Patterns of orofacial movement velocity across variations in speech rate.

To understand the clinical aspects of speech rate control, a clearer picture is needed of how orofacial structures are coordinated across variations in speech rate. To address this problem, patterns of orofacial tangential velocity or speed were analyzed in a group of 9 normal speakers as they produced the utterance "a bad daba" at fast, normal, and slow speech rates. An electromagnetic system was used to record the movements of the upper lip, lower lip, jaw, and tongue. Measures of the magnitude of peak tangential velocities were obtained across the four structures. Orofacial velocities consistently decreased at slow rates relative to normal rates, whereas at fast rates increased and decreased velocities were observed in an equivalent number of cases. Significant correlations frequently were obtained across speech rate between lip, tongue, and jaw velocities. Upper and lower lip velocities showed consistent positive correlations with one another, whereas marked intersubject differences were observed in the sign of jaw-related correlations. Repeated testing on 3 subjects indicated a high degree of consistency within subjects in the overall patterns of mean velocity for the different structures. Results are discussed in relation to possible motor control differences underlying fast and slow speech, neural coupling of muscle systems, and jaw-related individual differences in speech motor coordination.