Instability of syllable repetition in Parkinson's disease—Impairment of automated speech performance?

Steady vocal pace performance according to a multi-level syllable repetition paradigm has been previously shown to be compromised in patients in advanced stages of Parkinson's disease (PD) under chronic deep brain stimulation. The aim of the present study was to implement the same syllable repetition test in mildy to moderately affected patients in order to survey if (a) the characteristic pattern of abnormalities can be observed even in early stages of PD and (b) if there are correlations to motor symptoms of PD. Sixty-four patients with PD who were on stable dopaminergic medication and 40 healthy subjects had to repeat a single syllable (/pa/) or a pair of alternating syllables (/pa-ti/) in a self chosen steady pace or in a given pace of 80/min. As main results, PD patients’ pace performance was (1) already irregular in the simple steady syllable repetition task, (2) even more impaired when a given pace had to be kept and (3) showed a further decline when two alternating syllables had to be repeated in a given pace; (4) measures of irregular pace performance showed a correlation to axial motor symptoms. This pattern of impaired vocal pace performance was similar, although less pronounced, to the previously reported findings in advanced PD and therefore might be a characteristic feature of PD. The results could be interpreted as a hint for increasing disruption of automaticity in speech performance with rising complexity of the task which can be found even in early stages of PD and correlate to axial motor symptoms.