Dynamic aspects of lower lip movement in parkinsonian and neurologically normal geriatric speakers' production of stress.

Lower lip+jaw movement was evaluated for parkinsonian dysarthric and age-matched, neurologically normal speakers during the production of alternating stress contrasts. Discrete measures of movement, including displacement amplitude, peak velocity, the relation of amplitude to peak velocity, and movement durations were compared across groups for stressed and unstressed syllables. Additionally, quantitative (parameter c and the ratio of acceleration to deceleration) and qualitative indices of dynamic characteristics of velocity profiles for lower lip+jaw opening and closing gestures were compared across the subject groups. Within the dysarthric group, the relation between each discrete and dynamic kinematic parameter and the perceived severity of the dysarthria was investigated. The discrete measures confirmed previous findings of reduced displacement and peak velocity for the parkinsonian speakers during opening and closing gestures for both the stressed and unstressed syllables. However, the relation between amplitude and velocity did not differ for the two subject groups for any gesture. Movement durations were equivalent for the two groups during the production of opening gestures, but were significantly shorter for the parkinsonian speakers during closing gestures. Quantitative indices of the velocity profiles also failed to differentiate between the subject groups or between dysarthric speakers as a function of severity. By contrast, the qualitative descriptions of the velocity profiles showed between-speaker differences that were more pronounced for subjects with more severe dysarthria. These qualitative differences were evident in opening gestures toward an unstressed vowel, only.