Task-dependent variations in parkinsonian motor impairments. Studies of visually-guided arm movements in patients with Parkinson's disease (PD) have suggested a relationship between slowness of movement and a reduction in the ratio of movement amplitude and peak velocity. Recent studies indicate, however, that PD impairments may be different for well-learned, natural movements performed without visual guidance, or movements associated with sequential motor behaviours such as speech. To address this issue, PD subjects and age/sex-matched controls were compared on the performance of three tasks, all of which required lowering the jaw: (1) single, rapid, visually-guided movements; (2) equivalent movements associated with a single speech syllable (inherently without visual guidance), and (3) well-learned speech movements produced in a natural sequence. PD subjects manifested similar deficits for visually-guided jaw lowering as those previously reported for arm movements, namely reduced velocity/amplitude ratios and increased movement durations. By contrast, analogous jaw movements during the sequential speech tasks were unimpaired on these measures. These results suggest that PD motor impairments are influenced by a variety of factors, including the degree to which tasks are familiar and natural, and the availability of visual information. 