Effect of Speaking Task on Intelligibility and Naturalness in Speakers with Parkinson's Disease and Cerebellar Disease

Abstract

It is well established that individuals with Parkinson's disease (PD) have difficulty performing skilled movement when forced to rely on internal cues versus responses guided by external stimuli. This discrepancy has been attributed to the dysfunctional basal ganglia lacking an adequate, internally generated model causing persons with PD to become overly reliant on external cues to guide skilled movement. The extent to which speech production aligns with theories of internal versus external cuing is not well understood. The purpose of this study was to examine the effect of an internally versus externally cued speech task on the understandability and naturalness of speakers with PD and a clinical comparison group of speakers with cerebellar disease (CD) as perceived by 10 experienced speech-language pathologists. A direct comparison was made between sentences extracted from a covertly recorded conversational sample (internally cued) and the reading of those same sentences by the speakers (externally cued). The listeners rated the speech samples using a visual analog scale for the perceptual dimensions of understandability and naturalness. Results suggest that experienced listeners perceived the speech of participants with PD as more natural and more understandable during the reading condition. The cerebellar group also demonstrated a difference between speaking conditions, but only for understandability. Thus, the percept of naturalness appeared to be sensitive to capturing the differences between speakers with Parkinson's disease and cerebellar disease.