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 prehens ile action to objects requiring different levels of precision. Thus, we sou ght to determine if movement-accuracy requirements affect the control of mo vement path. Thirteen PD patients and 13 age-matched controls served as par ticipants. In addition to having prolonged movement times, PD patients show ed differences in the kinematic patterns of the transport and grasp compone nts. For the transport component, relative time to maximum deceleration and relative time to maximum elbow velocity occurred earlier for the PD patien ts than the controls. Analyses of wrist paths indicated that, when accuracy requirements were increased, patients produced paths that appeared more se gmented 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 devi ations 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 initia l movement in the horizontal plane regardless of accuracy constraints. Howe ver, PD patients had a significantly longer interval between initial moveme nt in the vertical plane and subsequent movement in the horizontal plane wh en reaching to the small object. Higher accuracy constraints also resulted in PD patients achieving relative time to maximum elbow velocity significan tly earlier than controls. For the grasp component, PD patients produced mo vement patterns in which the amplitude of and relative time to maximum aper ture were less sensitive to object size. In addition, patients exhibited gr eater variability in the time to maximum aperture. Additional analyses of t he grasp component indicated that control participants exhibited a stable p osition, relative to object location, in which aperture began to close. Con versely, PD patients showed little consistency in where aperture began to c lose with respect to object location. Irregularities in the transport compo nent suggest that PD patients have a reduced capability to precisely coordi nate joint segments, particularly under high accuracy requirements. Variabi lity in where aperture began to close and disruptions in transport-grasp co ordination suggests that the basal-ganglia dysfunction, as exhibited in PD, affects the specification of these movement parameters used to produce a c onsistent pattern of coordination between prehensile components.