Bidirectional interference between speech and postural stability in individuals with Parkinson's disease.

Because people frequently talk while engaged in other activities, and because Parkinson's disease (PD) is known to diminish multi-tasking performance, this study examined dual task interference between speaking and postural stability in nine individuals with PD, seven age-matched, and 10 healthy young controls. Participants repeated a target utterance and performed a rise to toes task in both single and dual task conditions. Diphthong transitions were measured from audio recordings and postural variables reflecting planning, coordination, and stability were derived from a multi-camera motion capture system and force plate recordings. Thus, sensitive measures of both speech and postural control were obtained. The group with PD performed more poorly than both control groups for the isolated postural task, but their single task speech measures did not differ from the controls, in spite of listener ratings which indicated mild-to-moderate dysarthria severity. The group with PD showed evidence of bidirectional dual task interference in that there were reduced diphthong extents and slopes along with smaller, slower, and less stable postural movements. These results indicate that concurrent performance of speech and a challenging postural control task impairs speech and postural stability in persons with PD and may result in greater risk during daily activities.