Visually and posturally mediated tilt illusion in Parkinson's disease and in labyrinthine defective subjects.

We tested 24 normal subjects, 24 patients with idiopathic Parkinson's disease (PD), and eight patients with bilateral absence of vestibular function (labyrinthine defective [LD] subjects) in their ability to set a straight line to the perceived gravitational vertical (visual vertical). Measurements were taken in static conditions, sitting upright, and lying down on the right side, and during visual background motion at constant angular velocities around the line of sight (roll-motion) in both sitting upright and sideways position. Aims of the study were to determine if the reported increased "visual dependence" in PD was present in a psychophysical task that is independent of motor performance, and to examine the interaction between visual motion and proprioceptive cues in the perception of verticality, in the absence of vestibular function. LD patients showed abnormally large deviations of the visual vertical induced both by lateral body tilt and by visual roll-motion. This suggests that vestibular cues play a significant part in counterbalancing visually and proprioceptive mediated biases on the perception of verticality. In contrast, PD patients were normal in all these tasks indicating that visual dependence in PD is not present at an afferent/perceptual level.