combination facilitates feature extraction while retaining the domain-specific expertise of a human analyst. We demonstrate that this pipeline generalises well across datasets covering behaviours such as speech, vocal size exaggeration, laughter, and whistling, as well as producing reliable outcomes across analysts, particularly among users with domain-specific expertise. With this article, we make this pipeline available for immediate use by the research community, and further suggest that it may contribute to the continued development of fully automated methods based on deep learning algorithms.

Real-time speech MRI datasets with corresponding articulator ground-truth segmentations

The use of real-time magnetic resonance imaging (rt-MRI) of speech is increasing in clinical practice and speech science research. Analysis of such images often requires segmentation of articulators and the vocal tract, and the community is turning to deep-learning-based methods to perform this segmentation. While there are publicly available rt-MRI datasets of speech, these do not include ground-truth (GT) segmentations, a key requirement for the development of deep-learning-based segmentation methods. To begin to address this barrier, this work presents rt-MRI speech datasets of five healthy adult volunteers with corresponding GT segmentations and velopharyngeal closure patterns. The images were acquired using standard clinical MRI scanners, coils and sequences to facilitate acquisition of similar images in other centres. The datasets include manually created GT segmentations of six anatomical features including the tongue, soft palate and vocal tract. In addition, this work makes code and instructions to implement a current state-of-the-art deep-learning-based method to segment rt-MRI speech datasets publicly available, thus providing the community and others with a starting point for developing such methods.

Neural oscillatory activity and connectivity in children who stutter during a non-speech motor task

BACKGROUND

Neural motor control rests on the dynamic interaction of cortical and subcortical regions, which is reflected in the modulation of oscillatory activity and connectivity in multiple frequency bands. Motor control is thought to be compromised in developmental stuttering, particularly involving circuits in the left hemisphere that support speech, movement initiation, and timing control. However, to date, evidence comes from adult studies, with a limited understanding of motor processes in childhood, closer to the onset of stuttering.

METHODS

We investigated the neural control of movement initiation in children who stutter and children who do not stutter by evaluating transient changes in EEG oscillatory activity (power, phase locking to button press) and connectivity (phase synchronization) during a simple button press motor task. We compared temporal changes in these oscillatory dynamics between the left and right hemispheres and between children who stutter and children who do not stutter, using mixed-model analysis of variance.

RESULTS

We found reduced modulation of left hemisphere oscillatory power, phase locking to button press and phase connectivity in children who stutter compared to children who do not stutter, consistent with previous findings of dysfunction within the left sensorimotor circuits. Interhemispheric connectivity was weaker at lower frequencies (delta, theta) and stronger in the beta band in children who stutter than in children who do not stutter.

CONCLUSIONS

Taken together, these findings indicate weaker engagement of the contralateral left motor network in children who stutter even during low-demand non-speech tasks, and suggest that the right hemisphere might be recruited to support sensorimotor processing in childhood stuttering. Differences in oscillatory dynamics occurred despite comparable task performance between groups, indicating that an altered balance of cortical activity might be a core aspect of stuttering, observable during normal motor behavior.

Temporal organization of syllables in paced and unpaced speech in children and adolescents who stutter

PURPOSE

Speaking with an external rhythm has a tremendous fluency-enhancing effect in people who stutter. The aim of the present study is to examine whether syllabic timing related to articulatory timing (c-center) would differ between children and adolescents who stutter and a matched control group in an unpaced vs. a paced condition.

METHODS

We recorded 48 German-speaking children and adolescents who stutter and a matched control group reading monosyllabic words with and without a metronome (unpaced and paced condition). Analyses were conducted on four minimal pairs that differed in onset complexity (simple vs. complex). The following acoustic correlates of a c-center effect were analyzed: vowel and consonant compression, acoustic intervals (time from c-center, left-edge, and right-edge to an anchor-point), and relative standard deviations of these intervals.

RESULTS

Both groups show acoustic correlates of a c-center effect (consonant compression, vowel compression, c-center organization, and more stable c-center intervals), independently of condition. However, the group who stutters had a more pronounced consonant compression effect. The metronome did not significantly affect syllabic organization but interval stability improved in the paced condition in both groups.

