Speech timing in ataxic disorders: sentence production and rapid repetitive articulation.

We studied syllabic timing in patients with ataxia (10 with cerebellar atrophy, 6 with Friedreich's ataxia) under two conditions: in a "natural" sentence production context and in the context of a rapid syllable repetition task. The two tasks included comparable articulatory maneuvers. We measured syllable durations from the speech signal and analyzed variables describing average syllabic rate and within-trial variation of syllable durations. Among the observed measures, slowed syllable repetition was a particularly powerful predictor of the severity of dysarthric impairment. In sentence production, patients often performed at normal syllabic rates. Irregular pacing of syllable repetitions was frequent. Different patterns of between-articulator variation emerged in the two tasks. All patients except one were slower in rapid repetitive articulation than in sentence production. These data suggest that sentence production and rapid repetitive articulation are governed by basically different motor processes. The disproportionate slowing of ataxic patients in the repetitive task can be ascribed to adaptation to novel motor tasks being impaired in cerebellar disease.