Disruptions in the reach-to-grasp actions of Parkinson's patients.

Previous research has shown that Parkinson's-disease (PD) patients produce irregular movement paths during a rapid arm pointing task. The aim of this study was to investigate the movement paths of PD patients during a prehensile action to objects requiring different levels of precision. Thus, we sought to determine if movement-accuracy requirements affect the control of movement path. Thirteen PD patients and 13 age-matched controls served as participants. In addition to having prolonged movement times, PD patients showed differences in the kinematic patterns of the transport and grasp components. For the transport component, relative time to maximum deceleration and relative time to maximum elbow velocity occurred earlier for the PD patients than the controls. Analyses of wrist paths indicated that, when accuracy requirements were increased, patients produced paths that appeared more segmented than controls. For PD patients, reaches to a small object resulted in wrist paths that were significantly less smooth, as reflected by higher jerk values, and were less continuous, as indicated by larger standard deviations in curvature. A temporal analysis of movement-initiation patterns in the vertical and horizontal planes indicated that control participants had a minimal offset between initial movement in the vertical plane and initial movement in the horizontal plane regardless of accuracy constraints. However, PD patients had a significantly longer interval between initial movement in the vertical plane and subsequent movement in the horizontal plane when reaching to the small object. Higher accuracy constraints also resulted in PD patients achieving relative time to maximum elbow velocity significantly earlier than controls. For the grasp component, PD patients produced movement patterns in which the amplitude of and relative time to maximum aperture were less sensitive to object size. In addition, patients exhibited greater variability in the time to maximum aperture. Additional analyses of the grasp component indicated that control participants exhibited a stable position, relative to object location, in which aperture began to close. Conversely, PD patients showed little consistency in where aperture began to close with respect to object location. Irregularities in the transport component suggest that PD patients have a reduced capability to precisely coordinate joint segments, particularly under high accuracy requirements. Variability in where aperture began to close and disruptions in transport-grasp coordination suggests that the basal-ganglia dysfunction, as exhibited in PD, affects the specification of these movement parameters used to produce a consistent pattern of coordination between prehensile components.