CONCLUSION

Children and adolescents who stutter and matched controls have a similar syllable organization, related to articulatory timing, regardless of paced or unpaced speech. However, consonant onset timing differs between the group who stutters and the control group; this is a promising basis for conducting an articulatory study in which articulatory (gestural) timing can be examined in more detail.

Real-time magnetic resonance imaging reveals distinct vocal tract configurations during spontaneous and volitional laughter

A substantial body of acoustic and behavioural evidence points to the existence of two broad categories of laughter in humans: spontaneous laughter that is emotionally genuine and somewhat involuntary, and volitional laughter that is produced on demand. In this study, we tested the hypothesis that these are also physiologically distinct vocalizations, by measuring and comparing them using real-time magnetic resonance imaging (rtMRI) of the vocal tract. Following Ruch and Ekman (Ruch and Ekman 2001 In Emotions, qualia, and consciousness (ed. A Kaszniak), pp. 426-443), we further predicted that spontaneous laughter should be relatively less speech-like (i.e. less articulate) than volitional laughter. We collected rtMRI data from five adult human participants during spontaneous laughter, volitional laughter and spoken vowels. We report distinguishable vocal tract shapes during the vocalic portions of these three vocalization types, where volitional laughs were intermediate between spontaneous laughs and vowels. Inspection of local features within the vocal tract across the different vocalization types offers some additional support for Ruch and Ekman's predictions. We discuss our findings in light of a dual pathway hypothesis for the neural control of human volitional and spontaneous vocal behaviours, identifying tongue shape and velum lowering as potential biomarkers of spontaneous laughter to be investigated in future research. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'.

Characteristics of articulatory gestures in stuttered speech: A case study using real-time magnetic resonance imaging

INTRODUCTION

Most of the previous articulatory studies of stuttering have focussed on the fluent speech of people who stutter. However, to better understand what causes the actual moments of stuttering, it is necessary to probe articulatory behaviors during stuttered speech. We examined the supralaryngeal articulatory characteristics of stuttered speech using real-time structural magnetic resonance imaging (RT-MRI). We investigated how articulatory gestures differ across stuttered and fluent speech of the same speaker.

METHODS

Vocal tract movements of an adult man who stutters during a pseudoword reading task were recorded using RT-MRI. Four regions of interest (ROIs) were defined on RT-MRI image sequences around the lips, tongue tip, tongue body, and velum. The variation of pixel intensity in each ROI over time provided an estimate of the movement of these four articulators.

RESULTS

All disfluencies occurred on syllable-initial consonants. Three articulatory patterns were identified. Pattern 1 showed smooth gestural formation and release like fluent speech. Patterns 2 and 3 showed delayed release of gestures due to articulator fixation or oscillation respectively. Block and prolongation corresponded to either pattern 1 or 2. Repetition corresponded to pattern 3 or a mix of patterns. Gestures for disfluent consonants typically exhibited a greater constriction than fluent gestures, which was rarely corrected during disfluencies. Gestures for the upcoming vowel were initiated and executed during these consonant disfluencies, achieving a tongue body position similar to the fluent counterpart.

CONCLUSION

Different perceptual types of disfluencies did not necessarily result from distinct articulatory patterns, highlighting the importance of collecting articulatory data of stuttering. Disfluencies on syllable-initial consonants were related to the delayed release and the overshoot of consonant gestures, rather than the delayed initiation of vowel gestures. This suggests that stuttering does not arise from problems with planning the vowel gestures, but rather with releasing the overly constricted consonant gestures.

Structural brain network topological alterations in stuttering adults

Persistent developmental stuttering is a speech disorder that primarily affects normal speech fluency but encompasses a complex set of symptoms ranging from reduced sensorimotor integration to socioemotional challenges. Here, we investigated the whole brain structural connectome and its topological alterations in adults who stutter. Diffusion weighted imaging data of 33 subjects (13 adults who stutter and 20 fluent speakers) was obtained along with a stuttering severity evaluation. The structural brain network properties were analyzed using Network-based statistics and graph theoretical measures particularly focusing on community structure, network hubs and controllability. Bayesian power estimation was used to assess the reliability of the structural connectivity differences by examining the effect size. The analysis revealed reliable and wide-spread decreases in connectivity for adults who stutter in regions associated with sensorimotor, cognitive, emotional, and memory-related functions. The community detection algorithms revealed different subnetworks for fluent speakers and adults who stutter, indicating considerable network adaptation in adults who stutter. Average and modal controllability differed between groups in a subnetwork encompassing frontal brain regions and parts of the basal ganglia.

