The subjective visual vertical and the perceptual upright in Parkinson’s disease

Previous reports have suggested that patients with Parkinson’s disease (PD) have increased visual dominance for spatial perception tasks. We therefore assessed the relative roles of visual and non-visual cues in determining the subjective visual vertical (SVV) and the perceptual upright (PU) in patients with PD using a luminous line and the newly developed OCHART test (Exp. Brain. Res. 173; 612-622). Visual cues were manipulated by filling the visual field with a static 2D image containing either a frame (tilted ± 18º) or polarized room (tilted ±18º and ± 112.5º). Images were viewed on a laptop through a circular shroud (diameter: 42º). The SVV was measured by asking whether a line was tilted left or right of vertical. The PU was measured by identifying a character whose identity depended on its orientation and locating its most ambiguous orientations. To measure the relative contributions of visual, vestibular and body orientation cues, cues were separated by testing while upright or lying right side down. Patients with PD were tested on and off (12 hour washout) of their medications and compared to an age-matched control group. No differences were found attributable to medication. Patients with PD had a significantly higher variance for both the SVV and PU tasks compared to controls. When upright, patients with PD were more influenced by visual cues than controls for the SVV task. However, when tested lying on their sides, the control group was more influenced by visual cues for the SVV task. When both body orientations were taken together patients with PD were found to be more influenced by body orientation than visual cues for the SVV task. The PU in patients with PD was also found to be less influenced by visual cues than controls. This pattern of results suggests that PU and SVV may be determined by different mechanisms, with only the SVV showing an increased visual dependence in PD and then only when upright. We conclude that across different body postures, patients with PD are more influenced by the orientation of their body than visual cues in spatial perception tasks, that is, in general the body’s orientation is more heavily weighted in patients with PD than in age-matched controls