We developed a novel method to dissociate motor commands for visually guided smooth pursuit movement of the wrist joint into three functionally distinct components. The primary component that belongs to the lower frequency range (0~0.5Hz; F1 component) represents an output from a predictive controller that synchronizes movement of the wrist joint with motion of the target continuously. The second component that belongs to the higher frequency range (0.5~3Hz =F2 component) represents an output from a feedback controller that corrects positional errors intermittently. The third component that belongs to even higher frequency range (3~8Hz =F3 component) represents small oscillatory or step-wise movement. We name this component microsteps. Despite some overlap of the frequency range with the resting tremor, the microsteps should be distinguished from the resting tremor because it is observed during tracking movement. In this study, we evaluated the wrist tracking movement of patients with Parkinson's disease (PD) in terms of the three components (F1, F2 and F3), and examined relationship between the three components and the motor symptoms of PD. Nine PD patients participated in this study, and their motor symptoms was assessed by the Unified Parkinson's Disease Rating Scale (UPDRS) Part III (motor examination).
UPDRS Part III was strongly correlated with the prediction error from the predictive controller (R2=0.73), suggesting that UPDRS Part III comprehensively reflects the state of the predictive controller. On the other hand, UPDRS Part III was poorly correlated with the accuracy of the feedback controller and the amount of the microsteps. The result appeared to represent diversity of pathophysiological conditions of PD patients. Namely, PD patients with the same score of UPDRS Part III could be divided into subgroups with respect to the F2 and F3 components of movement.
Overall, our proposed method allowed us to study various aspects of the pathophysiological condition of PD patients in terms of three different motor functions; predictive control, feedback control, and involuntary microsteps movement.