The results revealed extensive structural network alterations and substantial adaptation in neural architecture in adults who stutter well beyond the sensorimotor network. These findings highlight the impact of the neurodevelopmental effects of persistent stuttering on neural organization and the importance of examining the full structural connectome and the network alterations that underscore the behavioral phenotype.

Speech kinematic variability in adults who stutter is influenced by treatment and speaking style

AIM

We tested whether completion of the Comprehensive Stuttering Program (CSP) is associated with a reduction in speech kinematic variability relative to pre-treatment when adults who stutter (AWS) use a casual speaking manner or fluency skills.

RATIONAL

Kinematic variability is higher in AWS suggesting a sensorimotor vulnerability; however, it is not clear whether high variability is a trait related to the underlying disorder or reflects the mutable state of stuttering. Speech restructuring intervention such as the CSP could support more consistent articulatory control and stable movement patterns.

METHODOLOGY

Thirteen AWS were tested before and after completing the CSP while 11 adults who do not stutter (AWNS) completed a single session. Participants were instructed to use a casual manner of speaking in the first post-treatment session. In the second post-treatment condition, the AWS employed their fluency skills at a control speaking rate. An optical tracking system captured lower lip movements while participants spoke two English phrases and a complex nonword. Across-utterance kinematic variability was measured using the spatiotemporal index (STI) and within-utterance variability was measured with recurrence quantification analysis (RQA).

RESULTS

There was a positive treatment outcome based on significant reductions in percentage syllables stuttered (%SS) during speaking and reading, decreases in stuttering severity and improved perceptions of stuttering and communication confidence. The STI of the AWS decreased significantly after treatment for both speaking styles. The RQA variables indicated that AWS used a less stereotyped and more flexible manner of speaking in the casual condition after treatment, but speech movement regularity increased when using fluency skills.

CONCLUSIONS

The AWS showed a significant decrease in labial kinematic variability alongside a successful treatment outcome involving speech restructuring and cognitive behavioral techniques. These changes in across-utterance and within-utterance kinematic indices demonstrate that effective stuttering treatment can promote speech motor stability along with fluent speech.

Individual differences in vocal size exaggeration

The human voice carries socially relevant information such as how authoritative, dominant, and attractive the speaker sounds. However, some speakers may be able to manipulate listeners by modulating the shape and size of their vocal tract to exaggerate certain characteristics of their voice. We analysed the veridical size of speakers’ vocal tracts using real-time magnetic resonance imaging as they volitionally modulated their voice to sound larger or smaller, corresponding changes to the size implied by the acoustics of their voice, and their influence over the perceptions of listeners. Individual differences in this ability were marked, spanning from nearly incapable to nearly perfect vocal modulation, and was consistent across modalities of measurement. Further research is needed to determine whether speakers who are effective at vocal size exaggeration are better able to manipulate their social environment, and whether this variation is an inherited quality of the individual, or the result of life experiences such as vocal training.

Contemporary Theories of Stuttering Development

The article presents a theoretical analysis of contemporary models of persistent stuttering development in children and adults at the current period of development of science. The accumulated amount of scientific knowledge suggests that stuttering has a neurological basis: it is associated with disorders in the structure and function of the brain. On this basis, there have been emerged models of stuttering that link the cause of a speech disorder with an unstable speech motor system. Theories and models of stuttering based on cognitive and language processing are likely to be useful in that they have explanatory power in relation to the mechanisms that play an important role in the production of key symptoms of stuttering. Considering that stuttering is a complex disorder, the logical result of this was the proposal of multifactorial models of impaired speech fluency. The presented overview may be useful to psychiatrists, clinical psychologists, speech therapists, teachers, and practitioners interacting with children and adults with stuttering and other speech fluency disorders.