The control of equilibrium in Parkinson's disease patients: Delayed adaptation of balancing strategy to shifts in sensory set during a dynamic task.

Processing of sensory information, timing operations and set-shifting can be affected in Parkinson’s disease (PD) patients.We investigated their
capacity and swiftness to pass from a kinaesthetic- to a vision-dependent behaviour during dynamic balancing on a continuously moving support
base. Nineteen on-phase PD patients and 13 age-matched normal subjects stood on a platform continuously translating in the antero-posterior
direction at 0.2 Hz. Body segment oscillations were identified by a stereophotogrammetric device and electromyogram (EMG) was recorded
from tibialis anterior and soleus. Under constant visual conditions, both patients and normal subjects roughly stabilised head and trunk in space
with eyes open (EO) but followed the platform displacement with eyes closed (EC). Amplitude and variability of the periodic EMG bursts were
smaller with EO than EC. Constant visual-condition trials were intermingled with trials in which subjects opened (EC–EO) or closed (EO–EC)
the eyes in response to an acoustic signal. Both patients and normal subjects changed kinematics and EMG patterns to those appropriate for the
new visual condition. However, PD patients were slower in changing their behaviour under the EC–EO condition. These findings show abnormal
temporal features in balancing strategy adaptation when shifting from kinaesthetic to visual reference in PD. The delay in the implementation of
the vision-dependent behaviour was unexpected, given the advantage vision is supposed to confer to motor performance in PD. This condition may
play a major role in the instability of patients performing dynamic postural tasks under changing sensory conditions.