Association of orofacial with laryngeal and respiratory motor output during speech.

Speech motor coordination most likely involves synaptic coupling among neural systems that innervate orofacial, laryngeal, and respiratory muscles. The nature and strength of coupling of the orofacial with the respiratory and laryngeal systems was studied indirectly by correlating orofacial speeds with fundamental frequency, vocal intensity, and inspiratory volume during speech. Fourteen adult subjects repeated a simple test utterance at varying rates and vocal intensities while recordings were obtained of the acoustic signal and movements of the upper lip, lower lip, tongue, jaw, rib cage, and abdomen. Across subjects and orofacial speed measures (14 subjects x 4 structures), significant correlations were obtained for fundamental frequency in 42 of 56 cases, for intensity in 35 of 56 cases, and for inspiratory volume in 14 of 56 cases. These results suggest that during speech production there is significant neural coupling of orofacial muscle systems with the laryngeal and respiratory systems as they are involved in vocalization. Comparisons across the four orofacial structures revealed higher correlations for the jaw relative to other orofacial structures. This suggests stronger connectivity between neural systems linking the jaw with the laryngeal and respiratory systems. This finding may be relevant to the frame/content theory of speech production, which suggests that the neural circuitry involved in jaw motor control for speech has evolved to form relatively strong linkages with systems involved in vocalization.