Instability of syllable repetition as a model for impaired motor processing: is Parkinson's disease a "rhythm disorder"?

Alterations of speech rate and rhythm are well-known features in parkinsonian patients suffering from hypokinetic dysarthria and are thought to be induced by complex dysfunction of planning, preparing and executing of motor speech sequences. Since speech can be subdivided down to the level of single utterances, the aim of our study was to test the hypothesis of a fundamental impairment of vocal pace performance in parkinsonian patients based on a syllable repetition paradigm. Furthermore, the overall integrity of acoustical feedback mechanisms was surveyed by testing the ability to correctly identify pace and steadiness of the presented audio examples. N = 73 patients with Parkinson's disease (PD) and n = 43 age-matched healthy controls were tested. Participants had to repeat the syllable/pa/at least 30 times at a "comfortable" self-chosen steady (isochronous) pace. Percent coefficient of variance (COV) of interval length and the change in interval length with successive utterances were measured for the description of pace stability throughout the performance. Then, participants had to identify irregularities of 30 played-back audio tests. The number of incorrect classification was defined as the error rate (ER). Patients with PD showed significant difficulties in steadily executing a syllable repetition task with a significant elevation of COV and showed a clear tendency to pace acceleration in the course of the performance. However, we found no differences in the correct auditory identification of rhythm irregularities between the PD group and controls. As compared to healthy controls, the PD group features disabilities in performing a steady sequence of utterances, which cannot be explained solely by impaired acoustical feedback mechanisms. The pattern of pace disturbance shows similarities with the finding of speech acceleration and rhythm irregularity in the course of reading or more complex conversational speech and therefore might share the same pathophysiology. Further studies are necessary to reveal the correlations between speech rhythm and motor performance to identify the underlying mechanisms that lead to "dysrhythmia" in PD.