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.
