The effects of varying lower-lip displacement on upper-lip movements: implications for the coordination of speech movements.

Upper-lip and lower-lip movements were transduced in the inferior-superior dimension in five normal-speaking subjects during four tasks. In task 1 visual feedback was used to manipulate the maximum displacement of the lower lip during speech. The upper lip elevated significantly less for the opening gesture when the amount of opening from the lower lip was increased. The upper lip moved to significantly lower positions for bilabial closure when the distance to be moved by the lower lip was increased. In task 2 the same procedures were followed with a bite block between the teeth. The bite block did not significantly change the interactions between lips for the opening gesture. The interactions were larger for bilabial closure with the bite block. In task 3 different vowels instead of visual feedback were used to manipulate lower-lip displacement. The relations between lips were similar to those found in task 1. In task 4 it was shown that these relations between lips are not found in nonspeech lower-lip movements. The interactions between lips are discussed in relation to models of speech motor control, including spatial targets, mass-spring systems, and planned trajectories.