Submovements during pointing movements in Parkinson's disease.

Velocity irregularities frequently observed during deceleration of arm movements have usually been interpreted as corrective submovements that improve motion accuracy. This hypothesis is re-examined here in application to movements of Parkinson's disease (PD) patients in which submovements are specifically frequent. Pointing movements in patients and age-matched controls to large and small targets in three movement modes were studied. The modes were discrete (stop on the target), continuous (reverse on the target), and passing (stop after crossing the target). Two types of submovements were distinguished, gross and fine. In both groups, gross submovements were more frequent during the discrete and passing than continuous mode, specifically for large targets. This suggested that gross submovements were fluctuations accompanying motion termination (stabilization at the target) that was included in discrete and passing but not continuous movements. Gross submovements were specifically frequent in patients, suggesting that PD causes deficiency in smooth motion termination. Although in both groups fine submovements were more frequent for small than large targets, this relation was also observed in passing movements after crossing the target, i.e., when no corrections were needed. This result, together with higher jerk of the entire trajectory found for smaller targets, indicates that fine submovements may also be not corrective adjustments but rather velocity fluctuations emerging due to low speed of movements to small targets. This interpretation is consistent with the recognized inability of PD patients to promptly change generated force as well as to quickly re-plan current motion. The results suggest a need to re-examine the traditional interpretation of submovements in PD and the related theory that the production of iterative submovements is a strategy used by patients to compensate for a decreased initial force pulse.