Rate-specific template effects on the STI (Wisler et al., 2025)

<p dir="ltr"><b>Purpose: </b>The spatiotemporal index (STI) is widely used to measure variability across repeated speech stimuli. One of the most well-established findings is that STI values increase when speakers are cued to slow their articulatory rate. However, the mechanism behind this increase—and how it reflects changes in motor control—remains unclear. Recent evidence suggests STI differences may stem from template motion patterns as well as changes in motor stability. This study examines how variations in template motion patterns across speaking rates influence STI values.</p><p dir="ltr"><b>Method: </b>Kinematic data from 20 participants at three speaking rates (fast, habitual, slow) were used to estimate template motion patterns. These patterns were then used to simulate rate-specific template effects on STI values while holding motor stability constant.</p><p dir="ltr"><b>Results: </b>When motor stability is held constant, simulations show that STI values increase during slow speech (p = .003) and decrease during fast speech (p < .001), driven solely by variations in template motion patterns. In slow speech, these template effects increased the STI by 1.754, accounting for a sizable portion (31.9%) of the increased STI values observed in participants’ real speech data. In fast speech, template effects lowered the STI by 2.085.</p><p dir="ltr"><b>Conclusions: </b>These findings provide strong evidence that template motion patterns contribute to the elevated STI values observed in slow speech. Since these effects act in the opposite direction for fast speech, they may also be masking potential increases in motor instability when speaking quickly. While the results do not dispute that reduced motor stability contributes to increased STI values in slow speech, they suggest its role has likely been overestimated in prior research.</p><p dir="ltr"><b>Supplemental Material S1. </b>Justification for correlation-based template selection.</p><p dir="ltr">Wisler, A., Fletcher, A., & Mefferd, A. (2025). Does the spatiotemporal index (STI) overestimate instability in slow speech? Investigating template effects on the STI across different speaking rates. <i>Journal of Speech, Language, and Hearing Research. </i>Advance online publication. <a href="https://doi.org/10.1044/2025_JSLHR-25-00279" rel="noreferrer" target="_blank">https://doi.org/10.1044/2025_JSLHR-25-00279</a></p>