Control of repetitive lip and finger movements in Parkinson's disease: influence of external timing signals and simultaneous execution on motor performance.

This study addresses the question of whether external timing signals and/or simultaneous rhythmic movements of other limbs can alleviate sequencing motor deficits associated with Parkinson's disease (PD). Subjects performed rhythmic lip and finger movements simultaneously or in isolation. In addition, they had to self-pace their movements or match them to an external signal. Our results are summarized as follows: (1) Seven of 12 patients had adequate mean repetition rates; that is, they fulfilled the task requirements on a global scale. The remaining five patients had various degrees of hastened responses and were not fully able to synchronize their movements to an external pacing signal. (b) PD patients exhibited hypometria in their finger tapping, but not in their lip movements. Their movements were not abnormally slowed, but peak velocity was appropriately scaled, even to reduced movement amplitudes. (c) Mean repetition rates, stability of frequency response, and kinematics did not differ between conditions of external and internal stimulation within the PD group, but were different form the control group performance. (d) Kinematic measures were not improved during dual-task execution. PD patients were not able to maintain a 1:1 rhythm between effectors. The incidence of hastening increased during simultaneous motor execution. We conclude that the use of external pacing signals might aid movement initiation of PD patients, but does not improve their temporal or spatial coordination when generating repetitive movements. Simultaneous execution does not necessarily enhance motor performance, but might actually have detrimental effects in patients prone to hastening.