Motor equivalence coordination in the labial-mandibular system

Abstract

Upper lip, lower lip, and jaw displacements were recorded for repetitive productions of [æ, i, and ɛ] in [hV1bV2b] context. These data were analyzed to determine the displacement contribution of each of these articulators to the vertical dimension of oral opening. These analyses revealed that while the vertical oral opening remained relatively constant across repetitions, the individual contributions of the upper lip, lower lip, and jaw to that opening varied considerably. These observations suggested that the specific motor programs for these articulators varied reciprocally across repetitions, i.e., repetitions where the jaw had large displacements, the lower lip had correspondingly small displacements, and conversely. Such variation implies that programming of the labial-mandibular system for speech occurs functionally at two major levels (at least)—one which specifies overall oral opening and a second which generates specific commands for individual articulators. The second, and apparently lower level of programming, appears dependent upon the peripheral status and the motor programs of other coparticipating articulators. Hypotheses concerning neuromuscular mechanisms underlying these observations are presented. [Supported by NINDS grants N8-11780 and NS-10247.]