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

Abstract

Purpose People who stutter (PWS) have more unstable speech motor systems than people who are typically fluent (PWTF). Here, we used real-time magnetic resonance imaging (MRI) of the vocal tract to assess variability and duration of movements of different articulators in PWS and PWTF during fluent speech production. Method The vocal tracts of 28 adults with moderate to severe stuttering and 20 PWTF were scanned using MRI while repeating simple and complex pseudowords. Midsagittal images of the vocal tract from lips to larynx were reconstructed at 33.3 frames per second. For each participant, we measured the variability and duration of movements across multiple repetitions of the pseudowords in three selected articulators: the lips, tongue body, and velum. Results PWS showed significantly greater speech movement variability than PWTF during fluent repetitions of pseudowords. The group difference was most evident for measurements of lip aperture using these stimuli, as reported previously, but here, we report that movements of the tongue body and velum were also affected during the same utterances. Variability was not affected by phonological complexity. Speech movement variability was unrelated to stuttering severity within the PWS group. PWS also showed longer speech movement durations relative to PWTF for fluent repetitions of multisyllabic pseudowords, and this group difference was even more evident as complexity increased. Conclusions Using real-time MRI of the vocal tract, we found that PWS produced more variable movements than PWTF even during fluent productions of simple pseudowords. PWS also took longer to produce multisyllabic words relative to PWTF, particularly when words were more complex. This indicates general, trait-level differences in the control of the articulators between PWS and PWTF. Supplemental Material https://doi.org/10.23641/asha.14782092.

Figure 1.

Image analysis pipeline. (A) Example (single frame) of the reconstructed image. (B) Using the air–tissue boundary toolbox (Kim et al., 2014), the airway was identified manually by drawing a line through the open vocal tract (yellow line). The lowest point of the upper lip, back of the hard palate, and larynx were also identified manually (red dots). (C) Equally spaced gridlines were placed orthogonal to the yellow line and centered on it. Gridlines highlighted in red were the ones used for tracking the tongue body and velum separately (see text). (D) Tracking of air–tissue boundaries. Upper boundary shown in green; lower boundary shown in red.

Figure 2.

Examples of movement traces. Each plot shows 10 repetitions of the words (A) “mab,” (B) “mabshaytiedoib,” and (C) “mabteebeebee” for a single representative participant. Each line represents one repetition. The start and end points are defined as the frame where lip aperture departs from zero for the /m/ and returns to zero for the final /b/, respectively.

Figure 3.

Variability of articulator movements over repeated utterances of the pseudoword set. CoV = coefficient of variation; PWS = people who stutter; PWTF = people who are typically fluent; 4c = four-syllable, complex word (“mabshaytiedoib”); 4 s = four-syllable, simple word (“mabteebeebee”). Violin plots are shown to visualize the distribution of data and its probability density for each group separately for each syllable set. Solid horizontal lines represent the median, and dashed lines show the interquartile range.

Figure 4.

Individual variability scores for pseudowords with one to three syllables and the complex (4c) and simple (4s) four-syllable pseudowords. Red participants = people who stutter; blue participants = people who are typically fluent. Data from some participants are missing due to speech errors (see the Analysis Plan section). SSI scores are shown above individual data plots for people who stutter. * indicates data missing for one pseudoword. CoV = coefficient of variation.

Figure 5.

Duration of responses. PWS = people who stutter; PWTF = people who are typically fluent; 4c = four-syllable, complex word (“mabshaytiedoib”); 4s = four-syllable, simple word (“mabteebeebee”). Violin plots are shown to visualize the distribution of data and its probability density for each group separately for each syllable set. Solid horizontal lines represent the median, and dashed lines show the interquartile range.