Investigating functional unit interactions via Granger causal analysis using diffusion and tagged MRI

The human tongue is a muscular hydrostat that performs critical roles in speech, swallowing, and mastication through highly coordinated muscle activity. Previous research applied Granger causality analysis to time-series strain values from individual tongue muscles, revealing predictive relationships that shed light on their sequential interactions during protrusive and speech-related tasks. Building on these findings, this study shifts the focus from single muscles to functional units of tongue motion—groups of cohesive local muscle regions—to better understand how they collaborate to produce articulate speech. We collected diffusion MRI to capture muscle fiber orientation and tagged MRI to capture motion dynamics from four participants while they articulated the phrase “a kouk.” After validating the stationarity of the strains along fiber orientations with statistical tests, we identified optimal time lags and applied Granger causality analysis to evaluate predictive relationships among these functional units. Our pipeline provides new insights into tongue-movement coordination, enabling improved predictions of motion patterns and articulatory behaviors. These findings not only enhance our understanding of the biomechanics of speech but also hold promise for informing rehabilitative strategies for individuals with speech disorders.