Speech-like movements emerge from simulated peri-oral muscle activation space without neural control

Muscle synergies have been proposed as a solution to the degrees of freedom problem inherent in the biomechanics of speech production [Gick & Stavness, 2013, Front. Psych, 4, 977]. However, the majority of experiments involving the extraction of muscle synergies prove theoretically insufficient to determine whether such synergies are of neural origin or simply reflect the lower dimensionality of an under-sampled biomechanical/neural task space [Kutch & Valero-Cuevas, 2012, PLoS computational biology, 8(5), e1002434]. Therefore, to what extent biomechanics of the human vocal tract may constrain the “ecological state space” of a speaker during locution remains uninvestigated. As a proof of concept, we created a simplified version of the peri-oral region using FEM modeling in a physics-based simulator (ArtiSynth [Lloyd et al., 2012, Soft tissue biomech…surgery, pp. 355]). Systematic simulations enabled us to model the full kinematic/biomechanical space. Visualization of the resulting biomechanical state space using t-SNE [van der Maaten & Hinton, 2008, J.MLR., 9, 2579] illustrates that speech-like movements (e.g., lip rounding or spreading) emerge as self-organizing structures (muscle synergies) critically without a direct neural controller. These results are discussed in the context of current and future explorations of motor control of speech. [Funded by NSERC.]