Effects of Speaking Rate on Word Recognition in Parkinson's Disease and Normal Aging

Current theories of basal ganglia function emphasize their role in the integration of sensory information into motor activities, particularly in the control of movement timing. People with basal ganglia disorders such as Parkinson's disease exhibit poor temporal control of movements, in general and articulation in particular, as demonstrated by irregular speaking rate, reduced stress contrasts, and reduced movement durations and velocities. Previous research has implicated sensory deficits as contributory factors in limb movement control in patients with Parkinson's disease; however, the relation between sensory deficits and speech-movement abnormalities has not been documented. In the present study, the existence of perceptual processing difficulties of speaking rate was investigated in subjects with Parkinsonian dysarthria (PD). Comparisons in perception were made between subjects with PD, neurologically normal geriatrics (GN) and neurologically normal young adults (YN) for accuracy in identification of words presented at different speaking rates. We hypothesized that word-identification scores would be lower for PD and GN subjects compared to the YN subjects, an effect that was supported by the data. We also expected that there would be differences between the GN and PD subjects in their accuracy of word identification at a faster speaking rate, an hypothesis that was not supported by the data. Rather, GN and PD subjects differed in identification scores for words spoken at a slow rate. PD subjects who had faster habitual speaking rates (HSR) had significantly lower word-identification scores in the slow compared to conversational rate conditions, a relation that was significant r = +0.64). These data suggest the need to consider perceptual deficits as an additional factor that contributes to rate variations in PD speech.