Granger causality analysis of internal tongue muscles in tongue protrusion from diffusion and tagged MRI

The human tongue is a muscular hydrostat, comprising a group of orthogonally oriented and interdigitated muscles without skeletal support. Key characteristics of the tongue as a muscular hydrostat include that the tongue’s volume is constant, and any decrease in one dimension will be compensated by an increase in another dimension. This work aims to identify causal relationships among internal muscles in protrusive muscle contractions, which is vital to understand their underlying muscle coordination patterns. We investigated four muscles known to contribute to tongue protrusion, including transverse, verticalis, superior longitudinal, and genioglossus muscles. To capture tongue structure and function, we acquired diffusion MRI and tagged MRI to estimate the muscle fiber geometry of the four muscles and the motion patterns of tongue protrusion, respectively. We first computed Lagrangian strains along muscle fiber direction in each voxel, reflecting muscle fiber shortening. Average strain values in each muscle, defined by a vocal tract atlas, were then input into a linear vector autoregression Granger causality model, to investigate their causal relationships. Our findings on the strain patterns over time as well as the pairwise causal relationships were consistent in part with the muscular hydrostat theory, which helps elucidate the tongue muscle coordination patterns.