Influences of Methodological Decisions on Assessing the Spatiotemporal Stability of Speech Movement Sequences

Abstract

Purpose: The spatiotemporal index (STI) is a widely used approach for measuring speech pattern stability across multiple repetitions of a stimulus. In this study, we examine how methodological choices in the implementation of the STI (including the number of repetitions, length of stimuli, and parsing procedure) can affect its value. Method: To evaluate how each methodological decision affects the STI, we use a synthetic data framework that allows for the generation of random productions of the template phrase “Buy Bobby a Puppy” at different stability levels. Within this framework, we conduct three experiments: Experiment 1 investigates the effects of the number of repetitions, Experiment 2 investigates the effects of stimulus length, and Experiment 3 investigates the effects of parsing errors. Results: In Experiment 1, we observed that STI values based on fewer repetitions will systematically underestimate larger repetition estimates. Experiment 2 showed that STI values will tend to be higher when calculated on longer (multimovement) stimuli independent of any differences in the stability of the underlying speech patterns. Finally, in Experiment 3, we showed that even minor parsing errors (≈ 10 ms) increase the value of the STI. Conclusions: The results of this study illustrate that even minor choices in the implementation of the STI can have a noticeable impact on the resulting value. These findings highlight the care that needs to be taken when designing studies and comparing STI values across studies to ensure that different STI values are capturing real differences in motion pattern stability rather than trivial methodological variation.

Tongue and Lip Acceleration as a Measure of Speech Decline in Amyotrophic Lateral Sclerosis

“…We must note that our study included fewer repetitions than the original formulation of STI, which was calculated based on 15 repetitions of a single phrase [22]. Although there is not an established minimum number of repetitions that should be used when calculating STI, having more repetitions improves reliability [58]. In our analysis, STI acceleration of the tongue and LL showed a statistically significant association with intelligible speaking rate.…”

Oromotor Nonverbal Performance and Speech Motor Control: Theory and Review of Empirical Evidence

“…Variability in speech production occurs at many levels, including articulator positions for different consonants (e.g., Refs. [80][81][82]), coordination between two articulators for a particular segment and even across segments (coarticulation, also referred to as coproduction), sentence-level timing (see review in [83]), and the many acoustic phonetic outputs related to segmental and prosodic characteristics of the speech signal. Many of the articulatory variabilities for vowel segments such as /u/ and the lax vowels [84] and rhotics and liquids [85,86] are in the service of acoustic stability; in this sense, these are "good" variabilities.…”

Abstracts of the 8th International Conference on Speech Motor Control Groningen, August 2022

“…Well-controlled articulatory movement patterns are believed to be highly consistent, whereas deviations in neuromuscular activity decreases movement stability. Although prior work has primarily attributed instability to increased demands on the motor system (e.g., task complexity, motor learning, disordered speech; McHenry, 2003;Saletta et al, 2018), it has recently been shown that in absence of behavioral changes, increased length (in syllables) in stimuli increases STI values in simulated data (Wisler et al, 2022), suggesting STI is more sensitive to temporal instability in longer stimuli. Less is known about how articulatory variability is impacted by syllable length in individuals with dysarthria due to ALS.…”