Dynamic estimation of hand position is abnormal in Parkinson's disease

Parkinson's disease (PD) is widely viewed as a disorder of central motor control. However, recent studies suggest that disordered kinesthetic processing may also contribute to bradykinesia and hypometria in PD. To examine the hypothesis that abnormal kinesthesia in PD would result in impaired hand motion estimation used for motor control, we tested PD patients, elderly people, and young adults in an active, multi-joint kinesthetic-to-visual matching task. To minimize initial localization errors, visual information about the starting position was always available. The participants performed center-out drawing movements to visual targets in the absence of visual feedback of hand/pen motion at their preferred speed. Movement time (MT), end-point position error (EPE), and initial directional error (IDE) were measured. No detrimental effects of aging were observed; however, the PD group showed prolonged MTs and higher EPE scores as compared to the elderly and young groups. Principal component analysis of the end-point error distributions showed that the PD patients had larger variability in both the extent and direction axes. These results suggest that PD patients have abnormal proprioception and deficits in the central processing and integration of kinesthetic signals, resulting in the incorrect assembly of multiple sensorimotor inputs into a motor plan. It is hypothesized that altered kinesthesia in PD causes improper estimation of hand motion used for motor control due to the degraded maintenance of a dynamic internal hand representation.