Labial-mandibular coordination in the production of speech: implications for the operation of motor equivalence.

The purpose of this study was to explore select properties of speech movement coordination. Values of displacement for the upper lip, lower lip and jaw were obtained during repetitive productions of three vowels (ae, i, xi, in CVCVC contexts) by six subjects at two speaking rates. These data were analyzed to determine the relative contribution of each of these articulators to the superior-inferior distance between the upper and lower lips. The results of these analyses provided substantive evidence for the operation of motor equivalence in the speech movement coordination of the labial-mandibular system. The theoretical implications of these findings for the neural control mechanisms underlying speech movements are discussed.