Visuomotor control in patients with Parkinson's disease.

Previous studies have suggested that the deteriorated visuomotor control in patients with PD (Parkinson's disease) is due to deficits in various aspects of the sensory-motor processing rather than motor control itself. In the current study, by taking a control-theoretic approach, we systematically examined how PD and antiparkinsonian medication affect visuomotor control and the underlying sensory-motor system. We tested 20 PD patients in both ON and OFF medication states and 20 demographically matched healthy controls with a commonly used manual control task. Specifically, in each 95-s trial, participants were instructed to use a joystick to control a randomly moving target to keep it centered on a computer display. We found that although antiparkinsonian medication improved visuomotor control in PD patients, they still showed significantly decreased control precision (measured by RMS error) and response amplitude (gain) as well as increased response delay (phase lag) compared with healthy controls. Our model-driven analysis revealed that PD impairs the responsiveness and the predicting ability of the sensory-motor system as well as the stability of the neuromuscular system. Taking antiparkinsonian medication improves the responsiveness of the sensory-motor system. More importantly, it improves the ability of the sensory-motor system to make sensory predictions of the current control actions (see Wolpert et al., 1995) to anticipate the input error signals and generate control responses ahead of time up to the level of healthy controls. However, taking antiparkinsonian medication does not improve the stability of the neuromuscular system. These results support the claim that the effect of antiparkinsonian medication on visuomotor control is mainly through improving visual-stimulus-dependent sensory-motor processing. The present study provides the first quantitative examination of the effects of PD and antiparkinsonian medication on the visual-stimulus-dependent sensory-motor and visual-stimulus-independent neuromuscular systems underlying visuomotor control. The findings have practical implications for developing sensitive assessment tools to evaluate the efficacy of different therapies for PD and preliminary screening and training tools for fitness-to-drive in PD patients.