In this preliminary report we investigate the effects of emotion on the lower lip movements during the production of a monosyllabic word in two different prosodic contexts: phrase initial and phrase final. The purpose is to examine the rigidity of the phrase boundary effects under emotional perturbation of the normal, or neutral, speech production. It is found that the effect of emotion is ubiquitous in that it affects all kinematic and dynamic parameters of the lower lip movements considered in this study. The effects of emotion are not arbitrary, however, and some common characteristics of emotional speech articulation can be identified. Firstly, speakers maintain the phrase boundary effects on the duration and movement amplitude of the lower lip gesture under emotional variations. Secondly, when considering all emotion types together in a given speaker, an approximately linear relationship holds between movement amplitudes and maximum lip opening velocities when emotion varies. Therefore, the stiffness of the lower lip opening gestures seems almost invariant under emotional variations, implying that the lower lip gesture is regulated by a simple harmonicoscillator like system with a constant stiffness. These and other findings support the hypothesis that speech production constraints imposed by the phrase boundary condition are maintained against emotional perturbations of speech articulations. In addition, the findings in the study may point to a general emotional speech production mechanism that the effects of emotion on the kinematics and dynamics of the oral speech articulators are manifested mainly in the arousal dimension of emotion.

Index Terms: emotion, lip movement, stiffness, phrase boundary